Facile preparation of optically transparent film of acetylated cellulose nanofiber-reinforced poly(methyl methacrylate) utilizing cosolvency in aqueous ethanol

Composite films of cellulose nanofibers (CNFs) and poly(methyl methacrylate) (PMMA) are fabricated typically by solution-casting using an organic solvent (e.g., chloroform) that can dissolve PMMA. However, the preparation of the casting dope using the previously reported method requires multistep procedures for the solvent exchange of the CNF suspension from water to the organic solvent. Here we developed a facile method utilizing the cosolvency of PMMA in aqueous ethanol (EtOH aq.) to fabricate an optically transparent CNF-reinforced PMMA film. In this study, an acetylated-CNF (Ac-CNF) was used as a hydrophobic filler for compatibility with a hydrophobic PMMA matrix. First, an appropriate amount of EtOH was added dropwise to Ac-CNF aq. suspension to adjust the EtOH concentration to 80 vol%. PMMA was then added to the solution and dissolved under heating above the upper critical solution temperature to yield a clear casting dope. The dope is cast at 65 degrees C and then dried at 45 degrees C for 24 h in a closed oven to obtain an Ac-CNF/PMMA composite film. The composite film with 1% Ac-CNFs exhibited high total-light transmittance (93%) and low haze (4.4%), while 1.3-times higher tensile strength and Young's modulus than the neat PMMA film, indicating the effective reinforcement of PMMA by the well-dispersed Ac-CNFs. Thus, this method enables the facile preparation of an Ac-CNF/PMMA composite film with excellent transparency and enhanced mechanical performance.