Model butanediol–MDI–polypropylene oxide polymers have been synthesized to explore the structure–property relationships in urethane/polyether polymers. The results of mechanical, thermal, and spectroscopic analyses agree remarkably well. The phase mixing in these polymers decreases with increasing hard‐segment length, while the hard‐segment stability increases with increasing hard‐segment length. This is demonstrated clearly by dynamic mechanical, differential scanning calorimetry, infrared, and NMR studies. The importance of hydrogen bonding to the stability of the hard segment is strongly supported by the variable‐temperature infrared experiments. The critical hard‐segment size for phase segregation was shown to be two butanediols per hard segment. The temperature limit of the flatness of the storage modulus was tied to the thermal stability of the hard‐segment hydrogen bonding which is controlled largely by the length of the hard segment and the extent of the hydrogen bonding.

Model urethane elastomers consisting of 4,4'-methylene diisocyanate, l,Cbutanediol, and a 2000 molecular weight poly(oxypropylenedio1) were examined by solid-state carbon-13 NMR spectroscopy. T, values were determined for the aromatic and butanediol carbons as a function of cross polarization fiefd strength and temperature. The results for the aromatic carbons correlate well with observed hightemperature behavior of the elastomer as determined by dynamic mechanical spectroscopy. The biexponential decay of the butanediol resonances indicated that two types of motional populations can be associated with the phase-segregated portion of the hard segment. These results demonstrate the intimate connection between molecular motion and mechanical properties in urethane polymers. This understanding provides an important framework for optimizing physical properties in urethane polymers.