The onset of polymer diffusion in a drying acrylate latex: how water initially retards coalescence but ultimately enhances diffusion

Abstract

The diffusion of polymer chains across the interface between distinct latex particles is the final step in latex film maturation. This step drives the transformation of a honeycomb of compacted latex particles bound by weak surface forces into a mechanically robust film. Knowledge of the onset of this diffusion process is limited. We have examined film formation in butyl acrylate-methyl methacrylate copolymer latex containing 1 wt% methacrylic acid. These films dry via a propagating drying front. We were able, via fluorescence resonance energy transfer measurements, to determine the extent of polymer interdiffusion at 23°C as a function of distance from the edge of the drying front for a series of partly wet latex films. Our apparatus allows us to arrest the latex drying process and to extract interdiffusion information from sub-millimeter regions of the drying film. We have tracked the latex drying process and subsequent polymer diffusion as a function of humidity. We find that adjacent to the drying front, increasing humidity initially delays the onset of interdiffusion, but once this initial barrier is overcome increasing humidity increases the rate of diffusion. This transition occurs within 1-2 mm of the drying front.

Interdiffusion during film formation of ionically cross‐linked acrylics investigated with Förster resonance energy transfer (FRET)

“…Increasing rH from 10 to 75% accelerates interdiffusion in all films (Figure ). This acceleration can be attributed to hydroplasticization, which has been observed for films from water‐borne coatings and PSA formulations as well. The main repetition unit in the polymers investigated in Figure is BA.…”

“…Intuitively, a value close to zero is expected because no interdiffusion has taken place. The reason for a value above zero is associated with the formation of labeled, water‐soluble oligomers which exchange between the particles after donor‐ and acceptor‐labeled dispersions are intermixed . Water‐soluble oligomers are a known by‐product in emulsion polymerization .…”

“…It has been shown by differential scanning calorimetry (DSC) measurements that polymer films made from dispersions have a reduced glass transition temperature, T g , when water is dissolved in their matrix (hydroplasticization) . FRET studies have shown that hydroplasticization accelerates polymer interdiffusion . Recent studies addressed polymer interdiffusion in film forming water‐borne PSA dispersions .…”

Influence of Ethylene Glycol and Propylene Glycol on Polymer Diffusion in Poly(butyl acrylate-co-methyl methacrylate) Latex Films

“…26 We found that adjacent to the drying front, increasing humidity initially delays the onset of interdiffusion, but once this initial barrier is overcome, high humidity plasticizes the polymer and increases polymer diffusivity. To explain these results, we proposed that after the drying front has passed, there still remains a very thin film of water between the compacted latex particles that blocks polymer diffusion.…”

Watching paint dry; more exciting than it seems

“…For simple viscous liquids this process occurs on very short time scales, accessible only with high-speed imaging 18 or highfrequency conductometry 19 and can be described by hydrodynamics alone. By contrast, polymer intermixing upon coalescence can be studied at the molecular scale using Förster Resonance Energy Transfer (FRET) [29][30][31] in which lateral and depth information are usually lost. As a result, the mixing process following film rupture occurs over much longer time scales, and may continue well after the coating has dried.…”