Uniform amphiphilic cellulose nanocrystal films

We describe the synthesis of highly amphiphilic cellulose nanocrystals (CNCs) and their assembly into ultrathin smooth monolayers and multilayers by the Langmuir–Blodgett technique. The amphiphilicity of the CNCs and hence their solubility in chloroform was tuned by controlling the extent of surface functionalization with hydrophobic –C 8 groups and the concentration of the carboxylic acid groups at the reducing end. For the most amphiphilic CNCs, a well-defined LB isotherm with three distinct phases was observed without the use of any additives. The assembled monolayers were exceptionally stable at the air/water interface for over 3 h, allowing their facile transfer to both hydrophilic and hydrophobic substrates, resulting in smooth films. A monolayer of amphiphilic CNCs deposited by the upstroke on a hydrophilic substrate showed good alignment along the substrate, while random orientation was observed on a hydrophobic substrate. Atomic force microscopy study of the monolayer transferred at a surface pressure of 15–20 mN/m showed complete monolayer coverage. Smooth multilayers were also fabricated by the sequential deposition of up to six monolayers. The excellent control over film formation afforded by the combination of the amphiphilicity of the CNCs, LB assembly, and the use of organic solvent make this methodology useful for emerging optical and electronic applications of CNCs.