Temperature-Controlled Molecular Bonding Hysteresis: Interphase Dynamics of a Nanoparticle-Modified Polymer Network

Abstract

This study demonstrates the existence of temperature-induced molecular bonding hysteresis at nanoparticle−polymer interfaces in a highly cross-linked epoxy-based polymer, modified with core−shell rubber nanoparticles. This thermally induced bond hysteresis manifests itself in a hysteresis-like change of the strength of the electrical bond polarization between epoxy molecules and surface molecules of the core−shell nanoparticles. This kind of dynamic bond behavior can be controllably switched from one bond state to the other by a sufficient temperature change. The related optical remanence is evidenced by a refractive index hysteresis independent of the temperature change using the new experimental technique of temperature-modulated optical refractometry (TMOR). From the investigation of quasi-static and dynamic thermal expansion separately, TMOR allows for the conclusion that the observed hysteresis is caused by the specific refractivity and not the dipole number density.C ross-linked polymers, such as epoxy-based thermosets (EP), are indispensable in modern applications and serve, e.g., as adhesives or coatings or are used as polymer matrices for fiber-reinforced composites. 1,2 These materials are amorphous in the cross-linked state and have isotropic symmetry. Above the canonical glass transition temperature