Study On The Formation And Regulation Of Bimodal Polyethylene Shish-kebab Crystals Under Shear Flow Field

During the processing of semi-crystalline polymers,there is a flow field induced crystallization effect,and it has a significant impact on the performance of polymer products.Therefore,it has been the focus of academic research and technical application for a long time.Shish-kebab crystal is a highly oriented structure generated by flow field induced crystallization,which can significantly enhance part of the mechanical and heat resistance of the final product.Therefore,through the research in the effect of different conditions on the formation mechanism of shish-kebab,we can not only further understand the flow field induced crystallization,but also may be used to improve the performance of semi-crystalline polymer products.In this paper,the in-situ SAXS/WAXD study at SSRF of two types of PE samples with C4 short chain branch at high and low shear temperature was used to study the formation of shish-kebab crystals.The main findings include:1.By studying the effect of high content of C4 short chain branch in the process of shearinduced crystallization on the formation of shish-kebab crystals under high temperature shear conditions(selected superheats of 22.5 ° C and 19.5 ° C).By analyzing the SAXS results,it was found that both samples formed shish crystals in the shearing stage.Kebab crystals and partially oriented platelets were formed later in the subsequent isothermal crystallization stage.Analysis of WAXD results showed that the two samples had an orientation degree as high as 0.75 ? 0.95 during the shearing process,and the crystallinity was lower than the detection limit of the WAXD method.In the subsequent 121 ° C isothermal crystallization stage,due to the formation of kebab crystals and a large number of oriented platelets,a higher degree of crystallinity and a lower degree of orientation were obtained.The formation mechanism of the shish crystals of the two PE samples under high temperature shear is the coil-stretch transition(CST).On the one hand,the presence of higher content of C4 short chain branch will hinder the formation of shish crystals,but on the other hand,the presence of higher content of C4 short chain branch can effectively increase the relaxation time of the sample,so that the two PE samples can still form a certain amount of shish crystals and allow epigenetic growth of kebab crystals,but higher C4 short chain branch will make shish crystals less regular,making the period of epiphytic kebab crystals lower and worse,and also the distribution is wider.2.By studying the effect of the higher content of C4 short chain branch in the shearinduced crystallization process on the formation of shish-kebab crystals in two PE samples under low temperature shear conditions(selected superheats of 13.5 ° C and 10.5 ° C).By analyzing the SAXS results,it was found that both samples formed shish-kebab crystals in the shearing stage,and kebab crystals and more oriented platelets were formed in the subsequent isothermal crystallization stage.Analysis of WAXD results showed that the two samples had crystallinity varying from 0.2% to 20% during the shearing process,and the degree of orientation was 0.5 to 0.86.In the subsequent isothermal crystallization stage,due to the stronger mechanical work acting on the melt during low-temperature shear,platelets with better overall orientation are obtained,and a higher degree of orientation is obtained than the result of shear under high-temperature.At this time,the formation mechanism of shish-kebab crystal is the stretched network model.More C4 short chain branch hinder crystallization during shearing.Due to the strong shearing at low temperature,shish-kebab crystals can also generated based on the tensile network,but resulting in poor regular shish-kebab crystals.In summary,we adjusted the entanglement degree of PE by changing the shear temperature to achieve the regulation of different formation modes of the shish-kebab crystals which are attributed to the CST mechanism or the tensile network mechanism.We conducted an in-depth study on the effect of short chain branch on the formation of shish-kebab crystals at different shear temperatures during shear-induced crystallization,and found that although higher levels of C4 short chain branch hindered the overall PE to a certain extent crystallization,but a small amout of shish-kebab crystals can still be formed.The effect of C4 short chain branch is mainly reflected in the reduction of the regularity of shish crystals.The higher the content of C4 short chain branch,the greater the impact on the regularity of shish crystals.