Carbon fiber reinforced polymer matrix composites are commonly used to fabricate energy-absorbing structures expected to experience impact loading. As such, a detailed understanding of the response of the constituent materials is necessary. Since the rate, temperature, and pressure dependence of fiber reinforced polymer matrix composites is a manifestation of the rate, temperature, and pressure dependence of the polymer matrix, it is crucial that the constitutive behavior of the polymer be accurately characterized. In this work, an existing viscoplastic polymer constitutive formulation is extended to more accurately account for the tension-compression asymmetry observed in the response of polymeric materials. A new plastic potential function is proposed, and elementary loading conditions are utilized to determine relations between model constants to ensure the potential function is positive valued. Expressions for the tensile and compressive plastic Poisson’s ratios are derived and used to determine bounds on model constants to ensure physically realistic plastic flow. The model is calibrated against experimental data across a range of strain rates, temperatures, and loading cases for a representative thermoset epoxy; good correlation between simulations and experimental data is obtained. Temperature rises due to the conversion of plastic work to heat are computed via the adiabatic heat energy equation. The viscoplastic polymer model is then implemented into the generalized method of cells micromechanics theory to investigate the effects of adiabatic heating on unidirectional composite response. Significant thermal softening due to the conversion of plastic work to heat is observed for matrix dominated deformation modes.