Fig. 3
Factors influencing the formation of β-phase during processing. a Comparison of the phase transition in samples (670–700 kg mol−1) with monolayer and six multi-stacked layers at three different pressures (force 300 kN), 120, 667 and 3000 MPa, followed by pressure annealing at 165 °C for 5 min, which indicates that high pressure and layered structure favour the formation of β-phase. b Fraction of β-phase for samples (670–700 kg mol−1) with the same dimensions, but composed of different numbers of layers that were pressed once at 667 MPa (force 300 kN) followed by pressure annealing at 165 °C for 5 min and cold water quenching. c FEM simulations of film compression in the cases of monolayer film, four-layer film with same friction coefficients µ between layer and platen and between layers, and four-layer film with µ between layer and platen higher than µ between the layers. The bar scale represents the overall strain deformations. d FTIR of samples with different Mw, which indicates the formation of γ-phase in PVDF (low Mw) at higher Tanneal (denoted by the γ-characteristic band at 1234 cm−1) and high Mw favours the transformation of β-phase (inset). For PVDF with Mw of 180, 534 and 670 kg mol−1, Tm was 169, 172 and 172 °C, respectively. e Demonstration of phase evolution with Tanneal and Mw during P&F and schematic diagram of chain conformation, crystal structure and polarization of PVDF. The HP (no folding) films are mainly α-phase, with two trans-gauche-trans-gauche′ (TGTG′) chains anti-parallel packed in a pseudo-orthorhombic unit cell. The symbols and arrows represent the in-plane and out-of-plane contributions to the dipole moments. After P&F at different temperatures (room temperature to Tm), films formed into β-phase (>90 wt %) with all-trans (TTT) chain conformation. Low Tanneal and high Mw favour the formation of β-phase
