Internal Composition versus the Mechanical Properties of Polyelectrolyte Multilayer Films: The Influence of Chemical Cross-Linking

Different types of polyelectrolyte multilayer Films composed of poly(L-lysine)/hyaluronan (PLL/HA), chitosan/hyaluronan (CHI/HA) and poly(allylamine hydrochloride)poly(L-glutamic acid) (PAH/PGA) have been investigated Cor their internal composition, including water content, ion pairing, and ability to be covalently cross-linked, as well as for their mechanical properties. Film buildup under physiological conditions was monitored by the quartz crystal microbalance with dissipation monitoring (QCM-D) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), which allows unambiguous quantification of the different groups present in the polyelectrolytes. (PAH/PGA) films emerged as the most dense films with the lowest hydration (29%) and the highest COO- molar density. In addition, PAH is greatly in excess in these films (3 PAH monomers per PGA monomer), The formation of amide bonds during Film cross-linking using the water-soluble carbodiimide EDC was also investigated, All of the films could be cross-linked in a tunable manner, but PAH/PGA exhibited the highest absolute number of amide bonds created, similar to 7 times more than for (PLL/HA) and (CHI/HA) Films. The Young's modulus E of the films measured by AFM nanoindentation was shown to vary over I to 2 orders or magnitude for the different systems. Interestingly, a linear relationship between E and the density of the covalent cross-links created was observed for (PLL/HA) and (CHI/HA) films whereas (PGA/PAH) films exhibited biphasic behavior. The mean distance between covalent cross-links was estimated to be similar to 11 nm for (PLL/HA) and (CHI/HA) films and only similar to 6 nm for (PAH/PGA) Films for the maximum EDC concentration tested (100 mg/mL).