Rheology of a HASE Associative Polymer and Its ... - ACS Publications

Chapter 22

Rheology of a HASE Associative Polymer and Its Interaction with Non-Ionic Surfactants 1

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Downloaded by NANYANG TECHNOLOGICAL UNIV on June 21, 2016 | http://pubs.acs.org Publication Date: August 10, 2000 | doi: 10.1021/bk-2000-0765.ch022

R. J. English , R. D. Jenkins , D. R. Bassett , and Saad A.

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Khan

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Department of Color Chemistry, The University of Leeds, Leeds, United Kingdom Union Carbide Asia Pacific Inc., 16 Science Park Drive, Singapore 118227 U C A R Emulsions Systems, Union Carbide Corporation, Cary, N C 27511 Department of Chemical Engineering, North Carolina State University, Raleigh, N C 27695-7905

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Solutions of a hydrophobically modified alkali soluble emulsion (HASE) polymer in alkaline media are seen to behave as reversible networks in small amplitude oscillatory shear, but their response is seen to deviate from the simple Maxwellian response exhibited by telechelic HEUR associative polymers. In this polymer, which contains a relatively small number of complex alkylaryl hydrophobes bound to each polymer chain, stress relaxation is considered to be influenced by the both the disengagement rate of the hydrophobes from their junction domains and topological constraints arising from physical entanglements of the chains. Several unusual phenomena are observed in steady shear, including shear induced structuring and stress saturation. Possible microstractural interpretations of these phenomena are discussed. The hydrophile - lipophile balance of non-ionic surfactants is seen to exert a profound influence on the nature of the polymer surfactant interaction. Rheological differences between solutions of the polymer containing a nonylphenol ethoxylate of higher HLB (NP + 10 EO) and a more hydrophobic surfactant (NP + 6 EO) are described and interpreted in terms of possible differences in surfactant phase behaviour.

Introduction In spite of their commercial utilisation [1], understanding of the dynamics and topology of HASE (Hydrophobic Alkali Soluble Emulsion) polymer networks is less well developed than that of simple, telechelic HEUR (Hydrophobic Ethoxylated Urethane) associative polymers [2-7]. This arises largely as a consequence of the

Corresponding author. © 2000 American Chemical Society

Glass; Associative Polymers in Aqueous Media ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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more complex architecture of HASE systems, lack of knowledge regarding the state of supramolecular assembly of the polymer in the dilute regime and the number of chain states possible at higher concentrations. In particular, the assignment of key parameters, such as the mean number of hydrophobes residing in a junction domain (^aggregation number) and their rate of disengagement, presents a difficult task. Recent experimental work on HASE polymers has considered the effects of hydrophobe size and side chain length on solution rheology [8-10], behaviour in parallel superposed steady/dynamic shear [11,12], physicochemical changes during solubilisation [13] interactions with surfactants [14,15] and the effects of added electrolytes on the behaviour in the dilute regime [16]. In the present study, we concentrate on some of the rheological phenomena observed with solutions of HASE polymers and how these are affected on addition of non-ionic surfactants. As majority of the water-soluble, polymers are employed in formulations containing surfactants or other amphiphilic species, polymer-amphiphile interactions are of considerable technological and theoretical interest [17,18]. The nature of the polymer-surfactant interaction may involve conformational transitions, owing to adsorption of surfactant microphase separation due to complexation, or modification of network formation via formation of mixed micelles at network junctions [19]. The latter form of interaction is particularly relevant to HASE polymers, as network junctions involve dynamic association of hydrophobic groups. In the case of nonionic amphiphiles of low water solubility, the phase structure of the system may be more complex and modified by inclusion of polymeric species [20]. Recent studies have concentrated on the role of surfactant phase behavior on the rheology of polymer-amphiphile systems and also the development of systems that exhibit gelation. Panmai et al. [21] examined the rheology of hydrophobically modified hydroethyl cellulose and hydrophobically modified poly(acrylamide) in the presence of a range of surfactants. Under conditions where the surfactants formed spherical micelles, interchain hydrophobic interactions were screened at the highest surfactant concentrations. Kaczmarski et al. [22] found the presence of both SDS and an octylphenol ethoxylate to promote stnicturing of HEUR polymers bearing large terminal hydrophobes. Deguchi et al. [23] demonstrated the gelation of solutions of cholesterol modified pullulan in the presence of SDS. The thermoreversible gelation of hydrophobically modified poly(acrylic acid)s in the presence of linear alcohol ethoxylates was studied by Loyen et al. [20, 24]. Interactions of a HASE polymer with non-ionic surfactants is considered in the present study, using alkylphenol ethoxylates of different hydrophile-lipophile balance and different degrees of aqueous solubility. In this respect, we attempt to demonstrate more clearly the role of surfactant phase behavior in dictating the mode of interaction with a hydrophobically modified polymer of relatively complex architecture.

Experimental The polymer examined here is identical to the material employed in our previous publications [11,14] and has the idealised constitution depicted in Figure 1. The hydrophobes comprise oligomeric condensates of nonylphenol, resulting in a


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relatively high hydrophobe molar volume. Full details of the emulsion polymerization process employed in the synthesis of model HASE polymers of this type are described in detail elsewhere [25]. The mean degree of ethoxylation in the macromer, n, is -80 and the weight fractions of ethyl acrylate, methacrylic acid and macromer employed in the polymer synthesis were 0.4, 0.4 and 0.2, respectively. This corresponds to a mole fraction of macromer of ~ 0.0055. Dilute solution viscometry showed the mtrinsic viscosity, [η] of the polymer to be ~ 4.8 dig" , at 25°C in 0.05 M NaCl at pH 9. The HASE polymer latex was purified by dialysis [Spectropore 7 cellulosic membrane - 50000 Da M cut-ofl], in order to remove serum electrolyte and excess anionic stabilizer. The non-ionic surfactants employed were commercial materials supplied by Union Carbide - Tergitol NP6, a nonyl phenol ethoxylate with a mean degree of ethoxylation of-6, and Tergitol NP10 a nonylphenol ethoxylate with a mean degree of ethoxylation of 10. Both surfactants were used without further purification. The polymer latex was solubilized in the presence of 2-amino-2-methyl1-propanol (AMP-95), at a level of 6.0 χ 10" mol. of die amine per gram of polymer (pH~9). Samples where prepared at a constant ionic strength (0.05 M NaCl), by combining the required amounts of purified latex, distilled-deionised water, 0.5 M NaCl and 1.0 M AMP and, where appropriate, non-ionic surfactant Although HASE polymers are polyelectrolytes in their native state, any polyelectrolyte behavior (or electrostatic effects) were eliminated/screened under these experimental conditions [16]. The HASE polymer concentration of samples containing NP6 was fixed at 0.6 g df , whilst the concentration of samples containing NP10 was fixed at 10 gl" . The concentration of NP6 in the system, CNP, was varied between 0.5 and 15 gl* and the concentration of NP10, CNPIO, between 1 and 11 gl" . All samples were œntrifuged (2500 rpm, 5 mia) in order to remove entrained air and allowed to stand for several days prior to rheometrical characterizatioa Rheometrical experiments were carried out in steady and dynamic shear using a Rheometric Scientific DSR controlled stress rheometer, fitted with appropriate cone and plate and concentric cylinder geometries. Steady shear data at low rates of deformation (γ < 10* s") where derived from sequential creep experiments, thus ensuring that the duration of the experiment was sufficient to for attainment of a steady state strain rate. High frequency dynamic data where obtained on a Rheometrics Scientific RMS800 controlled deformation rheometer,fittedwith a conicylinder geometry. All experiments were conducted at 25 ± 0.1 °C. Pre-shearing was carried out for 180s, followed by a rest period of 120s, prior to commencing the steady shear experiment Evaporation of the samples was prevented by coating the exposed edges with a thin film of low viscosity PDMS fluid (Dow Corning DC200, 10 cS).


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Results and Discussion We consider initially the behavior of the HASE polymer before embarking on the effects of adding non-ionic surfactants to the system. Figure 2 depicts the steady shear viscosity of the HASE polymer solutions as a function of the applied shear stress. For each of the polymer concentrations studied, a linear response (i.e. η - » η ) is only 0


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H A S E Polymer - Constitution EA(40wt%)

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Figure 1.Idealized constitution ofthe HASE polymer considered in the pre

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Rheology of a HASE Associative Polymer and Its ... - ACS Publications

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