Viscoelasticity, stiffness gradient and their effects on adhesion of an epoxy shape memory polymer

Abstract

The shape memory polymer based adhesives have demonstrated excellent programmable switchable adhesion, in which the material properties play an important role. Here, the viscoelasticity, stiffness gradient, and their effects on adhesion of an epoxy shape memory polymer (ESMP) were studied. The ESMP sample and polydimethylsiloxane (PDMS) control sample were fabricated firstly by mold casting and curing techniques. The sample was fixed on an electric heating plate with double-sided tape to conduct the adhesion measurements against a hemispherical glass indenter under different temperature and displacement conditions by using an adhesion tester and the temperature measurements on the double-sided tape and ESMP sample surfaces by using a digital thermodetector. Based on the measured force-displacement-time data, the hysteresis behavior and internal dissipation were evaluated; the peak force relaxation was studied; the reduced modulus was calculated and the stiffness gradient features were evaluated. The results show that the ESMP sample exhibits strong viscoelasticity and significantly enhanced adhesion that strongly depends on the compressive displacement in the glass-rubber transition zone near it's glass transition temperature (T_g), while weaker viscoelasticity and lower adhesion in the soft rubbery state. The reduced modulus of the ESMP sample obviously decreases with the increasing compressive displacement at a preset temperature below 70 ℃. It is found that there exists a linear relation between the pull-off force, effective work of adhesion, reduced modulus, and contact area. This study helps deeply understand the material properties and adhesion mechanism of the ESMP, which can guide the design of switchable adhesives.