Electrostatic Effect on the Solution Structure and Dynamics of

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Electrostatic Effect on the Solution Structure and Dynamics of PEDOT:PSS Michael A. Leaf and Murugappan Muthukumar* Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, Massachusetts 01002, United States S Supporting Information *

ABSTRACT: Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) is a material consisting of two oppositely charged polymers and is available as aqueous dispersions. Various annealing and doping methods dramatically enhance PEDOT:PSS conductivity, rendering it competitive with inorganic conductors like indium tin oxide. Yet a comprehensive understanding of PEDOT:PSS conductivity enhancement remains elusive. To unravel the chief physical interactions at play, we explore aspects of solution-state PEDOT:PSS through light scattering, X-ray scattering, rheological measurement, and the construction of a partial phase diagram. We show these features in neat water and with the addition of several conductivity enhancement agents: dimethyl sulfoxide, ethylene glycol, 1-ethyl-3-methylimidazolium tetrafluoroborate, or sodium chloride. We find that PEDOT:PSS solutions exist as a dispersion of charged, many-chain microgels with structural features and dynamics strongly influenced by electrostatic interactions. Two distinct phase transitions occur with high sensitivity to ionic strength: a coexistence of a dilute and a concentrated PEDOT:PSS phase and a physical gelation. Despite the rich solution properties uncovered here, we find no connection between them and the enhancement of film conductivity.

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INTRODUCTION The need for high-performance organic electronic materials is spurred in part by increasing public interest in solar energy initiatives and the demand for other cost-effective electronic devices. Ideal properties of device components are applicationspecific, but many devices require a component with high conductivity. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) is a polymeric material1,2 extensively used in devices such as solar cells,3−6 light-emitting diodes,7 thermoelectrics,8−11 and field-effect transistors12,13 for its high conductivity. It is under widespread investigation as a potential substitute for the standard inorganic electrode indium tin oxide (ITO). PEDOT:PSS is typically sold and stored as aqueous suspensions of oligomeric PEDOT, in a cationic oxidation state, bound electrostatically to PSS. The typical repeat structures of this material are shown in Figure 1. The introduction of PSS stabilizes the otherwise insoluble PEDOT in water, allowing for the economical casting of PEDOT:PSS into films with high transparency and conductivity. Pristine PEDOT:PSS films possess bulk direct current conductivity as high as about 1 S/cm, which is significantly lower than the conductivity of ITOabout 3000 S/cm.14 Fortunately, there are a wide variety of treatments to enhance PEDOT:PSS conductivity.15 These include thermal annealing,16−18 mechanical shear,19,20 strong acids,21,22 cosolvents to water such as dimethyl sulfoxide (DMSO) or ethylene glycol (EG),5,23−30 imidazolium-based ionic liquids, 31−34 and salts.35,36 Incredibly, conductivities as high as 4600 S/cm have been reported by some of these methods,20,22 which is an © XXXX American Chemical Society

improvement of 4 orders of magnitude over untreated PEDOT:PSS films. It is unsurprising that PEDOT:PSS is under extensive study to understand the mechanism of conductivity enhancement. Some studies report an increase in PEDOT rigidity, resulting in its structural transition from coil-like to rod-like.37 Many others mention the partial phase segregation between PEDOT and PSS.28−30,35,36,38−41 In both cases, a structural rearrangement of PEDOT is believed to reduce local barriers to charge transport,42 thus increasing conductivity. The typical film morphology of PEDOT:PSS, consisting of PEDOT-rich, semicrystalline conducting nanodomains separated by insulating PSS-rich coronas,26,39,43−45 is consistent with this picture. In situ grazing incidence wide-angle X-ray scattering measurements suggest that the solvent evaporation process dictates much of the resulting film structure.46 Despite these many efforts, a holistic description of why these structural changes occur, and why they occur for such a variety of treatments, remains elusive. This is in part because we expect the physical behavior of this material to be quite complex. For instance, it may be dictated by a combination of electrostatic interactions, conformational flexibility of PSS, solvency of each component, and PEDOT crystallinity. These effects will influence the equilibrium film morphology and also the processing from solution to film, so fundamental knowledge Received: April 9, 2016 Revised: May 14, 2016

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Figure 1. Chemical structure of PEDOT:PSS. mM and the cosolvent species (EG, DMSO) in concentrations from 100 to 1500 mM (1−10 wt %). Miscibility of each of the four components in water was verified by the lack of an increase in scattered light intensity. Milli-Q water (Millipore) was used as a diluent and toluene (Sigma-Aldrich) was used as calibration standard in light scattering measurements. No further purification was performed on any materials unless explicitly noted. Phase Space Determination. For a PEDOT:PSS concentration of 0.5 mg/mL, solutions of various dopant concentration were prepared and allowed to equilibrate for several days at room temperature. If any phase segregation occurred, samples were centrifuged at 5000 rpm (estimated acceleration of 2800g) for 30 min in 15 mL of polypropylene tubes. The relative masses of the supernatant and sediment were measured, and the supernatant was extracted. The concentration of PEDOT in the supernatant was determined by a UV−vis absorbance at 600, 650, 700, and 750 nm by a UV−vis spectrophotometer with a quartz 10 mm cuvette using calibration standards of known PEDOT:PSS concentration in neat water for each wavelength. An average of the four apparent concentrations was taken as the true measured PEDOT concentration. The absorbance of PSS, which exhibits a notable UV absorbance profile, was found not to impact absorbance at these wavelengths. Similarly, the concentration of PSS in the supernatant was determined by UV absorbance at 250, 257, 263, and 268 nm, which all correspond to a broad peak associated with absorbance of PSS. An average of the four apparent concentrations was taken as the true PSS concentration using the same calibration standards. PEDOT was assumed to not contribute significantly to the absorbance at this wavelength. For all measured absorbance values, the contribution from the solvent was subtracted. We assume weak coupling between the absorbance contributions of the polymer and the solvent in applying our polymer concentration calibrations. By mass conservation, the PEDOT and PSS concentrations in the sediment were determined, and a coexistence curve was plotted with varying dopant concentration. The dopant concentration in each phase was undetermined, and the total ratio of PEDOT to PSS was not varied, so we consider this construction to be a partial phase diagram which illustrates the phenomenological phase behavior. In certain homogeneous phase conditions, a gelation transition was observed. In these instances, rheometry was employed to distinguish liquid-like from gel-like behavior and is discussed in the Experimental Section. Light Scattering. Light scattering measurements were made by an ALV-5000E correlator (ALV, Langen Germany), which contained 288 channels and was mounted on a goniometer. A variable-power argon laser with a power of 600 mW was used as the incident light source. Measurements were taken at angles ranging from 45° to 120°. Scattered light intensity (I) and the normalized intensity autocorrelation function g(2) = ⟨I(t)I(t + τ)/I(t)I(t)⟩ were simultaneously measured. Scattering intensity was converted to the Rayleigh ratio R by subtracting contributions from the background and solvent, followed by calibration with a toluene standard. Data for g(2) were converted to the absolute values of the normalized field amplitude (E) autocorrelation function g(1) = ⟨E(t) E(t + τ)/E(t)E(t)⟩ by the Siegert relation.50 This relationship is

of them is crucial. The majority of research in this area focuses on the film state, rendering it difficult to assess the role of underlying effects. The solution state, which facilitates the assessment of fundamental interactions, has been briefly studied through a particle size characterization47 and spectroscopy analysis.48 Also, a small-angle neutron scattering analysis on solution PEDOT:PSS with an ionic liquid has revealed a structural character like that of semidilute polyelecrolyte solutions.49 However, more study is needed to understand the fundamental behavior of this solution. This knowledge will equip us to better understand PEDOT:PSS in both films and solutions and may offer explanatory power for the curious enhanceability of film conductivity. For this reason, our primary goal is to understand the solution behavior of PEDOT:PSS; we specifically consider the phase behavior, morphology, and dominant intermolecular interactions. In this work, we explore phase behavior and some properties of PEDOT:PSS solutions through scattering techniques and rheometric measurement in order to unravel the interactions at play. We show these features in neat water and highlight changes that occur with the addition of DMSO, EG, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4), or sodium chloride (NaCl), which are considered secondary dopants to PEDOT:PSS conductivity. We classify the former two as cosolvents and the latter two as ion pairs, and all except NaCl are known to improve film conductivity.15,31,35 In particular, we note any phase transitions that occur using UV−vis spectroscopy to quantify polymer concentration and rheometry to demonstrate an interesting physical gel transition. We perform rigorous dynamic light scattering of dilute PEDOT:PSS solutions for different dopant concentrations to understand dynamic processes. Laser light scattering in dilute solution and X-ray scattering in semidilute solution are also applied to reveal some structural features. Interparticle interactions are probed by determining the effect of PEDOT:PSS concentration on the solution viscosity in the presence of each dopant. Lastly, any changes in solution are then correlated with film conductivity improvement due to the addition of each dopant.

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EXPERIMENTAL SECTION

Materials. Using Milli-Q water (Millipore, Billerica, MA, conductivity of 55 nS/cm), samples were prepared by dilution of the PEDOT:PSS product Orgacon ICP1050 (Agfa Gevaert NV) to various polymer concentrations ranging from 0 to 11 mg/mL. This grade is rated for ultrahigh conductivity and possesses a PEDOT:PSS mass ratio of 5:8 and a polymer concentration of 11 mg/mL. Solution conditions were altered as needed by the addition of another solute. These were composed of EMIM BF4, EG, DMSO, and NaCl (SigmaAldrich, St. Louis, MO). The ionic species (EMIM BF4, NaCl) were added to ICP1050 solutions in concentrations ranging from 5 to 500 B

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Macromolecules g(1)(τ ) =

|g(2)(τ ) − 1|

to convert to relative intensity to absolute intensity, the two curves corresponding to different sample−detector distances were superimposable on each other without the use of a shift factor. The two regimes met at a q value of 1 nm−1. Viscometry. The capillary elution time of diluted PEDOT:PSS and extracted PSS was measured at various concentrations using a type 75 Ubbelohde viscometer (Cannon, State College, PA). All experiments were performed at room temperature, which was consistently measured as 20 °C. The viscosity ratio of ICP1050 compared to the solvent was used to calculate the reduced viscosity ηr by the expression

(1)

The decay time here is τ and the observation time is t. Data for g(1) were then fit to multiple exponential functions such that the fit residuals were negligible and unsystematic. The functional form of this fit is given by

g(1)(τ ) = A 0 +

∑ Ai exp(−Γiτ) i

(2)

Here, A0 is a baseline, and Ai is the intensity weighting of mode i, each having a characteristic decay rate Γi. For modes of field amplitude relaxation that correspond to diffusive processes, Γ is related to the scattering vector q by Γ = Dq2, where D is the diffusion coefficient. In the case of this relation’s validity, D was calculated by averaging the quantity Γ/q2 across all q. It is alternatively possible to calculated D as the linear slope of Γ with q2. Both methods yielded the same results for these experiments. For diffusive modes of relaxation, D was converted to a hydrodynamic radius Rh by the Stokes−Einstein relation. Rh =

kBT 6πηD

ηr =

⎞ 1 ⎛ tp ⎜ − 1⎟ cp ⎝ t0 ⎠

(4)

The PEDOT:PSS mass concentration is cp, the elution time of the PEDOT:PSS solution is tp, and the elution time of the solvent is t0. Bulk density changes between the various solutions were assumed negligible for a given solvent. Five solvent conditions were selected: neat Milli-Q water, 15 mM EMIM BF4, 15 mM NaCl, 500 mM EG, and 400 mM DMSO. These experiments were also used to determine the viscosity of the cosolvent solutions for dynamic light scattering analysis. Rheometry. All measurements were made on a TA Instruments AR-2000 rheometer using a parallel plate geometry with a 40 mm aluminum top plate, a gap thickness of 1 mm, and a maintained temperature of 20 °C. While accounting for frictional losses and instrument inertias, the mechanical response was measured in small, oscillatory strain conditions for various concentrations of ICP1050 in pristine aqueous conditions and with the addition of various concentrations of either EMIM BF4 or NaCl. Pristine ICP1050 solution was added directly to the rheometer bottom plate (2 mL), and the desired quantity of a 1 M solution of either EMIM BF4 or NaCl was added and stirred for several minutes before the top plate was positioned and excess material was trimmed. A frequency range of 0.1−100 Hz was considered, with a strain of 1%. At each frequency, the sample was allowed 20 s of equilibration under the controlled strain condition, and the resulting stress response was subsequently measured for 40 s. Film Preparation and Conductivity Measurement. The film substrates were prepared from microscope glass slides, cleaned by sequential sonication in a Mucasol (Sigma-Aldrich)/deionized water solution, acetone, and isopropanol, dried in an oven, and subjected to a 15 min UV-ozone treatment. Four small square gold electrodes were attached on the cleaned substrate by thermal evaporation such that the electrodes were collinear and equally spaced. The gold layer was 50 nm in thickness, and a 7 nm chromium layer was deposited between the gold and glass to improve cohesion. The substrate was then subjected to another 15 min UV-ozone treatment, and solution PEDOT:PSS films were prepared on top by spin-coating. The spincoating was performed under a nitrogen blanket at room temperature with a spinning speed of 2000 rpm for 2 min. The wet film was then dried at 150 °C for 30 min on a hot plate and stored under nitrogen until testing. Films cast from five solutions were prepared: pristine ICP1050 (11 mg/mL PEDOT:PSS in water), ICP1050 with 640 mM DMSO, ICP1050 with 800 mM EG, diluted ICP1050 (5 mg/mL) in 35 mM EMIM BF4, and diluted ICP1050 (5 mg/mL) in 35 mM NaCl. Components were first filtered before mixing using 0.8 μm cellulose acetate membranes (Millipore) with a Nalgene housing. Films were prepared within minutes of mixing to minimize the influence of phase transition events that may have occurred for the case of 35 mM EMIM BF4 or NaCl. Bulk conductivity values for each film were determined at room temperature by measurement of film thickness using a profilometer (KLA Tencor, Milpitas, CA) and of sheet resistance using a Keithley 4200 SCS characterization system (Keithley Instruments, Cleveland, OH). Sheet resistance was determined using tungsten probes attached to the gold electrodes by the standard collinear four-point probe technique. Sheet resistance was measured using a range of applied

(3)

The solvent viscosity is η, T is temperature which was kept at 20 °C, and kB is the Boltzmann constant. PEDOT:PSS was diluted to a polymer concentration of 0.3 mg/mL in all light scattering experiments. This solution was filtered by a cellulose acetate membrane with a Nalgene housing and a pore diameter rating of 0.8 μm (Thermo Fisher Scientific, Waltham, MA). This filter size was selected as a compromise between a larger filter which ineffectively removed dust and other contaminants and a smaller filter which removes a large portion of PEDOT:PSS from the filtrate and significantly decreases the average particle size. Since the filters may have removed some material from the sample, the true concentrations were probably somewhat lower than 0.3 mg/mL. The filtrate was directly added to a dust-free glass culture tube and immersed in the p-xylene bath of the light scattering instrument. Any dopants were separately filtered and added to the postfiltered PEDOT:PSS so that any dopant-induced aggregates would not be removed by the filter. Results from both static and dynamic light scattering were collected for PEDOT:PSS in the presence of each of the four considered additives in various concentrations. Dynamic light scattering data were similarly collected for poly(styrenesulfonic acid) (PSS), which was extracted from PEDOT:PSS by addition of NaCl. The PSS extraction was performed by adding a solution of 1 M aqueous NaCl to ICP1050 such that the polymer concentration was 7.7 mg/mL and the NaCl concentration was 0.3 M. After 24 h of mixing, the solution was spun in an Eppendorf centrifuge at 13 000 rpm (estimated acceleration of 7500g) for 30 min. The supernatant, which contained no PEDOT because it did not absorb visible light, was extracted, filtered through a membrane with a pore diameter rating of 0.8 μm, and dialyzed against Milli-Q water for 4 days. The dialyzed supernatant was lyophilized in a 0.07 mbar vacuum for 2 days. The resulting powder, which was pure PSS as verified by UV−vis spectroscopy, was weighed and resuspended in Milli-Q water, and 800 mM NaCl was added in order to screen electrostatic interactions for scattering measurements. X-ray Scattering. X-ray scattering was performed using a Ganesha SAXS instrument (Molmex Scientific, Northampton, MA) on aqueous PEDOT:PSS with a concentration of 10 mg/mL in the presence of five aqueous solvent conditions: neat Milli-Q water, 50 mM EMIM BF4, 50 mM NaCl, 1600 mM EG, and 1300 mM DMSO. Each sample was contained in a button cell with mica windows through which the beam passed. Measurements were collected for 10−50 min each and, depending on signal noise, at two different sample−detector distances to provide a broad q range (0.06 < q < 30 nm−1). This range covers both a small-angle and wide-angle scattering regime. Radial averages of sample intensity were calculated and were corrected based on sample thickness, beam transmission through the sample, and incident beam intensity. The corrected scattered intensity of the solvent was subtracted from the solution signal. Since all corrections were applied C

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Macromolecules voltages, 10−100 mV, to verify that the film yielded an Ohmic response. For the sake of comparison with films, the conductivity of an 11 mg/mL PEDOT:PSS gel with 70 mM EMIM BF4 was measured using a Tetracon 325 conductivity cell (WTW, Weilheim, Germany) at room temperature.

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RESULTS AND DISCUSSION Phase Behavior. We find that two distinct phase transitions occur; PEDOT:PSS undergoes a phase segregation at high ionic strength, through adding either NaCl or EMIM BF4, and undergoes a gelation at moderate ionic strength in sufficiently high concentration. Figure 2 shows this phase separation

Figure 2. Partial phase diagram of PEDOT:PSS, showing the coexistence (red circles) of a polymer-rich and polymer-poor phase at high ionic strength and also the gel transition, which distinguishes liquid-like behavior (blue triangles) from gel-like behavior (violet inverted triangles). Data for both EMIM BF4 and NaCl additions are included, resulting in identical phase behavior. The dotted lines represent visually determined estimates for the transition thresholds. The inset plots the mass ratio of PEDOT to PSS in each phase for different ionic strength.

Figure 3. (a) Magnitude of the complex shear modulus G and (b) the tangent of the phase lag δ for various oscillatory strain frequencies with a magnitude of 1%. Samples are 10 mg/mL PEDOT:PSS with various quantities of either EMIM BF4 or NaCl. Error bars reflect the standard deviation from measurements of three replicate gels.

behavior in detail for various ionic strengths, contributed from NaCl or EMIM BF4. The Figure 2 inset shows the mass ratio of PEDOT to PSS in each phase for different ionic strengths, which gives an indication of the polymer composition. It shows that essentially all of PEDOT and much of PSS partitions into a polymer-rich phase upon phase separation, and some PSS still remains in the polymer-poor phaseapproximately 0.1 mg/ mL. Also included in Figure 2 are data in the one-phase region where we draw a distinction between liquid-like and gel-like behavior, as determined by rheometry. From these measurements, we take the liquid-gel transition point as when tan δ = 1 at 10 Hz. Typical linear viscoelastic responses used in the construction of the liquid/gel regions of the phase diagram are shown in Figure 3 for various EMIM BF4 and NaCl concentrations. A distinct increase in the complex shear modulus G and a decrease in the tangent of the phase lag δ is observed. There is a weak frequency dependence at high ionic strength, indicating that this material is physically crosslinked. There is a strong frequency dependence at low ionic strength which is consistent with a liquid-like response. The phase space is identical using either NaCl or EMIM BF4, but no phase transition is observed for DMSO or EG additions. This highlights the electrostatic nature of this system. To our

knowledge, the peculiar physical cross-linking of PEDOT:PSS has not been previously discussed in the literature. We further study this phenomenon in the remainder of this article by exploring the structure and dynamics in both the liquid and the gel regions of the phase space. Structure. Given the range of compositions and viscoelastic properties exhibited by PEDOT:PSS, it is natural to ask what the polymer structure resembles. We first consider dilute and low ionic strength conditions, which falls into the liquid region of the phase diagram. Dynamic light scattering measurements of Rh for dilute PEDOT:PSS shows a slow mode of motion that scales with q in a manner consistent with the Brownian motion of large particles. However, we observe a second, faster dynamic process that is not diffusive in nature and may be due to the complex internal motions of the particles. A typical g(1)(τ) function, and its decomposition into two exponential decays, is provided in Figure 4. The dependence of the fit parameter Γ on q is given as an example in Figure 5 for both the slow and fast modes. We find that the Rh for the large particles in neat water is 250 nm. Such large, well-defined particles must be composed of many polymer chains. To give a size comparison between primary particles and individual PSS chains, dynamic light scattering characterization of the same PSS in 800 mM NaCl D

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Figure 4. Autocorrelation of the electric field amplitude g(1) at 90° for 0.3 mg/mL PEDOT:PSS in neat water (red) with a biexponential fit to the data (black). The two exponential curves that compose this fit are shown. The dominant mode (blue) corresponds to the Brownian motion of large particles, and the secondary mode (pink) corresponds to a faster nondiffusive process.

yields an Rh of only 9.3 nm. Because these particles are so large, and exhibit gel-like viscoelastic properties, we designate them as charged microgels. We also find that the microgels can change in size upon solute addition. The Rh of PEDOT:PSS microgels is plotted against aqueous solute concentration in Figure 6. All four dopants are considered as solutes: NaCl, EMIM BF4, DMSO, and EG. The data show that Rh decreases with ionic concentration. This decrease is consistent with either deswelling or a breakup of microgels into smaller entities, but we cannot distinguish the two here. In either case, the substantial shrinkage due to such a small addition (5 mM) suggests that electrostatic screening influences microgel size. By contrast, Rh has no significant dependence on cosolvent concentration, even for generous additions (up to 1500 mM). Because they are so large, we probe the internal structure of the microgels by laser light scattering at a dilute PEDOT:PSS concentration (0.3 mg/mL) and X-ray scattering at a semidilute concentration (10 mg/mL) in both the one-phase regime and in the polymer-rich phase that emerges from phase segregation. A summary of these results is given in Figure 7. The X-ray scattering intensity profile for neat PEDOT:PSS monotonically decreases with the q but includes a broad shoulder around 0.5 nm−1. This scattering pattern is consistent with a semidilute polyelectrolyte mesh in addition to a fractal scaling of intensity at low q which we believe is characteristic for these microgels. Fits consistent with this interpretation are plotted with the Xray scattering data of PEDOT:PSS. The four-parameter form of this fit is given as follows for low-salt conditions. I(q) = Aq−n +

Figure 5. Decay rate Γ versus squared scattering vector q2 for the (a) slow and (b) fast modes of relaxation for 0.3 mg/mL PEDOT:PSS in neat water. Error bars reflect standard deviation from four replicate samples over 10 angles. The red line in (a) is a best fit with an intercept at the origin and illustrates that this mode of relaxation is diffusive in nature. The value of this slope can be used to determine the diffusion coefficient D, which in this case agrees with alternatively calculating D as an average of Γ/q2 over all data points. The data in (b) show that this mode is not diffusive.

The parameters A and B are weightings, ξ is the polyelectrolyte mesh size, κ is the inverse Debye length, and n is the fractal dimension of the scattering object for 1/l > q > 1/Rg, where l is the chain segment length and Rg is the radius of gyration. The second term of this equation is the theoretical expectation of a semidilute polyelectrolyte. The best fit for PEDOT:PSS in neat water yields the following parameters: n = 2.6 and ξ = 4.0 nm. Parameters for the other data sets are similar. The partial deviations in this behavior from theoretical predictions is permissible when considering the several nonidealities of this material: two species of oppositely charged polymers are present, and PEDOT has inherently poor solvency. Here, we merely point out the consistency of the data with this structural description. Laser light scattering data are also included on the graph for much lower q, and the data for intensity are arbitrarily scaled since the source of scattering contrast is different. The scale factors are chosen to illustrate the apparent continuity between the light scattering and X-ray scattering regimes. The

Bq2 1 + (qξ)4

(5)

A five-parameter fit must be used for the high-salt conditions, given as follows. I(q) = Aq−n +

B (q 2 + κ 2 ) 1 + q 2 (q 2 + κ 2 )ξ 4

(6) E

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viscoelastic properties and phase behavior. We speculate that the internal microgel structure is also similar in dilute conditions at large q, though we could not access these conditions due to contrast limitations. Most likely, the microgels simply aggregate when they phase separate at high ionic strength. An illustration of our morphological interpretation of PEDOT:PSS is provided in Figure 8. This is

Figure 6. Hydrodynamic radius Rh of PEDOT:PSS microgels in water for various solute concentrations. Error bars reflect the standard deviation from three replicate samples with 10 angles per replicate.

Figure 8. Cartoon of the morphology of PEDOT:PSS in the liquid and gel states, where red indicates PEDOT oligomers and blue indicates PSS chains. In both cases, PEDOT:PSS collects into manychain charged microgels whose structure is largely invariant. Physical cross-links exist in both states, represented as yellow circles, but in the gel case these cross-links connect the microgels into a macroscopic network.

consistent with prior interpretation of PEDOT:PSS structure,46 but the microgel nature of this material is our new contribution. There is an interesting resemblance between these swollen microgels in solution and the dehydrated core−shell sphere model proposed by Takano et al. in films.39 Good agreement also exists between our interpretation and the structured aggegates discussed by Gangopadhyay et al. in solution and upon lyophilization,47 giving further support to both interpretations. At wide angles, no interesting observations are noted for the excess X-ray scattering intensity. The inset of Figure 7 shows total absolute scattering intensity without background subtraction for PEDOT:PSS and compares it with that of pure water. There is a sharp peak at 15 nm−1 which is from the mica sheet encasement and can be ignored. The curves possess a broad peak and are nearly identical, indicating no significant change in the angstrom-scale arrangement of water. Wide-angle X-ray scattering curves for different solvent conditions, provided in the Supporting Information, also show little change due to the presence of PEDOT:PSS. The lack of well-defined ordering at this length scale implies that PEDOT crystallization, which is known to occur in the film state, is absent in solution. Interparticle Interactions. We probe the nature of interaction between microgel particles in dilute conditions by considering the concentration dependence of PEDOT:PSS reduced viscosity, shown in Figure 9 for the presence of each dopant. For neutral particles in dilute solution, we expect a positive, linear slope with a positive intercept that is described by the Huggins equation.

Figure 7. Excess scattering intensity of PEDOT:PSS in various solvent conditions by light scattering (0.01 < q < 0.03 nm−1) in 0.3 mg/mL dilute solution and X-ray scattering 0.04 < q < 4 nm−1) in 10 mg/mL semidilute solution. Data are vertically shifted for visual clarity. The intensity of the light scattering data is arbitrarily shifted to align with the X-ray scattering data to illustrate profile continuity. A composite fractal scattering and polyelectrolyte mesh model is applied to each data set and shown as a best fit line. The inset shows the total X-ray scattering intensity at high angles for the cases of aqueous PEDOT:PSS and pure water. The broad peak corresponds to water structure. The sharp peak is an artifact from the mica sample casing.

fractal dimension measured in the light scattering regime is n = 2.7, which agrees with that obtained from X-ray scattering. We expect to be in a fractal scattering regime for polyelectrolyte microgels, since qRg > 1 even in light scattering experiments. The data are in excellent agreement with small-angle neutron scattering data recently published for PEDOT:PSS.49 Data for PEDOT:PSS with each of the secondary dopants are also shown and shifted for clarity. Despite the large rheological changes observed upon the introduction of ions to semidilute PEDOT:PSS, the scattering profiles are largely identical. Even the profiles from the polymer-rich phases, collected from phase segregation in dilute conditions, are nearly identical to the others. These data demonstrate that no significant structural change to the microgels occurs, even with profound changes to

ηr = [η] + kH[η]2 c p F

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the same, and no phase separation occurs. By contrast, PEDOT phase segregation from water occurred in dilute conditions above about 100 mM of either NaCl or EMIM BF4, and complete physical gelation was observed in semidilute conditions for ionic concentrations as low as 34 mM. The difference in sensitivity reinforces the significance of electrostatic interactions on this material. It also offers no correlation between behavior in solution and conductivity in films, since DMSO, EG, and EMIM BF4 all show conductivity enhancement, but NaCl does not. The film conductivities for each of these dopants are listed in Table 1. The literature reflects the same trends in conductivity enhancement for other PEDOT:PSS commercial grades as well. Table 1. Electronic Conductivity (κ) and Thickness (d) of Films Prepared from Aqueous Solutions of PEDOT:PSS with Various Solvent Compositionsa Figure 9. Reduced viscosity ηr for various concentrations cp of PEDOT:PSS in five different aqueous solvent conditions. Error bars reflect the standard deviation of six independent measurements on the same stock solution.

The coefficients in this expression are the intrinsic viscosity [η] and the Huggins coefficient kH. This is applicable when hydrodynamic coupling dominates between particles. For polyelectrolytes in solution, we instead expect the following general trend for small values of cp. ηr ∝ c p

−1/2

solvent composition

cp (mg/mL)

pristine water DMSO, 640 mM (5 wt %) EG, 800 mM (5 wt %) EMIM BF4, 35 mM NaCl, 35 mM

10 10 10 5 5

κ (S/cm) 0.9 180 290 50 0.8

± ± ± ± ±

0.2 30 30 15 0.1

d (nm) 56 51 50 29 22

± ± ± ± ±

5 5 6 6 3

a Errors reflect the standard deviation of measurements made on six replicate films.

The conductivity of a typical PEDOT:PSS gel (11 mg/mL + 70 mM EMIM BF4) is 0.012 S/cm, which is much lower than what is seen in films. This gel, once the water is evaporated, has a nearly matching composition with one of the films in Table 1 (prepared from 5 mg/mL PEDOT:PSS and 35 mM EMIM BF4), whose conductivity is 50 S/cm. This difference by a factor of over 4000 is expected, since electronic transport is accomplished through overlapping π orbitals in the film and probably through directed ion motion in the solution. The stark contrast between conduction in films and solutions is further indication that changes to solution properties do not affect film conductivity enhancement.

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This power law dependence of cp on ηr is generally expected for single-chain polyelectrolytes in low-salt conditions according to experiments51−54 and theoretical arguments55−58 and is consistent with electrostatic interactions between polyelectrolytes while also taking into account the strong concentration dependence on their spatial distribution. The exact value of the exponent varies across both experiments and theory but is always negative and is typically about −0.5. This behavior is expected not only for polyelecrolytes but also for charged microgels as described by Hess and Klein59 for large Debye lengths. Antonietti et al. generalized this expression60 to include a high-salt limit, where the Debye−Hückel interactions are screened and hydrodynamic interactions dominate, and this agrees with eq 7. For high ionic strength, by adding either EMIM BF4 or NaCl, we see a trend in agreement with eq 7. For low ionic strength, either in neat water or by adding DMSO or EG, ηr depends on cp−0.4, which agrees with eq 8. The agreement indicates that the dominant interparticle interaction is electrostatic in nature and that free ions screen these interactions. For all conditions, there is an upturn in PEDOT:PSS viscosity at a concentration of 1 mg/mL which corresponds to the microgel overlap concentration. But overall, interparticle interactions are dominated by electrostatics in low-salt conditions. Correlation of Solution Properties with Conductivity Enhancement. While the effect of NaCl and EMIM BF4 on the interactions and phase behavior of PEDOT:PSS solutions is rich, we have shown that the influence of DMSO and EG is less dramatic. For the additions of these cosolvents to aqueous PEDOT:PSS, which are chosen in these experiments to be typical concentration targets for film conductivity enhancement, virtually no physical change is observed. The microgels have no noticeable size change, interparticle interactions appear

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CONCLUSIONS We have studied structural, dynamical, and phase behavior aspects of PEDOT:PSS dispersions upon the addition of these dopants: EMIM BF4, NaCl, DMSO, and EG. PEDOT:PSS collects into many-chain charged microgels that are hundreds of nanometers in scale. The observed sensitivity to ionic strength for dynamics and phase behavior underscores the dominance of electrostatic forces in PEDOT:PSS solutions. However, the internal structure of these entities appears largely unchanged even in phase separation events and agrees with the picture of a semidilute polyelectrolyte mesh that is expected of a charged microgel. Our identification of a phase instability is a key consideration when preparing PEDOT:PSS films containing ionic dopants; care should be taken to avoid heterogeneities caused by this phase separation, which may then impact film performance. We observe no apparent correlation between the extent of film conductivity enhancement and changes to microgel dynamics, structure, or interparticle interactions. The apparent lack of correlation strengthens the widely held belief that the mechanism for conductivity enhancement is more closely linked to the crystalline ordering of PEDOT in films rather than the material’s solution properties. More broadly, the key to unveiling the mechanism of conductivity enhancement G

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appears to be understanding the environmental factors that induce and influence the ordering of PEDOT as it is processed into the film state. This work contributes to a piece of those factors by laying out the rich structural features of PEDOT:PSS and by demonstrating the relevance of electrostatic interactions and their influence on solution state properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00740.

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Figures S1−S7 and Tables S1−S3 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS It is a pleasure to thank Solvay for initial financial support on the theory and modeling of the generic system studied here, and Drs. Ryan Murphy and Larry Hough for stimulating discussions. Additionally, the authors acknowledge the financial support provided by the National Science Foundation (DMR1504265), and facilities provided by the National Science Foundation Materials Research Science & Engineering Center (MRSEC).

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