J. Phys. Chem. B 2007, 111, 8567-8571
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Influence of Chain Charge and Complexation on the Overlap and Entanglements Formation in Poly(acrylic acid) Salt-Containing Aqueous Solutions† Ekaterina A. Litmanovich,* Svetlana O. Zakharchenko, and Georgi V. Stoichev Department of Polymer Sciences, Faculty of Chemistry, M.V. LomonosoV Moscow State UniVersity, Leninskie Gory, V-234, Moscow 119992, Russia ReceiVed: January 4, 2007; In Final Form: May 24, 2007
Dilute-semidilute regime crossover in aqueous solutions of partly neutralized poly(acrylic acid) and of its complex with tetradecyltrimethylammonium bromide was studied by light scattering and viscometry methods. The chain charge growth causes the decrease of overlap concentration (c*) and the increase of the entanglements formation concentration (ce), hence, the semidilute unentangled regime of solution expands. Complexation of the polyelectrolyte with an oppositely charged surfactant leads to c* increase and to ce decrease. It is shown that in semidilute entangled solutions the surfactant acts as an effective structuring agent because of the binding of polyelectrolyte chains via surfactant micelles.
Introduction Concentration regimes of polyelectrolyte solutions are intensively studied now because of the originality and complexity of properties of charged chains solutions. The main concentration regimes in salt-containing aqueous media are dilute, semidilute unentangled, semidilute entangled, and concentrated solutions.1 The crossover from dilute to semidilute regime occurs when the average concentration of monomer units in solution becomes equal to the concentration within a polymer coil (c*). It is well-known that in solutions of neutral polymers the fluctuating network of entanglements begins to form in the crossover area. The change of thermal motion mechanism from diffusion of isolated polymer coils to cooperative diffusion occurs at c*; therefore, c* may be determined by scattering methods.2 The entanglements formation is accompanied by a change of the viscous flow mechanism from a translational (in unentangled solutions) to a reptational one (in entangled solutions);3,4 hence, the concentration of entanglements formation (ce) may be found from viscometry measurements. Uncharged polymers c* and ce differ slightly. In the case of polyelectrolytes, the situation is more complicated because of electrostatic interactions, which hinder the interpenetration of coils and entanglements formation. For polyelectrolytes, c* and ce differ strongly, and this expanded semidilute unentangled regime existing between c* and ce is a unique one, being a characteristic feature for polyelectrolytes only. Polyelectrolytes with variable charge density are the most interesting objects for study of the peculiarities of concentration regimes of charged polymer solutions. In the present work, aqueous solutions of poly(acrylic acid) (PA) were studied by light scattering and viscometry methods. The choice of PA was caused by the easy controlled change of polymeric chain charge: in acidic form, PA is weekly charged, while the conversion of carboxylic groups to carboxylate ones leads to the chain charge growth. Therefore, the study of PA with † Part of the special issue “International Symposium on Polyelectrolytes (2006)”. * Corresponding author. E-mail:
[email protected]. Phone: +7-4959395419. Fax: +7-495-9390174.
different degrees of neutralization enables one to investigate the influence of chain charge on the overlap and entanglements formation in polyelectrolyte solutions. Besides, the addition of oppositely charged surfactant to the PA aqueous solution, leading to the formation of polyelectrolyte-surfactant complex (PSC), may influence the structure of the semidilute polyelectrolyte solution. Reactions between polyelectrolytes and surfactants have usually been investigated in dilute solutions. It is known that, in the presence of polyelectrolyte, surfactants form micelles on the polymer chain; the size of micelle may either coincide with the size of micelle in a solution without polymer or exceed it. Additionally, the PSC particle may contain one or several polymer chains.5 In all cases, PSC formed in dilute solutions are individual particles, and their formation does not lead to structuring of the solution. In contrast to the dilute solutions, reactions of polyelectrolytes with surfactants in semidilute solutions are investigated insufficiently. Thus, a complex of PA and tetradecyltrimethylammonium bromide (C14AB) was used to study the effect of surfactant on the properties of PA semidilute solutions. Experimental Section Materials. PA with a weight-average molecular weight (Mw) of 2.5 × 105 as determined from light scattering experiments was obtained by Polysciences Inc. (Warrington, PA). C14AB, + CH3(CH2)12CH2N(CH3)3Br-, by Tokyo Kasei, Inc. was used without further purification. All solutions were prepared using bidistilled water and contained 0.05 M NaBr. The neutralization degree (R) was adjusted by addition of NaOH solutions; R was determined as the ratio of NaOH molar concentration to the molar concentration of polyacid carboxylic groups. Over the range of R studied (0 e R e 0.5), R growth is in accord with chain charge increase. The procedure of PSC initial solution preparation requires accuracy because of the necessity of operating at high polymer concentration. In order to avoid any local supersaturating, PSC formation was performed in two stages: first, required amounts of PA and C14AB were dissolved in a NaBr-water solution for 14 days. Complex formation is suppressed in an acidic
10.1021/jp070070t CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007
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medium, so it was necessary to achieve uniform distribution of the surfactant in the sample. Then, a calculated amount of 2 M NaOH solution was added, and the viscous PSC solution obtained was kept for 14 days under stirring. The ratio of surfactant molar concentration to the molar concentration of polyelectrolyte monomeric units in the solution was 0.25. The critical micelle concentration (CMC) value for tetradecyltrimethylammonium bromide is 3.5 × 10-3 M,6 and the critical aggregation concentration value in the presence of sodium polyacrylate is 1 × 10-4 M.7 In this work, the minimal concentration of surfactant used was 2 × 10-2 M, so all experiments were carried out at concentrations exceeding CMC. Rheological Measurements. Relative viscosity measurements were carried out in an Ubbelohde capillary viscometer with a suspended meniscus (solvent flow time was equal to 48 s). Dynamic viscosity was measured using a rotational viscometer Rheotest 2.1 (Germany) with a cylinder-cylinder working cell at constant shear rates regime. The range of shear rates lies from 0.1 to 1300 s-1. For calculations of the specific viscosity from rotational viscometry data, the dynamic viscosity was extrapolated to zero shear rate and normalized to the dynamic viscosity of the solvent. Light Scattering. Light scattering measurements were carried out with scattered laser light goniometer Photocor Complex (Photocor Instruments, U.S.A.). A 25 mW He-Ne laser operating at a 633 nm wavelength was used as the light source. Static light scattering data processing was performed using toluene as a reference. Refractive index increments used to determine the optical constant K were measured by a KMX-16 differential refractometer (Milton Roy, U.S.A.) with a 0.5 mW He-Ne laser as a light source. A solvent equilibrated with the solution studied by dialysis was used as the reference sample. All solutions were cleared of dust particles by filtering two or three times through a Millipore GS 0.22 µm pore size filter. In dynamic light scattering experiments, the time autocorrelation function of the scattered light intensity fluctuation g2(τ) was measured by digital real-time 288 channel correlator Photocor-FC (Photocor Instruments, U.S.A.). Data processing was performed using DynaLS software. Results and Discussion Light scattering is one of the most useful methods of investigation of transfers from one concentration regime to another because of the possibility of study of equilibrium properties of solutions in the absence of any external field. Dynamic light scattering (DLS) gives the time-dependent autocorrelation function of scattered light intensity fluctuation g2(τ), which allows for the calculation of the diffusion coefficient D of scattering particles:
g2(τ) ) 1 + C[
∫DD
Z(D) exp(-q2Dτ) dD]2
max
(1)
min
where q ) (4π/λ) sin(θ/2) is the scattering wave vector, τ is the correlation time, and D is the diffusion coefficient. Figure 1 gives, as an example, autocorrelation functions (a) and distributions by diffusion coefficients (b) for solutions of PA of two different concentrations at pH ) 3, when the PA ionization degree does not exceed 0.01. Curve 1 in Figure 1a is typical for solutions of concentrations lower than 0.01 g cm-3. In these conditions, the autocorrelation function decreases monotonically, and the correlation time is determined by the translational diffusion of isolated polymer coils. At concentrations equal to 0.01 g cm-3 or higher, the autocorrelation function contains regions of fast and slow decay (curve 2 in Figure 1a).
Figure 1. (a) Autocorrelation functions, g2(τ), and (b) distributions by diffusion coefficients in PA solutions. [PA] ) 3 × 10-3 g cm-3 (1) and 2.5 × 10-2 g cm-3 (2). [NaBr] ) 0.05 M; R ) 0; scattering angle θ ) 90°.
It means that both fast and slow diffusion processes in solution proceed simultaneously, as it is shown in Figure 1b. It is important to notice that neither amplitudes nor diffusion coefficients of modes change at scattering angle variation, confirmed by measuring at scattering angles from 50 to 140°. The slow diffusion mode may be caused either by the aggregation process or by the long-distance correlation which appeared as a result of the polymer coils overlap. A static light scattering (SLS) study may provide an understanding of the nature of a slow diffusion process. SLS data for dilute solutions of nonassociating particles are characterized by the Debye equation:
1 K‚c ) + 2A2‚c Rθ Mw
(2)
where K is the optical constant, c is the concentration, Rθ is the intensity of scattered light, Mw is the weight-average molecular mass, and A2 is the second virial coefficient. In the case of aggregation, static scattering data in Debye coordinates are described by two linear parts; the initial ordinate of the second part lies lower compared with the initial ordinate
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Figure 3. Specific viscosity dependence on PA concentration, R ) 0 (1) and 0.5 (2); [NaBr] ) 0.05 M. Figure 2. SLS data of the PA solutions, R ) 0 (1), 0.1 (2) and 0.5 (3). [NaBr] ) 0.05 M; θ ) 90°.
TABLE 1: Comparison of c* Values Calculated as from Viscometry and from SLS Data for PA at r ) 0-0.5 R
c* ) [η]-1 × 103 (g cm-3)
c*SLS × 103 (g cm-3)
0 0.05 0.1 0.3 0.5
9.1 6.7 2.3 1.4 0.8
10.0 5.0 1.5 1.1 0.8
of the first part, because it is determined by aggregates, which have much larger mass than nonassociated particles. In contrast to aggregation, the long-distance correlation leads to nonlinearity of (K‚c/Rθ) versus concentration. Figure 2 presents SLS data for PA solutions with different values of neutralization degree (R). At low concentrations of PA, all curves are linear; however, with an increase of PA concentration, the deviation from linearity is observed. SLS data are found to be angular independent. The concentration at which nonlinearity starts (c*) correlates well with the concentration at which the slow mode appears in DLS experiments. It indicates that the slow mode is caused by the long-distance correlation of concentration fluctuations. Such correlation is the characteristic feature of semidilute solutions, in which the mechanism of macromolecules thermal motion becomes cooperative because of the overlapping of the coils. Summarizing the DLS and SLS data, c* may be interpreted as the dilute-semidilute regime crossover concentration. The traditional way to determine the overlap concentration is the measurement of intrinsic viscosity of the polymer solution. Since the condition of the dilute-semidilute regime crossover is the equality of the average monomeric units concentration in the solution and the concentration of units in a coil, the crossover concentration c* should be approximately equal to the reciprocal intrinsic viscosity: c* ≈ [η]-1. So, we estimated c* using this approach; that is, we calculated [η] by extrapolation of the concentration dependence of reduced viscosity to zero concentration, and the results obtained are shown in Table 1. The values of c* obtained from SLS data show good correlation with the one calculated by the viscometry method. As we have shown before,8,9 the change in the shape of concentration dependencies of the SLS intensity at c* is a typical phenomenon for various polyelectrolytes (sodium polymeth-
acrylate, poly(diallyldimethylammonium chloride), and poly(N-ethyl-4-vinilpyridinium bromide)). For all of the mentioned polyelectrolytes, the change of growth rate of concentration dependence of scattered light intensity was observed at the same concentration, at which DLS experiments show the appearance of two modes of diffusion. In all cases, this critical concentration was close to the value of c* calculated as [η]-1. The data shown in Table 1 indicate that the growth of PA chain charge leads to the decrease of the c* value. The growth of the chain charge increases the electrostatic repulsion of monomer units; hence, the volume of polymer coil arises, and correspondingly, the polymer concentration within a coil decreases. Whereas diffusion process in polymer solutions can be studied by scattering methods, the information on the flow mechanism may be obtained from viscometry data. Figure 3 presents the specific viscosity versus concentration in logarithmic coordinates. This dependence is described by the power-law function:4
η ) kcm
(3)
where the value of k is determined by hydrodynamic interactions and the exponent m depends on the mechanism of mass transfer in the flow process. In the regime of dilute solutions, the mechanism of mass transfer is translational. The transition from the translational to the reptational mechanism of chain movement occurs at entanglements formation. So the curve bend corresponds to the concentration of entanglements network formation (ce). The slopes of initial linear parts of the curves are equal to 0.6 (the traditional value is 0.5 for polyelectrolyte solutions without added salt and 1.25 for a high salt concentration10). At high concentrations, the slopes are equal to 4.4 at R ) 0 and to 2.4 at R ) 0.5 (theoretical values are 1.5 for entangled semidilute solutions of polyelectrolytes without added salt and 3.75 for high salt concentration10). Thus, the experimental conditions are intermediate between low- and high-salt regimes. In the case of non-neutralized PA (curve 1 in Figure 3), the value of ce nearly coincides with the value of c*. This is in a good agreement with the fact that in solutions of uncharged polymers overlapping of coils is accompanied by entanglements formation. The increase of chain charge (curve 2 in Figure 3) leads to the rise of the ce value. A possible explanation for the ce increase with chain charge growth is the following. The criterion of entanglements formation is11 ce ≈ n4c*, where c* is overlap concentration and n is the number of chains overlapping with
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Figure 4. Concentration regimes diagram for PA solutions. See the text for details.
Figure 5. Dynamic viscosity dependence on concentration for PSC (1) and PA (2) solutions. [NaBr] ) 0.05 M.
the selected chain. Assuming n to be constant, we could expect the decrease of ce with decreasing c*. However, chain charge growth is accompanied by the increase of the second virial coefficient (compare slopes of curves 1-3, Figure 2). Such an increase of A2 confirms that the probability of monomermonomer contact decreases (both for single-chain units and for units which belong to different chains). In the first case, it will lead to the swelling of the charged coil, while in the second case, the decrease of the monomer-monomer contact probability can lead to the situation when not all of the n chains surrounding the given chain will overlap with it. Hence, to achieve the entanglement criterion, more neighboring chains are required. Since c* is the upper concentration limit of dilute solutions regime and ce is the starting concentration of entanglements formation, the data given above allows for the plotting of the concentration regimes diagram of PA salt-containing aqueous solutions (Figure 4). Concentrations below c* correspond to the dilute solution regime; a discrete system of macromolecules moving independently is a distinctive feature of the dilute solution. Concentrations between c* and ce correspond to the semidilute unentangled regime. In this regime, the cooperative diffusion process is realized; indeed, polymer coils fill up all of the volume of solution, and so the motion of any coil influences the motion of neighboring ones. In the semidilute unentangled regime, the mechanism of the viscous flow remains translational, because macromolecular coils interpenetrate insufficiently to entangle. When the concentration exceeds ce, the solution passes into thesemidilute entangled regime. Entanglements formation leads to a change of the viscous flow mechanism from translational to reptational. The limits of concentration regimes are determined by the strength of electrostatic interactions. For partly neutralized PA, an increase of R results in a c* decrease and a ce growth, hence, in the expansion of the semidilute unentangled regime. For PA in the acidic form (at R ) 0), c* and ce differ very slightly, so the solution properties of non-neutralized PA become close to those of an uncharged polymer solution. As long as the properties of polyelectrolyte solutions are largely determined by electrostatic interactions, any influence on the ionogenic groups of polyelectrolyte (such as the addition of oppositely charged objects, e.g., micelle-forming surfactants) may essentially change the properties of the solution as a whole. In our previous work, it was clearly shown that the surfactant may play the role of effective structure-forming agent in semidilute solutions.12 In this study, PA-C14AB PSC solutions were investigated in a large range of concentrations, including
the regime of semidilute solutions. In dilute solutions, these PSC particles contain only one PA chain; that is, particles are chaindispersed.12 The formation of PSC in dilute solutions leads to compacting of the PA chain: the hydrodynamic radius of a PSC particle (10 nm) is noticeably lower than that of the individual PA coil (18 nm), as determined from DLS measurements. This compacting is accompanied by the increase of concentration of monomeric units within PSC particles compared with the PA coil; as a result, the intrinsic viscosity falls dramatically at complex formation. The overlap concentration c* calculated as reciprocal intrinsic viscosity for PSC (3.7 × 10-3 g cm-3) is nearly five times larger than c* in the PA solution (8 ×10-4 g cm-3). Figure 5 presents the dependence of the dynamic viscosity on concentration for PSC (curve 1). The corresponding dependence for PA is given for comparison (curve 2). Each curve consists of two linear parts with a different angle of inclination. The initial linear part (observed at low concentrations) corresponds to an unentangled solution. In this concentration regime, the viscosity of the PSC solutions is essentially lower than the viscosity of the PA solutions. The curve bend corresponds to the beginning of a network formation. Structuring in PSC solutions begins at a lower concentration compared with PA solutions. As was shown before12 in dilute solutions, PSC is formed as a result of interaction of one PA macromolecule and one surfactant micelle. In semidilute solutions, the macromolecules contact with each other, so it may result in interaction of one micelle with various PA chains. Thus, it may be assumed that the structuring of a semidilute solution of PA in the presence of the surfactant occurs because of the formation of electrostatic connections of various PA chains with one micelle. This transition from an intra- to an interchain mechanism of complexation is analogous to the well-known transition from an intra- to an interchain cross linking with an increase of polymer concentration observed in the formation of polymer nets. So if we consider the semidilute solution of PSC as a polymer net, then we may say that micelles play the role of mesh points, being electrostatically connected with several PA chains. As a result, the structure of this net differs from that one formed in the PA solution in the absence of the surfactant; in the last case, the mesh points are formed simply because of topological entanglements. The higher growth rate of PSC solution viscosity observed after the curve bend compared with the PA solution confirms the hypothesis of the additional structuring of the PSC solution. With an increase of the PSC concentration in addition to electrostatic mesh points, the formation of topological entanglements of unbounded segments of PA chains should start.
Poly(acrylic acid) Salt-Containing Aqueous Solutions
J. Phys. Chem. B, Vol. 111, No. 29, 2007 8571 macromolecules. Chain charge growth leads to the decrease of the c* value because of the swelling of charged coils and leads to the increase of the ce value. As a result, the extension of the semidilute unentangled regime increases noticeably with the raising of chain charge density. Complexation of PA with an oppositely charged surfactant leads to a c* increase and a ce decrease. In semidilute solutions of PSC, the special network is formed in which surfactant micelles play the role of mesh points. So, the surfactant acts as an effective structuring agent for semidilute polyelectrolyte solutions. Acknowledgment. The authors are grateful for the support of this work provided by Russian Basic Research Foundation (RFBR, Grant 06-03-32964-a).
Figure 6. Dynamic viscosity dependence on shear strain for PCS (1) and PA (2) solutions. [PA]) 8 × 10-2 g cm-3; [NaBr] ) 0.05 M.
As a result, in the semidilute entangled regime, both micellar and topological mesh points exist simultaneously in the PSC solution. The information about the structure of the entangled PSC solution can be obtained from the flow curves, given in Figure 6. The initial parts of flow curves (at low shear strain values) correspond to the flow of solution with the nonbroken structure; the viscosity in these conditions (ηmax) is independent of shear strain. The comparison of ηmax values for PA and PSC shows considerable strengthening of the network in the case of PSC. The destruction of the network in the PSC solution, leading to the decrease of viscosity, starts at the same values of shear stress as in the PA solutions. This fact indicates that similar structure elements (as we suppose, the topological entanglements) exist in both PA and PSC solutions. However, the higher values of the PSC viscosity observed even in the range of network destruction allows for the conclusion that a certain component of the network (e.g., it might be the micellar mesh points, which “stick together” PA chains) is not destroyed. Conclusions Dilute-semidilute regime crossover in aqueous solutions of PA is governed strongly by the electrostatic interactions of
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