Polyurea hard segment model compounds and a series of segmented poly(urethane urea) (SPUU) copolymers, with hard segment content ranging from 18.8-91.0%, were synthesized based on 4,4’-diphenylmethane diisocyanate (MDI), 3,5-diethyltoluenediamine (DETDA), and ethylene-oxide-capped poly(propylene oxide) polyether diol (PPO). By analysis of model compounds, crystallization of DETDA-MDI hard segments was confirmed and relationships between urea hydrogen bonding with morphological changes were suggested. The crystallization, however, was not complete in the copolymers even when the hard segment content was high. New spectral evidence for phase inversion is presented. Fourier transform infra-red analysis suggested that the equilibrium of urea hydrogen bonding between crystalline region and amorphous regions was affected mainly by morphological changes (phase inversion) and was almost independent of chemical composition of the SPUU copolymers under certain morphological conditions both before and after phase inversion.