Molecular Orientation Evolution Simulation And Birefringence Prediction Method For Polymer Forming

Polymers are key materials for the manufacture of lightweight and impact-resistant optics,whose orientation during forming has an important influence on the optical,force and electrical properties of the final product.Molecular orientation simulation is the key to regulate the performance of polymer products.However,due to the complex molecular orientation process in the shear flow,the existing polymer molecular orientation evolution models are still limited to single molecular chain in a simple flow field,the prediction of optical properties such as birefringence is based on the indirect relationship model of stress-optical law,and the cross-scale coupled simulation of molecular orientation and macroscopic flow field has not been achieved,making it difficult to accurately predict the performance of polymer products.Therefore,the molecular orientation evolution simulation and birefringence prediction of polymer forming process are carried out in this paper.The main work is as follows:The orientation behavior of flexible entangled and rigid polymers in shear flow are explored.To solve the problem of inaccurate description of transient rheological behavior by existing models,a molecular orientation evolution model considering the effect of dynamic entanglement is established.The maximum deviation of stress relaxation predictions in an example is reduced to 13.8%,improving the accuracy of the model in describing the transient rheological behavior of the polymer.It is found that the molecular orientation degree of flexible polymers in the shear flow increased with the increase of the maximum stretch length,entanglement-dependent factor and relaxation time,and decreased with the increase of the volume-dependent factor.Meanwhile,the tumbling orientation behavior of rigid molecules is studied,and a rigid molecular orientation evolution model considering intermolecular forces is established,which successfully describes the rigid molecular orientation degree increases with the shear rate.Based on the thermodynamic method,the molecular orientation simulation model of flexible and rigid-flexible polymers are established.Compared with the XPP model based on molecular theory,the model proposed in this paper can more accurately predict the rheological properties and molecular orientation of polymer melts at high shear rates(>400s^-1),and the viscosity calculation error at a shear rate of 10000 s^-1 is reduced from 33.8%to5%in a case.For block polymer materials,a simulation model of rigid-flexible molecular orientation evolution is proposed by combining molecular theory and thermodynamics.The calculation results of the above orientation evolution model is in good agreement with the experimental measurements of the dielectric anisotropy in the rotating Couette flow.The mathematical models between molecular orientation and rheological properties,dielectric anisotropy and birefringence are established.The experimental measurements of steady viscosity and dynamic complex modulus are consistent with the model predictions,verifying the accuracy of the orientation-rheology relationship in predicting steady and dynamic rheological properties.Using the molecular polarizability as a bridge,a mathematical model between molecular orientation and dielectric anisotropy is established,which provides an experimental verification method for molecular orientation evolution models.A relationship model between molecular orientation degree and birefringence properties is proposed,which comprehensively considers the contribution of molecular morphology and orientation changes,and the accuracy of the model is verified by Raman and birefringence experiments.A coupling simulation method of molecular orientation and forming flow field is established by taking macroscopic stress as a bridge,and the prediction of the birefringence is realized.The precited birefringence of the injection-compressed polystyrene products exhibits bimodal distribution,which is consistent with the experimental results,and the simulation error at the shear layer is within 5%.Compared with the stress-birefringence method,the orientation-birefringence model reduces the birefringence simulation error at the core layer of the injection-molded block polymer plate from 72.3%to 2.7%,and successfully predicts the unimodal distribution of rigid-flexible polymer birefringence.