Fabrication of micropillars on nanocellulose films using a roll-to-roll nanoimprinting method

This paper presents a simple method to modify biobased nanocellulosic films with thermal roll-to-roll nanoimprinting lithography (R2RNIL) to produce microstructured films. In the NIL process a patterned roll and an elastic backing roll are pressed against each other at elevated temperatures, and the pattern is replicated onto the film structure. Here we demonstrate the fabrication of patterned nanocellulosic films prepared using mechanically disintegrated cellulose nanofibrils (CNF) and TEMPO-oxidised cellulose nanofibrils (TEMPOCNF). Although both CNF films are brittle and nanocellulosic material itself is not thermoplastic with a clear softening point, it was shown that the mechanical patterning of both CNF films was possible. Imprinted pillar structures with a width of 7.5 µmand a height of 1.5 µmwere achieved and both of the patterned CNF films showed a clear optical effect. The patterns are formed probably due to the compressibility of the CNF films and due to the film softening effect brought about by sorbitol, which is a commonly utilised softener for biopolymer films. In addition, the polyvinyl alcohol (PVA) used as a strength additive in the case of the films prepared using TEMPOoxidised CNF possesses a thermoplastic feature. Distinct temperature dependency was observed with both films above 70 °C. The height of the pillars increased almost linearly up to 155 °C with a constant pressure of 8 MPa. The temperature increase seems to improve the replication of the pillar shape. Simultaneously the nanoimprinting process smoothed the film surface roughness. Recovery and relaxation properties of the NIL-patterned CNF films were studied after one week. Height changes were as high as 36% for CNF film and 20% for TEMPO-CNF film.