剪切均匀性对流动诱导等规聚丙烯结晶的影响

固定应变和最终应变速率,采用瞬时和缓慢2种电机加速方式对样品施加剪切,研究了流场加载模式对样品流变和结晶行为的影响.实验结果显示缓慢加速能够消除剪切过程中流场的非均匀性,使样品取向度增加,提高流场对聚合物熔体的作用效果.同时,流动诱导结晶对于加速时间有依赖性.对于速率为17.7 s-1的剪切,加速时间为1 s时,熔体流动均匀且流动诱导的晶体取向最强,短加速时间(0.5 s)和长加速时间(1.5 s)样品的流动诱导结晶效果都弱于加速时间为1 s的样品.但是,对于不同剪切速率,其对应的最优加速时间不同.对于流动诱导结晶来说,加速时间应当作为一个重要参数来考虑,其背后的真实物理含义还需要进一步研究来说明.

Influence of Shear Homogeneity on Flow-induced Crystallization of Isotactic Polypropylene

Influence of shear homogeneity on rheology and flow-induced crystallization (FIC) behavior of polymer is investigated by contrasting the sudden startup shear with the slow shear rate ramp-up procedure.The shear experiments are performed via changing only the rate ramp-up time at a fixed shear rate of 17.7 s-1 and a constant strain of 20.And the corresponding motor acceleration time is set to 0,0.5,1.0 and 1.5 s,respectively.Combining wide angle X-ray scattering and polarized microscopy characterization,this set of experiments is designed to study the rate ramp-up time dependence of the rheological properties and FIC at the same shear rate and strain.When the rate ramp-up time is 0.5 s,it is found that a slow acceleration cannot completely eliminate the flow inhomogeneity.The stress overshoot still occurs in the stress-strain curve and the sample orientation is only slightly improved.As the rate ramp-up time is further increased to 1.0 s,the stress overshoot disappears,indicating that the inhomogeneity induced by destruction of melt structures vanishes.In this case,the degree of sample orientation is further improved,and the amount of oriented crystals is increased.However,when the rate ramp-up time is 1.5 s,the effect of flow induced crystallization is weakened,which is indicated by a decrease in orientation degree and the amount of oriented structures.Results obtained sufficiently illustrate that the FIC is dependent on the rate ramp-up time.However,a slower rate ramp-up procedure is not always associated with a better the flow-induced crystallization.Actually,there exists an optimal rate ramp-up time for a fixed shear rate.In addition,the optimal rate ramp-up time is not the same for different shear rates.These results suggest that rate ramp-up time should be considered as a critical parameter for FIC.And the underling mechanism still needs further investigation.