We have demonstrated that 1H spin–spin relaxation time (T2), which is measured by using low field NMR techniques, is an effective parameter to elucidate the degradation behavior of poly (urea-urethane). It is difficult to reveal the degradation behavior of poly (urea-urethane), when the degradation proceeds in urea or urethane bond units.
We prepared two types of poly (urea-urethane), which were Urethane-A and B, and aged them at 100 °C for up to 10,000 h. After that, we measured tensile strength, elongation at break and T2. We also observed 13C DP/MAS and 15N CP/MAS NMR spectra.
The tensile strength of Urethane-A decreased, increased and decreased again, and that of Urethane-B decreased slowly. 13C and 15N NMR spectra revealed the degradation behavior of Urethane-A and B. The degradation of Urethane-A was caused by molecular chain scission and cross-linking in urea bond units, and that of Urethane-B proceeded with molecular chain scission in urethane bond units.
T2 had three components (T2S, T2I and T2L). The results showed that T2S and T2I described the molecular mobility of urea and urethane bond units, respectively. Molecular chain scission in urea bond units reduced the fraction of T2S (xS), and the cross-linking reaction in them increased xS. Molecular chain scission in urethane bond units decreased T2I value itself, not the fraction of T2I. Molecular chain scission in urea bond units causes the destruction of hard segment phase. It is because molecular mobility of fragments in-between the scission points becomes high so that it contributes to the fraction of T2L. In contrast, molecular chain scission in urethane bond units increases T2I value itself, but the increase does not become significantly large owing to the remaining urea bond units. Therefore, the degradation sites can be specified by measuring T2, when it proceeds in-between urea or urethane bond units.