CN102615885B - Preparation method of reversible photoinduced deformation liquid crystal high polymer and carbon nano tube composite thin film - Google Patents
Preparation method of reversible photoinduced deformation liquid crystal high polymer and carbon nano tube composite thin film Download PDFInfo
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- 230000002194 synthesizing effect Effects 0.000 claims description 3
- NIXOWILDQLNWCW-UHFFFAOYSA-M acrylate group Chemical group C(C=C)(=O)[O-] NIXOWILDQLNWCW-UHFFFAOYSA-M 0.000 claims description 2
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- KRHYYFGTRYWZRS-UHFFFAOYSA-M Fluoride anion Chemical compound [F-] KRHYYFGTRYWZRS-UHFFFAOYSA-M 0.000 claims 1
- RIQVSVGXJVQRNM-UHFFFAOYSA-N cyclopenta-2,4-dien-1-ylbenzene titanium(2+) Chemical group [Ti++].c1cc[c-](c1)-c1ccccc1.c1cc[c-](c1)-c1ccccc1 RIQVSVGXJVQRNM-UHFFFAOYSA-N 0.000 claims 1
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- DMLAVOWQYNRWNQ-UHFFFAOYSA-N azobenzene Chemical compound C1=CC=CC=C1N=NC1=CC=CC=C1 DMLAVOWQYNRWNQ-UHFFFAOYSA-N 0.000 abstract description 9
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Abstract
The invention belongs to the technical field of an intelligent material and particularly relates to a reversible photoinduced deformation liquid crystal high polymer and carbon nano tube composite thin film and a preparation method thereof. A liquid crystal high polymer in the composite thin film is a cross-linked type liquid crystal elastic body containing a photosensitive azobenzene liquid crystal element; and a carbon nano tube is a highly and sequentially oriented carbon nano tube thin film. In the composite thin film, a carbon nano tube orienting structure induces the liquid crystal element to be oriented along the axial direction of the carbon nano tube; and other orienting layers are not needed. When the prepared composite thin film is in ultraviolet illumination, the composite thin film is bent towards lights and can be recovered to the original state when the composite thin film is irradiated by visible lights. Reversible deformation generated by alternatively illuminating the composite thin film by ultraviolet lights and visible lights can be repeated for hundreds of times and has no obvious attenuation and fatigue. Moreover, the introduction of the carbon nano tube greatly enhances the mechanical performance of the material, and the material has excellent electric performance.
Description
Technical Field
The invention belongs to the technical field of intelligent materials, and particularly relates to a preparation method of a liquid crystal polymer and carbon nanotube composite film.
Background
Due to the huge application prospect in the fields of artificial muscles, optical drivers, micro optical mechanical systems and the like, the photoinduced deformation high polymer material has been greatly developed in recent years. Among them, a crosslinked liquid crystal polymer containing a photosensitive group such as azobenzene is one of the most studied materials, and deformation of the material is achieved by photoisomerization of the azobenzene group. By introducing cross-linked liquid crystal polymer materials, people have realized many light-driven flexible devices, including plastic motors, inchworm-type walkers, flexible micro-robots, high-frequency oscillators, artificial fibers, and the like. In the above applications, it is critical to control the bending direction of the liquid crystal polymer material, and the bending direction depends on the alignment structure of the mesogen. For example, a liquid crystal polymer film of uniform parallel alignment is bent to light, while a liquid crystal polymer film of vertical alignment is bent to backlight. In order to achieve liquid crystal alignment, the surfaces of the substrates of the liquid crystal cells used generally need to be modified to enhance the interaction with the liquid crystal molecules. The most common method is mechanical friction. The method mainly comprises the steps of coating a layer of polyimide on the surface of a glass substrate and then mechanically rubbing the glass substrate along the same direction. The parallel grooves generated by the rubbing process are used to induce the alignment of the liquid crystal molecules. However, this method has some problems, such as introduction of broken fragments and static electricity on the surface, and a certain degree of damage to the structure, which limits the properties and applications of liquid crystalline polymers.
On the other hand, carbon nanotubes have been studied extensively since their discovery. Because of their excellent mechanical and electrical properties and their good absorption in the visible-near infrared region, attempts have been made to disperse carbon nanotubes in heat-sensitive liquid crystalline polymers to form a network of photopolymers that shrink upon irradiation with near infrared light. The carbon nano-tube can absorb near infrared light and convert the near infrared light into heat energy, so that the phase transition of the thermosensitive liquid crystal polymer from a liquid crystal state to an amorphous state is induced. However, it is still difficult to control and improve the orientation of liquid crystal polymer because the carbon nanotubes are randomly distributed, and because of indirect light driving, the sensitivity and stability of the material are low, and other mechanical and electrical properties need to be further improved.
Disclosure of Invention
The invention aims to provide a reversible photoinduced deformation liquid crystal polymer and carbon nano tube composite film with good mechanical property and electrical property and a preparation method thereof.
According to the preparation method of the reversible photo-deformable liquid crystal polymer and carbon nanotube composite film, the liquid crystal polymer containing photosensitive azobenzene groups is used as a research model, and is compounded with the highly oriented carbon nanotube film, so that the oriented structure of the carbon nanotubes effectively induces the liquid crystal elements to be oriented along the axial direction of the carbon nanotubes, and other oriented layers are not needed, and the reversible photo-deformable liquid crystal polymer/carbon nanotube composite material is prepared.
The invention directly spins the carbon nanotube film from the carbon nanotube array synthesized by the chemical vapor deposition method, and compounds the carbon nanotube film with the liquid crystal polymer containing the photosensitive azo element to prepare the composite film; then, the flat composite film is irradiated by ultraviolet light, the composite film bends towards the light, the same surface of the bent composite film is irradiated by visible light, the composite film is restored to the flat state, and the reversible photoinduced deformation can be repeated for hundreds of times without obvious attenuation.
The invention relates to a preparation method of a photoinduced deformation liquid crystal polymer/carbon nano tube composite film, which comprises the following specific steps:
(1) synthesizing a carbon nanotube array by a chemical vapor deposition method;
(2) spinning by a dry method to obtain a carbon nanotube film, and adhering and fixing the carbon nanotube film on a glass substrate;
(3) fixing two glass substrates coated with carbon nanotube films in parallel to prepare a liquid crystal box;
(4) mixing a liquid crystal monomer, a cross-linking agent and a photoinitiator according to a certain mass ratio, heating the mixture to a certain temperature to melt the mixture, and injecting the molten mass into the liquid crystal box;
(5) cooling the melt to a certain critical point, wherein the liquid crystal is in a nematic phase; maintaining the temperature, and carrying out photopolymerization; after polymerization, the cell was opened and the resulting composite film was directly peeled off from the glass substrate.
The composite film is alternately irradiated on the same surface by ultraviolet light and visible light, so that the phenomena of bending towards light and flattening recovery alternately occur, and the reversible photoinduced deformation can be repeated for hundreds of times without obvious attenuation.
In the step (2), the number of layers of the carbon nanotube film fixed on the glass substrate can be more than 1, the carbon nanotubes in the film are highly oriented along the same direction, and the carbon nanotube gap is more than 50 nanometers.
In the step (3), when the liquid crystal box is manufactured, two glass sheets coated with the carbon nanotube film are fixed along the parallel direction of the carbon nanotube; the thickness of the liquid crystal cell is 1-1000 μm, which corresponds to the thickness of the desired composite film.
In the step (4), the liquid crystal monomer, the cross-linking agent and the photoinitiator are mixed according to a certain proportion; wherein, the molar ratio of the liquid crystal monomer to the cross-linking agent is 1:99-99:1, and the photoinitiator is 0.01-10% of the total molar percentage of the liquid crystal monomer and the cross-linking agent; the mixture is heated to a temperature (typically 80-200 degrees celsius to bring it to a molten state).
In step (4) of the present invention, the crosslinking agent is a compound having a polymerizable double bond structure at both ends, such as an acrylate crosslinking agent, and the photoinitiator may be a visible photoinitiator, such as diphenyltitanocene fluoride or bis (pentafluorophenyl) titanocene.
In the step (5), the temperature of the certain critical point is 50-77 ℃ and is in a liquid crystal state; carrying out photopolymerization, wherein the wavelength of light is 400-700 nm, the power is 1-30 mW/cm, and the light irradiation polymerization time is 1-24 h.
In the composite film prepared by the method, the liquid crystal polymer can be a cross-linked liquid crystal elastomer containing photosensitive azobenzene liquid crystal elements, and the carbon nano tubes are highly ordered oriented carbon nano tube films; the carbon nano tube alignment structure induces the liquid crystal element to be aligned along the axial direction of the carbon nano tube; the composite film is bent to light under ultraviolet illumination and is restored to the original state under visible illumination. The reversible deformation that occurs under alternating irradiation of ultraviolet and visible light can be repeated hundreds of times without significant attenuation and fatigue.
The invention has the advantages that:
the carbon nanotube film is highly oriented, and the oriented structure can effectively induce the orientation of liquid crystal molecules and realize the photoinduced deformation of the liquid crystal material containing the photosensitive elements. Other orientation layers are not required to be introduced, so that the problems of structural defects, electrostatic aggregation and the like caused by the traditional mechanical friction method are avoided. Meanwhile, the mechanical property of the composite material is further improved by introducing the carbon nano tubes, and the composite material is endowed with higher conductivity, while the pure liquid crystal high polymer material is non-conductive. The method is simple to prepare, and can be popularized and applied to prepare various high-performance liquid crystal polymer composite materials.
Table 1 shows the mechanical properties of the composite material of the present invention, measured in the parallel (/) and perpendicular (#) directions, respectively, to the carbon nanotubes. The mechanical properties of the liquid crystal film aligned using the conventional mechanical rubbing method were compared in the parallel (/) and perpendicular (×) rubbing alignment directions and the randomly aligned liquid crystal film.
Drawings
FIG. 1 shows the chemical structures of a liquid crystal monomer and a crosslinking agent. Wherein, K: crystalline state, S: smectic state, N: nematic, I: in an amorphous state.
FIG. 2 is a schematic diagram of a process for preparing an oriented liquid crystal polymer/carbon nanotube composite film. The process is as follows: 1) synthesizing a carbon nanotube array by a chemical vapor deposition method, 2) pulling out a carbon nanotube film from the array and fixing the carbon nanotube film on a glass substrate, 3) preparing a liquid crystal box, and 4) melting and injecting a liquid crystal monomer mixture into the liquid crystal box.
FIG. 3 is a scanning electron micrograph of carbon nanotubes. Wherein, a) the side surface of the carbon nano tube array (the upper right corner is a high-resolution picture), b) the carbon nano tube film.
FIG. 4 shows the liquid crystal molecule orientation in the liquid crystal polymer/carbon nanotube composite film. Wherein, a) a polarization microscope image, b) a polarization absorption spectrum. White arrows indicate the optical axis directions of the two polarizers, and dotted lines indicate the carbon nanotube alignment direction. A/and A ≠ respectively represent that the polarized light is parallel to and perpendicular to the orientation direction of the carbon nanotube.
FIG. 5 is a photograph showing the bending and recovery process of the liquid crystal polymer/carbon nanotube composite film under the alternate irradiation of ultraviolet light and visible light.
Detailed Description
The liquid crystal monomers and crosslinkers used are shown in FIG. 1, A11AB6, A9Bz9, and C9A, synthesized and purified according to literature reported methods. The highly oriented carbon nanotube film is prepared by dry spinning. Firstly, the carbon nano tube array is synthesized by a chemical vapor deposition method, and a uniform carbon nano tube film is directly pulled out from the array and fixed on a glass substrate. Two glass substrates coated with carbon nanotube films were fabricated into a liquid crystal cell as shown in fig. 2. Mixing the monomer, the cross-linking agent and the photoinitiator according to a certain proportion, heating the mixture to a molten state, and injecting the mixture into a liquid crystal box. The temperature was reduced to 77 c at a rate of 0.1 c/s, at which time the liquid crystal was in the nematic phase. The temperature is then maintained and light is filtered through a high pressure mercury lamp to obtain monochromatic light of a given wavelength for polymerization, typically 2 hours. And finally opening the liquid crystal box, and peeling the composite film from the glass substrate.
FIG. 3a is a scanning electron micrograph of a carbon nanotube array, which shows that the carbon nanotubes are highly oriented perpendicular to the glass substrate, and a high resolution transmission electron micrograph indicates that the carbon nanotubes have a multi-walled structure and a diameter of about 10 nm. Fig. 3b is a thin film of carbon nanotubes fixed on a glass substrate, and it can be seen that the carbon nanotubes are also highly oriented in the nanometer scale.
The orientation of the mesogen in the LC polymer/CNT composite film can be determined by measuring the transmittance of the film when it is placed between two perpendicular polarizers. Fig. 4a is a photograph of a polarizing microscope, which shows the maximum transmittance when the alignment direction of the carbon nanotubes is at an angle of 45 degrees to the polarizing plate, respectively, and shows the minimum transmittance when the alignment direction of the carbon nanotubes is parallel or perpendicular to the polarizing plate. Therefore, by rotating the composite film at intervals of 45 degrees, periodic bright and dark images can be observed. This indicates that the mesogen is aligned along the carbon nanotube alignment direction.
Further, the spectral absorption dichroism of the composite film can be detected by polarized light, the directions of which are respectively parallel and perpendicular to the carbon nanotubes. As shown in fig. 4b, it is apparent that the absorption strength in the direction parallel to the carbon nanotubes is greater than that in the perpendicular direction. This result further demonstrates that the azobenzene mesogen is aligned along the alignment direction of the carbon nanotubes. The results of a polarizing microscope and a polarization absorption spectrum show that the highly oriented carbon nanotube structure can induce the orientation of liquid crystal elements in liquid crystal macromolecules.
Similar to the liquid crystal polymer film containing azobenzene prepared by the traditional mechanical friction method, the liquid crystal polymer/carbon nano tube composite film has quick and reversible photoinduced deformation. As shown in fig. 5, under the irradiation of ultraviolet light, the composite film is bent toward light and is along the alignment direction of the mesogen, that is, the alignment direction of the carbon nanotubes. It is known that, when irradiated with ultraviolet light, the azobenzene liquid crystal cell undergoes trans-to-cis photoisomerization, the azobenzene cell becomes shorter, and the irradiated surface shrinks in the same direction, resulting in bending of the composite film along the direction of the carbon nanotubes. When the film is irradiated by visible light, the azobenzene mesogen returns from cis to trans, and the bent film returns to the original flat state. The above-described light-induced reversible deformation can be repeated hundreds of times without significant attenuation.
The liquid crystal polymer/carbon nano tube composite film shows different mechanical properties in different directions. The stress-strain curves of the composite films in the directions parallel and perpendicular to the carbon nanotubes were measured, respectively. For comparison, the stress-strain curves of the liquid crystal polymer film prepared by the conventional mechanical rubbing method and the randomly oriented liquid crystal polymer film were measured at the same time. As shown in table 1, the mechanical strength of the composite film along the carbon nanotube orientation direction reached 31.2 mpa, while the strength in the vertical direction was 6.4 mpa, and it is clear that the composite film has better mechanical properties in the carbon nanotube orientation direction. In addition, the strength in the alignment direction of the composite film is higher than that of a purely aligned or non-aligned liquid crystal polymer film. These results indicate that highly aligned carbon nanotubes not only induce effective alignment of liquid crystal molecules, but also enhance the mechanical properties of the material.
The introduction of the oriented carbon nanotubes also endows the composite material with higher electrical conductivity. The room-temperature conductivity of the composite film along the direction of the carbon nano-tubes reaches 270S/cm. The good conductivity can effectively avoid the electrostatic aggregation easily occurring in other liquid crystal polymer materials, and greatly improve the responsiveness and stability of deformation, for example, reversible photoinduced deformation can be repeated for hundreds of times without attenuation. The conductivity of the composite film can be further improved by optimizing experimental parameters. The excellent electrical property enables the composite film to be applied to the fields of deformable photoelectricity, electronic devices and the like.
The invention reports a new method for preparing a photoinduced deformation liquid crystal polymer composite film, and the rapid reversible deformation can be realized by the alternate irradiation of ultraviolet light and visible light. Compared with other liquid crystal polymer materials prepared by using a mechanical friction method, the liquid crystal polymer/carbon nanotube composite film has higher mechanical strength and excellent conductivity. Due to these excellent properties, the composite film can be widely used as a high-performance driving material in the fields of drivers, micro robots and the like.
Specific example 1
The highly oriented carbon nanotube film is prepared by dry spinning and fixed on a glass substrate. The two glass substrates are respectively covered with 2 layers of carbon nanotube films, the glass sheets are fixed in parallel, the interval is 30 microns, and a liquid crystal box is manufactured. Two monomers a11AB6, A9Bz9 and a crosslinker C9A were mixed in a molar ratio of 1: 1: 3 and 1 percent of photoinitiator are mixed evenly, heated to 90 ℃, melted and injected into a liquid crystal box, and then the liquid crystal material is in an amorphous state. The temperature was reduced to 77 c at a rate of 0.1 c/s and the liquid crystal was in the nematic phase. Then, the temperature was maintained at 77 ℃, monochromatic light with a wavelength of 547 nm was obtained by filtering a high-pressure mercury lamp light source, and polymerization was carried out by irradiation for 2 hours. Then, the temperature is reduced to room temperature, the liquid crystal box is opened, and the composite film is peeled off from the glass substrate. The taken-down composite film was in a flat state, and the composite film was irradiated with ultraviolet light having a wavelength of 365 nm and an intensity of 100 mw/cm for 45 seconds, and bent toward the ultraviolet light along the carbon nanotube alignment direction. And then irradiating the same surface and the same area of the composite film by using visible light with the wavelength of 560 nanometers and the intensity of 35 milliwatts per square centimeter, and recovering the bent composite film to the original state, so that the composite film is irradiated by alternately using ultraviolet light and visible light, and the reversible photoinduced deformation can be repeated for hundreds of times.
Specific example 2
The highly oriented carbon nanotube film is prepared by dry spinning and fixed on a glass substrate. Two glass sheets coated with 1 layer of carbon nanotube film are fixed in parallel at an interval of 20 μm to form a liquid crystal cell. Two monomers a11AB6, A9Bz9 and a crosslinker C9A were mixed in a molar ratio of 1: 1: 3 and 1 mol percent of photoinitiator are mixed evenly and heated to 89 ℃, and the mixture is melted and injected into a liquid crystal box, and then the liquid crystal material is in an amorphous state. The temperature was lowered to 76 degrees celsius at a rate of 0.1 degrees celsius/second and the liquid crystal was in the nematic phase. Then, the temperature was maintained at 76 ℃, monochromatic light with a wavelength of 547 nm was obtained by filtering a high-pressure mercury lamp light source, and polymerization was carried out by irradiation for 2 hours. Then, the temperature is reduced to room temperature, the liquid crystal box is opened, and the composite film is peeled off from the glass substrate. The taken-down composite film was in a flat state, and the composite film was irradiated with ultraviolet light having a wavelength of 365 nm and an intensity of 100 mw/cm for 50 seconds, and bent toward the ultraviolet light along the carbon nanotube alignment direction. And then irradiating the same surface and the same area of the composite film by using visible light with the wavelength of 560 nanometers and the intensity of 35 milliwatts per square centimeter, and recovering the bent composite film to the original state, so that the composite film is irradiated by alternately using ultraviolet light and visible light, and the reversible photoinduced deformation can be repeated for hundreds of times.
In conclusion, the novel liquid crystal polymer/carbon nanotube composite film is prepared, and the high-orientation structure of the carbon nanotube effectively induces the liquid crystal molecules to be oriented along the direction of the carbon nanotube, so that the composite film has good reversible photoinduced deformation performance and excellent mechanical and electrical properties. The composite films can be widely used in the field of driving functional materials. The invention also provides a universal method for preparing the high-performance liquid crystal polymer material.
TABLE 1
| Tensile Strength (MPa) | Elongation at Break (%) | |
| Composite film (/) | 31.2 | 2.7 |
| Composite film (T) | 6.4 | 5.6 |
| Pure orientation liquid crystal film (/) | 15.7 | 4.0 |
| Pure oriented liquid crystal film (T) | 5.5 | 6.2 |
| Non-oriented liquid crystal film | 8.5 | 3.6 |
Claims (5)
1. A preparation method of a reversible photoinduced deformation liquid crystal polymer and carbon nano tube composite film is characterized by comprising the following specific steps:
(1) synthesizing a carbon nanotube array by a chemical vapor deposition method;
(2) spinning by a dry method to obtain a carbon nanotube film, and adhering and fixing the carbon nanotube film on a glass substrate;
(3) fixing two glass substrates coated with carbon nanotube films in parallel to prepare a liquid crystal box;
(4) mixing a liquid crystal monomer, a cross-linking agent and a photoinitiator, wherein the molar ratio of the liquid crystal monomer to the cross-linking agent is 1:99-99:1, and the photoinitiator is 0.1-10% of the total molar ratio of the liquid crystal monomer to the cross-linking agent; heating the mixture to 80-200 ℃ to melt the mixture, and injecting the molten mass into the liquid crystal cell; the crosslinking agent is a compound with a polymerizable double bond structure at two ends, and the photoinitiator is a visible light initiator;
(5) cooling the melt to a certain critical point temperature, wherein the liquid crystal is in a nematic phase; maintaining the temperature, and carrying out photopolymerization; after polymerization is finished, opening a liquid crystal box, and directly stripping the obtained composite film from the glass substrate;
wherein,
the number of the carbon nanotube film layers fixed on the glass substrate is more than 1, the carbon nanotubes in the film are highly oriented along the same direction, and the carbon nanotube gap is more than 50 nanometers.
2. The method of claim 1, wherein: when the liquid crystal box is manufactured, two glass sheets coated with carbon nanotube films are fixed along the parallel direction of the carbon nanotubes; the thickness of the liquid crystal cell is 1-1000 μm, which corresponds to the thickness of the desired composite film.
3. The method of claim 1, wherein: in the step (4), the cross-linking agent is an acrylate cross-linking agent, and the photoinitiator is diphenyl titanocene fluoride or bis-pentafluorophenyl titanocene.
4. The method of claim 1, wherein: in the step (5), the temperature of the certain critical point is 50-77 ℃; the photopolymerization is carried out, the wavelength of light is 400-700 nm, the power is 1-30 milliwatts per square centimeter, and the light irradiation polymerization time is 1-24 hours.
5. A reversible photo-deformable liquid crystal polymer and carbon nanotube composite film prepared by the preparation method according to any one of claims 1 to 4, wherein the composite film is bent to light under ultraviolet irradiation and restored to its original shape under visible light irradiation.
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