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Abstract: Four kinds of well defined poly(N-methacryloyl-L-amino acid)s were prepared via reversible addition fragmentation chain transfer polymerization. The effects of molecular structure, molecular weight, and end-group on their responsive properties in water were investigated. We found that the monomer structure is critical for the polymer to exhibit stimuli-responsive properties in water; only polymers derived from aspartic acid showed defined dual thermo- and pH-responsive properties. To study the effects of molecular weight and end group on the lower critical solution temperature behavior of poly(N-methacryloyl-L-β-isopropyl aspartic acid) (PMAIPAC), a series of PMAIPAC homopolymers with controlled molecular weights and narrow polydispersities were prepared and treated with excess radical initiators to give defined end groups. The cloud point of PMAIPAC was found to increase with the increase of solution pH, due to the ionization of carboxylic acid groups, and decrease with the increase of molecular weight. The structure of end groups also affects the cloud point of PMAIPAC particularly for low molecular weight samples at low pH. [Copyright &y& Elsevier]

Tensile deformation behavior of bimodal polyethylene (PE) was investigated by employing coarse-grained (CG) molecular dynamics simulations based on a united atom model. The bimodal PE was modeled by blending two linear polyethylene chains with different molecular weights. The mechanical response and corresponding conformational behaviors of the polymer melt during stretching were recorded as a function of strain under both high and low strain rates. We find that the tensile toughness was enhanced in an additive fashion by increasing the fraction of polymers with high molecular weight in bimodal PE. During elongation, the polymer chain extends and orients itself along the loading direction. Surprisingly, varying the bimodal distribution and the strain rates, the disentanglement does not synergistically follow the tendency of chain expansion and ordering along the tensile direction as the strain is gradually raised, implying that multiple deformation modes might exist during the plastic flow process. The molecular origin of this unique development of the entanglement network in bimodal PE is discussed in detail. Image 1 • Tensile mechanical response of bimodal PE melts is elastomer-like. • Bimodal distribution and strain rate both influence degree of orientational order. • Disentanglement is strongly coupled with intermolecular non-bond energy. [ABSTRACT FROM AUTHOR]

The properties and applications of silicone elastomers depended on the molecular structure and composition of polysiloxane precursors, which were the most technologically important silicon-based polymers. Preparation of perfect diphenyl silicone elastomers was challenging due to the unexpected branches formed during the ring-opening copolymerization (ROCP) of phenylcyclosiloxane monomer. In this contribution, diphenyl silicone elastomer precursors with controlled structure were successfully prepared by cyclic trimeric phosphazene base (CTPB) catalyzed ROCP of octamethylcyclotetrasiloxane (D 4), octaphenylcyclotetrasiloxane (D 4 8Ph), and 2,4,6,8-tetravinyl-2,4,6,8-tetramethylcyclotetrasiloxane (D 4 Vi) with high molecular weight up to 551.0 kg/mol and dispersity (Đ) ≤ 2.50. 29Si NMR characterization verified the random microstructures of the resulting diphenyl containing polysiloxanes without chain branching. Moreover, the silica-filled elastomer prepared from poly(dimethylsiloxane- co -vinylmethylsiloxane) (PMVS) and poly(dimethylsiloxane- co -diphenylsiloxane- co -vinylmethylsiloxane) (PMPVS) precursors exhibited very high elasticity, which can be stretched to 1043% with a high tensile strength of 7.7 MPa. Thermal oxidation aging test proved the excellent thermal aging resistance of the phenyl-containing silicone elastomers. This work demonstrated facile method to prepare high quality diphenyl containing polysiloxane precursor toward robust silicone elastomers. [Display omitted] • Diphenyl silicone elastomer precursors with controlled structure were successfully prepared by CTPB catalyzed ROCP. • Silica-filled elastomer prepared from PMVS and PMPVS precursors exhibited very high elasticity. • Thermal oxidation aging test proved the excellent thermal aging resistance of the phenyl-containing silicone elastomers. [ABSTRACT FROM AUTHOR]