Phase behaviour of thermoplastic interpenetrating polymer networks by thermal and mechanical measurements

The phase behaviour of thermoplastic interpenetrating polymer networks based on semi-crystalline polyurethane and a styrene/acrylic acid block-copolymer has been investigated by differential scanning calorimetry and dynamic mechanical spectroscopy. Distinct calorimetric and mechanical transitions point to an inherent heterogeneity characterizing the structure of thermoplastic networks, mainly due to a weak affinity of the two components. At variance with these observations, deviations from a simple additive law are revealed from the composition behaviour of the melting endotherms associated to polyurethane component and of the gamma- and alpha(a)-relaxation strengths. Quantitative analysis of low temperature gamma-relaxation indicates that the fraction of amorphous chains of polyurethane is enhanced on blending, this result being in close agreement with calorimetric results. The findings suggest the formation of a mixed amorphous microphase as a consequence of specific weak interactions (H bonds) between the functional groups of the two components.