The Effect of Chain Topology on the Crystallization and Polymorphism of PVDF: Linear versus Star Molecules

Abstract

Well-defined linear, 3-, 4-, and 6-arms polyvinylidene fluoride (PVDF) stars are synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization using mono-, tri-, tetra-, and hexa-functionalized chain transfer agents, respectively, and 1,1-bis-(tert-butylperoxy)-cyclohexane as initiator. The crystallization kinetics and the polymorphic character of PVDF stars are investigated by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR). Using a cooling rate of 10 °C min−1, all samples form α- and β-phases. However, the amount of β-phase increases with respect to the α-phase as the number of arms in the PVDF stars increases. This results from the increased topological complexity in the stars, which leads to the preferential formation of the less thermodynamically stable ferroelectricβ-phase. When the cooling rate is decreased (1 °C min−1) 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 i faster than the linear as a result of their speedier nucleation, possibly promoted by their particular topology where the arms radiate from a common junction point.