This study presents the effect of bending strain upon the structure and mechanical behavior of poly(vinylidene fluoride) (PVDF). Using the ISO 22088-3 Bent Strip Method, the PVDF samples were submitted to 3.5 % and 7.0 % strain in bending for 21 days. The unstrained and strained samples were characterized by Fourier Transformed Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) and tensile tests at 23 °C.
The applied bending strain of 3.5% and 7.0 % resulted permanent deformation for both strains applied, which curvature was very close to 3.5% and 7%. The 7% strained samples showed stress-whitening, most likely due to the well-known cavitation phenomenon; while 3.5% strained samples no cavitation was seen.
Infrared spectroscopy analysis indicates a mechanically induced α to β crystalline phase transition on the outer fibers as the bending strain increases. DSC measurements showed no change on crystallinity volume fraction and melting temperature as deformation increased. The DMA showed a shift on glass transition temperature (TG) to higher temperatures and the Elastic Modulus decreased as the bending strain increased for all temperatures.
Reverse deformation had to be applied on the plastically deformed bending samples in order to straight them out to do the tensile tests. The tensile properties measured at room temperature showed the Young’s Modulus and Yield Stress decreasing as the applied bending strain increases. On the other hand, the yield strain rose as the bending strain increased.