Molecular Engineering of Rheology and Pinching Dynamics of Formulations

Hydrophobic stickers, charged micelles, and charged particles provide hydrophilic flexible polymers with temporary, dynamic junctions with association-disassociation time that changes with polymer concentration and deformation. Macromolecular engineering of formulations typically relies on the characterization of response to shear flow, with velocity gradients perpendicular to flow direction, emulating processing flows through channels and drag flows near moving solid surfaces. However, characterizing extensional rheology response has remained a longstanding challenge, even though streamwise velocity gradients, associated with extensional flows, often arise during processing especially during dispensing and liquid transfer by dripping, jetting, or spraying. In this contribution, we examine the contrasting influence of dynamic associations on macromolecular dynamics and formulation rheology in response to shear flows using torsional rheometry and extensional flows with dripping-onto-substrate (DoS) rheometry. We find that dynamic associations help to tweak the rate-dependent rheological response, to facilitate easier dispensing and coating, better sagging, leveling, and storage while reducing misting and stringiness.