Optimization of the annealing procedure for thermoplastic polyurethane systems using <i>in situ</i> dynamic mechanical analysis

Thermoplastic polyurethanes (TPUs) are versatile polymers widely used in various industries due to their lightweight and good chemical and abrasion resistance properties. However, their high-temperature mechanical performance remains a challenge, limiting their potential to replace metal counterparts. Annealing offers a cost-effective approach to enhance TPUs' high-temperature properties by promoting microphase separation, yet optimizing annealing parameters such as temperature and duration is critical for maximizing performance. This study employs an innovative in-situ dynamic mechanical analysis technique to optimize annealing conditions for a commercially available TPU. The high sensitivity of DMA to viscoelastic changes enabled real-time monitoring of storage modulus (E′) during isothermal annealing at temperatures ranging from 120 to 160 °C for up to 8 hours. The Avrami equation was used to model and extrapolate the time-dependent exponential growth of E′, providing insights into the kinetics of microphase separation. The optimal annealing duration was determined as the time required to reach 95% of the maximum achievable phase separation extent [PSE(t∞)] at each temperature. Modeling results revealed that PSE(t∞) increased with annealing temperature up to 140 °C, but declined at higher temperatures, likely due to hydrogen bond dissociation and phase mixing. Thermomechanical testing of annealed samples confirmed the highest performance improvement for TPU annealed at 140 °C, validating the model predictions. This study establishes a robust framework for optimizing annealing conditions, demonstrating the importance of precise control over temperature and time in tailoring the thermomechanical properties of TPUs for demanding applications.