CN112341652B

CN112341652B - Chiral azobenzene polymer film and preparation method and application thereof

Info

Publication number: CN112341652B
Application number: CN202011323992.3A
Authority: CN
Inventors: 张伟; 缪腾飞; 程笑笑; 张正彪; 朱健; 周年琛; 朱秀林
Original assignee: Suzhou University
Current assignee: Suzhou University
Priority date: 2020-11-23
Filing date: 2020-11-23
Publication date: 2022-06-07
Anticipated expiration: 2040-11-23
Also published as: CN112341652A

Abstract

The invention discloses a chiral azobenzene polymer film and a preparation method and application thereof; preparing a side chain type azobenzene polymer into a film; then, the chiral azobenzene polymer film is obtained through the induction of a chiral reagent. Firstly, synthesizing an azobenzene random copolymer with a hydroxyl group at the tail end of a side chain through a series of organic synthesis reactions and RAFT polymerization, and carrying out detailed investigation on the molecular weight and the liquid crystal performance of the polymer by using characterization means such as nuclear magnetism, GPC, DSC, POM, XRD and the like; and then, preparing a polymer film by a spin coating mode, and selecting chiral limonene steam to carry out chiral induction on the polymer film to obtain the optically active polymer film. Therefore, the chiral is transferred from the chiral solvent to the achiral substance, the chiral solvent induction method is simple to operate, and the complex synthesis steps of chiral polymers and the use of expensive chiral reagents are avoided.

Description

Chiral azobenzene polymer film and preparation method and application thereof

Technical Field

The invention belongs to the technical field of supramolecular chiral fixation, relates to supramolecular chiral induction and crosslinking fixation of an achiral side chain type azobenzene random copolymer, and particularly relates to a chiral azobenzene polymer crosslinked film and a preparation method and application thereof.

Background

In the prior art, a supermolecule chiral self-assembly is based on supermolecule weak acting forces such as hydrogen bonds, pi-pi accumulation, acid-base action, metal-coordination action, subject-guest action and the like, a constructed chiral assembly has the driving force of reversible noncovalent weak interaction no matter whether a constructed element is a chiral small molecule or a polymer, the noncovalent weak interaction force has poor stability due to weak energy (generally less than 10 KJ/mol), and a chiral supermolecule ordered structure is easy to respond to external stimulation (light, heat, pH, a solvent, metal ions and the like) and even dissociates irreversibly, so that the formed chiral supermolecule structure is damaged, and the application of chiral supermolecule materials is limited to a great extent.

In recent years, due to the good application prospects of chiral polymers in the fields of chiral identification, photoinduced polarized fluorescence, chiral catalysis and the like, the chiral polymers attract the general attention of researchers. However, most chiral polymers obtained by directly organic synthesis methods use expensive chiral reagents, and the variety of the synthesized chiral polymers is very limited, thereby greatly restricting the development of the chiral polymers. Therefore, if the supermolecule chirality can be constructed in an achiral polymer system through a certain induction mode, the use of expensive chiral reagents and a more complicated synthesis process can be avoided, the structural range of the chiral polymer can be expanded, and the supermolecule chirality construction method has very important significance.

Disclosure of Invention

Aiming at the situation, the invention firstly synthesizes azobenzene random copolymer with hydroxyl at the end of a side chain through a series of organic synthesis reactions and RAFT polymerization, and utilizes characterization means such as nuclear magnetism, GPC, DSC, POM, XRD and the like to carry out detailed investigation on the molecular weight and the liquid crystal performance of the polymer; then, a polymer film is prepared by a spin coating mode, and chiral limonene steam is selected to carry out chiral induction on the polymer film to obtain an optically active polymer film; furthermore, the obtained chiral film is placed in a steam environment of formaldehyde and hydrochloric acid for crosslinking reaction, so that the supramolecular chirality fixation is realized, and particularly, the solvent resistance of the crosslinked film is improved through molecular design. The specific technical scheme is as follows:

the preparation method of the chiral azobenzene polymer film comprises the following steps of preparing a side chain type azobenzene polymer into a film; then, the chiral azobenzene polymer film is obtained through the induction of a chiral reagent.

In the invention, the side chain type azobenzene polymer has the following chemical structural formula:

such as:

in the invention, a side chain type azobenzene polymer is prepared from a monomer, an RAFT reagent and an initiator, wherein the molar ratio of the monomer to the RAFT reagent to the initiator is 50-500: 3: 1, preferably 100: 3: 1; the monomer is Az and AzOH, preferably, the molar ratio of Az to AzOH is (0.1-10) to 1, preferably (0.5-4) to 1. In the side chain type azobenzene polymer, the ratio of x to y is 1 to (0.1-3), and n is 3-15, preferably 5-10.

Further, the initiator is selected from any one of azobisisobutyronitrile, azobisisoheptonitrile and dimethyl azobisisobutyrate, preferably Azobisisobutyronitrile (AIBN); the RAFT agent comprises isobutyronitrile α -dithionaphthoate.

In the present invention, the AzOH is prepared by reacting compound a with compound B, preferably in the presence of a base and a halide under reflux conditions.

The reaction process is shown as follows, wherein the characters below the structural formula represent the name of the structural formula in the invention:

according to the invention, Az is an achiral methoxy azobenzene monomer, AzOH is an achiral hydroxy-terminated azobenzene monomer, the two react to form a random copolymer, namely an achiral side chain azobenzene polymer, after the film is prepared, a chiral solvent is used for inducing an achiral substance to generate chirality, and the method is a flexible and effective method.

In the invention, a solution spin coating method is adopted to prepare the side chain type azobenzene polymer into a film.

In the invention, chiral azobenzene polymer film is obtained by adopting chiral solvent thermal induction.

The preparation method of the chiral azobenzene polymer crosslinked film specifically comprises the following steps:

1) synthesis of achiral methoxy azobenzene monomer Az

Adding raw materials of methoxyaniline and concentrated hydrochloric acid into a deionized water solution, dropwise adding a sodium nitrite aqueous solution under an ice salt bath, and stirring for half an hour to prepare a diazonium salt aqueous solution of p-methoxyaniline;

phenol was dissolved in deionized water, again under ice salt bath conditions, and sodium hydroxide (NaOH) and sodium bicarbonate (NaHCO) were added₃) Mechanically stirring the solid evenly, and then dropwise adding the diazonium salt solution of the p-anisidine obtained before; after the dropwise addition, reacting at normal temperature to obtain a khaki turbid liquid, and performing suction filtration, drying, recrystallization and vacuum drying on the obtained turbid liquid to finally obtain a brownish yellow compound 1;

adding the compound 1, potassium carbonate, potassium iodide and a solvent DMF (dimethyl formamide) into a 500 mL dry round-bottom flask, carrying out reflux reaction, then dropwise adding a halogen alcohol DMF solution, cooling to room temperature after reacting overnight, carrying out suction filtration to remove redundant solid, pouring filtrate into a large amount of ice water, precipitating a crude product, carrying out suction filtration to collect solid, recrystallizing for further purification, and drying in vacuum to obtain a pure product, namely a compound 2;

adding the compound 2 and a catalyst into dry tetrahydrofuran, stirring under argon, and dropwise adding methacryloyl chloride under the constant temperature condition of an ice salt bath. After the addition, the reaction was kept at room temperature overnight. After the reaction, the solid was removed by suction filtration, the filtrate was collected and the solvent was spin-dried, redissolved with dichloromethane, washed with saturated sodium bicarbonate water several times and then with saturated brine. The organic phase was dried over anhydrous sodium sulfate. The solvent is dried by spinning, and the yellow solid is obtained by column chromatography. Further recrystallization purification gave an azobenzene monomer (Az) as a terminal methoxy group as yellow crystals.

The above-mentioned halogen alcohol is selected from any one of 6-chlorohexanol, 12-bromo-1-dodecanol, 8-bromo-1-heptanol, 4-bromobutanol and 2-bromoethanol, and 6-chlorohexanol is preferred. The catalyst is selected from any one of sodium hydroxide, triethylamine, sodium bicarbonate and potassium carbonate, and triethylamine is preferred.

The above reaction is as follows:

2) synthesis of Azobenzene monomer AzOH with hydroxyl at the end

Adding raw materials of p-nitrophenol, potassium carbonate and DMF into a 500 mL round-bottom flask, refluxing to form potassium salt, adding a halogen alcohol DMF solution, reacting overnight, pouring the reaction solution into a large amount of water after the reaction is finished, precipitating a crude product, and further purifying by recrystallization to obtain yellow crystal powder as a compound 3;

adding the compound 3 into a three-neck flask, adding excessive anhydrous tin dichloride, heating and refluxing for reaction, directly adding into a large amount of ice water after the reaction is finished, and adjusting the pH value to 7-8. Then extracting with ethyl acetate, washing with water, drying, and purifying by spin-drying solvent column chromatography to obtain a compound A;

the raw materials of halogen alcohol and dry THF are added into a 500 mL round-bottom flask, triethylamine is added at the same time to be used as a catalyst, stirring is carried out under argon, and methacryloyl chloride is dropwise added under the constant temperature condition of an ice salt bath. After the addition, the reaction was kept at room temperature overnight. After the reaction, the solid was removed by suction filtration, the filtrate was collected and the solvent was spin-dried, redissolved with dichloromethane, washed with saturated sodium bicarbonate water several times and then with saturated brine. The organic phase was dried over anhydrous sodium sulfate. Spin-drying the solvent, and performing column chromatography to obtain a colorless transparent liquid as a compound B;

and finally, adding the compound A into a flask, adding potassium carbonate, potassium iodide and a solvent DMF, carrying out reflux reaction, dissolving the compound B in a DMF solution, dropwise adding the solution into the solution, reacting overnight, cooling to room temperature, carrying out suction filtration to remove redundant solid, pouring the filtrate into a large amount of ice water, precipitating a crude product, carrying out suction filtration to collect the solid, recrystallizing for further purification, and carrying out vacuum drying to obtain a pure product, namely the azobenzene monomer (AzOH) with the terminal of hydroxyl.

The halogen alcohol is selected from any one of 3-bromopropanol, 4-bromobutanol and 6-chlorohexanol, and 6-chlorohexanol is preferred.

The above reaction is as follows:

3) synthesis of achiral side-chain azobenzene homopolymer and random copolymer

Adding the monomer AzOH obtained in the step 1), the monomer AzOH obtained in the step 2), a RAFT reagent alpha-isobutyronitrile dithionaphthoate (CPDN), an initiator Azobisisobutyronitrile (AIBN) and a solvent into a reaction vessel, deoxidizing with inert gas, and polymerizing under a heating condition for 1-8 hours. The reaction was stopped to obtain an achiral side chain type azobenzene polymer (PAz)_x-r-AzOH_y). Wherein the total molar amount of the two monomers and the molar ratio of the RAFT reagent to the initiator are 50-500: 3: 1, and preferably 100: 3: 1.

The solvent is selected from Tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), N-Dimethylformamide (DMF), and anisole, preferably anisole. The inert gas is selected from any one of argon, nitrogen, helium and neon, and preferably argon.

4) Preparation of achiral azobenzene polymer film and induction of chiral steam

Weighing a polymer, and dissolving the polymer in an organic solvent to prepare a polymer solution. And (3) taking a clean thin quartz plate, placing the quartz plate in a rotary film coating machine for sucking and fixing the quartz plate, sucking a small amount of polymer solution by using a liquid-transferring gun, slowly dripping the polymer solution on the surface of the quartz plate, and starting a spin coating machine to start film coating. And after the spin coating is finished, the film is placed in a vacuum oven for vacuum heating and annealing treatment. And taking out the test piece after the test is finished, and placing the test piece in a dark place to wait for testing.

And (3) suspending the annealed polymer film in a closed cuvette, adding a chiral solvent, placing the mixture at the bottom of the cuvette, heating and cooling to obtain the chiral azobenzene polymer film, taking out and drying the residual solvent, and placing the film in a dark place for testing.

The organic solvent for dissolving the polymer is selected from Tetrahydrofuran (THF), N-Dimethylformamide (DMF), and chloroform (CHCl)₃) One of them, Tetrahydrofuran (THF) is preferred. The coating condition is that after low-speed glue homogenizing, high-speed glue homogenizing is carried out, for example, the low-speed glue homogenizing is carried out for 6 s, the rotating speed is 0.5 kr/min, the high-speed glue homogenizing is carried out for 20s, and the rotating speed is 1.9 kr/min. The chiral solvent is selected from one of chiral limonene, chiral carvone, chiral sec-butyl alcohol and chiral sec-octyl alcohol, and preferably chiral limonene.

Further, the chiral azobenzene polymer film obtained by the invention is subjected to supermolecular chiral film crosslinking

And (3) pouring a formaldehyde solution and a hydrochloric acid solution into a beaker, suspending the chirality azobenzene polymer film after chirality induction to be above the liquid level, and sealing the whole system and placing the whole system in a dark place for reaction. And (3) taking out the film after the reaction is finished, washing with water to remove residual hydrochloric acid and formaldehyde, drying to obtain the chiral azobenzene polymer crosslinked film, and placing the chiral azobenzene polymer crosslinked film in a dark place for testing.

After completion of each of the above-mentioned processes, purification steps including, but not limited to, chromatography, dissolution/precipitation separation, filtration, etc. may be carried out in order to obtain a product of higher purity.

The invention introduces active groups which can be crosslinked or polymerized on small molecules or polymer structures, and crosslinks and fixes supermolecule chiral structures under specific conditions, so that the covalent bond is more important to fix the assembly, and the problem that the existing assembly formed by non-covalent bond cannot be stabilized is solved.

In the prior art, a method for fixing an original structure of an assembly by utilizing a covalent bond is similar to a coumarin method, a styrene crosslinking method, a cinnamic acid method, a diacetylene crosslinking method and the like, however, ultraviolet illumination is usually used as an initiating means of a crosslinking reaction in the method, or a larger crosslinking reaction group needs to be introduced into the structure, so that on one hand, the difficulty of supramolecular chiral construction is increased, and in an azobenzene polymer system, 365nm ultraviolet illumination easily causes cis-trans isomerization to destroy the assembly structure. The invention discloses an azobenzene polymer structure with a hydroxyl at the tail end of a side chain and performs chiral induction for the first time, a cross-linked microstructure fixing technology is applied to the field of chiral supermolecules for the first time, cross-linking is utilized, conditions are mild, the use of a group which is relatively rigid and large in volume is avoided, the successful induction of supermolecule chirality is improved, and supermolecule chiral fixation is realized in a polymer system; after the crosslinking reaction, the one-dimensional characteristic of the polymer main chain can be kept, and the polymer chain can also keep the orientation when being irradiated by UV light or heated at high temperature, so that the preparation method has the advantages of simplicity, high efficiency and the like.

Drawings

FIG. 1 is a nuclear magnetic diagram of an achiral monomer Az;

FIG. 2 is a nuclear magnetic map of achiral monomer AzOH;

FIG. 3 is a nuclear magnetic map and GPC outflow curve for different polymers;

FIG. 4 is a DSC curve for different polymers;

FIG. 5 is a polarization microscope (POM) photograph of different polymers;

FIG. 6 is a plot of small angle X-ray scattering versus wide angle X-ray diffraction for a polymer;

FIG. 7 is a CD spectrum, UV spectrum, POM and XRD pattern of the polymer film before and after induction;

FIG. 8 shows the chiral expression of copolymer films with different hydroxyl contents;

FIG. 9 is a film of a copolymer with different hydroxyl contentg _CDA spectrogram;

FIG. 10 is copolymer PAz₁-r-AzOH_0.62Crosslinking process of the chiral film;

FIG. 11 is copolymer PAz₁-r-AzOH_0.62Chiral thinSolvent resistance examination before and after crosslinking of the membrane;

FIG. 12 is a drawing of copolymer PAz₁-r-AzOH_0.62Investigating heat resistance before and after crosslinking of the chiral film;

FIG. 13 is copolymer PAz₁-r-AzOH_0.62Investigating the light resistance before and after crosslinking of the chiral film;

FIG. 14 is a CD review of UV illumination and heating-cooling cycling of a crosslinked chiral film;

FIG. 15 is a schematic view of a manufacturing process according to an embodiment of the present invention.

Detailed Description

The raw materials involved in the invention are all the existing products, and except for special instructions, the preparation is carried out under the conventional conditions; the specific operation method and the test method are the prior art.

The invention will be further described with reference to specific embodiments and drawings.

Chemical reagent:

p-anisidine, 95%, aladin;

phenol, a.r., Aladdin;

6-chlorohexanol, 95%, Acros;

methacryloyl chloride, 95%, aladin;

95% of p-nitrophenol and Aladdin;

97% of 2-bromoethanol and Acros;

tin dichloride, 98%, Energy Chemical;

azobisisobutyronitrile (AIBN), chemically pure, Shanghai reagent IV plant, recrystallized twice before use;

isobutyronitrile ester of alpha-dithionaphthoic acid (CPDN), 99%, carbofuran;

anisole, 99.5%, Shanghai chemical Agents Corp;

(R) - (+) -limonene [ alpha ]] ²⁴ ₅₈₉ =99.62°，TCI；

(S) - (-) -limonene [ alpha ]] ²⁴ ₅₈₉=97.72°，TCI；

Formaldehyde solution, AR, permu chemical limited;

hydrochloric acid, analytically pure, Jiangsu Qiangsheng functional chemistry GmbH;

sodium nitrite, analytically pure, Jiangsu Qiangsheng functional chemistry GmbH;

potassium iodide, analytically pure, Jiangsu Qiangsheng functional chemistry GmbH;

triethylamine, analytically pure, Jiangsu Qiangsheng functional chemistry GmbH;

anhydrous sodium sulfate, 98%, national drug group chemical reagents ltd;

potassium carbonate; analytically pure, Jiangsu Qiangsheng functional chemistry corporation;

sodium hydroxide; analytically pure, Jiangsu Qiangsheng functional chemistry GmbH;

sodium bicarbonate; analytically pure, Jiangsu Qiangsheng functional chemistry GmbH;

ethyl acetate, 99.5%, Jiangsu Qiangsheng functional chemistry GmbH;

petroleum ether, analytically pure, Jiangsu Qiangsheng functional chemistry GmbH;

tetrahydrofuran, 99.5%, Nanjing chemical reagents, Inc.;

ammonium chloride, analytically pure, Jiangsu Qiangsheng functional chemistry GmbH;

thin layer chromatography silica gel, CP, Qingdao ocean chemical ltd.

Testing instruments and conditions:

nuclear magnetic resonance hydrogen spectrum (¹H-NMR): using a Bruker 300MHz NMR spectrometer with CDCl₃And DMSO-d ₆As solvent, TMS as internal standard, copolymer nuclear magnetization was measured at high temperature, the rest at room temperature.

Gel Permeation Chromatography (GPC): molecular weight and molecular weight distribution A gel permeation chromatograph with TOSOH TSKgel SuperHM-M was used, which is an automatic advancement type, polymethyl methacrylate (PMMA) was used as a standard to calculate the molecular weight of the polymer, N, N-Dimethylformamide (DMF) was used as a mobile phase at a flow rate of 0.65 mL/min and a temperature of 40 deg.C^oC。

Uv-visible absorption spectrum: an ultraviolet absorption signal is measured by using an ultraviolet-2600 ultraviolet-visible absorption spectrometer produced by Nippon Jinshima, and the scanning range is 250-600 nm.

Round dichroism (CD): measured on a J-815 circular dichrograph (JASCO, (Hachioji, Tokyo, Japan)) with a Peltier temperature control assembly to control the measured temperature. The scanning speed is 200 nm/min, the scanning range is 250-600 nm, and the bandwidth is 2 nm.

Differential Scanning Calorimeter (DSC): the first temperature rise and fall rate is 20 by using TA DSC 250^oC/min, the second temperature rise and fall rate is 10^oC/min。

Polarizing microscope (POM): the test was performed using a CNOPTEC BK-POL polarizing microscope, equipped with a (Linkam THMS 600) hot stage.

Small angle X-ray scattering (SAXS): an Anton Paar SAXSess MC2 diffractometer, Cu Ka radiation source, wavelength 0.154 nm was used.

Wide angle X-ray diffraction (WAXD): an X-ray diffractometer from bruker D8 ADVANCE, Cu ka radiation source, wavelength 0.154 nm was used.

X-ray diffraction photoelectron spectroscopy (XPS): the change in binding energy of the polymer film before and after crosslinking was examined by photoelectron spectroscopy at Thermo Fisher Scientific ESCALAB 250 XI.

Synthesis example

1) Synthesis of achiral methoxy azobenzene monomer

Adding raw material methoxyaniline (12.32 g, 0.1 mol) and concentrated hydrochloric acid into deionized water solution (80 mL), magnetically stirring for 0.5 h under ice salt bath, and dropwise adding 30 mL NaNO₂(7 g, 0.1 mol/L) aqueous solution, and after the dropwise addition, the reaction is continued for 0.5 h under an ice salt bath to prepare a diazonium salt aqueous solution, wherein the system is red.

Phenol (16 g, 0.15 mol), NaOH (8 g, 0.2 mol), NaHCO₃(8.4 g, 0.1 mol) is dissolved in 50 mL deionized water, mechanically stirred under the ice salt bath condition (the temperature is controlled to be 0-5 ℃), and then the obtained diazonium salt solution of the p-anisidine is dripped into the phenol solution, wherein the ice salt bath condition is still required to be maintainedThe solution gradually changed from colorless to yellow and finally to brown-yellow. After the dropwise addition, the reaction is carried out for 4 hours at room temperature to obtain a khaki suspension, the obtained suspension is subjected to suction filtration, washed by water to remove salt impurities, dried, recrystallized by ethanol and dried in vacuum to finally obtain the brown yellow compound 1 (14.78 g, 64.84%).

A500 mL dry round bottom flask was charged with Compound 1 (12.0 g, 52.6 mmol), potassium carbonate (29.0 g, 0.21 mol) and DMF solvent (150 mL) at 80%^oAnd C, refluxing, magnetically stirring for 30 min, and adding 20 mg of KI as a catalyst after the solid is completely dissolved. Then hexachlorohexanol DMF solution (10.5 mL in volume, diluted with 10 mL of DMF) was added dropwise to the above solution, reacted for 18 hours and cooled to room temperature, filtered to remove excess solids, the filtrate was poured into a large volume of ice water to precipitate the crude product, which was collected by suction filtration and further purified by ethanol recrystallization, and dried in vacuo to give the pure product compound 2 (11.15 g, 64.6%).

Compound 2 (5.0 g, 15.24 mmol), triethylamine (25 mL) were added to dry tetrahydrofuran (100 mL), cooled in a ice salt bath and stirred under argon atmosphere for 30 min. Methacryloyl chloride (2.5 g, 24.0 mmol) was diluted with anhydrous tetrahydrofuran and added dropwise to the above solution, maintaining the temperature in a ice salt bath. After the dropwise addition, the reaction is kept at room temperature, and a point plate tracks the reaction progress. After the reaction, the solid was removed by suction filtration, the filtrate was collected and the solvent was spin-dried, redissolved with dichloromethane, washed with saturated sodium bicarbonate water several times and then with saturated brine. The organic phase was dried over anhydrous sodium sulfate. The solvent is dried by spinning, and the yellow solid is obtained by column chromatography. Further purification by ethanol recrystallization gave azobenzene monomer (Az) (4.23 g, 70.2%) as terminal methoxy groups as yellow crystals.

2) Synthesis of hydroxy-terminated azobenzene monomer

The starting materials p-nitrophenol (6.95 g, 50.0 mmol), potassium carbonate 27.6 g, 200.0 mmol) and 150 mL DMF were added to a 500 mL round bottom flask, 80^oRefluxing for 6 hours under C to form potassium salt. 2-Bromoethanol (9.38 g, 75.0 mmol) dissolved in DMF (50 mL) was added to the above mixtureIn (1). After 20 hours of reaction, the reaction was poured into a large amount of water, and the crude product was precipitated and further purified by ethanol recrystallization to give compound 3 (5.58 g, 80.2%) as a yellow crystalline powder.

The above compound 3 (5.49 g, 30 mmol) was charged into a three-necked flask, and anhydrous ethanol (50 mL) and then an excess amount of anhydrous tin dichloride (67.5 g, 300 mmol) were added thereto, followed by heating under reflux to react for 3 hours. After the reaction was completed, the reaction mixture was directly added to a large amount of ice water, and the pH was adjusted to 7.5 with potassium carbonate. Then extracting with ethyl acetate, washing with water, drying, and purifying by spin-drying solvent column chromatography. Compound 4 (4.32 g, 78.7%) was obtained.

A500 mL round bottom flask was charged with hexachlorohexanol (5.0 g, 36.6 mmol) as starting material and dry THF (100 mL) along with triethylamine (12 mL, 86.3 mmol) as catalyst, stirred under argon for 30 min, and methacryloyl chloride (7.6 g, 73.1 mmol) was added dropwise at constant temperature to the ice salt bath. After the addition, the reaction was kept at room temperature for 20 hours. After the reaction, the solid was removed by suction filtration, the filtrate was collected and the solvent was spin-dried, redissolved with dichloromethane, washed with saturated sodium bicarbonate water for 3 times and then with saturated brine. The organic phase was dried over anhydrous sodium sulfate. The solvent was dried by spinning, and column chromatography gave compound 5 (6.02 g, 80.6%) as a colorless transparent liquid.

Finally, compound 4 (3.0 g, 11.6 mmol) was added to the flask, potassium carbonate (6.4 g, 46.5 mmol), 30 mg KI and solvent DMF (35 mL) were added at 80 deg.C^oRefluxing for 30 min under C. Then, a certain amount of compound 5 (3.21 g, 15.7 mmol) was dissolved in DMF solution and added dropwise to the above solution, after reacting for 15 hours, cooled to room temperature, filtered to remove excess solid, the filtrate was poured into a large amount of ice water to precipitate a crude product, filtered to collect solid, recrystallized from ethanol to further purify, and dried under vacuum to obtain pure product, i.e., hydroxyl-terminated azobenzene monomer (AzOH) (3.62 g, 73.3%).

FIG. 1 is a nuclear magnetic diagram of the achiral monomer Az, wherein the nuclear magnetic peak corresponds to the monomer, and no hetero-peak is present, indicating that the monomer is relatively pure. FIG. 2 is a nuclear magnetic diagram of the achiral monomer AzOH, wherein the nuclear magnetic peak corresponds to the monomer, and no hetero-peak is present, indicating that the monomer is relatively pure.

3) Synthesis of achiral side-chain azobenzene homopolymer and random copolymer

Adding the monomer AzOH obtained in the step 1), the monomer AzOH obtained in the step 2), alpha-isobutyronitrile dithionaphthoate (CPDN) (8.08 mg, 0.029 mmol), an initiator Azobisisobutyronitrile (AIBN) (1.64 mg, 0.010 mmol) and a solvent anisole (1.5 mL) into a five-mL ampoule, wherein the ratio of the total molar amount of the monomers to the RAFT reagent and the initiator is 100: 3: 1. and after the sample addition is finished, performing freezing-air extraction-inflation-unfreezing circulation for three times by using the double-row pipe to remove oxygen, sealing the bottle mouth after the completion, and heating and stirring the bottle mouth at the temperature of 70 ℃ for reaction for 3 hours. The polymerization takes place under heating for 3 h. The reaction was stopped, the reaction was diluted with 2 mL of THF, precipitated twice in methanol, and the solid was collected to give an achiral side-chain azobenzene polymer (PAz)_x-r-AzOH_y）。

When the monomer is only the monomer AzOH obtained in the step 2), the rest is unchanged, and the obtained polymer is PAzOH; when the monomer Az obtained in step 1) was the only monomer, the remainder was unchanged, and the polymer obtained was PAz.

According to different molar charge ratios of the monomer Az and the monomer AzOH, homopolymerized polymers and copolymerized polymers can be obtained.

FIG. 3 shows the nuclear magnetic diagram and GPC outflow curve of different polymers, and the ratio of two monomers in the copolymer is obtained by comparing the peak of hydroxyl in nuclear magnetic spectrum with the peak of hydrogen on benzene ring of azobenzene. FIG. 4 is a DSC curve of different polymers. FIG. 5 shows a photograph taken by a polarizing microscope (POM) of different polymers, wherein the polymer solid powder is heated to a temperature exceeding the transition temperature of the clearing point, and then slowly cooled to a temperature range of the liquid crystal phase, and the results are shown in the following order: PAz, PAz₁-r-AzOH_0.19，PAz₁-r-AzOH_0.25，PAz₁-r-AzOH_0.62，PAz₁-r-AzOH₂PAzOH. FIG. 6 shows the small-angle X-ray scattering and wide-angle X-ray diffraction patterns of the copolymer under the same conditions as those of POM. The nematic liquid crystal structure of the polymer is determined by figures 4, 5 and 6,the direct liquid crystal element spacing is about 0.44 nm calculated by the Bragg equation.

Table 1 shows the charge ratio, molecular weight and hydroxyl content of different polymers.

Examples

Preparing an achiral azobenzene polymer film and preparing the chiral azobenzene polymer film by chiral steam induction

The polymer was weighed and dissolved in THF to prepare a 12 mg/mL polymer solution (clear solution). Taking a clean thin quartz piece, placing the quartz piece in a rotary film coating machine for sucking and fixing the quartz piece, and adjusting the rotating speed and time of low-speed glue homogenizing and high-speed glue homogenizing. Sucking 1mL of polymer solution by using a liquid-transferring gun, dropwise adding the polymer solution to the surface of a quartz plate, starting a spin coater to start coating, and uniformly coating the quartz plate at a low speed for 6 s and a rotating speed of 0.5 kr/min and then at a high speed for 20s and a rotating speed of 1.9 kr/min; after the spin coating is finished, the film is placed in a vacuum oven to be heated for 12 hours at the temperature of 110 ℃ in vacuum, and annealing treatment is carried out. And taking out the test piece after the test is finished, and placing the test piece in a dark place to wait for testing.

Suspending the annealed polymer film in a closed cuvette, adding chiral limonene, placing the polymer film at the bottom of the cuvette (not contacting the film), heating to 93 ℃, keeping for 5 minutes, naturally cooling to obtain a chiral azobenzene polymer film, taking out and drying the residual solvent, and placing the chiral azobenzene polymer film in a dark place for testing.

Fig. 7 shows CD spectra, uv spectra, POM and XRD patterns before and after induction of the polymer film, and shows that the polymer film forms a Chiral Nematic phase (Chiral Nematic) by fumigation with high temperature Chiral vapor through the above tests. FIG. 8 shows the chiral expression of different copolymer films, and from the CD spectrum, the chiral signal intensity obtained by the copolymer gradually changes with different monomer ratios. FIG. 9 shows films of different copolymersg _CDThe spectrum of the light beam is shown,g _CDis calculated as [ ellipticity/32,980]Absorbance.

5) Cross-linking preparation of chiral azobenzene polymer cross-linked film by supermolecule chiral film

The formaldehyde solution and the hydrochloric acid solution are poured into a beaker. Suspending the film after chiral induction and keeping the liquid level above the air, and sealing the whole system in the shade for 24 hours; and (3) taking out the film after the reaction is finished, washing with water to remove residual hydrochloric acid and formaldehyde, drying to obtain the chiral azobenzene polymer crosslinked film, and placing the film in a dark place to wait for testing.

FIG. 10 is copolymer PAz₁-r-AzOH_0.62The cross-linking process of chiral thin film, FIG. 10 (a) is a schematic diagram of a cross-linking reaction apparatus; FIG. 10 (b) is the CD spectrum change before and after crosslinking, from which it can be seen that the supramolecular chirality of the polymer film remains after crosslinking; FIGS. 10 (c), (g) are narrow scan spectra in XPS of elemental oxygen and elemental carbon before and after crosslinking, wherein a shift in binding energy corresponding to hydroxyl oxygen and a disappearance of binding energy to carbon adjacent to hydroxyl indicates that the crosslinking reaction is almost completely proceeding; it can be seen from FIGS. 10 (d) - (f) that the chiral nematic phase structure remains after crosslinking, and the diffraction peak at the wide XRD angle decreases after crosslinking.

FIG. 11 is copolymer PAz₁-r-AzOH_0.62Solvent resistance of the chiral film before and after crosslinking is inspected, and a good solvent THF of the polymer before crosslinking is selected as an object to be inspected. It can be seen from the graphs (a) and (d) that the solubility of the membrane before and after crosslinking is substantially changed, the polymer membrane before crosslinking is very soluble in THF, and the solution does not show CD signal (fig. 11 (b)), and the polymer membrane after crosslinking is insoluble in THF, and the CD signal can be restored to the original intensity only by a rapid temperature increase and decrease treatment (20 s to room temperature after 10 ℃/min to 100 ℃) without chiral source, see fig. 11 (c) - (f). Homopolymer PAz chiral films remained readily soluble in THF after the same cross-linking treatment.

Taking PAz-rPutting the chiral cross-linked film of the-AzOH polymer into THF, taking out after 30 minutes, heating to 100 ℃ at the temperature of 10 ℃/min, cooling to room temperature for 20s, observing the swelling change of the film through a spectrum, wherein PAz shows that all the films are not dissolved₁-r-AzOH_0.62CD signal of chiral cross-linked film is swollen-quickly heated and cooledThe CD spectrum can be restored to the original signal intensity without changing the previous and the next, PAz₁-r-AzOH_0.25、PAz₁-r-AzOH₂The CD signal of (1) decreases after swelling-rapid warming and cooling, and PAz₁-r-AzOH₂CD signal reduction of PAz₁-r-AzOH_0.25Is obvious.

FIG. 12 is copolymer PAz₁-r-AzOH_0.62Examination of heat resistance before and after crosslinking of the chiral film. From fig. (a) - (d), it can be seen that the CD signal stability of the film before and after crosslinking shows obvious difference at 100 ℃, and the POM test of cooling after heating to clearing point can also see that the film texture and CD after crosslinking both have good recovery performance, as shown in fig. 12e, g.

FIG. 13 is copolymer PAz₁-r-AzOH_0.62And (3) observing the light resistance of the chiral film before and after crosslinking. It can be seen from the figure that the film before crosslinking can not recover after the CD signal is completely disappeared under 365nm illumination because of no chiral source, while the film after crosslinking can still induce disordered parts through the internal crosslinking ordered structure even without the chiral source, so that the disordered parts can form a chiral structure again, and the CD spectrum can recover to the original signal intensity.

FIG. 14 is a CD investigation of UV illumination (365 nm, 92 min) and heating-cooling cycling (20 s to room temperature after 10 ℃/min to 100 ℃ and 5 cycles) of a crosslinked chiral film. It can be seen from the figure that the chiral film after crosslinking has good chiral self-healing properties.

FIG. 15 is a schematic view of the preparation process of the present invention, which comprises first synthesizing an azobenzene random copolymer having a hydroxyl group at the end of a side chain by a series of organic synthesis reactions and RAFT polymerization, and examining the molecular weight and liquid crystal properties of the polymer in detail by characterization means such as nuclear magnetism, GPC, DSC, POM and XRD; then, a polymer film is prepared by a spin coating mode, and chiral limonene steam is selected to carry out chiral induction on the polymer film to obtain an optically active polymer film; the obtained chiral film is placed in the steam environment of formaldehyde and hydrochloric acid for cross-linking reaction, so that the fixation of supramolecular chirality is realized, and the defects of instability and easy dissociation of the traditional assembly are overcome.

Claims

1. The preparation method of the chiral azobenzene polymer film is characterized by comprising the following steps of preparing a side chain type azobenzene polymer into a film; then, obtaining a chiral azobenzene polymer film through the induction of a chiral reagent; the side chain type azobenzene polymer has the following chemical structural formula:

。

2. the chiral azobenzene polymer film according to claim 1, wherein the side chain type azobenzene polymer is prepared into a film by a solution spin coating method.

3. The chiral azobenzene polymer film according to claim 2, wherein the spin coating is a low-speed spin coating followed by a high-speed spin coating.

4. The chiral azobenzene polymer film as claimed in claim 1, wherein the chiral azobenzene polymer film is obtained by thermal induction with a chiral solvent.

5. The chiral azobenzene polymer film as claimed in claim 4, wherein the chiral solvent is selected from one of chiral limonene, chiral carvone, chiral sec-butanol, and chiral sec-octanol.

6. The use of the chiral azobenzene polymer film of claim 1 in the preparation of chiral materials.

7. Use of the chiral azobenzene polymer film of claim 1 in the preparation of chiral crosslinked films.

8. The use according to claim 7, wherein the chiral cross-linked film has a self-healing function.