Inducing Crystallization of Polymer through Stretched Network
Inducing Crystallization of Polymer through Stretched Network
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DOI：
10.1021/ma802679h
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发表时间：
2009-02
期刊：
影响因子：
5.5
通讯作者：
Baijin Zhao;Xiangyang Li;Youju Huang;Yuanhua Cong;Zhe Ma;Chunguang Shao;Haining An;T. Yan;
中科院分区：
文献类型：
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作者：
Baijin Zhao;Xiangyang Li;Youju Huang;Yuanhua Cong;Zhe Ma;Chunguang Shao;Haining An;T. Yan;
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Introduction. Flow-induced crystallization of polymers is not only a fundamental nonequilibrium thermodynamic problem but also of importance for polymer processing and the final properties of polymer products. During polymer processing, such as extrusion, injection molding, and fiber spinning, polymer materials are subjected to various flow fields. Flow can enhance the crystallization rate up to several orders of magnitude and induce the formation of the so-called row nuclei or shish-kebab structure, which significantly increases the stiffness and thermo deformation temperature. Although the benefit from flow field is widely recognized in academia and polymer industry, no satisfactory molecular theory has been achieved yet. To explain flow-induced crystallization of polymer, entanglement-disentanglement (EDT) and coil-stretch (CS) transitions have been the basic ideas dominated in the community date back to Keller and de Gennes. They suggested that the flowinduced shish-kebab structure composes of extended long chains as row nuclei or shish and folded-chain crystals as kebab. Some recent elegant experiments and computer simulations further support the nucleation role of the oriented long chains. However, a different picture presented by Kimata et al. shows that the short chains or medium chains are more involved in the formation of shish. Indeed, more experimental evidence does not fit in the initial shish-kebab mechanism. Hsiao et al. observed that a single lamella can grow from multiple shish instead of a single shish. Zhang et al. showed that it is possible to produce row nuclei with a small shear rate and suggested EDT may not be a necessary condition for the formation of row nuclei. Without EDT, the stretched transient network is sufficient to accelerate crystallization and induce shish-kebab structure. With evidence from different facets, the two essential questions still remain as open debates, which prevent from further pursuing the molecular mechanism of the flow-induced crystallization of polymer. (i) What is the role of long and short chains in the formation of row nuclei or shish (or oriented nuclei in general)? (ii) What is the role of EDT in the formation of the oriented nuclei? We use “oriented nuclei” to emphasize on the enhancement of flow on nucleation in this work, as different structures such as point precursor, row nuclei, or shish-kebab can be produced by flow. In this Communication, two simplified mixtures containing short free chains and chemically end-linked network were designed to answer the above-mentioned two questions, which resemble the transient network of polymer blend with short and long chains. A schematic picture is illustrated in Figure 4A. These systems minimize the interferences of relaxation and macroscopic inhomogeneous of flow field, which may complicate the interpretation on the observed experimental phenomenon. Similar systems had been employed to verify the initial reptation theory and other mechanism related to polymer dynamics. Low molecular weight poly(ethylene oxide) (PEO) with a molecular weight of 2000 g/mol (PEO2F) which has two acetyl end groups was used as the free chains. To build the end-linked network, the mesh size with molecular weights M of 2000 and 6000 g/mol, denoted as PEO2N and PEO6N, were chemically cross-linked with 1,3,5-benzenetricarbonyl trichloride, respectively. As the critical entanglement molecular weight Me of PEO is about 2000 g/mol, 23 and the chemical cross-linking points in network were considered as the permanent entanglements points, PEO2N resembles the high molecular weight PEO with permanent “entanglements”, while PEO6N represents a polymer melt with about two possible disentanglement points.