Effect of Polyelectrolyte Molecular Weight on the Formation and Properties of Polyelectrolyte Multilayers

Polymers whose repeating units bear an electrolyte group, so-called polyelectrolytes, are the main building blocks of polyelectrolyte multilayers (PEMs).These thin organic films are versatile nanomaterials with applications in many fields.The main reason for their widespread applications lies in the ability to fine-tune their properties simply by adjusting the experimental conditions of their preparation.One of these parameters is the molecular weight of polyelectrolytes utilized for PEM preparation.Although about three decades have passed since the preparation of the first PEM, consensus among scientists about the influence of polyelectrolyte molecular weight on the properties of PEMs still has not been achieved.For example, several studies have shown that PEM thickness increases with the increase of polymer molecular weight, others that PEM thickness decreases with polymer molecular weight, and third that molecular weight does not influence film thickness.To shed some light on these somehow contradictory findings, we report here the results of a study in which we explore the influence of polyelectrolyte molecular weight on the properties of PEM not as an independent parameter but as a variable related to the polyelectrolyte degree of charging.For that purpose, we prepared PEMs via the layer-by-layer assembly with two distinct molecular weights of poly(allylamine hydrochloride) and poly(acrylic acid) at pH = 5.0, 8.0 (with and without NaCl), and 10.0.In the end, we found that the thickness and, for the most part, the surface roughness of prepared films increased with higher molecular weights of polyelectrolytes in all examined conditions except at pH = 8.0.In that condition, the molecular weight of polymers did not affect the PEM properties because polyelectrolytes are highly charged at that pH and adsorb to a surface in a linear conformation.In other conditions, the impact of polyelectrolyte molecular weight on growth and properties of PEM has been explained by the packing of chain molecules at the film surface (pH = 5.0 and pH = 8.0 with NaCl) or by film erosion that was noticed during PEM build up (pH = 10.0).