A set of flexible water-blown slab stock polyurethane foams and their respective compression molded plaques as well as a chemically similar polyurea-urethane elastomer, PUUE, were studied to better understand the relaxation behavior and the molecular orientation upon deformation in these systems. The two main experimental techniques used in this investigation were stress relaxation in tension and deformation-IR dichroism. The stress relaxation in the foams and their respective plaques increased with hard segment content. The stress relaxation in the foams also appeared to depend very little on its anisotropic cell geometry and hence, mainly on the material comprising, the cell-wall struts and windows. Segmental orientation was measured as a function of elongation and relaxation, as well as hysteresis behavior for several of the plaques and the PUUE elastomer. The orientation changes upon deformation in the soft segments of both materials were small. Small changes in orientation with time and upon cyclic straining were also observed for the soft segments. The orientation at the interface of the hard and soft segments was influenced more by the soft segments in comparison to the hard segments in the plaques and in the PUUE elastomer. Significant transverse orientation upon deformation was observed in the hard segments of the plaques and up to elongations of 100 percent for the PUUE elastomer. Based on this transverse orientation behavior, the polyurea aggregates in the plaques were thought to possess a lamellar-like structure with the long axis of the aggregates aligning in the stretch direction. Relaxation and hysteresis behavior were observed upon following the orientation of the hard segments of the PUUE elastomer, but were negligible in that of the hard segments of the plaques. / Master of Science