Softening and hardening of thermal plastic polyurethane blends by water absorbed

In this work, the effect of water on the structure and mechanical properties of thermoplastic polyurethane (TPU) blends with different ratios of a high and low hardness TPU composition was explored by experiment and simulation. The decrease of modulus of the TPU blends after immersion in water (softening of TPU), and the interesting increase of modulus of wet samples after losing some free water (hardening of TPU) compared to the dry samples, were observed in this work. By analyzing the change of glass transition temperature, the degree of phase separation of samples after immersion in water, and the types of water and hydrogen bond in samples, the possible mechanism behind the above softening and hardening of wet TPU samples was proposed. The softening and hardening of wet TPU samples were attributed to the change of structure in the interface between hard and soft domains in TPU, which is a combined effect including the plasticization of free water, breakage and/or decreasing the strength of original hydrogen bond by water, and formation of new hydrogen bond (elastic active cross-linking point) by free carbonyl groups in TPU and water etc.