Property-Structure Relationships in Polyurethanes

Abstract

As previously described in Chapter 1 the polyurethanes can contain a high concentration of polar groups, in particular the urethane groups resulting from isocyanate—hydroxyl reactions, as well as ester, urea and other groups. The interactions between these polar entities are of great importance in determining the properties of polyurethanes of all types, and especially the polyurethane block copolymers where local concentrations of polar groups occur together. An empirical estimate of the energies of interaction between some common groups can be seen in Table 3.1, and the high cohesion energy of the urethane and urea groups will be noted. The summed interactions between hard segments containing many urethane and urea groups (hydrogen bonding and dipole—dipole interactions) can be great enough to provide a pseudo crosslinked network structure between linear polyurethane chains, so that the polymer has the physical characteristics and mechanical behaviour at room temperature of a covalently crosslinked network. The rigid segments in polyurethane block copolymers particularly affect the modulus, hardness and tear strength, and determine the upper-use temperature by their ability to remain associated at elevated temperatures.