講者：鄒海雄博士 演講題目：Sequence Distribution and Elasticity of Propylene-Based Elastomers 演講時間：3/3（五）15:30~17:20 演講地點：博雅館 201 主持人：李篤中教授 Dr. Andy H. Tsou (鄒海雄博士) 現職 : Senior Research Associate ExxonMobil Chemical Co. 經歷 : Purdue Ph.D.; 125篇學術期刊，85項專利，30多年之高分子研發實務經驗及榮獲多項國際獎章。
Sequence Distribution and Elasticity of Propylene-Based Elastomers
Andy Tsou, Alex Norman, and John Hagadorn
ExxonMobil Chemical Company
Baytown, Texas, USA
Conventional propylene-based elastomers are ethylene-propylene random copolymers synthesized by coordinative insertion polymerization using C2-symmetric metallocene catalysts. Ethylene co-monomer defects, at 10 to 20 wt% of ethylene, are utilized to break up the isotactic propylene backbone sequences so to restrict polypropylene crystallization, lower the polypropylene crystallinity, isolate individual polypropylene crystallites, and minimize crystallite assembly. By suppressing crystallization and crystallinity in propylene-based elastomers, elastomer network morphology consisting spatially dispersed and isolated polypropylene crystallite cross-links connected via entropic springs of amorphous ethylene-propylene copolymer segments can be established and resulting elastic properties can be obtained. These propylene-based elastomers are random copolymers with their reactivity ratio products (r1r2) being 1 and have random backbone sequence distributions. Propylene based elastomer copolymerized by non-random copolymerization, where r1r2 ≠ 1, have not been synthesized and evaluated before. In order to understand the effects of non-random sequence distribution in propylene based elastomers on their crystalline morphology and elastic properties, three iso-specific organometallic catalysts for ethylene and propylene copolymerization with varying r1r2 parameters were utilized to synthesize equal-molecular-weight propylene-based elastomers (PBEs) of alternating, random, and blocky backbone sequences. To compensate for the variations in catalyst iso-specificity and thus to maintain a constant amount of 50% isotactic propylene trimer concentration, the ethylene content of PBEs was varied between 11 wt % and 16 wt %. Although all three PBEs have an equal amount of crystallizable sequences, the high-C2 (high-ethylene) blocky PBE, bPBE, was found to have the highest crystallinity and crystallization rate, while the low-C2 alternating PBE, aPBE, possesses the lowest crystallinity and rate. However, a higher crystallinity in bPBE does not lead to more crystallite assembly. Instead, bPBE has the least crystallite assembly whereas aPBE, having the lowest crystallinity, has the most crystallite assembly. At the same time, both aPBE and bPBE have thicker crystallites with corresponding higher melting temperatures and crystallite strengths than those found in rPBE, a randomly copolymerized PBE, with bPBE having the thickest crystals. This suggests that non-random sequence distribution of a PBE affects its crystallization and crystalline morphology development which deviate from those in random copolymerized PBEs and do not follow crystallization theory developed for random copolymerization. With its thickest crystallites and tightest lamellae (strong crosslinks) and its least crystallite assembly (least plastic flow), bPBE has the lowest elastic set and hysteresis among the three PBEs evaluated. It does have the undesirable highest top load resulting from its high crystallinity. aPBE has the lowest top load from its low crystallinity and has lower hysteresis than that of rPBE for its thicker crystals. Thus, it is desirable to blend bPBE with aPBE for a bimodal PBE provided that the molecular weight and ethylene content of aPBE are raised. Increasing ethylene content in an aPBE is to lower the crystallinity so to prevent crystallite assembly, but this reduction in crystallite crosslinks needs to be balanced by an increase in entanglement crosslinks through the increase in molecular weight. Bimodal and co-continuous blends, designed in accordance to phase continuity criterion, of high MW alternating PBE and moderate MW blocky PBE both of the same ethylene content were prepared with bimodalities in MW and in sequence distributions. It was found that these bimodal and co-continuous PBE blends possess excellent elastic properties with low tension set, hysteresis, and top load.