Influence of the Hydrophobe Structure on Composition, Microstructure, and Rheology in Associating Polyacrylamides Prepared by Micellar Copolymerization

Abstract

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.

Rheological properties of multisticker associative polyelectrolytes in semidilute aqueous solutions

“…Neutralization at pH ϭ 9 was achieved by the slow addition of AMPS to an aqueous sodium hydroxide solution, and this produced NaAMPS. The hydrophobic comonomer Di-HexAM was prepared and purified according to the technique of Valint et al 10,50 Water was purified with a Millipore Milli-RO/Milli-Q system. Sodium dodecyl sulfate (SDS; Acros) and 4,4Ј-azobis(4-cyanovaleric acid) (ACVA; Acros) were used as received.…”

“…10,42 The polymerization procedure was similar to that described elsewhere. 49,50 The monomer concentration in the feed was kept at 2 wt %, and the initiator concentration was set to 0.8 mol % with respect to the monomer feed. Such a high initiator concentration was required 53,62 because of the rather low dissociation constant of ACVA at 50°C (k d,50°C Ϸ 3 ϫ 10 Ϫ6 s…”

“…50 The measurements were performed in methanol-d 4 /D 2 O mixtures (8/2 v/v) at a polymer concentration of 1 wt %.…”

“…It has been inferred from photophysical and rheological experiments that the length of these hydrophobic segments corresponds roughly to the average number of hydrophobic monomers per micelle (N H ). 18,[43][44][45][46][47][48][49][50][51] It has also been clearly established that the thickening efficiency is directly related to the polymer microstructure: the longer the hydrophobic blocks, the greater the thickening efficiency at a constant level of hydrophobe incorporation. 48 -53 In this report, we describe the micellar synthesis and aqueous solution properties of multiblock polyelectrolytes of AM, sodium 2-acrylamido-2-methylpropanesulfonate (NaAMPS), and N,N-dihexylacrylamide (DiHexAM).…”

“…60 The choice of a disubstituted AM derivative as the hydrophobic monomer (DiHexAM) is justified by the fact that it minimizes or even suppresses the compositional drift that is observed when monosubstituted AM derivatives are used. 50,61 After describing the synthesis and characterization of the polymers, we focus on their rheological properties in semidilute, salt-free aqueous solutions. The study mainly deals with polymer solutions in the unentangled semidilute regime; the impact of the polymer microstructure on the rheological behavior is described in detail, along with a comparison of the dynamic properties with available theories.…”