Theoretical concepts and techniques in polyurethane science
There is very good physical understanding behind polyurethane structure, properties, and the chemical processes leading to material properties. Perhaps unique among modern polymers, theoretical understanding of polyurethanes has had to await developments in different fields of materials science. Theoreticians then applied those developments to polyurethanes, almost as an afterthought. Thus, modern polyurethane science has dined well at the theoretician's table serving other materials such as composites, polymer blends, and block copolymers. Even as our qualitative appreciation of polyurethanes has grown very rapidly, so has understanding that some aspects of quantitative understanding are limited due to potentially uncontrolled parameters of polyurethane polymerization. These uncontrolled variables can have very significant effects on final polymer structure and properties. Thus, theoretical foundations of polyurethane science are sound and predictive; however, they do not as yet allow one to know ahead of an experiment if one of myriad influences on polymerization will invalidate a predicted result. Current theory allows one to look back after materials analysis and make sense of the result but not predict ahead of time if that particular result will occur.
The purpose of this chapter will not be to derive the equations that are currently well applied to polyurethane science. Instead the basis for those equations and their physical meaning and limitations will be presented. An intuitive understanding of the equations and their extension to predicting polyurethane performance will also be provided. The best understanding of polyurethane structure comes from a simultaneous appreciation of the thermodynamic concepts underpinning the drive from reactants to final polymer structure coupled with the micromechanical models that allow estimation of properties from that structure. Tensile properties of elastomers can be predicted applying micromechanical results to calculations with assumptions of polymer extensibility. Bulk polymer structure can also be propagated into foam properties assuming well-known relationships associated with properties as a function of bulk density and cell structure.