WO2015026928A1 - Electrospinning to form nanofibers - Google Patents

Abstract

A nanofiber has a conductive filler and a functional polymer. The functional polymer is, for example, poly[(vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)), polyvinylidene fluoride (PVDF), or a co-polymer of PVDF. The conductive filler may be an organosilicate or graphene. Electrospinning a solution of the conductive filler, a solvent, and the functional polymer can form a thin sheet or membrane having multiple fibers.

Description

ELECTROSPINNING TO FORM NANOFIBERS

Cross-Reference to Related Applications

[0001] This application claims priority to U.S. provisional patent application no. 61/867,648, filed August 20, 2013, the disclosure of which is hereby incorporated by reference. Field of the Disclosure

[0002] This disclosure relates to electrospinning to form a nanofiber or nanofibers.

Background of the Disclosure

[0003] In recent years, considerable attentions and efforts have been directed to the development of multifunctional electronic textiles (e-textiles) or wearable electronics to meet the ever-increasing demands in civil and defense applications. These e-textiles may be required to be wearable, but also capable of detecting changes in environmental conditions, monitoring human body functions, local computation as well as wireless communication. Piezoelectric materials, which generate electric outputs in response to an applied mechanical stress, offer great promise to be used as reliable sensors and power generators in e-textiles. Generally speaking, piezoelectric materials can be divided into two main groups: ceramics and polymers. Although piezoelectric ceramics have been used successfully in many applications, they have some obvious drawbacks as wearable electronics, including that they are difficult to deform, brittle and heavy. Moreover, many

piezoelectric ceramics contain Pb, which has serious damaging effects on human health.

[0004] Piezoelectric polymeric films have been developed and are commercially available. However, they only have very limited flexibility and are impermeable to human perspiration, making them less desirable as components of wearable electronics. Electrospinning is an effective technique to prepare flexible nanofibrous membranes. Recently, electrospun nanofibrous

membranes based on fluoropolymer have attracted much attention because of their potential in many areas including as a power nanogenerator, sensor, tissue engineering component, filtration membrane, battery separator, and polymer electrolyte. However, the poor mechanical properties of such electrospun membranes have limited their applications because even partial failure can lead to serious consequences in the end-use. For example, a deficiency of electrospun polyvinylidene fluoride (PVDF) membranes as separators of Li-ion batteries was low mechanical performance, which induced partial short circuits inside the cells. Improvements to mechanical properties of electrospun PVDF membranes have been attempted by hot-pressing. This method sacrifices many desirable characteristics of an electrospun membrane, including high porosity, light weight, good breathability, and piezoelectricity. The fabrication of durable, porous and highly flexible

piezoelectric materials is, therefore, still a bottlenecking technology to be addressed for e-textiles, wearable electronics or other applications. Brief Summary of the Disclosure

[0005] Piezoelectric polymers, such as PVDF and its copolymer poly(vinylidene- trifluoroethylene) [P(VDF-TrFE)] are lightweight, spinnable, soft and flexible, making them potential candidates for functional fibers and wearable electronics. Fabrication of a novel P(VDF- TrFE)/organosilicate composite membrane prepared by electrospinning is disclosed. The composite membrane containing 4 wt% of organosilicate demonstrated dramatic improvements in strength, modulus, extensibility, and toughness by about 880%, 270%, 100%, and 1860%), respectively, when compared with those of electrospun pure P(VDF-TrFE) membrane. It is also significant to note that the electrospun P(VDF-TrFE)/organosilicate membrane possessed high porosity, low density, good breathability and piezoelectricity. Such an organosilicate-reinforced durable porous P(VDF-TrFE) membrane may be an excellent material not only for wearable electronics, but also for other applications, such as filter membranes, tissue engineering, battery separators, and polymer electrolytes.

[0006] P(VDF-TrFE)/organosilicate composite membrane was fabricated by electrospinning.

The electrospun composite membrane containing 2 wt.% of organosilicate demonstrated dramatic improvements in strength, modulus, extensibility, and toughness by about 1510, 210, 53, and

2020%, respectively, when compared with those of electrospun pure P(VDF-TrFE) membrane. High porosity, low density, good breathability and piezoelectricity are found in the electrospun composite membrane. An organosilicate-reinforced durable, porous and piezoelectric P(VDF-TrFE) membrane has huge advantages in various applications such as flexible sensors, wearable electronics, filter membrane, tissue engineering, battery separator, and polymer electrolyte.

[0007] In an aspect, the present disclosure provides a nanofiber comprising a conductive filler and a functional polymer, where the functional polymer is a copolymer comprising

polyvinylidene fluoride (PVDF). In an embodiment, the functional polymer comprises

poly[(vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)), polyvinylidene fluoride (PVDF), or a co-polymer of PVDF. In an embodiment, the functional polymer comprises P(VDF-TrFE) and wherein a ratio of PVDF in the P(VDF-TrFE) is from 20% to 80%. In an embodiment, the nanofiber is piezoelectric. [0008] In an embodiment, the conductive filler comprises an organosilicate, graphene, or carbon nanotubes. In an embodiment, the conductive filler has a CO or OH group configured to stabilize an interface between the conductive filler and the functional polymer. In an embodiment, the nanofiber comprises an amount of conductive filler greater than 0% and less than or equal to 10%. In an embodiment, the nanofiber comprises 4% conductive filler. In an embodiment, the organosilicate is a porous organosilicate (POS).

[0009] In an embodiment, a membrane comprises a plurality of the nanofibers and the nanofibers are disposed in contact with each other to form a membrane. In an embodiment, the conductive filler of the membrane is an organosilicate (e.g., a POS) and the membrane comprises 4% weight of POS to 10% weight of POS. In an embodiment, the membrane has a density of approximately 0.33 g/cm³ to 0.38 g/cm³ and a porosity between approximately 70% to 82%. In an embodiment, the membrane has a tensile strength of approximately 12.8 + 1.2 MPa to 13.0 + 1.1 MPa, a modulus of approximately 12.8 + 1.2 MPa to 22.4 + 2.3 MPa, and an elongation at break of 89.6 + 10.5% to 111.0 + 13.9%. In an embodiment, the membrane has a toughness of 7.13 J/m³ to 9.21 J/m³. In an embodiment, the membrane has a thickness between approximately 110 μιη and approximately 130 μιη. In an embodiment, the nanofiber has a diameter between 10 nm and 1 μιη. In an embodiment, the nanofiber has a diameter between 10 nm and 900 nm. In an embodiment, the nanofiber has a diameter between 54 nm and 595 nm. In an embodiment, the nanofiber has a length of between 1 cm and 1 m. [0010] In an aspect, the present disclosure provides a method for forming a nanofiber or plurality of nanofibers (e.g., a membrane comprising a plurality of fibers). In an embodiment the method comprising: electrospinning a solution of a conductive filler, a solvent, and a functional polymer to form a thin sheet comprising a plurality of fibers. In an embodiment, the solvent comprises a mixture of dimethylformamide (DMF) and acetone. In an embodiment, the mixture comprises 75% volume DMF and 25% volume acetone. In an embodiment, the solvent dissolves the functional polymer and evaporate during or after the electrospinning. In an embodiment, the functional polymer comprises poly[(vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)), polyvinylidene fluoride (PVDF), or a co-polymer of PVDF. In an embodiment, the conductive filler comprises an organosilicate, graphene, or carbon nanotubes. [0011] In an embodiment, the method further comprises adding the functional polymer into a suspension comprising the conductive filler and the solvent to form the solution prior to the electrospinning. In an embodiment, the method further comprises subjecting the suspension to ultrasonication at room temperature prior to the adding. In an embodiment, the adding comprises magnetic stirring at a temperature above room temperature. In an embodiment, the method further comprises subjecting the solution to ultrasonication at room temperature prior to the electrospinning. [0012] In an embodiment, the electrospinning comprises applying a DC voltage to a tip of a syringe containing the solution. In an embodiment, the electrospinning comprises projecting the mixture from the syringe onto a rotating drum collector. In an embodiment, the tip is separated from the rotating drum collector by approximately 15 cm and wherein the DC voltage is approximately 14 kV. In an embodiment, the thin sheet comprises a membrane. [0013] In an embodiment, the method comprises: adding organosilicate into a solvent mixture to form an organosilicate suspension, wherein the solvent mixture comprises 75% volume dimethylformamide (DMF) and 25% acetone; subjecting the organosilicate suspension to

ultrasonication at room temperature; dispersing poly[(vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)) into the organosilicate suspension by magnetic stirring at a temperature above room temperature to form a solution; subjecting the solution to ultrasonication at room temperature; and electrospinning the solution to form a membrane by applying a voltage to a device containing the solution and projecting a fiber from the device to a collector.

Description of the Drawings

[0014] For a fuller understanding of the nature and objects of the disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates the intercalation of P(VDF-TrFE) chains into organosilicate interlayer and the toughening mechanism of organosilicate for P(VDF-TrFE) nanofiber;

FIG. 2 illustrate SEM images, densities, porosities, and WVTRs of electrospun membranes; FIG. 3 represent the WAXD patterns of organosilicate and electrospun composite

membranes in low-angle range, (b) The FTIR spectra of electrospun membranes, (c) The WAXD patterns of electrospun membranes in high-angle range, (d) The DSC curves of electrospun membranes;

FIG. 4 represent the stress-strain curves of electrospun membranes; FIG. 5 represent the piezoelectricity curve and peak value of charge output of (a) non- piezoelectric PET solid film (b) commercial piezoelectric PVDF solid film, (c) electrospun PVDF membrane (d) electrospun P(VDF-TrFE) membrane (e) POS4, and (f) POS10;

FIG. 6 are TEM images of (a),(b) electrospun membranes and (c),(d) POS4;

FIG. 7 is FTIR spectra of electrospun PVDF and P(VDF-TrFE), POS4 and POS 10;

FIG. 8 are WAXD patterns of electrospun membranes in high-angle range;

FIG. 9 are stress-strain curves of electrospun membranes; and

FIG. 10 represent the piezoelectricity curve and peak value of charge output of (a) non- piezoelectric PET solid film (b) commercial piezoelectric PVDF solid film, (c) electrospun PVDF membrane (d) electrospun P(VDF-TrFE) membrane (e) POS2, (f) POS4, and (g) POS 10.

Detailed Description of the Disclosure

[0015] Although claimed subject matter will be described in terms of certain embodiments, other embodiments, including embodiments that do not provide all of the benefits and features set forth herein, are also within the scope of this disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the disclosure.

Accordingly, the scope of the disclosure is defined only by reference to the appended claims.

[0016] Organically modified layered silicate is an excellent nanoscale reinforcing agent which has good compatibility with hydrophobic polymer. These organosilicates are composed of approximately lnm-thick silicate nanoplates stacking layer by layer with organic modifier in between the interlayer. Polymer chains can intercalate inside the layers when there are strong interfacial interactions between the polymer chains and the interlayer surface of organosilicate (see Figure 1). If organosilicates are dispersed well in the fluoropolymer matrix, a remarkable improvement in mechanical properties of resultant composites can be achieved.

[0017] Electrospun nanofibrous thin sheets, such as membranes, based on fluoropolymer (such as polyvinylidene fluoride (PVDF) and its copolymer poly(vinylidene-trifluoroethylene) P(VDF-TrFE)) have attracted much attention because of their potential in many areas including power nanogenerators, sensors, tissue engineering, filtration membranes, battery separators, and polymer electrolytes. However, the poor mechanical properties of such electrospun membranes have limited their applications because even partial failure can lead to serious consequences in the end- use.

[0018] The present disclosure provides a method for making novel PVDF/organosilicate and

P(VDF-TrFE)/organosilicate composite membranes by electrospinning. The electrospun composite membranes demonstrate dramatic improvements in strength, modulus, extensibility, and toughness when compared with those of electrospun pure PVDF and P(VDF-TrFE) membrane. It is also significant to note that the electrospun PVDF/organosilicate and P(VDF-TrFE)/organosilicate membranes possess high porosity, low density, good breathability and piezoelectricity.

[0019] For the preparation of electrospun PVDF/organosilicate or P(VDF-

TrFE)/organosilicate composite membranes, organosilicate is added into a mixed solvent of DMF- acetone and subjected to ultrasonication at room temperature. Next, PVDF or P(VDF-TrFE) is dispersed in the organosilicate suspension by magnetic stirring at elevated temperature. The

PVDF/organosilicate or P(VDF-TrFE)/organosilicate solution is then ultrasonicated at room temperature again to obtain a homogenous mixture. The resultant solution is put into a plastic syringe. A DC voltage was applied to the needle tip of the syringe to electrospin

PVDF/organosilicate or P(VDF-TrFE)/organosilicate nanofibers on a rotating drum collector.

[0020] In one example, the parameters for preparing P(VDF-TrFE)/organosilicate composite membrane include (i) a loading amount of organosilicate of 4 wt%; (ii) use of a mixed solvent of DMF-acetone (75 vol% : 25 vol%); (iii) application of 14 kV of DC voltage to the needle tip of the syringe to electrospin P(VDF-TrFE)/organosilicate nanofibers on a rotating drum collector; and (iv) a distance from the needle tip of the syringe to the collector of about 15 cm.

[0021] Electrospun pure P(VDF-TrFE) and P(VDF-TrFE)/organosilicate nanofibers with different amounts of organosilicate (POS4: 4 wt% of organosilicate, POS10: 10 wt% of

organosilicate) were formed. In all three electrospun samples, nanofibers contained very little beads and are randomly distributed to form nanofibrous webs. The densities of electrospun P(VDF-TrFE) membrane, POS4 and POS10 were measured to be 0.37 g/cm³, 0.38 g/cm³, 0.33 g/cm³, respectively, which are about one-fifth of the density of bulk P(VDF-TrFE) (1.85 g/cm³). Accordingly, their porosities are in the range of approximately 79-82%. The inclusion of organosilicate may have little effect on the nanofiber structure and the porosity of the membrane and, consequently, the composite membranes can have similar vapor permeability or breathability (measured in terms of water vapor transmission rates (WVTRs)) to the electrospun P(VDF-TrFE) membrane.

[0022] The electrospun P(VDF-TrFE) membranes may exhibit a tensile strength of

1.32=1=0.15 MPa and an elongation at break of 55.0±6.2%. The modulus of electrospun P(VDF-TrFE) membrane obtained from the initial stage of stress-strain curve may be only 6.0±2.0 MPa. In contrast, the tensile strength, modulus, and elongation at break of POS4 and POS10 are greater. The tensile strength, modulus, and elongation at break of POS4 was 13.0±1.1 MPa, 22.4±2.3 MPa, and 111.0±13.9%, respectively, and those of POS10 are 12.8±1.2 MPa, 26.8±3.8 MPa, and 89.6±10.5%, respectively. Furthermore, the toughness can also be compared by calculating the area under the stress-strain curve, which represents the work required per unit volume to fracture the sample. The calculated toughness of electrospun P(VDF-TrFE) membrane is only 0.47 J/m³ whereas the corresponding values for POS4 and POS10 are 9.21 J/m³ and 7.13 J/m³, respectively.

[0023] Although the dipole density of electrospun porous P(VDF-TrFE) membrane is lower than that of commercial piezoelectric PVDF solid film, its charge output is higher. It is possible that more deformation was induced when the compressive force was applied on the electrospun P(VDF- TrFE) membrane. Moreover, the piezoelectricity of electrospun P(VDF-TrFE) is also higher than that of the electrospun PVDF membrane because it contains higher amount of β phase crystal.

However, it can be easier for the electrospun P(VDF-TrFE) membrane to become damaged when continuously subjected to stress because of its poor mechanical properties. The charge output of the more durable P(VDF-TrFE)/organosilicate membrane POS4 is about 38% lower than that of electrospun P(VDF-TrFE) membrane but still relatively close to that of electrospun pure PVDF membrane. The reduction in charge output from electrospun P(VDF-TrFE)/organosilicate membranes is probably due to their relatively higher modulus, giving rise to less deformation when compared to electrospun P(VDF-TrFE) membrane at the identical pressure. With such significant improvement in mechanical properties and still good piezoelectricity, the P(VDF- TrFE)/organosilicate membrane disclosed herein is an improved material for practical applications such as e-textiles and tissue engineering.

[0024] The fibers disclosed herein may be used in, for example, e-textiles as a sensor or power source for small electronics. In clothing or shoes, these fibers may be flexible, conformal to the body, strong, and breathable. These fibers also may enable improved water vapor transmission. The fibers can be arranged to function as, for example, a pressure sensor in clothing or shoes, which in one instance is connected to wireless technology and can monitor a wearer's lifestyle. These fibers also may be used in, for example, batteries as battery separator or polymer electrolyte, filter membranes in the filtration system, tissue engineering, scaffolds for cell propagation, or other applications known to those skilled in the art. The piezoelectric properties of these fibers may enable cells on a scaffold to grow faster.

[0025] In an example, the fibers disclosed herein were used as sensors or electrical generators in shoes.

[0026] In an alternate embodiment, electrospun membranes based on fluoropolymer and functionalized carbon nanotubes or graphene may be fabricated. This is because (1) the interfacial interaction between the huge surfaces of functionalized carbon nanotubes or graphene and fluoropolymer can significantly induce the formation of the piezoelectric β-phase crystalline in fluoropolymer; (2) the drawing and control of fiber orientation in electrospinning can align dipoles without the poling process and further enhance the formation of β-phase crystalline in

fluoropolymer; and (3) the reinforcement effect of functionalized carbon nanotubes or graphene as a nanofiller for fluoropolymer to improve mechanical properties.

[0027] Higher porosity in a membrane may reduce the piezoelectric property. However, the piezoelectric output of a membrane is similar to a film, which may be a ceramic, of equal thickness. Per weight, the membrane can provide better piezoelectric output compared to the film because it is porous.

[0028] The fiber orientation or alignment in a membrane may have an effect on piezoelectric properties. The rotation speed of a drum can affect this orientation or alignment because faster rotation speed may enable more parallel or organized fibers on the drum.

[0029] Reducing the diameter of a fiber may equate to a higher solvent percentage. This may improve piezoelectric properties.

[0030] A membrane fabricated of the fibers disclosed herein may have varying dimensions.

For example, the membrane may be 8.5" x 11", though other dimensions are possible.

[0031] The conductive filler disclosed herein may contain functional groups (OH, OOH, etc.) which can induce β form of crystallinity. But pure carbon nanotubes (CNTs) or graphene without functional groups may also induce β form due to the electron interaction.

[0032] The solvent disclosed herein is configured to dissolve the functional polymer and evaporate during or after the electrospinning. The solvent may be a mixture of dimethylformamide (DMF) and acetone. Other solvents also can be used alone or in a mixture. These solvents include, for example, Ν,Ν-dimethylacetamide (DMA), Ν,Ν-dimethylformamide (DMF ),

dimethylsulphoxide (DMSO ), hexamethylphosphoramide (HMPA), N-methyl-2-pyrrolidone (NMP), tetramethylurea (TMU ), triethyl phosphate (TEP ) and trimethyl phosphate (TMP).

Different solvents can be selected to control the nanofiber morphology.

[0033] The ratio of polymer per solvent may vary. Typically, the polymer (including organosilicate) is about 14-20 wt%, but the adding of clay may affect electrospinning. In one particular example, 4.8g polymer and 25.2g solvent was used to form a 30g solution. Thus, 16% polymer was used. The actual ratio may depend on the design of an electrospinning machine or may be adjusted to obtain a uniform membrane. [0034] A ratio of PVDF in the P(VDF-TrFE) functional polymer may vary. For example, the amount of PVDF in the P(VDF-TrFE) may be 20%, 80%, or any value in between approximately 20% to 80%. Other ratios are possible and these are merely listed as examples.

[0035] The steps of the method described in the various embodiments and examples disclosed herein are sufficient to carry out the methods of the present disclosure. Thus, in an embodiment, a method consists essentially of a combination of the steps of the methods disclosed herein. In another embodiment, a method consists of such steps.

[0036] The following example is presented to illustrate the present disclosure. They are not intended to be limiting in any manner. EXAMPLE 1

[0037] The P(VDF-TrFE) (80/20) copolymer was provided by Piezotech France. The organically modified clay (DK4 OMMT) was provided by Fenghong Clay Company in the form of yellow powder. The commercial PVDF solid film (110 micron in thickness) was provided by Measurement Specialties in USA. Ν,Ν-dimethylformamide (DMF) and acetone were provided by Dongzhen Chemical Reagent Company.

[0038] For the preparation of electrospun P(VDF-TrFE)/organosilicate membrane, organosilicate was added into a mixed solvent of DMF-acetone (75 vol%> : 25 vol%>) and subjected to ultrasonication at room temperature for 1 hour. Next, the P(VDF-TrFE) copolymer was dispersed in the organosilicate suspension by magnetic stirring at 50 °C for 3 hours. The P(VDF- TrFE)/organosilicate solution was then ultrasonicated at room temperature for another 1 hour to obtain a homogenous mixture of P(VDF-TrFE) and organosilicate in the solvent. The resultant P(VDF-TrFE)/organosilicate solution was put into a plastic syringe. 14 kV of DC voltage was applied to the needle tip of syringe to electrospin P(VDF-TrFE)/organosilicate nanofibers on a rotating drum collector. The distance from the needle tip of the syringe to the metal collector was fixed to be 15 cm. Similar experimental conditions were used to prepare the electrospun pure PVDF and P(VDF-TrFE) membranes. The thicknesses of resultant electrospun membranes were about 110- 130 microns.

[0039] FTIR spectra were recorded on a Perkin Elmer spectrometer. A JSM-6490 scanning electron microscope (SEM) and a FEI Tecnai T-12 Spirit transmission electron microscope (TEM) were employed to observe the morphology of electrospun membranes. Thermal analysis was carried out by a Perkin Elmer DSC-7 equipment with a heating rate of 10 °C/min from 0 to 170 °C in nitrogen atmosphere. Wide angle X-ray diffraction (WAXD) patterns were obtained on a Scintag PAD X theta-theta diffractometer. Tensile testing was carried out by using an Instron 5566 machine at a strain rate of 5 mm/min, in which several strips of each electrospun membrane were measured and an average value was taken. In an example, the strips had lengths of 12 mm and widths of 5 mm. Water vapor permeability was measured using the cup test method according to BS 7209 (temperature: 21.1 °C, relative humidity: 65%). The density of electrospun membrane was calculated according to a method reported in the literature. The electrospun membrane was cut into a rectangle shape and calculated the volume (V) by lengthxwithxthickness. The mass of the electrospun membrane (M) was weighed by the electronic balance and the density of the electrospun membrane can be obtained from M/V. Piezoelectric responses were measured using customized equipment, in which an Instron 5566 machine in compression mode produced the compressive pressure and a Fluke 8846 A digital multimeter combined with a lab charge amplifier (Measurement Specialties) was used to measure the charge output. Two metal foils were attached on both sides of the electrospun membrane as electrodes. The applied pressure was 2000 Newton per 10 cm² and the working area was 6 cm².

[0040] Figure 2 shows SEM images of electrospun pure P(VDF-TrFE) and P(VDF-

TrFE)/organosilicate composite nano fibers with different amounts of organosilicate (POS4: 4 wt% of organosilicate, POS10: 10 wt% of organosilicate). In all three electrospun samples, nano fibers contained very little beads and are randomly distributed to form nanofibrous webs. For POS4 and POS10, it appears that the organosilicate nanoplates have been embedded in the P(VDF-TrFE) nanofibers. TEM images (see Figure 6) further confirm the existence of small aggregations of organosilicate nanoplates in the P(VDF-TrFE) nanofibers and most of them are oriented along the fiber axis. The alignment of organosilicate nanoplates was due to the high extension of the electrospun jet resulting in the orientation of organosilicate along the motion direction of the solution jet. According to the diameter quantification, POS4 and POS10 had smaller nanofiber diameters than electrospun P(VDF-TrFE) membrane, which is similar to other reported electrospun composite membranes. The decrease in nanofiber diameter in POS4 and POS10 is believed to be attributed to the enhanced electrical conductivity of the electrospinning solution containing organosilicate.

[0041] In one experiment, the densities of electrospun P(VDF-TrFE) membrane, POS4 and

POS10 were measured to be 0.37 g/cm³, 0.38 g/cm³, 0.33 g/cm³, respectively, which are about one- fifth of the density of bulk P(VDF-TrFE) (1.85 g/cm³). Accordingly, their porosities are in the range of 79-82%. As shown in Figure 2, the inclusion of organosilicate had little effect on the nanofiber structure and the porosity of the membrane, consequently, the composite membranes had very similar vapor permeability or breathability (measured in terms of water vapor transmission rates (WVTRs)) to the electrospun P(VDF-TrFE) membrane.

[0042] In another experiment shown in Table 1 , the densities of electrospun P(VDF-TrFE) membrane, POS2, POS4, POS6, POS8 and POSIO were measured to be 0.48, 0.44, 0.43, 0.43, 0.46, 0.43 g/cm³, respectively, which are about one-fourth of the density of bulk P(VDF-TrFE) (1.85 g/cm3). Accordingly, their porosities are in the range of 74-78%. Since the inclusion of

organosilicate had little effect on the nanofiber structure and the porosity of the membrane, the composite membranes had very similar vapor permeability or breathability (measured in terms of water vapor transmission rates (WVTRs)) to the electrospun P(VDF-TrFE) membrane.

[0043] Table 1 - values of clay content, membrane density, porosity and WVTRs for electrospun membranes.

Sample Clay content Membrane density Porosity WVTRs

(wt.%) (g/cm³) (%) [g/(hourm²)]

P(VDF-TrFE) 0 0.48 74.1 16.8

POS2 2 0.44 76.4 17.7

POS4 4 0.43 77.1 15.1

POS6 6 0.43 77.2 16.4

POS8 8 0.46 75.8 17.1

POS10 10 0.43 77.5 17.1

[0044] Figure 3a shows the WAXD patterns of organosilicate, POS4 and POS10 in an experiment where the first peaks in the low-angle range correspond to the characteristic (001) plane reflections of organosilicate. It means that, for both POS4 and POS10, organosilicate has been successfully incorporated into the P(VDF-TrFE) nanofiber. The diffraction-peak intensities of composite membranes decreased significantly when compared to organosilicate, indicating a more disordered layered structures or even partial exfoliation. Organosilicate has a dOOl -spacing of 2.8 nm (2 theta=2.47 degree) while POS4 has a larger dOOl -spacing of 3.2 nm (2 theta=2.20 degree), implying the intercalation of highly hydrophobic polar P(VDF-TrFE) chains inside the organic layered galleries of organosilicate. This can be explained by the good compatibility and strong interaction between P(VDF-TrFE) and organosilicate. Whereas in the case of POS10, no shift of diffraction peaks can be observed, this may be due to the agglomeration of organosilicate at a higher percentage.

[0045] In another experiment, The (001) plane distances of samples were calculated by the

Bragg's law and the results are listed in Table2. For P(VDF-TrFE)/organosilicate membranes, the (001) plane distances decrease with the increment of organosilicate content. Organosilicate has a dOOl -spacing distance value of 3.57 nm while POS2 and POS4 have larger dOOl -spacing distance values of 4.09 nm and 4.0 lnm, respectively, implying the intercalation of highly hydrophobic polar P(VDF-TrFE) chains inside the organic layered galleries of organosilicate. This can be explained by the good compatibility and strong interaction between P(VDF-TrFE) and organosilicate. Whereas in the case of POS10, no shift of diffraction peaks can be seen, this may be due to the agglomeration of organosilicate at a higher percentage.

[0046] Table 2 - Values of dooi-spacing distance, X_c, T_c and T_m for samples

Sample dooi -spacing distance Xc T_c T_m

(nm) (%) (°C) (°C)

Organosilicate 3.57 — — —

P(VDF-TrFE) — 51.9 127.7 141.9

POS2 4.09 55.0 128.9 142.1

POS4 4.01 55.9 129.7 141.9

POS6 3.82 56.1 129.2 143.1

POS8 3.76 53.4 128.9 143.1

POS10 3.57 57.8 129.1 142.6

[0047] Figure 3b and Figure 7 show the Fourier-transform infrared (FTIR) spectra of electrospun membranes. Both POS4 and POS10 exhibit two peaks at 1038 and 1007 cm^"1 and the intensity increases as organosilicate concentration increases. These two peaks are the major features of the organosilicate spectra originated from Si-0 stretching vibrations, which further confirms the presence of organosilicate in the composite membranes. Moreover, it is known that PVDF has five different crystal structures including α, β, γ, ξ, and ε phase. The non-piezoelectric a phase is the most stable one but the β phase may possess the strongest piezo activity. The FTIR result of electrospun PVDF membrane shows a combination of non-piezoelectric a phase and piezoelectric β phase, which means that the a phase may not completely transfer into β phase by electrospinning. If the β phase cannot be effectively formed in the electrospun PVDF membrane, the piezoelectricity decreased significantly. P(VDF-TrFE) has an advantage over PVDF in that it can form high fraction of piezoelectric β crystalline automatically at room temperature. From Figure 3b, only strong peaks of β form crystalline at 843 and 1283 cm^"1 can be observed for electrospun P(VDF-TrFE) membrane, POS4 and POS10, regardless of the existence of organosilicate, suggesting that P(VDF- TrFE) can effectively crystallize into the piezoelectric β phase during electrospinning.

[0048] The effect of organosilicate on the crystallization behavior of P(VDF-TrFE) was investigated by WAXD and differential scanning calorimeter (DSC). Figure 3c presents the WAXD patterns of electrospun membranes. The electrospun P(VDF-TrFE) membrane showed a

characteristic diffraction peak at 2 theta=20.2° and a halo shoulder between 15 to 19 °C, which represent (220)/(l 10) lattice planes of the β form crystallite and the amorphous region, respectively. For POS4 and POS10, only the strong diffraction peak at 2 theta=20.2° can be observed. It is thus possible that organosilicate has a good nucleation effect to facilitate the crystallization of β phase owing to its strong interfacial interaction with P(VDF-TrFE) chains. Therefore, pure P(VDF-TrFE) does not crystallize as effectively as POS4 and POS10 during the quick electrospinning process. Figure 3d shows the DSC thermograms of electrospun samples. Electrospun P(VDF-TrFE) membrane exhibits two peaks at 117.9 and 145.9°C, which correspond to the transition temperature of ferroelectric to paraelectric phase and the melting temperature of the β form crystalline, respectively. POS4 and POS10 have the similar DSC curves to the electrospun P(VDF-TrFE) membrane. The endothermic enthalpy of electrospun P(VDF-TrFE) membrane was measured to be 33.2J/g, equaling to 74.0% of crystallinity degree (Xc), but those of POS4 and POS10 increased to 34.8 (Xc =77.3%) and 35.6 J/g (Xc =79.1%), respectively. It implies that organosilicate acts as the nucleation agent to improve the crystallinity degree of P(VDF-TrFE), which is consistent with the WAXD result.

[0049] Figure 8 presents the WAXD patterns of samples. It can be seen that PVDF solid film obtained from hot-pressing evidently exhibits a phase crystallite as shown from the characteristic peaks at 2 theta of 18.5°, 20.0° and 26.7° corresponding to a(100), a(l 10) and a(021), respectively. For the electrospun PVDF membrane, the intensity of a phase crystallite obviously reduces, and a new reflection peak at 2 theta of 20.7° emerges in the WAXD pattern which is relative to the β phase crystallite of PVDF. It implies that the much more stable a phase crystallite in PVDF could partly transform into the β phase crystallite by electrospinning. The electrospun P(VDF-TrFE) membrane showed a characteristic diffraction peak at 2 theta=20.2°, which represents the (220)/(l 10) lattice plane of the β phase crystallite. For electrospun P(VDF-TrFE)/organosilicate composite membranes, the diffraction peak at 2 theta=20.2° can also be observed. According to the WAXD results, the values of percent crystallinity (X_c) of all electrospun P(VDF-TrFE)/organosilicate composite membranes are higher than that of electrospun P(VDF-TrFE) membrane (see Table 2). It is possible that organosilicate has a good nucleation effect to facilitate the crystallization of β phase owing to its strong interfacial interaction with P(VDF-TrFE) chains. Therefore, organosilicate can act as the nucleation agent to improve the crystallinity degree of P(VDF-TrFE). Meanwhile, it is worth noting that the X_c value (38.2%) of the electrospun PVDF membrane is much lower than those of electrospun P(VDF-TrFE)/organosilicate composite membranes. Figure 9 shows the DSC thermograms of electrospun samples. Electrospun P(VDF-TrFE) membrane exhibits two peaks at 127.1 and 141.9 °C, which correspond to the transition temperature of ferroelectric to paraelectric phase (Tc, F-P transition) and the melting temperature (T_m), respectively. Electrospun P(VDF-

TrFE)/organosilicate composite membranes have the similar DSC curves to the electrospun P(VDF- TrFE) membrane. The transition of ferroelectric to paraelectric phase further confirms the existence of the β form crystalline in the electrospun membranes. Moreover, it is noted that the values of the F-P transition and melting peaks for electrospun P(VDF-TrFE)/organosilicate composite membranes are slightly higher than that of electrospun P(VDF-TrFE) membrane (see Table 2), which may be due to the larger size of crystal regions in polar and nonpolar phases for composite membranes.

[0050] Figure 4 shows the representative stress-strain curves of electrospun P(VDF-TrFE) and P(VDF-TrFE)/organosilicate membranes and the mechanical data based on stress-strain curves are listed in Table 3. The electrospun P(VDF-TrFE) membranes are weaker, exhibiting a very low tensile strength of 1.32±0.15 MPa and an elongation at break of 55.0±6.2%. The modulus of electrospun P(VDF-TrFE) membrane obtained from the initial stage of stress-strain curve is only 6.0±2.0 MPa. Such mechanical performance of electrospun P(VDF-TrFE) membrane may adversely affect its application in various areas. In contrast, the tensile strength, modulus, and elongation at break of POS4 and POS10 are much greater. The tensile strength, modulus, and elongation at break of POS4 was 13.0±1.1 MPa, 22.4±2.3 MPa, and 111.0±13.9%, respectively, and those of POS10 are 12.8±1.2 MPa, 26.8±3.8 MPa, and 89.6±10.5%, respectively. Furthermore, the toughness can also be compared by calculating the area under the stress-strain curve, which represents the work required per unit volume to fracture the sample. The calculated toughness of electrospun P(VDF- TrFE) membrane is only 0.47 J/m³ whereas the corresponding values for POS4 and POS10 are 9.21 and 7.13 J/m³, respectively. The enhancement of mechanical properties of electrospun P(VDF- TrFE)/organosilicate membranes is significant when compared to other reported electrospun composite membranes using other nanoscale reinforcing agents. The tensile strength, modulus, and elongation at break of electrospun poly(ethylene terephthalate) (PET) membrane were 1.56 MPa, 10.8 MPa, and 163%, while in the case of electrospun PET/multi- walled carbon nanotube (MWNTs, 3 wt%) membrane the corresponding values were 2.05 MPa, 21.8 MPa, and 96%, respectively.

[0051] Table 3 - Values of Young's modulus, tensile strength, elongation at break, toughness and charge output for electrospun membranes

Sample Young's Tensile Elongation at Toughness Charge modulus strength break output

(MPa) (MPa) (%) (J/m³) (nC)

P(VDF-TrFE) 5.99±2.05 1.33±0.16 55.11±6.28 0.47±0.08 8.1

POS2 18.69±1.69 21.51±1.72 84.59±7.56 9.69±1.79 6.0

POS4 22.40±2.31 13.03±1.11 111.09±13.85 9.20±1.83 5.0

POS6 23.17±3.41 19.92±0.95 73.25±4.65 7.76±0.89 4.8

POS8 24.75±2.32 17.95±0.90 69.32±4.11 6.39±0.72 4.3

POS10 26.82±3.81 12.89±1.18 89.59±10.45 7.13±0.74 3.9 [0052] In another example, the tensile strength, modulus, and elongation at break of POS2 are 21.51±1.72 MPa, 18.69±1.69 MPa, and 84.59±7.56%, respectively, and those of POS4 are 13.03±1.11 MPa, 22.40±2.31 MPa, and 111.09±13.85%, respectively. The calculated toughness of electrospun P(VDF-TrFE) membrane is only 0.47±0.08 J/m³ whereas the corresponding values for POS2 and POS4 are 9.69±1.79 and 9.20±1.83 J/m³, respectively, which are more than 19 and 18 times higher than that of electrospun P(VDF-TrFE) membrane, respectively.

[0053] The modification of crystalline for PVDF from a to β phase can effectively improve its mechanical properties. Since the main crystal structure of POS4 and POS10 is β phase which is the same as electrospun P(VDF-TrFE) membrane, the improved mechanical performance may be mainly attributed to the reinforced effect of organosilicate. Generally speaking, the incorporation of rigid nano fillers into polymeric nano fibers can increase the modulus and strength if the applied stress can be effectively transferred from polymer matrix to nanofillers across their interface.

However, this increase is usually accompanied with a considerable reduction in the elongation at break. As for our electrospun P(VDF-TrFE)/organosilicate composite membranes, simultaneous improvement of strength and elongation at break were achieved. The improvement of modulus and strength can be ascribed to the incorporation of stronger organosilicate into P(VDF-TrFE) nanofibers and the enhanced stress transfer across the interface as a result of the excellent compatibility between P(VDF-TrFE) chains and organosilicate. As far as the enhanced elongation at break and toughness are concerned, as demonstrated in Figure 1 , it is possible that the organosilicate nanoplates can form a temporary crosslinked network during deformation. As the sample is subjected to tensile drawing, the P(VDF-TrFE) chains preferentially align parallel to the stretching direction. Because of the strong interfacial interaction, organosilicate nanoplates move together with the polymer chains and act as crosslinked points during mechanical stretching, which leads to more efficient energy dissipation and a delay of crack formation. In particular, because POS2 and POS4 have a better intercalated structure, namely much more interfacial interaction, its elongation at break or toughness are higher than those of POS10.

[0054] The compressive piezoelectric responses of a non-piezoelectric PET solid film, a commercial piezoelectric PVDF solid film, and electrospun membranes are shown in Figure 5 and Figure 10. The PVDF solid film, electrospun PVDF and P(VDF-TrFE) membranes, and P(VDF- TrFE)/organosilicate composite membranes (e.g., POS4 and POS10) exhibited clear charge outputs, but almost no signal can be found for the PET solid film, which confirmed the piezoelectricity of the PVDF solid film, electrospun PVDF, P(VDF-TrFE) membranes, and P(VDF-TrFE)/organosilicate composite membranes. It can also be seen that the piezoelectric charge-output signs changed alternatively showing two opposite peaks, which correspond to the imparting and releasing of repetitive external stress, respectively. Although the dipole density of electrospun porous P(VDF- TrFE) membrane is lower than that of commercial piezoelectric PVDF solid film, its charge output is much higher. It is possible that much more deformation was induced when the compressive force was applied on the electrospun P(VDF-TrFE) membrane. Moreover, the piezoelectricity of electrospun P(VDF-TrFE) is also higher than that of the electrospun PVDF membrane because it contains higher amount of β phase crystal. However, an electrospun P(VDF-TrFE) membrane can be damaged when continuously subjected to stress because of its poor mechanical properties. The charge output of the much more durable P(VDF-TrFE)/organosilicate composite membrane POS2 is about 26% lower than that of electrospun P(VDF-TrFE) membrane (see Table 3), but still higher than that of electrospun pure PVDF membrane. The charge output of the more durable P(VDF- TrFE)/organosilicate membrane POS4 is about 38% lower than that of electrospun P(VDF-TrFE) membrane but still close to that of electrospun pure PVDF membrane. The reduction in charge output from electrospun P(VDF-TrFE)/organosilicate membranes is probably due to their relatively higher modulus, giving rise to less deformation when compared to electrospun P(VDF-TrFE) membrane at the identical pressure. With such significant improvement in mechanical properties and still good piezoelectricity, the P(VDF-TrFE)/organosilicate membrane disclosed herein is an improved material for practical applications such as e-textiles and tissue engineering. [0055] High-performance multifunctional P(VDF-TrFE) membranes containing organosilicate have been developed by an electrospinning technique. The organosilicate-reinforced electrospun membrane shows more than an order of magnitude increase in toughness. Meanwhile, the low density, high porosity, good breathability, and piezoelectricity still can be found in the electrospun P(VDF-TrFE)/organosilicate composite membrane. These mechanical properties of the composite membrane can potentially be further optimized by (1) exfoliation of organosilicate in the nanofibers, (2) control of nanofiber orientation, and (3) mechanical drawing. In an example, it was found that electrospun P(VDF-TrFE)/organosilicate composite membrane had the best mechanical properties and the relatively good piezoelectricity when the content of organosilicate is 2 wt.%. [0056] Although the present disclosure has been described with respect to one or more particular embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the spirit and scope of the present disclosure. Hence, the present disclosure is deemed limited only by the appended claims and the reasonable interpretation thereof.

Claims

What is claimed is:

1. A nanofiber comprising:

a conductive filler and a functional polymer, wherein said functional polymer comprises poly[(vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)), polyvinylidene fluoride (PVDF), or a co-polymer of PVDF, and wherein said conductive filler comprises an organosilicate.

2. The nanofiber of claim 1, wherein said functional polymer comprises P(VDF-TrFE) and wherein a ratio of PVDF in said P(VDF-TrFE) is from 20% to 80%.

3. The nanofiber of claim 1, wherein said conductive filler contains a CO or OH group configured to stabilize an interface between said conductive filler and said functional polymer.

4. The nanofiber of claim 1, wherein said nanofiber comprises an amount of said conductive filler greater than 0% and less than or equal to 10%.

5. The nanofiber of claim 1, wherein said nanofiber is piezoelectric.

6. The nanofiber of claim 1, wherein said organosilicate is a porous organosilicate (POS).

7. The nanofiber of claim 1, further comprising a plurality of said nano fibers, wherein said

plurality nano fibers are disposed in contact with each other to form a membrane.

8. The nanofiber of claim 1, wherein said nanofiber has a diameter between 10 nm and 1 μιη.

9. A nanofiber comprising:

a conductive filler and a functional polymer, wherein said functional polymer comprises poly[(vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)), polyvinylidene fluoride (PVDF), or a co-polymer of PVDF, and wherein said conductive filler comprises graphene.

10. The nanofiber of claim 9, wherein said functional polymer comprises P(VDF-TrFE) and wherein a ratio of PVDF in said P(VDF-TrFE) is from 20% to 80%.

11. The nanofiber of claim 9, wherein said conductive filler contains a CO or OH group configured to stabilize an interface between said conductive filler and said functional polymer.

12. The nanofiber of claim 9, wherein said nanofiber comprises an amount of said conductive filler greater than 0% and less than or equal to 10%.

13. The nano fiber of claim 9, wherein said nano fiber is piezoelectric.

14. The nanofiber of claim 9, further comprising a plurality of said nanofibers, wherein said

plurality nanofibers are disposed in contact with each other to form a membrane.

15. The nanofiber of claim 9, wherein said nanofiber has a diameter between 10 nm and 1 μιη.

16. A method comprising:

electrospinning a solution of a conductive filler, a solvent, and a functional polymer to form a thin sheet comprising a plurality of fibers, wherein said functional polymer comprises poly[(vinylidenefluoride-co-trifluoroethylene] (P(VDF-TrFE)), polyvinylidene fluoride (PVDF), or a co-polymer of PVDF, and wherein said conductive filler comprises an organosilicate or graphene.

17. The method of claim 16, wherein said solvent comprises a mixture of dimethylformamide

(DMF) and acetone.

18. The method of claim 16, further comprising adding said functional polymer into a suspension comprising said conductive filler and said solvent to form said solution prior to said

electrospinning.

19. The method of claim 16, wherein said thin sheet comprises a membrane.