Surface Pressure-Induced Isotropic−Nematic Transition in Polymer MonolayersEffect of Solvent Molecules

The coupling of hydrodynamics to molecular orientation has been studied for a Langmuir film of the hairy-rod polymer poly( p -phenylene)sulfonic acid mixed with either arachidyl alcohol or 1,2-dimyristoyl- sn -glycero-3-phosphatidylcholine. These latter materials are small amphiphilic molecules. The orientational dynamics of the polymer under an extensional flow is examined using UV dichroism. The results reveal a surface pressure-induced isotropic−nematic transition, which is affected by polymer−solvent interactions as well as polymer−polymer interactions. An interfacial stress rheometer was used to further examine the existence of the surface pressure-induced transition. These structural and liquid crystalline transitions are difficult to observe using traditional pressure−area isotherm analysis.