PROPERTIES OF HYDROPHOBICALLY ASSOCIATING POLYACRYLAMIDES - INFLUENCE OF THE METHOD OF SYNTHESIS

Hydrophobically modified water-soluble polymers have been prepared by radial copolymerization of acrylamide and ethylphenylacrylamide as the hydrophobic comonomer. Three methods of synthesis in aqueous media have been investigated: (i) a ''micellar'' process in which the presence of a surfactant ensures the solubilization of the hydrophobic monomer; (ii) a ''homogeneous'' process wherein a miscible cosolvent is used; (iii) a ''heterogeneous'' process, without additive to solubilize the insoluble monomer. The properties of the copolymers in dilute and semidilute aqueous solutions strongly depend on the conditions of the synthesis. Copolymers prepared by the homogeneous and heterogeneous processes behave like homopolyacrylamide; i.e., hydrophobic interactions do not occur significantly. Copolymers obtained by micellar copolymerization exhibit improved thickening properties due to intermolecular hydrophobic associations. These differences can be directly related to the copolymer microstructure, i.e., to a random or blocky distribution of the hydrophobic units. The blockiness of the copolymer can be adjusted by varying the [hydrophobe]/[micelle] ratio at a constant hydrophobe level. Thus, it is possible to control the association degree and therefore the rheological properties. Fluorescence studies, using pyrene as a probe, reveal the formation of hydrophobic microdomains which corroborate the rheological results.