The behavior of fatty acid modified poly(glycerol adipate) at the air/water interface

The influence of hydrophobic modification with fatty acids of various lengths on the behavior of poly(glycerol adipate) (PGA) at the air/water interface is investigated. The Langmuir isotherm of unmodified PGA reveals a liquid expanded (LE) phase followed by film collapse as indicated by the plateau starting at πc (collapse pressure) ∼13 mN/m. The film stability increases significantly by grafting fatty acids to the PGA backbone; PGA-graft-acyl. However, it tends to decrease again for higher degree of substitution due to the decreasing number of pendant hydroxyl groups on PGA backbone, which stabilize the monolayer by forming hydrogen bonds with water molecules. For samples with higher degree of substitutions, the LE phase could not be detected in the Langmuir isotherm which only shows a sharp increase in surface pressure, corresponding to the liquid condensed (LC) phase, followed by the film collapse. Atomic force microscopy images of the Langmuir–Blodgett films depict the formation of 3D nuclei of different heights at surface pressure, π > πc and the Brewster angle microscopy images reveal the formation of buckles in the collapse region for PGA-graft-tridecyl films. The formation of a solid phase in the Langmuir film is observed only for PGA grafted with longer alkyl side chains, including, heptadecyl and heneicosanyl. Overall, it is observed that with increasing length of the acyl chains the stability and compressibility of the film on the water surface increases because of enhanced coherence between the alkyl side chains that overcomes the obstruction effect of the PGA backbone.