Preparation And Photoresponsive Behavior Of Novel Azobenzene-Functionalized Polymers

Polymers bearing azobenzene moieties (azobenzene polymers) combine the unique optical trans-cis-trans isomerization behavior of azobenzene and the good processability and mechanical properties of polymer materials. These azobenzene polymers can be potentially applied in fascinating photo-responsive variations, such as liquid crystal displays, optical data storage, nonlinear optical materials and so on, which have attracted increasing attention in the past few years. Therefore, develop and design of new azobenzene polymers with the novelty structure and excellent optical activity has been an important assignment for polymer chemistry.In this thesis, we designed and synthesized a series of azobenzene polymers with the novel structures. Their trans-cis isomerization and so caused properties change were investigated. The detailed researches were summarized as the following:(1) A novel azobenzene-containing dithiocarbamate, 1-phenylethyl N, N'-(4-phenylazo)phenylphenyldithiocarbamate (PPADC), was successfully synthesized. PPADC was used as RAFT agent, the reversible addition-fragmentation chain transfer (RAFT) polymerization was well controlled in the case of MA. However, there was the slightly ill-controlled in the case of St, which was affected by a less reactive double bond due to the delocalization of the nonbonded electron pair on the nitrogen with the thiocarbonyl group. Interestingly, the polymerization of St could be well-controlled when using PPADC as the initiator via atom transfer radical polymerization (ATRP) technique. The obtained polymer was characterized by 1H NMR analysis, ultraviolet absorption, FT-IR spectra analysis and chain-extension experiments. The polymerizations by RAFT and ATRP system were both well controlled. Furthermore, the photoresponsive behaviors of azobenzene-terminated poly(methyl acrylate) (PMA) and polystyrene (PS) were similar to PPADC. The isomerization rate constants (kexps) of PPADC, PMA and PS were 0.00651 s-1, 0.00515 s-1 and 0.00733 s-1, respectively.(2) The well-defined block copolymers, poly(vinyl acetate)-b-poly(styrene)s (PS-b-PVAc) containing middle azobenzene moiety were successfully synthesized by a combination of RAFT and"Click"chemistry. This novel method provided an efficient way to prepare middle functionalized block copolymer: firstly,α-alkyne and azobenzene chromophore terminated poly(styrene) (PS), andω-azido-terminated poly(vinyl acetate) (PVAc) were designed via RAFT technology. Secondly,''Click''reaction was performed using the combination of CuBr and PMDETA as catalyst system. Thus, well-controlled block copolymer poly(vinyl acetate)-b-poly(styrene) (PS-b-PVAc) was obtained. Block copolymers (PS-b-PVAc) were demonstrated by GPC, 1H NMR, FT-IR spectra and differential scanning calorimetry (DSC) analysis. Furthermore, the trans-cis-trans isomerization of PS-b-PVAc was also observed in chloroform solution. The ke and kH of PS-b-PVAc was 0.0027 and 2.2×10-4 s-1, respectively.(3) The well-defined photo-responsive alternating copolymer containing azobenzene chromophore, poly(4-(N-maleimido)azobenzene-alt-styrene) (PMSt) was successfully synthesized from the copolymerization of 4-(N-maleimido)azobenzene (MAB) and styrene (St) via RAFT polymerization using CPDN as the RAFT agent and AIBN as an initiator in 1,4-dioxane solution. The copolymerization was well controlled, which showed polymers with controlled molecular weight and narrow molecular weight distribution. The alternating structure of the copolymers PMSts were characterized by 1H NMR, 13C NMR, UV-vis absorption and FT-IR. Furthermore, The highly branched azobenzne copolymers with high molecular weight were successfully obtained after adding N, N-4,4-diphenylmethyenebismaleimide (BMI) into the polymerization system. The copolymers showed high glass transition temperature (Tg = 174 ~ 250 oC) and decomposition temperature (Tg = 174 ~ 250 oC). On irradiation with a linearly polarized Kr+ laser beam, the diffraction efficiency of SRG decreased along with increasing the molecular weight of polymer film. For the same polymer, film with high thickness showed high SRG diffraction efficiency. The formed SRG on PMSt1 film was stable even at 150 oC due to the high Tg of PMSt1.(4) A novel methacrylate monomer containing azobenzene chromophore and tetrazole moiety, 4′-(2-methacryloxyethyl)methylamino-4- (5-chlorotetrazol-1-yl)azobenzene (MACA), was synthesized and polymerized to form homopolymers (PMACA) and block copolymers (PMMA-b-PMACA) via reversible addition-fragmentation chain transfer (RAFT) polymerization. The structures of these polymers were characterized by 1H NMR spectroscopy and FT-IR spectra, and gel permeation chromatography (GPC) characterization, which indicated that polymers with controlled molecular weights and narrow molecular weight distributions (Mw/Mns <1.30). Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) results indicated these polymers were amorphous. Furthermore, the photoisomerization of azobenzene polymers was investigated, it should be indicated that in the beginning period of irradiation (3 minutes), the absorption (430 nm) of trans-form azobenzene slightly increased upon UV irradiation, which may due to the bulky tetrazole substitution on the azobenzene. After UV irradiation of 3 minutes, the trans- form content of azobenzene was increased irradiation time of 365 nm UV light. Furthermore, during the tran-cis isomerization, the absorption peak at 430 nm was a"red-shift"peak from 413 to 513 nm. The electrochemical behavior of polymers in solution before and after UV irradiation was investigated using the cyclic voltammetry (CV), the oxidation peak in CV curve obviously shifted to 0.6V. The electrochemical behavior can be adjusted by the UV irradiation, which may find their applications in photo electronic materials.(5) A novelα-azide andω-alkyne A-B type azobenzene monomer, 3'-enthynylphenyl[4-(4-azidobutoxy)phenyl]azobenzene (EAPA), was synthesized and used to generate a novel polymer via step-growth polymerization using"Click"chemistry. The structure of the resultant main-chain azobenzene polymer, PEAPA, was characterized by GPC, 13C-NMR, ultraviolet and FT-IR spectra. Thermal stability and crystallinity of PEAPA powder was studied by TGA and WAXD. The obtained linear main-chain azobenzene polymer PEAPA containing 1, 2, 3-triazol group showed a good thermal stability and crystallinity, for example: PEAPA1, Tg = 134 oC, Td = 357 oC, which was due to the introduction of the triazole ring in the polymer backbone. Comparing the rate constant of trans-cis photoisomerization of monomer EAPA (ke = 0.022 s-1), the corresponding value of PEAPA was much slower (ke = 0.0088 s-1). The reason was considered due to sterically hindering effect of the main-chain configuration and the triazole group in PEAPA2. However, the cis-trans thermal isomerization behavior of PEAPA2 (7.4×10-4 s-1) was similar to EAPA's (5.4×10-4 s-1).(6) The main-chain azobenzene polymer PEHPA2 was obtained through thermal 1, 3-dipolar cycloaddition technique. All of these polymers showed high thermally resistance (Td above 330 oC). The polymer PEHPA2 showed good solubility in common organic solvents due to the formation of regiorandom structures, and the 1,4-regioisomeric ratios (F1,4, F1,4') of PEHPA2 were calculated to be 78 % from NMR spectrum of PEHPA2. The photo-induced trans-cis isomerization of the polymers in CHCl3 solution was examined, and the photoisomerization rate constant was 0.010 s-1. The film prepared from PEHPA2 showed efficient surface relief gratings (SRGs) formation ability. The diffraction efficiency from SRG with film thickness of 465 nm was measured to be about 0.82 %.