剪切条件下等规聚丙烯的结晶行为

Crystallization Behavior of Isotatic Polypropylene Under Shearing

聚丙烯是工业中应用最广泛的聚合物。聚丙烯在静态结晶时生成球晶 ,剪切后会生成排核 ,最终长成柱晶或纤维晶 ;在强剪切条件下会生成串晶。聚丙烯是一种存在多种晶型的聚合物 ,α型晶是热动力学最稳定的晶型 ,在一般实验条件下很难得到其他晶型 ;在剪切条件下聚丙烯会生成 β晶 热动力学亚稳晶型。剪切会显著影响聚丙烯的结晶动力学 :增加球晶的生长速率 ,缩短结晶诱导时间 ,增加活化晶核密度。人们提出了许多模型来解释剪切加速结晶动力学的实验现象 ,但是都有不足之处。

Isotatic polypropylene(i-PP)is a polymorphic material with several known crystal modifications,such as monoclinic(α),hexagonal(β)and triclinic(γ).Of these polymorphs,the α form is the thermodynamically stable crystalline modification and is predominant under common processing conditions.But iPP may form the β-phase under shear conditions.By tensile and shear loads,polymeric molecules are stretched and oriented into rows in the molten state,the row nucleus transform into cylindrites or fibril crystallites.The effect of shear on crystallization kinetics is characterized in the following aspects:shear accelerates the crystalline growth rate,decreases the induction time and increases the activated nuclei density.The effect of flow on the kinetics can be understood by considering that the increase of the degree of order due to flow results in an effective change of the melt energy.On the basis of this concept,several empirical approaches have been proposed,but these models have some limits.