Azobenzene block copolymer supramolecular assembly with multi-level chirality and adjustable liquid crystal properties and preparation method thereof

Abstract

The present invention discloses a supramolecular assembly of azobenzene block copolymers with multi-level chirality and adjustable liquid crystal properties and a preparation method thereof, and synthesizes chiral azobenzene monomers (AzoxMA*- R / S ), in a single ethanol solvent, the dispersion polymerization of solvophobic chiral azobenzene-containing monomers, AzoxMA*‑R / S, was initiated using the initiation sites of a macromolecular chain transfer agent. Ethanol is a good solvent for the macromolecular chain transfer agent but a poor solvent for the azobenzene segments. As the polymerization proceeds, the degree of polymerization of the solvophobic azobenzene segments gradually increases, and the amphiphilic block copolymer undergoes microphase separation in the ethanol solution, resulting in supramolecular chiral assemblies with varying morphologies and liquid crystal properties. Furthermore, the chiral expression of the assemblies differs for chiral azobenzenes with different alkyl chain lengths, providing an effective method for regulating the liquid crystal order and chiral expression of the assemblies.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer synthesis, and particularly relates to the synthesis and chirality control of chiral azobenzene molecules. Background Art

[0002] The rapid development of polymerization-induced self-assembly (PISA) has provided a more efficient route for synthesizing well-defined assembly structures. Inspired by the chirality transfer process in naturally occurring chiral nanostructures, researchers have constructed a variety of artificial nanostructures with supramolecular chirality through chiral self-assembly. In this regard, chiral helical superstructures can be constructed through non-covalent interactions between adjacent azobenzene units associated with an excitonic coupling mechanism. It has also been reported that azobenzene units can selectively helically stack due to cooperative and / or antagonistic coupling between directional order and asymmetric distortion. Although studies on chirality regulation and size control have been extensively reported in main-chain helical polymers and supramolecular polymers, understanding the chirality evolution at different levels in azobenzene block copolymers, such as the chirality transfer mechanism from stereocenter to supramolecular structure, the formation of liquid crystal phases, and the macroscopic assembly process, remains challenging. Summary of the Invention

[0003] In response to the above situation, the present invention designs a solvent-philic macromolecular chain transfer agent obtained by solution polymerization, whose side chain carries solvent-philic methacrylic acid (MAA) to improve the solvent-philicity of the macromolecular chain transfer agent. Next, the synthesized macromolecular chain transfer agent is used to successfully initiate the dispersion polymerization of chiral azobenzene monomers with flexible alkyl chain spacers of different lengths in ethanol through thermal initiation. During the polymerization process, self-assembly is carried out to construct a supramolecular chiral liquid crystal assembly containing azobenzene. At the same time, the supramolecular chirality of the azobenzene polymer assembly is regulated by controlling the repeating units of the polymer.

[0004] The present invention adopts the following technical solutions:

[0005] A supramolecular assembly of azobenzene block copolymers with multi-level chirality and adjustable liquid crystal properties is obtained by initiating a polymerization reaction of chiral azobenzene monomers with a solvent-philic macromolecular chain transfer agent; the azobenzene block copolymer is PMAAm- b -PAzoxMAn, m is 30-70, n is 3-80, preferably 5-60, and x is 3-20, preferably 4-16. Most preferably, m is 50-55, n is 35-45, and x is 10-12.

[0006] In the present invention, the polymerization reaction is carried out in the presence of an initiator in an alcohol solvent, and the alcohol solvent is any one of methanol, ethanol, propanol, and butanol, preferably ethanol.

[0007] In the present invention, the polymerization reaction is carried out at 60 to 80° C. for 12 to 18 hours, preferably at 70° C. for 15 hours.

[0008] In the present invention, the molar ratio of the chiral azobenzene monomer to the solvophilic macromolecular chain transfer agent is 3 to 80:1.

[0009] In the present invention, the chemical structural formula of the chiral azobenzene monomer is as follows:

[0010]

[0011]

[0012] The chemical structure of the solvent-philic macromolecular chain transfer agent is as follows:

[0013]

[0014] m is 30-70, preferably 40-60; a is 3-20, preferably 5-15.

[0015] In the present invention, compound 1 is prepared using p-nitrophenol and a chiral alcohol as raw materials; compound 1 is sequentially aminated and diazotized, and then reacted with phenol to obtain compound 3; compound 3 reacts with a halogen alcohol to obtain compound 4; and methacryloyl chloride and compound 4 are used as raw materials, and a reflux reaction is carried out under an inert gas to prepare a chiral azobenzene monomer.

[0016] In the present invention, a macromolecular chain transfer agent is prepared by taking a hydrophilic monomer and a small molecular chain transfer agent as raw materials.

[0017] The present invention discloses the use of the azobenzene block copolymer supramolecular assembly with multi-level chirality and adjustable liquid crystal properties in the preparation of high-ellipticity materials. Preferably, the high ellipticity exceeds 2000 mdeg.

[0018] The present invention discloses the use of the above-mentioned azobenzene block copolymer supramolecular assembly with multi-level chirality and adjustable liquid crystal properties in the preparation of ultraviolet-resistant chiral materials. Preferably, ultraviolet resistance means that the assembly structure of the chiral material does not change under ultraviolet irradiation.

[0019] Due to the implementation of the above technical solution, the present invention has the following advantages compared with the prior art:

[0020] (1) This invention is the first attempt to use the length of the flexible spacer group of azobenzene units to regulate the supramolecular chiral helical direction and liquid crystal order of azobenzene polymer assemblies.

[0021] (2) Azobenzene polymers with different liquid crystal phase structures are obtained by polymerization, and the stacking mode of their azobenzene building blocks is then regulated. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Figure 3 shows the self-assembly route of amphiphilic azo BCP supramolecular assembly as well as the adjustable chiral stacking mode and assembly morphology. It can be seen that different x has an impact on the assembly.

[0023] Figure 2 This is a synthetic route for chiral azobenzene monomers.

[0024] Figure 3 These are the NMR images of chiral monomers with flexible spacer groups of different alkyl chain lengths.

[0025] Figure 4 These are the NMR images of chiral monomers with flexible spacer groups of different alkyl chain lengths.

[0026] Figure 5 Figure 3 is the NMR spectrum and GPC elution curve of the macromolecular chain transfer agent (post-modified with benzyl group).

[0027] Figure 6 This is the GPC elution curve of the macromolecular chain transfer agent (post-modified with benzyl group).

[0028] Figure 7 TEM images and assembly schematics of different block copolymer assemblies.

[0029] Figure 8 TEM images and assembly schematics of different block copolymer assemblies.

[0030] Figure 9 The nanofibers are 2-8S-20 (m) and 2-8S-60 (n).

[0031] Figure 10 Schematic diagram of morphological transformation and superposition mode transformation.

[0032] Figure 11 AFM images of block copolymer assemblies containing flexible spacers of different lengths at different polymerization degrees, the average height and width distribution of different assemblies, and PMAA 51 - b Phase diagram of the morphology of the PAzoxMA* diblock copolymer assembly.

[0033] Figure 12 The circular dichroism and UV spectra of block copolymer assemblies containing flexible spacers of different lengths are shown. The supramolecular chirality of the assemblies can be determined by both the flexible spacers and the degree of polymerization of the azobenzene blocks.

[0034] Figure 13 Figure 3 shows the circular dichroism (CD) and UV spectra of block copolymer assemblies containing flexible spacers of different lengths. With increasing DP, the absolute maximum CD value first increases and then decreases, which is also affected by the corresponding morphological transformation.

[0035] Figure 14 CD and UV-visible spectra of azo BCP assemblies. Inset: Schematic diagram of different assembly modes.

[0036] Figure 15 The UV-light and thermal recovery spectra of block copolymer assemblies containing flexible spacers of different lengths, as well as images of the morphological transformation of 4-8R / 8S-DP before and after UV irradiation, show that the assembly 4-8R / 8S-DP exhibits a dynamic chiral response with a significant morphological change.

[0037] Figure 16 The circular dichroism spectra and UV spectra of block copolymer assemblies containing flexible spacers of different lengths under UV illumination are shown. As the UV illumination time increases, the chiral signal of the assembly gradually weakens and eventually disappears.

[0038] Figure 17 The recovery of the maximum CD value of block copolymer assemblies containing flexible spacers of varying lengths under UV irradiation and heating-cooling conditions. By alternating UV irradiation and heating-cooling treatments, the device can be switched on and off more than five times, successfully constructing a dynamic, reversible chiral optical switch.

[0039] Figure 18 TEM images of the morphological transformation of azo BCP assemblies of a) 4-8S-5, b) 4-8S-10, c) 4-8S-20, d) 4-8S-30, e) 4-8S-40, and f) 4-8S-60 before and after UV irradiation.

[0040] Figure 19 The WAXD and SAXS spectra of block copolymer assemblies containing flexible spacers of varying lengths at different degrees of polymerization show that the assemblies contain a stacked azobenzene bilayer structure with a twisted grain boundary A* phase.

[0041] Figure 20 The DSC curves of block copolymer assemblies containing flexible spacers of varying lengths are shown below. The DSC results show that the assemblies with the shortest flexible spacer (x = 3) exhibit weaker liquid crystallinity, while those with longer flexible spacers (x = 4, 5, 11, 12, and 16) exhibit stronger liquid crystallinity.

[0042] Figure 21 POM images of homopolymers containing flexible spacers of varying lengths at a degree of polymerization of 60 are shown. The images reveal weakly birefringent aggregates with a filamentous structure. This suggests that the chirality of the azo assemblies arises from the twisted grain boundary A* phase formed by the side-chain azobenzene units. DETAILED DESCRIPTION

[0043] To achieve the above objectives, the present invention first synthesizes liquid crystal small molecule units with different segment lengths, then in situ prepares block copolymers with different polymerization degrees and azobenzene supramolecular chiral liquid crystal assemblies with different morphologies, and then regulates the repeating units of the azobenzene polymer assembly to achieve multiple flips in the chirality of the azobenzene polymer assembly by regulating the number of repeating units. Figure 1 Schematic diagram of the present invention.

[0044] As an example, the steps for synthesizing chiral azobenzene monomer are as follows:

[0045] The raw materials p-nitrophenol, chiral alcohol (such as R 2-octanol), diisopropyl azodicarboxylate, and ether were added to a three-necked flask, and triphenylphosphine was added to the flask. The mixture was then allowed to react at room temperature for 12 hours. After the reaction, the mixture was filtered, the solvent was dried, and then purified by column chromatography and dried to obtain compound 1.

[0046] Add the above compound 1 to a three-necked flask, then add tin dichloride and heat to react for 3 hours. After the reaction is completed, add directly to a large amount of ice water and adjust the pH to 7-8. Then extract with ethyl acetate, spin dry the solvent, purify by column chromatography, and dry. Compound 2 is obtained;

[0047] Under low temperature conditions, sodium nitrite is dissolved in 100 ml of water, hydrochloric acid is diluted with water, and the diluted hydrochloric acid is added to compound 2. After completing the above steps, the sodium nitrite aqueous solution is added dropwise to the hydrochloric acid solution while maintaining the temperature at low temperature, thereby obtaining a diazonium salt solution of compound 2;

[0048] At temperature 0 oC Under the following conditions, phenol is dissolved in 300-400 ml of water, and sodium hydroxide (NaOH) and sodium bicarbonate (NaHCO3) are added. The diazonium salt solution of compound 2 obtained previously is then added dropwise to the phenol solution, still maintaining low temperature. The solution gradually changes from colorless to yellow, and eventually to brownish yellow over time. After reacting for 4 hours under this condition, a khaki turbid liquid is obtained. The resulting turbid liquid is then filtered, extracted, dried, purified by column chromatography, and vacuum dried to finally obtain the yellow compound 3.

[0049] Add catalyst (such as potassium carbonate), compound 3, potassium iodide (KI) and halogen alcohol (such as 3-bromopropanol) into a 500 ml dry round-bottom flask and heat to 85 o C. Tetrahydrofuran was added to the round-bottom flask and stirred at high speed to completely dissolve the potassium carbonate. The milky white turbid liquid in the round-bottom flask turned into a brown turbid liquid. After vigorous stirring for 4 hours, the mixture was cooled to room temperature and extracted with ethyl acetate and water. The resulting oil phase was dried over anhydrous sodium sulfate, then purified by column chromatography using rotary evaporation and then dried to obtain compound 4.

[0050] Triethylamine, methacryloyl chloride, and compound 4 were added to a tetrahydrofuran solution and refluxed under argon. After 24 hours, a 10% aqueous solution of ammonium chloride (NH4Cl) was added. The mixture was then extracted with dichloromethane, and the combined organic extracts were washed with water and dried over anhydrous sodium sulfate. The crude product was then purified by flash chromatography and dried to yield the azobenzene monomer.

[0051] The synthesis process of the solvophilic macromolecular chain transfer agent is as follows:

[0052] Add the solvent-friendly monomer (such as methacrylic acid), a small molecule chain transfer agent (such as 4-cyano-4-(thiobenzoyl)valeric acid), a free radical initiator (such as 4,4'-azo(4-cyanovaleric acid)), and ethanol as solvent to a reaction vessel. Control the reaction temperature at 70-80°C and stir for 4-8 hours. Stop the reaction, then dilute with ethanol. Settle the mixture three times in n-hexane, then dialyze in a dialysis bag for three days. After dialysis, settle the mixture in ether and dry it completely. This yields a solvent-friendly macromolecular chain transfer agent (PMAA macro-CTA). The molar mass ratio of the monomer to the small molecule chain transfer agent is 50-500:1, preferably 60:1.

[0053] The general process of chiral self-assembly induced by the polymerization of chiral azobenzene monomers with different spacer lengths (x) in ethanol solution is as follows:

[0054] The chiral azobenzene monomers with different alkyl chain flexible spacer lengths, macromolecular chain transfer agents, and free radical initiators such as (4,4'-azo(4-cyanovaleric acid)) were added to a mixed solution of ethanol, and the mixture was deoxygenated with inert gas at 70 o The polymerization was carried out at 400 °C for 15 hours to obtain an in situ azobenzene block copolymer assembly. The molar ratio of the chiral azobenzene monomer to the macromolecular chain transfer agent was 5-60:1.

[0055] Furthermore, the halogen alcohol is selected from any one of 2-bromoethanol, 4-bromo-1-butanol, 5-bromo-1-pentanol, 11-bromo-1-undecanol, 12-bromo-1-dodecanol and 16-bromo-1-hexadecanol. The catalyst is selected from any one of sodium hydroxide, triethylamine, sodium bicarbonate and potassium carbonate, preferably potassium carbonate.

[0056] Furthermore, the free radical initiator is selected from any one of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, and 4,4'-azo(4-cyanovaleric acid), preferably 4,4'-azo(4-cyanovaleric acid) (ACVA).

[0057] Furthermore, the solvent-friendly monomer is selected from any one of methacrylic acid, acrylic acid, 4-vinylpyridine, and N-isopropylacrylamide, preferably methacrylic acid (MAA). The small molecule chain transfer agent is selected from any one of 4-cyano-4-(thiobenzoyl)valeric acid, 2-methyl-2-(dodecyltrithiocarbonate)propionic acid, 4-cyano-4-(((ethylthio)carbonylthio)thio)valeric acid, and 4-cyano-4-[[(dodecylthio)thioketomethyl]thio]valeric acid, preferably 4-cyano-4-(thiobenzoyl)valeric acid (CPADB).

[0058] Furthermore, the inert gas is selected from any one of argon, nitrogen, helium and neon, preferably argon.

[0059] As is common sense, each reaction step can be followed by a purification step to obtain a product of higher purity. The purification step includes (but is not limited to) chromatography, recrystallization, dissolution / precipitation separation, filtration, and the like.

[0060] The present invention will be further described below with reference to specific embodiments and drawings. The specific operating steps and testing methods are conventional techniques.

[0061] Chemical reagents: 4-cyano-4-(thiobenzoyl)valeric acid, 97%, Aladdin; 4,4'-azo(4-cyanovaleric acid), 98%, J&K Chemical; recrystallized twice before use; 2-bromoethanol, 95%, Acros; 3-bromo-1-propanol, 4-bromo-1-butanol, 5-bromo-1-pentanol, 11-bromo-1-undecanol, 12-bromo-1-dodecanol, 16-bromo-1-hexadecanol, p-nitrophenol, 95%, Aladdin; phenol, AR, Aladdin; tin dichloride, 98%, Energy Chemical; diisopropyl azodicarboxylate, 98%, 3A Chemicals; triphenylphosphine, 99%, Greagent; benzyl chloride, 99%, Macklin; methacrylic acid, 99%, Aladdin; chiral octanol ( R and S ), 99%, TCI; tetrahydrofuran, 99.5%, Nanjing Chemical Reagent Co., Ltd.; ethanol, analytical grade; methacryloyl chloride, 95%, Aladdin; hydrochloric acid, analytical grade; sodium nitrite, analytical grade; 1,4-dioxane, analytical grade; potassium iodide, analytical grade; triethylamine, analytical grade; potassium carbonate, analytical grade; sodium hydroxide, analytical grade; sodium bicarbonate, analytical grade; ethyl acetate, 99.5%; petroleum ether, analytical grade; ether, analytical grade; ammonium chloride, analytical grade, all from Jiangsu Qiangsheng Functional Chemical Co., Ltd.; anhydrous sodium sulfate, 98%, Sinopharm Chemical Reagent Co., Ltd.

[0062] Test instrument and conditions: Gel permeation chromatography (GPC): Molecular weight and molecular weight distribution were determined using a gel permeation chromatograph with TOSOHTSKgel SuperHM-M in an automatic injection mode. Polymethyl methacrylate was used as a standard to calculate the polymer molecular weight. N, N-dimethylformamide (DMF) was used as the mobile phase at a flow rate of 0.65 mL / min and a temperature of 40 o C. H NMR spectroscopy ( 1 H-NMR): Bruker 300 MHz NMR was used with CDCl3 and DMSO- d 6 as solvent, TMS as internal standard, measured at room temperature. Transmission electron microscopy (TEM): A HITACHI HT 7700 transmission electron microscope was used, with an accelerating voltage of 120 kV. Atomic force microscopy (AFM): A Bruker Multimode 8 atomic force microscope was used, with the tapping mode. Circular dichroism (CD): A JASCO J-815 circular dichroism spectrometer was used, with a 25 o C measurements were performed with a scan rate of 200 nm / min, a scan range of 300–600 nm, and a bandwidth of 2 nm. UV-visible spectroscopy (UV-vis): Shimadzu UV-2600 spectrometer was used with a scan range of 300–600 nm. Differential scanning calorimetry (DSC): TA DSC 250 was used with a heating and cooling rate of 10 o C / min. Small-angle X-ray scattering (SAXS) was performed using an Anton Paar SAXSess MC2 diffractometer with a Cu Kα radiation source at a wavelength of 0.154 nm. Polarized optical microscopy (POM) was performed using a CNOPTEC BK-POL polarizing microscope.

[0063] Example 1 Synthesis of Chiral Azobenzene Monomer

[0064] See also Figure 2 , a schematic diagram of the synthesis of chiral azobenzene monomers.

[0065] To synthesize Azo3MA-8 S Take the raw materials p-nitrophenol (13.9 g, 0.1 mol), R 1-Octanol (13.0 g, 0.1 mol), diisopropyl azodicarboxylate (20 mL) and 300 mL of ether were added to a three-necked flask, and triphenylphosphine (26.2 g, 0.1 mol) was dissolved in 50 mL of ether and then added to the above flask; the reaction was then carried out at room temperature for 12 hours; after the reaction was completed, the product was first filtered, then the solvent was dried, and then purified by column chromatography and dried to obtain compound 1 (19.27 g, 0.075 mol).

[0066] The above compound 1 (10.8 g) and 100 mL of ethanol were added to a three-necked flask, followed by the addition of 40.5 g, 0.542 mol, of tin dichloride. The mixture was reacted at 70°C for 3 hours. After the reaction, the reaction solution was poured into 600 mL of ice water, and potassium carbonate was added to adjust the pH to 7. The mixture was then extracted with ethyl acetate, dried over anhydrous sodium sulfate, filtered, the solvent was evaporated, purified by column chromatography, and dried to obtain compound 2 (9.3 g, 0.046 mol).

[0067] At 0°C, sodium nitrite (5.4 g, 0.078 mol) was dissolved in 25 mL of water to obtain an aqueous sodium nitrite solution. 22.5 mL of hydrochloric acid was diluted with 60 mL of water, and the diluted hydrochloric acid was added to compound 2 (9.3 g, 0.046 mol). Then, the sodium nitrite aqueous solution was added dropwise while maintaining the temperature at 0°C to obtain a diazonium salt solution of compound 2.

[0068] At 0°C, phenol (8.3 g) was dissolved in 500 mL of water, and sodium hydroxide (NaOH) (4.5 g) and sodium bicarbonate (NaHCO3) (4.5 g) were added. Then, the diazonium salt solution of the above-mentioned compound 2 was added. The reaction was maintained at 0°C for 4 h to obtain a khaki turbid liquid. The obtained turbid liquid was filtered, extracted, dried over anhydrous sodium sulfate, purified by column chromatography, and dried in vacuo to obtain a yellow compound 3 (8.0 g, 0.028 mol).

[0069] Potassium carbonate (14.5 g, 0.11 mol), compound 3 (8.0 g, 0.028 mol), 0.5 g potassium iodide KI and 3-bromopropanol (3.9 g, 0.028 mol) were added to a 500 mL dry round-bottom flask and heated to 85°C. 300 mL of tetrahydrofuran was added to the round-bottom flask and stirred for 4 h before cooling to room temperature. The mixture was extracted with ethyl acetate and water, dried over anhydrous sodium sulfate, and the oil phase was rotary evaporated. It was then purified by column chromatography and dried to obtain compound 4 (8.5 g, 0.026 mol).

[0070] Triethylamine (25 mL), methacryloyl chloride (4.3 g, 0.041 mol), and compound 4 (8.5 g) were added to 300 mL of tetrahydrofuran solution and refluxed under argon for 24 hours. Then, 20 mL of 10% NH4Cl aqueous solution was added; then, the mixture was extracted with dichloromethane, and the combined organic extracts were washed with water and dried over anhydrous sodium sulfate. The crude product was then purified by flash chromatography to obtain a yellow solid after drying. S -Azobenzene monomer (8.4 g, 0.022 mol).

[0071] R -Synthesis steps of azobenzene monomer and S -Azobenzene monomer is the same except that in step R -Octanol to S -octanol, yield 8.1 g R -Azobenzene monomer (Azo3MA-8 R ).

[0072] Extended implementation examples

[0073] Based on Example 1, 3-bromopropanol was replaced by C(CH2) x OH (where C is a halogen and x is a different number of methylene repeats) to obtain monomers AzoxMA-8 with different methylene lengths S 、AzoxMA-8 R , such as 12-bromo-1-dodecanol, to obtain Azo12MA-8S and Azo12MA-8R.

[0074] On the basis of Example 1, R -Octanol is replaced by other chiral small molecule alcohols with different carbon chain lengths to obtain chiral azobenzene monomers with different alkyl chain lengths.

[0075] The NMR characterization of different monomers can be seen in Figure 3 and Figure 4 .

[0076] Example 2: Synthesis of Solvent-Affective Macromolecular Chain Transfer Agent

[0077] See also Figure 5 Methacrylic acid (5.16 g, 60.0 mmol), the small molecule chain transfer agent 4-cyano-4-(thiobenzoyl)valeric acid (0.28 g, 1.0 mmol), 4,4'-azo(4-cyanovaleric acid) (56.1 mg, 0.2 mmol), and ethanol (10.32 g) were added to a reaction vessel. The reaction temperature was controlled at 70°C and stirred for 5 hours. The reaction was stopped and diluted with 2 mL of ethanol. The mixture was then decanted in 500 mL of n-hexane three times. The mixture was then placed in a dialysis bag and dialyzed against 1000 mL of ethanol for three days. After dialysis, the mixture was decanted in 500 mL of n-hexane and dried completely. A solvophilic macromolecular chain transfer agent (PMAA macro-CTA) (4.32 g, 84% yield, degree of polymerization 51) was obtained. The molar ratio of monomer, small molecule chain transfer agent, and initiator was 60:1:0.2.

[0078] Example 3: General process of polymerization-induced chiral self-assembly

[0079] The chiral monomer (0.1 mmol) obtained in step 1), the macromolecular chain transfer agent (93.3 mg, 0.02 mmol) obtained in step 2), 4,4'-azo(4-cyanovaleric acid) (1.16 mg, 0.004 mmol) and ethanol (1.19 g) were added to a reaction vessel, deoxygenated with argon, and polymerized at 70°C for 15 h to obtain a supramolecular assembly of azobenzene block copolymers with multi-level chirality and adjustable liquid crystal properties, named 3-8. S -5, wherein 5 is the degree of polymerization of the azobenzene polymer segment, calculated as the molar ratio of monomer to chain transfer agent.

[0080] The molar ratio of the chiral azobenzene monomer to the macromolecular chain transfer agent can be 5 to 60:1.

[0081] 4-8 S -60 as an example, PMAA51 macro-CTA (93.3 mg, 0.02 mmol), ACVA (1.16 mg, 0.004 mmol), Azo4MA-8 S (559.2 mg, 1.2 mmol) and EtOH (7.43 mL) were added to the reaction vessel, deoxygenated with argon, and polymerized at 70 °C for 15 h to obtain a supramolecular assembly of azobenzene block copolymers with multi-level chirality and adjustable liquid crystal properties, named 4-8. S -60. By changing the amount of monomer, polymers with different degrees of polymerization are obtained. Figure 6 GPC curves of different polymers.

[0082] Example 4 Assembly Characterization

[0083] The morphological transformation of the azo component is regulated by the alkyl spacer and the length of the azo segment. Figure 7 、 Figure 8 The morphological transitions of azo BCP assemblies are illustrated. For x = 3, 4, 5, micelles with an average diameter of approximately 40 nm were observed for DP = 5, and increasing the degree of polymerization to 10 resulted in the formation of soft fibers (worms). Increasing DP resulted in the formation of the only fiber morphology with a length exceeding 5 μm. When the DP reached 30, well-defined vesicles were observed with a membrane thickness of 35 nm. The vesicles became larger with increasing azo segment length. For x = 11, 12, 16, larger vesicles were formed, and the range of vesicle sizes became narrower with increasing x, with azo 16MA* forming larger complex micelles (LCM) at degree of polymerization = 40. The various morphologies presented in azo BCPs with longer spacers (x>2) differ from previously reported results (x=2), which showed pure nanofibers ( Figure 9m and 9n). The morphological transition from nanofibers to vesicles occurs at a relatively low DP due to enhanced solvent hydrophobicity and LC ordering ( Figure 10 ).

[0084] By using AFM, the morphological transition can be further detected. Taking 4-8R as an example ( Figure 11 a) When the degree of polymerization was 5, small micelles with a diameter of 40 nm were observed. When the degree of polymerization increased to 10, the morphology transformed into soft fibers, while when the degree of polymerization was further increased to 20, rigid fibers with longer lengths were formed. Finally, in the case of 4-8R-40, a pure vesicular phase was observed. AFM results showed that the average height distribution and diameter of these vesicles were 145 nm and 245 nm, respectively ( Figure 11 b and 11c). LCM is also observed in the 11-8R-100 azo assembly due to the increased asymmetry of the solvophobic and solvophilic segments. The morphological changes of the azo BCP assemblies prepared by the PICSA strategy are clearly defined in the phase diagram ( Figure 11 d). First, as the alkyl spacing increases, the morphology evolves from micelles to fibers, vesicles, and liquid crystal molecules. Furthermore, the morphological transition is controlled by the degree of polymerization of the azo segment. Fibers appear in systems with low degrees of polymerization, while vesicles and liquid crystal molecules are the main nanostructures in systems with medium and high degrees of polymerization, respectively. This morphological transition may lead to hierarchical chirality in azo BCP assemblies ( Figure 11 e).

[0085] The chiral properties are driven by the synergistic effects of liquid crystal order (x size) and solvent hydrophobicity (DP). Azo aggregates with local chiral orientation can interact electrically with each other, generating the exciton-coupled CD effect ( Figure 12 a). During the PICSA process, living polymerization, self-assembly, and LC ordering occur simultaneously. Figure 12 b shows the changes in turbidity and color of the polymerization solution, indicating the formation of chiral assemblies. More importantly, the molecular chirality of the stereocenter is transferred to the azo supramolecular structure, leading to chiral LC ordering within the azo core-forming block. A broad absorption band of the π-π* transition can be observed at ~360 nm ( Figure 12c and 13), indicating that the azo units exist in various aggregation states (H aggregation, J aggregation and free arrangement). In particular, the absorption of the functional oriented H aggregate is located at 345 nm relative to the maximum absorption of the isolated azo unit (357 nm). In addition, all series of azo assemblies show aggregation-induced split CD bands with the exciton coupling center at around 340 nm, indicating that the same H band splitting is caused by the chiral H aggregation between the interchain azo units. In this regard, the alkyl spacer introduced between the main chain and the azo unit also maintains the flexibility of the main chain. Therefore, the chiral property is more likely to originate from the chiral supramolecular structure of the azo unit (aggregation-induced circular dichroism). Taking 4-8R / 8S as an example ( Figure 12 c) The R-type assembly shows a positive Cotton band at 390 nm followed by a negative Cotton band at 320 nm, while the S-type assembly shows a mirror-image Cotton effect. An exciton coupling centered at 337 nm was detected, which is blue-shifted to the transition band of the interchain H aggregate. The mirror-image CD signal indicates the chirality transfer of the stereocenter to the azo supramolecular stack, which may be further amplified to LC assembly through interchain cooperative asymmetric interactions. The LC order and solvent photophobicity of the azo core are determined by the length of the alkyl spacer and the DP of the azo segment, which can synergistically affect the supramolecular chirality expression ( Figure 12 d and 12e). The absolute maximum CD value first increases and then decreases with increasing DP, which is also affected by the corresponding morphological transformation. First, the CD value increases significantly with DP due to the increasing number of azo supramolecular units participating in the construction of the helical structure. However, as the azo DP continues to increase, the CD value of the assembly decreases. When the morphology of the azo compound changes, the absolute CD value also experiences a turning point. For example, regardless of whether the worm is transformed into a vesicle (x = 3, 4, 5, Figure 12 d) Or vesicles are converted into LCM (x = 11, 12, 16, Figure 12 d), all CD values ​​will be weakened. The synergistic effect of LC order and helical polarization along the fiber enhances the CD signal of the fiber assembly. Compared with vesicles, the disordered distribution of the Azo blocks forming the core within the LCM makes the polarization direction of the helical structure more isotropic and random. Therefore, the morphological transformation of the Azo assembly affects the process of chirality transfer, stacking and expression to a certain extent. Inserting a flexible spacer between the main chain and the mesogen unit can effectively decouple the dynamics of the two components. The increase in length helps the Azo building block to participate in chiral H aggregation, but the longer the spacer, the more Gaussian conformations are adopted and the chiral properties are correspondingly reduced. Therefore, the CD maximum value shows irregular changes with respect to the spacer length, indicating that small changes in the spacer length can significantly affect the chiral structure and expression of the Azo BCP assembly ( Figure 12 e). The hierarchical chirality and various morphologies can be controlled by the number of helical azo stacks (mainly DP) and the coupling effect of the side chains (mainly x) ( Figure 12 f). Unexpectedly, when the interval length x = 12, the highest ellipticity (± 2.3 × 10 3 mdeg, 12-8R / 8S-40), and the Azo stacking has appropriate conformational freedom in the vesicle morphology.

[0086] These results indicate that, regardless of DP and morphological variations, relatively abundant thermodynamically stable H aggregates dominate the Azo assemblies at spacer lengths x > 2, which is significantly different from previously reported work in which Azo assemblies displayed multiple chiral inversions with three stacking modes of transitional dipole moments, from intrachain π-π stacking to interchain H aggregation and J aggregation, depending on the DP of the Azo (also as shown in

[15] ). Figure 14 shown).

[0087] Dynamic chiral response with distinct morphological transformations. Due to non-covalent interactions and unique reversible trans-cis isomerization properties, the chirality and morphology of azo polymer assemblies can be regulated by external stimuli. Under UV irradiation, the absorption peak at 365 nm associated with the π-π* electronic transition of the trans isomer decreases as the intensity of the cis isomer at 475 nm increases ( Figure 16 The CD intensity of the azo assembly series gradually decreases with UV irradiation and completely disappears in the photostable state (PSS) ( Figure 15 a). The transformation of the coplanar form of trans-Azo to the non-coplanar and bent form of cis-Azo disrupts the long-range chiral H aggregation between Azo supramolecular molecules. Heating-cooling treatment (HC) can achieve cis-trans isomerization of Azo units while restoring the damaged supramolecular helical structure to some extent. All CD-free Azo BCP assemblies showed obvious CD signals with increasing aging time and remained stable for around 15 minutes ( Figure 15 b). The x-spacers have different spatial degrees of freedom, allowing azo building blocks to stack on each other, which makes them important in the recovery rate of thermally induced helical rearrangement and supramolecular chirality. 3-8R / 8S, 4-8R / 8S, and 5-8R / 8 can achieve high chiral recovery rates ( Figure 15 c, higher than 90%), 11-8R / 8S, 12-8R / 8S, and 16-8R / 8S showed relatively low chiral recovery rates (less than 40%). The present invention has successfully constructed a dynamic reversible chiral switch of the Azo component series, and the "switch" operation can be performed more than 5 times during the alternating transition process of UV irradiation and HC treatment ( Figure 15 d, 17).

[0088] After irradiation with 365 nm UV light for 30 min, the small micelles of 4-8R-5 gradually transformed into nanospheres with more uniform size and shape, and the diameter gradually increased from 30 ± 5 nm to 45 ± 5 nm ( Figure 15 e). In particular, the soft fibers of 4-8R-10 were significantly transformed into rare necklace-like assemblies composed of spherical micelles with an average diameter of approximately 40 nm. In addition, the rigid nanofibers of 4-8R-20 with higher surface free energy were transformed into cleanly cut spherical vesicles with lower surface free energy. Unexpectedly, the azo assemblies of 4-8R-30, 4-8R-40, and 4-8R-60 retained their vesicle structure without morphological changes ( Figure 15 f). The S-type assembly exhibits similar morphological changes during the trans-to-cis isomerization process ( Figure 18 ).

[0089] WAXD images of all Azo BCP series ( Figure 19 a) Indicates the formation of LC phase structure. The two typical scattering peaks in the SAXS pattern can prove the layered structure (q2 / q1=2:1, Figure 19 b). The periodicity d001 calculated from SAXS data ranges from 4.76 nm to 7.76 nm, which is less than twice the fully extended length of the side chain 2L (5.24 nm to 8.58 nm calculated by CASTEP) ( Figure 19 c), indicating the presence of a bilayer smectic a* (SmA*) phase whose domains are relatively interdigitated in terms of H-aggregation between Azo units. The corresponding homopolymer Poly(x-8R) is transparent ( Figure 19 d, prepared by omitting the chain transfer agent), POM detected weakly birefringent aggregates with a filamentous texture ( Figure 21 When the spacer length x>2, the interchain H aggregates dominate the chiral expression in the Azo assembly series ( Figure 19 e and 19f).

[0090] The above results indicate that the exciton coupling chirality of all azo polymer series is determined by the interchain stacking in the helical geometry of the H aggregation. The trans-azo compounds with rigid rod structures have mesogenic properties, while the bent cis isomers are amorphous. In the PICSA process, chiral liquid crystal asymmetry is the intrinsic driving force for the formation of azo BCP assemblies with unique chiral expressions. The chiral liquid crystals of azo BCPs were studied in detail using differential scanning calorimetry (DSC), POM, small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD). DSC results show that the azo BCP with the shortest spacing (x=3) appears to be amorphous ( Figure 20), while azo BCPs with longer spacings (x = 4, 5, 11, 12, and 16) clearly exhibited a mesophase isotropic transition during secondary heating. DSC results further analyzed the liquid crystal temperature region of the block copolymers. The Tg values ​​for 12-8R / 8S-60 and 16-8R / 8S-60 were 55°C and 65°C, respectively, while the Tg values ​​for the other polymers were undetectable. For x = 4, 5, 11, 12, and 16, the mesophase isotropic transition Ti temperatures were 83°C, 67°C, 50°C, 57°C, and 71°C, respectively.

[0091] This invention combines the advantages of polymerization-induced chiral self-assembly, in situ supramolecular interactions, and chiral stacking to successfully prepare azobenzene block copolymer supramolecular assemblies with multi-level chirality (from stereocenter to polymer assembly) and tunable liquid crystal properties. The coupling effect of achiral alkyl spacers, the non-covalent interaction of azobenzene units, and the assembled chiral liquid crystal field are key factors in the expression of chirality and liquid crystal order in block copolymer self-assembly. In addition, the trans-cis isomerization of azobenzene units can regulate the morphological transition and chiral response of the assembly. Surprisingly, when the spacer length x = 12, the assembly of the invention produces a structure with the highest ellipticity (±2.3×10 3 mdeg, 12-8R / 8S-40), and when x=6, the ellipticity is similar to that of x=5, and mdeg is less than 200; when the interval length x=2, the ellipticity is also very small and the positive and negative are reversed, which is different from the present invention.

Claims

1. A supramolecular assembly of azobenzene block copolymers with multi-level chirality and adjustable liquid crystal properties, characterized in that: The chiral azobenzene monomer is obtained by initiating a polymerization reaction of a solvophilic macromolecular chain transfer agent; the chemical structure of the chiral azobenzene monomer is as follows: ； The chemical structure of the solvent-philic macromolecular chain transfer agent is as follows: ； m is 51, x is 12, and a is 5; The polymerization reaction is carried out at 60-80°C for 12-18 hours; The molar ratio of chiral azobenzene monomer to solvophilic macromolecular chain transfer agent is 40:1; Azobenzene block copolymer supramolecular assemblies with multi-level chirality and tunable liquid crystal properties have an ellipticity of ±2.3×10 3 mdeg.

2. The azobenzene block copolymer supramolecular assembly with multi-level chirality and adjustable liquid crystal properties according to claim 1, characterized in that: The polymerization reaction is carried out in the presence of an initiator and an alcohol solvent.

3. The azobenzene block copolymer supramolecular assembly with multi-level chirality and adjustable liquid crystal properties according to claim 1, characterized in that: Compound 1 is prepared using p-nitrophenol and chiral alcohol as raw materials; compound 1 is sequentially aminated and diazotized, and then reacted with phenol to obtain compound 3; compound 3 reacts with halogen alcohol to obtain compound 4; chiral azobenzene monomer is prepared using methacryloyl chloride and compound 4 as raw materials through reflux reaction under inert gas; a macromolecular chain transfer agent is prepared using a hydrophilic monomer and a small molecule chain transfer agent as raw materials.

4. Use of the azobenzene block copolymer supramolecular assembly with multi-level chirality and adjustable liquid crystal properties according to claim 1 in the preparation of high ellipticity materials.

5. Use of the azobenzene block copolymer supramolecular assembly with multi-level chirality and adjustable liquid crystal properties according to claim 1 in the preparation of ultraviolet-resistant chiral materials.

6. A high ellipticity UV-resistant chiral material, characterized in that: The invention is prepared from the supramolecular assembly of azobenzene block copolymer with multi-level chirality and adjustable liquid crystal properties as described in claim 1.

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