On the crystallization mechanism of poly(ethylene terepthalate) in its blends with poly(vinylidene fluoride)

The overall crystallization rates of poly(ethylene terepthalate) (PET) in its blends with poly(vinylidene fluoride) (PVF2) were studied by differential scanning calorimetry. At a fixed temperature the crystallization rate of PET decreased with increase in φPVF2 (φPVF2=volume fraction of PVF2) in the blend. However, at a fixed undercooling, initially there was almost an invariant rate with φPVF2, but it increased at higher PVF2 concentrations. The Avrami analysis indicated that the nucleation process of PET changed from three-dimensional heterogeneous nucleation to two-dimensional heterogeneous nucleation due to blending in major cases. Analysis of crystallization rate according to the extended form of Lauritzen–Hoffman (L–H) growth-rate theory indicated regime-I to regime-II transition both in pure PET as well as in the blends. The undercooling (ΔT) required for the regime transition was almost the same for both the pure polymer and the blends except for the blend composition φPET=0.35. A jump in the crystallization rate was observed at the regime transition temperatures for the blends with higher φPVF2. This phenomenon has been attributed to the different diffusion processes occurring in the two regimes. Analysis of the lateral surface free energy (σ) obtained from the nucleation constant (Kg) indicated that there might be some chain extension of PET due to blending.