Research On The Wall Slip Of SIS And The Viscoelastic Properties Of Its Composities

The wall slip phenomenon was a special viscoelastic rheological phenomenon in which polymer melts began to slip on the surface of solid medium when wall shear stress exceeded a critical value.The research on wall slip was mainly focused on the polyolefin melts with linear structure and high degree of entanglement and mechanisms of“chain detachment/desorption”and“chain disentanglement”were proposed.Thermoplastic elastomers as a phase separation system,microphase separation structure would be changed by increasing temperature,shear stress or adding fillers,which would lead to a change in the viscoelastic rheological properties of the melt.At present,there were few studies on the relationship between the phase structure transition of the thermoplastic elastomer and the viscoelastic rheological behavior of melt,and no unified theoretical explanation has been formed.In this thesis,the viscoelastic rheological behavior of triblock thermoplastic elastomer(SIS)and its composite melts was studied by using a constant-speed capillary rheometer and a rotary rheometer.Phase structure transition was studied by means of small angle X-ray scattering(SAXS)and transmission electron microscopy(TEM).The relationship between micro-morphology of the thermoplastic elastomer and the viscoelasticity rheological behavior of melts was established.It was found that SIS melt exhibited special rheological phenomena during extrusion.At low temperature(<170~oC),the flow curve of the melt had an obvious turning point:the shear stress first increased with the shear rate and was basically constant when the shear rate reached the critical value.When the temperature was over170~oC,the turning point of the flow curve would disappear.According to the Cox-Merz rule,the phenomenon was attributed to the wall slip behavior,which was directly related to the melt phase structure transition.The wall slip velocity which was calculated by Mooney method first increased and then decreased with the increase of shear stress.SAXS results showed that the orientation degree of phase structure increased with the increase of shear stress.Based on two classical wall slip mechanisms and combined with SAXS data,the possible mechanism of wall slip phenomenon in SIS melts was proposed.In addition,the effect of molecular parameters(block ratio,molecular weight,etc.)on the melt viscosity was studied and found that the melt viscosity exhibited different dependence properties.Under the viscosity transition temperature of the PS phase,the viscosity of the SIS melt depended mainly on the ratio of S/I,when the temperature exceeded the viscosity transition temperature of PS phase,the melt viscosity of SIS depended mainly on the molecular weight.The effects of fillers with different scales on the viscosity of SIS melts was also studied.It was found that the addition of trace fillers(0.1wt%)could cause a strong decrease in the melt viscosity,both for nanofillers and non-nanofillers.The reason for the decrease of melt viscosity was probably that the addition of filler promoted the orientation of PS phase of SIS and weakened the interaction between the melt and the inner wall of the capillary.