ABSTRACT
Polyurethanes (PUs) were first discovered by Bayer and coworkers in 1937 [1-3] as a competitive response to the work on nylon by scientists from DuPont de Nemours. Because of the versatility of PU chemistry, this class of polymer can be used to produce an extremely broad spectrum of materials ranging from flexible foams to thermoplastic elastomers [4-9]. Thermoplastic polyurethanes (TPUs) were developed in the 1950s as a new type of PU in Germany by Bayer-Fabenfabriken and in the United States by B.F. Goodrich. The Alliance for the Polyurethane Industry (API) describes TPUs as “bridging the gap between rubber and plastics” because TPUs offer the mechanical properties of rubber but can be processed as thermoplastics [10]. The TPUs consist of linear soft segments (long-chain diols) and linear hard segments (diisocyanates and chain extenders). The wide applicability of TPU materials arises from their high modulus, drawability, high strength, and good toughness. In general, phase separation occurs in most TPUs because of the intrinsic incompatibility between the hard and soft segments. The hard segments, composed of polar materials such as urethane and/or urea, can form hydrogen bonds between carbonyls and amino groups and thus tend to cluster or aggregate into ordered hard domains, whereas the soft segments form amorphous domains. Improving the microscale phase separation of hard and soft segments during preparation and processing can influence the mechanical properties of the material. The domain size of the hard segments is on the order of 10 nm, which is similar to the domain size in the crystalline lamellae of typical thermoplastics such as polyethylene and polypropylene.
