Joseph Selb

Hydrophobically-modified polyacrylamides prepared by micellar polymerization

Abstract The characteristic features of hydrophobically-modified polyacrylamides (HMPAM) prepared by a micellar polymerization technique are reviewed. This method of synthesis leads to copolymers in which the hydrophobic units are randomly distributed as small blocks in the acrylamide backbone. Special emphasis is put on the improvement of the technique so that well characterized and homogeneous samples are synthesized. The effect of the various parameters controlling the rheological behavior of HMPAM is thoroughly analyzed. In particular, it is shown that a determining factor for a good thickening ability is the copolymer microstructure, i.e. the hydrophobe distribution. Some recent advances are presented towards a better understanding of the association structure, thus allowing the design of tailored materials with good controllable rheological properties.

Associating behaviour of polyacrylamides hydrophobically modified with dihexylacrylamide

Abstract Polyacrylamides hydrophobically modified with a small amount of dihexylacrylamide (HMPAM) have been prepared by means of an aqueous micellar polymerization technique. This process leads to copolymers in which the hydrophobic molecules are randomly distributed as blocks in the acrylamide backbone. Keeping constant the hydrophobe level (1 mol%), the length of the blocks was adjusted by varying the surfactant content used in the synthesis, i.e. by varying the number of hydrophobes per micelle, NH. The resulting copolymers are homogeneous in composition, but have a different microstructure (blockiness). The rheological properties of the samples in aqueous solution have been investigated as a function of polymer concentration, polymer conversion, temperature and in the presence of surfactant, using steady flow and oscillatory experiments. The hydrophobic modification of the polyacrylamide backbone induces a showing down of the reptation movement of the polymer chains. The results stress the major role of the hydrophobe distribution in the copolymer for a good control of the thickening efficiency of associating HMPAM.

Copolymers of Acrylamide/n-Alkylacrylamide in Aqueous-Solution - the Effects of Hydrolysis On Hydrophobic Interactions

Abstract Two copolymers of acrylamide N-4- ethylphenylacrylamide have been prepared by an aqueous micellar copolymerization technique with two different amounts of an anionic surfactant (sodium dodecyl sulphate). Such changes in the polymerization medium give copolymers with different microstructures. Samples of each copolymer were subsequently hydrolysed to 10, 20 and 40% completion. Hydrophobic interactions of all the samples in aqueous solution were then studied in the semi-dilute concentration range near to and above the chain overlap concentration, c∗ , via rheological and fluorescence techniques. The samples were each studied as a function of concentration, salt concentration, surfactant concentration and temperature. In all cases, the rheological and the fluorescence data were consistent and directly related to the number of interchain liaisons in solution. The number of interchain liaisons increased with increase in the copolymer hydrophobicity and were hence related to the hydrolysis level and the blockiness of the copolymer.

Compositional heterogeneity effects in hydrophobically associating water-soluble polymers prepared by micellar copolymerization

Abstract An aqueous micellar radical copolymerization technique has been used to prepare water-soluble copolymers of acrylic acid/styrene, acrylamide/styrene and acrylic acid/ethylphenylacrylamide. In this process, the hydrophobic monomer (1–2 mol% based on the monomer feed) is dispersed within surfactant micelles while the water-soluble monomer is dissolved in the aqueous medium. This synthesis leads to block copolymers in which the number and length of the hydrophobic blocks vary with the degree of conversion. Kinetic studies have shown that the compositional heterogeneity results from the balance between the values of the reactivity ratios of the monomer pairs and the micellar effect inherent to the technique. The latter always leads to an enhanced apparent reactivity for the hydrophobic monomer. It is shown that an optimization of the process can be achieved which eventually corrects for the drift in the average copolymer composition with conversion. The thickening ability of the hydrophobically modified water-soluble copolymers is directly affected by the extent of their compositional heterogeneity. Analysis of the data provides some criteria for selecting appropriate systems with good controllable rheological properties.

Terpyridine-functionalized imidazolium ionic liquids

This paper reports the synthesis and the physical characterization of a new family of chelating ionic liquids carrying a terpyridine fragment suitable for metal extraction.

Characterization of high charge density ampholytic copolymers prepared by microemulsion polymerization

Abstract A series of copolymers of sodium 2-acrylamido-2-methylpropanesulfonate (NaAMPS) and 2-methacryloyloxyethyltrimethylammonium chloride (MADQUAT) prepared by microemulsion polymerization was characterized by means of several techniques. After dialysis, the samples self-neutralize with formation of monomer pairs in the copolymer. The molecular weights determined by light scattering are high (⩾ 107) and were found to be relatively unaffected by monomer feed ratios. The copolymer microstructure was calculated from the reactivity ratios and compared to that obtained for copolymers synthesized in aqueous solution. The microemulsion polymerization process yields copolymers more homogeneous in composition ( r A r B ⋍ 1.6 ) than those obtained by polymerization in solution which have a strong tendency to alternation ( r A r B ⋍ 0.05 ). These differences are discussed in terms of microenvironment and polymerization mechanisms.

A study of multicompartment polymeric micelles

Abstract A series of novel polymerizable hydrocarbon (H) and fluorocarbon (F) cationic acrylamido surfactants were prepared, characterized and used as comonomers for the synthesis of multicompartment polymeric micelles. The nature of the amido group of the surfactant, either N-monosubstituted (CONRH) or disubstituted (CONRC 2 H 5 ) is one of the key parameters governing their solution behavior. Thus, micellar growth for pure F-surfactants and miscibility/incompatibility for mixed systems depend on the possibility of H-bond formation. Radical polymerization of acrylamide in aqueous solution in the presence of demixing hydrocarbon and fluorocarbon polymerizable micelles leads to water-soluble polymers containing well segregated hydrocarbon and fluorocarbon domains as confirmed by fluorescence spectroscopy.

Polymerization of acrylamide in solution and inverse emulsion: Number molecular weight distribution with chain transfer agent

The molecular weight distributions of the free-radical polymerization of acrylamide in solution and in inverse emulsion are examined as a function of concentrations of initiator and of amount of added chain transfer agent (sodium formate). When these distributions are displayed as In(number molecular weight distribution), readily obtained from the GPC trace (essentially by dividing by the square of molecular weight), the results can be semi-quantitatively interpreted in terms of the amounts of chain stoppage by transfer and by termination.

Synthesis and Properties of Hydrophobically Modified Polyampholytes

Abstract Hydrophobically modified, low charge density polyampholytes were prepared by copolymerization of acrylamide, [2-methacryloyloxy)ethyl] trimethylammonium chloride, sodium 2-acrylamido-2-methylpropane sulfonate and a hydrophobic monomer (N,N-dihexylacrylamide) in a micellar aqueous medium. The resulting polymer is soluble in pure water and in sodium chloride aqueous solutions and exhibits improved thickening properties with respect to the unmodified polyampholyte due to intermolecular association. This behavior manifests itself particularly in high-salinity media.

Polymerization in nanostructured media: applications to the synthesis of associative polymers

Some recent developments obtained by free radical polymerization in nanostructured media are presented. The characteristic features of the confined media like the large overall interfacial area and the particular microenvironment are taken advantage to yield novel materials with interesting properties. The examples considered here include the synthesis of i) thermosensitive polyampholytes by polymerization in bicontinuous microemulsions, ii) ionomers by polymerization in inverse microemulsions and iii) multicompartment polymeric micelles by means of an aqueous micellar polymerization process. The characteristics inherent to each of these syntheses are discussed.