Local and cooperative molecular mobilities in thermoplastic polymers

Comparative measurements of calorimetry and low-frequency mechanical spectroscopy (temperature interval between 120 and 400 K) in a thermoplastic interpenetrating polymer network based on semicrystalline poly(urethane) and a styrene–acrylic acid block copolymer show the existence of distinct calorimetric and mechanical transitions associated with the two components, as a clear indication of a multiple-phase heterogeneous structure. In contrast with the apparent thermodynamic incompatibility of the components, significant deviations in the magnitudes of local and cooperative transitions from a simple dilution effect are revealed. These findings are associated with a limited miscibility due to weak interactions (hydrogen bondings) between the functional groups of the two polymeric components.