The Effect of Drying Conditions on the Morphology and Optical Properties of All-Cellulose Composite Films

All-cellulose composite (ACC) films, with their excellent tunable optical and mechanical properties, combined with biodegradability, represent a highly promising material for applications in the packaging and flexible electronics sectors. The optical properties of ACC films are critically governed by their microstructure, which is determined by drying conditions. In this study, the effects of drying conditions on the structure–property relationships of ACC films were systematically investigated. First, ACC films were fabricated via the partial dissolution of microcrystalline cellulose powder in ionic liquids, followed by a film-casting process. Subsequently, various drying conditions under different temperatures and pressures were applied to finalize the films. XRD characterization demonstrated the coexistence of cellulose I and cellulose II structures. Optical and morphological tests revealed that (1) drying without pressure resulted in obvious shrinkage and deformation, with the diameter reduced by 70%; (2) the high-temperature/high-pressure drying method promoted a dense structure, resulting in ACC films with high transmittance (>90%) and low haze (<10%); and (3) ACC films dried under different hot-press temperature conditions showed similar transmittance and a large difference in haze, which could be related to the micro-pores formed within films. The systematic correlation between structure and optical properties established in this work provides a clear pathway for the tailoring of the optical performance of cellulose films through controlled drying conditions.