PROPERTIES OF HYDROPHOBICALLY ASSOCIATING POLYACRYLAMIDES - INFLUENCEOF 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 ha ve 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 sem idilute aqueous solutions strongly depend on the conditions of the syn thesis. Copolymers prepared by the homogeneous and heterogeneous proce sses behave like homopolyacrylamide; i.e., hydrophobic interactions do not occur significantly. Copolymers obtained by micellar copolymeriza tion exhibit improved thickening properties due to intermolecular hydr ophobic 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 adjuste d by varying the [hydrophobe]/[micelle] ratio at a constant hydrophobe level. Thus, it is possible to control the association degree and the refore the rheological properties. Fluorescence studies, using pyrene as a probe, reveal the formation of hydrophobic microdomains which cor roborate the rheological results.