Strain induced strengthening of soft thermoplastic polyurethanes under cyclic deformation

TL;DR

This study reveals that thermoplastic polyurethanes (TPUs) exhibit a unique strain-induced stiffening effect under cyclic tension, contrasting with typical softening in filled rubbers, due to hard domain fragmentation and formation of new physical crosslinks.

Contribution

It introduces a novel self-reinforcement mechanism in TPU caused by hard domain fragmentation, differing from traditional Mullins effect in filled elastomers.

Findings

TPUs show stiffening after cyclic loading, unlike softening in filled rubbers.

Hard domain fragmentation creates smaller units acting as physical crosslinks.

The mechanism is akin to strain-induced crystallization but is persistent and localized.

Abstract

We investigate the cyclic mechanical behavior in uniaxial tension of three different commercial thermoplastic polyurethane elastomers (TPU) often considered as a sustainable replacement for common filled elastomers. All TPU have similar hard segment contents and linear moduli but sensibly different large strain properties as shown by X-Ray analysis. Despite these differences, we found a stiffening effect after conditioning in step cyclic loading which greatly differs from the common softening (also referred as Mullins effect) observed in chemically crosslinked filled rubbers. We propose that this self-reinforcement is related to the fragmentation of hard domains, naturally present in TPU, in smaller but more numerous sub-units that may act as new physical crosslinking points. The proposed stiffening mechanism is not dissimilar to the strain-induced crystallization observed in stretched natural rubber, but it presents a persistent nature. In particular, it may cause a local reinforcement where an inhomogeneous strain field is present, as is the case of a crack propagating in cyclic fatigue, providing a potential explanation for the well-known toughness and wear resistance of TPU