Synthesis, properties and applications of functional polymer nanocomposites

Abstract

Polymer nanocomposites have been extensively studied and have found numerous applications as they provide versatile materials with substantially enhanced properties. Despite the enormous developments, design of novel value-added polymer nanocomposites with new superior functional properties through convenient low-cost synthesis remains a continuous challenge. This thesis demonstrates the alternative, simple design of several novel polymer nanocomposite systems with enhanced mechanical, surface, or catalytic properties. Firstly, a new method for the modification of cellulose nanocrystals (CNCs) is developed with the use of hyperbranched polyethylene ionomers containing cationic quaternary ammonium ions through an ionic interaction mechanism. A systematic study has been undertaken on the modification process and the modified CNCs. In contrast to original CNCs that can only disperse in water or few highly polar solvents, ionomer modified CNCs are able to disperse in several nonpolar or low polarity organic solvents. Dispersions of several modified CNCs in THF exhibit the unique thixotropic rheological behavior. The modified CNCs have also become dispersible in a commercial non-polar hydrophobic ethylene-octene copolymer (EOC) elastomer due to the presence of nonpolar polyethylene modification layer. Based on thermal, rheological, and tensile mechanical characterizations, EOC nanocomposites filled with the modified CNCs are significantly reinforced with nearly doubled tensile modulus relative to neat EOC while with a much better-maintained elongation at break relative to those filled with unmodified CNCs or surfactant-modified CNCs. iv Secondly, a class of CNC-sodium alginate (SA) nanocomposites derived exclusively from sustainable biopolymers has been designed to fabricate tough-strong nanocomposite film. A systematic study on the effects of composite composition on the optical, thermal, and mechanical properties of the prepared films has been undertaken. The calcium ion crosslinked composite films maintain high film transparency with higher thermal and mechanical properties than the uncross-linked films, indicating that the calcium ions play an important role in the enhancement of mechanical and thermal properties. The effects of various metal ions on film mechanical properties have also been studied. As a result, the bivalent calcium ions show the most optimum effect to render strong-tough composite films. Thirdly, hybrid composites of multi-walled carbon nanotubes (MWCNTs) decorated with polycyclopentene crystals have been synthesized by a novel in situ Pd-catalyzed cyclopentene polymerization technique. It is demonstrated that the method offers a convenient, large-scale, one-pot noncovalent surface decoration of polycyclopentene crystals on the MWCNTs. Controlling the catalyst loading and/or polymerization time in the polymerization effectively tunes the composition and morphology of the as-prepared hybrid composites. Interestingly, films made of the composites show the characteristic lotus leaf-like superhydrophobicity featured with high water contact angle (> 150), low contact angle hysteresis (< 10), and low water adhesion, while being electrically conductive. Lastly, a systematic study on ligand-assisted selective hydrogenation of alkynes (phenylacetylene and diphenylacetylene) over three Pd nanocatalysts have been presented with the purpose of identifying the most optimum ligands. Five ligands, including quinoline, pyridine, DMSO, 3,6-dithia-1,8-octanediol (DTO), and triphenylphosphine, have been v screened with their performance compared. It is demonstrated that the sulfur-containing DTO and phosphine-containing triphenylphosphine are the more efficient and practical ligands in improving the alkene selectivity of the catalysts.