Crystal Structure and Morphology of Linear Aliphatic n-Polyurethanes

The crystal structure and morphology of a series of linear aliphatic polyurethanes with generic formula -(CH2)(n)OCONH- and containing from 5 to 12 methylene units have been investigated by using TEM combined with X-ray and electron diffraction analysis. Lamellar crystals with a thickness ranging between 5 and 10 nm displaying a variety of crystal habits were obtained by isothermal crystallization from diluted solution in different solvents. A crystal structure similar to that described for the layered alpha-form of nylons seems to be commonly adopted by these polyurethanes. It essentially consists of a triclinic lattice made of hydrogen-bonded sheets with chains in fully extended conformation. Chains within the sheets are arranged in the antiparallel mode giving a normal scheme of hydrogen bonds. Sheets are stacked along the b-axis and progressively shifted along both a and c axes. The interchain and intersheet distances in this lattice are of values similar to those found in nylons. Energy calculations were in agreement with the arrangement proposed for chain packing. X-ray diffraction recorded from heated samples at real times showed that none of these polyurethanes undergoes Brill transition although odd members displayed structural changes concomitant with the occurrence of an unfinished ongoing process. The IR analysis at variable temperature revealed that approximately 50% of hydrogen bonds were retained upon heating at temperatures fairly above melting.