  We embed the equations of polyconvex thermoviscoelasticity into an augmented,
symmetrizable, hyperbolic system and derive a relative entropy identity in the
extended variables. Following the relative entropy formulation, we prove the
convergence from thermoviscoelasticity with Newtonian viscosity and Fourier
heat conduction to smooth solutions of the system of adiabatic thermoelasticity
as both parameters tend to zero. Also, convergence from thermoviscoelasticity
to smooth solutions of thermoelasticity in the zero-viscosity limit. Finally,
we establish a weak-strong uniqueness result for the equations of adiabatic
thermoelasticity in the class of entropy weak solutions.
