| Summary: | In this work the structural differences in binary latex composites are investigated, with a particular focus on the effect of changing size ratio. It is demonstrated that, in composites cast from low and high Tg latex dispersions, stratified layers of the smaller species may be present in the dry composite. It is highlighted that, stratification is heavily influenced by changing the size ratio of the two latex species, with smaller particles able to stratify at lower volume fractions relative to the larger species. It is also demonstrated that, stratification is less likely in composites cast from more highly concentrated dispersions. In the case of composites cast from more concentrated dispersions, stratification is only observed in films with a high size ratio and high volume fraction of the small species. The tensile properties of the same composites were investigated in films with soft/hard size ratios of 1:1 and 7:1. Two distinct routes to brittleness, as the volume fraction of hard particles are increased, are identified. A 1:1 size ratio leads to a sharp transition from ductile to brittle, whereas a 7:1 size ratio leads to a more gradual decline from ductile to brittle. The proposed reason for the two distinct routes is a structural one, in which the microstructure of the binary composites prevents continuity of the low Tg phase to varying extends depending on both volume fraction and size ratio. The structural investigation of binary composites is advanced by investigating the structures and mechanical properties of latex/calcium carbonate composites over a range of size ratios, incorporating sizes of filler particles from nano-sized to micro-sized. It is demonstrated that smaller filler particles lead to the onset of brittleness at lower volume fractions than larger ones, however smaller particles also provide greater mechanical reinforcement at lower volume fractions in latex/calcium carbonate composites.
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