剪切流场诱导iPP等温结晶行为的研究

利用流变学方法研究了剪切诱导等规聚丙烯(iPP)的等温结晶行为.在稳态剪切流场作用下,结晶初期的法向压应力和粘度基本为一定值,一定时间后,会出现增大并迅速上升的现象.在高剪切速率下,与粘度相比较,法向压应力突变的时间要早;剪切速率减小,二者趋于一致;当剪切速率很低时,法向压应力的值很小且超出了仪器的量程,只有粘度的值是可信的.针对该现象提出了一种新的结晶速率表征法,即利用法向压应力和粘度二者中较早的突变时间表征结晶诱导时间(ton).诱导应变γ=.γ.ton,q是决定剪切影响结晶行为的一个无量纲参数,可以将不同温度下的结晶诱导时间曲线组合成一条主曲线.

SHEAR-INDUCED CRYSTALLIZATION OF ISOTACTIC POLYPROPYLENE

Shear-induced crystallization behavior of isotactic polypropylene(iPP)was investigated using a rotational rheometry covering a shear rate range of 1.2×10-4～1 s-1 and temperatures between 135℃ and 145℃.As the specimen was sheared at a constant rate,the viscosity and normal force were constant at first,then started to increase sharply after a certain time.At higher shear rates the transition of the normal force occurred considerably earlier than that of the viscosity,and at medium shear rates the two transition times were coincident.However at extremely low shear rates the data of normal force became noisy and only the data of viscosity was reliable.It was suggested that the earlier time in transitions for the normal force and viscosity was used to define an onset time(t_ on)which characterized the crystallization process.Furthermore,it is proposed that the plot of t_ on versus shear rate can be converted into a temperature-invariant curve by dividing the y axis by quiescent onset time t_ on,q and multiplying the x axis by t_ on,q.