Probing the Elasticity of Langmuir Polymer Films of High Molecular Weight Poly (γ-benzyl-L-glutamate)

Poly (γ-benzyl-L-glutamate) (PBLG) Langmuir Polymer films at high surface densities were compressed and expanded repeatedly. During expansion a very rapid initial drop of surface pressure occurs at areas per molecule which are smaller than expected for a closely packed array of helical rods of PBLG. This is followed by an extremely slow pressure decrease represented by a nearly horizontal line when the area is expanded to values even larger than the initial value obtained by material deposition. The same behavior of expansion curves is observed when the films are exposed to successive cycles of compression and expansion. However, with each cycle the shallow decreasing part of the curve is shifted to lower surface pressures, while essentially keeping the same slope. Even more surprising is the observation that re-compression traces show a seemingly negative compressibility i.e., a decrease in pressure during compression. The present study focuses on understanding this surprising behavior. Snapshots of the morphologies during these cycles were taken to correlate the information from the compression/expansion curves with structural information. Applying the Langmuir-Schäfer technique, we transferred the films to solid substrates at different surface densities during compression and also at different surface densities during expansion. We also changed the lateral position of the transferred film fragment with respect to the barriers and Wilhelmy device to investigate the lateral homogeneity of the film. The transferred film fragments were probed with Atomic Force Microscopy. Our findings suggest the formation of a stable solidified layer at high surface pressures, which apparently did not disassemble upon expansion as the microscopic morphologies of compressed and expanded state resemble strongly when investigated at the microscale. However, probing the lateral homogeneity on the entire trough area shows, that the Langmuir film suffers local ruptures during expansion.