Humidity- and surfactant-accelerated aging in poly(vinyl alcohol)-based thin films

This work aims to understand how relative humidity (RH) and chemical composition influence the diffusion of guest molecules in formulated polymer films replicating packaging materials for detergent products. Surface morphology of thin poly(vinyl alcohol) films containing glycerol and surfactants of various headgroup chemistry was examined using Atomic Force Microscopy under controlled RH conditions (15% – 55% RH). Diffusion was found to be controlled by both surfactant “blooming” from the polymer matrix and RH, leading to changes in film morphology. Anionic surfactants bloomed in the films at low concentration (1 wt%) above a threshold of 35% RH. Higher concentrations (above 3 wt%) were required for blooming of cationic surfactant, while nonionic surfactants showed no changes in film morphology regardless of concentration or RH. It is likely that chemical compatibility and charge-pairing effects are the main driving forces for these changes. Increased RH and presence of glycerol could enhance the free volume within the polymer matrix, accelerating diffusion and therefore achieving equilibrium (surfactant blooming), as confirmed by Dynamic Vapour Sorption measurements.