Unusual features in the surface pressure-area isotherms in the Langmuir monolayer of a siloxane polymer

Abstract

Here we report some interesting features seen in the surface pressure (π) – area isotherms of the Langmuir monolayer of a liquid crystalline polymer LCP1. Our studies on the compression – expansion hysteresis in the π – area isotherms indicate some unusual features. Usually, in the compression cycle, one obtains a steep increase in π corresponding to the liquid expanded (LE) phase and then a plateau. However, for LCP1, at room temperature, the LE region terminated with an unusual decrease in π. After this there was a plateau and then a rise. The expansion cycle carried out from this point showed a sharp fall in π and then a slight increase, followed by a plateau. This increase was of the order of 0.5 to 0.7 mN/m, much higher than the instrumental resolution. On increasing the temperature, the behaviour of the expansion cycle was somewhat different. Our epifluorescence microscopic studies revealed some new phases. We have correlated the unusual features of the isotherm with the formation of these phases.