CHARACTERIZATION OF THE AQUEOUS PHASE AND THE WATER-POLYMER INTERFACE IN LATEX SUSPENSIONS BY DIFFERENTIAL SCANNING CALORIMETRY

The behavior of water in film-forming and non-film-forming latexes, with a mean diameter of 200-240 nm, has been studied in the temperature range -80 to +20°C by differential scanning calorimetry (DSC). Only the film-forming latexes show the existence of a fraction of water freezing at -40°C. It has been related to water molecules entrapped in disconnected microdomains, created by the deformation of the latex particles occurring during the film-formation process. For a film-forming latex, the surface of which is covered with sodium dodecyl sulfate, equivalent mean diameters, varying from 107 to 54 nm as a function of the water content, have been evaluated for the microdomains. The water-latex interface was investigated especially for non-film-forming latexes either prepared with functional comonomers (sulfopropylmethacrylate, methacrylic acid) or poststabilized with polyvinyl alcohol (PVA) of various molecular weights. For the former, two different processes have been evidenced for ice melting; in excess water, the amount of water in the polymer layer has been found to be constant. However, it depends on the functional properties of the comonomer. For the latter, a fraction of water molecules is rendered incapable of freezing and is related to PVA concentration. From calorimetric data, the amount of adsorbed polymer (Γ) per unit area of polystyrene surface was accurately determined and a minimum thickness of hydrated PVA (δDSC) was deduced and discussed with respect to the thicknesses (δPCS) obtained on the same latexes, by photon correlation spectroscopy. Γ and δ are compared with literature data. The mean number of water molecules per PVA segment in the adsorbed layer was evaluated and compared with the mean number of water molecules per unperturbed PVA segment in solution. © 1991.