The Research Of Polymer Melts' PVT Under Ultrasonic Field

The PVT relationship is a process that describes the change of the specific volume of a substance with changes in pressure and temperature.As the inherent physical properties of the melt,it can be used to simulate the volume change of the melt under different temperature and pressure conditions in the process of forming.The volume compressibility of the melts in different temperature and pressure ranges is used to analyze whether there will be quality problems such as volume shrinkage during the molding process of the product.It has important applications in the molding process of polymer products.Because the processing of polymers is usually a process of melt plasticization,in order to obtain the PVT relationship of the melt in a wide range of pressure and temperature,the most commonly used polyolefin is selected as the main raw material for the experiments,and the apparent shear viscosity of the samples are measured,explains the macroscopic performance from the internal movement of the molecule.(1)This study predicts the PVT relationship of the LDPE melt to the group contribution correction model state equation(GCMCM).By comparing the measured and predicted PVT relationship data,it is shown whether the group contribution correction model state equation can be used for the LDPE melt to predict the PVT relationship.Through the power law equation and the Andrade equation to study the shear viscosity,analyzed the causes of the shear rate and temperature influence on the shear viscosity,and calculated the relevant parameter values of the equation.The results show that: LDPE is a pseudoplastic fluid,and the shear viscosity decrease in rising temperature and the decrease in shear rate decrease the effect of shear viscosity by changing the free volume of the melt and the entanglement between the macromolecular chains.The PVT relationship data of the LDPE melt was predicted by the group contribution correction model equation of state.According to the comparison between the predicted curve and the measured curve,it was found that the predicted curve generally moves upwards from the measured curve,but the deviation of the upward movement is smaller,which shows that The group contribution correction model has certain practicality for predicting the PVT relationship of LDPE melt,and is of great significance for calculating the specific volume value under wide temperature and pressure conditions.(2)Through a large number of experiments,it was found that the relationship between the specific volume of the polymer with pressure and temperature can be reflected by an isothermal curve and an isobaric curve,respectively.The two form a PVT relationship characteristic curve,and follow the following PVT relationship characteristic law: When increased,the specific volume of the polymer increases due to thermal expansion;when the pressure increases,the specific volume of the polymer decreases due to compressibility.However,when some polyolefin grade samples were tested through a capillary rheometer,it was found that the melt exhibited an abnormal phenomenon of "heat shrinkage and cold expansion" under a certain pressure within a certain temperature range,and then the root cause of the molecular chain was analyzed in depth.And mechanism,it is believed that the melt volume is determined by such parameters as the degree of unwrapping of the molecular chain and the distance between the molecular chains,and a physical model based on the level of molecular motion that characterizes the change in the PVT relationship of the polyolefin melt is established.After fitting analysis Mathematical analysis is obtained,and the experimental data is consistent with the model calculation results,indicating that the volume of polyolefin melt increases first,then decreases,then increases,and then decreases again with the increase of temperature.The experimental results are consistent with the theoretical calculations.Finally,the test analyzes the apparent viscosity of the polyolefin melt at different temperatures.The results show that as the temperature increases,when the volume becomes smaller,the melt viscosity will increase.Conversely,when the volume increases,the melt viscosity decreases,again revealing The molecular chain movement principle of the "heat shrinkage and cold expansion" phenomenon supports the correctness of the model.The research results can be used to guide the formulation of polyolefin material modification formula and the setting of processing parameters,so as to improve the accuracy of products and expand the scope of use.(3)In order to promote production and processing,in this paper,through the self-designed PVT measuring instrument that can apply external field ultrasound,the molten polyethylene glycol is the experimental object to study its PVT characteristics under ultrasound.The volume expansion rate of polyethylene glycol at different temperatures,different pressures,different molecular weights and different ultrasonic powers was discussed.Experiments show that the volume expansion rate of polyethylene glycol under the application of ultrasonic waves increases significantly,indicating that ultrasonic waves can promote the flow of polymer melts,which is conducive to production and processing.