振荡剪切场下PS/PVME共混物的相分离动力学研究——相分离的依时性及应力响应

利用自制的激光散射流变仪研究了聚苯乙烯/聚甲基乙烯基醚(PS/PVME)(重量比30/70)二元共混物在振荡剪切场下的相分离动力学过程.在特定的振荡频率和应变振幅条件下,共混物的相分离具有显著的周期特性和各向异性.在同一个振荡相位角条件下,相分离在早期符合经典的Cahn-Hilliard线性理论.在相分离过程中,流动方向上的特征波数qxm表现出明显的受迫振荡特征而涡流方向上的qzm却没有;同时,两个方向上的特征波数qm平均值都未随时间发生变化,说明特定振荡频率和应变振幅的振荡剪切对特定尺寸的浓度涨落具有选择作用.研究还发现,体系的应力响应可以反映出相分离的阶段性过程,例如当相分离早期结束时,表征弹性的第一法向应力差出现了峰值.

STUDY ON THE PHASE SEPARATION KINETICS OF PS/PVME BLENDS UNDER OSCILLATORY SHEAR——THE TIME DEPENDENCE AND STRESS RESPONSE OF PHASE SEPARATION

The phase separation kinetics of polystyrene/poly(vinyl methyl ether) (PS/PVME) blends under oscillatory shear was studied on a customized Rheo-SALS tester.It was found that,under fixed frequency and strain amplitude,the phase separation exhibited anisotropy and periodicity.At a given phase angle,the early-stage kinetics of phase separation followed the classical Cahn-Hilliard theory.The average of characteristic wavenumber q_m,where the maximum scattering locates,stayed unchanged during the phase separation,implying that only the composition fluctuations with specific size will be favored under a oscillatory shear with given frequency and strain amplitude.Under shear,q_ xm ,the characteristic wavenumber along flow direction,experienced a compulsive oscillation with a frequency same as that of shear,while q_ zm ,the characteristic wavenumber along vorticity direction,dose not.Through simultaneous rheological measurements,it was also found that stress could appropriately reflect the stage of phase separation.For example,the first normal stress difference reached maximum when the early stage of phase separation ended.