Clarifying the Origin of Multiple Melting of Segmented Thermoplastic Polyurethanes by Fast Scanning Calorimetry

Segmented thermoplastic polyurethanes (TPU) often show multiple endothermic signals during melting in conventional differential scanning calorimetry (DSC). Repeated reorganization during heating and separate crystal melting and phase mixing have been proposed as the origin of this phenomenon. We used fast scanning calorimetry on a series of TPUs with polyether soft segments and hard segments consisting of 4,4'-methylenediphenyl diisocyanate and 1,4-butanediol to differentiate between the two possibilities. Fast heating scans (>500 degrees C s(-1)) of isothermally crystallized samples exhibited only a single melting peak, while complementary experiments on quenched, fully amorphous samples showed a single glass transition step indicating a homogeneous mixed phase. These results prove that the multiple melting peaks observed in DSC are related to reorganization during heating, i.e., repeated melting and recrystallization. Phase separation during cooling of such TPUs is induced by crystallization. A comparison with conventional DSC scans shows the consistency of both calorimetric techniques in the limit of low heating rates.