Improved Nanocomposite Performance Using Carboxylated Cellulose Nanocrystals

Abstract

This thesis aims to develop sustainable polymer nanocomposites by capitalizing on the unique properties of carboxylated cellulose nanocrystals (cCNCs)—a bio-based nanomaterial derived from wood pulp via an environmentally friendly hydrogen peroxide oxidation process. cCNCs are hydrophilic, making dispersion in hydrophobic polymers challenging. This thesis explores strategies to enhance polymer performance while addressing dispersion issues through surface changes and processing techniques. Two important industrial polymer systems were selected: polylactic acid (PLA) and latex-based pressure-sensitive adhesives (PSAs), which both stand to benefit from renewable nanofillers that can boost performance without compromising sustainability. In the first project, a commercial PSA formulation was introduced into a seeded, semibatch emulsion polymerization (2-ethyl hexyl acrylate/methyl methacrylate/styrene), which successfully achieved high tack and peel strength. Yet, the shear adhesion remained insufficient for industrial applications. By integrating cCNCs, the PSA films demonstrated simultaneous increases in tack, peel strength, and shear adhesion, avoiding the property trade-off between tack and peel strength, and shear adhesion. Key to these gains was cCNC reassembly: driedredispersed cCNCs, especially under sonication, were more prone to forming nanofibril-like networks, reinforcing the adhesive film and substantially boosting shear adhesion (by >1,300% in some cases). Systematic experiments revealed that cCNC dispersion and subsequent end-toend realignment into nanofibrillar networks proved vital to maximizing adhesive performance. Building on this framework, the second study investigated whether cCNCs' compatibility with the hydrophobic polymer matrix could be further improved by acetylation. Through a systematic variation of degrees of substitution (DS), the hydrophilic cCNC surfaces became increasingly hydrophobic. cCNCs with the highest DS (0.42) demonstrated noticeably better PSA performance than unmodified cCNCs. Improved dispersion and latex particle coalescence were encouraged by lower interfacial tension, which ultimately led to higher tack, peel strength, and shear adhesion. Overall, these findings reinforce how fine-tuning cCNC surface chemistry can enhance nanoparticle networks and mechanical performance in latex-based PSAs. To investigate the further potential of cCNCs and their surface modification, we explored their influence on the crystallization behaviour of polylactic acid (PLA). While PLA’s renewable origin and biodegradability make it attractive, commonly used solvents for dispersing CNCs—such as chloroform and dimethyl formamide—undercut its environmental appeal. Here, ethyl lactate (EtLa) was adopted as a safer, bio-based solvent that also acts as a plasticizer, enhancing chain mobility and lowering PLA’s cold crystallization temperature. Incorporating cCNCs in EtLa not only accelerated PLA crystallization but also improved mechanical performance, as evidenced by polarized optical microscopy, X-ray diffraction, and differential scanning calorimetry results. Acetylated cCNCs (AcCNCs) further refined nucleation efficiency, with lower DS AcCNCs achieving superior dispersion and fostering rapid spherulitic growth. Though higher DS AcCNCs increased the storage modulus, they were comparatively less effective in nucleation efficiency due to suboptimal interactions in EtLa. Consequently, balancing DS emerged as the key to delivering enhanced crystallization, better mechanical robustness, and an overall more sustainable PLA nanocomposite—achieved without relying on harmful solvents. The adaptability of cCNC-based nanocomposites is highlighted overall in this thesis, which demonstrates how specific surface alterations and processing condition adjustments might narrow the performance and sustainability gap. The insights here offer a foundation for future research, from large-scale acetylation protocols to alternative green solvents, ensuring that cCNCs continue to drive innovation across various polymer applications.