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Modulation of Hydrophobic Interactions in Associative Polymers Using Inclusion Compounds and Surfactants

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Modulation of Hydrophobic Interactions in Associative Polymers Using Inclusion Compounds and Surfactants Macrom olecules 2003, 36, 7833-7841 7833 Modulation of Hydrophobic Interactions in Associative Polymers Using Inclusion Compounds and Surfactants Ahmed A. Abdala,†,‡ Alan E. Tonelli,‡ and Saad A. Khan*,† Departm ent of Chem ical Engineering, N orth Carolina S tate University, R aleigh, N orth Carolina 27695-7905, and Fiber and Polym er S cience Program , N orth Carolina S tate University, R aleigh, N orth Carolina 27695-8301 R eceived February 10, 2003; R evised Manuscript R eceived J uly 29, 2003 ABSTRACT: We report the modulation of the solution rheology of a comblike, hydrobhobically modified alkali-soluble emulsion (HASE) associative polymer through addition of R-and!-cyclodextrins (CDs). The ring-shaped CDs with hydrophobic inner cores interact with the pendant macromonomer segments of the associative polymer containing hydrophobic end groups, leading to reduction in polymer solution viscosity and dynamic moduli by several orders of magnitude. We find no interactions between the CDs and the polymer backbone as substantiated by the fact that an analogous parent polymer without hydrophobes reveals no changes in the solution rheology in the presence of CDs. In contrast, the CDs encapsulate the hydrophobic groups on the associative polymer. This is confirmed by the complexation between the CD and a surfactant modified to resemble the hydrophobic macromonomer of the associative polymer as observed using 1H NMR, differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). The stoichiometric ratio of complexation between the CD and the hydrophobic macromonomer is determined independently from both NMR and yield data to be 5 mol of CD/mol of hydrophobe. Interestingly, the reduction in polymer viscoelasticity in the presence of CD is reversibly recovered upon subsequent addition of different nonionic surfactants that have a higher propensity to complex with the CD than the hydrophobic segments of the HASE polymer. 1. Introduction with difficulty in handling during solution preparation Associative polymers are macromolecules with attrac- and prior to end use. The hydrophobic interactions also tive groups either attached to the ends or randomly make extracting information from characterization tech- distributed along the backbone.1 Hydrophobically modi- niques, such as light scattering and gel permeation fied alkali soluble emulsion (HASE) polymers are one chromatography (GPC), cumbersome and less accurate. class of water-soluble associative polymers that have a The removal of the hydrophobic interactions would comblike structure with pendant hydrophobic groups simplify the information gained from these techniques randomly distributed along a polyelectrolyte backbone. and assist in understanding the behavior of these HASE polymers have several advantages over other polymers. In addition, the properties of the hydropho- associative polymers in terms ofcost and wide formula- bically modified polymers are usually compared to those tion latitude.2 Consequently, they are currently being of the unmodified parent polymer without hydrophobes used in a range ofapplications, including paint formula- to gain understanding about their microstructures and tions, paper coatings, and recently as glycol-based associating abilities. However, such an assessment may aircraft antiicing fluids3-5 and also have potential for not be realistic because modified and unmodified poly- mers may differ by more than just the hydrophobic use in enhanced oil recovery and personal care products. 6 These polymers are usually added to either modify the modification. The ability to compare modified polymers rheology of aqueous solutions or increase the stability with both active and deactivated hydrophobes provides of dispersions. Because of their high thickening ability, a plausible basis for understanding their behavior. a few percent of HASE polymers can increase the In this study, we examine a powerful method to solution viscosity by several orders of magnitude. This control the solution rheology of HASE polymers by thickening ability is predominantly the result of the means of removing the hydrophobic interactions using 6 molecular hydrophobic associations that occur to mini- cyclodextrins to form inclusion compounds with the mize contact between the aqueous medium and the macromonomer part of the HASE polymer. Cyclodex- hydrophobicsegments ofthe polymer;the hydrodynamic trins (CDs) are ring-shaped oligosaccharides consisting volume expansion upon neutralization of the carboxylic of 6, 7, or 8 glucose units (corresponding to R-, !-, and groups on the polymer backbone also plays a minor role γ-CD) joined by R-1,4-glycosidic linkages. They have a in this regard. hydrophobic inner core and a hydrophilic outer shell, Despite the importance of hydrophobic interactions thus making it possible for the hydrophobic segments to promote viscosity enhancement in this polymer of the polymer to reside inside them and form a complex system, there is also a need to remove these interactions referred to as an inclusion compound. Such a notion is in many instances. For example, the high solution supported from previous studies which reveal cyclodex- viscosity of a concentrated solution is always associated trins to have superior tendencies to interact with the hydrophobic segments ofdifferent hydrophobically modi- † fied water-soluble associative polymers, including hy- Department of Chemical Engineering. 7-12 ‡ Fiber and Polymer Science Program. drophobically end-capped poly(ethylene oxide), poly- *Correspondingauthor: phone919-515-4519;fax919-515-3465; (ethylene glycol)s (PEGs) bearing hydrophobic ends e-mail [email protected]. (naphthyl and phenyladamantyl),13 N ,N -dimethylacryl- 10.1021/ma034173v CCC: $25.00 © 2003 American Chemical Society Published on Web 09/06/2003 7834 Abdala et al. Macrom olecules, Vol. 36, N o. 20, 2003 ethylene oxide (EO) units. Details of the preparation method can be found in a previous publication.40 In addition to the hydrophobically modified polymer, an unmodified polymer that has the same structure as the modified polymer with the C22H 45 hydrophobes replaced by an equivalent amount of methyl groups was also used. Both the modified and the unmodified polymers were prepared in an identical manner and are believed to have the same molecular weight. The polymer latexes were dialyzed against deionized water using a cellulosictubular membrane for at least 3 weeks with daily change of water. After dialysis, the polymer was freeze-dried, and 5%solutions were prepared and neutralized topH of9 ( 0.1 using1 N NaOH with the ionicstrength adjusted to0.1 M KCl. C22EO40 surfactant under the commercial name of Rhoda- Surf was provided by Dow Chemical Co. The surfactant was Figure 1. Schematic representation of a HASE associative modified to resemble the macromonomer part (MW 2287 Da) polymer and the molecular constitution of the HASE polymer of the HASE polymer through reaction with R,R-dimethyl used in this study.R refers tothe C22H 45 hydrophobe, p ) 40, meta-isopropenyl benzyl isocyanate (TMI (meta), American and x/y/z ) 43.57/56.21/0.22 by mole. Cyanamid) as shown in Scheme 1. The nonionic surfactant, nonylophenol poly(ethylene glycol) ether with degree ofethox- amide-hydroxyethyl methacrylate copolymer hydro- ylation of 4 (NP4), was provided by Dow Chemical Co. phobically modified with adamantyl groups,14-16 hydro- Industrial grade R-and!-cyclodextrins were supplied by phobically modified ethyl(hydroxyethyl) cellulose,6 hy- Cerestar and used as received. drophobically modified, degradable, poly(malic acid),17 2.2. Methods. The steady and dynamic rheological behavior isobutene maleate polymer with pendant hydrophobic of the polymer solutions were measured using a stress- 4-tert-butylanilide,18 hydrophobically modified ethoxy- controlled rheometer (Rheometrics DSR II) fitted with ap- lated urethanes,19 hydrophobically modified alkali- propriate cone and plate, parallel plates, and couette geom- 20,21 etries. Details on the rheological techniques are provided in soluble emulsion polymers, and hydrophobically previous publications.41-44 22 modified Dextran. Cyclodextrins have also been re- 1H NMR data were obtained using a 500 MHz Bruker DRX ported to form inclusion compounds with many nonionic NMR spectrometer. All spectra were acquired at 298 K using surfactants.23-39 tetramethylsilane (TMS)as internal standard, and all samples In this work, we focus on investigating the effects of were prepared in DMSO-d6. The instrumental parameters for R-and!-cyclodextrin addition on the rheology of HASE acquisition of the one-dimensional proton spectra were as polymer solutions, understanding the mechanism of follows: tuning frequency 500.128 MHz, spectral width 13.2 cyclodextrin polymer complexation and evaluating the ppm, number of data points 32K, relaxation and acquisition times 1.0 and 2.47 s, respectively, pulse width 10.5 µm, tip reversibility of these interactions. As such, we examine angle 90°, and number of transients 16. initially the extent of rheology changes upon CD addi- Differential scanning calorimetry (DSC) was carried out on tion and the existence, if any, of quantitative relation- 3-8mgsampleswithaPerkin-ElmerDSC-7thermalanalyzer ship between the molar ratio of CDs to the polymer equipped with a cooler system. A heating rate of 10 °C/min hydrophobes on solution rheology. To isolate whether was employed, and an indium standard was used for calibra- the observed changes are due to interactions of the CD tion. Before each scan,
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