The influence of microcrystalline structure and crystalline size on visible light transmission of polyvinyl alcohol optical films

The crosslinking degrees and microcrystalline structure determine the light transmission of polyvinyl alcohol (PVA) optical films and mechanical properties. Preparing a PVA optical film satisfying these two aspects with a low swelling propensity is still challenging. Herein, PVA optical films were prepared by solution casting with environment-friendly water as a solvent. Based on PVA's unique monoclinic lamellar crystal structure, the optical properties, such as light transmission and haze, were revealed by changing the original molecular weight, polymer concentration, and heat treatment conditions. We systematically investigated the effect of the original molecular weight and polymer concentration on the PVA crosslinking degrees. The crystal size effect on the optical properties was investigated through a heat treatment for the crosslinked PVA films. The rotational rheometer and swelling tests revealed the PVA crossling degrees. DSC and XRD are utilized to systematically investigate the relationship between the crystalline structure of PVA and its optical transparency and mechanical properties. The PVA optical films (F2099-8%) derived from 8 wt% PVA2099/water solution exhibit excellent light transmission with transparency of 90.2 % and haze of 0.4 % due to the smallest crystal size of 3.29 nm. An appropriate heat treatment process could further improve the mechanical and swelling-resistant properties. The F2099-8% sample after heat treatment (<200 °C, 30 min) had a significant enhancement of mechanical strength (200 % increase) and swelling resistance (300 % decrease), but only 6 % transmittance decreased, and 2 % haze increased because of crystal size increasing. The PVA optical films prepared in this work showed relatively high light transmission and high mechanical and swelling-resistant properties, which have a good potential application in display panels and camera lenses.