Study On Nonlinear Viscoelasticity And Confined Flow Of Disentangled Polystyrene

Polymer materials are widely used in life and high-tech fields due to their low cost and adjustable performance characteristics.In the past,the study of the linear viscoelasticity and equilibrium state of entangled polymers has been relatively complete,but the actual processing rate is often much higher than the reciprocal of the equilibrium relaxation time.The current research on the dynamics of entangled polymers is expanding from linear to nonlinear,from equilibrium to non-equilibrium state.Both disentangled polymers and confined polymers are non-equilibrium states that deviate from the properties of the bulk.Their glass transition,crystallization,rheology and other properties exhibit different behaviors from those of the bulk.In order to further explore the nonlinear shear dynamics of polymer and the mechanism of micro-nano processing,we studied the nonlinear rheology and two-dimensional nano-confined flow process of disentangled polymer in this work.Firstly,we prepared disentangled polystyrene(PS)of different molecular weights by freeze-drying benzene solutions,and prepared"solutions"of PS and oligostyrene with different concentrations.Comparing the disentangled PS with bulk which have the same molecular weight,it is found that the disentangled PS obviously suppress the stress overshoot.Compared with the bulk,the stress maximum,steady stress and overshoot amplitude of disentangled PS are all reduced.We believed that the steady state of the polymer under nonlinear shear may be related to the initial degree of entanglement,because the different plateau of PS with different entanglement degrees after stress overshoot.with the increase of the shear rate,the overshoot amplitude of the disentangled PS gradually approaches to the bulk,which may be due to the easier orientation of disentangled PS at low shear rate.However,at high speed,the stretch ability of the polymer with the same chain length is basically the same.In addition,we compared the nonlinear results of samples with similar entanglement numbers at 150?and PS200k at different temperatures.After normalization,it is found that the results at different temperatures have better overlap,while those with similar entanglement numbers showed more different.This may be related to the changes of monomer friction coefficient and molecular tumbling mentioned in previous simulation studies.Then,we studied the flow behavior of PS100k and PS200k with different entanglement degrees at different conditions.The results were compared with Lucas-Washburn eqution and the rheology results.It is found that the bulk PS100k confined in the medium pore(4<R/R_g<10)conforms to the h?t^1/2 relationship but the prefactor exhibits deviation,and the effective viscosity shows 2-3 times higher than the rheological measurement;and in the smaller pore(R/R_g<4),the initial flow is stronger than the medium pore,but the power law relationship is about t^0.1?t^0.3.Comparing the flow behavior with different entanglement degrees,it is found that the[?]c?1 exhibit an effective viscosity that is 30-50%lower than the bulk under same conditions,whatever the molecule weight and pore diameter.In addition,the disentangled sample gradually recovered to a flow rate similar to the bulk,which is consistent with the results of rheology experiment.In the study of molecular weight and temperature,significant differences were still observed after normalization of viscosity and surface tension.The relationship between effective viscosity and molecular weight may be greater than the power in macroscopic flow.And in addition to apparent viscosity and surface tension,temperature also affects other parameters related to chain motion.