Downloaded by UNIV OF MISSOURI COLUMBIA on May 21, 2013 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch015
15 Hydrophobic and Electrostatic Interactions in Water-Soluble Associating Copolymers Joseph Selb, Simon Biggs, Delphine Renoux, and Françoise Candau Institut Charles Sadron (Centre de Recherches sur les Macromolécules-Ecole d'Application des Hauts Polymères), 6 rue Boussingault, 67083 Strasbourg Cédex, France
Associative polyacrylamide derivatives containing both ionic sites and small numbers of hydrophobic groups were prepared, and their thickening properties in aqueous solution were investigated. Two different radical micellar copolymerization processes in aqueous media were used: The comonomer of acrylamide was either a hydrophobic monomer (Nethylphenylacrylamide) solubilized within surfactant micelles (sodium dodecyl sulfate) or a micelle-forming cationic polymerizable surfactant (n-hexadecyldimethyl-4-vinylbenzylammonium chloride). Relationships between the copolymerization mechanism and the copolymer microstructure are proposed. Owing to the competition between attractive hydrophobic interactions and repulsive electrostatic interactions, such hydrophobically modified polyacrylamides exhibit various rheological behaviors in aqueous solution depending on shear rate, shear time, ionic strength, and copolymer characteristics.
modified with relatively low amounts of a hydrophobic comonomer (10 mol%). However, such compounds are generally not effi cient thickeners because the intramolecular hydrophobe associations are responsible for a hypercoiled conformation and intermolecular as sociations are not favored even in the semidilute range. We report here some results of an investigation into the synthesis, characterization, and solution properties of two classes of ion-contain ing hydrophobically modified polyaerylamides. The specific features of the copolymerization methods used as well as their influence on the copolymer microstructure were examined. The aqueous solution properties of the copolymers were studied in the absence and the presence of salt. We compared the thickening abilities of the various samples, giving special attention to the effect of introducing charges onto the polymer chain; the nature of the hydrophobe groups, their number, and their distribution along the polymer backbone were taken into account. Shear rate and shear time effects were also investi gated. Additional information on the association process was provided by fluorescence studies.
Experimental Methods
Materials. Acrylamide (Merck) was recrystallized twice from chloro form. N-4-Ethylphenylacrylamide (βΦΑΜ) was synthesized by the reac-
In Hydrophilic Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
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Downloaded by UNIV OF MISSOURI COLUMBIA on May 21, 2013 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch015
tion of 4-ethylaniline with acryloyl chloride according to a procedure de scribed in the literature (36). Both reagents (Aldrich) were distilled just prior to use. n-Hexadecyldimethyl-4-vinylbenzylammonium chloride (N16) was prepared by quaternization of hexadecyldimethylamine (Hoechst, distilled) with 4-vinylbenzyl chloride (Kodak, used as received) in ethanol at 50 °C for 1 day and in the presence of tert-butylcatechol as inhibitor of polymerization. Ν16 was purified by recrystallization from ethyl acetate. Copolymer Synthesis. Acrylamide-Sodium Acrylate-Ethylphenylacrylamide Series. Terpolymers of acrylamide (AM), sodium acrylate (NaA), and ethylphenylacrylamide (βΦΑΜ) were prepared in a two-step procedure. ΑΜ-βΦΑΜ copolymers were first obtained by radical copoly merization using an aqueous micellar method well detailed elsewhere (6, 36). Typical experimental conditions were as follows: The total concentra tion of monomers in water was 3 wt%; the proportion of βΦΑΜ in the monomer feed varied from 1 to 3 mol% (i.e., 2.4—7 wt%); sodium dodecyl sulfate (SDS) was used as surfactant at concentrations ranging from 1 to 5 wt% of the total recipe; the concentration of K 2 S 2 O 8 used as watersoluble initiator was 0.3 wt% based on monomers. The reaction was al lowed to proceed at 50 °C for 7 h under nitrogen. About 90% conversion occurred. The copolymer was recovered by precipitation in methanol and purified by repeated filtrations and washings in methanol before being dried under vacuum at 50 °C. Subsequent partial hydrolysis (10—40% of A M units) to create carboxylate groups was carried out at a polymer con centration of 0.7 wt% in water under alkaline conditions ([NaOH] = 0.25 M); the reaction times and temperatures were 1 h at 30 °C, 2 h at 30 °C, and 2.5 h at 50 °C for 10, 20, and 40% hydrolysis, respectively. The recov ery and purification of hydrolyzed samples were like those just described. Some AM-NaA-βΦΑΜ samples were prepared in a single step by direct copolymerization of the three monomers under similar micellar condi tions. For some polymerization kinetics studies, the reaction solution was divided into several botdes by a double-needle transfer technique before K 2 S 2 O 8 was added. The separate reactions were terminated at various reaction times by addition of hydroquinone and cooling in an ice-water bath (37). AM—N16 Series. Typical experimental conditions for A M - N 1 6 copo lymerization were as follows: The concentration of monomers in water was 3 wt%; the proportion of Ν16 in the monomer feed varied from 0.5 to 5 mol% (i.e., 2.9-27 wt%); and 4,4'-azobis(4-cyanovaleric acid) (ACVA) (Aldrich), at 3 wt% based on monomers, was used as water-soluble initiator instead of K 2 S 2 O 8 because the latter strongly reduces the solubility of N16. Other experimental conditions (temperature, reaction time, and recovery procedure of the polymer) were like those for the process just described. Copolymer Characterization. The hydrophobe content in A M — NaA-θΦΑΜ copolymers was determined by U V spectrophotometry using the calibration established with ethylphenylpropionamide (36, 37). The level of carboxylate groups was determined by potentiometry and sodium elemental analysis. The copolymer molecular weights before hy-
In Hydrophilic Polymers; Glass, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
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drolysis were measured in formamide by static light scattering and were in the range 1.6-2.6 x 10 . The composition of A M - N 1 6 copolymers was determined from the chlorine content. 6
Rheological Measurements. The viscosity of aqueous polymer solu tions was measured at 25 °C with either a controlled stress rheometer (CarriMed) using cone plan geometry or a Contraves Low-Shear 30 instru ment. The polymer concentration ranged from 0.25 to 3% wt/wt, and the shear rate ranged from 0.01-10 to 1000-2500 s ~ depending on the viscos ity of the sample. Downloaded by UNIV OF MISSOURI COLUMBIA on May 21, 2013 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch015
1
Other Methods. Conductometry was used for determining the criti cal micellar concentration (cmc), as described elsewhere (38). Details on the experimental procedure for fluorescence measurements were given in a previous paper (6).
Copolymer Synthesis Two classes of associating polyacrylamide derivatives were synthe sized: one contains anionic-type derivatives and one contains cationictype derivatives. The former class (structure 1) contains terpolymers of A M with NaA as the ionic component and βΦΑΜ as the hydrophobe (AM-NaA-βΦΑΜ series). The second class (structure 2) consists of copolymers of A M with a comonomer containing both hydrophobic and ionic groups, the Ν16 ( A M - N 1 6 series). Samples of various hydrophobicities and ionic levels were prepared by varying the copolymer composition. Two particular "micellar" radical copolymerization pro cesses were used. In both cases the kinetics of copolymerization and hydrophobe incorporation were investigated, allowing us to propose the most probable reaction mechanism. Such information was neces sary for a clear understanding of the solution properties, because varia-
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