聚合物流变学导论2005秋-6new.ppt

具有时间依赖性的流动行为1. Thixotropy (触变性) A thixotropic material becomes more fluid with increasing time of applied force. The applied force could be stirring, pumping or shaking. This effect is sometimes called ‘work softening’. It is often reversible, so that if left undisturbed for some time a thixotropic slurry regains its viscosity. Mechanism of thixotropy Superstructure decomposed during shearing process; Structure recovery or complete regeneration of superstructure Equilibrium Structure Thixotropy / Hysteresis Loop Sample 3: Unsaturated Polyester Composite Resin: Unsaturated Polyester Cross-linker: Styrene Glass Fiber Filler: Calcium Carbonate Bulk Mold Composite (BMC) Sheet Mold Composite (SMC) Sample 4: Thixotropy of Paint Poor leveling（流平性）： Sagging（流淌）on a vertical wall or thickness of wet layer： Comparison of two brushing paints Comparison of paints Rheopexy (work hardening, 震凝性) A rheopectic material becomes more viscous with increasing time of applied force. This effect is the opposite of thixotropy, and is sometimes called ‘work hardening’. PVC 塑化、分解 Thixotropy and Shear Thinning Thixotropy, Rheopexy and Hysteresis Loop Viscosity and Mol. Weight At low shear rates (or stresses) a “lower Newtonian” region is reached. The limit is the zero-shear viscosity, ho The zero shear viscosity is a function of polymer molecular weight. Viscosity and MWD Broader MWD polymers are easier to process at high shear rates Temperature Dependence Temperature Dependence - WLF Equation Newtonian Fluids Recall Stress (应力) Stress is the intensity of a force Stress = force/area Divide the force into components normal and tangential to the area Stress Tensor (应力张量) Shear Stress (剪切应力) and Normal Stress (法向应力) Stress tensor for static liquid (静态流体的应力张量) Hydrostatic and Deviatoric Stress(偏应力张量) Simple Shear （简单剪切） Normal stress difference (法向应力差) in simple shear flow Relationship between Normal Force Difference and Shear Rate Reason for Normal Stress Difference Die sw