Gradient Copolymers: Controlled Preparation, Properties And Shape Memory Functions

Chain sequence structure (CSS), as a new micro-structural parameter, has received considerable attensions since living polymerization techniques was invented. Along with chemical composition, molecular weight and molecular weight distribution (MWD), it would open up a wide space for tuning polymeric properties. Gradient copolymer, CSS of which is just between random and block copolymers, is such a new kind polymer whose monomer compositional profile along the copolymer chains is gradually varied from one end to the other. It holds promising as compatilizers of polymer blends, emulsifiers, and damping materials. Currently, the gradient copolymers have been tailor-made via a model-aided monomer feeding strategy in a controlled/living radical solution polymerization. The method suffers from some problems like low molecular weight of product, high dead chain percentage, and long polymerization time. Also, the influence of CSS on the polymeric properties has been scarcely investigated in a symstematic way.In the current dissertation, we invented a new facile method tailor-making gradient copolymers via RAFT emulsion polymerization. Then, the influecence of CSS on viscoelasticity, mechanical and dielectric properties was investigated. Finally, a general strategy based on gradient polymer to synthesize multi-shape memory materials was proposed and explored. The main conclusions are as follows.(1) The phenomeneon of broadened MWD was investigated in the preparation of PS-PnBA-PS triblock copolymers by RAFT emulsion polymerization. It was found that the broadened MWD was caused by radical transfer reactions to PnBA leading to the formation of long-chain branches. The mass transfer properties of the poly(acrylic acid) (PAA) interfacial layer of the latex particles was found to exert significant influence on the formation of long-chain branches. Lowering the neutralization level of AA, which decreased the diffusion barrier of the small radicals from the aqueous phase into particles, could suppress the formation of long-chain branches. As a result, the polydispersity of MWD (PDI) could be lowered to a reasonable level.(2) A many-shot RAFT emulsion polymerization was invented to tailor-make gradient copolymer. Styrene/n-butyl acrylate gradient copolymers with molecular weight of 90kg/mol could be tailor-made in 7 hours. The molecular weights and compositions agreed well with design values. The principle behide the method could be extended to other monomer pairs. This new method offered a facile and efficient way to tailor-make gradient copolymers of high molecular weight.(3) Five kinds of styrene/n-butyl acrylate copolymers with the same composition, molecular weight and PDI but different CSS including random, diblock, triblock (S-b-nBA-b-S, S for polystyrene, BA for polybutyl acrylate), linear gradient (S-gradient-nBA) and V-shaped gradient (S-gradient-nBA-gradient-S) were made by design. The influence of CSS on material properties was investigated. The following conclusions were drawn:(a) Glass transition temperatures (Tg) determined by DSC suggested an obvious effect of CSS. The block copolymers had two distinct TgS, located nearby the corresponding homopolymers’Tgs, respectively. The random copolymer showed only one Tg in the middle of the homopolymers’Tgs. The gradient copolymers had a very broad and continuous glass transition region from PnBA’s Tgto PS’s one.(b) Phase separation was directly observed by AFM. The random copolymer was homogeneous but the block copolymers showed obviously separated nano-phases with clear and sharp phase boundaries. The gradient copolymers showed intermediate nano-phase separations. Two-phases with domain sizes similar to block copolymers were clearly observed. However, the phase boundaries were rather vague and broad. There existed a transitional region composed of the copolymers with different compositions.(c) Dielectric properties could be impacted by the CSS. The block copolymers showed strong interfacial polarization under low frequencies (<1Hz), which led to a sudden jump of dielectric constant and loss. However, the broad and gradually composition-changed phase boundaries of the gradient copolymers suppressed the interfacial polarization, suggesting better dielectric stability.(d) CSS could exert significant influence on mechanical properties. On one hand, because of the formation of physical crosslinking networks, the triblock and V-shaped gradient copolymer showed much better mechanical properties in terms of tensile strength and elongation at break than the diblock and linear gradient copolymer. On the other hand, the broad phase transitional regions in the gradient copolymers could help transfer the stress during deformation, which could increase the ductility. The linear gradient copolymer showed much higher elongation at break than its diblock counterpart. Moreover, the mechanical properties of the gradient copolymers exhibited obvious temperature sensitivity.(4) The rational designed V-shaped gradient copolymer (S-gradient-nBA-gradient-S) exhibited a good multi-shape memory polymer. It was demonostrated that four shapes could be memorized in one memory cycle. The outest poystyrene segments introduced physical cross-links into the gradient copolymers, which strongly enhanced the shape memory performances. The broad Tg offered many transitions to trigger shape lock and recovery.