Whirang
Cho
a,
Jinghang
Wu‡
a,
Bong Sup
Shim§
a,
Wei-Fan
Kuan
b,
Sarah E.
Mastroianni
b,
Wen-Shiue
Young¶
b,
Chin-Chen
Kuo
a,
Thomas H.
Epps, III
ab and
David C.
Martin
*ac
aDepartment of Materials Science & Engineering, University of Delaware, 201 DuPont Hall, Newark, DE 19716, USA. E-mail: milty@udel.edu; Fax: +1-508-256-8352; Tel: +1-302-831-2062
bDepartment of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716, USA
cDepartment of Biomedical Engineering, University of Delaware, 125 E. Delaware Avenue, Newark, DE 19716, USA
First published on 12th January 2015
We describe the synthesis and characterization of bicontinuous cubic poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer gels prepared within lyotropic cubic poly(oxyethylene)10 nonylphenol ether (NP-10) templates with Iad (gyroid, GYR) symmetry. The chemical polymerization of EDOT monomer in the hydrophobic channels of the NP-10 GYR phase was initiated by AgNO3, a mild oxidant that is activated when exposed to ultraviolet (UV) radiation. The morphology and physical properties of the resulting PEDOT gels were examined as a function of temperature and frequency using optical and electron microscopy, small-angle X-ray scattering (SAXS), dynamic mechanical spectroscopy, and electrochemical impedance spectroscopy (EIS). Microscopy and SAXS results showed that the PEDOT gels remained ordered and stable after the UV-initiated chemical polymerization, confirming the successful templated-synthesis of PEDOT in bicontinuous GYR nanostructures. In comparison to unpolymerized 3,4-ethylenedioxythiophene (EDOT) gel phases, the PEDOT structures had a higher storage modulus, presumably due to the formation of semi-rigid PEDOT-rich nanochannels. Additionally, the storage modulus (G′) for PEDOT gels decreased only modestly with increasing temperature, from ∼1.2 × 105 Pa (10 °C) to ∼7 × 104 Pa (40 °C), whereas G′ for the NP-10 and EDOT gels decreased dramatically, from ∼5.0 × 104 Pa (10 °C) to ∼1.5 × 102 Pa (40 °C). EIS revealed that the impedance of the PEDOT gels was smaller than the impedance of EDOT gels at both high frequencies (PEDOT ∼102 Ω and EDOT 2–3 × 104 Ω at 105 Hz) and low frequencies (PEDOT 103–105 Ω and EDOT ∼5 × 105 Ω at 10−1 Hz). These results indicated that PEDOT gels were highly ordered, mechanically stable and electrically conductive, and thus should be of interest for applications for which such properties are important, including low impedance and compliant coatings for biomedical electrodes.
Several previous studies have demonstrated that a variety of conducting polymers such as polythiophene, polyaniline, and polypyrrole can be deposited into hydrogels to create soft, electrically, and ionically conductive materials.7–12 However, these gels typically exhibit disordered morphologies and, correspondingly, poorly defined conductive pathways. We hypothesized that templating PEDOT gels within a self-assembled bicontinuous gyroid (GYR) phase might lead to particularly well-defined and isotropic nanostructures, thus improving control of electrical properties by providing charge diffusion pathways within the resulting nanoscale channels.
Previously, several groups13–18 have explored the use of amphiphilic surfactants as structure-directing templates for conjugated polymers and shown the control and lock-in of nanoscale self-assembly in conducting polymers transferred from the nanostructure of the surfactant molecules. Our group has electrochemically deposited ordered PEDOT using self-assembled non-ionic poly(oxyethylene)-oleyl ether (n ∼ 10) surfactants from a hexagonal mesophase, resulting in highly anisotropic microdomains.6
Bicontinuous cubic phases, such as the gyroid (GYR) with Iad symmetry, are structured materials containing three-dimensionally (3-D) interconnected nanochannels.19–27 In self-assembling amphiphilic systems, the GYR morphology has high interconnectivity of both hydrophobic (oily) and hydrophilic (watery) constituent phases within discrete channels.28–36 Possible applications of these materials include photonic crystals,37 mechanical actuators,38 drug delivery carriers,39 and selectively permeable membranes.40 Khiew et al. previously described the polymerization of polyaniline within a bicontinuous cubic non-ionic surfactant nanostructure.41 However, their results regarding the symmetry of the ordered cubic phase were inconclusive, and the physical properties of the materials were not examined in detail.
In this study, we report the successful preparation of PEDOT gels employing templates consisting of poly(oxyethylene)10 nonylphenol ether (NP-10) surfactants, in which EDOT monomer in octane was preferentially incorporated into the hydrophobic GYR bicontinuous channels (Fig. 1). NP-10 surfactant was used as the template for our experiments because of its relatively large lattice parameter (d ∼ 3–10 nm) as well as its precisely-defined gyroid symmetry near room temperature at concentrations of ∼63% by weight in water.41,42 UV-initiated polymerization of EDOT was performed slowly using the mild oxidant AgNO3 to insure that the NP-10 GYR phase would not be significantly disrupted by the polymerization. The resulting PEDOT structures were characterized by optical microscopy, thermogravimetric analysis (TGA), Fourier Transform infrared spectroscopy (FTIR), small-angle X-ray scattering (SAXS), and dynamic mechanical spectroscopy. Also, we have explored the temperature and frequency dependent conductivity of the samples using electrochemical impedance spectroscopy (EIS).
Fig. 2 Chemical structures of 3,4-ethylenedioxythiophene (EDOT, top) and poly(oxyethylene)10 nonylphenol ether (NP-10, bottom). |
To form the nanotemplated PEDOT constructs, a 10 wt% EDOT–octane solution was added to the NP-10 surfactant and mixed using a vortex mixer for 1 min. Then, DI-water was added to the mixture of NP-10/EDOT–octane, and the resulting mixture was vortexed for ∼5 min. The mixture was refrigerated for 1 day to stabilize as a clear transparent sample (NP-10 surfactant gels and EDOT gels, respectively). Following stabilization, a 4-times molar excess of AgNO3 initiator relative to the EDOT was added and allowed to diffuse into the pre-formed mixture over 1 week. Subsequently, UV light (Spectroline® model EN-140L Long Wave [365 nm] UV-A/Ultraviolet lamp, Spectronics Corporation) was used to polymerize the EDOT phase (into PEDOT) at 4 °C, as the AgNO3 was reduced from silver ions to silver metal after exposure to the UV-source.45,46 The UV polymerization of the EDOT gel was done for 6 h (PEDOT gels), which was determined to be sufficient time for the absorption properties to reach steady-state. We chose AgNO3 as the initiator because we found that it caused the least amount of disruption to the GYR phase after being added to the mixture, in comparison to other initiators that we investigated (including FeCl3). AgNO3 also dissolved less exothermically in water than FeCl3, which helped insure that the ordered EDOT phases would not be thermally disrupted before their conversion to PEDOT.47
Synchrotron SAXS experiments were conducted on the X27C beamline at the National Synchrotron Light Source of Brookhaven National Laboratory (NSLS-SAXS) using an incident beam of wavelength λ = 1.371 Å and a 1823 mm sample-to-detector distance. Sample temperatures were controlled using a Linkam HFS91 CAP stage while acquiring in situ scattering data. One dimensional SAXS data are presented as azimuthally integrated logarithmic intensity versus the scattering vector, q = |q| = 4πλ−1sin(θ/2), where θ is the scattering angle.48 The in situ SAXS data were acquired at each temperature after annealing for at least 15 min, and acquisition times were ∼1 min.
An AR-2000 rheometer (TA instruments) with a 40 mm 1° cone-and-plate fixture was used to measure the storage/elastic modulus (G′) and loss/viscous modulus (G′′) of the samples. The sample temperature was controlled by a water bath that circulated through a Peltier plate. Temperature scans at a fixed frequency of 6.28 rad s−1 were carried out at a heating rate of 1 °C min−1 and the equilibration time was 5 min. The torque was fixed at 10 μN m for the NP-10 and EDOT samples and at 100 μN m for the PEDOT samples, which was small enough to ensure a linear viscoelastic response in all cases.
A custom built microindentor was used to measure the compressive modulus. A 1.6 mm indenter tip was driven by a piezoelectric actuator with a total moving range of 1 mm and a resolution of 60 nm. During indentation, a 50 g load cell recorded force. A ball head was used to make the sample perpendicular to the indenter tip. Subsequently, indenter tip was lowered manually until it makes solid contact with the sample, then 0.5–1 g preload was applied to ensure flat contact. Indentation was then performed at 10 μm s−1 to a total placement of 1000 μm.
A Princeton Applied Research PARSTAT 2273 frequency response analyzer (AC Impedance Spectrometer) with a built-in test cell on a Linkam HFS91 CAP stage was used to measure charge transport properties as a function of temperature. The morphology of each sample was confirmed at each temperature tested using simultaneous SAXS in a lab-scale Rigaku instrument with a 1.6 kW sealed-tube X-ray source (Cu Kα, λ = 1.54 Å) and 1200 mm sample-to-detector distance. Samples were sandwiched between two ion blocking aluminum foil electrodes using a 0.5 mm thick Teflon O-ring as a spacer. The contact area (A) between sample and each electrode was 0.32 cm2. The impedance measurements were conducted on heating between 10 °C and 50 °C. Two impedance measurements were taken at each temperature with 5 and 8 min annealing times. The AC frequency range and voltage amplitude were 10−1–105 Hz and 10 mV, respectively. The effective conductivities (σ) at both high (105 Hz, σh) and low (10−1 Hz, σl) frequencies were calculated using σ = L/(|Z|A), for which L (0.5 mm) is the sample (Teflon O-ring) thickness and |Z| is the amplitude of the impedance at that frequency.48
Fig. 3 The initially colorless and optically transparent EDOT gels (left) change to a deep, dark bluish black (right, PEDOT gel) after the UV-initiated polymerization. |
The thermal behavior of the gels was investigated by thermogravimetric analysis (TGA) as shown in Fig. 6. In all of samples, the mass loss below 100 °C was primarily attributed to the evaporation of water. From 100–150 °C the PEDOT samples retained just less than 70% of their initial mass, whereas the NP-10 and EDOT gels retained ∼60% of their initial mass over this same temperature range. The PEDOT samples lost ∼5% of their mass around 160 °C, perhaps from low molecular weight oligomers. The bulk of the mass was lost from 250–400 °C for all three gels. The PEDOT gels showed a residual mass of about 7% after heating to 500 °C, whereas for the EDOT and NP-10 gels the mass loss was essentially complete by that temperature.
Fig. 6 TGA spectrum of NP-10 gel (dashed line), EDOT gel (dot-dashed line), and PEDOT gel (solid line). |
The temperature-dependent variations in nanostructure were examined by in situ SAXS. Fig. 7 shows SAXS profiles of neat NP-10 surfactant gels (with no EDOT monomer, Fig. 7(a)), NP 10 surfactant gels containing 10 wt% EDOT monomer (Fig. 7(b)), and PEDOT gels (after polymerization, Fig. 7(c)). We chose 10 wt% EDOT monomer because it allowed us to introduce a significant amount of monomer into the NP-10 template yet still retain the desired bicontinuous cubic symmetry (Fig. S2, ESI†).
Characteristic Bragg peaks of phases with bicontinuous cubic (Iad) (GYR) symmetry were observed at for which d(hkl) is the Bragg spacing, a is the unit cell size (lattice constant), and h, k, and l are the Miller indices.53 In the 10 °C profiles of NP-10 surfactant, EDOT monomer gels, and PEDOT gels, the ratio of the scattering vector moduli for the first two reflections was ; these are associated with the (211) and (220) spacings. In the NP-10 and EDOT gels, higher-order peaks at and in certain cases even corresponding to the (420), (332), (422), and (431) spacings of the GYR structure also were observed. The lattice constant, a, at 10 °C for the neat NP-10, EDOT monomer-containing, and PEDOT-containing cubic mesophases was 18.7 nm, 18.1 nm, and 18.3 nm, respectively. The cubic structure of the neat NP-10 surfactant gel was stable through 20 °C. However, when the temperature was increased to 25 °C, an additional peak was evident (qL: 0.97 nm−1). At 30 °C, another peak at twice the value of the qL emerged. This second set of peaks with a relative peak ratio of 1:2 at 30 °C indicated the formation of a lamellar (LAM) morphology with a domain spacing d = 6.5 nm. The GYR peaks decreased in intensity with further increases in temperature (from 30–40 °C), with only the LAM mesophase remaining at 45 °C. Because of the narrow width of both the temperature and concentration region over which the GYR phase is stable in the NP-10–water system, the GYR phase is never far from a phase boundary with the LAM phase. The proximity of these phase boundaries evidently gives rise to mixed phase structures under these experimental conditions.54 Two-phase coexistence regions also have been reported at the boundaries of the cubic phase in several poly(oxyethylene)-alkyl ether surfactant–water systems.55–57 At 15 °C the neat NP-10 gel sample exhibited only the GYR mesophase, while at this temperature a LAM peak had already appeared in the EDOT samples. The incorporation of EDOT into the oily phase evidently allows for the LAM phase to appear at lower temperatures. This may be due to the relative ease of incorporating EDOT into the layered structure of the LAM phase, rather than the highly curved domains of the GYR sample. In the EDOT samples from 20 °C to 35 °C, the two mesophases continued to coexist, and the samples transitioned entirely to the LAM phase at 40 °C.
After PEDOT polymerization, the PEDOT GYR structure persisted below 25 °C and underwent a transition from GYR to LAM beginning at 30 °C. These results demonstrate that the stable temperature range of the GYR phase was wider in the polymerized sample compared to the neat NP-10 surfactant and EDOT monomer gels. In-house SAXS experiments were conducted to examine the effect of increased annealing time (several hours acquisition time) at each temperature (Fig. S3, ESI†). The GYR to LAM phase transition happened at lower temperatures in all cases, indicating the importance of the kinetics on this process. The thermal stability of GYR morphology in PEDOT gels relative to NP-10 gels and EDOT gels also was confirmed by relatively long-time (1 h) SAXS annealing experiments. The (211) d-spacing of the GYR structure (and hence the lattice parameter) increased with increasing temperature, as shown in Fig. 8(a). The d-spacings were relatively insensitive to temperature at low temperatures, but began to expand significantly at higher temperatures. The temperature corresponding to this change in behavior was higher for PEDOT gels (∼30 °C) than for the neat NP-10 surfactant (∼20–25 °C) and EDOT monomer containing gels (∼15 °C). The temperatures at which this transition occurs correspond with the appearance of LAM peaks in each sample. Also, the thermal onset of the GYR-to-LAM phase transition of the PEDOT gels was higher than that for EDOT monomer gels as shown in Fig. 8(b).
Fig. 8 (a) Change of d-spacings determined from the primary (211) cubic peak q* and the lamellar peaks from each synchrotron SAXS pattern (Fig. 7), (b) nanostructure changes as a function of temperature. |
Fig. 9 Storage modulus (G′, filled circles) and loss modulus (G′′, empty circles) as a function of frequency (ω) during heating sweep (from low to high frequency) of EDOT gels. |
Fig. 10 Storage modulus (G′, filled circles) and loss modulus (G′′, empty circles) as a function of frequency (ω) during heating sweep (from low to high frequency) of PEDOT gels. |
Fig. 11 presents the changes in G′ (closed symbols) and G′′ (open symbols) as a function of temperature during a dynamic temperature sweep at a constant frequency of 6.28 rad s−1 for the neat NP-10 surfactant gel, EDOT monomer containing gel, and PEDOT gel. The G′ and G′′ of the PEDOT gel exceeded those of both the NP-10 and EDOT monomer gels, confirming the substantial enhancement of stiffness by PEDOT polymerization. In comparison to unpolymerized NP-10 and EDOT cubic phases (with storage modulus G′ ∼ 104 Pa at 10 °C), the PEDOT cubic structures had a much higher storage modulus (G′ ∼ 105 Pa at 10 °C), presumably due to the formation of semi-rigid PEDOT-rich nanochannels. Also, no gel-to-sol crossover point of the G′ and G′′ curves59 occurred between 10 °C and 60 °C in the polymerized PEDOT gel. Additionally, the G′ for the polymeric PEDOT gels decreased only modestly with increasing temperature, from ∼1.2 × 105 Pa (10 °C) to ∼7 × 104 Pa (40 °C), whereas G′ for the NP-10 surfactant and monomeric EDOT phases decreased dramatically, from ∼5 × 104 Pa (10 °C) to ∼1.5 × 102 Pa (40 °C). The structural reinforcement of the PEDOT GYR gels also was confirmed by the one order of magnitude higher compressive modulus of PEDOT (∼105 Pa) compared to NP-10 surfactant gels and EDOT monomer gels (∼104 Pa) (see Fig. S5, ESI†).
Fig. 11 Changes in G′ and G′′ of NP-1, EDOT, and PEDOT gels as function of temperature during dynamic temperature sweeps with a fixed frequency of 6.28 rad s−1. |
Fig. 14 Effective conductivity comparison between EDOT and PEDOT gels at 105 Hz (top) and 10−1 Hz (bottom). |
These results clearly show that forming PEDOT GYR from the bicontinuous EDOT GYR nanotemplates enhances charge transport properties at both high and low frequencies. The data suggest that the more rapid electron transport and the slower ionic transport processes have both been enhanced by the formation of the ordered PEDOT interconnected domains. The increased high frequency (electron) transport likely is due to the formation of conjugated PEDOT chains that are known to accommodate easy movement of charge along the molecular backbone and by lateral hopping between molecules. The enhanced low frequency (ion) transport may be due to the increased rigidity of the locally porous structure provided by the PEDOT polymerization process (as was seen by mechanical testing), perhaps because the more rigid PEDOT framework is not able to dissipate local ion diffusion processes as efficiently as the unpolymerized EDOT. It may also be a synergistic effect related to the enhanced electronic conductivity of the PEDOT network. Further studies are underway to examine the detailed origin of these observations.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cp04426f |
‡ Present address: The Dow Chemical Company, Midland, MI, USA. |
§ Present address: Department of Chemical Engineering, Inha University, Incheon 402-751, South Korea. |
¶ Present address: The Dow Chemical Company, Spring House, PA, USA. |
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