High-Stress Shear-Induced Crystallization in Isotactic Polypropylene and Propylene/Ethylene Random Copolymers

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• 作者：Zhe Ma ; Lucia Fernandez-Ballester ; Dario Cavallo ; Tim Gough ; Gerrit W. M. Peters
• 刊名：Macromolecules
• 出版年：2013
• 出版时间：April 9, 2013
• 年：2013
• 卷：46
• 期：7
• 页码：2671-2680
• 全文大小：521K
• 年卷期：v.46,no.7(April 9, 2013)
• ISSN：1520-5835

文摘

Crystallization of an isotactic polypropylene (iPP) homopolymer and two propylene/ethylene random copolymers (RACO), induced by high-stress shear, was studied using in situ synchrotron wide-angle X-ray diffraction (WAXD) at 137 掳C. The 鈥渄epth sectioning鈥?method (Fernandez-Ballester Journal of Rheology 2009, 53 (5), 1229鈭?254) was applied in order to isolate the contributions of different layers in the stress gradient direction and to relate specific structural evolution to the corresponding local stress. This approach gives quantitative results in terms of the specific length of fibrillar nuclei as a function of the applied stress. As expected, crystallization becomes faster with increasing stress鈥攆rom the inner to the outer layer鈥攆or all three materials. Stress-induced crystallization in a RACO with 7.3 mol % ethylene content was triggered at only 1 掳C below its nominal melting temperature. The comparison of iPP and RACO鈥檚 with 3.4 and 7.3 mol % ethylene monomer reveals the effect of ethylene defects on high-stress shear induced crystallization at 137 掳C. It is found that, for a given applied stress, the specific nuclei length formed by flow increases with ethylene content鈥攚hich is attributed to a greater high molecular weight tail. However, the linear growth rate is significantly reduced by the presence of ethylene comonomers and it is found that this effect dominates the overall crystallization kinetics. Finally, a time lag is found between development of parent lamellae and the emergence of daughter lamellae, consistent with the concept of daughter lamellae nucleated by homoepitaxy on the lateral faces of existing parent lamellae.