Conformational changes and phase transformation mechanisms in PVDF solution-cast films

• Published in: Journal of polymer science. Part B, Polymer physics Vol. 42; no. 18; pp. 3487 - 3495
• Main Authors: Salimi, A., Yousefi, A. A.
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 15.09.2004; Wiley
• Subjects: 19F NMR; Applied sciences; chain conformation; Crystallization; Exact sciences and technology; FTIR absorption bands; Organic polymers; phase transformation; Physicochemistry of polymers; piezoelectric coefficient; Properties and characterization; PVDF; piezoelectric coefficient; Crystal form; Film; Piezoelectric properties; Experimental study; Fabrication property relation; Polymorphic transformation; PVDF; chain conformation; 19F NMR; Alpha form; Structure processing relationship; Hot drawing; Beta form; Oriented polymer; Vinylidene fluoride polymer; phase transformation; FTIR absorption bands; Piezoelectric constant; Solution crystallization; Isothermal crystallization
• ISSN: 0887-6266; 1099-0488
• DOI: 10.1002/polb.20223

The supramolecular crystal structure in poly(vinylidene fluoride) (PVDF) solution‐cast films is studied through changing crystallization conditions in two solvents of different structures and polarities. The crystalline‐state chain conformations of isothermally solution‐crystallized PVDF in N, N‐dimethylacetamide (DMAc), and cyclohexanone are studied through the specific FTIR absorption bands of α, β, and γ phase crystals. There are no changes in the FTIR spectra of cyclohexanone solution‐crystallized films in the temperature range of 50–120 °C. In the case of DMAc solution‐crystallized films, low temperature crystallization mainly results in formation of trans states (β and γ phases), whereas at higher temperatures gauche states become more populated (α phase). This is due to the variations in solvent polarity and ability to induce a specific conformation in PVDF chains, through the changes in chain coil dimensions. This indicates that in spite of cyclohexanone solutions, the intermolecular interactions between PVDF and DMAc are temperature‐sensitive and more important in stabilizing conformations of PVDF in crystalline phase than temperature dependence of PVDF chain end‐to‐end distance . The high‐resolution 19F NMR spectroscopy also showed little displacement in PVDF characteristic chemical shifts probably due to changes in PVDF chain conformation resulting from temperature variations. Upon uniaxial stretching of the prepared films under certain conditions, contribution of trans state becomes more prominent, especially for the originally higher α phase‐containing films. Due to formation of some kink bands during film stretching and phase transformation, α phase absorption bands are still present in infrared spectra. Besides, uniaxial stretching greatly enhances piezoelectric properties of the films, maybe due to formation of oriented β phase crystals, which are of more uniform distribution of dipole moments. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3487–3495, 2004