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.

Acrylamide polymers modified with low amounts of alkyl- or alkylarylacrylamides (1−5 mol %) have been prepared by an aqueous micellar copolymerization technique. This method is known to lead to multiblock copolymers in which the number and length of the hydrophobic blocks vary with the initial number of hydrophobes per micelle. The incorporation behavior of different types of hydrophobes and their effects on the rheological copolymer properties have been investigated. Interestingly, the use of disubstituted acrylamides leads to an average copolymer composition independent of the degree of conversion, in contrast to what is observed with monosubstituted acrylamides. Solubility measurements of both types of hydrophobes indicate that the micellar dynamics is not responsible for this behavior, but rather the difference in polarity between the bulk phase and the micellar phase. This microenvironment effect modifies the reactivity ratios of those hydrophobes capable of forming hydrogen bonds, whereas the reactivity of the other hydrophobes remains unaffected. The rheological properties of the samples are discussed in terms of copolymer microstructure and type of hydrophobe used (bulkiness, degree of branching, and alkyl chain length). For example, at similar hydrophobe levels, double-chain hydrophobes considerably enhance the thickening efficiency with respect to single-chain hydrophobes.

Multisticker polymer chains consisting of water-soluble polyacrylamides hydrophobically modified with low amounts of N,N-dihexylacrylamide have been prepared by a free radical micellar polymerization technique. This process gives multiblock copolymers in which the number and length of the hydrophobic blocks can be tuned by varying the surfactant over hydrophobe molar ratio. The viscoelastic behavior of semidilute solutions of various series of copolymers with variable molecular weights (M w ≈ 4.2 × 104−2.7 × 106), hydrophobe contents ([H] = 0.5−2 mol %), and hydrophobic block lengths (N H = 1−7 units per block) has been investigated as a function of polymer concentration, C, using steady-flow and oscillatory experiments. In the semidilute range, two different regimes can be clearly distinguished in the zero-shear viscosity η 0 = f(C) curves:  a first unentangled regime where the viscosity increase rate strongly depends on N H and [H]; a second entangled regime where the viscosity follows a scaling behavior of the polymer concentration with an exponent close to 4, whatever [H] or N H. The linear viscoelasticity can be described by (i) a slow relaxation process with a plateau modulus that only depends on polymer concentration and (ii) other faster complex relaxation processes. In the latter regime, the results can be quite well accounted for by a hindered reptation model.

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