Anisotropic Structure and Properties of Chitin and Chitosan Nanofibril-Supported Starch Foams

Highly surface-charged rigid bionanofibrils are difficult to be processed into integrated foams or aerogels owing to lack of good interfibrillar adhesion. An alternative solution is to involve flexible hydrophilic polymers for gluing fibril bundles, allowing a perfect alignment under ice induction in the lyophilized foams. In this work, we incorporated the defibrillated chitin/chitosan nanofibrils (CTF/CSF) into gelatinized starch foams and investigated the effects of surface charge, solid content, nanofibril loading, and directional orientation on the dimensional stability, morphology, crystallinity, compressive stress, and flame-retardant properties of lyophilized composite foams. Results show that CTF with low positive surface charges presents stable electroattractions to negatively charged starch and also exhibits sustainable reinforcement effects on the composite foam. What is more, compact, small porous, and lamellar CTF-supported foams exhibit relatively high resistance to flammable combustion. In the case of high loading of polycationic CSF, directional freeze-drying successfully induced the channel orientation along the vertical, horizontal, spiral, or bi/triphase junctional direction in the anisotropic foams, which revealed variable compression properties according to the fibril alignment.