Surface tension and surface dilatational elasticity of associating hydrophobically modified polyacrylamides in aqueous solutions

The effects of the variation of different parameters in some hydrophobically modified polyacrylamides in aqueous polymer solution on their surface properties have been investigated. Two different hydrophobes, N,N-dihexylacrylamide, (DiHexAM) and N-(4-butylphenyl)acrylamide (BPAM), have been used; the former was most extensively investigated. The molecular weight (50 000-1000 000), amount of hydrophobes (0.5-2 mol %), and block length of the hydrophobic groups (1-7 units) have been varied. The dynamic surface pressure has been measured by means of drop shape analysis of a sessile bubble, and the surface dilatational elasticity and viscosity have been obtained during the adsorption process by the oscillating bubble technique, II-A isotherms of the adsorbed polymers have also been measured by using compression and decompression cycles on the sessile bubble. All the hydrophobically modified polymers show surface activity, but the adsorption rate is very low; it usually takes more than 24 h to obtain surface pressures beyond 20 mN m(-1) . The adsorption is therefore believed to be controlled by unfolding and reconfiguration of the bulk polymer and the penetration of the surface layer by individual hydrophobic blocks, Adsorption rate generally decreases with increasing molecular weight, except where a synergistic effect is believed to be present. Increasing block length (keeping the amount of hydrophobe constant) leads to a lower adsorption rate, while increasing amount of hydrophobe (keeping the block length constant) has the opposite effect. The assumption of the penetration of individual hydrophobic blocks into the surface layer accounts for both these effects. All polymers show high surface dilatational elasticity, up to ca. 80 mN m(-1), and almost zero surface viscosity. The surface elasticity as a function of the surface pressure follows a linear relationship over practically all the surface pressure range with an average slope of 3.6. This agrees well with a theoretical derivation based on scaling theory, and it is thus possible to calculate the nu exponent for the polymers at the interface. An average of 0.69 is obtained, which shows that the surface layer is a medium to good solvent for the block copolymer.