Grazing incidence X-ray diffraction studies on the structures of polyurethane films and their effects on adhesion to Al substrates

Grazing incidence X-ray diffraction was carried out to analyze the structure of polyurethane films as a function of X-ray penetration depth by varying the angle of incidence. Coherence lengths, interplanar spacings, and crystallinities were obtained for non-aged and aged films of OH numbers of 120, 375, and 600 bonded to an aluminum substrate. Aging led to the improvement of bulk crystallinity of all the samples, particularly in the case of the aged PU-375 film, for which a dramatic increase of the bond strength was observed. The crystallinity of non-aged samples varied from the air/polymer surface down to the polymer/aluminum interface. Invariance of coherence length from air surface to interface was observed for PU-120 and PU-375 samples, implying that substrate-induced ordering has little effect on the average crystallite size. As the X-ray penetration increases near the polymer/Al interface, interplanar spacing of (021) reflection in all the films approach the value found for a bulk polyurethane-urea by Ishihara et al. The bond strength of the polyurethane film to the aluminum was exponentially proportional to the crystallinity including the crystalline interphase formed near the substrate. It is also found that the polymer film containing more (100) planes provided higher bond strength.