Phase separation in urethane elastomers as judged by low-angle X-ray scattering. I. Fundamentals

In thermoplastic urethane elastomers, phase separation between hard and soft segments resulting in a domain structure is an essential condition for a sufficiently high heat distortion temperature. The state of segregation, i.e., the domain structure, is characterized by the following aspects: 1) the size distribution, i.e., the dispersion level of the domains; 2) the sharpness of diffuseness of the domain boundaries, and 3) hard segments dissolved in the soft segment matrix or soft segments included in the domains. The state of segregation may be evaluated by means of low-angle X-ray scattering. The fundamental theory, based on Porod's theory of low-angle scattering, is given in this paper. Experimental findings will be discussed in a later paper.