(19a) Harnessing Unsaturated Surfactant Tails in Tailoring Polyelectrolyte/Surfactant Complex Properties

Polyelectrolyte/surfactant complexes are used in a diverse array of household, process, and healthcare technologies and, consequently, have been a subject of substantial and systematic investigation. Many works have examined the effects of various molecular/formulation parameters, such as the linear charge density of the polymer, hydrophobic tail length of the surfactant, and the surfactant:polyelectrolyte charge ratio on properties such as phase behavior, microstructure, and rheology. Less examined, however, has been the effect of the surfactant hydrophobic tail unsaturation. To this end, here we explore polycation complexation with a homologous series of fatty acids: steric acid, oleic acid, and linoleic acid, which contain zero, one, or two cis double bonds while maintaining identical numbers of aliphatic carbon atoms. Using polyallylamine (PAH) as the primary model polycation, we show that, while PAH/stearate complexation forms brittle precipitates in both water and ethanol, the introduction of double bonds in the aliphatic fatty acid tail converts the insoluble polyelectrolyte/surfactant complexes into either moldable adhesive putties (when they are aqueous) or liquid coacervates (when their water content is replaced with ethanol). Small-angle X-ray scattering and polarized light microscopy analyses reveal this solvent-triggered moldable solid-to-liquid transition to coincide with a loss of liquid crystalline order, and the rheological properties of these putties and liquids can be extensively tailored by varying the number of cis double bonds and the fatty acid:polycation charge ratio. Besides these physicochemical properties, we show how the adhesiveness and stimulus-sensitivity of these polyelectrolyte/surfactant complexes enable the design of reversible tie-layers for multilayer plastic (MLP) films that can be dissociated on demand during their recycling. Specifically, we show how polyelectrolyte/unsaturated fatty acid coacervates can be used to bond dissimilar film layers together (producing peel strengths comparable to those reported for commercial tie-layers) and then be dissociated on demand using an aqueous NaOH solution (such as used in conventional recycling processes) as a pH-trigger. The subsequent delamination of the MLP film layers then enables their organic solvent-free separation for recycling.