Synthesis And Performance Of Cyclic Azobenzene-Containing Amphiphilic Diblock Copolymers

Compared with the linear azobenzene molecules, cyclic azobenzene provides unique photo-responsive behaviors and unparalleled physical properties because of the ring strain in a cyclic structure. In recent years, some researches about cyclic azobenzene have been documented, which promotes the development of azobenzene functional materials.In this work, a novel amphiphilic block copolymers PDMAEMAm-b-Pcyclic Azon, composed of hydrophilic PDMAEMA and hydrophobic Pcyclic Azo with cyclic azobenzene pendants were successfully synthesized via Reversible addition-fragmentation chain transfer(RAFT) polymerization. The self-assembly process of block copolymers in solution and the stimuli-response behavior to light, p H and temperature were investigated. Furthermore, compared with the homopolymer, we studied the surface relief grating(SRG) formation and birefringence of the block polymer. The detailed researches are as follows:(1) Firstly, we synthesized cyclic monomer containing azobenzene unit. Then, homopolymer PDMAEMA was synthesized via the RAFT polymerization of DMAEMA using 2-cyanoprop-2-yl 1-dithionaphthalate(CPDN) as a chain transfer agent, and then different kinds of well-controlled diblock copolymers with the same DMAEMA block length(m) and different cyclic Azo MMA block lengths(n) were prepared via RAFT chain-extension in anisole using PDMAEMA as the macro-RAFT agent and cyclic Azo MMA as monomer. The structures of resulting copolymers were confirmed via Gel permeation chromatography(GPC), 1H NMR, 13 C NMR and FT-IR spectra.(2) The self-assembly of PDMAEMA101-b-Pcyclic Azo31 was performed in the mixed solution of DMF and HAc/Na Ac solution to generate the thermodynamic stable micelles. The photoisomerization kinetics of cyclic azobenzene-containing diblock copolymer in THF solution and in its micelle solution were performed by UV-vis spectra, under 365 nm light irradiation and staying in the dark, respectively. We noted that the both k H of the copolymer in THF and in its micelle solution were much smaller than respective ke this can be attributed to the more stable conformation of cis-isomeric cyclic azobenzene pendants compared with that of its trans-analogue. The dipole moments of cis and trans isomers of the cyclic azobenzene pendants of the hydrophobic block were calculated, the result showed a decrease of the dipole moment of the trans and cis configuration of the cyclic azobenzene pendants through trans-to-cis photoisomerization of azobenzene moieties, in which the cis configuration is 4.0 D while trans form is 5.3 D. It becomes clear that the cyclic topology results in a decrease in the dipole moment of the trans and cis configuration of cyclic pendants through trans-to-cis photoisomerization of azobenzene segment. This implies a decrease of polarity(hydrophily) of the hydrophobic Pcyclic Azo block in the micellar cores, resulting in the hydrophilic/hydrophobic balance shifts in an opposite direction under UV light irradiation. As a result, it is reasonable that trans-to-cis photoisomerization of azobenzene segments in cyclic pendants is favorable for compact packing of micelles aggregates. Furthermore, the micelles can returned to the initial sizes in the dark and multi-responses of the formed sphere micelles to external stimuli, i.e. p H and light were also investigated.(3) The SRG formations of the films of homopolymer and diblock copolymers containing the cyclic azobenzene were investigated, which showed a fact that the modulation depth(M.D.) of homopolymer was larger than that of the diblock copolymers under identical experiment conditions. Moreover, the M.D. diminished with the decrease of azobenzene unit content in polymers. The grating relative diffraction efficiency was also decreased in shorter hydrophobic block with the same exposure time. Furthermore, birefringence value of homopolymer was higher than that of diblock copolymers. Interestingly, in diblock copolymers, higher azobenzene content reduced the birefringence value, revealing that the number and volume size of azobenzene functioned together as a consequence.