Size-dependent and spatial variations in the structural properties of spin-coated poly(Vinylidene Fluoride) films

Abstract

The uniformity of structural properties in large spin-coated poly(vinylidene fluoride), or PVDF, films is crucial due to their widespread applications and thus requires careful investigations. In this study, variations in structural properties across PVDF films of different sizes and positions were examined. Thin PVDF films were fabricated by the spin-coating method onto rectangular substrates with lengths varying from 10 mm to 40 mm while maintaining a constant width of 10 mm. Three key characteristics – thickness, phase, and crystallinity – were characterized and analyzed. Two effects were investigated – the size-dependent effects, i.e., the property variations at a specific position due to the increasing film length, and the spatial effects, i.e., the property variations along distances from the film center within a film of a specific size. The average thickness of the fabricated film was in the range of 5.00 μm to 6.00 μm, while the crystalline size was in the range of 1.00 nm to 3.00 nm. For the size-dependent effects, at a specific position on the film, increasing the film length did not significantly affect the thickness; however, the phase of PVDF shifted toward a more chain-like β phase, while the crystalline size decreased. At the film center, the crystalline size decreased by 60.7% when the film size increased from the smallest to the largest. These changes resulted from four combined mechanisms: centripetal force, viscosity, evaporation rate, and shear force. For the spatial effects, in a film with a specific size, when the distance from the center increased, the thickness decreased, the phase remained β, and the crystalline size was smaller. For the largest 40 × 10 mm² film, the thickness and crystalline size decreased by 16.7% and 3.6%, respectively, from the center to the edge of the film. These trends were attributed to the three combined mechanisms: centripetal force, viscosity, and evaporation rate. The findings of this study provide critical insights into a deeper understanding of property variations in spin-coated PVDF films among both different film sizes and different positions on a film, which is essential for optimizing their applications.