Viscoelastic relaxation attributed to the molecular dynamics of polyrotaxane confined in an epoxy resin network

Abstract

The molecular dynamics of polyrotaxane (PR) dispersed homogeneously in a cross-linked epoxy resin were studied using dynamic mechanical analysis (DMA) and pulsed NMR spectroscopy. In PR, poly-e-caprolactone (PCL)-grafted α-cyclodextrins (CDs) are threaded on a polyethylene glycol (PEG) axis. At low temperatures, the PEG and PCL chains of the PR embedded in the epoxy network are in a glassy state. With increasing temperature, the PEG in the PR undergoes a glass-to-rubber transition and fluctuates in the glassy PCL-grafted CDs confined by the epoxy matrix, which causes viscoelastic relaxation. The glass transition temperature, Tg, of the PEG in the PR is much higher than that of pure PEG because of the strong confinement effect in the epoxy network. In addition, the Tg of the PEG drastically changes with coverage by the CDs on the PEG, suggesting that the topological constraint by the CDs also substantially influences the PEG dynamics. The viscoelastic relaxation ascribed to PEG enhances the deformability and toughness of the epoxy resin containing PR under uniaxial stretching. The molecular dynamics of PCL-grafted polyrotaxanes (PRs) homogeneously dispersed in a cross-linked epoxy network were investigated using viscoelastic mechanical measurements and relaxation time measurements with pulsed NMR spectroscopy. With increasing temperature, the PEG axial chains in the PRs exhibit a glass-rubber transition and start fluctuating in the CDs with glassy PCL graft chains, which causes viscoelastic mechanical relaxation.