Strengthening polylactic acid (PLA) composites with poly(methyl methacrylate)-functionalized cellulose nanofibrils created through grafting-through emulsion polymerization

Abstract

Cellulose nanofibrils (CNFs) were surface modified with poly(methyl methacrylate) (PMMA) in water by a grafting-through surfactant free emulsion polymerization scheme resulting in reinforcements that could be straightforwardly dried while maintaining a high specific surface area. These PMMA modified CNFs contained 40 wt% PMMA, could be filtered to remove most the of water, and subsequently dried under vacuum to yield powders that could be directly used as reinforcements for composites. The PMMA modification prevented fibrillar collapse upon drying yielding high specific surface area (ca. 50 m² g⁻¹) and surface energy similar to PMMA. Once melt compounded into PLA, PMMA modified CNFs led to composites with a tensile strength of 79 MPa, a nearly 30% increase over neat PLA, at 20 wt% loading of the reinforcement. The mechanism of improvement was attributed to the improved interfacial compatibility between the PMMA modified CNFs and the PLA as confirmed by surface energy measurements and the ability of the reinforcement to disperse within the PLA matrix as confirmed by imaging and rheological measurements. Overall, this work demonstrates that a scalable water-based modification can be used to create CNF reinforcements for PLA composites that significantly improve mechanical properties without complex drying and solvent exchange processes.