Structure Formation And Evolution Of Polymorphic Polymers In The Process Of Casting And Stretching

Understanding of the crystalline structural evolution process of crystalline polymers in the presence of stress field is of great significance for the performance manipulation in the processing of polymer film materials.During the processing of polymer films,the stress field often affects the crystalline structure of the polymer.The semicrystalline polymer can undergo multilevel structural evolution under the stress field,such as crystal slip,melt-recrystallization and cavitation,which has an important impact on the final shape and performance of the polymer products.For the polymorphic polymers,the existence of stress fields also affects their crystallization paths and leads to the change of molecular chain conformation,wich further induces the variation of crystal polymorphs of polymers.In addition,the addition of nucleating agent can induce the crystallization of the polymer under relatively “mild” conditions.The nucleating agent has a specific nucleation effect on the polymorphic polymers and is often used as an additive in the processing and molding of polymer materials.In this thesis,to understand the microstructural evolution behavior and mechanism of polymorphic polymers under stress fields,we chose the common polymorphic polymers,isotactic polypropylene(i PP)and polyvinylidene fluoride(PVDF),as the research objects.The effect of casting conditions on the crystallization structure of polymorphic polymers with the addition of specific nucleating agent,and the multilevel microstructure evolution of polymorphic polymers during post-stretching process were studied.Firstly,we introduced the rare earth ? nucleating agent(WBG)into i PP and prepared i PP/WBG cast films with different casting roller temperatures and draw ratios by blending casting method,and studied the effects of casting roller temperatures and draw ratio on the crystalline strcutue of i PP.It is found that the crystallinity of i PP/WBG casting film was enhanced at a high draw ratio and at a low casting roller temperature;this led to that the mechanical properties of casting film was improved,having higher tensile strength and elastic modulus.In the poststretching process,the tansformation from ? form to ? form for the i PP/WBG casting films was favored at a high stretching temoerature and strain.Meanwhile,the lamellar structure showed a highly orientated structure and the formation of cavitation was observed during the post-stretching of i PP/WBG casting films.The microporous membrane was also produced successfully.Secondly,tetraphenylphosphonium bromide(TPPB)was selected to regulate the crystalline structure of polyvinylidene fluoride(PVDF)after comparing a variety of nucleating agents.PVDF/TPPB casting film with different draw ratios were prepared.The crystalline structure of PVDF/TPPB casting films was obviously affected by the draw ratio.The long period of PVDF/TPPB casting films decreased with the increase of draw ratio.Besides,the increasing of the draft ratio is beneficial to prepare high hard-elastic casting film,and their elastic recovery ratio was higher than 90%.In the post-stretching process,the formation of ? crystals for PVDF was achieved at a mild condition with the addition of nucleating agent TPPB.The formation of ? crystal was facilated at a high post-stretching temperature.Meawhile,the stretching-induced formation of cavitaion was also observed in the post-stretching process of PVDF/TPPB casting film.The quantitatively calculated results of the cavitation degree show that the formation of cavitaion was facilitated at a low temperature.