KR101363759B1 - Microfiber for gas sensing - Google Patents

Abstract

Provided is a microfiber for a gas sensor which includes: a core having liquid crystal portion; a macromolecular sheath which surrounds the core, and which is in contact with gas molecules in order for the gas molecules to be dispersed into the liquid crystal portion. The phase of the liquid crystal changes so as to decrease scattering of light by the change in alignment structure triggered by an infiltration of the gas molecules.

Description

Microfiber for gas sensing

The technology disclosed herein relates generally to microfibers for gas sensors.

The harmful biological effects of many organic vapors can be a significant issue for human health. In addition, because these organic vapors are flammable, there is a risk of explosion in the air, so detecting them quickly is very important for safety. For this reason, several detection methods have been developed using electrochemical methods (PN Bartlett, et al., Electronic Noses: Principles and Applications ; Oxford University Press: Oxford, UK, 1999., RH; NS Lewis, et al., Anal Chem. 2000, 72, 3181.).

However, the detection methods require driving power for the operation of the sensor, a device capable of reading data, and a problem such as a complicated manufacturing process.

According to an embodiment of the present invention, a core having a liquid crystal region; And a polymer sheath surrounding the core and capable of contacting external gas molecules to diffuse into the liquid crystal region, wherein the liquid crystal has a change in alignment structure due to penetration of the external gas molecules and reduction of light scattering. There is provided a microfiber for a gas sensor whose phase changes.

According to another embodiment of the present invention, there is provided a fiber mat woven from the fine fibers for the gas sensor described above.

According to another embodiment of the invention, providing a coaxial microfiber comprising a liquid crystal core (core) and a polymer sheath surrounding the liquid crystal core (sheath); Exposing the coaxial microfibers to external gas molecules to allow the gas molecules to penetrate the liquid crystal core; And observing a change in light transmittance of the coaxial microfibers due to the penetration of the gas molecules.

1 shows a microfiber for a gas sensor according to an embodiment of the present invention.
Figure 2 is a schematic diagram showing the phase change of the liquid crystal generated when the gas is sensed using the microfiber according to an embodiment of the present invention.
Figure 3 shows two forms of the microfibers for the gas sensor according to an embodiment of the present invention.
4 shows an apparatus for producing a microfiber for a gas sensor by an electrospinning method.
5 is an optical micrograph of coaxial fine fibers obtained by electrospinning.
6 is an electron micrograph of coaxial fine fibers obtained by electrospinning.
7 is a gas reaction test result for confirming the applicability of the fine fiber to the gas sensor.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The following implementations are provided by way of example so that the teachings of the persons skilled in the art are fully conveyed. Accordingly, the disclosed technique is not limited to the embodiments described below and may be embodied in other forms. In the drawings, the width, length, thickness, etc. of components may be exaggerated for convenience.

1 shows a microfiber for a gas sensor according to an embodiment of the present invention. Referring to FIG. 1, a microfiber for a gas sensor includes a core having a liquid crystal region and a polymer sheath surrounding the core.

There is no restriction | limiting in particular in the kind of said liquid crystal, A liquid crystal conventionally known, such as an aromatic type, an aliphatic type, and a polycyclic type compound, is included. The kind of the liquid crystal is preferably a (non-chiral) nematic liquid crystal or a chiral nematic liquid crystal (cholesteric liquid crystal).

Nematic liquid crystals are agglomerates composed of organic molecules with long, thin rods or flat plate-like geometrical shapes.The statistical distribution of the molecular center shows fluid-like flow, but the molecular direction spontaneously It refers to a material or a phase thereof arranged to exhibit macroscopic anisotropy different from the liquid. Molecular polarization is canceled because the directions of molecules are equal up and down, so they do not generally exhibit ferroelectricity.However, properties such as refractive index, permittivity, magnetization, conductivity, and viscosity are perpendicular to the direction parallel to the long axis of the molecule. Have different anisotropy. In particular, when an electric field is applied from the outside, the liquid crystal molecules are rearranged in a direction parallel to (or perpendicular to) the direction of the electric field due to dielectric anisotropy. Such a change in electric field induction array causes a change in the effective refractive index to change the path or polarization direction of the incident light.

On the other hand, cholesteric liquid crystal is a liquid crystal showing a nematic phase of the spiral structure when the chiral structure of the mirror structure is broken in the molecular structure of the molecules forming the nematic phase. Locally, cholesteric liquid crystals, like nematic liquid crystal molecules, are oriented in one direction, but macroscopically, there is a spiral axis perpendicular to the director, and the director is rotated about this axis.

Specific examples of the nonchiral nematic liquid crystal include 4-butyl-N- [methoxy-benzylidene] -aniline (MBBA) and 4-pentyl-4'-cyanobiphenyl (5CB). Specific examples include cholesteryl benzoate and cholesteryl pelagonate.

The content of the liquid crystal may be 10 to 90% by weight, preferably 20 to 80% by weight, more preferably 40 to 80% by weight based on the total weight of the microfibers for the gas sensor. If the liquid crystal content is less than the above range, the gas sensing ability is lowered. If the liquid crystal content is more than the above range, it may be difficult to properly function as a microfiber when liquid crystal oozes out of the fiber.

The polymer shell supports the liquid crystal and forms a shell of the microfibers for the gas sensor. The type of the polymer is polyurethane, polyethylene oxide, polyacrylonitrile, polyvinyl acetate, cellulose acetate, polyaniline, polyvinylpyrrolidone, polypyrrole, polythiophene, polyphenol, polyacetylene, polypetylene, poly (lactic acid ), Poly (methyl methacrylate), poly (glycolic acid), polyacrylate, polyester, polyamide, poly (ethylene oxide), polyolefin, polyvinyl chloride, polyimide, polyether, polysulfone and any of these And combinations thereof.

Preferably, the polymer shell is transparent or translucent, ie at least translucent, in order to visually observe the sensing of a particular gas, such as a volatile organic substance. By "at least translucent" is meant at least 50%, preferably at least 60%, more preferably at least 70%, even more preferably at least 80% of the incident light when measuring the transmittance of the polymer according to ASTM-D1003. , Particularly preferably at least 90% is transmitted.

In addition, the polymer shell may surround the core and may be diffused into the liquid crystal region by contact with the external gas molecules.

Preferred types of polymers are polyvinylpyrrolidone, polyacrylates, polyethylene terephthalate, polyethylene, polypropylene, polysulfones, polyethersulfones, polycarbonates, cyclic olefin copolymers, polyarylates and polyimides and their It may be a derivative.

In the case of chiral nematic liquid crystals, the helical structure selectively reflects light to give color. In the case of nonchiral nematic liquid crystals, the effective refractive index spatial change or the refractive index between the liquid crystal and the polymer shell inside The light is scattered by the car. When the microfibers of this type are exposed to a specific gas, gas diffusion through the polymer coating occurs and mixed with the liquid crystal. As a result, in the case of chiral or nonchiral nematic liquid crystals, the liquid crystal phase changes to an isotropic state. In this case, the chiral nematic liquid crystal undergoes a process of changing the twist period of the spiral structure.

This type of liquid crystal phase can be easily observed with the naked eye, and this property can be used as a gas sensor.

Figure 2 is a schematic diagram showing the phase change of the liquid crystal generated when the gas is sensed using the microfiber according to an embodiment of the present invention. (A) of FIG. 2 shows the phase change of a non-chiral nematic liquid crystal, and FIG. 2 (b) shows the phase change of a chiral nematic liquid crystal. In (a) of FIG. 2, it can be seen that the aligned liquid crystals are broken due to the diffusion of an external organic vapor. In addition, in FIG. 2B, organic vapor is introduced between the aligned liquid crystals of the spiral structure to change the twisting period, and then the alignment is broken and isotropic.

When the liquid crystal in the core region is a chiral nematic liquid crystal, it shows color at a low gas concentration and becomes transparent at higher concentrations because the alignment of liquid crystal molecules is broken and light scattering is reduced as it changes to an isotropic structure. . This phenomenon of light scattering can be observed in nonchiral nematic liquid crystals.

By using the above-described properties, microfibers having a liquid crystal core-polymer shell structure can be applied as a gas sensor. For example, the microfibers may be manufactured so that the reaction can be maximized only in a specific gas by adjusting the type of liquid crystal or the mixing ratio of various types of liquid crystals.

According to one embodiment of the present invention, a gas sensing method using coaxial microfibers is provided. The gas sensing method may be configured in the following steps. First, a coaxial microfiber including a liquid crystal core and a polymer sheath surrounding the liquid crystal core is provided. The coaxial microfibers are then exposed to external gas molecules to allow the gas molecules to penetrate the liquid crystal core. Subsequently, the change in the transmittance of light to the coaxial microfibers as the gas molecules penetrate is observed.

In this case, the transmittance may become opaque as the concentration of the gas molecules increases. In addition, the permeability may be reversibly changed depending on the presence or absence of the gas molecules.

According to an embodiment of the present invention there is provided a fiber mat woven from the fine fibers for the gas sensor described above. For example, fabricated fiber mats using these microfibers are initially opaque due to light scattering between the internal liquid crystal and the polymer sheath, but contact with toluene vapor breaks the alignment of the internal liquid crystal and reduces the light scattering effect. This makes the fiber mat transparent. If you stop contacting the toluene vapor again, you can see that it returns to its original opaque state.

The liquid crystal region may have a continuous shape along the long axial direction of the fiber. Alternatively, the liquid crystal region may have a discontinuous shape along the long axial direction of the fiber.

Figure 3 shows two forms of the microfibers for the gas sensor according to an embodiment of the present invention. 3 (a) shows a continuous form of the liquid crystal region constituting the core in the fiber, and (b) shows a discontinuous form of the liquid crystal region constituting the core in the fiber.

The diameter of the total microfibers may be 1 to 3㎛ and the diameter of the core may be 0.5 to 2㎛ when the liquid crystal region has a continuous form. When the liquid crystal region has a discontinuous form, the liquid crystal region may include a plurality of spherical shapes. At this time, the diameter of the core may be 0.5 to 2㎛ and the spacing between discontinuous cores may be 1 to 2㎛. Having a discontinuous core shape can further increase the surface area in contact with the external gas.

Polymer microfibers can be produced in a variety of ways. For example, methods for producing polymeric microfibers include melt spinning, wet spinning, melt blowing, gel spinning, dry-jet wet spinning, dry spinning and electrospinning. Microfiber production method may be selected in consideration of the properties of the polymer and the specifications and physical properties of the final fiber. In the process of producing microfibers, another polymer or small molecule component may be introduced into the fiber separately. The spinning process allows the introduction of another polymer by mixing it with the main polymer or by forming a separate domain from the polymer.

Microfibers for co-axial gas sensors according to an embodiment of the present invention can be produced by an electrospinning method. The microfibers of the liquid crystal core-polymer shell composite structure by the electrospinning method may be prepared, for example, as follows. By changing the relationship between the flow rate of the external polymer solution and the flow rate of the liquid crystal, the morphology of the core can be varied step by step from discontinuous to very thin but continuous. At high liquid crystal flow rates, continuous bulk cores and thick fibers are produced. Increasing the voltage beyond a certain threshold at which the fibers begin to form during electrospinning can reduce the fiber diameter. Discontinuous cores can be made by adjusting the interfacial tension of the liquid crystal and polymer solution with the addition of surfactants.

By combining various variables such as two flow rates, molecular weight of the polymer, and viscosity of the polymer solution, a composite microfiber of a desired shape can be obtained.

4 shows an apparatus for producing a microfiber for a gas sensor by an electrospinning method. Figure 4 (a) is a schematic diagram of the electrospinning apparatus, (b) is the appearance of a co-axial needle (co-axial needle) structure of the electrospinning apparatus actually used in the experiment. First, a polymer solution forming a sheath is supplied by connecting a Teflon tube and an aluminum needle. The core liquid crystal (LC) was poured after inserting a silica capillary into the aluminum needle. The needle of this structure has the advantage that the core material and the shell material can be transported without mixing with each other. When a high voltage is applied to the aluminum needle and the collector, charges are collected at the tip of the needle, and the surface tension and the electrostatic force of the solution balance each other to form a Taylor cone. After a certain voltage is exceeded, the solution is ejected toward the collector and at the same time the solvent evaporated to fly away, leaving only the fiber. In this experiment, the interval between the needle and the collector was maintained at 15 seconds and the voltage was applied at 10 kV.

As described above, the microfibers for the gas sensor according to the exemplary embodiment of the present invention are fibers filled with a liquid crystal material inside and wrapped with a polymer on the outside. In particular, when the internal liquid crystal region is formed in a discontinuous sphere shape, the contact area between the liquid crystal and the gas may be further increased, and the light scattering effect may also be increased. The manufacturing method of such fibers is not particularly limited, but an electrospinning method may be used.

The microfibers for the gas sensor of the present invention have a large response area with a large surface area to volume, and do not have a large difference in concentration gradient between the microfiber surface and the inside, so that the sensor response can be accurately determined. In addition, if you make porous fiber mats through electrospinning like non-woven fabrics and make them into garments, it will be possible to produce a gas sensor that can be worn.

Hereinafter, the present invention will be described with reference to various embodiments, which are only intended to help understanding of the present invention, and the technical spirit of the present invention is not limited by the following examples.

[Example]

5 ml of ethanol was added to 0.556 g of polyvinylpyrrolidone (PVP) and stirred at room temperature for 12 hours to completely dissolve the PVP. Then, 0.5 wt% NaCl was added and stirred at room temperature for 12 hours to prepare a pure PVP solution of 10 wt%. It was. 150 μl of titanium (IV) isopropoxide, 100 μl of acetic acid and 5 ml of ethanol were added to 0.44 g of PVP, followed by stirring at room temperature for 12 hours to prepare PVP-TiO ₂ solution. As chiral nematic liquid crystal, (S) -4-cyano-4- (2-methylbutyl) -biphenyl ((S) -4-cyano-4- (2) which is a chiral dopant -methylbutyl) -biphenyl) and nematic liquid crystals, RO-TN-605, were mixed immediately at room temperature in a weight ratio of 40:60 and immediately used. The nematic liquid crystal 4'-pentyl-4-biphenylcarbonitrile (4 '-Pentyl-4-biphenylcarbonitrile) was also used without addition of other materials. When preparing a discontinuous core, 1 wt% of myristyltrimethyl-ammonium bromide, a cationic surfactant, was added to adjust the interfacial tension between the liquid crystal and the polymer solution.

Liquid crystal flows into the silica capillary, PVP solution flows into the aluminum capillary, the pumping pressure is adjusted, and the voltage is about 15 kV using a high voltage power supply. You can get a coaxial nano fiber mat that is well aligned with the collector. As a result of analyzing the fiber mat made by the polarizing microscope, it was confirmed that the liquid crystal was contained in the fiber, and through the SEM analysis, a uniform fiber was obtained. there was.

Poly (vinyl pyrrolidone) used Across's MW 1,300,000, and Titanium (IV) isopropoxide, acetic acid, ethanol, toluene, sodium chloride (NaCl) were purchased from Aldrich. As a liquid crystal material, (S) -4-cyano-4- (2-methylbutyl) -biphenyl, 4′-Pentyl-4-biphenylcarbonitrile was purchased from Synthon Chemicals, and RO-TN-605 from Hoffmann-La Roche. The needles used to make the coaxial fibers were inserted with BGB Analytik's polyimide coated silica capillary into an aluminum capillary with an internal diameter of 0.7 um. The flow rate was controlled by Fluigent's MFCS-4C, and the high-voltage power supply was made by Gamma High Voltage Research. The electrospun fiber was analyzed using Nikon's Eclipse polarizing optical microscope (POM), and SEM's JSM-7500F was used.

5 is an optical micrograph of coaxial fine fibers obtained by electrospinning. (a) to (c) each represent (a) a fiber consisting solely of PVP, (b) a fiber having a liquid crystal core in a continuous form, and (c) a fiber having a liquid crystal core in a discontinuous form.

In the non-polarized state (a), (b), it can be seen that all of the (c) form a certain fiber form. The presence or absence of the liquid crystal in the core can be confirmed by observing the birefringence of the liquid crystal in the polarization state. In addition, since PVP is in an amorphous state, it does not show birefringence in a polarized state. (a) is a fiber composed only of PVP and could not identify internal birefringence. In the case of (b) and (c), birefringence was observed by the internal liquid crystal, (b) shows a continuous liquid crystal state, and (c) shows a discontinuous liquid crystal state.

6 is an electron micrograph of coaxial fine fibers obtained by electrospinning. (a) is a low magnification image, and (b) is a high magnification image. Referring to FIG. 6, it was confirmed that a fiber having a diameter of about 1.8 μm was obtained by electron microscopic observation of the coaxial microfibers. It was also confirmed that the fibers maintained a uniform sheath shape.

Microfiber according to an embodiment of the present invention can be used as a gas sensor.

7 is a gas reaction test result for confirming the applicability of the fine fiber to the gas sensor. In FIG. 7, (a) shows the gas reaction of the fiber consisting only of PVP, and (b) shows the gas reaction of the fiber having a core liquid crystal.

Toluene vapor was used for gas sensor experiments. Toluene steam was used as the steam generated by introducing air into the toluene solution. (a) is a fiber mat made of fiber consisting only of PVP without liquid crystal in the core, and no difference in transparency was found before and after exposure to toluene vapor. However, as in the case of (b), the liquid crystal in the core was opaque by light scattering before exposing toluene vapor to the fiber mat, but when exposed to steam, it could be confirmed that it became transparent within 2 to 3 seconds. This is because the toluene vapor diffuses into the core liquid crystal due to the difference in concentration, and thus the liquid crystal phase is changed into an isotropic state and causes a refractive index change. Stopping the vapor exposure again returns the liquid crystal phase from isotropic to the original phase within 2-3 seconds, making the fiber mat opaque.

Claims (11)

A core having a liquid crystal region; And
And a polymer sheath surrounding the core and capable of contacting external gas molecules to diffuse into the liquid crystal region.
The liquid crystal is a microfiber for a gas sensor that the phase changes so that the alignment structure is changed by the penetration of the external gas molecules and the scattering of light is reduced. The method according to claim 1,
And the liquid crystal region has a continuous shape along the long axial direction of the fiber. The method according to claim 1,
And the liquid crystal region has a discontinuous shape along the long axial direction of the fiber. The method of claim 3,
The microfiber for a gas sensor, wherein the liquid crystal region comprises a plurality of spherical shapes. The method according to claim 1,
The liquid crystal is a non-chiral nematic liquid crystal or chiral nematic liquid crystal microfibers for gas sensors. The method according to claim 1,
The microfiber for the gas sensor is the liquid crystal isotropically changed as the gas molecules penetrate. The method according to claim 1,
The polymer shell is at least translucent microfibers for gas sensors. Fiber mat woven from the fine fibers for gas sensors according to any one of claims 1 to 7. Providing a coaxial microfiber comprising a liquid crystal core and a polymer sheath surrounding the liquid crystal core;
Exposing the coaxial microfibers to external gas molecules to allow the gas molecules to penetrate the liquid crystal core; And
Comprising the step of observing the change in the transmittance of light to the coaxial microfibers as the gas molecules penetrate. 10. The method of claim 9,
The permeability of the gas sensing method is opaque the higher the concentration of the gas molecules. 10. The method of claim 9,
The permeability is a gas sensing method that varies reversibly depending on the presence or absence of the gas molecules.

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