Cyclo-olefin polymers (COP) have been applied to optical materials because of their high moisture resistance and high transparency. However, they also possess poor mechanical properties and insufficient thermal stability for some industrial purposes. Compounding TEMPO-oxidized cellulose nanofibers (TOCNs) is considered as one of the effective ways to improve the mechanical properties of polymer materials. TOCNs, however, tend to aggregate in hydrophobic COP due to their hydrophilic surface, thus preventing the improvement of the mechanical properties. We focused on a Pickering emulsion as an effective and simple method to prepare polymer composites with well-dispersed reinforcing agents. In the Pickering emulsion process, aqueous suspensions of TOCNs and polymer solutions are mixed and ultrasonicated, where TOCNs adhere to the surface of the polymer-solution droplets serving as a surfactant to stabilize the resulting emulsion. After drying the emulsion, we attempted to fabricate polymer composites with well-dispersed TOCNs in COP by compression molding (Fig). It was found that the fabricated TOCNs/COP composite films were transparent. UV-vis results exhibited that TOCNs/COP with the TOCNs content of less than 2 wt% maintained the visible-light (400-800 nm) transmittance of more than 85%. The result indicated that the composites could be potentially used for optical applications. The Young’s modulus of TOCNs/COP increased up to 1327 MPa at the TOCNs concentration of 0.5 wt%, which was actually ~51% higher than that of pure COP (881MPa). The coefficient of thermal expansion of TOCNs/COP films at the TOCNs concentration of 5 wt% became ~11.7×10-5K-1, decreasing by 30% compared to pure COP (16.5×10-5K-1). The deformation temperature increased from ~117°C to ~124°C. According to the morphological analyses, TOCNs were uniformly dispersed inside COP by the Pickering emulsion method, contributing greatly to the improvement of both the mechanical properties and the thermal stability of TOCNs/COP, while maintaining the transparency of COP.

Mechanical and thermal properties of TEMPO-oxidized cellulose nanofibers/cyclo-olefin polymer nanocomposites fabricated via Pickering emulsion process

ACS Spring 2022

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