Interfacial Compression and Polymerization of 4- and 9-((Butoxycarbonyl)methyl)urethane−Diacetylene Monomolecular Films:  A Fluorescence Microscopy Study

The isotherms of monomolecular films of monomers of 3-BCMU, 4-BCMU, and 9-BCMU all reveal a phase transition which resembles that of the corresponding polymers. However, the transition exhibited by the 3-BCMU monomer is not well-defined, and the isotherm is somewhat metastable. The isotherms of the monomers of both 4-BCMU and 9-BCMU show a metastable peak at the onset of the transition due to difficulty in achieving the transition from a bipolar (expanded) to a pseudo-monopolar (condensed) attachment to the water interface. Fluorescence microscopy enabled the two-phase region of the transition to be visualized for 4-BCMU and 9-BCMU but not that of 3-BCMU. The resultant micrographs reveal dark regions representing the condensed monomer phase from which the probe is effectively excluded. The condensed phase domains consist of curved leaf or needlelike regions whose precise size and texture depend on both the molecular composition and the rate of compression of the film. It was also observed that the condensed phase was highly rigid and crystalline, in that the domains showed sharp fractures when brought into contact. Polymerization at the air/water interface was brought about by exposing the films to an appropriate UV frequency through the objective of the microscope through which the process was observed. Exposure within expanded regions did not result in any observable polymerization but appeared to occur primarily at condensed/expanded interfaces and to initiate within hairline crystals. We conclude that monomers of 4-BCMU and 9-BCMU polymerize best when both condensed and expanded states are present.