Role of the Hydrophilic Latex Particle Surface in Water Diffusion into Films from Waterborne Polymer Colloids

The diffusion mechanism and growth of large-scale domains during the immersion of latex films in water have been thoroughly investigated with scattering techniques in a combination with the gravimetric method. Latex dispersions for film formation studies had identical main monomer compositions and only differ in the hydrophilic comonomers that result in distinct "hairy" layer structures of the particles. The major effects of the presence and the structure of the surface layers were identified: (1) Introducing the hydrophilic surface layer in the binder structure results in a more uniform penetration of water and a reduction in the water domain growth. (2) The nature of the particle shell defines the rate of the formation of the first hydration layer and the beginning of the large cluster formation. Poly(acrylamide) in the particle shell promotes the formation of the homogeneously swollen film and slows down the development of water "pockets." Poly(acrylic acid) leads to a more heterogeneous material and accelerates water uptake and cluster growth. (3) The thickness of the particle hairy layer regulates the thickness of the interstitials in the dry film and the number of the chemical groups involved in H-bonding with water molecules without a cluster formation. The amount of water that was absorbed before large domains start evolving increased with the growth of the particle shell thickness.