Annealing effects of phase I poly(vinylidene fluoride)

The effects of annealing temperatures on stretched poly(vinylidene fluoride) film were systematically studied from the “as-received” condition up to the melting range (180°C). X-ray diffraction studies indicate that the annealing process brings about better chain packing and increased crystallite perfection. The elastic modulus and piezoelectric strain and stress constants, d₃, and e₃₁, decrease as the annealing temperature T_a increases up to 160°C, while the remanent polarization P_r remains almost constant. Some of these characteritics may be interpreted in terms of a morphological transformation of microfibrils. The values of P_r, d₃₁, and e₃₁ increase dramatically as T_a increases from 160 to 180°C; P_r increases from 56 to 85 mC/m², d₃₁ from 20.2 to 27.7 pC/N, and e₃₁ from 51.4 to 65.2 mC/m². As a result, the values of P_r and d₃₁ were the largest recorded from any of the samples used in the present study. e₃₁ showed a value close to the largest one; this usually occurs in unannealed samples. Samples annealed in the melting range also exhibit significantly improved ageing characteristics. The large value of P_r and the small relaxation strength of both the dielectric constant and elastic modulus indicates that the largest crystallinity obtained is approximately 70%.