US20170130366A1

US20170130366A1 - Nanofiber structure and manufacturing method thereof

Info

Publication number: US20170130366A1
Application number: US15/342,954
Authority: US
Inventors: Kaori Yasufuku; Tetsuo Hino; Kenji Takashima; Kazuhiro Yamauchi
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2015-11-06
Filing date: 2016-11-03
Publication date: 2017-05-11
Also published as: JP2017089028A

Abstract

In a nanofiber structure in which a nanofiber A and a nanofiber B are tangled with each other, the softening point of the nanofiber A is different from that of the nanofiber B, a cross-section along a surface of the nanofiber A orthogonal to a longitudinal direction thereof has a shape having a concave portion, and the nanofiber A and the nanofiber B are fused with each other at this concave portion.

Description

BACKGROUND OF THE INVENTION

Field of the Invention
The present disclosure relates to a nanofiber structure and a manufacturing method thereof.
Description of the Related Art
In recent years, attention has been paid to a nanofiber structure, such as a polymer nanofiber sheet (hereinafter, referred to as “nanofiber sheet” in some cases), which is formed in such a way that at least two nanofibers each containing a polymer material are integrated and are tangled with each other in a three-dimensional manner.
As one characteristic of the polymer nanofiber structure, a large specific surface area may be mentioned. In order to obtain an excellent effect using the above characteristic, as a constituent material of the polymer nanofiber structure, at least two types of materials are used in some cases. For example, a polymer nanofiber sheet in which a nanofiber primarily formed from a hydrophilic polymer and a nanofiber primarily formed from a hydrophobic polymer are laminated to each other is used as a leakage prevention sheet having flexibility and a high water absorbability. In addition, when a polymer nanofiber sheet is formed using a nanofiber formed of a material in which a functional group is introduced into a part of a mother polymer and a nanofiber formed of this mother polymer material, a polymer nanofiber sheet having a desired function can be obtained while the mechanical strength thereof is maintained. This polymer nanofiber sheet may be used, for example, as a constituent material of a drug delivery system or the like.
In addition, the nanofiber contained in the nanofiber sheet has been formed to have a special shape other than a cylindrical or an oval shape. When at least a part of the nanofiber is formed to have a special shape, for example, advantages in that nanofibers are likely to be tangled with each other, the touch of the nanofiber sheet is changed, and very small dust and dirt are effectively collected can be expected.
However, in a related sheet-shaped nanofiber structure, although nanofibers were tangled with each other in a three-dimensional manner, this tangled structure was formed by the physical entanglement therebetween. Hence, the mechanical strength and the durability of the nanofiber structure itself were inferior. As a method to overcome this problem, there has been used a method in which after a cross-linking agent is added or applied to the inside of nanofibers or the surfaces thereof, portions at which the nanofibers are intersected with each other are chemically cross-linked by application of physical energy so that the nanofibers are bound with each other. In addition, as another method, for example, there may be used a method in which by application of physical energy, at least some portions at which nanofibers are intersected with each other are fused. Japanese Patent Laid-Open Nos. 2-57254 and 2-234965 have disclosed a sheet manufactured using at least two types of nanofibers formed from different types of polymers and a method in which when a sheet is manufactured, at least some portions at which nanofibers are intersected with each other are fused.
However, when at least two types of nanofibers are contained in a nanofiber structure, and when polymer materials which are constituent materials of the nanofibers are different from each other, the materials to be used and the combination therebetween may be restricted in some cases. In addition, in the case as described above, by the methods disclosed in Japanese Patent Laid-Open Nos. 2-57254 and 2-234965, adhesion at the portions at which the nanofibers are intersected with each other may not be sufficient in some cases. Incidentally, by the method disclosed in Japanese Patent Laid-Open No. 2-57254, the material which can be used is limited to a material having a hydrophilic functional group. In addition, in Japanese Patent Laid-Open No. 2-234965, at the portion at which the nanofibers are intersected with each other, although thermal pressure bonding of the nanofibers is performed using a convex portion of a thermal emboss besides the adhesion at the portions described above, by the method described above, the durability of the nanofiber structure itself may not be sufficient in some cases.

SUMMARY OF THE INVENTION

The present disclosure provides a nanofiber structure having excellent durability.
A nanofiber structure of the present disclosure comprises: a nanofiber A and a nanofiber B tangled with the nanofiber A, wherein:
the softening point of the nanofiber A is different from that of the nanofiber B, a cross-section along a surface of the nanofiber A orthogonal to a longitudinal direction thereof has a shape having a concave portion, and in the concave portion, the nanofiber A and the nanofiber B are fused with each other.
According to the present disclosure, a nanofiber structure having excellent durability can be provided.
Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view showing one example of an embodiment of a nanofiber structure.

FIG. 1B is a cross-sectional view showing a lump portion surrounded by a dotted line represented by 1B shown in FIG. 1A.

FIG. 2A is a schematic view showing a concrete example of a method for observing a nanofiber structure.

FIG. 2B is a partially enlarged view of a IIB portion shown in FIG. 2A.

FIG. 3 is a schematic view showing one example of a cross-sectional shape of a nanofiber A.

FIG. 4 is a schematic view showing one example of an apparatus of forming a precursor.

FIG. 5 is a photo obtained by laser microscope observation.

FIG. 6 is a view showing a sample to be used when the tensile elastic modulus of a nanofiber structure is measured.

DESCRIPTION OF THE EMBODIMENTS

The present disclosure relates to a nanofiber structure in which at least two types of nanofibers, that is, a nanofiber A and a nanofiber B, are tangled with each other. In the present disclosure, the softening point (softening temperature) of the nanofiber A is different from that of the nanofiber B, a cross-section along a surface of the nanofiber A orthogonal to a longitudinal direction thereof has a shape having a concave portion, and at this concave portion, the nanofiber A and the nanofiber B are fused with each other.
Hereinafter, with reference to the drawings, the nanofiber structure of the present disclosure will be described.

[Nanofiber Structure]

FIG. 1A is a schematic view showing one example of an embodiment of the nanofiber structure of the present disclosure, and FIG. 1B is a cross-sectional view showing a lump portion surrounded by a dotted line represented by 1B shown in FIG. 1A. A nanofiber structure 1 shown in FIG. 1A includes two types of nanofibers, that is, a nanofiber A represented by reference numeral 3 in FIG. 1A and a nanofiber B represented by reference numeral 4 in FIG. 1A. In addition, as shown in FIG. 1A, the nanofiber structure 1 is a structure in which the nanofiber A represented by reference numeral 3 and the nanofiber B represented by reference numeral 4 are tangled with each other in a three-dimensional manner.
A cross-section along a surface of the nanofiber A included in the nanofiber structure 1 shown in FIG. 1A has a shape having a concave portion 3 a as shown in FIG. 1B, the surface of the nanofiber A being orthogonal to a longitudinal direction thereof. In the present disclosure, although the concave portion 3 a of the nanofiber A is primarily provided in an edge portion of the cross-section along the surface of the nanofiber A orthogonal to the longitudinal direction thereof, the concave portion 3 a is not limited thereto and may be provided inside the cross-section along the surface of the nanofiber A orthogonal to the longitudinal direction thereof. The concave portion 3 a is preferably provided in the edge portion of the cross-section along the surface of the nanofiber A orthogonal to the longitudinal direction thereof and is particularly provided in an edge portion of the cross-section in a direction orthogonal to a longitudinal direction thereof or in an edge portion of the cross-section in the longitudinal direction thereof. In addition, the concave portions 3 a may also be provided in the edge portion of the cross-section in the direction orthogonal to the longitudinal direction thereof and in the edge portion of the cross-section in the longitudinal direction thereof.
Since being intersected with the nanofiber A at at least one position, the nanofiber B included in the nanofiber structure 1 shown in FIG. 1A is tangled with the nanofiber A. In addition, in the present disclosure, the intersection between the nanofibers included in the nanofiber structure includes not only the intersection between the nanofiber A and the nanofiber B but also the intersection within the nanofiber A and that within the nanofiber B.
In the nanofiber structure 1 shown in FIG. 1A, at a portion at which the nanofiber A and the nanofiber B are intersected with each other, for example, at a portion surrounded by a dotted line represented by 1B shown in FIG. 1A, a lump portion 2 is formed. When the nanofiber A and the nanofiber B are intersected with each other, the nanofiber A and the nanofiber B present at the portion at which the nanofibers are intersected with each other are each deformed as shown in FIG. 1B, so that the lump portion 2 is formed. The case shown in FIG. 1B is an example in which the nanofiber B is largely deformed as compared to the nanofiber A, and the deformed nanofiber B forms a part of the lump portion 2. In this case, each nanofiber is deformed when large energy is applied thereto. This deformation may also be called “fusion” of the nanofiber, and since this fusion occurs at the portion at which the nanofibers are intersected with each other, adhesion (or fusion) between the nanofibers, for example, between the nanofiber A and the nanofiber B, occurs, so that the lump portion 2 is generated at a portion at which this adhesion occurs.
As shown in FIG. 1B, in the nanofiber structure 1 shown in FIG. 1A, at at least a part of the lump portion 2 formed at the portion at which the nanofiber A and the nanofiber B are intersected with each other, the nanofiber B is filled in the concave portion 3 a of the nanofiber A. Since the nanofiber B is at least partially filled in the concave portion 3 a, the nanofiber A and the nanofiber B are fused with each other at this concave portion 3 a.
In the case described above, the nanofiber A has the concave portion 3 a as shown in FIG. 1B. Hence, when the nanofiber A and the nanofiber B are intersected with each other so that the nanofiber B is present on this concave portion 3 a, by an irregular shape of the nanofiber A, the area at which the nanofibers are intersected with each other is increased, and the nanofiber B partially catches on the concave portion of the nanofiber A. Hereinafter, a phenomenon in which the nanofiber B partially catches on the concave portion of the nanofiber A is called an anchor effect. As described above, since the anchor effect is obtained by the special shape of the nanofiber A, at the portion at which the nanofibers are intersected with each other (for example, the nanofiber A and the nanofiber B are intersected with each other), the lump portion 2 is formed, and the nanofibers are adhered to or fused with each other at this lump portion 2; hence, the nanofiber structure is strengthened, and the durability thereof is improved.
In addition, the lump portion 2 is formed by applying energy, such as by heat and/or pressure application, in particular, to the portion at which the nanofibers are intersected with each other. In a particular example in which the nanofiber A and the nanofiber B are intersected with each other, when energy is applied to the intersection portion between the nanofiber A and the nanofiber B, a plurality of molecules of the nanofibers are physically associated with each other, so that the lump portion 2 is formed. In addition, when a chemical reaction occurs between a plurality of molecules of the nanofibers, as is the case of the above physical association, the lump portion 2 is also formed.
In addition, when the nanofiber structure of the present disclosure is observed by a scanning electron microscope (SEM) or a laser microscope, for example, a micro structure and/or a micro environment of the nanofiber structure can be confirmed. For example, the state in which the nanofibers are intersected with each other, the fusion between the nanofibers at this intersection portion, and the formation of the lump portion 2 at a portion at which the nanofibers are fused with each other can be confirmed. In addition, by the observation using a SEM or a laser microscope, the shapes of the nanofiber A and the nanofiber B can be confirmed, that is, in particular, for example, the cross-sectional shape and the diameter of each nanofiber can also be confirmed. In this case, the cross-sectional shape of the nanofiber can be confirmed in such a way that after the nanofiber structure is cut, the cross-sections of the nanofiber A and the nanofiber B which are exposed to the cut portion and the cross-section of the portion at which the nanofiber A and the nanofiber B are intersected with each other are observed. In addition, by this cross-section observation, the widths of the nanofiber A and the nanofiber B, and the height of the nanofiber in this cross-section can be confirmed. In addition, according to the observation using a laser microscope or the like, when the surface of the nanofiber structure is observed from an upper side thereof, the cross-sectional structure of the nanofiber of the nanofiber structure can be confirmed without cutting the nanofiber structure itself. Accordingly, for example, the width and the height of the nanofiber can be confirmed.
FIG. 2A is a schematic view showing one concrete example of an observation method of the nanofiber structure, and FIG. 2B is a partially enlarged view of a IIB portion shown in FIG. 2A. FIG. 1A is a schematic view obtained when the nanofiber structure 1 is observed from a surface side thereof, and when the nanofiber structure 1 is observed from the surface side thereof, the view point of an observer is in a direction orthogonal to the surface of the nanofiber structure 1 as shown in FIG. 2A, that is, is right above the nanofiber structure 1. In addition, for example, when the IIB portion in FIG. 2A is observed, if the observation result is as shown in FIG. 2B, from this observation result, it can be confirmed whether the nanofiber having a concave portion, that is, in particular, whether the nanofiber A represented by the reference numeral 3, is included in the structure or not. In addition, from this observation result, it can also be confirmed whether a portion at which the nanofiber A (reference numeral 3) and the nanofiber B (reference numeral 4) are intersected with each other (for example, a portion surrounded by a dotted line represented by β1 in FIG. 2B) is present or not.
In the nanofiber structure of the present disclosure, the portion at which the nanofibers are intersected with each other and the lump portion 2 generated at this intersection portion are present not only on the surface of the nanofiber structure but also present therein. In addition, the nanofiber A and the nanofiber B forming the nanofiber structure of the present disclosure each have a length larger than the width thereof. In the present disclosure, the length of each nanofiber is generally 10 times or more the average diameter of the nanofiber, that is, the fiber diameter.

[Nanofiber A]

Hereinafter, the nanofiber A forming the nanofiber structure of the present disclosure will be described in detail. FIG. 3 is a schematic view showing one example of a cross-sectional shape of the nanofiber A. In addition, FIG. 3 is a view showing the shape of a cross-section along a surface of the nanofiber A orthogonal to the longitudinal direction thereof. Hereinafter, the “cross-section along the surface of the nanofiber A orthogonal to a longitudinal direction thereof” is referred to as “the cross-section” in some cases. In addition, in the present disclosure, the shape of the cross-section of the nanofiber A is not limited to an approximately I shape shown in FIG. 3, and any shape may be included in the present disclosure as long as the cross-section thereof has a concave portion.
Hereinafter, with reference to FIG. 3, the shape of the cross-section of the nanofiber A, in particular, an irregular shape of the cross-section, will be defined.
As shown in FIG. 3, the shape of the cross-section along the surface of the nanofiber A orthogonal to the longitudinal direction thereof has a shape having a concave portion. In addition, by the cross-sectional shape of the cross-section, the width and the height of the nanofiber A in the cross-section can be defined. In addition, since the width indicates the dimension in a direction orthogonal to a height direction, and the height indicates the dimension in a direction orthogonal to a width direction, when one of the width and the height is defined, the other can be defined. In the following description, the definition of the width will be described.
The width of the cross-section can be defined by a relative position of the concave portion 3 a confirmed in the cross-section. In particular, when the concave portion 3 a is provided in an edge portion of the cross-section, the following definition is effective. In this case, this relative position can be determined using the longitudinal direction of the cross-section as the reference. When the cross-sectional shape of the nanofiber A is, for example, the shape as shown in FIG. 3, the longitudinal direction of this cross-sectional shape is a horizontal direction shown by an arrow. As described above, since the longitudinal direction of this cross-section is defined, the width of the cross-section can be defined. In particular, the width can be defined as indicated by the following (i) or (ii).
(i) When the concave portion 3 a is provided in an edge portion of the cross-section in a direction orthogonal to a longitudinal direction thereof, as the “width of the cross-section,” the width of the cross-section in the longitudinal direction is defined.
(ii) When the concave portion 3 a is provided in an edge portion of the cross-section in the longitudinal direction thereof, as the “width of the cross-section,” the width of the cross-section in a direction orthogonal to the longitudinal direction is defined.
Since the case in which the nanofiber A has the cross-sectional shape shown in FIG. 3 corresponds to the above (i), the width of the cross-section in the longitudinal direction is defined as the “width of the cross-section.”
In the present disclosure, when the average value of the width of the nanofiber A is represented by L, L can be set in a range of from 1 nm to 8×10⁴nm. In this case, in consideration of easy handling of the nanofiber itself, L is preferably set in a range of from 50 nm to 7×10⁴nm. In addition, when the nanofiber structure of the present disclosure is used as a constituent member of a dust collection filter, in order to increase the specific surface area of the nanofiber structure for improvement of its function, L is more preferably set from 100 nm to 6×10⁴nm. In the present disclosure, L is particularly preferably set from 300 nm to 5.5×10⁴nm.
On the other hand, at the portion at which the width of the nanofiber A is from 1 nm to 8×10⁴nm, the cross-section preferably has a shape in which as shown in FIG. 3, at least two convex portions 3 b are provided. This convex portion 3 b indicates a portion in which the height thereof is equal to or more than the average height of the nanofiber A in this cross-section and is equal to or less than the maximum value of the height of the nanofiber A in this cross-section. That is, Z_ais in a range of from Z to Z_max.
In the present disclosure, it is preferable that at least two convex portions 3 b shown in FIG. 3 are present, and between those convex portions 3 b, at least one concave portion 3 a is provided.
In the present disclosure, the average Z of the height of the nanofiber A can be set from 1 nm to 8×10⁴nm. In this case, in consideration of easy handling of the nanofiber itself, Z is preferably set from 50 nm to 7×10⁴nm and more preferably set from 100 nm to 6×10⁴nm. In the present disclosure, in particular, Z is preferably set from 300 nm to 1×10⁴nm.
Incidentally, for example, as shown in FIG. 2A, Z and L can be evaluated by observation of the nanofiber structure. In particular, after one arbitrary place of each of segments of the nanofiber A confirmed by observation is selected, the shape of the cross-section thereof at each selected place is measured, and the height and the width can be obtained by averaging the measurement values thus obtained.

[Nanofibers B]

In the present disclosure, the cross-sectional shape of the nanofiber B is not particularly limited, and as a concrete shape, for example, a circular, an oval, a square, a polygonal, or a semi-circular shape may be mentioned. As is the nanofiber A, the cross-sectional shape of a surface of the nanofiber B orthogonal to a longitudinal direction thereof may be an irregular shape. In addition, the cross-sectional shape of the nanofiber B is not required to have an accurate shape as mentioned above and may be different between arbitrary cross-sections.
An average Y of the diameter (fiber diameter) of the nanofiber B of the present disclosure is preferably from 1 nm to 3,000 nm. The fiber diameter of the nanofiber B can be obtained by measurement of the maximum length in a direction parallel to the cross-section of the nanofiber B confirmed by observation of the nanofiber structure of the present disclosure. In addition, the fiber diameter can also be obtained by averaging heights and widths of the nanofiber B obtained by measurement of predetermined cross-sectional shapes thereof. In this case, in order to impart sufficient durability to the nanofiber structure, Y is preferably 10 nm or more. Furthermore, Y is more preferably 100 nm or more.
Incidentally, Y can be evaluated by a method similar to that of the above Z and L. In particular, the fiber diameter can be obtained in such a way that after one place of each of segments of the nanofiber B confirmed by observation of the nanofiber structure is selected, the values of the fiber diameters obtained therefrom are averaged.

[Relationship Between Width of Nanofiber A and Fiber Diameter of Nanofiber B]

In the present disclosure, a ratio L/Y, that is, a ratio of a parameter L of the nanofiber A to a parameter Y of the nanofiber B, is preferably from 3.3×10⁻⁴to 8×10³. An L/Y of more than 8×10³indicates that the fiber diameter of the nanofiber B is averagely too small. In this case, the durability of the nanofiber structure may not be sufficiently obtained in some cases. On the other hand, an L/Y of less than 3.3×10⁻⁴indicates that when the nanofiber A and the nanofiber B are intersected with each other, an area at which the nanofiber A and the nanofiber B are adhered to each other is significantly smaller than the cross-sectional area of the nanofiber B. In the case described above, a sufficient anchor effect by the nanofiber A may not be expected in some cases. In addition, in order to obtain sufficient durability of the nanofiber structure, L/Y is preferably from 1.6×10⁻²to 6×10³and more preferably from 0.1 to 5.5×10³.

[Constituent Materials of Nanofiber A and Nanofiber B]

Constituent materials of the nanofiber A and the nanofiber B are not particularly limited as long as the softening point of the nanofiber A is different from that of the nanofiber B. In addition, the constituent materials of the nanofiber A and the nanofiber B each may be one type of material or a mixed material containing at least two types of materials in combination. Since the lump portion 2 is likely to be obtained at the portion at which the nanofiber A and the nanofiber B are intersected with each other, as the constituent materials of the nanofiber A and the nanofiber B, organic resin materials are preferably used. Among the organic resin materials, a thermoplastic resin is preferable. In addition, when an organic resin material is selected as the constituent material of each nanofiber, in order to improve the mechanical strength of the nanofiber, an additive may be contained in the organic resin material. As the additive used in this case, for example, a low molecular weight organic compound, an inorganic material, fine particles, and a material containing a related known filler may be mentioned. Those additives may be used alone, or at least two types thereof may be appropriately used in combination.
As the organic resin material used as the constituent material of the nanofiber A or the nanofiber B, for example, there may be mentioned a fluorine-containing polymer (such as a polytetrafluoroethylene, a poly(vinylidene fluoride) (PVDF), or a copolymer (such as a copolymer (VDF-HFP) between vinylidene fluoride and hexafluoropropylene) with another monomer); a polyarylene (such as a polyarylene including poly(paraphenylene oxide), a poly(2,6-dimethylphenylene oxide), or a poly(paraphenylene sulfide); a polyimide; a polyamide; a poly(amide imide), a polybenzimidazole; a modified polymer in which a sulfonic acid group (—SO₃H), a carboxyl group (—COOH), a phosphoric acid group, a sulfonium group, an ammonium group, or a pyridium group is introduced into a polyolefin, a polystyrene, a polyimide, or a polyarylene; a modified polymer in which a sulfonic acid group, a carboxyl group, a phosphoric acid group, a sulfonium group, an ammonium group, or a pyridium group is introduced into a skeleton of a fluorine-containing polymer; a polybutadiene compound; a polyurethane compound (including an elastomer and a gel compound); a silicone compound; a poly(vinyl chloride); a polyarylate; a biodegradable polymer (such as a polycaprolactone (PCL) or a poly(lactic acid)); a polyester (PES) (such as a poly(ethylene terephthalate) (PET)); or a poly(meth)acrylic acid derivative (such as polymethylmethacrylate (PMMA)).
In addition, those organic resin materials mentioned above may be used alone, or at least two types thereof may be used in combination. In addition, a modified polymer in which a sulfonic acid group, a carboxyl group, a phosphoric acid group, a sulfonium group, an ammonium group, or a pyridium group is introduced into a polymer material other than a polyolefin, a polyimide, a polyarylene, and a fluorine-containing polymer may also be used. Furthermore, a copolymer obtained by copolymerizing at least two types of monomers may also be used. In addition, in the case of a material, such as a polyimide, a polyamide, a poly(amide imide) (PAI), or a polybenzimidazole (PBI), which is not likely to be fused, this material may be appropriately used in combination with a thermoplastic resin.
In addition, as the inorganic material used as the constituent material of the nanofiber, an oxide of a metal material selected from the group consisting of Si, Mg, Al, Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Cu, Sn, and Zn may be mentioned, and in more particular, a metal oxide, such as silica (SiO₂), titanium oxide, aluminum oxide, aluminum sol, zirconium oxide, iron oxide, or chromium oxide may be mentioned. In addition, a clay mineral, such as montmorillonite (MN), may also be used. In this case, if an inorganic material is contained in the nanofibers, when the nanofibers are adhered to each other, the mechanical strength tends to be significantly improved; hence, it is preferable in view of the improvement in mechanical strength of the nanofiber structure.

[Softening Point of Nanofiber]

In the present disclosure, the softening point of the nanofiber A and that of the nanofiber B each indicate a temperature at which when energy is applied to the constituent material of each nanofiber, the constituent material is deformed, that is, indicates a softening temperature. In particular, the softening point of each nanofiber may be represented, for example, by a deflection temperature under load, a Vicat softening temperature, or a glass transition temperature. In the present disclosure, although the softening point of the nanofiber A is different from that of the nanofiber B, this indicates that when the same measurement method is used for the constituent materials of the individual nanofibers, the temperature at which the material is deformed is different therebetween. That is, although the method for determining the softening point is not particularly limited, since the test method is defined, and since the thermal property of the material can be clearly obtained, a measurement method in accordance with that for the deflection temperature under load or the Vicat softening temperature is preferably used.
When the nanofiber A and the nanofiber B are each a thermoplastic resin, the difference in softening point between the constituent material of the nanofiber A and the constituent material of the nanofiber B is preferably from more than 0° C. to 100° C.
In addition, since the nanofiber itself has a fine structure having a large specific surface area, the softening point of the nanofiber A and that of the nanofiber B are each not exactly the same as the softening point of the thermoplastic resin itself used as the constituent material. However, when the nanofiber A and the nanofiber B each have the shape and the size as described above, the softening points of the constituent materials thereof are each a value which can be sufficiently used for selection of the materials for the nanofiber A and the nanofiber B.
In addition, when the fusion of the two types of fibers is confirmed at the portion at which the nanofiber A and the nanofiber B are intersected with each other by the anchor effect of the nanofiber A of the present disclosure, the difference in softening point between the nanofiber A and the nanofiber B is not particularly limited.
However, when the difference in softening point between the two types of nanofibers is more than 100° C., it indicates that the difference in softening point between the thermoplastic resins used as the constituent materials of the nanofibers is excessively large. In the case as described above, when the nanofiber A and the nanofiber B are fused with each other at the portion at which the two types of nanofibers are intersected with each other, one of the nanofiber A and the nanofiber B may be changed into a bulky shape in some cases. In order to form a nanofiber excellent in durability, the shape of the nanofiber is preferably maintained, and in order to increase the number of fusions at the intersection between the nanofiber B and the nanofiber A by the anchor effect thereof, the difference in softening point described above is preferably 80° C. or less and more preferably 50° C. or less.
In addition, in the present disclosure, when the softening point of the nanofiber A and that of the nanofiber B are different from each other, a repeating unit of the thermoplastic resin forming the nanofiber A may be exactly the same as a repeating unit of the thermoplastic resin forming the nanofiber B. That is, the combination between the nanofiber A and the nanofiber B includes a combination between materials having the same repeating unit but having different molecular weights and the like.

[Solubility Parameter]

In the present disclosure, although the difference in average solubility parameter between the constituent material of the nanofiber A and the constituent material of the nanofiber B is not particularly limited, the difference is preferably from 0 to 25 (J/cm³)^1/2.
The solubility parameter in this embodiment is Hansen solubility parameter. Hansen solubility parameter is formed of energy arising from an atomic dispersion force, a molecular permanent dipole-dipole force, and molecular hydrogen bonding, which are represented by δ_D, δ_P, δ_H[(J/cm³)^1/2], respectively. The solubility parameter δ [(J/cm³)^1/2] of this material is represented by the following formula (5).
δ=(δ_D ²+δ_P ²+δ_H ²)^1/2[(J/cm³)^1/2] (5)
A measured Hansen solubility parameter of a general material can be available as a literature value, and even if the literature value is not available since the material is special, the solubility parameter thereof can be calculated using a calculation software.
When a plurality of materials (for example, two types of materials) are mixed together, if the difference in Hansen solubility parameter among materials contained in a mixture to be obtained is large, energy required for dissolution or mixing is increased. Hence, when the difference in Hansen solubility parameter is large, since the solubility of a solute to a solvent is decreased, the mixing therebetween is not advanced. In this case, the difference in Hansen solubility parameter between a material a and a material b, that is, |Δδ_(a-b)| [(J/cm³)^1/2], can be calculated by the following formula (6).
|Δδ_(a-b)|{4(δ_aD−δ_bD)²+(δ_aP−δ_bP)²+(δ_aH−δ_bH)²}^1/2 (6)
In this case, when the difference in solubility parameter between the constituent material of the nanofiber A and that of the nanofiber B is more than 25 (J/cm³)^1/2, even if the nanofiber A and the nanofiber B are intersected with each other, among the portions at which the nanofibers are intersected with each other, the number of portions at which the two types of nanofibers are fused with each other is decreased. As a result, the durability of the nanofiber structure itself is degraded. In order to obtain a nanofiber structure having high durability, the difference in Hansen solubility parameter between the constituent material of the nanofiber A and that of the nanofiber B is preferably 18 (J/cm³)^1/2or less and more preferably 16 (J/cm³)^1/2or less.

[Presence Rate of Nanofibers]

As a quantitative index of a local structure of the nanofiber sheet of the present disclosure, a presence rate represented by the rate of the volume of the nanofibers to the volume (including void portions) of the sheet is used. This presence rate has a plurality of definitions in accordance with the method of obtaining the volume rate of the nanofibers. In particular, for example, there are a unit presence rate and an average presence rate. In this case, the unit presence rate is an area rate obtained in such a way that after a fracture surface of the nanofiber sheet is exposed, the rate of the fiber occupied in a portion having the same thickness as the fiber diameter in the laminate direction is calculated. On the other hand, the average presence rate indicates an average value of the unit presence rate in the thickness of a specific portion. In the following description, unless otherwise particularly noted, the presence rate indicates the average presence rate of an object portion.
Composition rates of the nanofiber A and the nanofiber B contained in the nanofiber structure of the present disclosure are not particularly limited. However, when the structure is used as a constituent member of a dust collection filter, the presence rate (average presence rate) of the nanofibers on the surface of the nanofiber structure is preferably from 10% to 60%. When the presence rate is less than 10%, the amount of the nanofibers present on the surface of the nanofiber sheet is decreased, and the advantage of the nanofibers, that is, the effect obtained by its high specific surface area, is reduced. On the other hand, when the presence rate is more than 60%, by adhesion of a small amount of materials to void portions, clogging of the voids may be generated in some cases.
As shown in FIG. 2A, the average presence rate of the nanofibers of the nanofiber sheet of the present disclosure is obtained by observation of a part of the nanofiber structure using a laser microscope and then can be evaluated.

[Thickness of Sheet Member]

The nanofiber structure of the present disclosure is a member formed of the nanofibers tangled with each other. In the present disclosure, although the shape of this structure is not particularly limited, for example, in view of easy handling, a member having a sheet shape is preferable. When the nanofiber structure of the present disclosure is a sheet-shaped member, although this member has a predetermined thickness, in the present disclosure, the thickness of this member is not particularly limited. However, in the present disclosure, the thickness of this sheet-shaped member is preferably from 1 μm to 1 mm and more preferably from 10 μm to less than 100 μm. The nanofiber structure of the present disclosure is excellent in mechanical strength and durability since many lump portions each having a predetermined size are present by adhesion between the nanofibers. Hence, when the structure itself is formed to have a sheet shape, the thickness of the sheet-shaped member can be set to the order of micrometers. In addition, when the thickness of this sheet-shaped member is smaller than 1 μm, since the nanofibers are not sufficiently tangled with each other, the number of the lump portions included in the nanofiber structure may not be increased in some cases. On the other hand, when the thickness of the sheet-shaped member is set to less than 100 μm, the air permeability of this sheet-shaped member is particularly excellent.
The thickness of the nanofiber structure can be confirmed by a method in which the cross-section of the nanofiber structure is directly observed using a scanning microscope (SEM) or a laser microscope or by a method in which measurement is performed using a micrometer.

[Physical Property of Nanofiber Structure (Tensile Elastic Modulus)]

In the nanofiber structure of the present disclosure, the number of nanofibers present on an arbitrary surface, the space between the nanofibers, and the number of the nanofibers laminated to each other can be appropriately selected and set in accordance with the characteristics of a desired nanofiber structure. For example, in the nanofiber structure 1 shown in FIG. 1A, the lump portions 2 are appropriately provided. Since nanofibers adjacent to each other are adhered therebetween by this lump portion 2, the nanofiber structure of the present disclosure is strengthened.
Incidentally, as a physical property for evaluating the durability of the nanofiber structure, the tensile elastic modulus may be mentioned. In the present disclosure, the tensile elastic modulus of the nanofiber structure is preferably 100 MPa or more. When the tensile elastic modulus is less than 100 MPa, the durability of the structure is degraded, and the structure cannot be used for a long time.
In addition, when the nanofiber structure of the present disclosure is used, for example, as a constituent member of a dust collection filter fitted to a ventilation hole, a predetermined mechanical strength or more is preferably imparted to the structure. In particular, besides a tensile elastic modulus of 100 MPa or more, the nanofiber structure is preferably able to withstand a predetermined wind pressure.

[Application of Nanofiber Structure]

Even when the nanofiber structure of the present disclosure is formed by laminating at least two types of nanofibers formed from different materials, the durability is excellent, and for example, when the nanofiber structure is used as a component, such as a dust collection filter fitted to a ventilation hole, to be used when a predetermined pressure is applied, this component can be advantageously used for a long time.
In addition, when the nanofiber structure of the present disclosure is a sheet-shaped member, for example, this sheet-shaped member can be used as a constituent member of a dust collection filter which is used at a ventilation hole and which forms a container collecting dust and the like. When this sheet-shaped member is used as a constituent member of a dust collection filter, in order to improve the durability thereof, at least two sheet-shaped members described above may be used in combination. In addition, the entire shape of the nanofiber structure of the present disclosure is not limited to a sheet shape, and for example, a mat shape (a sheet shape having a large thickness) or a block shape may also be mentioned.
In the nanofiber structure of the present disclosure, by the anchor effect of the nanofiber A contained in the structure, the adhesion at the portion at which the nanofiber A and the nanofiber B having a softening point different from that thereof are intersected with each other is improved. Hence, when nanofibers formed from different types of materials are used in combination, it can be said that the function of the nanofiber sheet can be improved. In particular, in the application of a dust collection filter, for example, there may be mentioned a combination between a nanofiber formed from a polymer having a special molecular structure capable of selectively collecting particles, such as dust, dart, and fine particles, and a nanofiber formed from a polymer having a sufficient strength capable of physically trapping or repelling collected materials. However, the application of the nanofiber structure of the present disclosure is not limited thereto. For example, the nanofiber structure of the present disclosure is also preferably used as a friction charging material of an electrostatic generation apparatus or an electric-field type particle sorting apparatus. In addition, although the use form of the nanofiber sheet of the present disclosure is not particularly limited, for example, a form in which the nanofiber sheet is wound around a roller-shaped member may be mentioned. In addition, when being formed into a mat shape or a block shape, the nanofiber structure of the present disclosure may be used as a constituent member for an acoustic insulating material, a mask, a cell scaffold material, or the like.

[Method for Manufacturing Nanofiber Structure]

Next, a method for manufacturing the nanofiber structure of the present disclosure will be described. In the following manufacturing method, for example, although a sheet-shaped structure can be manufactured, the nanofiber structure manufactured by this manufacturing method is not limited to the sheet structure. The manufacturing method of the present disclosure includes at least a step of forming a precursor by tangling the nanofiber A and the nanofiber B (precursor forming step) and a step of heating this precursor (heating step). The manufacturing method preferably includes the following steps (A) to (C).

(A) Solution Preparation Step

(B) Precursor Forming Step

(C) Heating Step

In the following description, the step (B) may also be called a “spinning step” in some cases. In the step (B), since the nanofiber A and the nanofiber B are tangled with each other, a sheet-shaped precursor is formed. In addition, in the step (C), the lump portion is formed at a portion at which the nanofibers are intersected with each other. Accordingly, in the manufacturing method of the present disclosure, a process of forming the structure is a combination between the step (B) and the step (C). Hereinafter, the individual steps will be described in detail.

(A) Solution Preparation Step

In the manufacturing method of the present disclosure, a solution containing the constituent material of the nanofiber A and a solution containing the constituent material of the nanofiber B are not always required to be prepared. However, since being easily handled in the following step (precursor forming step), the solution containing the constituent material of the nanofiber A and the solution containing the constituent material of the nanofiber B are preferably prepared.
when the solutions are prepared, materials each functioning as a solute are not particularly limited as long as the materials are the constituent materials of the nanofiber A and the nanofiber B and are able to form the lump portions in the following heating step.
In addition, a solvent used for preparation of the solution is not particularly limited as long as the solvent is able to dissolve the constituent material of the nanofiber A or the nanofiber B. In this case, as the solvent usable in this step, an organic solvent may be mentioned. In addition, the organic solvent may be appropriately selected in consideration of the polarity and the solubility parameter of the constituent material of the nanofiber A or the nanofiber B. The organic solvent used in this step may be a single solvent or may be a mixed solvent in which at least two types of solvents are mixed together at an appropriate rate.
When the nanofiber structure of the present disclosure is manufactured, a low molecular weight organic compound having a chemical reactivity may be contained in one of the constituent materials of the nanofiber A and the nanofiber B. When the low molecular weight organic compound described above is contained in the nanofiber A or the nanofiber B, in the step (C), the low molecular weight organic compound is able to react with one of the following (a1) to (a4).
(a1) An organic resin material functioning as the constituent material of the nanofiber A
(a2) An organic resin material functioning as the constituent material of the nanofiber B
(a3) The above low molecular weight organic compound
(a4) A low molecular weight organic compound other than the above low molecular weight organic compound
In particular, when the above low molecular weight organic compound reacts with (a1) or (a2), since a cross-linked structure is formed, the mechanical strength of the nanofiber structure thus manufactured is further improved.
In addition, whether the low molecular weight organic compound performs a chemical reaction or not can be confirmed, for example, by an infrared (IR) spectroscopy or a Raman spectroscopy. As a particular judgment method, an IR spectrum is measured before the chemical reaction occurs. In the IR spectrum at this point, there are observed absorption peaks of functional groups of the organic resin materials used as the constituent materials of the nanofibers and of the low molecular weight organic compound before the reaction. In addition, when the chemical reaction of the low molecular weight organic compound occurs, the chemical structure of the low molecular weight organic compound is at least changed, the change in IR spectrum derived from the low molecular weight organic compound is generated. Hence, the occurrence of the chemical reaction of the low molecular weight organic compound can be judged whether at least some of the absorption peaks of the IR spectrum observed before the reaction is decreased or not, and whether after the reaction, an adsorption peak not observed before the reaction appears or not.

(B) Precursor Forming Step (Spinning Step)

When the nanofiber structure of the present disclosure is formed, two types of nanofibers each forming the structure, that is, the nanofiber A and the nanofiber B, are necessarily formed. In this case, although a method for forming the nanofiber is not particularly limited, the nanofiber is preferably formed by spinning using an electron spinning method having the following characteristics (i) to (iii).
(i) Various nanofiber constituent materials can be formed into fiber shapes.
(ii) The fiber shape can be relatively easily controlled, and fibers each having a diameter of from a nanometer size to several tens of micrometers can be easily obtained.
(iii) The formation process is simple.
Hereinafter, a method for forming a precursor by spinning of nanofibers using an electron spinning method will be described appropriately with reference to the drawings. FIG. 4 is a schematic view showing one example of a precursor forming apparatus.
A forming apparatus 10 shown in FIG. 4 includes a head 17 having a storage tank 12 and a connection portion 11, a high voltage power source 16, and a collector 15. In the forming apparatus 10 in FIG. 4, the storage tank 12 functions to store a solution containing a nanofiber constituent material, has a spinning port 14, is connected to the connection portion 11, and is arranged at a predetermined position above the collector 15. In addition, as the material to be stored in the storage tank 12, besides the solution containing a nanofiber constituent material, for example, a fused nanofiber constituent material may also be contained.
In the forming apparatus 10 shown in FIG. 4, the connection portion 11 is a member electrically connected to the high voltage power source 16 via a wire 13. That is, in the forming apparatus 10 shown in FIG. 4, a voltage output from the high voltage power source 16 can be applied to the connection portion 11 or a member connected thereto, such as the spinning port 14.
In the forming apparatus 10 shown in FIG. 4, the collector 15 is a member collecting a nanofiber span from the spinning port 14 and is arranged to face the head 17 with a predetermined space provided therebetween. In addition, in the forming apparatus 10 shown in FIG. 4, the collector 15 is grounded by a wire 19.
In the forming apparatus 10 shown in FIG. 4, a voltage of from 1 to 50 kV is applied to the spinning port 14, and when an electric attraction force is more than the surface tension of the above mixture, a jet 18 of the above solution is ejected toward the collector 15. When the mixture in a solution form is ejected from the spinning port 14, the solvent contained in the jet 18 is gradually vaporized, and when the jet 18 reaches the collector 15, a nanofiber corresponding to the jet 18 can be obtained. In addition, when spinning is performed using the forming apparatus 10 shown in FIG. 4, for example, a solution containing the nanofiber constituent material is charged in the tank 12.
In addition, in the forming apparatus 10 shown in FIG. 4, a method in which the solution stored in the storage tank 12 is extruded through the spinning port 14 is employed.
Incidentally, at a stage of the solution preparation step, when the above low molecular weight organic compound having a chemical reactivity is contained in the solution, the low molecular weight organic compound is adhered to the inside or the surface of the nanofiber formed in this step.
In addition, when the nanofiber A having a concave portion in the surface orthogonal to a longitudinal direction thereof is formed, by appropriate adjustment of the following items, the nanofiber A can be formed.
(b1) A constituent material and a solvent used for preparation of a solution of the constituent material.
(b2) An application voltage and an ejection rate in the case in which spinning is performed using an electron spinning method.
In addition, when the nanofiber obtained by spinning is flexible, and voids are present therein, a concave portion may be formed by applying a physical force to the nanofiber from the outside.

(C) Heating Step

When the nanofiber structure of the present disclosure is manufactured, energy is applied to the precursor obtained in the spinning step. By application of energy, the following (C1) and/or (C2) is advanced.
(C1) Physical association, such as fusion, generated inside the nanofiber or on the surface thereof.
(C2) Chemical reaction generated inside the nanofiber or on the surface thereof.
As the above (C1) and/or (C2) is advanced, the lump portions are generated at portions at which the nanofibers are intersected with each other.
In the present disclosure, although a method for applying energy necessary to form the lump portion is not particularly limited, in consideration of the uniformity and the simplicity, a heat application method is preferable.
In this step, although a method for heating a precursor is not particularly limited and may be appropriately selected in accordance with the use conditions and the like, methods in each of which this precursor is heated without pressure application are preferable. In addition, among the methods described above, a heating treatment method using an oven is particularly preferable since lump portions each having an appropriately controlled area can be formed without deforming the shape of a nanofiber contained in the structure. In addition, when the precursor is heated, heating may be performed from one surface side thereof. The presence rate and the presence distribution of the lump portions of the nanofiber structure obtained thereby may be changed in a film thickness direction in some cases; however, when the nanofiber A and the nanofiber B contained in this nanofiber structure each have sufficient durability, any problems may not arise.
In this step, when the shape of the nanofiber can be maintained, and when the heating temperature is less than the decomposition temperature of the nanofiber constituent material, in accordance with the properties of the materials to be used, desired physical properties of the nanofiber structure, and the like, the manufacturing conditions can be appropriately selected. For example, as the constituent material of the nanofibers, when an organic resin material is used, the heating temperature of the nanofiber structure can be appropriately set in consideration of the softening point or the deflection temperature under load of the organic resin material. In particular, when the heating temperature of the nanofiber structure is set to be higher than the softening point or the deflection temperature under load thereof by at most 100° C. or less, it is more preferable since the lump portions can be easily formed in the nanofiber structure of the present disclosure.

EXAMPLES

Hereinafter, the present disclosure will be described in detail with reference to examples. However, the present disclosure is not limited to the following examples. In addition, appropriate modifications and changes of the modes described in the following examples within the scope of the present disclosure are also included in the present disclosure.

[Measurement Method and Evaluation Method]

Measurement methods of physical properties of a nanofiber structure formed in any one of the following examples and comparative examples and evaluation methods thereof will be described. In addition, the nanofiber structures formed in the examples and the comparative examples each have a sheet-shaped structure.

(1) Shape of Nanofiber A

The shape of the nanofiber A forming the nanofiber structure was confirmed by measurement using a laser microscope (manufactured by Keyence Corp.). In particular, a sheet-shaped nanofiber structure was observed by a laser microscope at a magnification of 200 times, and for example, a photo shown in FIG. 5 was obtained. In addition, the photo shown in FIG. 5 is a gray scale image. Next, after the gray scale image was enlarged at a magnification of 300 times by “Profile”, arbitrary three points of a predetermined nanofiber A were selected, and the cross-sectional shape of a surface of the nanofiber A orthogonal to the longitudinal direction thereof was measured at each of the three points. In addition, at the above three points, the heights of the above cross-sections were averaged, so that an average value Z of the height of the nanofiber A was obtained. In this example, it was confirmed that Z is from 1 nm to 8×10⁴nm. In addition, at each point at which the cross-sectional shape was observed, it was confirmed whether at least two convex portions having a height Z_awere present or not in the cross-section, the height Z_abeing in a range of from Z to Z_max(maximum value of the height). In addition, the width of the cross-section of the nanofiber A was measured at each of the three points at which the cross-section was measured. In addition, in this example, since it was confirmed that the position of the concave portion of the nanofiber A was located at an edge portion in a direction orthogonal to the longitudinal direction of the cross-section, the length in the longitudinal direction of the cross-section was measured as the width. In addition, the values of the widths measured at the points were averaged, so that L was obtained. In this example, L was in a range of from 1 nm to 8×10⁴nm.

(2) Presence Rate of Nanofibers on Surface of Nanofiber Structure

The presence rate of the nanofibers on the surface of the sheet-shaped nanofiber structure was evaluated by measurement using a laser microscope (manufactured by Keyence Corp.). In particular, a gray scale image obtained by measurement using a laser microscope was downloaded in an image analysis software (A-image-kun) (manufactured by Asahi Kasei Engineering Corporation), and the presence rate was obtained by performing area rate measurement.

(3) Average Film Thickness of Nanofiber Structure

The average film thickness of the sheet nanofiber structure was measured using a quick micro (manufactured by Mitsutoyo Corporation) at arbitrary three points, and the average value obtained therefrom was regarded as the average value.

(4) Evaluation of Peeling Resistance of Nanofiber Structure

A peeling resistance of the sheet-shaped nanofiber structure was evaluated using a nanofiber structure after a heat treatment was performed. In particular, when a nanofiber structure in which a layer formed by integrating the nanofiber A and a layer formed by integrating the nanofiber B were laminated to each other was peeled away from a supporting substrate by hand, evaluation was performed by visual inspection whether defects, such as separation between the two layers, are generated in the shape of the sheet or not. In addition, in the nanofiber structure of the present disclosure, it is important that peeling is not likely to occur since many contact points between the nanofiber A and the nanofiber B are present, and some of the contact points are bound or fused with each other by the anchor effect of the nanofiber A.

(5) Evaluation of Mechanical Strength (Tensile Elastic Modulus) of Nanofiber Structure

The mechanical strength, that is, the tensile elastic modulus, of the sheet-shaped nanofiber structure was evaluated by the following method.
A polymer nanofiber structure strippable from aluminum foil used as a substrate was partially cut away, and a sample 20 having a dumbbell shape shown in FIG. 6 was formed. In addition, when the sample 20 was formed from a polymer nanofiber structure obtained in one of the following examples and comparative examples, a, b, and c in FIG. 6 were set to 2 cm, 1 cm, and 0.5 cm, respectively. However, those dimensions mentioned above were merely shown by way of example, and the dimensions of the sample 20 shown in FIG. 6 may be freely set. Next, by the use of a micrometer, the film thicknesses at three points each circled by a chain line represented by reference numeral 21 in FIG. 6 were measured, and the average value obtained therefrom was regarded as a film thickness (t) of the polymer nanofiber structure. Next, by the use of a vernier caliper, the widths at the three points each circled by the chain line represented by reference numeral 21 were measured, and the average value obtained therefrom was regarded as a width (w) of the polymer nanofiber structure. Subsequently, the sample 20 (polymer nanofiber structure) shown in FIG. 6 was set in a tensile measurement device. In this case, as the portions of the sample 20 which were set in the measurement device, rectangular areas each represented by reference numeral 22 in FIG. 6 were used. The reason the portions of the sample 20 which are set in the measurement device are clearly defined as described above is to accurately measure the tensile elastic modulus without influenced by the change in area at which the dumbbell is held. Subsequently, by the use of a vernier caliper, the length of a portion represented by h₀shown in FIG. 6 was measured. In addition, h₀in FIG. 6 indicates a tensile initial length. Next, the film thickness (t), the width (w), and the tensile initial length (h₀) were input in a measurement software. Subsequently, the initial values of the height and the stroke of the tensile measurement device were each set to zero. Next, the sample 20 was pulled at a rate of 1 mm/min. In addition, the length of the sample 20 to be pulled, that is, the tensile length (Δh), was calculated in advance using the value obtained by the software. In addition, after the breakage of the sample 20 was confirmed by visual inspection, the pulling was stopped. A tensile force applied to the sample 20 when the breakage thereof was confirmed by visual inspection was regarded as a test force (N).
By the use of the film thickness (t), the width (w), the tensile length (Δh), and the test force (N) obtained by the process described above, the tensile elastic modulus (G) was obtained. When the tensile elastic modulus (G) was obtained, a strain (ε) and a stress (σ) were calculated from the following formulas (5-1) and (5-2), respectively, to draw the stress-strain curve, and the slope of the tangent line to the yield point was obtained using the following formula (5-3).
Δh/h ₀=ε (5-1)
N/(wt)=σ (5-2)
σ/ε=G (5-3)
The tensile test was performed on each sample twice, and the average value thereof was defined as the tensile elastic modulus of the polymer nanofiber structure. In addition, when the test was performed twice, if the difference in tensile elastic modulus therebetween was large, the tensile test was performed three times or more. In this case, among the values obtained by the test described above, as the tensile elastic modulus of the polymer nanofiber structure, the average value was obtained from two values having a small difference therebetween. In addition, in this measurement, the value was obtained without considering Poisson's effect generated in association with the change in film shape in tensile measurement.
In addition, as described above, although the lump portions are formed when nanofibers contained in the nanofiber structure are partially deformed, among the lump portions, when many lump portions in which the areas of inscribed circles thereof are in a predetermined range are present, the tensile elastic modulus of the nanofiber structure is improved. In this case, the mechanical strength of the nanofiber structure is increased, and the durability, such as a wind pressure resistance, is apparently excellent. That is, in the nanofiber structure having a high tensile elastic modulus, the mechanical strength of the nanofiber structure itself is improved, and hence, a sufficient wind pressure resistance is obtained. Accordingly, the nanofiber structure having a high tensile elastic modulus can be used for a long period of time.

(6) Evaluation of Wind Pressure Resistance of Nanofiber Structure

The wind pressure resistance of the nanofiber structure was evaluated in such a way that when a wind having a pressure of 0.1 MPa was blown for 30 seconds on one nanofiber sheet adhered to thick paper using an air gun, it was confirmed whether the nanofiber sheet was destroyed or not. In addition, when the experiment was performed, the distance between the nanofiber sheet and the air gun was set to 3 cm. In addition, as an evaluation method, after the blowing was performed using an air gun, whether the nanofiber structure was destroyed or fractured was confirmed by visual inspection. In this embodiment, the judgment criteria were as follows.
Excellent: Destruction and fracture of nanofiber structure are not confirmed.
Good: Destruction and fracture of nanofiber structure are not confirmed, but deformation thereof is confirmed.
No good: Destruction and fracture of nanofiber structure are confirmed.

(7) Evaluation of Air Permeability of Nanofiber Structure

The evaluation of the air permeability of the nanofiber structure was judged as excellent when an air permeability of 0.1 cc/cm²/sec at 125 Pa was obtained using an air permeability tester.

Example 1

(1) Solution Preparation Step

As described below, a solution A used to form the nanofiber A and a solution B used to form the nanofiber B were each prepared.

(1-1) Preparation of Solution A

A poly(methyl methacrylate) (PMMA, Sumipex MM, manufactured by Sumitomo Chemical Co., Ltd.) used as the organic resin material and methyl isobutyl ketone (MIBK, manufactured by Kishida Chemical Co., Ltd.) were mixed together to prepare an MIBK solution containing 17 percent by weight of PMMA.

(1-2) Preparation of Solution B

A polyoxymethylene (POM, Tenac LA543, manufactured by Asahi Kasei Chemicals Corporation) used as the organic resin material and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, manufactured by Tokyo Chemical Industry Co., Ltd.) were mixed together, so that an HFIP solution was prepared. In addition, in the HFIP solution thus prepared, 5.6 percent by weight of POM was contained.

(2) Precursor Forming Step (Spinning Step)

By an electron spinning method, the solutions prepared in the step (1) were ejected for spinning. Accordingly, a precursor in which nanofibers containing PMMA, POM, and unvolatilized solvents were physically tangled with each other was formed. In this step, in particular, the head 17 forming the electrospinning apparatus (manufactured by Mecc Co., Ltd.) shown in FIG. 4 was assembled in the order from the following (2-1) to (2-2). In addition, the head 17 was set at a position at which a measure provided in the apparatus indicated a value of 155 mm. In addition, on the collector 15, aluminum foil was provided.

(2-1) Fitting of Head Portion (Clip Spinneret)

(2-2) Fitting of Tank 12 to Head Portion (*)

(*In this example, a tank receiving the solution A prepared in the (1-1) in advance and a tank receiving the solution B prepared in the (1-2) in advance were both prepared.)
Next, a voltage of 22.5 kV was applied to the tank 12 receiving the solution A. Accordingly, when the solution A filled in the tank 12 was ejected toward the collector 15 for 30 minutes, the nanofiber A was obtained. In addition, the nanofiber A thus obtained was in an integrated state on the collector 15. Next, while an integrated body formed from the nanofiber A and containing unvolatilized MIBK was present on the collector 15, a voltage of 15 kV was applied to the tank 12 receiving the solution B. Accordingly, the solution B filled in the tank 12 was ejected toward the collector 15 for 30 minutes, so that on the above integrated body, the nanofiber B was integrated as was the nanofiber A. An integrated body including the nanofiber A and the nanofiber B obtained by the method as described above was used as the precursor in the following step.

(3) Heating Step

Aluminum foil on which the precursor obtained in the above (2) was placed was heat-treated at 165° C. for 2 hours using an oven. Accordingly, a sheet-shaped nanofiber structure was obtained in which fused portions were present at portions at which the nanofiber A and the nanofiber B were intersected with each other. In addition, this step may also be called a step of forming lump portions in which the lump portions are formed when nanofibers contained in the precursor are partially deformed.

(4) Evaluation of Nanofiber Structure

The physical properties of the nanofiber structure thus obtained were measured and evaluated based on the measurement methods and the evaluation methods described above. The results are shown in Tables 2 and 3.

Example 2

(1) Solution Preparation Step

As described below, a solution A used to form the nanofiber A and a solution B used to form the nanofiber B were both prepared.

(1-1) Preparation of Solution A

An MIBK solution used as the solution A was prepared by a method similar to that of Example 1(1-1).

(1-2) Preparation of Solution B

A poly(vinylidene fluoride) (PVDF, KF Polymer #1000, manufactured by Kureha Corporation) used as the organic resin material and N,N′-dimethylacetamide (DMAC, manufactured by Kishida Chemical Co., Ltd.) were mixed together, so that a DMAC solution containing 24 percent by weight of PVDF was prepared.

(2) Precursor Forming Step (Spinning Step)

Except that in Example 1(2), as the solution B received in the tank 12, the DMAC solution prepared in the above (1-2) was used, and the voltage applied to the spinning port 14 and the ejection rate were changed as shown in Table 1, a precursor was obtained by a method similar to that of Example 1(2).

(3) Heating Step

Except that in Example 1(3), the heating temperature was changed as shown in Table 1, a sheet-shaped nanofiber structure was obtained by heating the precursor by a method similar to that of Example 1(3).

(4) Evaluation of Nanofiber Structure

By methods similar to those of Example 1(4), the measurement and the evaluation of the nanofiber structure thus obtained were performed. The results are shown in Tables 2 and 3.

Example 3

(1) Solution Preparation Step

(1-1) Preparation of Solution A

A polyoxymethylene (POM, Tenac LA543, manufactured by Asahi Kasei Chemicals Corporation) used as the organic resin material and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, manufactured by Tokyo Chemical Industry Co., Ltd.) were mixed together, so that an HFIP solution was prepared. In addition, in the HFIP solution thus prepared, 8 percent by weight of POM was contained.

(1-2) Preparation of Solution B

A poly(methyl methacrylate) (PMMA, Sumipex MM, manufactured by Sumitomo Chemical Co., Ltd.) used as the organic resin material, N,N′-dimethylacetamide (DMAC, manufactured by Kishida Chemical Co., Ltd.), and tetrahydrofuran (THF, manufactured by Kishida Chemical Co., Ltd.) were mixed together to prepare a solution. In this solution, as the solvent forming this solution, DMAC and THF were mixed at a weight ratio of 1 to 1, and 17 percent by weight of PMMA was contained.

(2) Precursor Forming Step (Spinning Step)

In Example 1(2), as the solution A received in the tank 12, the solution prepared in the above (1-1) was used; as the solution B received in the tank 12, the solution prepared in the above (1-2) was used; and the voltage applied to the spinning port 14 and the ejection rate were changed as shown in Table 1. Except for those described above, a precursor was obtained by spinning the nanofibers using a method similar to that of Example 1(2).

(3) Heating Step

Except that in Example 1(3), the heating temperature was changed as shown in Table 1, a sheet-shaped nanofiber structure was obtained by heating the precursor using a method similar to that of Example 1(3).

(4) Evaluation of Nanofiber Structure

By methods similar to those of Example 1, the measurement and the evaluation of the nanofiber structure thus obtained were performed. The results are shown in Tables 2 and 3.

Example 4

(1) Solution Preparation Step

(1-1) Preparation of Solution A

By a method similar to that of Example 3(1-1), a HFIP solution was prepared as the solution A.

(1-2) Preparation of Solution B

By a method similar to that of Example 2(1-2), a DMAC solution was prepared as the solution B.

(2) Precursor Forming Step (Spinning Step)

In Example 1(2), as the solution A received in the tank 12, the solution prepared in the above (1-1) was used; as the solution B received in the tank 12, the solution prepared in the above (1-2) was used; and the voltage applied to the spinning port 14 and the ejection rate were changed as shown in Table 1. Except for those described above, a precursor was obtained by a method similar to that of Example 1(2).

(3) Heating Step

(4) Evaluation of Nanofiber Structure

Example 5

(1) Solution Preparation Step

(1-1) Preparation of Solution A

By a method similar to that of Example 1(1-1), an MIBK solution was prepared as the solution A.

(1-2) Preparation of Solution B

A poly(ethylene glycol) (PEG, weight average molecular weight: 900,000, manufactured by Sigma Aldorich Inc.) used as the organic resin material and ion exchanged water were mixed together, so that an aqueous solution containing 20 percent by weight of PEG was prepared.

(2) Precursor Forming Step (Spinning Step)

(3) Heating Step

(4) Evaluation of Nanofiber Structure

By methods similar to those of Example 1(4), the measurement and the evaluation of the nanofiber structure thus obtained were performed. The results are shown in Tables 2 and 3. In addition, when the nanofiber structure formed in this example was observed by a laser microscope, a photo shown in FIG. 5 was obtained.

Comparative Example 1

(1) Solution Preparation Step

A polymethylmethacrylate (PMMA, Sumipex MM, manufactured by Sumitomo Chemical Co., Ltd.) used as the organic resin material and methyl isobutyl ketone (MIBK, manufactured by Kishida Chemical Co., Ltd.) were mixed together to prepare an MIBK solution containing 17 percent by weight of PMMA.

(2) Spinning Step

Except that in Example 1(2), as the solution received in the tank 12, the solution prepared in the above (1) was used, and the voltage applied to the spinning port 14 and the ejection rate were changed as shown in Table 1, spinning was performed by a method similar to that of Example 1(2), so that an integrated body of a nanofiber was obtained. In this comparative example, the tank in which the solution prepared in the above (1) was stored was only prepared in this step.

(3) Heating Step

Except that in Example 1(3), the heating temperature was changed as shown in Table 1, a sheet-shaped nanofiber structure was obtained by heating the integrated body using a method similar to that of Example 1(3).

(4) Evaluation of Nanofiber Structure

Comparative Example 2

(1) Solution Preparation Step

(2) Spinning Step

(3) Heating Step

(4) Evaluation of Nanofiber Structure

Comparative Example 3

(1) Solution Preparation Step

(1-1) Preparation of PMMA-Containing Solution

A polymethylmethacrylate (PMMA (organic resin material), Sumipex MM, manufactured by Sumitomo Chemical Co., Ltd.), N,N′-dimethylacetamide (DMAC, manufactured by Kishida Chemical Co., Ltd.), and tetrahydrofuran (THF, manufactured by Kishida Chemical Co., Ltd.) were mixed together to prepare a solution. In this solution, DMAC and tetrahydrofuran, which were the solvents forming the solution, were mixed at a weight ratio of 1:1, and 17 percent by weight of PMMA was contained.

(1-2) Preparation of HFIP Solution

In addition, a polyoxymethylene (POM, Tenac LA543, manufactured by Asahi Kasei Chemicals Corporation) used as the organic resin material and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, manufactured by Tokyo Chemical Industry Co., Ltd.) were mixed together, so that an HFIP solution was prepared. In addition, in this HFIP solution thus prepared, 5.6 percent by weight of POM was contained.

(2) Spinning Step

In Example 1(2), a tank in which the PMMA-containing solution prepared in the above (1-1) was stored and a tank in which the HFIP solution prepared in the above (1-2) was stored were prepared. In addition, the voltage applied to the spinning port 14 and the ejection rate were changed as shown in Table 1. Except for those described above, an integrated body of nanofibers was obtained by spinning using a method similar to that of Example 1(2).

(3) Heating Step

Except that in Example 1(3), the heating temperature was changed as shown in Table 1, a sheet-shaped nanofiber structure was obtained by a method similar to that of Example 1(3).

(4) Evaluation of Nanofiber Structure

Comparative Example 4

(1) Solution Preparation Step

(1-1) Preparation of PMMA-Containing Solution

(1-2) Preparation of HFIP Solution

In addition, a poly(vinylidene fluoride) (PVDF, KF Polymer #1000, manufactured by Kureha Corporation)) used as the organic resin material and N,N′-dimethylacetamide (DMAC, manufactured by Kishida Chemical Co., Ltd.) were mixed together, so that a DMAC solution was prepared. In addition, in this DMAC solution thus prepared, 24 percent by weight of PVDF was contained.

(2) Spinning Step

In Example 1(2), a tank in which the PMMA-containing solution prepared in the above (1-1) was stored and a tank in which the DMAC solution prepared in the above (1-2) was stored were prepared. In addition, the voltage applied to the spinning port 14 and the ejection rate were changed as shown in Table 1. Except for those described above, an integrated body of nanofibers was obtained by spinning using a method similar to that of Example 1(2).

(3) Heating Step

(4) Evaluation of Nanofiber Structure

Comparative Example 5

(1) Solution Preparation Step

(2) Spinning Step

Except that in Example 1(2), as the solution received in the tank 12, the solution prepared in the above (1) was used, and the voltage applied to the spinning port 14 and the ejection rate were changed as shown in Table 1, spinning was performed by a method similar to that in Example 1(2), so that an integrated body of a nanofiber was obtained. In this comparative example, the tank in which the solution prepared in the above (1) was stored was only prepared in this step.

(3) Heating Step

(4) Evaluation of Nanofiber Structure

TABLE 1

		Difference
		in SP value		Application	Ejection	Application	Ejection
		between two		voltage in	rate in	voltage in	rate in
Nanofiber	Nanofiber	types of	\|Ta −	nanofiber	nanofiber	nanofiber B	nanofiber	Heating
A material	B material	materials	Tb\|	A spinning	A spinning	spinning	B spinning	temperature

Example 1	PMMA	POM	15.1	15° C.	22.5 kV	2.0 mL/h	15 kV	1.5 mL/h	165° C.
Example 2	PMMA	PVDF	7.45	10° C.	22.5 kV	2.0 mL/h	25 kV	2.0 mL/h	150° C.
Example 3	POM	PMMA	15.1	15° C.	16 kV	1.0 mL/h	18.5 kV	1.0 mL/h	180° C.
Example 4	POM	PVDF	9.39	25° C.	16 kV	1.0 mL/h	25 kV	2.0 mL/h	160° C.
Example 5	PMMA	PEG	9.39	35° C.	22.5 kV	2.0 mL/h	16 kV	1 mL/h	65° C.
Comparative	PMMA	/	/	/	22.5 kV	2.0 mL/h	/	/	150° C.
Example 1
Comparative	POM	/	/	/	16 kV	1.0 mL/h	/	/	180° C.
Example 2
Comparative	/	PMMA•POM	15.1	15° C.	/	/	PMMA: 18.5 kV,	PMMA: 1.0 mL/h,	160° C.
Example 3							POM: 15 kV	POM: 1.5 mL/h
Comparative	/	PMMA•PVDF	7.45	10° C.	/	/	PMMA: 18.5 kV,	PMMA: 1.0 mL/h,	155° C.
Example 4							PVDF: 25 kV	PVDF: 2.0 mL/h
Comparative	/	PMMA	/	/	/	/	18.5 kV	1.0 mL/h	160° C.
Example 5

TABLE 2

		Average	Average	Average
		value of width	diameter of	height of	Presence
Nanofiber A	Nanofiber B	of nanofiber A	nanofiber B	nanofiber A	rate of
material	material	(L)	(Y)	(Z)	nanofibers

Example 1	PMMA	POM	8.7 μm	1.3 μm	5.0 μm	26%
Example 2	PMMA	PVDF	8.7 μm	0.4 μm	5.0 μm	45%
Example 3	POM	PMMA	2.6 μm	0.9 μm	2.7 μm	39%
Example 4	POM	PVDF	2.6 μm	0.4 μm	2.7 μm	31%
Example 5	PMMA	PEG	8.7 μm	0.7 μm	5.0 μm	27%
Comparative	PMMA	/	8.7 μm	/	5.0 μm	33%
Example 1
Comparative	POM	/	2.6 μm	/	2.7 μm	35%
Example 2
Comparative	/	PMMA•POM	/	PMMA: 0.9 μm,	/	28%
Example 3				POM: 1.3 μm
Comparative	/	PMMA•PVDF	/	PMMA: 0.9 μm,	/	33%
Example 4				PVDF: 0.4 μm
Comparative	/	PMMA	/	0.9 μm	/	24%
Example 5

TABLE 3

			Tensile	Wind
Nanofiber A	Nanofiber B	Peeling	elastic	pressure	Air
material	material	resistance	modulus	resistance	permeability

Example 1	PMMA	POM	∘	258 MPa	excellent	excellent
Example 2	PMMA	PVDF	∘	186 MPa	excellent	excellent
Example 3	POM	PMMA	∘	214 MPa	excellent	excellent
Example 4	POM	PVDF	∘	157 MPa	excellent	excellent
Example 5	PMMA	PEG	∘	111 MPa	excellent	excellent
Comparative	PMMA	/	/	211 MPa	/	/
Example 1
Comparative	POM	/	/	189 MPa	/	/
Example 2
Comparative	/	PMMA•POM	x (interlayer	/	/	/
Example 3			peeling)
Comparative	/	PMMA•PVDF	x (interlayer	/	/	/
Example 4			peeling)
Comparative	/	PMMA	/	42 MPa	/	/
Example 5

As shown in Table 1, the difference in average solubility parameter (SP value difference) between thermoplastic resins used in each of Examples 1 to 5 and Comparative Examples 3 and 4 is from 0 to 25 (J/cm³)^1/2.
In addition, the softening point of the thermoplastic resin used as the constituent material of the nanofiber A in each of Examples 1 to 5 is represented by T_a, and the softening point of the thermoplastic resin used as the constituent material of the nanofiber B is represented by T_b. In this case, as shown in Table 1, in every Example, the difference between the two softening points, |T_a−T_b|, was from more than 0° C. to less than 100° C. In addition, as T_aand T_b(except T_bof PEG) of the thermoplastic resins, the deflection temperatures under load (ASTM D648) thereof were employed. In addition, as T_bof PEG used in Example 5, the melting point of PEG was employed. In addition, with reference to the softening point of the constituent material of the nanofiber which was used, a heat treatment temperature of the precursor (or the integrated body of nanofiber) in the heating step was appropriately determined.
The findings and confirmations obtained in Examples and Comparative Examples will be described.
In the nanofiber structure obtained in each of Examples 1 to 5 and Comparative Examples 1 and 2, the nanofiber A having a characteristic irregular shape could be confirmed (see Table 2). On the other hand, in the nanofiber structure obtained in each of Comparative Examples 3 to 5, a fiber having an irregular shape corresponding to that of the nanofiber A could not be confirmed.
In addition, from Table 2, it was confirmed that in all the nanofiber structures in each of which the presence of the nanofiber A was confirmed, the average value L of the width of the nanofiber A was from 1 nm to 8×10⁴(80,000) nm. In addition, it was also confirmed that the ratio (L/Y) of L to the average diameter Y of the nanofiber B was from 3.3×10⁻⁴to 8×10³, and the average height Z of the nanofiber A was also confirmed.
From Table 2, it was found that the presence rate of the nanofibers on the surface of the nanofiber sheet was from 10% to less than 60%.
In addition, the nanofiber structure formed in each of Examples 1 to 5 could be peeled away from aluminum foil as a sheet in which the nanofiber A and the nanofiber B were integrated with each other. However, although the nanofiber structure of each of Comparative Examples 3 and 4 was formed using the materials similar to those of each of Examples 1 to 3 and was processed by a heat treatment at a heating temperature similar to that described above, when the nanofiber structure was peeled away from aluminum foil, a loosened nanofiber portion arose, and a cotton shape was formed therefrom, so that the nanofiber structure could not be peeled away as a sheet. From the results thus obtained, it was found that in the nanofiber structure of the present disclosure, when different types of nanofibers were intersected with each other by a special shape of the nanofiber A contained in the structure, the nanofibers were sufficiently fused with each other at the points at which the nanofibers were intersected with each other.
It was found that when the tensile elastic modulus of the nanofiber structure formed in Example 1 was measured, the tensile elastic modulus was larger than that of the nanofiber structure of Comparative Example 1. From the result described above, it was found that by the shape of the nanofiber A contained in the nanofiber structure of the present disclosure, even when a nanofiber (nanofiber B or the like) having a different softening point was integrated and/or intersected therewith, a sufficient strength could be imparted to the nanofiber structure.
The tensile elastic modulus of the nanofiber structure of Example 2 was lower than that of the nanofiber structure of Example 1. The reasons for this are believed that the mechanical strength of the constituent material (PVDF) of the nanofiber B is low as compared to that of PMMA and POM, and that the average value Y of the fiber diameter of the nanofiber B is small as compared to that of Example 1.
Since the constituent material of the nanofiber A of the nanofiber structure of each of Examples 3 and 4 was POM, small voids were present in the surface of the fiber. However, as shown in Table 3, the peeling resistance was excellent, and the tensile elastic modulus was also 100 MPa or more. From the results described above, it is believed that in the present disclosure, the anchor effect obtained from the characteristic irregular shape of the nanofiber A can be sufficiently obtained without depending on the micro surface porous shape of the nanofiber A.
The mechanical strength (tensile elastic modulus) of the nanofiber structure obtained in Example 5 was low (111 MPa) as compared to that of each of Examples 1 to 4. The reasons for this are believed that the melting point of PEG used as the constituent material of the nanofiber B is low, such as 65° C., and that the mechanical strength of a bulk PEG is low.
The nanofiber structure obtained in each Example (Examples 1 to 5) has an excellent peeling resistance and a sufficient mechanical strength. The reason for this is believed that since the nanofibers are fused with each other in the heating step (precursor heating step), physical association between polymer molecules are generated inside the nanofiber or on the surface thereof. In addition, it is believed that the fusion between the nanofibers described above occurs not only by the intersection between the nanofiber A and the nanofiber B but also by the intersection within the nanofiber A and the intersection within the nanofiber B.
That is, in the nanofiber structure obtained in each Example (Examples 1 to 5), at the lump portions generated by the intersection and fusion between the nanofibers, a complicatedly tangled polymer network is present. Hence, it is believed that although the nanofiber structure of the present disclosure is formed of the nanofiber A and the nanofiber B having a softening point different from that thereof, since the nanofibers intersected with each other are sufficiently adhered to each other at the intersection point therebetween, the sheet shape can be maintained.
In addition, in the nanofiber structure obtained in each Example (Examples 1 to 5), the wind pressure resistance and the air permeability of the structure itself were measured and judged as excellent.
As has thus been described, it was found that according to the nanofiber structure of the present disclosure, the peeling resistance between nanofibers was excellent, the mechanical strength of the structure itself was high, and the specific surface area thereof was high. In addition, in the nanofiber structure of the present disclosure having the characteristics described above, the nanofibers contained in the structure are not likely to be loosened, and hence, the nanofiber structure can be advantageously used for a long time.
While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2015-218226, filed Nov. 6, 2015, which is hereby incorporated by reference herein in its entirety.

Claims

What is claimed is:

1. A nanofiber structure comprising:

a nanofiber A; and

a nanofiber B tangled with the nanofiber A, wherein:

the softening point of the nanofiber A is different from the softening point of the nanofiber B,

a cross-section along a surface of the nanofiber A orthogonal to a longitudinal direction thereof has a shape having a concave portion, and

at the concave portion, the nanofiber A and the nanofiber B are fused with each other.

2. The nanofiber structure according to claim 1, wherein the concave portion is provided in an edge portion of the cross-section in a direction orthogonal to a longitudinal direction thereof.

3. The nanofiber structure according to claim 1, wherein the concave portion is provided in an edge portion of the cross-section in a longitudinal direction thereof.

4. The nanofiber structure according to claim 1, wherein:

the average width of the cross-section of the nanofiber A is from 1 nm to 8×10⁴nm,

the cross-section of the nanofiber A at which the width thereof is from 1 nm to 8×10⁴nm has a shape having at least two convex portions,

the height of the convex portion is equal to or more than the average height of the cross-section of the nanofiber A and is equal to or less than the maximum height of the cross-section of the nanofiber A, and

the average fiber diameter of the nanofiber B is from 1 nm to 3,000 nm.

5. The nanofiber structure according to claim 1, wherein a ratio L/Y of an average value L of the width of the cross-section of the nanofiber A to an average value Y of the fiber diameter of the nanofiber B is from 3.3×10⁻⁴to 8×10³.

6. The nanofiber structure according to claim 1, wherein constituent materials of the nanofiber A and the nanofiber B are each a thermoplastic resin, and

the difference in softening point between the constituent material of the nanofiber A and the constituent material of the nanofiber B is from more than 0° C. to less than 100° C.

7. The nanofiber structure according to claim 1, wherein the difference in average solubility parameter between a constituent material of the nanofiber A and a constituent material of the nanofiber B is from 0 to 25 (J/cm³)^1/2.

8. The nanofiber structure according to claim 1, wherein an average presence rate of nanofibers on the surface of the nanofiber structure is from 10% to 60%.

9. The nanofiber structure according to claim 1, wherein:

the nanofiber structure is a sheet-shaped structure, and

the thickness of the sheet-shaped structure is from 1 μm to 1 mm.

10. A method for manufacturing a nanofiber structure, wherein the nanofiber structure comprises:

a nanofiber A; and

a nanofiber B tangled with the nanofiber A, wherein:

at the concave portion, the nanofiber A and the nanofiber B are fused with each other,

the method comprising:

forming a precursor by tangling at least the nanofiber A and the nanofiber B; and

heating the precursor.

11. The method according to claim 10, wherein the nanofiber A or the nanofiber B contains an organic resin material and a low molecular weight organic compound having a chemical reactivity.

12. The method according to claim 10, wherein in the heating, the precursor is heated without pressure application.