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

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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.

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Notes

  1. Theoretical descriptions of polymer interdiffusion anticipate non-Fickian interdiffusion over short lengthscales; and spin echo neutron scattering experiments seem consistent with this prediction. For a review of this topic, see McLeish, T.C.B., “Tube theory of entangled polymer dynamics,” Advances in Physics, 51, 1379 (2002). Sample polydispersity in both molecular weight and branching further complicates latex diffusion, as we expect that there is a broad distribution of tracer diffusion coefficients for individual chains in the system. A full treatment of latex interdiffusion for brached, polydispersed polymers is not yet possible.

  2. We also considered two other sources of the apparent energy transfer in the wet spot of the 50/49 latex, but ultimately ruled them out. The first possibility is that light scattering from the dispersion gives the appearance of energy transfer in the fluorescence decay. By carefully fitting fluorescence decays from the wet spot of donor only latex and donor/acceptor latex, we were able to rule this possibility out. The fluorescence decay of the donor was perfectly single exponential, with a lifetime of 44.5 ns. The fluorescence decay of the donor/acceptor latex was not single exponential, and even if we assumed that the early time deviation from single exponential decay was due to light scattering, the lifetime extracted from the tail of the decay was 43.3 ns. This shorter apparent lifetime cannot be due to light scattering. A second possible source of energy transfer in the wet spot could come from a small amount of skin formation on the wet spot of the latex. We also ruled this out. We aged a partly wet latex in the sample chamber at 23°C for 1 h. After this time period, we found no increase in ΦET measured from the wet spot. If skin formation was responsible for the energy transfer in the wet spot, then we would expect that polymer interdiffusion in the skin would make ΦET time dependent—there is no such time dependence. Since equation (3) treats all contributions that increase the donor decay rate to FRET, a faster decay rate of the Phe chromophore in this phase would lead apparent energy transfer; however, measurements of the Phe decay rate for the donor-only latex gave an exponential decay with a lifetime of 44.5 ns in the wet zone.

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Acknowledgments

The authors thank Rohm & Haas, Rohm & Haas Canada, and NSERC Canada for their support of this research. Y.L. appreciates scholarship support from Materials and Manufacturing Ontario.

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Correspondence to Mitchell A. Winnik.

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This paper was awarded First Place in the 2007 FSCT Roon Awards competition, held as part of the FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in Toronto, ON, Canada, on October 3–5, 2007.

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Haley, J.C., Liu, Y., Winnik, M.A. et al. The onset of polymer diffusion in a drying acrylate latex: how water initially retards coalescence but ultimately enhances diffusion. J Coat Technol Res 5, 157–168 (2008). https://doi.org/10.1007/s11998-007-9061-9

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