Preparation And Performance Of Photoresponsive Poly(Arylene Ether)s Containing Azobenzene Chromophores

Polymers bearing azobenzene moieties (azo-polymers) are fascinating materials and have attracted considerable attention in the past few years because of their unique reversible photoisomerization and photoinduced anisotropy of the azobenzene chromophores. The photoinduced isomerization and photoinduced anisotropy can cause significant bulk, surface property variation and polarity of the polymers, such as photoinduced phase transition, photoinduced birefringence and photoinduced surface relief gratings (SRGs) et al. They can be used in the fields of optical data storage, optical switch, electro-optical (EO) modulators, and nonlinear optical materials, etc.Research efforts to explore new azo-polymers have predominantly focused on the linear azo-polymers with side-chain or main-chain architectures through doped, covalently, H-bonded or ionic interactions, et al. However, most of polymer materials studied for optical storage were based on aliphatic backbones, which usually had poor thermal stability and low Tgs and prevented their application as optical and electronic devices using under a harsh environment. There is a continuous demand for high-performance polymers as optical and electronic devices using under a harsh environment. However, most of polymer materials studied for optical storage are based on aliphatic backbones, which usually have poor thermal stability and low Tgs. Poly(aryl ether)s (PAEs) are a family of high-performance engineering thermoplastics with excellent thermal, mechanical and electrical properties. Functionalized poly(aryl ether)s have been widely studied as proton exchange membranes (PEMs), light emitting materials and optical materials. To the best of our knowledge, few azobenzene functionalized PAEs (azo-PAEs) as photoactive materials have been reported so far. In this work, we report the synthesis and characterization of a series of azobenzene chromophore covalently or host-guest doped complexes of PAEs materials. A detailed evaluation of their physical properties, photoinduced birefringence and the formation of SRGs are also reported.First, a series of azobenzene functionalized poly(arylene ether)s (M-azo-PAEs) have been synthesized via nucleophilic aromatic substitution polycondensation and large amount of azobenzene chromophores could be introduced into the main chain of the polymer easily. Their chemical structure and properties were fully characterized and evaluated by means of IR, UV-vis, 1H NMR, DSC and TGA. The results suggest that azo-PAESs have high glass transition temperatures (Tg > 183 oC), good thermal stability (Td5 > 386 oC) and homogeneous photochromic behaviors. On irradiation with a linearly polarized 532 nm Nd: YAG laser beam, the azo-PAESs present stable photoinduced birefringence. By exposing the spin-coated films to an interference pattern laser beam, the azo-PAESs could be used for fabrication of thermal stable surface relief gratings. The SRGs have excellent thermal stability and can keep a good shape until 150 oC.Second,a series of azobenzene functionalized poly(arylene ether) copolymers (S-azo-PAEs) containing azo moieties on the pendants were prepared by direct copolymerization. Large amount of azobenzene chromophores could be introduced into the side chains of the polymer easily during the polycondensation step. Their chemical structure and properties were fully characterized by means of IR, UV-vis, 1H NMR, DSC, and TGA. The results suggested that azo-PAEs had high glass transition temperatures (Tg >159 oC), good thermal stability(DT5 > 403 oC) and homogeneous photochromic behaviors. The azo-PAEs present stable photoinduced birefringence upon irradiation with 532 nm Nd: YAG laser. By exposing the spin-coated films to an interference pattern laser beam, the azo-PAEs could be used for fabrication of thermal stable surface relief gratings (SRGs). Both the birefringence values and the relief depths of SRGs could be adjusted by controlling the content of the azo chromophores on the pandent of the azo-PAEs.In comparison with the linear poly(arylene ether)s, hyperbranched poly(arylene ether)s (HPAEs) possess highly branched structure besides good thermal properties. To the best of our knowledge, no azobenzene functionalized HPAEs as optical storage materials have been reported so far. The presence of highly branched structure may facilitate trans/cis isomerization of azo groups. Thus the combination of HPAEs backbone with photoactive azo groups could provide a new approach to develop novel high-performance materials with optical properties. Following this guidance, in the third part of this work, two kinds of HPAEs with different azobenzene chromophore substitute located in both main chain and side chain of the branched arms (azo-HPAEs) were synthesized by using B3+A2 methodology via a nucleophilic aromatic substitution polycondensation. Structures of the azo-HPAEs were characterized by means of IR, UV-visible, 1H NMR, and XRD. It is shown that both the azo-HPAEs exhibit high glass transition temperatures (Tgs), excellent thermal stability and homogeneous photochromic behaviors. By exposing their spin-coating films to an interference pattern of laser beam, both the azo-HPAEs could be used for rapid (within 30 s) fabrication of surface relief gratings (SRGs). The obtained SRGs present no shape changes even at temperature up to 200°C. Especially, the azo-HPAE with azobenzene moieties in the side chain shows larger photoinduced birefringence intensity and better reversible optical storage than the one with azobenzene moieties in the main chain upon irradiation with 532 nm Nd: YAG laser. The hyperbranched azo-polymers are expected to be promising materials with application potentials in holographic memories, reversible high density optical storage, optical switch, and other photo-driven devices.To avoid the tedious synthesis of azo-polymers, the method of doping normal polymers (host) with azobenzene-type compounds (guest) has been introduced for photoinduced birefringence and photofabrication of SRGs. Based on supramolecular interaction between the azobenzene chromophores and the polymer chains, uniform and transparent thin films of host-guest azo-polymer complexes were got. The host-guest doping method allows for easy variation of the contents and types of the azo chromophores. Host-guest complex films based on azo chromophores and polymers have been considered as promising materials for fabrication of SRGs, optical information storage, holographic recording and other optoelectronic areas. On irradiation with a linearly polarized 532 nm Nd: YAG laser beam, the host-guest complex films present fast and high photoinduced birefringence, but the stablility of the birefringence is very poor. When exposed to interferential illumination, these complexes can undergo reversible photoinduced mass migration to form SRGs that are stable over long time periods. These SRGs can be erased by exposing the patterned film under uniform circularly polarized laser light or by heating above the glass transition temperature of the polymer. The above SRGs technique also provides new methodology to obtain rewritable micropatterns. In the last part of this work, a facile method for fabricating fluorescent patterned polymer film by combination of the fluorophore-/azo- host-guest complexes and the holography technique was demonstrated for the first time. It is notable that the color of the fluorescent 2D-photopatterns could be easily tuned by changing the corresponding fluorophores. These studies provide a fascinating and viable strategy to design and fabricate patterned organic light-emitting device, provide a convenient method to detect the SRGs on the surface of the polymer film, and also open up a new way of recording and detecting optical information.