关键词: 壁面滑移, 应变不均匀, 大振幅剪切流, 应力分解

Abstract:

Wall slip of the melts of polydimethylsiloxane(PDMS), polymethylvinylsiloxane(PMVS), high-density polyethylene(HDPE) and polypropylene(iPP) were studied experimentally by using a rotational rheometer with parallel plate fixtures and checking the gap dependence of the stress/strain data. In the steady shearing flow it was found that, contrast to HDPE, there seems to be no critical shear stress for the onset of slip of PMVS. In the dynamic oscillatory shear flow, as the strain amplitude increasing, the stress data obtained at different gaps first overlapped well in the linear and the early part of nonlinear regions, then, at a certain strain amplitude, started to diverge, indicating that wall slip or strain stratification occurred. Employing the elastic/viscous stress-decomposition technique proposed by Cho et al.'s, we have examined possible candidates among various parameters that could be used to determine the onset of wall slip. The dimensionless parameter τ'_max/|G^*|, i.e., the ratio of the maximum elastic stress in a cycle to the linear complex modulus, was found to vary in the range from 0.26 to 0.49 for the four kinds of polymer melts when the wall slip or strain stratification occurred. In that range, the onset value decreases weakly as the angular frequency increases and is almost independent of the temperature in considerable large ranges. Thus elastic stress is the key factor causing the wall slip or strain stratification of polymer melts, which supports the conclusion of Vinogradov and Insarova′s earlier investigation.

Key words: Wall slip, Strain stratification, Large amplitude oscillatory shear, Stress decomposition