Researchers from the University of Maine, in collaboration with Oak Ridge National Laboratory (ORNL) have published a journal article titled, “Polymer-Grafted Cellulose Nanofibrils with Enhanced Interfacial Compatibility for Stronger Poly(lactic acid) Composites” in Applied Polymer Materials on their work developing polymer-grafted cellulose nanofibrils (CNFs) for stronger poly(lactic acid) (PLA) composites. This research, which is a part of the Hub & Spoke program, is significant as it could pave the way for the large-scale manufacturing of CNF/PLA composites, which could have numerous applications across various industries.
CNF, a type of nanocellulose and the most abundant biopolymer on earth have shown potential as a reinforcement for biodegradable composite matrices such as PLA, but they require commercially scalable drying methods that preserve their fibrillar morphology along with improved interfacial interactions with polymer matrices. In this study, researchers used a water-based grafting-through polymerization scheme to modify CNFs, which improved spray drying behavior and reinforcement capacity in PLA composites.
The researchers, Peter Kelly, Siamak Shams Es-haghi, Meghan Lamm, Katie Copenhaver, Soydan Ozcan, Douglas Gardner, and William Gramlich found that all polymer modifications yielded CNFs with a more fibrillar morphology, or the physical structure or shape of a material or substance that consists of long, thin fibers or fibrils after spray drying, increasing specific surface area by up to 490% compared to unmodified CNFs, values similar to conventional freeze drying. Moreover, polymer-grafted CNFs in PLA composites improved the tensile strength by 16% at 20 wt % loading and stiffness by 22% at a 10 wt % loading with two different graft-polymer chemistries compared to unmodified CNF composites.
To understand the improvements in composite properties, the researchers employed surface energy heterogeneity measurements of the reinforcements and PLA matrix. They found that polymer modifications lowered the total surface energy of the CNFs, and calculated ratios of work of adhesion to work of cohesion suggested improved interfacial compatibility for four of the modified CNFs with PLA.
The researchers also conducted rheological oscillatory shear studies of the composites, which correlated solid-like melt behavior, as demonstrated by storage moduli dominance, with higher tensile strength. Thermal analysis of the composites revealed that excessive plasticization by the poly(oligoethylene glycol methyl ether methacrylate)-grafted sample potentially offset mechanical property improvements imparted by the more fibrillar morphology.
The researchers’ work provides an opportunity for large-scale manufacturing of CNF/PLA composites via an entirely aqueous modification scheme and industrially relevant spray drying process. This study’s findings could have significant implications for the development of sustainable and biodegradable materials for various industries.
Advanced Structures & Composites Center