Fabrication and characterization of cellulose nanocrystal enhanced sustainable polymer nanocomposites through surface chemistry and processing
Cellulose nanocrystals (CNCs) belong to a class of cellulose based nanomaterials that are extracted from renewable and sustainable sources and have excellent mechanical and thermal properties. While applications for CNCs have been expanding, one of the challenges of utilizing CNCs is to overcome their low dispersibility in hydrophobic polymers. In the present work, several approaches are utilized to improve the interfacial compatibility and overall performance of CNC/epoxy and CNC/polyamide nanocomposite. For a two-part epoxy system, a novel approach was taken to disperse CNC in epoxy matrix by pre-formulating CNC into the hardeners. Three types of hardeners were evaluated for their compatibility with CNC. With less than 2 wt% CNC, Young’s modulus was increased by 16% in JD400 cured epoxy and by 19% in DETA cured epoxy. The DETA cured specimens had the highest increases in mechanical properties among the three hardeners evaluated. For all three hardeners cured specimens, residual water from the solvent exchange process plasticized the epoxy matrix. However, CNC was able to maintain and even improve the mechanical properties. To further improve compatibility, CNCs were surface modified through various esterification methods to graft side chains of different hydrophobicity. The grafting efficiency and preservation of structural morphology and crystallinity of CNCs were evaluated. Acid anhydride and CDI methods were found to be the most applicable reagents to graft short and long chain aliphatic carbons, respectively. The hydrophobicity of grafted CNCs was evaluated by dispersing them in organic solvents with different Hansen’s solubility parameters. The dispersibility of grafted CNCs in organic solvents was improved by using never-dried CNCs as source materials and keeping CNCs wet in their washing solvents after grafting, thus increasing the solvency range to disperse CNCs. Finally, surface modified CNCs were utilized to fabricate polyamide 11 (PA11) nanocomposites. The effects of CNC and plasticizing side chains on the mechanical and thermal properties of PA11 were evaluated. Using surface modified CNCs, the tensile modulus and strength both increased while the toughness of PA11 was preserved. In addition, the formation of shear band-like structures were also observed during uniaxial tensile testing. When PA11 and nanocomposites were annealed near their melting point, crystals under a certain size threshold were melted and potentially recrystallized on larger crystals that were above the size threshold. The larger crystals subsequently increased in size as annealing time increased. The addition of CNC inhibited this transformation, while surface modified CNC promoted it at early stage of annealing.
Moon, Purdue University.
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