INSTRUMENTAL ANALYSIS OF POLYURETHANES
Modern technology has developed the means to acquire data at relevant length scales such that many aspects of polyurethane morphology and function can be readily understood. Many techniques offer corroborating as well as extending information. For instance, it is often the case that characteristic phase separation lengths visibly discernable by atomic force microscopy (AFM) can be validated by small-angle X-ray scattering (SAXS). The SAXS data in addition can be further interpreted for information about the boundary between the hard and soft phases. An elastomer that exhibits slow elastic response by touch can have its room temperature glass transition temperature corroborated and quantified by dynamic mechanical analysis (DMA). Modern instrumental techniques have become so disseminated and accessible to experimentalists that it is now expected that data obtained by a single technique leaves too many questions unanswered to be compelling. Experimentalists now routinely apply numerous techniques to a single material [13-18]. The use of these techniques, apart and in unison, has allowed scientists unprecedented understanding of polyurethane morphology and enabled theoretical analyses that further deepen our understanding and predictive capabilities.
Microscopy of polyurethanes is of great interest because its evaluation can span length scales from millimeters to nanometers. Since important polyurethane structures express themselves across these lengths, the information obtained at one scale may often be corroborated at another length scale . For instance, a polyurethane specimen may be prepared and be either transparent or opaque. The cause of this optical distinction can usually be ascribed to phase characteristics of the urethane blocks , composite additives that when of sufficient size will scatter visible light or artifactual air bubbles. The samples can subsequently be observed by scanning electron microscopy (SEM) for the micron to 100 s of micron scale, which may confirm the presence of phase separated copolymer particles (see Chapter 2), soft segment crystals, composite additives, or gas bubbles generated by the inadvertent reaction of isocyanate and water to form CO2. Or the SEM may reveal no feature of sufficient contrast applied to an optically transparent specimen. Transmission electron microscopy (TEM) (10-1000s nm) may be employed with specialized stains to increase contrast between phases and to reveal phase separated hard segments that are spherulitic or lamellar. Lastly, AFM (1-100 s nm) may be applied to the sample to reveal phase separated spherical structures developed as a result of a nucleation and growth (opaque), the lacy structures developed from spinodal phase decomposition, or the stunted phase structures associated with incomplete phase separation resulting from polymer vitrification .