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

The effects of the variation of different parameters in some hydrophobicall y modified polyacrylamides in aqueous polymer solution on their surface pro perties have been investigated. Two different hydrophobes, N,N-dihexylacryl amide, (DiHexAM) and N-(4-butylphenyl)acrylamide (BPAM), have been used; th e former was most extensively investigated. The molecular weight (50 000-10 00 000), amount of hydrophobes (0.5-2 mol %), and block length of the hydro phobic groups (1-7 units) have been varied. The dynamic surface pressure ha s been measured by means of drop shape analysis of a sessile bubble, and th e surface dilatational elasticity and viscosity have been obtained during t he adsorption process by the oscillating bubble technique, II-A isotherms o f the adsorbed polymers have also been measured by using compression and de compression cycles on the sessile bubble. All the hydrophobically modified polymers show surface activity, but the adsorption rate is very low; it usu ally 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 reco nfiguration of the bulk polymer and the penetration of the surface layer by individual hydrophobic blocks, Adsorption rate generally decreases with in creasing molecular weight, except where a synergistic effect is believed to be present. Increasing block length (keeping the amount of hydrophobe cons tant) leads to a lower adsorption rate, while increasing amount of hydropho be (keeping the block length constant) has the opposite effect. The assumpt ion of the penetration of individual hydrophobic blocks into the surface la yer accounts for both these effects. All polymers show high surface dilatat ional 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 lin ear relationship over practically all the surface pressure range with an av erage 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 t he polymers at the interface. An average of 0.69 is obtained, which shows t hat the surface layer is a medium to good solvent for the block copolymer.