X-ray analysis has become a standard tool for developing in-depth understanding of polyurethane microstructure. It is particularly finding adherents in academic studies of structure-property relationships since scattering techniques can provide unique information on the angstrom to nanometer scale [27, 59, 60]. At this scale, a researcher can learn about packing within the hard segment, the ratio of crystal (or quasicrystalline) to amorphous phase volume, the periodicity of phase spacing, and the characteristics of the interface between the polyurethane soft segment and the hard segment. While X-ray methods are surely complementary to other less instrumentally demanding techniques (such as calorimetry, IR spectroscopy, AFM, and mechanical analyses), X-ray analysis, when done with proper care and control, provides a measurement that requires less inductive reasoning than other techniques applied for the same information. The most basic information provided by X-ray analysis is derived by the very simple Bragg equation 5.6 for determining the interdomain spacing X, regardless of the intra- or interphase nature of the scattering center:
qm is referred to as the scattering vector, X is the wavelength of the incident X-ray radiation (often 1.54 A for a Cu Ka source), and © is the scattering angle. The wealth of additional information contained within scattering diagrams depends on highly involved geometrical reasoning, and obtaining information depends critically on correct application of precise experimental techniques.
Wide-Angle X-ray Scattering
Wide-angle X-ray scattering (WAXS) is an X-ray diffraction technique used to characterize the crystallinity and characteristic spatial relationships between atoms within a crystalline phase . WAXS spectroscopy is performed with a sample between the X-ray source and the detector. While scattering is strictly from constituent electron density, this is usually well correlated to local density, making it a sensitive probe of local structural variations. WAXS is differentiated from SAXS by positioning the detector closer to the sample so that scattering at wide angles may still be captured. The detector may be a film or digitizing two-dimensional detector such as shown in Figure 5.13. Alternatively, the source may scan an angular range and the detector scan the diffracted intensity as a function of detector angle to provide the so-called 1-D spectrum. Such a 1-D spectrum would be the equivalent of recording the intensity along a single radius extending away from the center of the film to the film's edge.
Figure 5.13 Relationship of sample to detector for WAXS analysis.
Applied to many polyurethane structures, WAXS is often not particularly enlightening [13, 35, 52, 62, 63]. Since PU hard segment is disordered, sometimes compared to a nematic liquid crystalline structure (like matchsticks in a box), it often results in diffuse scattering. Thermoplastic polyurethanes (TPUs) are particularly diffuse in this regard; however, many TPUs are prepared with glycol soft segments capable of crystallizing. Among these are polybutylene adipate, polybutylene succinate (PBS), and polycaprolactone (see Chapter 2). WAXS of TPUs prepared with crystallizable soft segments may show up convoluted with the diffuse polyurethane hard segment as shown in Figure 5.14. The top image in Figure 5.14 shows a broad diffuse scattering with a well-known peak centered at about 20 = 20°. A two-dimensional photograph of this pattern would show up as a diffuse halo. Application of the Bragg equation indicates a characteristic scattering reflection of about 4.5 A. Progression of the sample shows that as design of the TPU is adjusted to encourage soft segment crystallization, the characteristic scattering from, in this case, PBS diol appears and begins to dominate the diffraction pattern.
The polyurea hard segment scattering emanating from polyurethane foams can show a greater amount of ordering and more reflections than observed for TPUs. This is as might be expected given the higher ordering power of the bidentate hydrogen bonding within the polyurea hard segment. Figure 5.15 is a representative WAXS from a water-blown foam. In this Figure , the characteristic 20 = 20° is prominent as well a shoulder at about 20 = 10° and 50°. A two-dimensional image of this pattern would show a less diffuse scattering intensity than observed in Figure 5.15 with sharper intensity maxima (still showing circular symmetry in the image) at the indicated 20 distances. The exact morphological nature associated with the scattering is not established but is believed to be characteristic of the hard segment hydrogen bonding network inferred from the commonality of many of the reflections independent of whether the isocyanate is MDI, pMDI, or TDI . In principle, processing variations such as compression molding, injection molding, annealing, etc. could affect WAXS patterns, but in practice, this is rarely observed. Soaking PU samples in strong solvents is known to decrease or even eliminate some of the WAXS, but this is certainly unsurprising. Large-scale sample deformation has been observed not to change peak maxima, but rather, in some cases, to change the overall peak shape .
Figure 5.14 WAXS data for polyurethane elastomers with polybutylene succinate (PBS) soft segments and MDI-BDO hard segments. As conditions are optimized for soft segment crystallization, the PBS diffraction becomes more prominent in the data. Reprinted with permission from Ref. . © Elsevier Pub.
Figure 5.15 WAXS from a polyurethane foam with a polyurea hard segment. Reprinted with permission from Ref. . © Elsevier Pub.