Unexpected shear dependence of pressure-induced gamma-crystals in isotactic polypropylene

Flow and pressure frequently coexist in practical polymer processing operations, but their combined influence on the microstructure of polymer parts has received very limited attention in the academic community. In the current work, we utilized a home-made pressuring and shearing device with a reliable dynamic sealing design to study the formation and microstructure of gamma-form isotactic polypropylene (iPP) obtained under the coexistence of flow and pressure. We observed a strong shear dependence of pressure-induced gamma-form iPP. There are three regions depending on shear flow intensity, i.e., facilitation (<3.7 s(-1)), suppression (3.7-9.1 s(-1)) and inexistence (>9.1 s(-1)) regions of the gamma-form. As the shear rate is below 3.7 s(-1), the pressure-induced gamma-form dominates and the shear flow slightly facilitates formation of gamma-form. Unexpectedly, above 3.7 s(-1), the shear flow is unfavorable for gamma-form growth. Even under a pressure of 100 MP, a flow field with a shear rate above 9.1 s(-1) could entirely suppress the gamma-form. Moreover, we did not observe any trace of the beta-form in the obtained iPP that is generally generated under shear flow alone. These interesting results have never been reported, which undoubtedly help manipulate the inner structure and thus enhance the performance of final iPP products.