The effect of chain topology on the crystallization and polymorphism of PVDF: linear versus star molecules

Well-defined linear, 3-, 4-, and 6-arms star polyvinylidene fluoride (PVDF) were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization using mono-, tri-, tetra-, and hexa-functionalized chain transfer agents (CTA), respectively, and 1,1-bis-(tert-butylperoxy)-cyclohexane (Luperox® 331P80) as initiator. The crystallization kinetics and the polymorphic character of PVDF stars were investigated by differential scanning calorimetry (DSC) experiments and Fourier transform infrared spectrometry (FTIR). Using a cooling rate of 10 °C/min, all samples formed α- and β-phases. However, the amount of β-phase created increases with respect to the α-phase as the number of arms in the PVDF stars increases. This could result from the increased topological complexity in the stars of several arms, which leads to the preferential formation of the less thermodynamically stable ferroelectric β-phase. On the other hand, when the cooling rate is decreased (1 °C/min) and/or an isothermal crystallization procedure is applied, polymorphism is inhibited in the PVDF stars, and only the paraelectric α-phase is formed. On the other hand, the linear PVDF sample is still capable of producing both paraelectric and ferroelectric phases after slow cooling or isothermal crystallization. The isothermal crystallization kinetics of the PVDF stars is faster than the linear PVDF as a result of their speedier nucleation, possibly promoted by their particular topology where the PVDF arms must radiate from a common center.