Relationship between the growth regime of polyelectrolyte multilayers and the polyanion/polycation complexation enthalpy

The alternate deposition of polyanions and polycations leads to the formation of films called polyelectrolyte multilayer films (PEMs). Two types of growth processes are reported in the literature, leading to films that grow either linearly or exponentially with the number of deposition steps. In this article we try to establish a correlation between the nature of the growth process and the heat of complexation between the polyanions and the polycations constituting the PEM film. Isothermal titration microcalorimetry experiments performed on several polyanion/polycation systems seem to indicate that an endothermic complexation process is characteristic of an exponential film growth, whereas a strongly exothermic process corresponds to a linear growth regime. Finally, weakly exothermic processes seem to be associated with weakly exponentially growing films. These results thus show that exponentially growing processes are mainly driven by entropy. This explains why the exponential growth processes are more sensitive to temperature than the linear growing processes. This temperature sensitivity is shown on the poly-L-glutamic acid/poly(allylamine) system which grows either linearly or exponentially depending on the ionic strength of the polyelectrolyte solutions.