Effects of strain rate and temperature on polymorphism in flow-induced crystallization of Poly(vinylidene fluoride)

The flow-induced crystallization of poly(vinylidene fluoride) (PVDF) was investigated over the broad strain rate and temperature ranges by combining the in situ synchrotron small- and wide -angle X-ray scattering with the extensional rheometer. The results show that temperature and strain rate collectively affect the crystallization polymorphism of PVDF. At 155-170 degrees C, the electroactive beta-phase was generated during flow (strain = 2.7 and strain rate = 10 s(-1)), without the appearance of customary kinetically favorable alpha-phase. For the same flow at a higher temperature of 180 degrees C, the fibrillar shish were formed but did not develop into crystallites yet. During the followed post-flow isothermal process at 155 degrees C, those flow-generated beta-phase grew pronouncedly and gamma-phase unexpectedly appeared at just 5 min. Differently, pure beta-phase existed at 160-170 degrees C, while no crystallites were detected at 180 degrees C. On the other hand, for the flow of strain 2.7, the broad strain rate range of 0.01-10 s(-1) could induce the formation of pure beta-phase during flow and the subsequent isothermal periods at 160 degrees C. When the partially crystallized PVDF was cooled down, alpha-phase was triggered by the increased undercooling. Interestingly, although the aforementioned variation of flow conditions significantly changes the formation of beta-phase, the corresponding appearance temperature of alpha-phase remains at a comparable temperature of around 149 degrees C, which is even similar with that of quiescent crystallization.