Water mobility during drying of hard and soft type latex: Systematic GARField 1H NMR relaxometry studies

GARField 1H NMR relaxometry experiments were done to study the drying process of two latices with different polymer Tg and to elucidate water mobility behavior during this process. It was found that the hard type latex, with a polymer Tg above room temperature, dries faster than the soft type latex, with a polymer Tg below room temperature. Diffusion measurements by means of echo time variations at different moments in the drying process show that water auto-diffusion decreases with increasing solid content of the latex independent of particle Tg. Two pools of protons with different mobilities were observed based on transversal relaxation T2. Determination of the long T2,long's and short T2,short's of both latex dispersions and their respective proton densities during drying at 80% RH showed evidence of particle deformation for the soft type latex and the absence thereof for the hard type latex. Additional drying of the resulting coatings with anhydrous CaCl2 showed a higher porosity for the hard type coating based on the proton distribution profile width. Moreover, two domains with different polymer proton mobilities are found for both coatings that are both plasticized by water at 80% RH. This is more apparent for the hard type coating, suggesting that a more hydrophilic polymer gives a higher degree of plasticization.