Iridescent cellulose nanocrystal/polyethylene oxide composite films with low coefficient of thermal expansion

Simultaneous control over optical and thermal properties is particularly challenging and highly desired in fields like organic electronics. Here we incorporated cellulose nanocrystals (CNCs) into polyethylene oxide (PEO) in an attempt to preserve the iridescent CNC optical reflection given by their chiral nematic organisation, while reducing the composite thermal expansion. The hydrophilic nature and long-range self-organisation of CNCs facilitated structural control in the PEO matrix. The coefficient of thermal expansion (CTE) was determined by using contrast-enhanced microscopy digital image correlation (CEMDIC) based on textural features revealed within CNC/PEO composites under polarised light. The attained composite films were iridescent and exhibited a significant reduction in CTE even with low CNC addition (∼50% CTE reduction at ∼10 wt.% CNC load). With further control over nanoparticle processing, such composites promise potentials for selective optical bandgap materials while tuning the CTE.