Langmuir 1988,4,689-693
689
Monolayer and Langmuir-Blodgett Multilayer Surface and Spectral Studies of Poly-3-BCMU J. E. Biegajski, D. A. Cadenhead,* and P. N. Prasad Department of Chemistry, State University of New York a t Buffalo, Buffalo, New York 14214 Received September 28, 1987. I n Final Form: December 29, 1987 Monolayer films of poly-3-BCMU are shown to form at the air-water interface,the surface pressure/area per residue (*/A) isotherms of which exhibit a horizontal plateau indicative of a phase change occurring on compression. Langmuir-Blodgett monolayer films, transferred while proceeding through this region, have visible absorption spectra indicating a conformational change from an amphipathic yellow to a nonamphipathic blue form having increased *-electron conjugation. A thermodynamic analysis of the temperature dependency of the compressional onset of the plateau (TJ, interpreted in terms of a corrected two-dimensional analogue of the Clapeyron equation, shows a slightly exothermic transition. A comparison of isotherms obtained both by continuous compression and by a stepwise 'equilibrium" method shows considerable relaxation can take place at areas per residue less than 100 A2.This is interpreted as reflecting the pronounced reorganization required for the highly ordered blue conformation to completely form. Multilayers of poly-3-BCMU and poly-4-BCMU transferred at different areas per residue show visible absorption spectra reflecting the different degrees of conformational order of the monolayer films from which they were fabricated. When monolayers of poly-4-BCMU and poly-3-BCMU are sandwiched, an alternating multilayer is formed in which the highly ordered blue conformation is forced to assume the slightly less ordered red conformation of poly-4-BCMU.
Introduction Previously' we have shown that films of the soluble diacetylenic polymer poly(dibutyl4,19-dioxo-5,18-dioxa3,20-diaza-10,12-docosadiynedioate), commonly known as poly-li-BCMU, undergo a transition from an expanded monomolecular yellow amphipathic film to a condensed multilayer (approximately bilayer) red nonamphipathic film. In solution, the observed change can be brought about by a changing solvent composition and has been interpreted as a change from a disordered yellow-coil form to a more ordered red-rod form because of an effective increase in the *-electron conjugation. We have similarly interpreted the observed film transition as that of a disordered yellow-coil form to a more ordered red-rod form. Duringthe transition, the absorption spectra of transferred Languir-Blodgett films showed a color shift from yellow to red through orange, with the interpretation being that the underlying monolayer retained its original form, while the second and subsequent layers assumed the red-rod form. We initially indicated' that this transition was slightly endothermic, but as will be pointed out,2when a correction term (dro/dTJ is included in the two-dimensional Clapeyron equation? the transition is indeed slightly exothermic. In this paper we wish to report results of a study of a similar polydiacetylene, poly-3-BCMU. The soluble polydiacetylene poly(dibutyl4,17-dioxo-5,16-dioxa-3,18-diaza-9,1l-eicosadiynedioate),or poly-&BCMU, shows a reversible phase transition from a yellow-coil form to a blue-rod form in both the solid state and in This change can be brought about either by decreasing the temperature or by changing the solvent composition. On the basis of our experience with poly-CBCMU, it seemed likely that the film behavior of poly-3-BCMU, including compressional and spectral properties, would show similar but not identical characteristics. The subsequent investigations revealed not only how minor structural modifications affect the behavior of such polymer films but also how these films affect each other in admixture. We have observed that a monolayer of poly-3-BCMU, a t the airwater interface, undergoes a similar yellow coil to blue rod *Author to whom correspondence may be sent. 0743-7463/88/2404-0689$01.50/0
transition to that of poly-4-BCMU upon compression to areas below 100 A2/residue. Again we interpret this as a transition from a less ordered to a more ordered form with increased *-conjugation. In addition, for both poly-3BCMU and poly-4-BCMU, we show that, by successively transferring single-monolayer films onto a hydrophobic glass slide, multilayer assemblies can be built that reflect the relative degree of order of the original area per residue of the individual monolayer films from which they were composed. Finally, the ability of poly-4-BCMU to force poly-3-BCMU to assume a red-rod conformation in an alternating sandwich-type multilayer is demonstrated.
Experimental Section The surface pressure ( T ) vs unit residue area (A) isotherms were obtained by utilizing a film balance, which has been previously described.' The balance uses the Wilhelmy plate method of measuring the surface pressure via a Cahn, R. G., automatically nulled electrobalance. The water used as a subphase was quadruply distilled with a pH of 5.6 and a resistivity of at least 15.0 MQ-cm.The subphase temperature was controlled to within hO.1 "C. The spreading solvent used was chloroform whose purity was checked for surface-active impurities by spreading it at the airwater interface and allowing adequate time for evaporation (6-10 min) prior to compressing the surface. No appreciable increase in surface pressure was observed upon Compression (
