The Effect of Crystallinity and Flexibility on the Photodegradation of Polyurethanes.

Using fluorescence spectroscopy and gel content measurements, the photolysis of simple polyurethanes based on methylene 4,4'-diphenyl diisocyanate (MDI) is shown to depend on both the flexibility and crystallinity of the polymer. Polyurethane films based on MDI and 1,2-dodecanediol (MDI-12) can be either annealed or quenched to yield a semi-crystalline (as measured by DSC and X-ray diffraction) or amorphous film, respectively. In the case of the quenched (amorphous) films, photolysis yields an ortho photo-fries product whereas the annealed (semi-crystalline) film does not. Similarly, for a given photolysis time the annealed film generates a lower extent of cross-linked gel. In addition, photolysis of amorphous polyurethane films derived from MDI and ethylene oxide oligomers shows that for relatively inflexible polymers with only three ethylene oxide segments, little or no ortho photo-fries rearrangement products are generated upon photolysis. In contrast, photolysis of more flexible polyurethane films with approximately thirteen ethylene linkages yields the ortho photo-fries product. Apparently, for aromatic diisocyanate based polyurethanes, the extent and mechanism of degradation is directly dependent on the crystallinity and main-chain flexibility of the polymer.