Formation of Tough and Deformable Films through Evaporating Dispersions of Elastomer Microspheres By Crosslinked with Rotaxane

Abstract

Downsizing polymer materials (<~10 micrometers) to the colloidal scale can be dispersed in the solvent and showed distinctly different behavior from the corresponding bulk materials. One of the most useful application of colloidal dispersions is that they are injectable, which significantly simplifies method of bulk materials. Compared to rigid microspheres based on polystyrene or silica, soft and deformable elastomer microspheres have been used to create colorless transparent films through solvent evaporation. In general, to obtain tough microsphere films, many additives are required, and/or post-polymerization modification is necessary. In this study, we synthesized elastomer microspheres cross-linked with rotaxane, and succeeded in getting tough films by simply evaporating water [1]. To obtain colloidal microspheres with flexible crosslinkers, a rotaxane crosslinker (RC), consisting of a crown ether wheel and an axle with one vinyl group per component. We selected mini-emulsion polymerization, in which an oil monomer droplet is polymerized to become a polymer microsphere, to sufficiently trap the RC within the microspheres because the RC hardly dissolves in water but can be dissolved in organic solvents or oil-monomers. Films were generated by depositing an elastomer microsphere dispersion on a silicon rubber sheet, followed by drying at room temperature and annealing effect. Tensile tests were performed at 25 °C at an elongation rate of 10 mm/min. The BM and BM-D0.05 films, in which the microspheres were crosslinked only physically and by conventional crosslinkers, were brittle because the chain entanglement between the microspheres was able to slip easily upon elongation, and the fracture energies (En ) of these films were <10 MJm-3. In contrast, BM-R0.05 film, where the microspheres were crosslinked by RC, reveal increased fracture stress and fracture strain (Figure 1). It suggested that the stress relaxation in each elastomer microsphere, originating from the flexibility of the crosslinking points, affected on the mechanical properties of the BM-R0.05 film, even though the post crosslinking reaction between the microspheres was not performed. The result of this study should find the advanced applications for such as in the areas of medicine and cosmetics, in which the minimization of impurities is desirable. Furthermore, the effect of film preparation condition will be discussed. Partially reproduced with permission from [1] Copyright (2018) John Wiley and Sons. Reference [1] S. Hiroshige, T. Kureha, D. Aoki, J. Sawada, D. Aoki, T. Takata, D. Suzuki, Chemistry - A European Journal, 2017, 23, 8405 – 8408. Figure 1