Relaxation suppression in a stretched copolymer matrix above T_g

Electrospun thermoplastic polyurethane (TPU) copolymer nanofiber mats are known to contract considerably upon heating up to ∼90 C, whereas cast TPU films expand as expected. This work examined contraction in single electrospun nanofibers. In contrast to nanofiber mats, where mat contraction appears like a critical phenomenon, no temperature threshold for contraction was measured for a single electrospun nanofiber. Unexpectedly, we demonstrate that cast TPU films can also massively contract upon heating, but only after thermomechanical programming which relies on film stretching (∼100%) at a high temperature (∼90 C). This nonequilibrium stretched state is highly preserved, despite sample temperatures that significantly exceeded the glass transition temperature of the soft phase, and hard segments concentration in the TPU macromolecules is too low to form a percolated "solid" system. A possible physical explanation of the observed phenomenon is proposed.