Insights into formation and relaxation of shear-induced nucleation precursors in isotactic polystyrene
Insights into formation and relaxation of shear-induced nucleation precursors in isotactic polystyrene
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DOI：
10.1021/ma071475e
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发表时间：
2008-02-26
期刊：
影响因子：
5.5
通讯作者：
Alfonso, Giovanni C.
中科院分区：
文献类型：
--
作者：
Azzurri, Fiorenza;Alfonso, Giovanni C.
对窄等规聚苯乙烯（i-PS）组分进行了流变光学实验，研究了剪切诱导成核前体寿命的温度和摩尔质量依赖性。与异聚（1-丁烯）和异聚丙烯类似，流动诱导结构的存活时间非常长，即使在远高于晶体测量熔点的温度下也是如此。已经观察到，成核前体浓度的降低遵循一级动力学，具有强烈的温度依赖性速率常数。整个弛豫过程的表观活化能约为400 kJ/mol，与以前发现的其他半结晶聚合物的数量级相同。对不同聚合物的表观活化能数据的比较分析，加上形态学证据，表明在熔体中系统向平衡状态演化的速率决定步骤是茎从流动诱导的定向束的侧表面的分离，在点状核的情况下，作为孤立的实体存在，或者作为由无序纳米畴改变的行，在长长的丝线里由于所研究的样品的单分散特性，取向成核前体的寿命的摩尔质量依赖性也已建立。
Rheooptical experiments on narrow isotactic polystyrene (i-PS) fractions have been carried out to investigate temperature and molar mass dependence of the lifetime of shear-induced nucleation precursors. Similar to i-poly(1-butene) and i-polypropylene, the survival of the flow-induced structures lasts very long, even at temperatures well above the measured melting point of the crystals. It has been observed that the decrease of concentration of nucleation precursors follows a first-order kinetics with a strongly temperature-dependent rate constant. The apparent activation energy of the overall relaxation process is around 400 kJ/mol, of the same order of magnitude previously found for other semicrystalline polymers. A comparative analysis of the apparent activation energy data for different polymers, coupled with morphological evidence, suggests that the rate-determining step in the evolution of the system toward the equilibrium state in the melt is the detachment of stems from the lateral surface of flow-induced oriented bundles, present either as isolated entities, in the case of pointlike nuclei, or as rows altemated by disordered nanodomains, in the long threads of the shish. Thanks to the monodisperse character of the investigated samples, the molar mass dependence of lifetime of oriented nucleation precursors has also been established.