Vapour-assisted roll-to-roll nanoimprinting of micropillars on nanocellulose films

Here we demonstrate a straightforward vapour-assisted roll-to-roll nanoimprinting lithography (R2RVANIL) patterning method to biobased nanocellulosic films with the aim to fabricate microstructured films with diffraction effect. In the R2RVANIL technique a roll with pillar structures is softly pressed against nanocellulose film in the presence of hot water vapour replicating the pattern onto the film. The printed pillar structures are compared with the structures produced using traditional thermal R2RNIL. The manufacturing method is demonstrated using optically transparent TEMPO-oxidised cellulose nanofibrils (TEMPO CNF). TEMPO CNF films possess high tensile strength and modulus whereas the tensile strain is low, they are very hygroscopic - a property which improves material processing at elevated humidifies, thus overcoming a lack of a clear softening point. In this context, the water molecules are acting as plasticising molecules thereby softening the relative brittle nanocellulose film structures. Patterned pillars were achieved due to the swelling and softening of the film which was contacted with moist air. R2RVANIL method appears to be more gentle and surface properties such as roughness are different from the ones achieved via conventional thermal NIL method. The diffraction grating is demonstrated using laser beam compared to calculated diffraction patterns. Water vapour assisted patterning methods are beneficial, for example, in the future optics and electronics where films cannot be processed under high pressure and, on the other hand, high production volumes are needed.