Mechanism of Flow Induced Crystallization of Polymers

[1]剪切流下结晶的直接观察（Takimoto）研制了一种原始的剪切流下结晶的直接观察装置。研究发现，随着剪切应力的增加（即剪切速率和/或分子量的增加），球晶的成核速率大大加快，但生长速率仅受微弱影响。在高剪切应力下，观察到排形核结构切变流动下结晶热与粘度的同时测量（Koyama）我们研制了一种切变流动热流变仪，可以同时测量切变流动下结晶热与粘度。当η(Xc)=η(0)exp（αXc）时，剪切粘度η随相对结晶度Xc的增大而增大。α常数取决于剪切速率，这表明在相同的Xc下，不同剪切速率下的形貌是不同的我们改进了一种PVT测量系统，使剪切流可以在高压下应用。结果表明：1)压力对结晶速率的影响可以用熔化温度的变化来描述；2)剪切速率的影响可以用不受结晶温度和结晶压力影响的感应应变来描述；3)压力和剪切速率的影响是可以分离的采用分子动力学模拟方法研究了剪切流动条件下聚乙烯低聚物的结晶过程。证明了分子的流动排列和成核的加速。

[1]Direct observation of Crystallization under shear flow (Takimoto)An original device for the direct observation of crystallization under shear flow has been developed. It has been found that the nucleation rate of spherulites is strongly accelerated as the shear stress increases (i.e., shear rate and/or molecular weight increase), but the growth rate is only weakly affected. At high shear stress, the row nucleated structure is observed.[2] Simultaneous measurement of heat of crystallization and viscosity under shear flow (Koyama)We have developed a shear flow thermal rheometer which can measure the heat of crystallization and viscosity under shear flow simultaneously. It has been found that the shear viscosity η increases with the relative crystallinity Xc as η(Xc)=η(0)exp(αXc). The constant α depends on the shear rate, which indicates that the morphology at the same Xc differs for different shear rates.[3] PVT measurement under shear flow (Koyama)We have modified a PVT measurement system so that shear flow can be applied under high pressure. It has been found that i) effect of pressure on the crystallization rate can be described by the shift of the melting temperature, ii) effect of shear rate can be described by a induction strain which is independent of crystallization temperature and pressure, and iii) effect of pressure and shear rate can be separated.[4] Molecular Dynamics Simulation (Masubuchi)Crystallization of polyethylene oligomer under shear flow was studied by molecular dynamics simulation. Flow alignme of the molecules and the acceleration of the nucleation have been demonstrated.