Effect of the geometry of cellulose nanocrystals on morphology and mechanical performance of dynamically vulcanized PLA/PU blend

Abstract

Effects of the addition of single but geometrically different cellulose nanocrystals (CNCs) namely cylindrical CNCs (ultrasonic treated), spherical CNCs (chemically treated) and rod-like CNCs (commercial) on microstructural development and mechanical performance of the thermoplastic vulcanizates (TPVs) based on polylactic acid/polyurethane prepolymer (80/20) were studied. It was shown that a fraction of PLA molecules reacted with PU molecules during the dynamic crosslinking process. This was confirmed by the results of gel content measurements which showed an appreciable amount of additional gel content. This phenomenon together with the dynamic vulcanization process facilitated the development of a dual-continuous morphology in the TPV sample which was confirmed by the melt viscoelastic results and TEM micrographs. Interestingly, the addition of spherical CNCs to the TPV led to a remarkable enhancement in its toughness. A similar effect but to a lesser extent was observed in the TPV sample filled with cylindrical CNCs. These results could be explained in terms of a 3D percolated or interconnected network microstructure formed between finely dispersed CNCs localized in PLA and/or between CNCs and the PLA matrix. This fact was evidenced by observation of strong liquid–solid transition in the melt viscoelastic results, as well as TEM and AFM micrographs. On contrary to other counterparts, the rod-like CNCs had a weakening effect on toughness of the TPV which was attributed to their poor dispersion and/or formation of their aggregates which could act as stress concentrators. Moreover, DMTA results showed an almost similar reinforcing effect for the three CNCs on increasing the modulus of the TPV below Tg of the PU and partially compensated the PU reducing effect on modulus in the temperature range between the Tg of PU and PLA.