Second order elasticity at hypersonic frequencies of reactive polyurethanes as seen bygeneralized Cauchy relations

Abstract

The non-equilibrium process of polymerization of reactive polymers can be accompanied bytransition phenomena like gelation or the chemical glass transition. The sensitivity of themechanical properties at hypersonic frequencies—including the generalized Cauchyrelation—to these transition phenomena is studied for three different polyurethanes usingBrillouin spectroscopy. As for epoxies, the generalized Cauchy relation surprisingly holdstrue for the non-equilibrium polymerization process and for the temperature dependence ofpolyurethanes. Neither the sol–gel transition nor the chemical and thermal glass transitionsare visible in the representation of the generalized Cauchy relation. Taking into account thenew results and combining them with general considerations about the elastic properties ofthe isotropic state, an improved physical foundation of the generalized Cauchy relation isproposed.