Enhanced dispersion of cellulose nanocrystals in melt-processed polylactide-based nanocomposites

Dispersion and distribution of cellulose nanocrystals (CNC) in a thermoplastic matrix is one of the most important issues in the development of CNC-based high performance composites. During melt processing, agglomeration of CNC is prone to occur due to poor polymer wetting on the hydrophilic CNC surface and to strong particle–particle interactions. Because of the high temperature and intensive mixing involved in melt-processing, degradation of the CNC is also possible. To avoid these problems, solvent mixing followed by solvent casting is the main processing route used in the majority of studies on polymer–CNC composites. In this work, we have explored a novel two-step process where solvent-mixing and melt-mixing were carried out sequentially to improve the overall dispersion of the CNC. The first step consisted in forming a CNC suspension into a polyethylene oxide (PEO) aqueous solution. In the second step, water was removed by freeze-drying to form a water-free well dispersed PEO/CNC mixture. The final step consisted in melt-mixing the PEO/CNC mixture into PLA for the preparation of the composites. PEO and PLA are known to be miscible in certain molecular weight and composition ranges, thus leading to a composite where the CNC particles are well dispersed into a homogeneous mixture of PLA and PEO. Two different PEO molecular weights were investigated in this study, and several formulations were compared under the same processing conditions. Direct blending of CNC and molten PLA was also carried out for comparison purposes. CNC particles tended to agglomerate during blending but the agglomerates were smaller and their number was considerably decreased when the PEO content increased in the formulation. At the highest PEO/CNC ratio, no agglomerates were observed. Thermomechanical and rheological properties of the PLA-based nanocomposites were also investigated.