Relation between acrylic-styrene colloid architecture and polymer film structure revealed by dry film rheology and calorimetry

As interest in waterborne coatings grows, a better understanding of the relationship between dispersion parameters and film microstructure and properties is needed. Although surfactant aggregates and inter-particle boundaries have been observed within continuous binder films, their formation mechanisms, dependency on dispersion parameters, and subsequent impact on film properties remain poorly understood. This study used linear oscillatory shear rheology and differential scanning calorimetry (DSC) to reveal the remnant internal structure of the solid, fully formed, free-standing films obtained from acrylic-styrene waterborne dispersions stabilized with anionic surfactants. The effect of molecular weight, hydrogen-bonding content, colloid size and annealing temperature on the remnant colloid–colloid interphase structure and network entanglement was further analysed. Thermal analysis revealed that surfactant aggregation is prevalent in films made from small colloids (100 nm). These aggregates remained resilient to thermal annealing up to 120 °C, suggesting that film homogenization is hindered either by the thermodynamic stability of the crystalline phase or by the high kinetic barrier presented by surfactant aggregates themselves. The M w appears as the dominant factor on the rheological properties; for colloids with similar M w , the viscoelastic behaviour of the film is instead dominated by the incompletely interdiffused colloid-colloid interphases and the presence of resulting water transport paths created during film formation. Larger colloid sizes (200 nm) showed plasticization of the inter-colloid regions in the dry film, evidenced by a small shift in terminal relaxation times, attributed to a better interfacial presence of surfactant. These findings reveal the fine but measurable relationship between colloidal suspension parameters affecting colloid packing and colloid-colloid interdiffusion and the ultimate viscoelastic behaviour of the resulting dried polymer film of relevance for coating design.