Strain and temperature induced phase changes in spin-coated PVDF thin films

Poly (vinylidene fluoride) (PVDF) is widely utilized for its unique pyro and piezoelectric properties related to its electro-active beta-PVDF. In the past few decades, methods of producing beta-PVDF by mechanical stretching of non-polar alpha-PVDF have been extensively documented. The aim of the present study is to understand the correlation between phase and mechanical behavior of beta-PVDF obtained directly from spin-coating, and its stretch-induced transformation from alpha-PVDF. The effect of thermal annealing and in-plane anisotropy on mechanical properties of spin-coated PVDF freestanding thin films is investigated, under in-situ tensile loading, in conjunction with digital image correlation, to measure the full-field deformation. Stress-strain behavior of spin-coated beta-PVDF and alpha-PVDF are correlated with mechanical stretch-induced phase transformation in alpha-PVDF. The mechanical strength and failure strain exhibited by spin-coated and annealed alpha-PVDF (similar to 35 MPa and similar to 5.5) is much higher than spin-coated beta-PVDF (similar to 10 MPa and similar to 0.45). The room temperature mechanical stretching caused alpha-PVDF to beta-PVDF beyond a stretch ratio of similar to 1.2, confirmed by both Fourier transform infrared and Raman spectroscopy. Furthermore, mechanical response and spectroscopic analysis confirmed in-plane anisotropy due to the spin-coating process in beta-PVDF.