1. Introduction. 2. Experiments

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1 Korea-Australia Rheology Journal, Vol.26, No.2, pp (May 2014) DOI: /s Precipitation polymerization of hydrophobically modified polyelectrolyte poly(aa-co-oda) in supercritical carbon dioxide and solution rheology properties Huaiping Zhang 1, *, Wei Li 1, Qing Cao 1 and Mingcai Chen 2 1 College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan , PR China 2 Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou , PR China (Received November 14, 2013; final revision February 27, 2014; accepted March 3, 2014) Hydrophobically modified (HM) polyelectrolytes were prepared by using precipitation polymerization of acrylic acid (AA) and octadecyl acrylate (ODA) in various molar ratios in supercritical carbon dioxide. The product was obtained in the form of a white powder and the micrographs show aggregates of primary particles < 1 µm in size. The effects of polymer concentration, ODA content in polymer, surfactant, shear time, shear rate on the apparent viscosity were investigated. The reason leaded to a significant viscosity enhancement was discussed. Steady-state and oscillatory tests of solution were also investigated. Solution exhibited shear thinning behavior and thixotropy. Polymers contain octadecyl acrylate (3.4 mol%) at 0.2 g/dl behaved as high entanglement structures or association gels, since the modulus G' were being higher than G" throughout the frequency range. The comparison of apparent and complex viscosities confirmed the association gel properties. Keywords: hydrophobically modified polyelectrolyte, supercritical carbon dioxide, solution properties, rheologic behavior, viscosity 1. Introduction Hydrophobical polyelectrolytes can be classified according to their chain-architecture as block copolymers, hydrophobically modified polyelectrolytes (associating polyelectrolytes) and polysoaps (Kotz et al., 2001). Hydrophobically modified polyelectrolytes contain a few hydrophobic groups incorporated into a charged hydrophilic backbone. HM polyelectrolytes that associate in solution via physical interactions are often efficient rheology modifiers. These are used as thickening agents in many fields of applications such as paint formulations, cosmetics, foods, paper coating and enhanced oil recovery (Tan and Tam, 2008; Hui et al., 2001; Tsitsilianis et al., 2000). HM polyelectrolytes solutions are known to exhibit unique rheological and interfacial properties (Tomatsu et al., 2005). The main microstructural feature of HM polyelectrolytes is their ability to give rise to weak intra and intermolecular hydrophobic interactions (Creuzet et al., 2006). In aqueous solution the hydrophobic groups selfassociate and form micellar type aggregates. In the semidilute regime, these aggregates act as cross-linkers between the polymer chains, and an enhancement in the rheological properties is observed. At high enough polymer concentration, the interchain associations cause HM polyelectrolytes solutions to gel (Malana et al., 2012). The reversible character of these physical cross-links imparts desirable rheological properties in solution such as shear *Corresponding author: huaipingzhang@gmail.com thinning or thixotropy (Mozumder et al., 2005). Hydrophobically modified polyelectrolytes can be obtained by a postpolymerization modification or by a direct free-radical copolymerization of monomers of various hydrophobicities (Juntao et al., 2002). In the present paper, we synthesized the hydrophobically modified polyelectrolytes poly(aa/oda) by using precipitation polymerization technique in supercritical carbon dioxide (Romack et al., 1995; Kendall et al., 1999; Xu et al., 2001; Zhang et al., 2008). Series copolymers with different composition are characterized. The aqueous behavior of HM polyelectrolytes was investigated using rheology experiments (the dynamic and static rheological measurements). 2. Experiments 2.1. Materials The octadecyl acrylate was obtained from Aldrich and used as received. Acrylic acid was purchased from Tianjing Chemical Reagents Co. (Tianjing, China) and purified by distillation under reduced pressure. The 2, 2-Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol. The CO 2 with purity of 99.9% was used as received Polymerization A series of copolymers (PAO) of acrylic acid with different amounts of comonomers octadecyl acrylate were synthesized by free-radical polymerization initiated by AIBN in supercritical carbon dioxide. The reactor is a The Korean Society of Rheology and Springer 141

2 Huaiping Zhang, Wei Li, Qing Cao and Mingcai Chen Scheme 1. A schematic depiction of chemical formulas of the copolymers used in the present study. Fig. 1. Flow chart of reaction (1-gas cylinder 2-pressure gauge 3- ice bath 4-syring pump 5-reactor 6-heater 7-separater). ml high pressure reactor with a magnetic stir bar. The high-pressure reactor was charged with acrylic acid (2.0 g), AIBN (0.02 g), and different amounts of comonomers, and then purged with 0.5 MPa CO 2 at 0.1 Lmin -1 for ca. 1 min to remove O 2. The reactor was sealed and enough liquid carbon dioxide was added through a syringe. The temperature was gradually increased to 60 C and carbon dioxide was added to 16 MPa. All reactions proceeded for 4 h. The reactor was cooled and vented slowly. The product was obtained in the form of a white powder. The products were extracted by supercritical carbon dioxide in-situ at 45 C and 15 MPa to remove the unreacted monomers till the mass of the products did not change. The yield of these polymerizations is calculated according to the mass of final products and the added monomer. The experimental setup is shown in Fig. 1. The arrows represent the direction of the CO 2 flow Characterization The morphology of the polymer powder was detected by XL-30 scanning electron microscopy (Philips). The contents of ODA in the final products were determined by acid base titration (Hongqi et al., 2003). FT-IR spectra of dry polymers were recorded on a RFX-65A FT-IR spectrometer (Analect, USA) using a KBr pellet method. The intrinsic viscosities of polymer were measured in 2 M NaOH aqueous solutions using a capillary viscometer (Ubbelohde type). The molecular weights were calculated using the equation [η]= M 0.43 (Yan et al., 1998). Apparent viscosity was measured by rotational viscometer using NDJ-1A (Shanghai) at shear rate=6 s -1. For all the experiments the temperature was fixed at 25 C. 2 M NaOH aqueous solutions were used to adjust the ph value of the solutions. The ph of the samples solution was fixed at 5.5 and measured with a PHS-3C instrument Rheological measurements Rheologic behavior tests were ran on a strain-controlled rheometer, American TA instruments ARES-RFS, equipped with a cone/plate geometry (diameter = 50 mm, Fig. 2. Scanning electron micrographs of sample PAO3. truncation = 1 mm). For all the experiments the temperature was fixed at 25 C. Care was taken not to stir the polymer solutions before measurements. The shear viscosities of polymer solutions were measured by using the steady-rate sweep method over a shear rate range from 0.1 to 100 s -1. A thixotropic loop measurement was made using a standardized shearing procedure for thixotropic materials that exhibit shear-dependent and time-dependent flow behaviors. 3. Results and Discussions 3.1. Characterization of copolymers The copolymerization method used in the paper was precipitation polymerization in supercritical carbon dioxide. A series of Poly(AA-co-ODA) copolymers were prepared by changing the hydrophobe level. Scheme 1 show a schematic depiction of chemical formulas of the copolymers used in the present study. The poly(acrylic acid) (PAA) and copolymers obtained at 60 C and 16 MPa are dry, white, free-flowing powders. Electron micrographs of products PAO3 are shown in Fig. 2. The micrographs show large aggregates of primary particles < 1 µm in size. There is no remarkable difference among homopolymer PAA and the copolymers with different ODA content. The copolymerization of 2.0 g acrylic acid with different amount of ODA produced a series of copolymers with different composition. Reaction conditions and properties of copolymers are shown in Table 1. It is evident that high ODA-containing monomer feed ratio leads to high ODA- 142 Korea-Australia Rheology J., Vol. 26, No. 2 (2014)

3 Precipitation polymerization of hydrophobically modified polyelectrolyte... Table 1. Reaction conditions and properties of copolymers produced in supercritical carbon dioxide a. Sample [M],b g/l ODA in feed, wt% mol% ODA In copolymer, c wt% mol% M d η Yield e 10-6 (%) PAA PAO PAO PAO PAO PAO a Copolymerizations were carried out at a temperature of 60 C, an initial vessel pressure of 16 MPa, concentration of initiator [I] 6.1 mmol/l, and reaction time 4 h; b Concentration of feed monomers; c Determined by acid base titration; d Viscosity-average molecular weight. [η]= M 0.43 ; e Defined as the weight percentage of the copolymer obtained with respect to the total amount of feed monomers Fig. 4. Plot of steady-state shear viscosity versus shear rate for aqueous solutions of PAO1 at different concentration. Fig. 3. FT-IR spectra of sample PAA (a) and PAO3 (b). Fig. 5. Plot of steady-state shear viscosity versus shear rate for aqueous solutions of different sample (C = 0.2 g/dl). containing monomer content in the copolymer. The content of ODA in sample PAO5 is 3.4 mol%. An interesting phenomenon is that the content of ODA in copolymers is lower than in feed, which indicates that acrylic acid is more active than ODA in such a copolymerization. Viscosity-average molecular weight M η of the copolymers increases with the increase of the content of ODA in copolymers. Yield of these polymerizations is above 92%. Fig. 3 shows the FTIR spectra of the sample PAA and PAO3. Disappear of the stretching vibration of C=C bond at cm -1 indicate in polymer has not any monomer. The IR spectra of PAO3 verified the existence of the stretching vibration and bending vibration of -CH2- at 2927 cm -1 and 1465 cm -1 respectively. The stretching vibration bands of C=O bond were found at ca 1720 cm -1, The stretching vibration bands of O H of COOH group were also found at ca 3435 cm Rheologic behavior Steady-state shear measurements Steady-state viscosity measurements were carried out in order to characterize the flow behavior of aqueous solutions of the HM polyelectrolyte. The variation in steady shear viscosity as a function of shear rate for solutions of PAO1 in different concentrations is shown in Fig. 4. It is shown that the viscosity of the solutions increases sharply in a relatively narrow concentration range. Tendency of viscosity variation is same to different concentrations. The most noticeable feature in the steady shear response of solutions is shear thinning effect. With increasing shear rate from 0.1 to 10 s -1 the viscosity decreases dramatically, and then gradually tends to be constant with shear rate increase from 10 to 100 s -1. The sharp decrease of viscosity with increasing shear is most likely due to the shear-induced damages of molecular networks built up by Korea-Australia Rheology J., Vol. 26, No. 2 (2014) 143

4 Huaiping Zhang, Wei Li, Qing Cao and Mingcai Chen Fig. 6. Plot of steady-state shear viscosity versus shear rate for PAO1 at various temperatures (C = 0.8 g/dl). Fig. 8. Plot of steady-state shear viscosity versus shear stress for PAO1 at different concentrations. Fig. 7. Plot of shear stress versus shear rate for PAO1at the different concentration. the relatively weak hydrophobic association. The variation in steady shear viscosity as a function of shear rate for the samples with different hydrophobic content was also investigated in Fig. 5. It is shown that a pseudoplastic behavior can be observed when the shear rate increased from 0.1 to 10 s -1. Fig. 6 plots the viscosity of PAO1 solutions against shear rate at various temperatures. The viscosity at 25 C is higher than that of 50 C. For hydrophobically modified polyelectrolyte, temperature increase made the movement of water molecules and hydrophobic groups increase, and the hydration spheres of the hydrophobic groups and/or in the structure of water itself are destroyed, which were unfavored for the intermolecular association of the copolymers. Hydration of -COO - weaken and polymer chain shrinkage, which make hydrophobic association and the solution viscosity decrease with increasing temperature. It can also be seen that shear thinning observed in the shear rate range below 10 s -1. According to the Fig. 4, 5 and 6, the aqueous solution has a Newtonian behavior in the range above shear rate greater than 10. Fig. 7 shows the shear stress versus shear rate plot at different concentrations. For all measured concentrations, the shear stress value increased with increasing concentration. The polymer solutions exhibited more dependence on shear rate at 0.2 g/dl. There was an increase on shear strain dependence with the polymer concentration over 0.4 g/dl. All polymer solutions were clearly thixotropic behavior. The dependence of the shear stress on shear rate also shows that the aqueous solution has a Newtonian behavior in the range above shear rate greater than 10. Such behavior of polymer solutions can be rationalized in terms of polymer entanglements. It exist intermolecular and intramolecular hydrophobical associations in the mean time for the aqueous solution. Wide-range dynamic crosslinking molecular networks form via nodes of hydrophobic microdomains, where the rate of disentanglement exceeds the rate at which the new entanglements form and this leads to a reduction in the cross-link density. Such studies provide evidence about the rate of destruction or rebuilding of the aggregates structures. The latter were shown to reform slowly. It reveals that this polymer exhibits strongly thickening efficiency. Plots of the steady-state shear viscosity as a function of shear stress for various polymer concentrations are presented in Fig. 8. The viscosity profiles exhibit similar characteristics in different concentrations, namely, a transition region (sharp thinning) at higher shear stress. This is in agreement with the structure of the present system since the physical cross-links network are aggregates of highly hydrophobic ODA. It shows the typical signature of a vis- 144 Korea-Australia Rheology J., Vol. 26, No. 2 (2014)

5 Precipitation polymerization of hydrophobically modified polyelectrolyte... curves form a hysteresis encloses an area. According to the thixotropic loops of shear stress depending on the shear rate, it indicated that solutions with concentrations 0.8 g/dl definitely exhibit thixotropic behavior nature. The shear process exist a kind of dynamic equilibrium between associating and unassociating of the polymer molecules, which has the character of time dependence. Both processes involve the alignment of polymer chains along the direction of shear. Fig. 9. Plot of viscosity versus shear time of sample PAO1 (shear rate = 5 s -1, C = 0.8 g/dl) Oscillatory shear measurements Oscillatory measurements were carried out over a frequency range of rad/s at a fixed strain value of γ = 1% using the dynamic frequency-sweep method. Oscillatory shear experiments were performed within the linear viscoelastic regime. Dependences of G' (storage or elastic modulus) and G" (loss or viscous modulus) on angular frequency are used to characterize the viscoelastic properties, where G' and G" reflect the elastic and viscous behavior of the sample solution, respectively (Duan et al., 2013; Kim and Ahn, 2012). The following functional form is often utilized to study viscoelasticity. It is generally valid for any isotropic material behavior (Ferry et al., 1980) G ( ω) = ω Gt () sin( ωt) dt 0 (1a) G ( ω) = ω Gt () cos( ωt) dt. (1b) 0 Fig. 10. Thixotropic loops of shear stress versus shear rate PAO1 (C = 0.8 g/dl). coelastic fluid. Fig. 9 shows the shear time versus viscosity plot of PAO1 solutions at constant shear rates 5 s -1. It is found that polymer solution exhibits thixotropy, i.e. at a constant shear rate, the viscosities of the polymer solution decrease with time. The homopolymer sample PAA does not show this phenomenon. The thixotropy is evidently due to the intermolecular association: the association is a kind of physical cross link that can be destroyed by shear stress, so at a constant shear rate the viscosity will decrease with shear time. Since thixotropy is commonly evaluated using a thixotropy loop method. The stresses and the shear rates of PAO1 solution were recorded in Fig. 10. The resulting up curve was obtained in the process of gradually increasing the shear rate. After reaching the assigned maximum rate, a gradual decrease gives the down curve. The down curve of thixotropic solutions is different from up curve. The The measured values of G' and G" as a function of frequency ω for aqueous solutions of PAO5 are plotted in Fig. 11. Over the entire frequency range studied ( rad/s). The values of the two modulus of the solution increased when the frequency ω increased. G' values were higher than G" in the investigated frequency range. At low frequencies, both elastic and viscous modulus are independent of frequency. G' exceeds G" by about 1 order of magnitude. It indicates that intermolecular associating form wide-range dynamic physical crosslinking networks via nodes. The solution shows the typical signature of a viscoelastic fluid. Note also that in our system the clear elastic character appears for polymer concentrations 0.2 g/dl. Such elastic behavior has few been observed with hydrophobically polymers at so low polymer concentrations. Obviously, the viscoelastic properties of the system are caused by formation of a physical crosslinking network. ODA content in sample PAO5 is 3.4 mol%. The higher content of ODA makes association stronger at low concentration. This is an indication that the average size of the stickers is more effective in the viscosity enhancement than their number. The tan δ (loss tangent or loss factor) obtained from the two G values provided additional information about the individual characteristics of the polymer solutions Korea-Australia Rheology J., Vol. 26, No. 2 (2014) 145

6 Huaiping Zhang, Wei Li, Qing Cao and Mingcai Chen Fig. 11. Storage (G') and loss (G") modulus dependence on the angular frequency for aqueous solutions of the PAO5 (C = 0.2 g/dl). Fig. 12. Cox Merz plot for rheological data on the PAO5 (C = 0.2 g/dl). (Markgraf et al., 2006; Kawakami et al., 2006). The tanδ was calculated according to the following equation: tanδ = G"/ G', where δ is the phase shift angle ( ). The tanδ < 1 of aqueous solutions of PAO5 show the elastic component predominates, confirming the characteristics close to gel behavior. It suggested that the solution was changed from a viscous fluid to an elastic gel. These few results demonstrate clearly that the hydrophobically modified polyelectrolyte can be used as thickeners in aqueous formulations. Even for concentration as low as 0.2 g/dl, HM polyelectrolyte can form association gels. The gellike properties are produced by a three-dimensional network that is physically crosslinked. Complex viscosity was calculated according to the following equation: η* = [(G') 2 + (G") 2 ] 1/2 /ω. For many polymer solutions, the frequency dependence of complex Fig. 13. Plot of viscosity versus polymer concentration (shear rate = 6 s -1 ). viscosity (η*) measured in oscillatory shear and the steady shear rate dependence of apparent viscosity (η a ) are observed to be closely superimposable. This connection, often called the Cox Merz rule, has been observed for a great number of polymer solutions. There are also a few exceptions, and these occur with association gel systems (Lizarraga et al., 2006). Fig. 12 shows this correlation for the PAO5 solution. At lower frequencies and shear rates, the complex viscosity lies above the apparent viscosity. With increasing frequency and shear rates, the former decreases faster than the latter, and the curves converge at higher frequencies and shear rates. Such behavior is usually associated with a tendency to form aggregated structures, which are then broken down under the applied strain. This behavior is a quite useful proof of the presence of association gel properties Solution behavior The dissolution of polymers in water becomes more difficult with the increasing of hydrophobe levels. High foaming ability is observed during the preparation of sample solutions and the measurement of viscosities. This behavior also reflects the amphiphilic nature of the copolymers. As shown in Fig. 13, the remarkable viscosity enhancement is observed with increasing solutions concentration. For comparison, the concentration dependence of the viscosity for PAA is also shown in Fig. 13. The viscosity of PAO5 is 700 times than PAA at 0.2 g/dl. The remarkable increase in solution viscosities of copolymers is attributed to the strong intermolecular associations. Wide-range dynamic physical crosslinking networks form via nodes of hydrophobic microdomains, which significantly increase the hydrodynamic volume and leads to the sharp ascending of the apparent viscosity. The apparent 146 Korea-Australia Rheology J., Vol. 26, No. 2 (2014)

7 Precipitation polymerization of hydrophobically modified polyelectrolyte... Fig. 14. Plot of viscosity versus content of ODA in copolymer (C = 0.2 g/dl, shear rate= 6 s -1 ). viscosity of PAO1 solution can reach 92.5 Pa.s at 0.8 g/dl. The viscosity property of associative polyelectrolytes in aqueous solution is strongly influenced by the intra- and intermolecular aggregation. The aggregation level depends largely on the content and structure of the bound hydrophobe. Fig. 14 show the results measured by the rotational viscometer at a shear rate of 6 s -1 and 25 C. It shows that there is a substantial viscosity enhancement with the increase of content of ODA from 2.3 to 3.4 mol%. With the content of ODA increasing, associating results enlarge hydrodynamic volumes of copolymers in aqueous solution. The hydrophobic association form physical networklike structures and this increase with the mole fraction of hydrophobic group increase. The viscosity of aqueous solution of associative polyelectrolytes increases dramatically. The solution properties of the water-soluble polymer can be changed by the addition of simple surfactants. Interactions between HM polyelectrolytes and surfactants are governed primarily by coulombic forces, dipole-ion interaction and the hydrophobic effect. Fig. 15 illustrates the viscosity behavior of PAO1 in the presence of cationic dodecyl trimethylammonium chloride (DTAC), anionic surfactants sodium dodecyl sulfate (SDS) and non-ionic surfactant sorbitan monolaurate (Span20). As exhibited in Fig. 15, the viscosity increases at lower DTAC concentration, reaches a maximum and then decrease at higher surfactant concentrations. The polymer surfactant interactions are probably hydrophobic and electrostatic in nature. Initial addition of cationic DTAC to the polymer solution leads to DTAC preferentially binds to the negatively-charged polymer chain in solution. Formation of hemimicelles along the polymer backbone develops intermolecular hydrophobicity. Upon further increasing the surfactant concentrations, association of DTAC and Fig. 15. Plot of viscosity versus surfacant of sample PAO1 (C = 0.1 g/dl, shear rate = 6 s -1 ). hydrophobes of the copolymer occurs, resulting in coil expansion. The formation of micelle will link the hydrophobic groups of different polymer molecular and result in dimensional network. The viscosity reaches its plateau value. However, further addition of DTAC just increases the number of surfactant micelle in the solution. These micelles cannot solubilize more than one hydrophobic group; DTAC micelles are in a large excess. Mixed micelles with hydrophobes from two or more chains are less prevalent and the network structure is destroyed. Associations of DTAC with the copolymer result in highly collapsed polymer coils and the viscosities decrease. The viscosity of the copolymer solution is essentially invariant with changes in non-ionic surfactant concentration, while a continuous decrease in the viscosity of the copolymer solution is observed with increasing concentration of the anionic surfactant SDS. The latter effect may be caused by the increase in the ionic strength of the solution upon addition of the anionic surfactant. 4. Conclusions A various molar ratios HM polyelectrolyte were synthesized by copolymerization of acrylic acid and octadecyl acrylate in supercritical carbon dioxide. The products are dry, white, free-flowing powders. Electron micrographs of products show large aggregates of primary particles < 1 µm in size. Yield of these polymerizations is above 92%. The remarkable viscosity enhancement is observed with increasing polyelectrolytes concentration and ODA content in the copolymer. The viscosity initially decreases dramatically, then gradually tends to be constant with increasing shear rate from 0.1 to 10 s -1, the aqueous solu- Korea-Australia Rheology J., Vol. 26, No. 2 (2014) 147

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