Scanning Tunneling Microscopic Studies of Epitaxial Films of Polyurethane, Polyester, and Polysiloxane Formed during Step Polymerization

Crystalline films of poly(hexamethylene-hexamethylenediurethane) were found to grow on the basal plane of graphite during polyaddition of diol and diisocyanate in a homogeneous solution containing a graphite substrate. Direct observations by scanning tunneling microscopy indicate that the film has been grown epitaxially. Also, an epitaxial film of poly(hexamethylene sebacate), an aliphatic polyester having the same number of bonds per monomeric unit as the polyurethane, was grown on the graphite surface during polycondensation of diol and dichloride. These results demonstrate that step polymerization induces epitaxy of polymers during polymerization. Then, the same polycondensation procedures were applied to study the films of an aromatic polyester, poly(hexamethylene terephthalate), and a polyester containing a triple bond, poly(butynyl sebacate) on graphite. The aromatic ring and the triple bond can be considered as the structural «kinks» on the aliphatic backbones that cause a misfit with a polyethylene-like repeat of the graphite hexagonal lattice. Formation of epitaxial films of these polyesters indicates the importance of having molecular structures capable of forming a close packed state in two dimensions for this epitaxial growth. Lastly, ring-opening polymerizations of cyclic dimethylsiloxanes were performed with the presence of a graphite substrate either in a good solvent at room temperature or in melt at 160 o C. Formation of crystalline films even at the temperatures much higher than the melting point of bulk polymer (-42 o C) and entropically driven polymerization of siloxanes suggest the two-dimensional nature of the crystallization process. On the basis of the present results and the previous studies, a mechanism of the late stage of epitaxial growth induced by polymerization is proposed