Viscoelastic Property Evolution of Thermoplastic Polyurethane during Annealing Treatment and Its Correlation with Segmental Crystallization

Viscoelastic properties of thermoplastic polyurethane (TPU) is of fundamental importance for its processing. In this work, we prepared different TPUs from polycaprolactone (PCL) diol, diphenylmethane-4,4 '-diisocyanate (MDI), and 1,4-butanediol (BDO), and investigated the viscoelastic behavior of three TPUs with different hard segment content during thermal annealing process. The storage modulus (G ') of TPU increases over time in a medium annealing temperature (Ta) region, but remains unchanged at both high and low temperature regions. The growth of loss modulus (G '') over time is slower than that of G '. At medium Ta, both G ' and G '' increase during the repeating frequency (omega) sweep, due to the gradual crystallization of hard segments. This indicates that the crystallites primarily restrain the relaxation of unit with large size. The increments of G ' and G '' are weakened when the content of hard segment in TPU is decreased. For TPU with high content of hard segments, a complete high elastic platform with a width of 3 orders of magnitude was observed only through one frequency scan test at medium Ta. In addition, the crystallites of hard segments grow up continuously during frequency scan test (isothermal annealing treatment) and cause the extreme increase in G ' and G '' with omega in low omega region.