Liquid crystallinity and thermal properties of polyhedral oligomeric silsesquioxane/side- chain azobenzene hybrid copolymer

Acrylic acid-modified polyhedral oligomeric silsesquioxane (AC-POSS) was synthesized by the reaction between the amine groups in polyhedral oligomeric silsesquioxane (POSS) and acrylic acid, which could dissolve in water and can be easily purified. Free-radical copolymerization was applied to synthesize azobenzene liquid crystalline polymer silsesquioxane (LCP-POSS) with different proportions of AC-POSS and liquid crystalline monomers. The trans-isomers of azobenzene moieties in LCP-POSS were gradually transformed to cisisomers with increasing ultraviolet irradiation time. The photoisomerization reaction of liquid crystalline polymer (LCP) and LCP-POSS showed the first-order dynamic reaction. Compared with the LCP, the photoisomerization rate constant of LCP-POSS was decreased due to the space steric hindrance of the POSS as a rigid segment. The phase transition temperature of liquid crystalline in LCP-POSS increased with increasing POSS content, and the liquid crystalline texture in LCP-POSS became smaller under the polarized light. With further increasing the POSS content (>50 wt%) in LCP-POSS, the ordered structure of the liquid crystalline phase was gradually affected, resulting in one-way liquid crystal (LC) phase behavior. The synthesized LCP-POSS has LC properties, light-responsive properties, and thermal stability. When the POSS is introduced into the LC material, the phase state of the LC material will become more abundant and the LC phase will become more stable. The significance of this study is to develop and extend its applications as stimuli-responsive materials and devices.

Liquid crystalline polymer (LCP) has attracted increasing attention due to its molecular orientation order, and POSS also has the ability of self-assembly with its regular structure. The incorporation of POSS may benefit the formation of the liquid crystal (LC) order. Kim et al. [23] prepared LCP-containing POSS by random copolymerization between vinyl-containing single functional group of POSS and one side chain type LC monomers. When the feed ratio of the POSS monomer is greater than 10%, the resulting LCP does not have LC properties. Compared with the LC homopolymer, the phase transition temperature of the LC copolymer decreased and the stability of the LC phase increased. Fan et al. [24] investigated that hybrid organic-inorganic jacketed polymers containing two POSS moieties in the side chains, denoted as P n POSS (n = 6 or 10, the number of methylene units between the terephthalate core and POSS moieties in the side chains), which were synthesized through conventional free radical polymerization. Compared with the triphenylene discotic LCs, crystalline POSS moieties have a stronger tendency of aggregation and can stabilize the LC phases formed by mesogenjacketed LCPs. Laine et al. [25] reported the POSS with average of four LC motifs in each molecule, which has only nematic phase. An LC material with both nematic and smectic crystalline phases containing an average of five LC elements in the molecule was prepared by controlling the feed ratio [26]. The incorporation of POSS into LCP increased the liquid phase transition temperature and enriched the LC phase.
The azobenzene polymer has attracted wide attention due to its photoluminescence property. Under ultraviolet (UV) irradiation, trans-cis configuration conversion could occur, while the molecular configuration restores to trans-structure under visible light or heated effect [27]. The structure and performance characteristics of azobenzene were studied deeply [28][29][30][31][32]. Chen et al. [33] prepared the functional POSS-based fluorinated azobenzene polymers, which were expected to be applied on the surface with light-responsive properties with controlled wettability. Miniewicz et al. [34] reported a novel polymer of polymethyl methacrylate composite dispersed with azo-functionalized POSS nanoparticles with photoresponsive properties.
However, there are no studies on the POSS and azobenzene LC hybrid nanomaterial with different POSS contents through free radical polymerization to discuss the azobenzene LC phase behavior under the confinement of different POSS contents [35]. The LCP-POSS has LC properties, light-responsive properties, and thermal stability. In this study, POSS with different proportions was incorporated into azobenzene LCP through freeradical polymerization. When the POSS is introduced into the LC material, the phase state of the LC material will become more abundant and the LC phase will become more stable. Meanwhile, the influence of POSS on the light-responsive properties of azobenzene has been studied, and LCP photoisomerization rate constant was investigated. Azobenzene LCP has photoisomerization properties, which could be applied in nanodevice, optical switch, information storage, and liquid crystal display. The LC phase and the thermal stability might be enhanced by the incorporation of low content of POSS to some extent, which may promote and extend its applications as stimuli-responsive materials and devices.

Synthesis of POSS and AC-POSS macromonomer
POSS was synthesized according to previous studies [37,38]. The synthesis of AC-POSS macromonomer with THF as a solvent, 100 mg of amino POSS, 10 mL of acrylic acid, 0.3 g DCC, and 0.1 g DMAP were weighed. The reaction mixture was stirred under the protection of nitrogen with magnetic stirring at room temperature for 24 h. After the reaction, the white solid was precipitated by distilled water, washed with a large amount of distilled water to remove the excess acrylic acid, and then washed with acetone to remove a small amount of catalyst (DCC) resulting in functionalized POSS (AC-POSS). The FTIR spectra of POSS and the acrylic acid-modified polyhedral oligomeric silsesquioxane (AC-POSS) are shown in Figure S3. The XRD result of POSS is shown in Figure S7. can be easily removed. A pure LCP-POSS was obtained by precipitation with methanol to remove excess azobenzene monomer and vacuum drying. Table 1 presents LCP-POSS copolymers with different synthesis ratios. The FTIR spectra of Azo-M, AC-POSS, LCP-POSS, and LCP are shown in Figure S4. Figure S6 shows the 1 H NMR spectrum of LCP-POSS.

LC behavior of LCP-POSS polymer
LC birefringence behavior is a phenomenon in which a beam of light is incident into an anisotropic crystal, which is decomposed into two beams and refracted in different directions. Figure 2 shows the polarized optical micrographs of LCP and LCP-POSS. Figure 2(a) and (b) shows the LCP at 90°C and 30°C, respectively. Figure  2(c-f) shows LCP-POSS-1, 2, 3, and POSS at 30°C, respectively. Per previous study [38], the birefringence phenomenon of LCP can be observed in the process of heating and cooling, and LCPs exhibit the schlieren texture ( Figure 2 Figure 3. On one hand, a small amount of POSS with a high thermal stability could improve the thermal stability of LCP. The LC phase transition behavior of LCP-POSS-1 and LCP-POSS-2 occurred in the heating and cooling processes. As shown in Figure 3(b), during the cooling process, the phase transition temperature of LCP-POSS-1 appears to be 110.5°C and 73.5°C, respectively; the phase transition temperature of LCP-POSS-2 LC appears to be 125.8°C and 83.5°C, respectively. With further increasing the POSS content, a larger steric hindrance in the LCP-POSS influences the formation of LC-ordered structure. As per the curve d shown in Figure 3(b), only a one-way LC behavior of the LCP-POSS could be found during the cooling process.   This phenomenon is consistent with the result of polarized optical microscope, where the crystallization texture could be observed only when cooling to 141.7°C. Figure 4 shows the thermal gravimetric (TG) curves of LCP, LCP-POSS-1, LCP-POSS-2, LCP-POSS-3, and POSS. In this study, the temperature at 10% decomposition is used as the initial decomposition temperature. The temperature at 10% decomposition shows the thermal stability of LCP and LCP-POSS, as presented in Table 3. Water can dissolve AC-POSS, which proves the synthesis of the LCP-POSS copolymer. The temperatures at 10% decomposition of LCP and POSS are 296.7°C and 343.4°C, respectively. The temperatures at 10% decomposition of LCP-POSS-1, LCP-POSS-2, and LCP-POSS-3 are 308.7°C, 317.4°C, and 327.8°C, respectively. It is clear that LCP has the lowest initial decomposition temperature, while POSS has the highest initial decomposition temperature. The result shows that the thermal decomposition temperature increases accordingly with the incorporation of the rigid cage-like POSS. Tanaka et al. [39] reported the use of unique organic-inorganic hybrid materials composed of octa-substituted polyhedral oligomeric silsesquioxane (POSS) cores as ionic liquid (IL) crystals. These materials could exist in the LC phase in a wide temperature range because of the stabilizing effect of the POSS core. The synthesized ion pairs composed of alkyl chain-substituted imidazolium and carboxylates of various lengths that were connected to the POSS core; then, the thermal properties of these materials were investigated. The highly symmetric structure of POSS contributes not only to the suppression of the molecular motion of the ion salts but also to the formation of regular structures, leading to thermally stable, thermotropic IL crystals [40]. The dispersion quality of nanoparticles has always limited the performance of polymer nanocomposites and coatings. Herein, the main purpose is to improve the dispersion quality of nanoparticles and overall properties in polyvinylidene fluoride (PVDF)/POSS nanocomposites fabricated through the spray-coating technique. POSS was added to PVDF/DMF solution at varying concentrations. The improved dispersion of POSS resulted in a significant enhancement in the crystallinity of PVDF from 29.8 to 59.5% according to the DSC results [41].

Photoresponsive properties
Different sample solutions were irradiated with 365 nm UV and visible light, and the variation of UV absorption spectrum with irradiation time was recorded. The  photoisomerization behavior of azobenzene and the effect of LCP content on absorbance at the same concentration were studied. The corresponding results were obtained at different UV irradiation times. Figure 5 shows the UV-vis absorption spectra of LCP and LCP-POSS with different ratios (POSS:LCP = 1:16, 1:5, and 1:1) under the UV irradiation of 365 nm. The transition characteristic absorption peak of azobenzene at 358 nm belongs to the π-π* electron transition. With increasing UV irradiation time, the absorbance at 358 nm decreases rapidly, while the absorbance of the peaks at 450 and 310 nm increases slowly. The results indicate that azobenzene gradually changed from trans-configuration to cis-configuration until it reached the stable state. Figure 5 shows that the absorbance of azobenzene characteristic peaks decreases significantly with decreasing LCP content for LCP-POSS-1, LCP-POSS-2, and LCP-POSS-3 before UV irradiation. With increasing UV irradiation time, the absorbance of the corresponding absorption peak at 358 nm decreases rapidly, and the characteristic absorption peak of azobenzene is extremely low. The result suggests that trans-azobenzene transformed into cis-azobenzene. With the lower content of LCP, the absorbance of azobenzene characteristic peak decreases obviously after UV irradiation. Figure 6 presents the UV-vis absorption spectra of the LCP and the LCP-POSS polymer solutions with different proportions (POSS-LCP-1, LCP-POSS-2, and  LCP-POSS-3) under visible light irradiation. The strong absorption peak at 358 nm corresponds to the π-π* electron transition of trans-azobenzene in LCP. The weak acromion at 450 nm belongs to the n-π* electron transition of cis-azobenzene. The weak absorption peak at 310 nm belongs to the n-π* electron transition of cisisomer (short-axis parallel direction of trans-isomer).
Under the visible light, the cis azobenzene transforms gradually, demonstrating that the isomerization of azobenzene is reversible. The true POSS percentage in the copolymer could be calculated based on the maximum absorption values at 358 nm according to the Beer law, as presented in Table 4. The azobenzene content in different polymers is calculated: 83.03%, 47.61%, and 8.67% in LCP-POSS-1, LCP-POSS-2, and LCP-POSS-3, respectively.
The first-order reaction kinetics was used to study the influence of the trans-cis isomerization reaction with the different POSS contents as presented in Table 5. A 0 is the absorbance before the light (t = 0) at 358 nm, A t is the absorbance at the time of the light t, A ∞ is the absorbance at the light t = ∞, and K is the first-order reaction rate constant of the trans-to-cis transformation (π-π* electron transition) [42].
where A ∞ is the absorbance at 360 nm with the UV irradiation time until the balance state, A t is the   absorbance at 360 nm with the UV irradiation time t, and A 0 is the absorbance at 360 nm without the UV irradiation. The aforementioned formula is the light reaction kinetics equation, as presented in Table 5. These approximately straight lines are obtained by the formula, and the slopes are the reaction rate constants (k) of polymer isomerization. Introducing POSS to LCP decreased the rate constant of cis-trans photoisomerization to some extent. It can be seen that the isomerization reaction rate constant decreases with the addition of POSS (Figure 7). The isomerization reaction rate constants of LCP, LCP-POSS-1, LCP-POSS-2, and LCP-POSS-3 are 0.0112 × 10 −4 , 0.0052 × 10 −4 , 0.008 × 10 −4 , and 0.0086 × 10 −4 , respectively. This result suggests that the addition of POSS to LCP structure, to some extent, weakened the isomerization reaction rate constant of LCP. Due to the structural ordering of POSS, the incorporation of POSS to LC has been extensively investigated. The LC element was incorporated into POSS to produce LCP-POSS hybrid, in which the degree of order increase, by Goodby and coworkers [43,44]. The LC phase was transformed from nematic phase to smectic phase with increasing LC temperature. Then, they incorporated chiral POSS molecules to LC, which increased the LC phase temperature [43,44]. The azobenzene LCP-POSS copolymers that have increased the LC temperature and reversible light-responsive properties were synthesized for the first time in our work. These hybrid copolymers with excellent LC behavior and light-responsive properties may be applicable in the LC display area. In our work, the incorporation of 47.61% azobenzene increased the structural ordering of LCP-POSS with the higher LC phase transition temperature, while the light-responsive property is basically unaffected.

Conclusions
The LCP-POSS with LC properties, light-responsive properties, and good thermal stability was synthesized through radical polymerization of modified AC-POSS and azobenzene LC, in which water-soluble AC-POSS is easy to remove after reaction. Due to the confinement of the rigid cage-like POSS, LCP-POSS exhibits better thermal stability and higher phase transition temperature. As the content of POSS gradually increases, the thermal stability of LCP-POSS gradually increased and the temperature at 10% decomposition of LCP-POSS-3 was 31.1°C higher than LCP. The LC phase transition temperature of LCP-POSS increased from 104.9°C to 139.9°C, and the polarized optical micrograph results further confirm the results. Incorporating about 53% POSS to LCP could not only keep the LC phase structure but also improve the thermal stability of LCP. As the content of azobenzene further decreased to 8.67%, the LC properties of the LCP-POOS-3 indicated the one-way LC phase behavior. Because of the steric hindrance effect, the addition of POSS to the LCP matrix reduces the cis-trans isomerization constant of azobenzene. However, the reversible photoresponsive behavior was still preserved, which has important application in nanodevice, optical switch, information storage, and liquid crystal display.