Temperature-dependent behavior of Langmuir monolayers of octadecyl-substituted preformed polyimides

The behavior of Langmuir monolayers of octadecyl-substituted preformed poly imide molecules exhibits a strong dependence on temperature when the isothe rms are measured using a Wilhelmy balance. This suggests a change in struct ure with temperature, though isotherms measured with a Langmuir balance cha nge only modestly with temperature. Brewster angle microscopy images of the monolayer morphology are consistent with the notion that the monolayer is extremely rigid at 20-21 degreesC, while it has somewhat more fluidity at 2 8 degreesC. Polarization-modulated infrared reflection absorption spectrosc opy results also indicate a qualitative difference between the characters o f the monolayer at the two temperatures. The Wilhelmy plate method is sensi tive to this difference, but its limited accuracy in the case of very rigid films results in a response that greatly amplifies the character of this c hange as compared to other means of probing the monolayer structure.