DSC Study of Morphological Changes in Segmented Polyurethane Elastomers

Abstract

Annealing studies were carried out to investigate the morphological changes in segmented polyurethane elastomers (PUR) induced by annealing as a function of temperature. Four series of PUR elastomers were studied by varying the hard segment type (MDI/BD and H₁₂MDI/BD), the content of the hard segment (35-57% by weight), the soft segment type (PTMO and PCL) and the soft segment molecular weight (PTMO 1000 and 2000, PCL 1250). Differential scanning calorimetry analyses, DSC, of these materials provided information about morphological changes during annealing at various temperatures (70, 130, 160 and 180°C). The effects of annealing process are visible in PUR elastomers based on the aromatic (MDI/BD) isocyanate and those based on the cycloaliphatic (H₁₂MDI/BD) isocyanate. The values of the soft segment glass transition temperature (T _g ) decreased on the higher annealing temperatures in PUR elastomers based on the aromatic isocyanate and on the cycloaliphatic isocyanate. This behaviour was connecting with better phase separation after annealing in both PUR elastomers.

A comparison of thermal results shows that the MDI/BD series elastomers exhibit a considerable degree of hard segment crystallinity whereas the H₁₂MDI/BD series elastomers are amorphous. Analyses of the DSC results indicate that increasing of the hard segment content leads to higher T _g values for control samples and samples after annealing, except for some samples whose T _g decreased after annealing at higher temperatures. This behaviour results from partial compatibility and therefore phase mixing of soft and hard segments. Endotherms due to the ordering in the hard segment domains appeared at higher temperature. The T _g of PUR elastomers based on polyether's soft segment phase increased with annealing process. The opposite effect was obtained in PUR elastomers based on polyester's soft segments. The same effect was obtained for elastomers with higher hard segment content. The results showed that the soft segment T _g decreased with increased molecular weight of the soft segments (PTMO 2000). This indicates that the greater degree of phase separation was allowed by the longer soft segment lengths. Endotherms resulting from hard segment ordering appeared at higher temperatures for higher annealing temperatures.