Research On Thermal Conductivity Models Of Crystalline Polymer

Polymer also known as macromolecular compound,generally refers to the compounds with relative molecular weight of several thousand to several million.Some of these polymer chains(such as polyethylene)have high regularity(chemical regularity and tacticity),and form a neatly arranged crystal structure at an appropriate temperature.Generally,the polymer with a crystal structure volume ratio of more than 40 % is called crystalline polymer.In the engineering application and daily life,polymers have a wide range of applications.Because polymer is characterized by good insulation and high resistance to corrosion as well as low cost,the polymer is widely used in electronic devices.However,polymers with relatively low thermal conductivity cannot satisfy the increasingly harsh heat dissipation conditions of electronic devices nowadays.Therefore,it is necessary to study the heat transfer properties of polymer.Heat transfer model is an efficient means to study polymer heat transfer mechanism,which is of great guiding significance to adjust and improve polymer thermal conductivity.In this paper,the physical significance of crystalline phase and discrete phase is determined by sorting out the evolution of crystalline polymer structure model.On this basis,Maxwell model is derived with heat transfer theory.Since Maxwell model does not take into account the influence of the interaction between discrete phases at a high discrete phase volume fraction,the Lord Rayleigh’s model was compared with the Maxwell model to determine the influence of such interaction on predicting thermal conductivity.The results show that the Lord Rayleigh’s model and Maxwell model are in good agreement even at high crystallinity.This indicates that Maxwell model does not require the condition that crystallinity should be less than 30% when applied to the prediction of isotropic crystalline polymers thermal conductivities.In order to further explore the validity of fitting results from Maxwell model,the fitting results of Maxwell model,Choy’s model and EMT model as well as their theoretical derivation process were sorted out and compared.The reasons for the deviations of these models in fitting experimental data of crystalline polymer thermal conductivity were explained from the derivation process.The results show that Maxwell model is more accurate in predicting the thermal conductivity of isotropic crystalline polymers.However,when the Choy’s model(Maxwell model of anisotropy)is applied to high tensile ratio(>5),the prediction of thermal conductivity of crystalline polymers cannot well reflect the change of thermal conductivity with the degree of crystallinity and orientation of crystals.This is because the Choy’s model does not take into account the change of orientation of amorphous phase,and the structure hypothesis in Choy’s model is not highly consistent with the oriented crystalline polymers.The data reported in the existing literature show that orientation of amorphous phase has an effect on the thermal conductivity of crystalline polymers.In order to further quantify the magnitude of the influence,a modified EMT model was derived based on the EMT model and Choy’s method of processing Maxwell model for materials with anisotropic two-phase media.The fitting results reflect the change of thermal conductivity of HDPE with the orientation of amorphous phase well.This explains,to some extent,why the orientation and crystallinity of crystals are almost unchanged in the process of stretching but the thermal conductivity of samples is still improved.However,the model cannot explain the situation that the thermal conductivity of the crystalline amorphous phase still increases when the orientation and crystallinity of the crystalline phase remain unchanged.This is due to the transition from folded chain crystal to elongated chain crystal,which resulting in the failure of the assumption of constant crystal phase thermal conductivity.The modified EMT model proposed in this paper mainly explains the influence of crystallinity,crystalline phase orientation and amorphous phase orientation on the thermal conductivity of crystalline polymers.When both phases are randomly oriented and isotropic,the model can be reverted to the traditional EMT model.The following are the main work and research results of this paper:(1)Maxwell model was derived from the structure model of crystalline polymer and heat transfer theory,and the physical significance of model parameters was discussed.(2)The data of crystallinity and thermal conductivity of polyethylene reported in literatures is collected.Maxwell model was fitted and compared with Choy’s model,Lord Rayleigh’s model and EMT model.The thermal conductivity of equivalent spherulite and amorphous phase in isotropic PE is determined,which is prepared for the subsequent anisotropy research.During the study,it was found that Maxwell model has high applicability and accuracy even with high crystallinity,which indicates that the interaction between discrete phases is not obvious in the field of heat transfer.(3)Taking PLA as an example,the applicability of Maxwell model near the glass transition temperature is discussed.(4)Referring to the modification of Maxwell model by Choy,the traditional EMT model is tried to extent to the form of anisotropic.The anisotropic EMT model is applied to predict the thermal conductivity of oriented crystalline polymers,and it has a higher prediction accuracy for samples with low tensile ratio.Because both of the orientation effect of crystalline phase and amorphous phase is considered,the prediction result of anisotropic EMT model is better than Choy’s model.