Designing yield stress fluids for advanced materials processing using derivatives of pH-responsive branched co-polymer surfactants

Abstract

Through careful formulation design from a structure–properties perspective, this work demonstrates the potential of pH-responsive branched co-polymer surfactants (BCSs) in emulsion engineering and advanced materials processing. A library of BCS derivatives, with controlled spatial distribution of hydrogen bonding motifs and different branching levels, is synthesised via a modified Strathclyde method with varying conditions in solids content and PEGMA chain length. High dilution during the co-polymerisation leads to linear co-polymers, while an increase of monomers concentration favours branching reactions. This diverse set of BCSs can be exploited to stabilise pH-responsive suspension and emulsions containing activated charcoal (AC) or strontium titanate (STO) to create an array of yield stress soft materials. Large amplitude oscillatory shear (LAOS) experiments reveal their diverse rheological properties and yielding behaviours, that correlate primarily with powders properties and concentration, the degree of branching of BCS macromolecules, and to some extent, with the PEGMA length or BCS molecular weight. Based on the rheological characterisation, two formulations are selected and optimised for direct coagulation casting of AC suspensions and direct ink writing (DIW) of STO to create macroscopic structures. The optimised STO emulsion gels for DIW show a dramatic shift in printing behaviour before and after triggering the pH-controlled assembly. LAOS analyses using Fourier-transform (FT) rheology and the sequence of physical processes (SPP) confirm that the pH triggered assembly of STO emulsion gels results in the transition from a stable microstructure that shows a smooth flow transition, to an aggregated and unstable microstructure that becomes easily disrupted under shear. The higher harmonics and SPP analysis enable the correlation of yielding and printing behaviours. Overall, the findings highlight the critical role that BCSs play in providing electro-steric stabilisation of suspensions and emulsion gels in the processing of advanced materials. Combining polymer chemistry, formulation design and rheology, we optimise responsive formulations to create complex macroscopic structures with hierarchical features.