Salt-Induced Changes in Triblock Polyampholyte Hydrogels

Article pubs.acs.org/Macromolecules

Salt-Induced Changes in Triblock Polyampholyte Hydrogels: Computer Simulations and Rheological, Structural, and Dynamic Characterization Margarita A. Dyakonova,† Anatoly V. Berezkin,† Konstantinos Kyriakos,† Sandra Gkermpoura,‡ Maria T. Popescu,‡ Sergey K. Filippov,§ Petr Štěpánek,§ Zhenyu Di,∥ Constantinos Tsitsilianis,*,‡ and Christine M. Papadakis*,† †

Fachgebiet Physik weicher Materie, Physik-Department, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany ‡ Department of Chemical Engineering, University of Patras, 26504 Patras, Greece § Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, v. v. i., Heyrovský Sq. 2, 162 06 Prague 6, Czech Republic ∥ Jülich Centre for Neutron Science at MLZ, Outstation at MLZ, Forschungszentrum Jülich GmbH, Lichtenbergstr. 1, 85748 Garching, Germany S Supporting Information *

ABSTRACT: We investigate the influence of ionic strength on the structural properties of stimuli-responsive hydrogels from triblock polyampholytes PAAb-P2VP-b-PAA (PAA and P2VP are negatively charged poly(acrylic acid) and positively charged poly(2-vinylpyridine)). In our previous studies, we found that the transition behavior depends on the charge asymmetry which is controlled by pH and which alters the degree of ionization of the two types of blocks [Dyakonova et al. Macromolecules 2014, 47, 7561]. The same triblock polyampholyte, but with chemically quaternized P2VP (QP2VP) instead of P2VP as the middle block, is highly positively charged, independently of pH. In the present investigation, PAA-b-P2VP-b-PAA at pH 3 and PAA-b-PQ2VPb-PAA at pH 5 were chosen to investigate the influence of the ionic strength on the micellar network morphology by adding NaCl at concentrations in the physiological range. Computer simulations of the latter system show that salt addition results in the formation of larger complexes due to increased hydrophobicity in the system upon screening of charges and that the distance between these complexes increases accordingly. Rheological studies reveal that the hydrogels from PAA-b-P2VP-b-PAA at pH 3 become softer when the ionic strength is above 0.10 M. Small-angle neutron scattering studies have indicated that, in salt-free solution, both systems form networks. Particularly, it was found that in PAA-b-PQ2VP-b-PAA, which has a high charge asymmetry, a variation of the ionic strength leads to significant changes in network architecture. In contrast, in PAA-b-P2VP-b-PAA at pD 3, which has a lower charge asymmetry and the morphology is less sensitive to salt, because the hydrophobic effect prevails. These findings demonstrate that the different response of the two systems to the variation of ionic strength is a consequence of the nature of the predominant interactions, namely charge screening and hydrophobic interactions.

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INTRODUCTION

in the formation of physical gels already at low concentrations.25,26 Being water-soluble and biocompatible, polyampholytes have many applications, e.g., for biochemical and medical purposes,27,28 such as artificial organs, muscles, implant coatings, and controlled drug release.29−33 The degree of swelling of polyampholyte chains depends on the ratio of oppositely charged monomers. Charge neutrality leads to the formation of globular particles, while the divergence from the neutral state leads to an increase of the

1,2

Polyampholytes represent charged associative copolymers, which have been synthesized either as random copolymers with oppositely charged monomers distributed randomly throughout the polymer3−6 or as block copolymers where the charged monomers are located in the different blocks.7−10 These monomers contain acidic or basic groups with variable degrees of ionization. The degree of ionization as well as the polymer architecture determines both the mechanical properties and the response to external stimuli, like temperature,11−14 ionic strength,15−17 and pH.18−24 The high degree of control of the properties of polyampholytes enables their use as responsive materials. Moreover, electrostatic interactions result © XXXX American Chemical Society

Received: August 6, 2015 Revised: October 24, 2015

A

DOI: 10.1021/acs.macromol.5b01746 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

of both ionizable blocks, PAA and P2VP, leading to variations in the structure of the hydrogels. A number of experiments varying ionic strength have been performed by different groups, and the solution behavior was found to depend on the architecture and the net charge on the polyampholyte.49−53 Peiffer and Lundberg have shown that charged polyampholytes which are insoluble in water may become soluble at rather high ionic strength.49 This is caused by the more pronounced intramolecular electrostatic interactions in salt-free solutions, whereas these break upon addition of salt, resulting in an increase of the viscosity. This supports the idea that it is predominantly the electrostatic interactions which determine the solution behavior of polyampholytes and, as a consequence, the mechanical properties. The effect of added salt on the swelling behavior of non-neutral polyampholytes has been studied by Ohlemacher et al.50 and English et al.51 It was shown that the swelling reaches a maximum at a relatively low salt concentration, whereas a further increase in ionic strength causes the shrinkage of the polyampholyte chain to a volume corresponding to the volume of a neutral chain. Tanaka and Tanaka investigated the behavior of randomly charged polyampholytes.52 They demonstrated that the initial decrease of the radius of gyration is due to screening of excess charges in the system, and therefore, the polyelectrolyte nature, which was predominant in salt-free solution, is weakened. A further increase in salt content causes an increase in the radius of gyration, which is due to a transition from the polyampholyte regime to the noncharged state. At this stage, all the electrostatic interactions are screened, and the contribution of the entropy of the counterions is negligible. Nisato et al. have reported that the shear modulus of polyampholyte hydrogels at equilibrium swelling reveals the same tendency upon varying salt concentration as polyampholytes with a significant net charge, namely polyelectrolyte behavior at low ionic strength, characterized by stretched conformation of chains, with a transition to a Gaussian chain conformation upon increasing ionic strength.53 The shear modulus was found to decrease at first as a function of the equilibrium swelling ratiowhich was attributed to a more compact chain conformationwith a subsequent increase at high swelling ratios. It was assumed that, at low ionic strength, domains rich in polyelectrolytes possess a high shear modulus and determine the rheological behavior of the gel. To summarize, altering the electrostatic effect in the system by addition of salt, one can significantly influence the internal structure of the system and its mechanical response. As shown below, in coarse-grained simulations, the PAA and QP2VP blocks can be considered as polymer chains with a solvophobic backbone carrying bound charges. Systematic simulations of such polymers with explicit counterions in poor implicit solvent were performed by Limbach and Holm.54 Depending on the degree of dissociation, α, and the Bjerrum length, lB, they found that a single chain may assume a stretched, a pearl-necklace, or a globular conformation, being insoluble in the latter state. Systematic simulations of similar polyelectrolytes, forming a hydrogel,55−58 elucidated a rich variety of internal gel morphologies,55 where regions of completely collapsed, partially collapsed (“sausage-like”), pearl-necklace, and stretched conformations of chains between cross-links were observed nearly in the same range of α and lB as for individual chains.54 Homopolymer nanogels in good solvent59 and in solvents of different quality60 were also investigated. In more recent simulations of Quesada-Pérez and

swelling ratio. An explanation has arisen from Donnan equilibrium which states that, in the latter case, the number of osmotically active ions in the hydrogel phase increases.34 Moreover, polyampholytes may carry an excess charge which cannot be screened and which tends to expand the chain.35 A number of studies have been conducted on the phase behavior of polyampholyte hydrogels.36 Bossard et al. presented the interesting case of such charge-driven association leading to the formation of a physical gel by ABA triblock copolymers with oppositely charged A and B blocks.37 The formation of a transient network by a triblock copolymer was also found by Lemmers et al.38 in solutions of ABA triblock copolymers with a charged A block and a neutral B block and oppositely charged homopolymers. Association of the two oppositely charged polyelectrolyte moieties leads to the formation of flowerlike micelles, which, at higher concentrations, form bridges, leading to a transient network. Several theoretical studies on the behavior of polyampholytes have been carried out.39−43 The scaling theory for polyelectrolytes and polyampholytes by Shusharina and Rubinstein44 predicts that diblock polyampholytes can form core−shell micelles with a partitioning of the blocks between the core and the shell. Oppositely charged blocks associate as a consequence of electrostatic attractions and form the core, while the corona contains the remaining blocks of the same charges, thus carrying uncompensated charges. The micelle formation has been experimentally observed in solutions of diblock polyampholytes.45,46 Furthermore, the following dependence of the aggregation number of the micelle on the net charge per chain was proven: A decrease of the net charge results in a weakening of the electrostatic repulsion in the corona, leading to an increase of the aggregation number of the micelles.44 In the present work, we focus on aqueous solutions and hydrogels from triblock polyampholytes featuring a relatively long poly(2-vinylpyridine) (P2VP) middle block and two short poly(acrylic acid) (PAA) end blocks, studied at pH 3, and its analogue having a quaternized middle block (QP2VP), studied at pH 5. A similar system has previously been investigated by some of us using small-angle neutron scattering (SANS) along with rheological methods.47,48 The behavior of the polymer is related to the particular response of each block to the variation of pH in H2O or pD in D2O (i.e., by the pKa values). Because of the high positive charge of the middle QP2VP block, which is independent of the pD, its significant electrostatic interaction with the PAA blocks has been shown to result in the formation of a network. In contrast, the charge of the nonquaternized P2VP middle block diminishes upon increase of pD. Despite the reduction of the electrostatic effect, the hydrophobic interaction between neutral P2VP segments increases the rigidity of the gel structure.48 The rheological properties reflect this behavior: At pH higher than 5, the system forms a viscous opaque liquid, whereas below pH 3, it undergoes a transition to a stiff gel. The main interest of the present study concerns the influence of the screening effect of the monovalent salt NaCl on the behavior of these polyampholyte networks, under the condition of a high net charge in salt-free solution. To this end, the structural properties of stimuli-responsive hydrogels from the triblock polyampholytes PAA109-b-P2VP819-b-PAA109 and the corresponding PAA109-b-QP2VP819-b-PAA109 are investigated in dependence on the ionic strength of the solution. It is expected that, at fixed pH, the addition of salt reduces the charge density B


Article

Macromolecules Martı ́n-Molina et al.,61−64 thermosensitive polyelectrolyte gels were simulated in detail. In the present work, the case of large values of α (α ≈ 1) and moderate Bjerrum length λB (λB ≈ 7.1 Å) is investigated. In hompolymer gels without added salt, under these conditions, strong electrostatic interactions cause counterion condensation, and one can expect a competition between “sausage-like” and pearl-necklace conformations.58 However, we consider the more complex physically cross-linked gels of amphiphilic polyelectrolytes at nonzero salt concentration, where the length of the chains between cross-links may vary, while the “pearls” may consist of stoichiometric complexes of PAA and QP2VP blocks with zero net charge. We are not aware of simulations of the salt effect in such gels, and in the present study, we use coarse-grained molecular dynamics with explicit counterions and salt ions as well as implicit solvent. Our coarse-grained model of hydrophobic polyelectrolyte is based on the work of Micka and Kremer.65 Moreover, we present results from rheological studies and SANS on PAA109-b-QP2VP819-b-PAA109 at pD 5 and PAA109-b-P2VP819-b-PAA109 at pD 3 upon addition of NaCl. We find that the two systems display different behavior: In both systems, screening of charges by NaCl leads to a partial restructuring of the physical network junctions, followed by their hydrophobically driven clustering which results in a less defined network. However, PAA109-bQP2VP819-b-PAA109 responds to smaller variations in ionic strength, while the structural rearrangements are more manifested. The paper is structured as follows: In the next two sections, the simulation technique, the samples, and the experimental methods are described. Afterward, we present the results from computer simulations, rheology, and SANS. Eventually, these results are discussed.

same coarse-grained potential was chosen in the mesoscopic model for both the PAA and the P2VP blocks. The simulation details are given in the Supporting Information. The hydrophobic and van der Waals interactions were modeled by the Weeks−Chandler−Andersen (WCA) potential that was chosen to be purely repulsive for counterion−counterion and monomer−counterion interactions, while to mimic hydrophobic attraction between chain backbones (both in PAA and P2VP), it has an attractive minimum. All particles carry positive or negative charges q = ±1, and their interactions are governed by the dimensionless Coulomb potential, written assuming the dimensionless Bjerrum length λB/σ = 3, that is standard in the polyelectrolyte simulations.65 The simulation boxes of volumes Vbox = 603 or 1203 with periodic boundary conditions contain 43 chains of N = 212 monomers, each having the structure A−20−B+172−A−20, where A− and B+ beads simulate PAA and P2VP monomers, respectively. The total numbers of A and B monomers in the box are nA = 1720 and nB = 7396. Additionally, the system contains an equivalent number of counterions C+ and D− (nC and nD) plus salt as follows: nC = nA + c(nA + nB),

nD = nB + c(nA + nB)

(1)

The parameter c is the relative concentration of monovalent salt, taking the values c = 0 (no salt) and 0.1−2.0. c is related to the ratio of concentrations of salt (csalt) and dissociated monomers by c = csalt/(αcm), where α and cm are the average degree of dissociation of the monomers and their molar concentration, respectively. In the case of α = 1, the model value c = 1 is proportional to the experimental salt concentration and corresponds to csalt = 0.305 M. Ionic strength is expressed through the concentrations of small monovalent ions as follows:

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MODEL AND SIMULATION TECHNIQUE The polyelectrolyte solution was modeled using the widely known mesoscopic model of strongly charged polyelectrolytes with hydrophobic backbone.65 In this model, the polymer chains and the counterions are represented as particles, while the surrounding water is treated implicitly by the proper parametrization of interparticle potentials. The interparticle potential combines two components: (i) a short-range potential including van der Waals repulsion and hydrophobic attraction of monomers and (ii) a Coulomb potential describing longrange electrostatic interactions. In some cases, interplay of these potentials can give positive values of excluded volume and good polymer solubility despite the hydrophobic nature of the polymer backbone. To account for this, we choose an attractive short-range potential for the nondissociated PAA monomers, while dissociated PAA is known to be water-soluble. To establish the choice of the attractive short-range potential in this case, we simulated hypothetical nondissociated PAA chains of 20 monomers in water. Simulations were performed in an NPT ensemble, where we fix the number of particles, temperature at 298 K, and pressure at 1 atm. Particle interactions are modeled with Optimized Potentials for Liquid Simulations (OPLS)66 and TIP3P67 force fields for polymer and water, respectively. Hydrophobic interactions between nondissociated PAA monomers were approved by the collapse and aggregation of the chains after 5 × 105 molecular dynamic steps. We did not perform this procedure for P2VP because its backbone seems to be even more hydrophobic than the one of PAA, as evident from its chemical structure. Therefore, the

I=

nC + nD ⎛1 ⎞ = ρ⎜ + c ⎟ ⎝ ⎠ 2Vbox 2

(2)

In the solutions used for experiments (see below), the polymer concentration of 3 wt % is equivalent to 0.306 M, leading to an ionic strength of counterions I = 0.153 M (without salt), and the Debye length κ−1 = 0.304/√I = 0.78 nm. Since the simulated chains are much shorter than those used in the experiments and each block contains a smaller number of charges, we modeled them as completely dissociated to compensate for this difference. In mesoscopic simulations of strong polyelectrolytes with a hydrophobic backbone, it is common practice to use a degree of dissociation α = 0.5 or even α = 0.333 to observe necklace conformations.65 In the present work, however, we aim at simulating a real system with α ≈ 1.0 for the QP2VP blocks, and necklace conformations were not observed. The system was considered at two number-average monomer densities (concentrations) ρ = (nA + nB)/Vbox = 0.0053 and 0.0422. The smallest value is considered to describe the salt effect in the dilute polyelectrolyte solution. The highest polymer concentration is close to an experimental mass fraction of 3 wt % (see below) and allows modeling of the gel structure in an experimentally interesting range of ionic strength.

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EXPERIMENTAL SECTION

The synthesis of the triblock polyampholyte as well as the procedure of quaternization were described previously.47,68,69 The obtained copolymers were characterized by static light scattering (SLS), size C


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into account. The data were azimuthally averaged. Data reduction was performed using the software QtiKWS provided by JCNS. SANS Data Analysis. The following expression was fitted to the SANS curves:

exclusion chromatography (SEC), and nuclear magnetic resonance (NMR), as reported earlier.48 The molecular characteristics of the obtained copolymers are summarized in Table 1.

Table 1. Molecular Characteristics of PAA-b-P2VP-b-PAA polymer P2VP PAA109-b-P2VP819-bPAA109

Mwa (g/mol) 86000 102000

Mw/Mnb

P2VPc (wt %)

P2VP DPw

1.12 1.23

100 80

819 109/819/109

′ (q) + SOZ(q) + Pagg(q) + Ibkg I(q) = I0P(q)SHS(q) + Psph

where P(q) is the form factor of the particle, describing its inner structure and shape and I0 its scaling factor. For all triblock polyampholyte solutions, the form factor for homogeneous spheres Psph(q) was used for P(q):

a By SLS. bBy SEC. cBy 1H NMR, DPw: weight-average degree of polymerization.

⎛ 3[sin(qR ) − qR cos(qR )] ⎞2 sph sph sph ⎟ Psph(q) = Isph(ρsph − ρs )2 ⎜⎜ ⎟ 3 ( ) qR ⎠ ⎝ sph

Sample Preparation. Samples were prepared by dissolution of PAA109-b-QP2VP819-b-PAA109 in D2O at 3 wt % at room temperature. The pD of the resulting solution was measured at 3.4. An appropriate amount of 0.01 M sodium hydroxide (NaOH) solution in D2O was added to install a pD value of 5.0. The gel from PAA109-b-P2VP819-bPAA109 was prepared in D2O at 3 wt % at room temperature at pD 3.0. Afterward, concentrations of 0.05, 0.1, and 0.15 M of sodium chloride (NaCl) in both gels were obtained by adding solutions of sodium chloride (NaCl) in D2O to both types of hydrogels. Dissolution of the polymer and homogenization of the gel were achieved by rigorous stirring and centrifugation. All samples were prepared in the same way to avoid a possible influence of the pathway of the gel formation process on the structure. For the rheological experiments, the 3 wt % PAA109-b-P2VP819-b-PAA109 hydrogel was prepared by direct dissolution of the polymer in aqueous 0.01 M HCl. Vigorous stirring and centrifugations were used for homogenization. Samples with different NaCl concentrations were prepared by adding appropriate amounts of salt. Finally, the pH values of the samples were adjusted to be 3. Rheometry. Rheological measurements were performed on a stress-controlled rheometer (AR-2000ex, TA Instruments) using a cone−plate geometry (diameter 20 mm, cone angle 4°, truncation 111 μm). Special care was taken during sample loading to exclude bubbles. After loading, a delay of 10 min was applied prior to any measurements. The rheometer was equipped with a Peltier control system that allows for accurate control of temperature. The linear viscoelastic regime was established by oscillatory strain sweeps using a frequency of 1 Hz. Dynamic shear moduli (G′ and G″) were examined in the linear viscoelastic regime at 25 °C. Creep experiments were performed by keeping the applied stress constant within the viscoelastic regime to determine the zero-shear viscosity, η0. From the linear part of the plot Je(t) over time, η0 and the plateau modulus G0 can be extracted according to the equation Je (t ) =

1 t + G0 η0

(6) where Isph = NsphVsph2, Nsph is the number of spheres, and Vsph and Rsph are their volume and radius, respectively. ρsph and ρs stand for the mean scattering length densities (SLD) of the sphere and the solvent, respectively. The values of 1.66 × 10−4, 1.69 × 10−4, and 1.26 × 10−4 nm−2 were calculated for PAA, P2VP, and QP2VP, respectively, using the mass densities of 1.2, 0.98, and 1.0 g/cm3. For D2O, the literature value of scattering length density of 6.36 × 10−4 nm−2 was used. The spheres are formed by interpolyelectrolyte complexes (IPECs) from PAA and P2VP chains, formed by electrostatic interaction. In line with our previous work,48 where it was established that PAA and P2VP blocks may form complexes in different ways, depending on the conditions, we allowed for polydispersity of the sphere radius, Rsph, using a Gaussian distribution: f (R sph) =

⎡ 1 ⎤ 1 exp⎢− 2 (R sph − R̅ sph)2 ⎥ ⎣ ⎦ δ 2π 2δ

(7)

where R̅ sph is the average radius, δ the width of the distribution, and p = δ/R̅ sph the corresponding polydispersity. The particle interactions were taken into account by the Percus− Yevick structure factor, SHS(q):70

SHS(q) =

1 1 + 24ηHSG(2RHSq)/(2RHSq)

(8)

with RHS being the hard-sphere radius, or half the average center-tocenter distance between interacting particles, and ηHS the fraction of particles that contribute to the formation of the network structure, or the hard-sphere volume fraction. The function G(x) reads

2x sin x + (2 − x 2) cos x − 2 sin x − x cos x + δ′ 2 x x3 4 2 3 − x cos x + 4(3x − 6 cos x + (x − 6x) sin x + 6 + ε′ x5 (9)

G(x) = γ ′

(3)

Terminal relaxation times (τR) can also be determined through the equation η0 = G0τR

(5)

with the help functions

(4)

Small-Angle Neutron Scattering (SANS). Measurements were carried out at the instrument KWS-2 at the JCNS outstation at MLZ in Garching, Germany. The neutron wavelength was chosen at λ = 0.45 nm with a spread Δλ/λ = 20%. The scattering signal was collected by a 128 × 128 scintillation detector having a pixel size of 0.5 × 0.5 cm2. Using sample−detector distances (SDDs) of 0.99, 3.63, and 19.63 m, a range of momentum transfers q = 0.022−5.5 nm−1 was covered. q = 4π sin(θ/2)/λ, and θ is the scattering angle. The samples were mounted in 0.5 mm quartz cuvettes (Hellma) and were measured at room temperature. The exposure times were 5, 10, and 20 min at SDD = 0.99, 3.63, and 19.63 m, respectively. For detector sensitivity measurements and for bringing the data to absolute scale, poly(methyl methacrylate) was used. From the measurements of boron carbide, the contribution of dark current and background were estimated. This background together with the scattering of D2O and the empty cell were subtracted from the sample scattering, taking the transmissions

γ′ =

(1 + 2ηHS)2 4

(1 − ηHS)

,

δ′ =

− 6ηHS(1 + ηHS /2)2 4

(1 − ηHS)

,

ε′ =

γηHS 2 (10)

In eq 5, Psph ′ (q) denotes the form factor of those spheres, which are not correlated with each other. It is expressed by the form factor for homogeneous spheres (eq 6) with a Gaussian distribution of the radii, eq 7. For fitting of the SANS curves from salt-free solutions, P′sph was not needed and was therefore set to zero. The excess scattering intensity at high q values (q > 0.15 nm−1) was attributed to the scattering from chains in the network having a correlation length ξ. The Ornstein−Zernike function was used for describing this contribution:71 SOZ(q) = D

IOZ 1 + (ξq)2

(11) DOI: 10.1021/acs.macromol.5b01746 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules with the intensity of the contribution IOZ and ξ the correlation length between strands. The strong increase in forward scattering at low q values (q < 0.05 nm−1) was modeled by a modified Porod law:72 Pagg(q) =

segments can be fixed by performing quaternization of the P2VP block in the triblock polyampholyte.75 In that case, the pD value determines the charge asymmetry in the system, leading to a large variety of structures.48 In the present study, we investigate PAA109-b-P2VP819-bPAA109 at pD 3 and PAA109-b-QP2VP819-b-PAA109 at pD 5, both in dependence on the ionic strength. While the degrees of polymerization of the blocks are lower than previously,48 the composition is nearly the same. At these pD values, the nonquaternized and the quaternized one form a stiff gel and a viscoelastic liquid, respectively. In the presence of charges, two counteracting forces are responsible for the network formation. The main stimulus is the presence of both types of charges along the chain, which induces the formation of neutral hydrophobic complexes due to electrostatic attraction and release of counterions. These complexes are connected by those parts of the P2VP blocks which are not involved in this complexation. Previous studies on similar polyampholytes at pH 3.0 have shown that, in the case of the nonquaternized polyampholyte, the gelation point lies at lower concentration than in the quaternized copolymer.48 This observation led us to conclude that the charge density of both blocks strongly influences the mechanical response of the system. The electrostatic repulsion between the P2VP segments is another crucial factor being at the origin of the stability of the hydrogel: Charged stretched blocks are able to connect interpolyelectrolyte complexes because they can bridge larger distances than uncharged coiled blocks, resulting in a high network connectivity. Upon addition of salt, we expect that the electrostatic attractive interactions between oppositely charged blocks are altered and that the repulsion between the connecting P2VP segments is screened; thus, these elastic strands are expected to become shorter. Results from Simulations. To gain an impression of the structures realized by the polyampholyte system and to characterize the behavior of structural parameters in dependence on polymer concentration and on ionic strength, computer simulations were carried out using the mesoscopic model of strongly charged polyelectrolytes with hydrophobic backbone. While the simulations take into account the interplay of hydrophobic attraction and electrostatic repulsion between chains and small ions, which are altered by the salt addition, the degree of dissociation of the model polyampholyte has to be fixed. Therefore, the model system resembles most closely the PAA109-b-QP2VP819-b-PAA109 system at pH < 7, but its hydrophobic collapse at pH > 7 observed experimentally is not mimicked. Thus, these simulations contribute to understand the effects of the polymer and the salt concentrations on the gel structure and on the conformational behavior of the copolymer blocks. In the polyampholyte system under study, notable structural changes are expected around the polymer overlap concentration ρ* ≈ 0.0057, which was obtained in our preliminary simulations. Systems with polymer concentrations below (ρ = 0.0053) and well above (ρ = 0.0422) ρ* and thus the percolation threshold were simulated. Snapshots of these systems are shown in Figure 1 for different salt concentrations. At the low polymer concentration (ρ < ρ*), dilute solution behavior of unimers with only intramolecular complexation of A (PAA) and B (P2VP) blocks is observed (Figure 1, left column). This structure is supported by the results of the cluster analysis, presented by open symbols in Figure 2. We present the average number of A blocks per complex (ntails), the

Iagg qαP

(12)

with the scaling factor Iagg and the Porod exponent αP. The latter indicates the surface roughness of the aggregate if αP ≤ 4 or a surface gradient near the interface if αP > 4.73 For the fitting of the data from the nonquaternized system ′ (q) was not needed and was set containing 0.05 or 0.10 M of salt, Psph to zero. Addition of a higher amount of salt (0.15 M) to this system leads to more drastic changes in the scattering curves. The form factor of homogeneous polydisperse spheres (eq 6) again with a Gaussian distribution of the radii (eq 7) was used as P(q), and the second term, ′ (q), was included using Psph ′ (q) = Psph(q), again describing Psph homogeneous spherical particles not contributing to the network formation. The same fitting function (polydisperse homogeneous spheres coupled by a Percus−Yevick structure factor, additional polydisperse homogeneous spheres, and the Ornstein−Zernike structure factor) was used for all salt-containing solutions of the quaternized polyampholyte system. For fitting, the NCNR SANS Analysis package within the Igor Pro environment was used.74 The incoherent background was calculated for each case from the incoherent scattering cross sections of the components and was found to be consistent with the experimental data.

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RESULTS This section is structured as follows: After a brief description of the systems under investigation, the results from computer simulations are described. The next part focuses on the rheological properties of the physical hydrogels formed via charge-driven association of the nonquaternized triblock polyampholyte at pH 3 in dependence on the NaCl concentration. The morphological changes of both hydrogelsthe nonquaternized one at pD 3 and the quaternized one at pD 5which occur upon addition of small amounts of NaCl are described. System. We briefly review the properties of the two systems under investigation. We have previously studied similar quaternized and nonquaternized triblock polyampholytes, PAA 163 -b-QP2VP 1397 -b-PAA 163 and PAA 163 -b-P2VP 1397 -bPAA163, in salt-f ree aqueous solution in D2O.48 The degrees of ionization of both the PAA and the P2VP blocks depend on the pH value (or, in D2O, on the pD value). The P2VP blocks are protonated (ionized) for pD < pKa = 5.0. The pKa of PAA is 4.2, and at low pD, ∼25% of the PAA segments are deprotonated. Thus, at low pD, PAA109-b-P2VP819-b-PAA109 is expected to feature a net positive charge and can be viewed as a weak cationic polyelectrolyte. This, in turn, leads to a chargedriven association between oppositely charged blocks and the formation of interpolyelectrolyte complexes. With increasing pD, a higher fraction of P2VP segments are not ionized and become hydrophobic, whereas more PAA segments are ionized. At pD 5, the degree of ionization of the PAA blocks reaches 55%. Around the isoelectric point, 4.8 < pD < 6.8, the equality in the number of opposite charges leads to phase separation. Above pD 7, PAA109-b-P2VP819-b-PAA109 is expected to carry a negative net charge because deprotonated PAA segments become predominant (∼80%), while all P2VP units turn hydrophobic. Moreover, the positive charges of the P2VP E


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number of chains to which these blocks belong (nchains), and the number of loops, i.e., chains having both A-blocks in the same complex (nloops). At ρ = 0.0053, ntails = 1.26 and nchains = 1.11, i.e., intramolecular complexation prevails. The high solubility of the polyampholyte is due to its high net charge, resulting in a long-range electrostatic repulsion of the positively charged P2VP blocks at a low ionic strength, where the degree of dissociation of the P2VP blocks is high. In the concentrated system, i.e. at ρ ≫ ρ*, the complexes involve on average more than two chains (closed symbols in Figure 2), which gives rise to gel formation. In all simulations, the mass fraction of the sol does not exceed 2.2 wt %. As seen from Figure 1, the gel consists of nearly spherical A/B complexes, connected by B strands. We now consider the impact of the ionic strength of the solution on the conformation of A and B blocks as well as on the size of the complexes and their distances. The radii of gyration of the A and B blocks as well as of the one of homopolymer chains B of the same length (NB = 172) are shown in Figure 3. The B blocks (P2VP), being exposed to the

Figure 3. Squared radius of gyration of the entire chain and the different polyelectrolyte blocks as a function of salt concentration. Figure 1. Snapshots from computer simulations of polyampholyte solutions at different polymer (ρ) and salt (c) concentrations. Monomers A (PAA) and B (P2VP) are shown with orange and blue color, respectively.

solution, are notably affected by the ionic strength. The size of the homopolymer chains of completely dissociated P2VP (green triangles in Figure 3) decays with ionic strength because of increased electrostatic screening. The size of the B blocks in the polyampholyte (green circles in Figure 3) displays a completely different dependence: These blocks are strongly stretched at low ionic strength, I ≈ 0.005. Detailed calculations, reported in the Supporting Information, demonstrate that under these conditions the radius of gyration scales as ⟨RgB2⟩1/2 ∼ N0.693±0.002. This swelling of the B blocks is apparently caused by electrostatic repulsion between monomers. In the range of I from 0.005 to 0.02, the size of the B blocks rapidly drops, and at I ≥ 0.063, it nearly follows the self-avoiding walk (SAW) scaling for the high polymer concentration (ρ = 0.0422) and is only slightly influenced by the salt concentration. The difference between the sizes of the B blocks and the B homopolymers can be related to the intramolecular (at ρ < ρ*) and intermolecular (at ρ > ρ*) complexation of the polyampholyte: B blocks are present not only between the complexes but also inside them. Within the complexes, the B blocks are more compact than in solution, which reduces their average size. For a given polymer concentration, the A blocks of PAA, being hidden within the complexes, have nearly the same size at any ionic strength. At ρ = 0.0053, their size is limited by the radii of small complexes, while in the larger complexes present at ρ = 0.0422, short block fragments scale nearly as in θ-

Figure 2. Effect of the polymer (ρ) and salt (c) concentrations on the average number of tails (A blocks) per complex (ntails), the number of chains to which these tails belong (nchains), and the number of loops, i.e., the number of chains having both A blocks in the same complex (nloops).

F


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Macromolecules conditions, i.e., ⟨RgB2⟩1/2 ∼ N0.504±0.004. It seems that within the complex electrostatic and hydrophobic interactions are ruled out. Further details are given in the Supporting Information. The size of the PAA and the P2VP blocks cannot be directly measured by SANS due to the weak contrast between these two polymers and also due to the low concentration of bridging P2VP blocks in solution. However, the size of the complexes and their distances can be experimentally determined, and the present simulations may help explain their behavior. The size of the spherical complexes with homogeneous density is related to the average number of tails therein, ntails, by ⎛ 3n N ⎞1/3 ⟨R ⟩ = ⎜⎜ tails A ⎟⎟ ⎝ 2πρc ⎠

blocks), true solutions of intramolecular complexes, stabilized by electrostatic repulsion, are found. At high polymer concentration and low salt concentration, only the gel phase appears. The size of the complexes in this gel and the distance between them are controlled by the balance of complexation and electrostatic repulsion of the bridging P2VP blocks. In the oversaturated gel, i.e., above the equilibrium polymer concentration, the complexes and distances between them grow upon salt addition, as observed in experiments (see below). Further salt addition (c ≥ 1.5) may lead to the formation of a separate supernatant phase. The gel attains its equilibrium structure, where the average number of A-tails in the complex is defined by the end-to-end distance of P2VP bridges ⟨RB2⟩ similarly to eq 14 as

(13)

where ρc is the average monomer density in the complex and NA the length of the A tail. In this equation, we take into account that each complex contains an equal number of A and B monomers. Figure 2 shows that in the gel (at ρ = 0.0422) the average number ntails, related to the complex size, passes through a maximum at a certain salt concentration (c ≅ 1.5). The absolute value of the crossover concentration depends on the model and will differ from experimental one. The maximum is the result of two competing effects: The first one is the growth of the complexes in size due to screening of electrostatic repulsion between bridging P2VP blocks. A weakening of the repulsion allows for a larger number of P2VP segments outside the complexes and, therefore, a larger number of tails per complex. This effect dominates at moderate salt concentrations (c < 1.5). The second effect is that, for a given polymer concentration in the gel, the distance between complexes increases with their size. However, too large distances are thermodynamically unfavorable because they involve strong stretching of the P2VP bridges. As seen from Figure 3, the salt addition leads to the contraction of these bridges. At higher salt concentrations (c ≥ 1.5), their size limits the maximal distance between the complexes and, consequently, the average complex size, simply because of mass conservation. If the polyelectrolyte complex contains a certain number of A-tails, ntails, the average number of macromolecules in a complex equals to ntails/2 (two tails per macromolecule), and the average distance between complexes is expected to be ⎛ n ⎞1/3 ⟨D⟩ = ⎜⎜ tails ⎟⎟ ⎝ 2ρp ⎠

⎛ n ⎞2/3 ⟨RB ⟩ ≈ ⎜⎜ tails ⎟⎟ ⎝ 2ρp ⎠ 2

(15)

In this case, the size of the complexes slowly decreases upon salt addition due to the contraction of the hydrophobic bridges, caused by the screening of electrostatic repulsion between P2VP monomers. Further salt addition and the decrease of the degree of polyampholyte dissociation can lead to the formation of dense gels, where the P2VP blocks are partially collapsed due to hydrophobic interactions and form a highly concentrated and nearly homogeneous mixture of polyampholytes and water. In simulations, this regime is not attained because a constant degree of dissociation was assumed. Rheological Properties. In order to investigate the effect of ionic strength on the mechanical response of the nonquaternized polyampholyte system, rheological measurements were carried out. The zero-shear viscosity was extracted from creep and steady-state shear viscosity (for low viscosities) measurements of 3 wt % aqueous solutions of PAA109-bP2VP819-b-PAA109 at pH 3. Oscillatory measurements were accomplished in the linear viscoelastic regime at different NaCl concentrations. In Figure 4a, the storage, G′, and loss, G″, modulus are plotted versus frequency. As seen, the hydrogels preserve their elastic response even at 0.5 M NaCl, since G′ remains higher than G″, and the moduli are independent of frequency. Yet, the terminal relaxation zones have been shifted to very low frequencies (f < 100 Hz), implying long relaxation times (τ > 100 s). This is consistent with the free-supporting behavior of the solutions and suggests the formation of a 3D network in all cases. In Figure 4b, the plateau modulus, derived from the oscillatory frequency sweep measurements at 1 Hz (Figure 4a), is shown in dependence on NaCl concentration. For the salt-free hydrogel, the elastic modulus is 121 Pa and initially increases up to 152 Pa at 0.05 M. Upon further addition of salt, the plateau modulus diminishes and becomes lower than that of salt-free solution above 0.2 M. This behavior implies that the expected screening of the electrostatic interactions (either repulsive along the P2VP segments or attractive between oppositely charged segments), upon increasing the ionic strength, imposes structural rearrangements, namely the number density of the elastically active chains of the network and the structure of the network junctions. Obviously, at low ionic strength, the number of elastic chains increases, regarding

(14)

Here, ρp = ρgel/N is the average concentration of polymers in the gel, and ρgel is the average concentration of monomers therein. To compare this distance to the radius of gyration of the bridging B blocks in our simulations, we assume that ρgel = ρ and calculate the value ⟨D⟩2/6 from the data of Figure 2 for the polymer concentration ρp = 0.0422/N = 2 × 10−4. The results are plotted in Figure 3 with black circles. At c ≥ 1.5, the calculated size of bridges coincides with the one obtained in the simulation. Discrepancies between ⟨RgB2⟩ and ⟨D⟩2/6 arise at lower salt concentrations, where the size of the B blocks notably exceeds the distance between complexes. Under these conditions, the P2VP blocks may be part of more than two complexes. Summarizing the simulation results, in dilute solutions of polyampholytes (below the overlap concentration of the P2VP G


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assume, however, that, at this high polymer concentration, the number of salt ions is not sufficient to screen all electrostatic interactions, and therefore the elastic character of the hydrogels is preserved even at 0.5 M NaCl (Figure 4a). To summarize, the ionic strength has a notable influence on the rheological behavior of the hydrogels above a NaCl concentration of 0.1 M, namely a significant reduction in viscosity and plateau storage modulus. Screening of both types of charges weakens both the attractive and repulsive electrostatic forces, resulting in softer hydrogels. Investigation of the Morphologies with SANS. SANS allows us to elucidate the mesoscopic structures of the hydrogels and their changes upon salt addition. The rheological studies described above have revealed a strengthening of the network up to a salt concentration of 0.1 M and a subsequent weakening, the latter being attributed to a reduced cross-link functionality upon increasing ionic strength. In order to characterize the underlying structural changes as a result of the screening of charges, we use SANS, focusing on low values of NaCl concentrations, i.e. up to 0.15 M, which covers the interesting rheological behavior (Figure 5). For the salt-free solution of PAA109-b-P2VP819-b-PAA109 at pD 3, the scattering curve shows a pronounced peak at q = 0.1 nm−1 and a shoulder at 0.2 nm−1, apart from forward scattering below 0.05 nm−1 (Figure 5a). The shoulder at q ∼ 0.2 nm−1 is associated with the particle form factor of the complexes, whereas the peak at q ∼ 0.1 nm−1 is ascribed to the interaction between them. The curves for solutions of PAA109-b-P2VP819-b-PAA109 with 0.05 and 0.1 M of NaCl have similar shape. All these scattering curves were successfully fitted with eq 5, which includes the form factor of homogeneous spheres with a Gaussian distribution of the radii, coupled with a hard-sphere structure factor, describing the correlation between them, a modified Porod law, describing the forward scattering, and an Ornstein− Zernike term describing the correlation between polymer strands (Figure 5b). The resulting particle radius is found at Rsph = 8.1 ± 0.6, 7.2 ± 0.6, and 8.3 ± 0.9 nm for the gel without salt and with 0.05 and 0.10 M of NaCl, respectively (Figure 6a); i.e., it does not vary significantly with increasing ionic strength. In our previous studies on similar salt-free triblock polyampholytes, we showed that, in dependence on the charge asymmetry, the triblock polyampholyte adopts different conformations.48 At pD 3, which is under consideration here, a 3-dimensional network of core−shell particles was found. The cores were associated with interpolyelectrolyte complexes, which form because of the attractive electrostatic interactions between oppositely PAA and P2VP charged blocks, while the remaining noncomplexed, ionized P2VP blocks are located in the shell. In the present study, the polyampholyte system has shorter block lengths and lower concentration, which, presumably, leads to a more random distribution of the complexed chains inside the particle, and a separate shell cannot be distinguished. The complexation goes along with the release of counterions from the polyampholyte chain and their following substitution with oppositely charged polymer segments. The excess positive charge along the ionized P2VP blocks makes them rather stretched, which is expected to result in a large average distance between the complexes. Indeed, the hard-sphere radius is found at RHS = 29.5 ± 0.3 and 31.5 ± 0.7 nm for the gel without salt and with 0.10 M of NaCl, respectively (Figure 6a), which is much larger than the radius of the particle. In general, the hardsphere radius depends only weakly on NaCl concentration.

Figure 4. Frequency dependence of storage (G′) and loss (G″) modulus at different NaCl concentrations (a) and plateau modulus, G0 (at 1 Hz) (b), along with the zero shear viscosity, η0 (c), as a function of NaCl concentration of 3 wt % PAA109-b-P2VP819-b-PAA109 aqueous solutions at pH 3. Error bars in (b) are the standard deviations which, however, are not visible in (c) due to the larger size of the symbols.

the salt-free solution, passing through a maximum prior to decrease to its half value at 0.5 M salt. On the other hand, the zero-shear viscosity, extracted from creep measurements in dependence on NaCl concentration, diminishes continuously with increasing ionic strength (Figure 4c). For the salt-free hydrogels, the viscosity is as high as 2.8 × 105 Pa·s, which corresponds to a rather stiff gel. The viscosity of the hydrogels with 0.3 M of NaCl is already 1 order of magnitude lower, and at 0.5 M, the viscosity drops even further, namely to 9.3 × 103 Pa·s. The continuous drop of the viscosity with ionic strength, despite the initial augmentation of the plateau modulus (number of elastic chains), is governed (at least in the low ionic strength) by the abrupt decrease of the terminal relaxation times as observed by creep measurements through the equation τR = η0/G0 (Figure S3). This effect seems reasonable, since the electrostatic screening, resulting from the salt addition, would weaken the integrity of the physical cross-links, facilitating therefore the exchanges of the PAA chain-ends from their junctions. Indeed, the stability of the interpolyelectrolyte complexes, i.e., the junctions, is affected by the NaCl ions, which are able to penetrate them and to screen the electrostatic interactions between oppositely charged PAA and P2VP blocks, thus leading to their partial disintegration and a softer gel. We H


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Figure 6. Parameters from fitting of the SANS curves of PAA109-bP2VP819-b-PAA109 at pD 3. (a) Sphere radius, Rsph (solid circles); radius of satellite spheres, R′sph (open circle); hard-sphere radius, RHS (solid triangles); and correlation length, ξ (stars). (b) Volume fraction ηHS.

form factor of polydisperse, homogeneous spheres for P(q), additional small spherical particles, P′sph(q), the Ornstein− Zernike structure factor, and the forward scattering (Figure 5c). The radius of the complexes is determined at Rsph = 17.4 ± 3.1 nm and the radius of the small spheres at R′sph = 6.5 ± 1.6 nm (Figure 6a). Thus, there is coexistence of small complexes of radius 6.5 nm (slightly smaller than the complexes at lower salt concentrations) with larger, homogeneous complexes of radius 17.4 nm. We suggest that these larger aggregates are composed of collapsed small initial complexes which are now more hydrophobic. Additionally, in accordance with the simulations, the salt ions in the solution screen the electrostatic repulsion of the ionized bridging P2VP blocks, which collapse and bring different micelles closer to each other, thereby forming larger complexes. However, not all small complexes are parts of a larger aggregate but rather stay apart. The aggregate formation is a result of the lower charge density in the system. At 0.15 M NaCl, the hard sphere radius is RHS = 29.9 ± 0.8 nm, i.e., the same as for lower salt concentrations (Figure 6a). The fact that this distance is not influenced means that it is mainly given by the degree of stretching of the P2VP blocks, which is lowered upon salt addition. Thus, it is an effect of two antagonistic effects: The salt addition increases the complex size, but the related increase of the average distance between complexes is suppressed by the simultaneous shrinkage of the P2VP bridges. The volume fraction is with ηHS = 0.32 ± 0.01 significantly higher than at lower salt concentrations (Figure 6b). This may seem contradictory to the fact that the peak at 0.1 nm−1 is not as pronounced as at lower salt concentrations (Figure 5a,c). This effect is due to the forward scattering which, at 0.15 M, is not separated from the peak as at lower NaCl concentrations. It can be fitted with eq 12 with an exponent αP = 2.7 ± 0.1, whereas at lower salt concentrations it is 4.4 ± 0.2, 4.1 ± 0.2, and 4.6 ± 0.1 at 0, 0.05, and 0.1 M of NaCl. Thus, at these lower salt concentrations, the aggregates are compact with a surface concentration gradient, whereas they are smaller and of

Figure 5. SANS curves of solutions of PAA109-b-P2VP819-b-PAA109 at 3 wt % in D2O at pD 3 and 26 °C for different NaCl concentrations together with the full fitting curves (a). (b, c) Same experimental data with the components of the fitting curves: with 0.1 M (b) and 0.15 M of NaCl (c). The solid lines are the full model fits (see text); noncontinuous lines are contributions to the model as indicated in the graph.

This architecture qualitatively agrees with those observed in our simulations of the polyampholyte gel at low ionic strengths (black circles in Figure 3). The slight increase of the volume fraction of micelles, contributing to the network, in a wide range of salt concentrations, from ηHS = 0.21 ± 0.05 in salt-free solution to 0.25 ± 0.02 at 0.10 M NaCl (Figure 6b), shows that there are no dramatic changes in the gel architecture, but rather a redistribution of chains between the complexes. Thus, in the range of NaCl concentrations up to 0.10 M, only internal structural changes within the complexes take place, without affecting the organization of the network at larger length scales. This may explain the slight increase of the plateau storage modulus (Figure 4b). Dramatic changes are observed in the scattering curve upon addition of 0.15 M of salt (Figure 5a,c). The shoulder at 0.2 nm−1 becomes less pronounced, the peak of the structure factor is not so well-defined any longer, and the forward scattering changes slope and dominates the scattering curve up to 0.1 nm−1. Again, the scattering curve was fitted using eq 5 with the I


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In our previous studies on similar quaternized and nonquaternized polyampholyte systems, we showed that the latter forms stiffer networks and has a lower gel point due to a higher number of P2VP bridges.47,48 Thus, being hydrophobic and therefore less mobile, these bridges contribute to the stiffening of the network in comparison with the quaternized system where the number of ionized P2VP segments is considerably higher. To investigate the influence of the hydrophobicity and of electrostatics on the response to external stimuli, in our case the ionic strength, we carried out SANS experiments on PAA109-b-QP2VP819-b-PAA109 at pD 5. At this pD, the QP2VP block is strongly charged (almost every monomer); at the same time, the fraction of ionized PAA monomers achieves 55%. Because of these high degrees of ionization of both blocks, we anticipate that the behavior upon salt addition is governed by the electrostatic interactions. In Figure 8a, the SANS curves of PAA109-b-QP2VP819-bPAA109 at pD 5.0 are presented for NaCl concentrations up to 0.15 M. At first glance, the curves look similar to the ones of the nonquaternized triblock polyampholyte (Figure 5a); however, for low NaCl concentrations, the peak is less pronounced than in the nonquaternized sample. For fitting of the curve of the salt-free hydrogel, eq 5 was again used. The size of the particle was found to be 7.1 ± 0.8 nm with a quite high polydispersity of 0.5 ± 0.1. The dimensions of the complex are similar to those of the nonquaternized sample, i.e., both salt-free systems consist of bridged spherical interpolyelectrolyte complexes. The hard-sphere radius is RHS = 33.6 ± 0.5 nm, i.e., slightly larger

more fractal nature at 0.15 M NaCl. The addition of salt thus leads to a breakup of the aggregates. This breakup at large length scales explains the decrease of the viscosity and the plateau storage modules discussed above. We discuss the possible origin of the large aggregates in the section on rotational DLS (Supporting Information). For all salt contents, fitting of the OZ term gave average distances between chains ξ which decrease from 1.1 to 0.8 nm (Figure 6a), a value which is similar to the previously determined one.48 These results confirm that at this low pD value the network organization is due to attractive electrostatic interactions.43,76 The structures of the hydrogels as deduced using SANS are shown in Figure 7.

Figure 7. Schematics of the structures of the hydrogels from the nonquaternized triblock polyampholyte without salt (a) and with 0.15 M of NaCl (b).

Figure 8. SANS curves of solutions of PAA109-b-QP2VP819-b-PAA109 at 3 wt % at pD 5 and 26 °C together (a) and separately (b−e) for different NaCl concentrations: salt-free (b), 0.05 M (c), 0.1 M (d), and 0.15 M (e). The solid lines are the models fits; see text. J


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to 0.11 ± 0.01 (Figure 9b). This decrease may be explained by the fact that a certain fraction of the QP2VP segments form the small spheres which do not contribute to the correlation between the larger complexes. Moreover, the QP2VP blocks bridging the complexes are now less stretched and more prone to back-folding than in salt-free solution, resulting in a reduced connectivity. At higher salt concentrations, the volume fraction increases again, and at 0.15 M of NaCl, the value is as high as ηHS = 0.3 ± 0.02; the network is thus strongly correlated, which may be due to the hydrophobic effect of the Q2VP blocks. The aggregate structure evolves in the same way as in the nonquaternized polyampholyte with increasing ionic strength. In the salt-free system, the exponent is αP = 3.4 ± 0.2 and weakly decreases to 3.1 ± 0.1, 3.0 ± 0.1, and 2.5 ± 0.1 after addition of 0.05, 0.1, and 0.15 M of NaCl, suggesting a diminishing degree of the density of packing in the aggregates and their partial disintegration. The Ornstein−Zernike correlation length, ξ, diminishes from 1.5 to 1.1 nm and is thus very similar to the one in the nonquaternized system. Comparing the two systems, one can state that both systems contain elastically active links from charged P2VP chains. Upon increasing the ionic strength, the electrostatic repulsion between the bridges decreases, and larger complexes may be formed. The hydrogel formed by the nonquaternized PAA109-bP2VP819-b-PAA109 at pD 3 maintains its structure for relatively high amounts of saltthe structural parameters hardly change up to 0.10 M of NaCl, and a difference is only encountered at 0.15 M (Figure 5a). This is due to the increase of hydrophobic segments in the P2VP blocks, which result in larger complexes and a better correlation between them. However, there is a high fraction of nonionized segments, and thus, the impact of screening is not so pronounced. Salt ions associating with charged units are not able to completely disrupt these junctions up to a concentration of 0.1 M. Amounts of salt higher than the ones used in the present study are presumably needed to screen sufficient charges along the chain to shorten it enough in order to break the connectivity in the system, as seen in the rheological studies (Figure 4). Thus, the hydrophobic interactions between nonionized P2VP segments hamper the effect of the salt addition. In the quaternized system, in contrast, strong effects of the salt on the network structure are observed even in the range of 0.05−0.15 M of NaCl. In this system, the fraction of noncharged and therefore hydrophobic P2VP segments is lower, and electrostatic interactions play a larger role.

than the one from the nonquaternized system, which is due to the electrostatically driven stronger stretching of the P2VP blocks. The volume fraction of micelles being part of the network is with ηHS = 0.19 ± 0.06 the same as for the nonquaternized system. In the scattering curves after addition of salt, as in the nonquaternized system, the peak moves to lower q values, and the shoulder becomes weaker (Figure 8a), which reflects the structural changes. In contrast to the nonquaternized triblock polyampholyte, the size of the homogeneous spherical particles starts to grow already upon addition of 0.05 M of NaCl and increases further with increase in ionic strength (Figure 8a), as expected from simulations. The complex radius is Rsph = 14.2 ± 0.3 nm at 0.05 M and 18.5 ± 0.4 nm at 0.15 M NaCl. This indicates the effect of increasing hydrophobicity of both blocks upon screening of charges by salt ions (Figure 9a). In analogy to the non-

Figure 9. Parameters from fitting of the SANS curves of PAA109-bQP2VP819-b-PAA109 at pD 5. (a) Sphere radius, Rsph (solid circles); radius of small spheres, R′sph (open circles); hard-sphere radius, RHS (solid triangles); and correlation length, ξ (stars). (b) Volume fraction ηHS.

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CONCLUSIONS The influence of the ionic strength on hydrogels of a nonquaternized PAA109-b-P2VP819-b-PAA109 triblock polyampholyte at pD 3.0 was investigated using computer simulations, rheology, small-angle neutron scattering, and dynamic light scattering. In this system, the prevailing nature of the interactions is of hydrophobic nature because the degrees of ionization of both blocks are rather low, and the uncharged monomers are hydrophobic. To prove this hypothesis, SANS was also carried out on a reference system, namely the analogous quaternized PAA109-b-QP2VP819-b-PAA109 in D2O at pD 5.0. In this system, the degrees of ionization of both blocks are significantly higher, and electrostatic interactions are more important than in the first system. We focus on the response of the two systems on variation of the ionic strength. Computer simulations mimic the completely dissociated polyampholyte chains both at concentrations below and well

quaternized, the hydrophobic complexes tend to agglomerate to larger globules. Moreover, additional smaller spheres are again present. Their size does not depend on NaCl concentration; it lies between R′sph = 7.3 ± 0.3 nm at 0.05 M and 8.6 ± 0.5 nm at 0.15 M. These smaller spheres are assigned to small globules of collapsed P2VP segments on bridging blocks. The hard-sphere radius follows the same tendency as the complex radius, but it is higher than in the salt-free solution. The value increases from RHS = 33.6 ± 0.5 nm in the salt-free solution to 57.9 ± 1.1 nm at 0.15 M. Thus, the distance between complexes is nearly proportional to the complex size. We assume that the excess positive charge of the complexes is at the origin of these well-defined distances. The volume fraction was found at ηHS = 0.19 ± 0.06 in the salt-free system, whereas at 0.05 M of salt, the value is reduced K


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Because of the high charge density in PAA109-b-QP2VP819-bPAA109, the impact of charge screening is found to be more pronounced; hence, a gradual weakening of the network upon addition of salt is more evident. In contrast, the high hydrophobicity in PAA109-b-P2VP819-b-PAA109 makes it less sensitive to a variation of ionic strength, and the system preserves its rigidity up to higher salt concentrations. Combining several methods enabled us to identify and separate these effects of varying ionic strength.

above the percolation threshold. At low polymer concentration, the solutions of the nonstoichiometric polyampholyte are dilute at all salt concentrations studied. In the gel at higher polymer concentration, the salt addition leads to a notable increase of both the size of complexes and their distance, which is due to the reduction of the electrostatic repulsion between the bridging P2VP fragments. The same behavior is clearly observed in the quaternized triblock polyampholyte. Because of the high and constant degree of dissociation, the model chains did not form necklace conformations or extended homogeneous aggregates, which are found experimentally. The rheological properties revealed that strong physical networks with high cross-link functionality are formed. The solution viscosity drops almost 1 order of magnitude when the NaCl concentration is increased from 0 to 0.3 M. The dynamic storage modulus has a weak maximum at 0.1 M of NaCl with a further decrease upon ionic strength increase and is almost half of its initial value at 0.5 M NaCl. We attribute the weak initial increase to the increase of the number of elastically active chains under shear, whereas at higher ionic strength, the network is gradually ruptured. As in ref 53, at low salt concentrations, the strong charge of the polyelectrolyte chains is mainly responsible for the enhanced elasticity in the system. The number of charges in the present system is relatively high up to 0.15 M NaCl. We investigated the structural and dynamic behavior in this range further using SANS and DLS. For both systems, the salt-free triblock polyampholyte network consists of spherical particles from interpolyelectrolyte complexes of oppositely charged P2VP and PAA blocks. The particle radius increases with NaCl concentration. The growth of the complexes corroborates the findings in ref 52 coupled with a transition from polyampholyte characteristics to a noncharged system. In the nonquaternized system, the complexes are significantly larger at 0.15 M NaCl due to the considerably reduced effect of electrostatic interactions. In contrast, in the quaternized system, where the degrees of ionization are higher, addition of salt leads to a growth of the complexes already at a NaCl concentration as low as 0.05 M. The network structure of the two systems behaves differently upon addition of NaCl. For the nonquaternized system, which features lower degrees of ionization, the structure of the network is more stable and stays close to its initial configuration; i.e., the distance of the complexes stays the same. For the quaternized system, in contrast, this distance increases strongly with NaCl concentration. This reflects the fact that the nonquaternized triblock polyampholyte gel does not only feature interpolyelectrolyte complexes but is also stabilized by the presence of hydrophobic units along the P2VP chain. This renders the complexes more insensitive to changes of the ionic strength, which is at the origin of the mechanical stiffness of the network. Moreover, in both systems, additional small globules are observed which are formed by collapsed hydrophobic P2VP units. These may restrict fluctuations of the length of the bridges between complexes. Rotational dynamic light scattering on the nonquaternized system revealed that the complexes are very stable. Thus, internal stress cannot relax on the time scale of the experiment; i.e., the complexes are quite stable. The present study of two polyampholyte hydrogels reveals complex behavior, which depends on the molecular characteristics and the charge conditions. Two counteracting interactions were found to be at the origin of the response of the system.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01746. Methods; description of the molecular model; rotational dynamic light scattering; results; simulation results on the conformational behavior of PAA20-b-P2VP172-bPAA20 polyampholytes and P2VP homopolymers in dilute solutions; dynamical studies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected], Fax +30 2610 997266 (C.T.). *E-mail [email protected], Fax +49 89 289 12 473 (C.M.P.). Funding

We thank DAAD and IKY for financial support of mutual visits in the framework of the program for the promotion of the exchange and scientific cooperation between Germany and Greece, IKYDA 2013, and the DAAD program PPP Tschechien, financed by the Bundesministerium für Bildung and Forschung (BMBF), and the Czech Academy of Sciences. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank M. Philipp for help with the measurements. This work is in part based upon experiments performed at the KWS2 instrument operated by JCNS at the Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany. We thank this facility for providing excellent equipment. We also thank Moscow State University Supercomputer Center for the access to the supercomputer “Chebyshev”.

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