Chiral Cellulose Nanocrystals with Intercalated Amorphous Polysaccharides for Controlled Iridescence and Enhanced Mechanics

A unique case of combined enhanced mechanical performance and tunable structural color for chiral composites from cellulose nanocrystals (CNCs) arises by adding nanofibrillar wood-derived polymers with similar chemical compositions. When amorphous polysaccharides (pullulan, dextran, and xylan) are added, they intercalate seamlessly into the original helicoidal organization via interstitial volumes within nanocrystals and between nematic monolayers. The polysaccharides fill the available free volume in between packed nanocrystals until 40 wt% content is reached, with no phase separation occurring. Due to the inter-nanocrystal intercalation, these natural polysaccharide-cellulose composites show a nearly twofold increase in toughness and modulus. Beyond improved mechanics, the preserved iridescence shows a dramatic red shift from blue to near-infrared region, expanding the initial pitch length without disturbing the long-range chiral ordering. In the mechanistic model suggested here, the individual backbones first form intercalated morphologies in the interstitial volumes between tightly packed nanocrystals. After this, the polysaccharides form a monolayer, and eventually double layer, between nanocrystal monolayers, thus incrementally unwinding initial chiral organization. The resulting CNC-polysaccharide films maintain their vivid iridescence with broad color appearance and are among the first entirely biobased composites to maintain iridescence with improved mechanics.