Hydrodynamically induced crystallization of polymers from solution
Hydrodynamically induced crystallization of polymers from solution
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DOI：
10.1007/bf02086621
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发表时间：
1970-02
期刊：
影响因子：
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通讯作者：
A. Pennings;J. Mark;H. C. Booij
中科院分区：
文献类型：
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作者：
A. Pennings;J. Mark;H. C. Booij
详细描述了在搅拌的线性聚乙烯溶液中结晶过程中观察到的几种现象。研究发现，简单的剪切流不能在高温下在线性聚乙烯（Marlex）的 5% 二甲苯溶液中加速初生核的形成。当库埃特仪器内筒的搅拌器速度超过对应于泰勒涡流开始的最小值时，注意到纤维状晶体形成的开始。围绕搅拌器周期性出现的纤维状晶体的闭环，在与泰勒涡中心重合的轨道上旋转。湍流似乎对诱导原纤维结晶最有效，并且使用 Marlex 的 5% 二甲苯溶液可达到 112°C 的最高结晶温度。提出了一种形成束状初级核的机制，该机制基于涡流之间的伸长流区域中聚集体的取向和变形。高分子量聚乙烯（Mw=1.5×106）溶液在第一根纤维出现之前表现出明显的魏森伯格效应。法向应力会降低库埃特流中发生涡流的搅拌速率。这可以解释在搅拌器速度下观察到的高分子量聚乙烯的原纤维结晶，在牛顿流体的情况下根据泰勒准则产生层流。纤维网络在搅拌器上的沉降是由二次流引起的。
A detailed description of several phenomena observed during crystallization in a stirred solution of linear polyethylene is presented. It is found that simple shear flow is incapable of accelerating the formation of primary nuclei at elevated temperatures in 5% xylene solution of linear polyethylene (Marlex). The onset of fibrillar crystal formation is noted when the stirrer speed of the inner cylinder of aCouetteinstrument exceeds a minimum value which corresponds to the onset ofTaylorvortices. Closed loops of fibrillar crystals occurring periodically around the stirrer, revolve in orbits which coincide with the centers of theTaylorvortices. Turbulent flow seems to be most effective in inducing fibrillar crystallization and a maximum crystallization temperature of 112°C was achieved with the 5% xylene solution of Marlex. A mechanism is proposed for the formation of primary nuclei of bundlelike type which is based upon orientation and deformation of aggregates in regions of elongational flow between the vortices. Solutions of high molecular weight polyethylene (Mw=1.5×106) exhibit a pronouncedWeissenbergeffect prior to the appearance of the first fibers. Normal stresses lower the rate of stirring inCouetteflow at which vortices occur. This may account for the observed fibrillar crystallization of the high molecular weight polyethylene at stirrer speeds which in case ofNewtonian fluids give rise to laminar flow according to theTaylorcriterion. Settlement of the fibrous network on the stirrer results from secondary flow.