Modulating Birefringence and Wavelength Dispersion of Biaxially Stretched Poly(vinyl acetal) Films

We present a poly(vinyl acetal) (PVAc)-based compensation material designed to offer tunable birefringence and controlled wavelength dispersion. In this study, we propose principles for modulating these optical properties based on the opposite birefringence signs between benzenoid groups and hydroxyl and the polymer chain arrangements induced by sequential (SEQ) biaxial stretching. Diverse PVAc derivatives, including poly(vinyl benzyl acetal) (PVBA), poly(vinyl 4-fluorobenzyl acetal) (PVFBA), and poly(vinyl p-methylbenzyl acetal) (PVMBA), were synthesized through the condensation of poly(vinyl alcohol) (PVA) with various aromatic aldehydes: benzaldehyde (BA), 4-fluorobenzaldehyde (FBA), and p-tolualdehyde (MBA), respectively. SEQ biaxial stretching was performed on these films, clearly mapping out the relationships between composition, processing conditions, birefringence, and wavelength dispersion. The results indicate that altering the type of benzenoid side groups and varying the mol % of vinyl acetal (AC) from 30% to 50% does not affect the positive intrinsic birefringence pattern of the modified PVAc derivatives. Upon SEQ biaxial stretching, samples with higher λ_y for a given λ_x exhibited lower Δn_in and higher Δn_th. The three-dimensional refractive index, calculated from Δn_in and Δn_th, changed accordingly: at constant λ_x, elongation in the y-direction increased n_y while decreasing n_x and n_z, resulting in a refractive index order of n_x > n_y > n_z. This transformation was attributed to the breakdown of single-direction orientation and alignment of polymer chains parallel to the film surface during biaxial stretching, as confirmed by polarized IR spectra. Furthermore, regarding wavelength dispersion, PVBA and PVFBA exhibit extremely flat wavelength dispersion with n_x > n_y > n_z, and are suitable for use as compensation films in vertical alignment liquid crystal displays (VA-LCDs). In contrast, PVMBA shows strong extraordinary wavelength dispersion, making it a potential candidate for quarter-wave plate applications. The primary merit of biaxially stretched PVAc films is their simplicity in synthesis and tunable optical properties, making them highly appealing for both academic exploration and industrial application.