Constraints and thermal expansion of the free volume in a micro-phase separated poly(ester-adipate)urethane

The free volume behavior of a thermoplastic polyesterurethane (TPU) versus temperature is investigated by positron annihilation lifetime spectroscopy and dilatometry. A discrepancy with the free volume predicted by the lattice-hole theory is found. The agreement is restored by assuming anisotropic expansion of the free volume holes, which in fact results in expansion mainly in two dimensions. This finding is perfectly compatible with a polymer structure based on rheological, thermal, and TEM data which envisage TPU as formed by short soft segments limited in their movements by chain connectivity and confined by physical crosslinks due to the hard segments.