http://www.e-polymers.org e-Polymers 2012, no. 009 ISSN 1618-7229

Influence of hydrophobe fraction content on the rheological properties of hydrosoluble associative polymers obtained by micellar polymerization

Enrique J. Jiménez-Regalado,1* Elva B. Hernández-Flores2

1Centro de Investigación en Química Aplicada (CIQA) Blvd. Enrique Reyna #140, 25253, Saltillo, Coahuila, México; e-mail: [email protected] bCentro de Investigación en Química Aplicada (CIQA) Blvd. Enrique Reyna #140, 25253, Saltillo, Coahuila, México; e-mail: [email protected]

(Received: 20 November, 2009; published: 22 January, 2012)

Abstract: The synthesis, characterization and rheological properties in aqueous solutions of water-soluble associative polymers (AP’s) are reported. Polymer chains consisting of water-soluble polyacrylamides, hydrophobically modified with low amounts of N,N-dihexylacrylamide (1, 2, 3 and 4 mol%) were prepared via free radical micellar polymerization. The properties of these polymers, with respect to the concentration of hydrophobic groups, using steady-flow and oscillatory experiments were compared. An increase of relaxation time (TR) and modulus plateau (G0) was observed in all samples studied. Two different regimes can be clearly distinguished: a first unentangled regime where the viscosity increase rate strongly depends on hydrophobic content and a second entangled regime where the viscosity follows a scaling behavior of the polymer concentration with an exponent close to 4.

Introduction Water-soluble polymers are of great interest from the scientific as well as the technological point of view. These polymers have been subjected to extensive research [1]. Water-soluble polymers are present in the composition of different types of industrial formulations as stabilizing, flocculants, absorbent, emulsifiers, thickeners, etc. These polymers have applications in, for example, paints, oil recovery, cosmetology, the paper industry and biomedical applications. Water-soluble associative polymers represent a particularly interesting alternative and have inspired a growing interest for several years [2-10] as thickener agents to control viscosity and rheological properties of different aqueous fluids (solutions, suspensions or emulsions). Its characteristic properties in aqueous solution result from the formation of intermacromolecular partnerships due to interactions between hydrophobic sites of macromolecular chains. The hydrophobically modified polyacrylamides represent an important class of associative polymers. These are commonly synthesized through micellar polymerization. This polymerization procedure has been the subject of numerous investigations since it is particularly suited to the structure and control of the rheological properties of the polymers obtained [11,12]. The water-soluble associative polymers, are modified by the presence of some hydrophobic sites statistically distributed throughout the skeleton (multisticker polymers) or ends of the chain (telechelic polymers). Such amphiphilic polymers in

1 aqueous solution form interactions between hydrophobic groups leading to intermacromolecular associations above a certain polymer concentration. When polymer chains associate become superstructures equivalent to higher molecular weights and lead to higher viscosities than the hydrophobically unmodified polymers. The increase in viscosity results from the decrease in the movement of the chains caused by the hydrophobic interactions (principle of reptation) [13, 14]. The properties and associative behavior of acrylamide with different quantities of comonomer were investigated. In this work we are interested in studying the influence of the spacer size between the hydrophobe groups in solution of acrylamide block copolymers. The hydrophobe used in this study was N,N-dihexylacrylamide. The block copolymer can be prepared by micellar radical copolymerization of N,N- dihexylacrylamide in an aqueous solution of acrylamide, to form a water-soluble multiblock copolymer. The length of the hydrophobic block (NH) was maintained constant at 3.2 due to the specificity of the free radical micellar copolymerization technique. In this process, that we have widely investigated in the past years [15, 16], the hydrophobe is solubilized within surfactant micelles whereas acrylamide is dissolved in the aqueous continuous medium. Due to their high density in the micelles, the hydrophobic monomers are randomly distributed as blocks in the acrylamide backbone. Hence, the rheological properties in aqueous solution of these multiblock copolymers are strongly dependent on the hydrophobe/surfactant ratio during the synthesis.

Results and discussion

Associative polymers The 1H NMR spectrum of PAM-co-DHAM(4) is presented in Figure 1, using 1 wt% solution in DMSO/D2O.

1 Fig. 1. The H NMR spectrum of the PAM-co-DHAM(4) in DMSO/D2O (85/15 wt%).

2 Figure 2 shows the variation of the steady-state viscosity, η, as a function of shear . rate, γ , for the PAM-co-DHAM(1) associative copolymer at different concentrations, in aqueous solution.

30

10

1 C (% en peso) 1 2 0.1 3

(Pa.s) 4 η 5 6 0.01 7 8 9 10 1E-3 1E-3 0.01 0.1 1 10 100 1000 10000 -1 .γ (s )

. Fig. 2. Log-log variation of the apparent viscosity (η) versus shear rate (γ ) of the copolymer PAM-co-DHAM(1) at different polymer concentrations.

The ratio of the two monomers in the copolymer, was determined by integrating the signals of the methyl proton (~ 0.8 ppm), and of the ethylene proton attached to the backbone (2 to 2.4 ppm), the hydrophobic monomer ratios in the final polymer are presented in Table 1. At concentrations ≤ 4 wt % the system is Newtonian; that is, there is no detectable . variation of η with γ . However, upon increasing the concentration further, ≥ 5 wt, the zero-shear viscosity increases, that is, the Newtonian behavior is followed by a shear thinning behavior; this shear thinning is due to the breakdown of the intermolecular hydrophobic interactions, which produce a decrease in viscosity.

Figure 3 shows the variation of the zero-shear viscosity, η0, as a function of polymer concentration in aqueous solution. When the concentration is Cη < C < CT; this regime is characterized by a rapid viscosity increase. It can be speculated that at the overlap concentration C* (which is very similar to Cη), intermolecular links are formed. These hydrophobic associations must be the main cause of the viscosity increase. This mechanism involves the disengagement of an associating sequence from a crosslink, followed by Rouse relaxation [17, 18]. In this regime, the viscoelasticity is possibly controlled by the effect of intermolecular hydrophobic associations, and it can be considered as the equivalent to the semidilute unentangled regime observed in unmodified polymers. In this respect, it is striking that CT closely coincides with the Ce value of the corresponding unmodified polymer. It should be noted that, for the multisticker chains such as these studied here, the disengagement of one

3 hydrophobic block from a cross-link does not allow the relaxation of the entire chain, since the chain is still “anchored” by many other stickers even in the absence of entanglements. Therefore, the chain cannot diffuse very far between two consecutive sticker releases [19]. 4 When C > CT. In this regime, the viscosity increases as a function of C . In fact, it can be safely assumed that, in the concentration range considered, the density of entanglements is much larger than that of hydrophobic associations. This is the case which has been treated in the sticky reptation model developed by Leibler et al. [19], which considers a concentrated solution of a monodisperse chain of N monomers with S stickers attached to each chain.

500 [H] (mol%) 100 0 1 2 10 3 4 1

(Pa.s) 0

η 0.1 m = 4 0.01

C = C 1E-3 e T 110 C* = C η C (wt%)

Fig. 3. Log-log variation of the zero shear viscosity (η0) versus polymer concentration (C) for the copolymers studied.

Figure 4 shows typical frequency dependences of the storage modulus G´(ω) and of the loss modulus G´´(ω). At low frequency, the behavior of the complex shear modulus is Maxwellian, as ascertained by the variations of G´(ω) and G´´(ω) versus ω that scale respectively like ω 2 and ω. The curves G´(ω) and G´´(ω) cross each other at circular frequency ω cross. The inverse of ω cross is often taken as the characteristic time of the system. As a matter of fact, the comparison between the actual experimental variations of G´(ω) and G´´(ω) and those calculated from the Maxwell model show deviations appearing already before the crossing frequency (Figure 4). The shape of the curves G´(ω) and G´´(ω) at higher frequencies is indicative of the occurrence of fast modes superimposing on a slow relaxation process. The current models describing the dynamics of associating polymers predict a multiple relaxation process (at least two characteristic times). It follows that the relaxation time determined from ω cross is smaller than the longest relaxation time, the latter being the physical quantity relevant for a comparison with the models. The behavior reported in Figure 4 is almost generally observed for all the samples. In fact, the slope of the G´(ω) and G´´(ω) curves are equal to 2 and 1, respectively, for all the samples studied.

4 From the analysis of the data in the low frequency range, we have attempted to determine the longest relaxation time, TR, plateau modulus, G0 and the dynamic complex viscosity, η*, associated with this slowest process. These are obtained from the following relationships:

2 2 ⎛ 1 G´ ⎞ 1 ⎛ G´´⎞ ()G´ + (G´´ ) TR = lim⎜ ⎟; G0 = lim⎜ ⎟; η * = ϖ →0 ϖ →0 ⎝ϖ G´´⎠ TR ⎝ ϖ ⎠ ϖ

200 100

10

1 G´, G´´ (Pa)

0.1

G´ G´´ 0.01 0.1 1 10 60 ω (rad/s)

Fig. 4. Storage (G´) and loss (G´´) moduli as a function of frequency for PAM-co- DHAM(4) sample (C = 4 wt%). The lines are the fit to the one-mode Maxwell model (G0 = 98.5 Pa, relaxation time = 0.12 s).

For entangled systems, the correlations between dynamic and steady-state measurements are generally well described through the empirical method of Cox- . Merz [20, 21]. This consists of comparing the steady shear viscosity η(γ ) as a function of shear rate with the modulus of the complex viscosity η*(ω) as a function of the circular frequency. For melts or entangled solutions of polymers, the two functions are found to coincide [21-23]. In particular, the crossover between the . Newtonian plateau and the shear-thinning regimen occurs at γ c = ϖ c , where ϖ c is the circular frequency at which G´(ω) and G´´(ω) cross each other. However, the Cox- Merz rule is usually not reliable for complex structured fluids [18, 24].

. The comparison between η(γ ) and η*(ω) illustrated in Figure 5 show that the Cox- . Merz rule fails for associating polyacrylamides. The η(γ ) and η*(ω) curves coincide . only in the Newtonian regime. We found in most cases that the shear-rate γ c at . which η(γ ) departs from the plateau behavior does not correspond to the inverse of

5 the terminal time as determined from the procedure describe above. Therefore, shear flow experiments conveniently used for measuring the zero-shear viscosity can only give here qualitative trends for the viscoelastic parameters.

60

(Pa.s) η∗ , η

10 0.01 0.1 1 10 100 . -1 γ (s ), ω (rad/s)

. Fig. 5. Steady-state viscosity η(γ ) and dynamic complex viscosity η*(ω) as function . of shear rate (γ ) or frequency (ω), sample PAM-co-DHAM(3) (C = 9 wt%).

0.5

0.1

(s) R T

[H] (mol%) 1 2 3 4 0.01 2345678910 C (wt%)

Fig. 6. Variation of the relaxation time, TR as a function of C, for all the systems investigated.

6 The variation of the terminal time TR are illustrated in Figure 6, in a semi-log representation against polymer concentration for all the systems investigated. As could be expected from the viscosity results, the relaxation process strongly slows down upon increasing hydrophobe concentration, [H]. The variation of TR with concentration can be described by roughly parallel straight lines, considering the experimental uncertainty on these measurements.

Finally, Figure 7 shows the variation of the plateau modulus, G0, as a function of the polymer concentration, C, for all the systems investigated. For C > CT it was found that G0 depends on polymer concentration and hydrophobe content within the experimental accuracy which suggests that the elasticity is dominated by entanglements.

1000 [H] (Mol%) 1 2 3 4

100

(Pa) 0 G

10 345678910 C (wt%)

Fig. 7. Plateau modulus, G0, as a function of C, for all the polymers studied.

Conclusions The results of this study refer to the rheological properties of aqueous solutions of associative hydrosoluble polymers with different hydrophobe content, prepared by a free radical micellar polymerization technique. We found two distinct regimes: a semidiluted unentangled regime Cη < C < CT dominated by intermolecular hydrophobic associations (where the chains are likely to obey Rouse dynamics), and a semidiluted entangled regime C > CT. The asymptotic behavior is described by parallel straight lines with an exponent of about 4. Here the hydrophobic associations might be fully intermolecular but the number of entanglements increases strongly with C. An increase of relaxation time (TR) and modulus plateau (G0) as a function of C and hydrophobe content was observed in all the samples studied.

Experimental part

Synthesis of the hydrophobic monomer The hydrophobic monomer was prepared via the reaction of acryloyl chloride with N, N-dihexylamine, according to the procedure previously described by Valint et al.[25].

7 The synthesis and characterization of this hydrophobe monomer has been described in detail in previous papers [26, 27]. Conversion was 70 %.

Synthesis of the associative polymers The associative polyacrylamides were obtained by micelar copolymerization [11, 28, 29]. The synthesis of the samples by means of a micellar polymerization technique has been described in detail in previous papers [30, 31]. The associating copolymers are polyacrylamides hydrophobically modified with different quantities of N,N- dihexylacrylamide (DHAM) (1, 2, 3 and 4 mol %). The molecular weight was decreased using 2-mercaptoethanol (1.92 ×10-4 mol/L) as a chain transfer agent. The initiator was 4,4´-azobis(4-cyanopentanoic acid) (ACVA). The hydrophobe/surfactant molar ratio was adequately adjusted in order to get the desired number of hydrophobe per micelle, NH. The total concentration of monomers was 3 wt%, the initiator concentration was 1×10-3 mol/L, the reaction temperature was 50 °C and the total reaction time was 7 h. In this process, the hydrophobic monomer is solubilized within sodium dodecyl sulfate (SDS) micelles, whereas acrylamide is dissolved in the aqueous continuous medium. The polymers obtained were purified with methanol and dried under reduced pressure at 40 °C. The characteristics of the samples investigated are given in Table 1. The hydrophobe / surfactant molar ratio was adequately adjusted in order to obtain the desired number of hydrophobes per micelle, NH, which was 3.2. NH was calculated from the following relationship:

NH=([H]×Nagg)/([SDS]-cmcSDS) where [SDS], cmcSDS and Nagg are the surfactant molar concentration, its critical micelar concentration and its aggregation number, respectively (cmcSDS is equal to -3 9.2×10 mol/L and Nagg is equal to 60 at the polymerization temperature of 50 °C [32, 33]). The compositions of the samples were determined by NMR.

Tab. 1. Polymer characteristics.

Sample Conversion [H]a) wt% (mol %) PAM 92.6 - PAM-co-DHAM (1) 92.3 1.1 PAM-co-DHAM (2) 91.6 1.6 PAM-co-DHAM (3) 98.8 2.1 PAM-co-DHAM (4) 93.1 3.1 a)Hydrophobe content in the polymer

These amphiphilic copolymers cannot be characterized by size exclusion chromatography (SEC) in water, due to aggregation phenomena. However, homopolyacrylamide prepared under identical experimental conditions, but without hydrophobe was determined by rheological experiments and found a molecular weight of 80,000 g/mol. Thus, similar molecular weights can be assumed for the copolymers. The sample code of the copolymers refers to the hydrophilic monomer, hydrophobic monomer and the quantities of the hydrophobic monomer, for example PAM-co-

8 DHAM(2) stands for a poly(acrylamide-co-dihexylacrylamide) copolymer obtained using 2 mol % of hydrophobic monomer.

Sample preparation Solutions at different concentrations were prepared by directly dissolving a known amount of polymer into deionized distilled water and gently stirred until the solution was homogeneous.

Rheological measurements Experiments were performed in a Paar Physica UDS200 controlled stress rheometer equipped with a cone and plate geometry (angle 2° and 50 mm diameter) or double gap geometry depending on the sample viscosity, at 25 °C. To prevent the evaporation of water, the measuring system was enclosed with a solvent trap. The zero-shear viscosity (η0) was obtained by extrapolation of the apparent viscosity at very low shear rates. The range of concentrations of the associative polymer aqueous solutions was 1 wt % < C < 10 wt %. Linear viscoelasticity experiments were performed on samples that were viscous enough to provide meaningful analysis.

Acknowledgements The authors wish to thank the Mexican National Council for Science and Technology (CONACYT) for the financial support to carry out this work, through project 46035/A- 1. The authors also thank M. Sanchez-Adame for her help in the experimental characterization.

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