A micromechanical approach to TPU mechanical properties: Framework and experimental validation

Mechanistic understanding of the mechanical properties of elastomeric thermoplastic polyurethane (TPU) rely on the hard segment content (HSC) as the sole predictor of mechanical properties for a given chemistry, regarding the TPU morphology as a nanocomposite of hard domains in a softer matrix. In order to go beyond this binary view of TPU structure-property relationships, the behavior of the soft segment-rich domain and the interphase between the hard and soft domains should be given more consideration. This work explores the application of several common micromechanical models to a series of TPUs with increasing HSC. A framework to determine model parameters (phase volume and modulus) for TPUs in general was developed. The ability of the model to capture the reinforcement behavior was evaluated in the context of the experimentally determined mechanical properties and morphology. An Eshelby double inclusion (EDI) model was used to compare two and three phase models. The soft matrix modulus, Em, was used as the fitting parameter to match the experimental Young's modulus for a series of TPUs with increasing HSC. The procedure reveals that Em must also increase as a function of HSC to realize the degree of reinforcement found experimentally. The design of TPU for specific applications would greatly benefit from the ability to predict stress-strain behavior from models which represent the complex TPU morphology.