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Electrostatic Effects on the Internal Dynamics of Redox-Sensitive Microgel Systems Simona Maccarrone,*,† Olga Mergel,‡ Felix A. Plamper,‡ Olaf Holderer,† and Dieter Richter† †

Outstation at MLZ, Jülich Centre for Neutron Science JCNS, Forschungszentrum Jülich GmbH, Lichtenbergstraße 1, 85747, Garching, Germany ‡ Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany S Supporting Information *

ABSTRACT: Microgels are flexible entities with a number of properties which can be tailored for a variety of applications. For redox-sensitive PNIPAM-based microgels involved in this study, the size and effective charge of microgels can be manipulated by electrochemical means. The electrochemical switching is implemented via interaction of redox-sensitive counterions (hexacyanoferrates: HCF) with oppositely charged (cationic) thermoresponsive microgels. Effects on the internal dynamics upon uptake of HCF and increased hydrophobicity with temperature are investigated with neutron spin echo spectroscopy. The polymer segmental dynamics is well described by the Zimm model. Unbalanced charges (in absence of HCF) apparently shorten the polymer length acting like confined discontinuity points (pinning). This effect vanishes in the presence of HCF. The ability of multivalent ferricyanides to bind several monovalent polymer charges at the same time produces an apparent secondary network. This effective bridging makes the dynamics slower analogous to an increase in cross-linker density. In support of this picture, an enhanced viscosity of the medium, where the polymer chains move, was obtained by the fitting.

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INTRODUCTION Microgels (μG) are polymer networks in the colloidal scale, typically in the range of several tens of nanometers up to several micrometers.1 They have attracted considerable interest in soft matter science2 due to their adaptability,3 stimuli sensitivity,4 catalytic properties,5 rheological properties,6,7 compartmentalization,8 and porosity required for uptake abilities.9 Functionalization opens up further applications.10−12 For instance, μG with cationic,13−21 anionic charges,22−24 and polyampholytic behavior25−27 are known. However, such polyelectrolyte μGs with a considerable amount of permanent charges are rare.28−30 These polymer-bound charges interact with multivalent counterions, like linear polyelectrolytes24,31−34 and smaller counterions,35−37 which alter the polymer properties (e.g., an upper critical solution temperature behavior within μG can be induced).38 Also, the specific uptake of ions into μGs was utilized to generate nanoparticles within the network.39 Recently, we investigated the uptake of electroactive hexacyanoferrates (HCF) into thermoresponsive, cationic μGs.30 A combination of hydrodynamic voltammetry and electrochemical impedance spectroscopy gave insight into the influence of the μGs on the electrochemistry, showing a pronounced incorporation of hexacyanoferrate(III) (ferricyanide) into the μGs. At the same time, the μG size can be modulated by mere electrochemical switching.40 As these examples show that μG interact with ions, such uptake is expected to influence the dynamics of the network © XXXX American Chemical Society

chains. However, there is hardly any investigation on the changes in the internal μG dynamics upon uptake. Hence, the present study addresses the segmental dynamics of thermosensitive microgels in the presence of multivalent, redoxsensitive counterions (here: HCF). As method, neutron spin echo (NSE) proved to be a suitable approach for μGs.41−46 Recently, the dynamical behavior of collapsed and swollen neutral microgels was comprehensively described within the theory of semidilute polymers in solutions, where hydrodynamic interactions are dominant.42,46 Since the uptake and binding of molecules affect the structural40 as well as the dynamical properties of the μGs, such investigations provide an effective method to probe the above-mentioned binding effects. By using switchable complexants, this can be again the basis for designing materials with switchable properties, envisioning an electrochemical manipulation of e.g. μG stiffness and rheology.47,48

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MATERIALS AND METHODS

Materials. Dideuterium oxide (D2O) was procured from Deutero, Kastellaun, Germany. Potassium hexacyanoferrate(III) (K3[Fe(CN)6]) was obtained from Merck. Hexacyanoferrate(II) trihydrate (K4[Fe(CN)6]·3H2O) was purchased from AnalaR NORMAPUR (all p.a. Received: November 24, 2015 Revised: January 12, 2016

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DOI: 10.1021/acs.macromol.5b02544 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules quality). The synthesis of μGs is described elsewhere.30 Shortly, the microgels are made of N-isopropylacrylamide (NIPAM; 85 mol %), cross-linker N,N′-methylenebis(acrylamide) (BIS; 5 mol %), and cationic monomer N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA, 10 mol %), which yields after quaternization with methyl iodide and counterion exchange methacrylamidopropyltrimethylammonium chloride (MAPTAC). The purified and lyophilized P(NIPAM-co-MAPTAC) μGs were used for preparation of samples together with twice distilled Milli-Q water. Dynamic Light Scattering (DLS). The experiment was performed on an ALV setup equipped with a 633 nm HeNe laser (JDS Uniphase, 35 mV), a goniometer (ALV, CGS-8F), digital Hardware correlator (ALV 5000), two avalanche photodiodes (PerkinElmer, SPCMCD2969), a light scattering electronics (ALV, LSE-5003), an external programmable thermostat (Julabo F32), and an index-match-bath filled with toluene. Angle- and temperature-dependent measurements were recorded in pseudo-cross-correlation mode varying the scattering angle from 30° to 140° at 10° intervals and variation of temperature in the range of 20−60 °C at 2 K intervals and measurement time of 60 s. The samples were highly diluted to avoid multiple scattering. For data evaluation, the first cumulant from second-order cumulant fit was plotted against the squared length of the scattering vector q2. The data were fitted with a linear regression, whereas the diffusion coefficient was extracted from the slope and the hydrodynamic radius Rh calculated by using the Stokes−Einstein equation. Rotating Disk Electrode (RDE). Electrochemical measurements were performed on the CH Instruments electrochemical workstation potentiostat CHI760D (Austin, TX). For rotating disk electrode measurements, the potentiostat was connected with a rotating ring disk electrode rotator (RRDE-3A from ALS Japan). The experiments were carried out at a fixed temperature of 23 °C in a conventional three-electrode setup in a water jacketed cell connected to a thermostat (Thermo Scientific Haake A28). A platinum (rotating) disk electrode, 4 mm disk diameter, was used as working electrode, and an Ag/AgCl electrode stored in 1 M KCl served as reference electrode. Electrochemical experiments were performed on the supernatant of a centrifuged μG P(NIPAM-co-MAPTAC) dispersion (2.5 wt %) containing 1 mM K3[Fe(CN)6] and 1 mM K4[Fe(CN)6] (1:1) mixture as redox probe in a supporting electrolyte solution of 0.1 M KCl. The centrifugation experiments were performed either at 20 °C or at 40 °C. Before performing each electrochemical measurement, the working electrode was polished first with 1 μm diamond and subsequently with 0.05 μm alumina polish, rinsed with water, and dried with a stream of argon. The solution was purged with Ar for 10 min to remove dissolved oxygen. Hydrodynamic voltammograms were recorded by sweeping the potential in the range of −0.1 to 0.5 V versus Ag/AgCl at a scan rate of 5 mV s−1, whereas the rotation rate was kept constant at 1000 rpm. To correlate the limiting currents iL with the concentration of the redox probe, concentration-dependent hydrodynamic voltammograms of K3[Fe(CN)6] and K4[Fe(CN)6] in 0.1 M KCl were recorded at a fixed rotation rate of 1000 rpm. Neutron Spin Echo (NSE). μG dispersions (2.5 wt % in D2O) were prepared 1 week previous to the experiment. The measured samples were mixed from these parent dispersions shortly previous to the experiment in order to avoid oxidation/reduction of the hexacyanoferrates by air. NSE measurements have been performed by using the J-NSE spectrometer at the FRM II research reactor in Garching, Germany.49 A wavelength of 8 Å was used to probe Fourier times up to 40 ns in the q range between 0.05 and 0.18 Å−1. The samples were mounted in a thermostat controlled sample environment. The internal dynamics of quaternized μGs with and without counterions was then measured at temperatures between 20 and 60 °C. Scattering from corresponding quartz cells containing the deuterated solvent has been subtracted as background from the NSE data.

acrylamide)-derived μGs (PNIPAM μGs), copolymerized with 10 mol % dimethylaminopropyl methacrylamide. The presence of the hydrophilic comonomer leads to a shift of the volume phase transition temperature (VPTT ∼ 40 °C) compared to the one of pure PNIPAM μGs (∼32 °C). NSE experiments were done on samples at reasonably high concentration (∼25 g/L μG) to have enough signal for NSE measurements. At the same time, we are still below the formation of colloidal crystals or glasses, as the viscosity, as measured by a capillary viscosimeter, increased only by a factor of 3.5 at 20 °C upon addition of 25 g/L μG.50 This indicates freely moving microgels. In all experiments, we used a 1:1 molar ratio of tetravalent hexacyanoferrate(II) ([Fe(CN)6]4− also called ferrocyanide) and trivalent hexacyanoferrate(III) ([Fe(CN)6]3− ferricyanide) and 0.1 M KCl aqueous solution as solvent. We relate the concentration of HCFs to the concentration of μG defining the initial charge-to-charge ratio icrtotal = [chargeHCF]/ [chargeμG] as the molar ratio between the hexacyanoferrate charges compared to the microgel charges. In most cases, we used icrtotal = 0 and icrtotal = 0.5 throughout this article. The nominal charge ratio ncr might deviate from the icr in the case of weak binding, indicating that the actual complex carries less complexant as added. The μG was characterized in the first place by DLS in dilute suspension. As seen in Figure 1, the μGs collapse with increasing temperature.

Figure 1. Hydrodynamic radius of diluted microgel (μG) dispersion in dependence of temperature (in 0.1 M KCl; pH ∼ 6 at 23 °C in the absence of HCF; open symbol: heating; solid symbol: cooling). Adapted with permission from ref 30.

In a further step, the actual amount of entrapped counterions and the ncr in dependence of temperature was determined. Hereby, we choose a simple method, which relies on the analysis of the supernatant.33−35 The dispersion (25 g/L μG in 0.1 M KCl containing 1 mM K3[Fe(CN)6] and 1 mM K4[Fe(CN)6]; icrtotal = 0.5) was centrifuged at a certain temperature, the supernatant was decanted, and the HCF concentration was determined in the supernatant by help of an electrochemical method (at 23 °C). We preferred to choose hydrodynamic voltammetry by use of a rotating disk electrode (RDE), applying the Levich equation for reversible systems.51,52 We established a calibration curve to determine the concentration of ferricyanide and ferrocyanide in the supernatant (ω = 1000 rpm; 0.1 M KCl, 23 °C; scan rate ν = 5 mV/ s). This eventually allowed the recalculation of the entrapped HCF. The results are summarized in Table 1. As seen in Table 1, the [Fe(CN)6]3− ions are preferentially complexed by the μG, which was found by other electro-

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RESULTS AND DISCUSSION Thermosensitive microgels (μGs) with permanent cationic charges were prepared by quaternization of poly(N-isopropylB


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Table 1. Measured HCF Concentrations in Supernatant csupernatant and the Therefrom Derived ncr, icr, and Estimated Average Concentration of Complexed HCF inside the μG cμG (All at Total icrtotal = 0.5; 25 g/L μG) c([Fe(CN)6]3−) c([Fe(CN)6]4−) c([Fe(CN)6]3−) c([Fe(CN)6]4−)

at at at at

20 20 40 40

°C °C °C °C

csuperna [mM]

cred,supnb [mM]

icrc

ncrtotald

ncre

cμGf [mM]

± ± ± ±

0.39 ± 0.06 0 0.45 ± 0.01 0

0.21 0.28 0.21 0.28

0.08 0.08 0.10 0.10

0.08 0 0.10 0

2 0 6 0

0.61 1.04 0.55 1.05

0.06 0.06 0.01 0.08

a Concentration in the supernatant csupern. bReduction of concentration cred,supn in the supernatant compared to original concentration (c0([Fe(CN)6]4− = 1 mmol/L, c0([Fe(CN)6]3− = 1 mmol/L) due to entrapment in μG. cIon-specific icr(T). dTotal ncrtotal(T) calculated by

ncrtotal(T ) = 4cred,supern([Fe(CN)6 ]4 − ) + 3cred,supern([Fe(CN)6 ]3 − ) 4c0([Fe(CN)6 ]4 − ) + 3c0([Fe(CN)6 ]3 − )

icrtotal

e Ion-specific ncr(T). fAverage concentration cμG of complexed HCF inside μG estimated by subdividing the calculated overall HCF concentration cμG([Fe(CN)6]4−/3−) inside μG using the formula

cμ G([Fe(CN)6 ]4 − /3 − ) w(Monocat)ρ(PNIPAM solid) ⎛ R h,collapsed ⎞ ncrtotal(T ) ⎜ ⎟ M(Monocat) ⎝ R h(T ) ⎠ z Fe, μG(T ) 3

=

with w(Monocat) being the weight percentage of cationic monomer with molar mass M(Monocat) in the used microgel, ρ(PNIPAMsolid) = 1.1 kg/L being the density of solid μG, which resembles the density of the collapsed microgel with hydrodynamic radius Rh,collapsed (∼70 nm), Rh(T) equals the hydrodynamic radius at the respective temperature T taken from Figure 1; ncr(T) is the nominal charge-to-charge ratio inside the μG, and zFe,μG(T) is the average valency of the HCF inside the μG at the respective T:

z Fe, μG(T ) = 4cred,supern([Fe(CN)6 ]4 − ) + 3cred,supern([Fe(CN)6 ]3 − ) cred,supern([Fe(CN)6 ]4 − ) + cred,supern([Fe(CN)6 ]3 − )

chemical techniques before.30 This is unexpected, since the entropic gain induced by the tetravalent ferrocyanide ions is expected to be more pronounced due to a release of a higher number of monovalent counterions.35,36 However, this behavior was found before for quaternized polyelectrolytes, discussing the ion-specific effects of HCFs in terms of different polarizabilities.37,53−56 Even more, the μGs exhibit a rather drastic discrimination between both HCF. At the conditions used, ferrocyanide is basically excluded from the μG. In contrast, the μGs are enriched with ferricyanide, especially at high temperature. About half of the ferricyanide is taken up by the μGs. At 20 °C, the local concentration of μG-bound ferricyanide cμG([Fe(CN)6]3−) is 2 times higher than the original bulk concentration c0([Fe(CN)6]3−). Upon heating from 20 to 40 °C, the local concentration triples due to increased uptake and increased confinement (μG collapses). We expect that cμG([Fe(CN)6]3−) even approaches concentrations of ∼30 mM in the fully collapsed case (considerably above 60 °C). Part of this increased uptake can be traced back to the increasing charge density inside the μGs with heating and a better “matching of charges”.57 Further mechanisms are discussed in the Supporting Information. Internal Dynamics. After having seen the uptake of ferricyanide, we address the chain dynamics in the complexed and uncomplexed state, at conditions which were of relevance for our previous investigations.30,46 Here, NSE spectroscopy allows to probe dynamic motions of the order of several nanoseconds on the nanometer length scale that correspond to the internal dynamic motions in the microgel particle. The intermediate scattering function S(q,t) (i.e., the Fourier

transform of the spectral function S(q,ω)) displayed in its normalized form S(q,t)/S(q,t = 0) is then directly measured. For neutral μGs of several tens of nanometers in radius,45 NSE scattering profiles are well described by a reduced mode spectrum of Zimm segmental dynamics. Only in the case of partially collapsed μG particles, it was possible to detect the transition from local chain (Zimm type) to collective dynamics; i.e., the relaxation rate (Γ) dependence passed from the q2 to q3 regime at q*≈ 1/ξ where ξ is the mesh size of the polymer network. In the present work, we introduce permanent charges along the polymer segments. In analogy to free polyelectrolytes in dilute and semidilute solution regimes,58 the electrostatic repulsion between charged monomers stretches the chain that finally becomes stiffer. In this case, the dynamics should slow down when compared to that one of neutral μGs. The length scale of the interaction between charges can be estimated by the Debye length λD calculated as follows: λD =

εrε0kBT N

NAe0 2 ∑ j = 1 nj0qj 2

where kB is Boltzmann’s constant, n0j is the mean concentration of charges of electrolyte species j, qj2 is the square of the charge of the electrolyte species j, NA is Avogadro’s constant, e0 is the elementary charge, εr is the relative permittivity of a material, and ε0 is the permittivity of vacuum. We estimate that in our case at 0.1 M KCl λD is around 1 nm. Donnan exclusion might even increase this value inside the gel. This allows reasonable electrostatic interaction between the charges, as the average C


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Macromolecules contour length between charges is around 2.5 nm, whereas the average contour length between chemical cross-linkers is around 5 nm (other experiments indicate a mesh size in the range of 10 nm probably due to lowered cross-linking efficiency).59 Therefore, λD is in the range of the average distance of charges within the network. The profile of S(q,t)/S(q,0) is usually well described by a stretching exponential of the type S(Q , t )/S(Q , 0) = exp[−(Γt )β ] = exp[−(Dqαt )β ]

(1)

where the exponent β is 2/3 and 1 and α is 3 and 2 for the Zimm single chain and diffusive motions, respectively. A double-logarithmic plot of the decay curve, ln[−ln[S(Q,t)/ S(Q,0)]] vs ln[t], provides β as a slope, which is shown in Figure 2 for the bare charged microgel at 20 °C as an example.

Figure 3. Intermediate scattering functions of bare microgel P(NIPAM-co-MAPTAC) in 0.1 M KCl in D2O at 20, 40, and 60 °C for different q values (from the top 0.05, 0.08, 0.11, 0.15, and 0.18 Å−1). The solid lines are the fitting curves with eq 2.

the Zimm model.60 The dynamics of a Gaussian chain is described in terms of a bead−spring model adding the hydrodynamic interaction between the chain segments in terms of a simple Oseen tensor approach. The internal motions including rotational diffusion of a finite chain consisting of N beads connected by entropic springs with a uniform bead distance l are described by relaxation modes with mode number p and characteristic times τp:

Figure 2. Double-logaritmic plot of S(q,t)/S(q,0) of bare microgel P(NIPAM-co-MAPTAC) in 0.1 M KCl in D2O at 20 °C for different q values (from the bottom 0.05, 0.08, 0.11, 0.15, and 0.18 Å−1).

τp =

At all three temperatures (20, 40, and 60 °C), we observe a crossover from diffusive motions to Zimm dynamics (from slope 1 to 2/3 ∼ 0.67) at t around 15 ns like in Figure 2. In Figure 3, we show the intermediate scattering functions S(q,t)/S(q,0) of bare cationic μGs for different q values between 0.05 and 0.18 Å−1 and three different degrees of swelling (three temperatures above, at, and below VPTT). The fitting was done with a linear combination of diffusive motions and Zimm single chain dynamics in the asymptotic form of a stretched exponential:

where Re corresponds to the mesh size of the polymer network and η is the solvent viscosity. Inserting v = 0.5, S(q,t) is computed for a Gaussian chain: N

S(q , t ) =

2

∑ e −q D

CM t − (q

2

/6)B(m , n , t )

(3)

m,n

with B(m , n , t ) = |n − m|2ν l 2 +

2

S(Q , t )/S(Q , 0) = A exp[−(Dq t )] + (1 − A) × exp[− (KZq3t )0.67 ]

η R e 3 − 3ν p 3π kBT

(2)

4R e 2 π2

pmax

∑ p=1

⎛ πpn ⎞ 1 ⎟ cos⎜ ⎝ N ⎠ p 2ν + 1

⎛ πpm ⎞ −1/ τp ⎟(1 − e × cos⎜ ) ⎝ N ⎠

where D represents diffusion coefficient associated with the short-time decay while KZ is the coefficient related to the local Zimm-like motions.60 From the fitting, we obtained D = 4.23 ± 0.12, 3.4 ± 0.13, and 2.60 ± 0.15 Å2/ns for 20, 40, and 60 °C, respectively. This trend reflects the (partial) collapse of the chains. The coefficient A decreases with T at a fixed q value; specifically at 20 °C this contribution accounts for the 30% of the total S(q,t), then decreases to 20% at 40 °C, and finally 10% at 60 °C. In order to obtain a more detailed description at longer time scales, we use in a second step the complete expression of

The center-of-mass diffusion assuming a Gaussian chain (Θsolvent) is61 DCM = 0.196

kBT R eη

for a good solvent the prefactor is 0.203 instead of 0.196. The maximal number of modes pmax was set to 20. Further increase of the number pmax did not change the results; higher modes do not contribute to the dynamics. D


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Macromolecules In order to improve the quality of the fit, we left DCM as the only fitting parameter; we fixed the viscosity η as the one of the solvent and Re = 3.1 nm as obtained from the 60 °C data set with the smallest error obtained of ±0.42. From a simultaneous fit, we obtained the center-of-mass diffusion coefficient DCM 3.27 ± 0.05, 2.81 ± 0.03, and 1.36 ± 0.13 Å2/ns for 20, 40, and 60 °C, respectively. It is interesting to note that Re is unlikely to correspond to the mesh size of the network as for neutral microgels. It rather corresponds to the distance between charges (estimated 2.5 nm). Charged monomers along the chain act like discontinuity points for the dynamics: they effectively reduce the Zimm length scale Re breaking the polymer chains in shorter segments. Additionally, those shorter segments move like free polymer in solutions without fixed ends: we did not need to cut out the odd modes in the Zimm summation like in ref 46 that are not possible for polymer with fixed ends (cross-links). We also tried to fit the data with a stretched exponential of the type like eq 1 with α = 2.6 and β = 0.75 which are typical parameters for a stiff chain. The quality of the fitting curves is worse especially at shorter times and higher q values. Because of the short distance between two charges (3 nm) and a comparable Debye length (1 nm), we are not able to more accurately distinguish between stiff dynamics and Zimm dynamics on this very short length scale of the order of few nanometers. In Figure 4 we show the intermediate scattering functions S(q,t)/S(q,0) of cationic μG with loaded counterions for

polymer segments move. The correspondent values for DCM at 20 and 40 °C are 3.08 ± 0.01 and 1.6 ± 0.1 Å2/ns. For the first time to the best of our knowledge, the internal dynamics of microgels with statistically distributed unbalanced charges was measured and characterized. The Zimm-like segmental dynamics is preserved, but the charged groups along the polymer segments act like discontinuity points reducing the polymer segment length that relaxes. In the time scale of the NSE experiment, an additional diffusive motion is necessary to describe the full decay of the intermediate scattering function. In our opinion, it cannot be identified with the “breathing modes” or collective dynamics of the network (as observed with dynamic light scattering by Tanaka62 and later by Geissler63) for the following reasons: (i) the microgel mesh size (estimated around 10 nm) is too big for NSE q-range to see them (the crossover between local and collective dynamics should occur at around 0.01 Å−1 ∼ 1/ξ), (ii) the values of D are 10 times bigger than the breathing modes observed for partially collapsed PNIPAM microgels,46 and (iii) the diffusive contribution we detect disappears completely at all q values with HCF absorption. This is certainly related to the presence of charges, and it can be explained as a cooperative motion of the virtually pinned network of charged monomers due to electrostatic repulsion of different charges. The addition of HCFs counterions maintains Zimm-like motions as expected, but the relaxation of the polymer chains occurs slower when S(q,t)/S(q,0) is compared at same q and T (Figure 5). The addition of HCF leads only to a total 36% rise

Figure 4. Intermediate scattering functions of bare microgel P(NIPAM-co-MAPTAC) in the presence of hexacyanoferrates (icrtotal = 0.5; c([Fe(CN)6]4−)/([Fe(CN)6]3−) = 1) in 0.1 M KCl in D2O at 20 and 40 °C for different q values (from the top 0.05, 0.08, 0.11, and 0.15 Å−1). The solid lines are the fitting curves with the Zimm model.

Figure 5. Comparison of intermediate scattering functions of bare microgel P(NIPAM-co-MAPTAC) with (solid symbols) and without hexacyanoferrates (empty symbols) at icrtotal = 0.5 and c([Fe(CN)6]4−)/([Fe(CN)6]3−) = 1 in 0.1 M KCl in D2O at 20 °C for different q values (from the top 0.05, 0.08, 0.11, and 0.15 Å−1).

different q values between 0.05 and 0.15 Å−1 and two different degrees of swelling (two temperatures). In this case, we performed the fitting using the full Zimm summation (eq 3) since the double-logarithmic plot showed only one slope of close to 2/3. The free parameters were the center-of-mass diffusion DCM, the viscosity η, and Re. The data are best fitted with Re 3.04 ± 0.22 and 3.88 ± 0.19 nm for 20 and 40 °C, respectively. The viscosity is almost 3 times the one of the solvent, 0.0029 ± 0.0009 and 0.0019 ± 0.0004 Pa·s for 20 and 40 °C, respectively. This can be justified by the presence of the HFCs inside the μG pores that effectively increase the microviscosity of the medium (solvent + HCF) where the

in ionic strength, translated in a mild 16% reduction of λD. Hence, the mild change in ionic strength does not account directly for the very different behavior of the μG dynamics. The major differences are seen at longer times and smaller q values (0.05 and 0.08 Å−1); that is where the Zimm motions are predominant. The preferred absorbed ferricyanide can partially compensate polymeric charges on the time scale of the NSE experiment by e.g. (dynamic) bridging/counterion condensation, reducing the effective charge of the backbone. Concomitantly, this creates a sort of localized secondary network that has an analogous effect of increased cross-linker density.44 Nevertheless, the reduction in effective charge E


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(SFB 985) “Functional Microgels and Microgel Systems”. The authors are thankful to Prof. Roland Winkler (IAS, Forschungszentrum Jülich) for fruitful discussions.

reduces the pinning effect, as seen in the disappearing of the diffusive mode in the absence of HCF. At the same time, incorporated HCF increase the microviscosity experienced by the polymer chains.

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CONCLUSION Redox-sensitive PNIPAM-based microgels are studied by combined electrochemical measurements and neutron spin echo spectroscopy. The internal dynamics of the charged polymer network inside μG particles is well described by the Zimm model and an additional diffusive mode. Unbalanced charges (in absence of HCF) apparently shorten the polymer length acting like confined additional cross-linkers (pinning effect). This effect does not vanish with increasing temperature, i.e., with increased hydrophobicity. The additional diffusive contribution can be identified with a cooperative motion of the pinned charged monomers onto a virtual lattice due to electrostatic repulsion of different charges (depicted in Scheme 1, left).

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Scheme 1. Pinned Network of Charged Monomers inside a Microgel Particle (Left) and Apparent Secondary Network of Absorbed Ferricyanide inside a Microgel Particle (Right)

The selective adsorption of trivalent ferricyanides which are able to bind at the same time several monovalent polymer charges produces an apparent secondary network (Scheme 1, right). This effective bridging makes the dynamics slower. Supporting this picture, the viscosity of the medium, where the polymer chains move, is enhanced, as obtained by the fitting.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02544. Comment on discrimination between HCF (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel +49-89-289-13805; Fax +49-89-289-10799; e-mail s. [email protected] (S.M.). Present Address

S.M.: Outstation at MLZ, Institute of Physical Chemistry, RWTH Aachen University, Lichtenbergstraße 1, 85747, Garching, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich F


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