A Convenient Route to Prepare Reactive Azobenzene‐Containing Liquid Crystal Polymers and Photodeformable Fibers

Azobenzene‐containing liquid crystal polymers (azo‐LCPs) with reactive groups attract much attention due to the combined properties of liquid crystallinity, photoresponsivity and reactivity, which make them promising in designing intelligent photo‐driven soft actuators. Herein, a post‐polymerization modification strategy is used to prepare a series of reactive azo‐LCPs using the trans‐esterification reaction of poly(pentafluorophenyl acrylate) with functional alcohols. These reactive azo‐LCPs show good capabilities in fabricating fibers via a simple thermal‐drawing method and subsequent crosslinking reaction with diamine under mild conditions. Furthermore, the prepared fibers exhibit reversible deformation behaviors under the alternate irradiation of UV and visible light (365/530 nm). This facile synthetic strategy is expected to open up new possibilities for fabricating photo‐driven actuators.

PPM reaction of PPFPA with functional alcohols (Figure 1a). With all copolymers synthesized from a single reactive precursor polymer, they exhibited similar degree of polymerization and chain-length distributions, which is important for studying the relationship between composition and properties. The reactive pentafluorophenyl (PFP) ester in PPFPA fulfills a dual role: 1) it can be substituted by azobenzene group to endow the polymer with both liquid crystalline and photoresponsive properties, and 2) it can further react with diamines to result in a crosslinked polymer network. Now, utilizing a simple thermal-drawing of the reactive azo-LCP and a subsequent crosslinking process, photodeformable fibers with reversible bending behavior were fabricated ( Figure 1b).
The design, synthesis, and chemical structure of the azo-modified PPFPA (PFAZO) are shown in Figure 1a, in which a hydroxyl-terminated azobenzene compound (AZO-OH) was chosen as functional alcohol. Through controlling the feed ratio of AZO-OH, three copolymers PFAZO-x (x ¼ 14%, 30%, 58%) with different azobenzene molar contents of 14%, 30%, and 58% were synthesized and their structural and component information was determined by 1 H NMR. As shown in Figure 2a, typical azobenzene proton signals (a, b) clearly emerged in all 1 H NMR spectra of the synthesized copolymers, demonstrating that the azobenzene moieties have been successfully attached to PPFPA. In addition, the absence of protons signal (c) of methylene attached to the hydroxyl group of AZO-OH at 3.6 ppm indicated that the residue AZO-OH had been removed from the products. The azobenzene contents (14%, 30%, and 58%, respectively) in three copolymers were calculated from the integration values of the resonance signals, and the details are shown in Figure S1-S3, Supporting Information. Furthermore, the existence of PFP esters in PFAZO, which is necessary for the post-functionalization of azo-LCPs, was verified by 19 F NMR spectra (Figure 2b) where all copolymers showed the typical PFP esters signals in the region of À150 to À165 ppm.
The successful PPM was also confirmed by Fourier transform infrared (FT-IR) spectroscopy ( Figure 2c). The FT-IR spectra of the homopolymer PPFPA presented a characteristic C═O stretching absorption of PFP ester groups at 1780 cm À1 . Once partially modified with AZO-OH, a new C═O stretching absorption at 1735 cm À1 appeared. This particular signal proportionally increased as the mole equivalent of AZO-OH was increased, accompanied by the gradual decrease in absorption at 1780 cm À1 . Similar changes were also found in the aromatic C═C stretching absorption of PFP at 1518 cm À1 and the aromatic C═C stretching absorption of azobenzene at 1500 cm À1 .
The mesomorphic properties of PFAZO-x (x ¼ 14%, 30%, 58%) were analyzed with a combination of polarized optical microscope (POM), small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). POM observation showed that all copolymers featured typical schlieren texture at room temperature, suggesting the formation of liquid crystal phase ( Figure 3a). The SAXS spectra of PFAZO-14%, PFAZO-30%, and PFAZO-58% were obtained at room temperature ( Figure 3b), in which all copolymers exhibited two scattering peaks (q 1 to q 2 being 1/2) and similar q 1 in the low-angle region, revealing the presence of a long-range ordered lamellar structure with layer spacing (d) around 4.10 nm (d ¼ 2π/q 1 ). The DSC study revealed the thermodynamic properties of polymers ( Figure 3c, Figure S4a, Supporting Information). Combining the observation of POM at varied temperatures ( Figure S4b, Supporting Information), it can be concluded that the endothermic peak around 120 C represents the transition between liquid crystal phase and isotropic phase. As the azobenzene contents were increased from 14% to 58%, the clearing temperatures of PFAZO increased from 116 to 130 C, indicating the improved stability of the liquid crystal phase. In addition, all obtained copolymers exhibited elevated glass transition (T g ) compared with the precursor polymer PPFPA ( Figure S4c, Supporting Information), which is due to the introduction of rigid azobenzene units.
The photoisomerization of azobenzene is one of the attractive properties of azo-LCPs. As shown in Figure 4a, upon irradiation with 365 nm UV light, PFAZO-58% in CH 2 Cl 2 solvent underwent a trans-cis isomerization. With the ongoing irradiation, the intensity of the π ! π* transition around 365 nm decreased, whereas the intensity of the n ! π* transition around 450 nm increased slightly until a photostationary state was eventually reached. The existence of isobestic points demonstrated the presence of two distinct absorbing species in equilibrium with each other and no side reaction took place during the photoisomerization process. [30] The cis-trans back isomerization was achieved through irradiating with visible light (λ ¼ 530 nm) (Figure 4b). A similar photoisomerization process was observed for PFAZO-14% and PFAZO-30%, as shown in Figure S5, Supporting Information.
To build actuators with good photomechanical motion, the presence of crosslinking networks is essential for it helps to spread the forces produced by the photoisomerization of azo groups to the whole system. [31] Noteworthy, this crosslinking structure can be easily realized in PFAZO utilizing a secondary functionalization of the remaining PFP ester group. Here, three freestanding fibers (PFAZO-14%, PFAZO-30%, PFAZO-58%) with different azobenzene contents and crosslinking extent were fabricated by the first fabrication of the uncrosslinked azo polymer fibers through a simple thermal-drawing method and subsequent crosslinking reaction under mild conditions (Figure 1b). The detailed crosslinking reaction of PFP groups with diamine is presented in a schematic illustration shown in Figure 4c. According to previous studies, in the post-crosslinking system, www.advancedsciencenews.com www.advintellsyst.com the extent of crosslinking in the fiber surfaces should be proportional to the contents of the crosslinkable group in the polymer. [15,31] Attenuated total reflection Fourier transforminfrared (ATR FT-IR) spectroscopy was performed to study the structural change on the PFAZO-58% fiber surface before and after its crosslinking with 1,12-dodecadiamine. As shown in Figure 4d, new absorptions around 1670 and 1542 cm À1 were observed after crosslinking, which corresponded to the C═O stretching and NH bending of the amide groups, respectively. This observation together with the disappearance of the absorption around 1780 and 1518 cm À1 (the PFP ester group from the uncrosslinked polymer) provides strong and direct evidence for the occurrence of the chemical crosslinking reaction of the PFP ester groups in the polymer via 1,12-dodecadiamine. The resistance of fibers against organic solvents also confirms the successful crosslinking ( Figure S6, Supporting Information). The uncrosslinked fibers were dissolved in CH 2 Cl 2 within 30 s, whereas the crosslinked fibers remained intact even after soaking in the same solvent for 30 min. The preferential orientation of mesogens is of vital importance for LCPs to deform in the anisotropic manner under external stimuli. [32] Here, 2D X-ray diffraction (2D-XRD) was used to investigate the orientation of mesogens in the fibers (Figure 4e, Figure S7a,b, Supporting Information). In the low-angle region, all fibers showed similar diffraction arcs on the equator line, implying the formation of a smectic phase with normal directions of the smectic layer perpendicular to the fiber axial direction. In the high-angle region, two diffraction arcs on the meridian demonstrated the smectic A phase with mesogens orientation along the radial direction of the fiber. A schematic diagram of mesogen orientations (in a single layer) in the fiber is shown in Figure S7c, Supporting Information.
With the alignment of mesogens in a direction perpendicular to the fiber axis, these fibers are expected to bend away from the light source when exposed to UV light for the anisotropic expansion caused by the trans-cis isomerization of the azobenzene mesogens and the resulted disruption of alignment in the surface region ( Figure S7d, Supporting Information). [33] The photoinduced bending behaviors of three crosslinked fibers (PFAZO-14%, PFAZO-30%, PFAZO-58%) are shown in Figure 5 and Movie S1-S3, Supporting Information, in which all fibers showed reversible photoinduced bending behaviors upon irradiation with UV light and subsequent visible light. As reported, the photodeformation behaviors of the crosslinked fibers are related to the azobenzene contents and crosslinking degrees. [34][35][36] In this work, the PFAZO-14% fiber only presented a slight deformation (bending angle around 5 ) because high crosslinking networks restrict the mobility of polymer chain and the low  azobenzene contents produce small photoinduced force. [34,35] Among all obtained fibers, PFAZO-58% fiber exhibited the largest bending angle, which is due to the synergistic effect between increased azobenzene contents and decreased crosslinking degree. When irradiated by UV light (365 nm, 10 mW cm À2 ), it took 30 s for PFAZO-58% fiber to reach its maximum bending angle of 30 . The bent fiber could revert to its initial state upon irradiation with visible light (530 nm, 10 mW cm À2 ) for 20 s. For comparison, the photodeformation behaviors of uncrosslinked fibers were also evaluated, which all showed negligible photomobility after irradiation with UV light. This phenomenon, along with other similar results in previously reported uncrosslinked fibers, [16,31] indicates the importance of crosslinking structure in the photo-driven actuating system. In summary, we have provided a facile strategy to synthesize reactive LCPs with different azobenzene contents (14%, 30%, 58%) by PPM of the reactive precursor PPFPA. These copolymers exhibited stable liquid crystal phases and reversible photochemical behaviors. Moreover, they showed great capabilities in fabricating photodeformable fibers with high alignment order of azo mesogens through a simple thermal-drawing and subsequent crosslinking process with diamine. Among the obtained fibers, PFAZO-58% fiber with the highest azobenzene content and lowest crosslinking degree exhibited a maximum bending angle at room temperature. Collectively, this straightforward synthetic strategy, along with the remaining active PFP groups, renders PFAZO attractive candidates for introducing additional functionalities to design a wide range of flexible and multi-functional actuators.

Experimental Section
Synthesis of PPFPA: Precursor polymer PPFPA was synthesized according to a reported method. [37] PFPA (2.4 g, 10.1 mmol), 1,4-dioxane (3.8 mL), azobisisobutyronitrile (3.3 mg, 0.02 mmol) were added into a Schlenk flask. Afterward, the reaction was frozen and thawed three times, where liquid N 2 and N 2 gas were used for freezing and protection, respectively. The reactant was then stirred at 70 C for 24 h. After cooling down with an ice bath, the crude product was added dropwise into methanol and then reprecipitated from tetrahydrofuran (THF) in methanol twice. The product was dried under vacuum to obtain a white powder product (Yield, 85%). The synthetic route is shown in Scheme S1, Supporting Information. 1  Synthesis of Compound 11-(4-((4-Butoxyphenyl)Diazenyl)Phenoxy) Undecan-1-ol (AZO-OH): Functional alcohol AZO-OH was synthesized according to the similar procedures reported previously, [38] with the synthetic routes as shown in Scheme S2, Supporting Information (Yield, 73%). Synthesis of PFAZO: The PFAZO were synthesized by PPM of PPFPA with AZO-OH following a recently developed trans-esterification strategy. [39] First, the PPFPA (100.0 mg, 0.42 mmol, 1.0 eq. of PFP group) was dissolved in 1.0 mL of dry N,N-dimethylformamide. Then, AZO-OH of varied equivalents to PFP groups of PPFPA (0.25, 0.50, and 0.75 eq.) and 0.2 eq. of 4-dimethylaminopyridine were added under inert N 2 atmosphere. The mixture was stirred for 15 h at 80 and then precipitated in n-hexane. After redissolving the precipitate in THF and then reprecipitating in n-hexane twice, PFAZO with three different azobenzene contents were obtained. Furthermore, these copolymers were dried under a high vacuum at 45 overnight. The relevant characterization data are summarized in Table S1, Supporting Information.
Fabrication of Crosslinked Fibers: In a typical experiment, a small amount of the copolymer (20 mg) was heated to 150 on a glass substrate placed on a hot stage (Mettler, FP-90 and FP-82), then the fibers were prepared by quickly drawing out the melt using a toothpick. The fibers were left at room temperature for about 1 h to be stabilized. The average diameter of the fibers was about 20 μm. Then the drawn fibers were immersed in a solution of 1,12-dodecadiamine in methanol (2 Â 10 À3 M) to undergo the chemical crosslinking reaction for about 1 h. After being washed with methanol several times and then dried at ambient temperature for 24 h, freestanding crosslinked fibers were finally obtained.
Characterization Methods: 1 H NMR and 19 F NMR spectra of the AZO-OH, PPFPA, and PFAZO were recorded on a Bruker DMX500 NMR spectrometer using tetramethylsilane as the internal standard and CDCl 3 as the solvent. FT-IR spectra and ATR FT-IR spectra were recorded on a Nicolet Nexus 470 spectrometer. The molecular weight as well as the polydispersity index were measured by gel permeation chromatography (GPC, Shimazu, LC-10ADvp) with THF as the eluent at a flow rate of 5 mL min À1 . The thermodynamic properties of PFAZO were determined by DSC (TA, Q2000) at a heating and cooling rate of 10 C min À1 . The textures of PFAZO were evaluated with a POM (Leika, DM2500p) equipped with a hot stage (Linkam THMS600). SAXS measurements were carried out on a Bruker NanoSTAR SAXS system using Cu Kα (λ ¼ 1.542 Å) as the radiation source. UV-Vis absorption spectra were measured with a UV-Vis absorption spectrophotometer (HITACHI, U-4100). 2D-XRD experiments of fibers were conducted on a small-angle/wide-angle diffractometer (Xeuss 2.0) with a 2D detector of Pilatus3R in transmission mode. Photographs and videos of the bending behaviors were taken by a super-resolution digital microscope (Keyence, VHX-1000C). 365 nm UV light was generated by an Omron ZUV-H30MC light source with a ZUV-C30H controller. Visible light (530 nm) was generated by a CCS HLV-24GR-3W.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.