A study of structure and temperature-triggered breakdown of particle gels prepared by pH-triggered heteroaggregation

Synthetic heteroaggregate particle gels are a relatively unexplored area of colloidal science. In our previous work pH-triggered formation of heteroaggregate gels prepared from mixed pigment/polystyrene (PS) dispersions were investigated [Hui et al., J. Colloid Interface Sci. 324 (2008) 110]. The pigment was beta-copper phthalocyanine (beta-CuPc). In this study that work is extended by investigating the structure of the heteroaggregates formed at room temperature in greater detail and also studying the temperature-triggered gel breakdown that occurs upon moderate heating for the first time. In this work the heteroaggregate structures were studied using TEM, atomic force microscopy (AFM) and also SEM. AFM showed the mesoscale structure to consist of highly branched space-filling heteroaggregates. They are proposed to be the building blocks of the heteroaggregate gels. TEM data supported our previous suggestion that the pigment particles act as a crosslink centre within the heteroaggregate gels. SEM images showed evidence for softening and partial coalescence of the aggregates at elevated temperatures which helped to explain the temperature-triggered gel breakdown that occurred upon heating. The temperature-triggered gel breakdown was directly correlated with the glass transition temperature of the PS particles. A study of the maximum storage modulus, G', achieved just prior to gel breakdown, revealed a maximum at a volume fraction of pigment of 0.033. The data from the study are used to propose a conceptual model for the temperature-triggered breakdown of the pigment/PS heteroaggregate gels.