Development of Cellulose Nanocrystal-Reinforced Polylactide

Recently, there has been an increasing interest to incorporate cellulose nanocrystals (CNCs), the rod-like nanoparticles obtained by acid hydrolysis of cellulose, into biopolymers, such aspolylactide (PLA), in a bid to expand their applications. Such an endeavor would result in improved mechanical and thermal properties over a wide range of temperatures. Interestingly, this combination of bio-based, biocompatible and biodegradable components would create entirely green composites to replace petroleum-based polymers, especially in packaging and automotive industries. Generally, neat PLA exhibits slow crystallization and consequently low degree of crystallization upon fast cooling and low heat resistance, a drawback that limits its application widely.

Fortunately, the use of reinforcing agents has presented a possible solution to this shortcoming. Nonetheless, the latter does not come without its challenges, particularly during the PLA–CNC nanocomposite preparation. In a recently published study, the application of a simple solvent casting to favor a good dispersion of CNCs within the PLA matrix has been reported and the strong effect of CNCs on the rheological behavior of the PLA even at a very low CNC content demonstrated. Still, a more efficient approach is needed.

Recently, Polytechnique Montréal scientists Dr. Davood Bagheriasl, Dr. Fatemeh Safdari, Professor Pierre Carreau, Professor Charles Dubois together with Professor Bernard Riedl at University of Laval developed a facile technique based on a previous approach for preparing PLA–CNC masterbatch. In particular, their main objective was to develop an efficient preparation method to prepare high performance PLA–CNC nanocomposites using both solution and conventional melt mixing methods that are more cost-effective, as compared to other PLA–CNC preparation methods involving only solvent-casting, in situ polymerization, or the use of a compatibilizer. Their work is currently published in the research journal, Polymer Composites.

Briefly, the research team used the solution-cast method to prepare a masterbatch of polylactide containing cellulose nanocrystals which was further diluted with neat PLA in the melt state using either a twin-screw extruder or an internal batch mixer to reach a final CNC content of 4 wt%. For comparison purposes, a direct melt mixing method was employed to prepare PLA–CNC composites. Lastly, characterization using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was undertaken.

The SEM imagery revealed the disappearance of large agglomerates when using the PLA-CNC masterbatch while as the TEM characterization revealed the existence of well-dispersed CNCs within the PLA matrix at a nanoscale for masterbatch-based nanocomposites. Surprisingly, the rheological properties of the nanocomposites containing the PLA-CNC masterbatch were significantly increased for both steady and small-amplitude oscillatory shear flow fields, compared to the composites prepared via direct melt mixing.

In summary, the Polytechnique Montréal study presented an in-depth comparison of the efficiency of various methods for preparing PLA-CNC composites in terms of the quality of dispersion of the CNCs in the PLA and the rheological, thermal and mechanical properties of the composites. Altogether, the study is important because it reported the most efficient method for the preparation of PLA-CNC composites was the use of the solution-based PLA-CNC masterbatch followed by melt-extrusion of the masterbatch and PLA.