Ionic Conductivities of Block Copolymer Electrolytes with Various

May 16, 2012 - analysis, a flow alignment technique was used to orient nanoscale domains in order to facilitate characterization of nanostructures suc...
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Ionic Conductivities of Block Copolymer Electrolytes with Various Conducting Pathways: Sample Preparation and Processing Considerations Wen-Shiue Young and Thomas H. Epps, III* Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: We explored the relationship between ionic conductivities and morphology in a lithium perchlorate-doped poly(styrene-b-ethylene oxide) (PS−PEO) system using ac impedance spectroscopy, in situ small-angle X-ray scattering, and transmission electron microscopy. To aid in morphological analysis, a flow alignment technique was used to orient nanoscale domains in order to facilitate characterization of nanostructures such as hexagonally perforated lamellae (HPL), hexagonally packed cylinders (HEX), and lamellae (LAM). Over the PEO volume fraction (f PEO) from 0.70 to 0.75, the neat PS−PEO exhibits a morphological transition from HPL to HEX, while the salt-doped PS−PEO shows morphological transitions from LAM to HPL to HEX. Additionally, experiments on hotpressed specimens show that 3-D conducting pathways (HPL and HEX) exhibit much higher normalized conductivities than the corresponding 2-D conducting pathway (LAM) even after accounting for nonrandom domain orientations and slight PEO molecular weight changes across samples. Our results further suggest that using block copolymer electrolytes with 3-D conducting pathways can prevent a decrease in through-plane conductivities caused by the partial nanostructure alignment during sample preparation.

I. INTRODUCTION Conducting block copolymers have attracted significant interest due to their independent manipulation of electric and mechanical properties, making them especially suitable for applications such as ion/electron/hole-conducting membranes in fuel cells, lithium batteries, and organic photovoltaic cells.1−8 These conducting membranes require defined conducting channels to transport charge carriers between electrodes and a supporting matrix to provide adequate mechanical integrity. High mechanical strength is especially important for lithium batteries in order to prevent dendrite growth on the anode surface during subsequent charge−discharge processes.9 Based on these requirements, block copolymers with both mechanically stable and ion-solvating components, such as poly(styrene-b-ethylene oxide) (PS−PEO),7,8 poly(methyl methacrylate-b-oxyethylene methacrylate) (PMMA−POEM),10 and poly(ethylene-b-ethylene oxide dendrons) (PE−PEOd),11 are of interest for lithium batteries. During the past decade, several studies investigating the mechanical strength of block copolymer electrolytes,7,10,12 morphology transitions upon salt-doping,3,4,6,11,13−18 and conductivity changes upon orderto-order transitions or order-to-disorder transitions have been reported.10,11,17,19−22 Those studies have provided a fundamental understanding of the mechanical, morphological, and electrical behavior of the salt-doped block copolymers; © 2012 American Chemical Society

however, the distinct relationship between the copolymer morphology and conductivity remains unclear. Several methods have been probed to study the morphology effects on the copolymer ionic conductivities. In 2007, Li et al. reported on a perpendicularly oriented cylindrical system using a liquid crystalline diblock copolymer membrane.23 This liquid crystalline diblock copolymer system enabled the authors to obtain isolated cylindrical PEO domains perpendicular to the membrane surface. The through-plane ionic conductivity was found to be one and a half orders of magnitude larger than the in-plane conductivity when the conducting channels were wellaligned, but the through-plane conductivity was identical to the in-plane conductivity when the conducting channels were isotropic. Compared to the well-aligned system, electrolyte systems with (relatively) randomly oriented domains may be more practical from a mass production standpoint. Thus, several electrolyte systems with randomly oriented nanoscale domains have been investigated. For example, Cho et al. reported the distinct conductivity changes at a cylinder-togyroid transition using LiCF3SO3-doped PE−PEOd.11 This result demonstrated the advantages of using block copolymers with 3-D conducting pathways (gyroid, GYR) over those with Received: February 21, 2012 Revised: April 13, 2012 Published: May 16, 2012 4689

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strength of block copolymer electrolytes is not the focus in this paper, we note that there are several ways to improve the mechanical integrity of the copolymer electrolytes, where PEO is the matrix in the morphology.25,26

1-D conducting pathways (hexagonally packed cylinders, HEX) in the case of randomly oriented domains. Similar results also were found by Weber et al.22 and Rubatat et al.19 in the polymerized ionic liquid (POIL) block copolymers and poly(sulfonated styrene-b-methyl methacrylate) (sPS− PMMA) systems, respectively, where samples with 2-D conducting pathways (lamellae, LAM) exhibited much higher conductivities than the corresponding samples with 1-D conducting pathways (HEX). Weber et al. also addressed the importance of long-range order of nanoscale domains in conductivity performance by comparing samples prepared by solvent-casting and melt-pressing.22 They found the conductivity of the solvent-cast sample, which had higher longrange order, was an order of magnitude greater than the corresponding melt-press sample. However, several studies also showed that no conductivity discontinuity was found across order-to-order transitions. Using a low molecular weight PS−PEO, Wanakule et al. found that the electrolyte conductivity exhibited no significant increase across the LAM-to-GYR transition.17 Morphology-independent conductivity also was reported by Naidu et al. in a LiClO4doped poly(styrene-b-2-vinylpyridine) (PS−P2VP) system.21 Despite the low conductivities, they found that samples quenched from different morphologies showed no significant difference in ionic conductivity. Additionally, the same argument has been made in the case of order-to-disorder transitions. While no distinct or gradual decrease in conductivity was found at order-to-disorder transitions in the case of salt-doped PS−PEO, 17 PS−P2VP, 21 PE−PEO dendrons,11 or PMMA−POEM systems,10 significant drops were found in the case of salt-doped poly(ethylene oxide-b(methacrylate-g-azobenzene)) (PEO−PMA(Az)) and ionic liquid-doped sPS−PMMA systems.19,23 The absence of conductivity discontinuities upon order-to-disorder transitions was rationalized by a persisting locally supramolecular structure above order-to-disorder transition temperature, even though the long-range order of the material disappeared.11 Furthermore, while studying ionic liquid-doped sPS−PMB systems, Kim et al. found that there was a distinct discrepancy between in-plane and through-plane conductivities for the LAM-forming sample due to the nonrandom orientation of copolymer domains, but smaller discrepancies were found for the HPL-forming and HEX-forming samples, where the conducting sPS domains are 3-dimensional.20 Similar results also were reported by Gwee et al. in an ionic liquid-doped poly(styrene-b-methyl methacrylate) (PS−PMMA) system.24 In the present work, we exclusively examine the morphological transitions upon a slight increase in PEO volume fraction ( f PEO) in a PS−PEO system, where PEO is the majority domain and the nondoped materials are located in the intermediate segregation regime. The salt-doping ratio ([EO]: [Li]) was fixed for all samples in order to compare the ionic conductivities of various conducting pathways. This work is different from those using low molecular weight or nonlinear (branched or brush) block copolymers, and the morphological transitions (LAM to HPL to HEX) were achieved without changing the polymer and salt chemistry. Comparing the ionic conductivities of samples with different morphologies, we have demonstrated that the morphology is a key factor in the electrolytes’ electric performance not only because of the dimensionality of the morphology but also because of the moderate alignment induced during modest sample preparations such as hot-pressing. Additionally, although mechanical

II. EXPERIMENTAL SECTION Materials. The PS−PEO diblock copolymers were synthesized via sequential anionic polymerization using the method described in ref 8. First, a poly(styrene) (PS-OH) block was synthesized from secbutyllithium and then end-capped with a hydroxyl end group using ethylene oxide. Subsequently, the PS-OH was reinitiated with potassium naphthalenide to add the PEO block via ring-opening polymerization of ethylene oxide. The number-average molecular weight (Mn), polydispersity index (PDI), and PEO volume fraction ( f PEO) were characterized via a combination of gel permeation chromatography (Viscotek 270max with Waters Styragel HR1 and HR4 columns using CHCl3 as mobile phase) and 1H nuclear magnetic resonance spectroscopy (Bruker AV-400) using homopolymer densities at 140 °C (ρPS = 0.969 g/mL, ρPEO = 1.064 g/mL).27 The

Table 1. Molecular Characteristics and Morphologies of Neat and LiClO4-Doped PS−PEO Samples

a

sample

Mn (g/mol)

f PEO

PDI

[EO]:[Li]

morphology

SO70 SO75 SO70P24 bSO73P24 SO75P24 PEOP24

44 300 53 100 44 300 --53 100 35 000a

0.70 0.75 0.70 0.73 0.75 1

1.06 1.09 1.06 --1.09 ---

----24:1 24:1 24:1 24:1

HPL HEX LAM HPL HEX ---

Obtained from Sigma-Aldrich.

characterization results are listed in Table 1 for all samples investigated in this study. Lithium perchlorate (LiClO4) and PEO homopolymer, obtained from Sigma-Aldrich, were dried under dynamic vacuum overnight before transfer into an argon-filled glovebox. PS−PEO and tetrahydrofuran (THF) also were dried rigorously before use due to the hygroscopic nature of the PEO:Li+ complex.7 All further sample handling was performed in a glovebox or vacuum chamber. The saltdoped PS−PEO samples were prepared by mixing premeasured amounts of PS−PEO and salt in dry THF, followed by solvent removal under dynamic vacuum. bSO73P24 was prepared by blending premeasured amounts of LiClO4, SO70, and SO75, resulting in f PEO = 0.73 and [EO]:[Li] = 24:1. Flow Alignment. The PS−PEO samples were partially flowaligned before morphological analysis by passing the molten sample thought a homemade Teflon channel (1.5 mm × 1.5 mm × 13 mm) in a vacuum chamber (as illustrated in Figure 1). This method, similar to channel die alignment,28 allows one to orient the copolymer nanostructure along the flow direction to facilitate morphological analysis. The aligned copolymer domains permitted the examination of morphologies such as hexagonally perforated lamellae (HPL).29,30 Typical sample preparation consisted of heating samples to 150 °C for 6 h under vacuum while passing through the flow channel and then allowing samples to cool to room temperature. After the alignment process, the samples at the end of flow channel were cut into 1.5 mm × 1.5 mm × 1.5 mm cubes for X-ray scattering and electron microscopy analysis. Note: the flow alignment process only was used to aid in nanostructure analysis and was not applied to the specimens examined in conductivity studies. Small-Angle X-ray Scattering (SAXS). SAXS experiments were conducted in the Department of Chemical and Biomolecular Engineering at the University of Delaware using a Rigaku SAXS instrument with 1.6 kW sealed-tube X-ray source (Cu Kα, λ = 1.54 Å) and a 2000 mm sample-to-detector distance. Sample temperature was controlled using a Linkam HFS91 CAP stage while acquiring in situ scattering data under vacuum. SAXS sample cells were sealed in the 4690

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III. RESULTS The morphologies of aligned neat and salt-doped PS−PEO samples were studied using a combination of SAXS and TEM on specific sample orientations. The 1-D SAXS profiles (Figure 2a) for neat SO70 were obtained with incident X-rays parallel to

Figure 1. Illustration of the flow alignment setup and the expected orientations of copolymer domains after alignment. The arrows passing through three morphologies represent the flow direction. For LAM and HPL, the lamellar normal is perpendicular to the flow direction. For HEX, the cylinder axial direction is parallel to the flow direction.

glovebox before transferring into the SAXS sample chamber. Twodimensional SAXS patterns were obtained with two sample orientations: X-ray incident beam parallel to flow direction (denoted as ||) and perpendicular to flow direction (denoted as ⊥). Onedimensional SAXS data are presented as azimuthally integrated logarithmic intensity vs the scattering vector, q = |q| = 4πλ−1 sin(θ/ 2), where θ is the scattering angle. Data reported here were all acquired at 100 °C. Transmission Electron Microscopy (TEM). A JEOL JEM2000FX TEM was used to study the block copolymer morphologies. Thin sections (∼70 nm thickness) of samples were obtained at −150 °C using a Leica Reichart Ultracut S microtome with a cryoattachment. Samples were cut under a nitrogen blanket. The cutting direction was perpendicular to the flow direction in the alignment setup, such that TEM images represent the cross-sectional morphologies along the flow direction. Samples were stored under a dry nitrogen environment until staining with ruthenium tetraoxide (RuO4) at room temperature. The typical staining time was 65 s. AC Impedance Spectroscopy. A Princeton Applied Research PARSTAT 2273 frequency response analyzer with a homemade test cell on a Linkam HFS91 CAP stage was used to conduct ionic conductivity measurements. Samples were hot-pressed into disks under vacuum in the antechamber of a glovebox (Supporting Information, Figure S1) and then sandwiched between two blocking aluminum (Al) foil electrodes using a 0.5 mm thick Teflon O-ring as spacer. The contact area (A) between sample and each electrode was 0.32 cm2. Samples were preannealed at 150 °C for 2 h to ensure good contact between the electrodes and the sample and then cooled to 20 °C at 30 °C/min and held for 1 h. The impedance measurements were conducted under vacuum, and the ionic conductivity was measured on heating between 20 and 150 °C. Two impedance measurements were taken at each temperature with 5 and 8 min annealing times. The first measurement is reported in the text, while the second measurement was used ensure that the ionic conductivity results were consistent during the experiment. The ac frequency range and voltage amplitude were 0.1−1 MHz and 10 mV, respectively. The bulk resistance of the electrolyte, R, was determined from the highfrequency plateau in the real impedance data, and the ionic conductivity, σ, was calculated using σ = L/(RA), where L is the sample (Teflon O-ring) thickness. To investigate the effects of nonuniformly oriented copolymer domains in the sample that may have been induced during the hot-pressing process, each measured sample was cut into ∼0.5 mm wide strips, turned 180° or 90° with respect to the axial direction of each strip (Supporting Information, Figure S2), repacked into the PTFE O-ring in the sample cell, and remeasured following the annealing procedure described above. In this work, the resulting conductivities are denoted as σ180° and σ90° according to the angle turned, and the original conductivity is denoted as σ0°.

Figure 2. (a) 1-D SAXS profiles, (b) 2-D SAXS patterns, and (c) characteristic TEM image of neat SO70. || and ⊥ represent the incident X-ray beam oriented parallel and perpendicular to the flow direction, respectively. 1-D SAXS profiles are shifted vertically for clarity. The arrows in the 2-D scattering pattern for SO70 ⊥ indicate the four weak off-meridional reflections. The TEM sample was stained with RuO4 vapor; thus, the PEO-rich regions appear dark.

flow direction (SO70 ||) and perpendicular to flow direction (SO70 ⊥). Both of the scattering traces exhibit Bragg reflection peaks located at q* and 2q* (where q* is the primary peak location). The primary peak intensity, I*, for SO70 ⊥ was weaker than that for SO70 || (I*⊥/I*∥ = 0.27), which was expected because fewer copolymer domains contributed to scattering for SO70 ⊥ due to the flow alignment. For both SO70 || and SO70 ⊥, the primary peaks were located at q = 0.0205 Å−1, giving a characteristic length (d* = 2π/q*) of 30.6 nm. The 2-D scattering patterns corresponding to SO70 || and SO70 ⊥ are shown in Figure 2b. For SO70 ||, the 2-D scattering pattern showed a slightly nonuniform primary ring, which was 4691

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reasonable as the flow channel was only 13 mm long and the flow alignment time might not be sufficient for the development of long-range order in this system.31,32 However, we note that the purpose of flow alignment in our case was solely to aid in morphology identification, not achieve long-range order. For SO70 ⊥, the 2-D scattering pattern showed two strong meridional reflections (perpendicular to the flow direction) and four weak off-meridional reflections (arrows) at ∼60° with respect to the meridian, which was consistent with scattering data reported for HPL-forming samples.33 The TEM image of SO70 (Figure 2c), cross-sectional along flow direction, showed that the lamellar PS domains (white) were perforated by PEO domains (dark), which was consistent with the 2-D scattering patterns in indicating a HPL morphology. The distance between two nearby PS layers in Figure 2c is ∼26 nm, which is slightly smaller than the value calculated from the SAXS profile. The deviation between SAXS and TEM results likely was due to the shrinkage of PEO domains under exposure to the electron beam in TEM.8 The SAXS and TEM results for the aligned SO75 sample are shown in Figure 3. The SAXS profiles of SO75 || and SO75 ⊥ (Figure 3a) were consistent with a HEX morphology with Bragg reflection peaks located at q*, √3q*, √7q*, and √9q*. The absence of a noticeable peak at √4q* was likely a result of a minimum in the scattering form factor due to the electron density profile of the sample.34−36 The primary peak was located at q = 0.0199 Å−1, giving a characteristic dimension, d10 (d*), of 31.6 nm. The 2-D scattering patterns of SO75 || and SO75 ⊥ shown in Figure 3b also were consistent with a HEX structure, where hexagonal six-spot and two-spot patterns were found in the || and ⊥ directions, respectively. The TEM image of the aligned SO75 sample (Figure 3c) showed hexagonally packed PS domains (white) isolated by a PEO matrix (dark), which was consistent with the HEX structure suggested by SAXS results. The inset in Figure 3c displayed the Fourier transform of the TEM image, which also indicated a hexagonally packed symmetry. The d10 measured from the TEM image of SO75 was 22.9 nm, which was smaller than the value obtained from SAXS profiles due to the PEO shrinkage effect as discussed previously. The morphology analysis of flow-aligned LiClO4-doped PS− PEO samples is summarized in Figure 4. The salt-doping ratio ([EO]:[Li]) for all specimens was all 24:1. The scattering profile of SO70P24 || in Figure 4a was consistent with a LAM structure, where Bragg reflection peaks were located at q*, 2q*, 3q*, 4q*, and 5q*. The primary peak was located at q = 0.0158 Å−1, giving a domain spacing of 39.8 nm. The 2-D scattering patterns of SO70P24 || and SO70P24 ⊥ are shown in Figures 4b and 4c, respectively. The results were consistent with an oriented lamellar structure, where ring and two-spot patterns were found for SO70P24 || and SO70P24 ⊥, respectively. The ratio of primary peak intensities of SO70P24 ⊥ and SO70P24 || was 0.3. The TEM image for SO70P24 (Figure 4d) also showed a lamellar morphology with a domain spacing of 17.5 nm. This value of domain spacing was much smaller than that obtained from SAXS. However, the layer thickness of a PS domain in Figure 4d was ∼12 nm as expected based on the PS volume fraction and the domain spacing from SAXS. Thus, we believe the much smaller domain spacing measured from Figure 4d likely was due to PEO shrinkage under electron exposure.8 The scattering profile of bSO73P24 || sample (in Figure 4a) exhibited several peaks, where the first two peaks were located at q* and ∼1.9q*. The primary peak was located at q = 0.0156 Å−1, giving

Figure 3. (a) 1-D SAXS profiles, (b) 2-D SAXS patterns, and (c) characteristic TEM image of neat SO75. || and ⊥ represent the incident X-ray beam oriented parallel and perpendicular to the flow direction, respectively. 1-D SAXS profiles are shifted vertically for clarity. The TEM sample was stained with RuO4 vapor; thus, the PEO-rich domains appear dark. The inset in (c) is the Fourier transform of the cropped TEM image.

a characteristic dimension of d* = 40.3 nm. The 2-D scattering patterns of bSO73P24 are shown in Figures 4e and 4f for || and ⊥ directions, respectively. Figure 4e (bSO73P24 ||) shows a ring pattern, and Figure 4f (bSO73P24 ⊥) shows a weak ring and two strong reflections located perpendicularly to the flow direction at the primary peak position (q = q*). The TEM image for bSO73P24 (Figure 4g) displayed a lamellar structure with perforations in the PS domains, which was consistent with a HPL structure. However, the TEM image for bSO73P24 showed less long-range order compared to the flow-aligned SO70 sample (Figure 2). The lack of long-range order may also explain the weak ring pattern observed in the 2-D scattering data, which appeared as a weak four-spot pattern for SO70 ⊥. Also, note that due to the lack of resolution in 1-D SAXS profiles for bSO73P24 (Figure 4a), we were not able to label the Bragg reflection peaks of our HPL structure due to the lack of long-range order in comparison to several literature reports,29,30 4692

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corresponding flow-aligned samples (Supporting Information, Figure S3), indicating that the sample morphologies were consistent. The salt-doping ratio for all conductivity samples was 24:1. The ionic conductivity profiles of salt-doped PEO and PS−PEO samples are summarized in Figure 5. The

Figure 5. Ionic conductivity profiles of LiClO4-doped PEO and PS− PEO samples at [EO]:[Li] = 24:1. Sample disks were prepared using a hot-pressing procedure (Supporting Information, Figure S1). Samples were annealed at 150 °C for 2 h, cooled to 20 °C at 30 °C/min, and then held at 20 °C for 1 h before measurement on heating. The annealing time before each measurement was 5 min. The reported values are the average of multiple samples at each condition (PEOP24: 2 samples; SO75P24: 4 samples; bSO73P24: 3 samples; SO70P24: 4 samples) with the standard deviation indicated by error bars on the data points.

samples were preannealed at 150 °C for 2 h followed by crystallization for 1 h at 20 °C before measurement on heating. Data points, shown in Figure 5, were the average values from several samples at each condition, where the error bars on each data point were indicative of the standard deviation of the data acquired at each condition. As the temperature increased, the ionic conductivities of samples increased. While the temperature-dependent conductivity showed a marked increase at lower temperatures, the higher temperature conductivities of each sample exhibited a more gradual increase with temperature. This steep trend in ionic conductivity at low temperature was due to contributions from the crystalline phase, in which the activation energy for ionic transport was large.38 The transition temperature between high- and low-temperature conductivity trends was between 60 and 70 °C for PEOP24 and between 50 and 60 °C for SO75P24, bSO73P24, and SO70P24. The relatively lower transition temperature for block copolymer electrolytes compared to homopolymer electrolytes was likely due to the relatively smaller crystallite sizes caused by the confinement of PS domains and limited time for crystallization.4,18,39,40 The ionic conductivities at 100 °C were 8.7 × 10−4 S/cm (for PEOP24), 4.5 × 10−4 S/cm (for SO75P24), 3.5 × 10−4 S/cm (for bSO73P24), and 8.8 × 10−5 S/cm (for SO70P24). This decrease in un-normalized conductivity was expected, as the reduction in PEO volume fraction should lead to a decrease in overall membrane conductivity.7 The raw conductivity data in Figure 5 were normalized based on the PEO volume fraction of each sample and the conductivity of the PEO homopolymer electrolyte at each temperature. The normalized conductivities (σ/f PEO/σPEO) are summarized in Figure 6. Only data above 70 °C were shown to avoid the PEO:LiClO4 crystallization effects observed at lower

Figure 4. SAXS profiles, 2-D scattering patterns in the || and ⊥ directions, and characteristic TEM images of flow-aligned LiClO4doped [24:1] PS−PEO samples. (a) 1-D SAXS profiles of samples in the || direction. (b−d), (e−g), and (h−j) are 2-D patterns in the || direction, 2-D patterns in the ⊥ direction, and TEM images for SO70P24, bSO73P24, and SO75P24, respectively. The arrows in (c, f, and i) represent the flow direction. The PEO domains in the TEM image were stained with RuO4 vapor.

but the 2-D scattering pattern in the ⊥ direction was consistent with those presented in the literature.37 The SAXS profile of SO75P24 || sample (in Figure 4a) was consistent with a HEX morphology where the scattering peaks were located at q*, √3q*, √4q*, √7q*, √9q*, √12q*, and √13q*. The primary peak was located at q = 0.0163 Å−1, giving a characteristic dimension, d10 (= d*), of 38.5 nm. Also, the 2-D scattering patterns of SO75P24 (Figure 4h,i) were consistent with a HEX morphology but with less long-range order than the SO75 sample as evidenced by the ring pattern in Figure 4h instead of the six-spot pattern seen for SO75 ||. In the TEM image of SO75P24 (Figure 4j), the hexagonally packed PS domains were surrounded by the PEO:LiClO4 matrix, which was also consistent with a HEX assignment. The characteristic length, d10, in the TEM image was 25.0 nm. To determine the ionic conductivity of salt-doped PS−PEO samples with various conducting pathways, ac impedance experiments were conducted on hot-pressed samples. Note that the flow alignment protocol described previously was used for assisting morphological analysis and was not applied to the conductivity samples. However, the (hot-pressed) conductivity samples showed similar scattering peak positions to the 4693

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Figure 6. Normalized ionic conductivity profiles of LiClO4-doped PS− PEO samples at [EO]:[Li] = 24:1. The reported values are the average of multiple samples at each condition with the standard deviation indicated by error bars on the data points.

temperatures. The normalized conductivity profile of each sample showed minimal deviation versus temperature, within experimental error. The normalized conductivities were ∼0.75 (for SO75P24), ∼0.55 (for bSO73P24), and ∼0.14 (for SO70P24). To determine if samples were partially oriented during hotpressing, σ180° and σ90° for SO70P24 and SO75P24 samples were measured and normalized with respect to σ0° (Figure 7). The specimen preparation and measurement procedure is described in the Experimental Section and Supporting Information (Figure S2). While σ 0° represented the through-plane conductivity of the sample, σ90° could be considered as inplane conductivity of the sample, which was measured by rotating sample instead of changing the experimental setup to those described in the literature.24,41 For SO70P24 (Figure 7a), the σ90°/σ0° was ∼2.5 and σ180°/σ0° was ∼1, and for SO75P24 (Figure 7b), both σ90°/σ0° and σ180°/σ0° were ∼1 at temperatures above 60 °C. The identical conductivities between σ0° and σ180° in both cases of SO70P24 and SO75P24 above 60 °C indicated that this cutting and turning method did not significantly affect the conductivities of the samples. For temperatures below 60 °C (crystallization region), σ90°/σ0° values for both SO70P24 and SO75P24 increased as temperature decreased. The increasing deviation between σ90° and σ0° was likely caused by crystallization of PEO domain and is discussed in the next section.

Figure 7. Ionic conductivity profiles of (a) SO70P24 and (b) SO75P24 for various sample orientations. Sample preparation is described in the Experimental Section. The ionic conductivities of samples following the hot-pressing procedure (as shown in Figure 5) are denoted as σ0° and normalized to 1 (red upward triangle). Samples after the first conductivity measurement were then cut to ∼0.5 mm wide strips, turned 180° or 90°, repacked into Teflon O-ring, and remeasured for conductivity. Conductivities of samples turned 180° and 90° are denoted as σ180° (blue downward triangle) and σ90° (green leftward triangle). The crystallization regions indicate the existence of semicrystalline PEO domains.

equilibrium phase in a blended system (diblock copolymer/ homopolymer).30,45 Also, a HPL-to-GYR transition was found in the shear-induced HPL-forming PS−PEO specimen (Mn = 28 kg/mol and f PEO = 0.365) upon heating above ∼150 °C,46 and the only reported GYR-forming salt-doped PS−PEO sample was obtained using a low molecular weight PS−PEO copolymer (Mn = 8.2 kg/mol, f PEO:LiTFSI = 0.64, and [EO]: [LiTFSI] = 20:1).17 Considering the relatively higher molecular weight (Mn = 44.3 kg/mol) in the present work, the morphology transition from HPL to GYR upon annealing at 150 °C is unlikely even if the GYR morphology is the equilibrium morphology of this LiClO4-doped PS−PEO system. (150 °C was the highest annealing temperature used in this work due to thermal stability considerations of the PEO:LiClO4 complexes.) Using a flow alignment technique, we were able to improve ordering in our samples to facilitate morphological analysis of our 2-D and 3-D conducting structures. The degree of orientation of flow-aligned samples was evaluated using second-order orientation factor (F2) as described in the

IV. DISCUSSION Morphology analysis of flow-aligned neat and salt-doped PS− PEO samples using SAXS and TEM revealed that SO70 transformed from HPL to LAM upon salt-doping at a ratio of 24:1, while SO75 retained HEX morphology upon doping. Blending of SO70 with SO75 to obtain an intermediate PEO volume fraction of 0.73 resulted in an HPL structure at [EO]: [Li] = 24:1. Thus, the small range of PEO volume fractions from 0.70 to 0.75 exhibited multiple structures with conducting pathways including 2-D LAM, 3-D HPL (lamellae with perpendicular cylinders connecting each layer), and 3-D HEX (the matrix for hexagonally packed cylinders). Note that although in a neat linear diblock copolymer system the HPL phase was predicted to be unstable using self-consistent meanfield theory,42 it was found to be a long-lived nonequilibrium phase in highly entangled systems37,43,44 and likely an 4694

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literature.28 Azimuthal scans were performed on 2-D SAXS patterns of the ⊥ direction along the primary peak position (q*). The intensity (I) was integrated within the full width at half-maximum (fwhm) of the peak, and azimuthal angle (β) was measured from the flow direction. The data were normalized using the normalized orientation distribution function, P(β), as given in eq 1. P(β) =

conductivity of a truly randomly oriented case should be somewhere between σ90° and σ0°. Unlike SO70P24, the in-plane and through-plane conductivities for SO75P24 samples were identical at temperatures above 60 °C, which was reasonable if one considers the 3-D conducting pathways in SO75P24 (with a conducting PEO matrix). We also note that at temperatures below 60 °C the in-plane conductivities of SO70P24 and SO75P24 both deviated from their corresponding through-plane conductivities. This behavior could be rationalized by the change in PEO volume fraction and nanostructure deformation upon PEO crystallization. This crystallization led to chain stretching that increased the copolymer domain spacing and altered the sample thickness. We did not account for this sample thickness change in our conductivity calculations; therefore, the conductivities may be underestimated in cases where a larger fraction of the lamellae (or cylinders) were oriented parallel to the electrodes (σ0°, σ180°) and overestimated in cases where a larger fraction of the lamellae (or cylinders) were oriented perpendicular to the electrodes (σ90°). The relationship between sample thickness changes and domain orientation upon the crystallization of PEO domain could explain the increasing deviation between inplane and through-plane conductivities as temperature decreases. The larger deviation between in-plane and through-plane conductivities below 60 °C in SO75P24 compared to SO70P24 could also be rationalized by different levels of sample crystallinity and domain spacing changes upon crystallization in different morphologies;49−51 however, further studies are needed to support this hypothesis. Considering the maximum conductivity of SO 70 P24 measured (σ90°) and additional 30% increase due to the molecular weight effect, the normalized conductivity for SO70P24 was ∼0.5. This overestimated normalized conductivity for SO70P24 was about 0.67 of the normalized conductivity of SO75P24 specimens (∼0.75). This result implied that, besides the dimensionality and the molecular weight, there were other factors that also contributed to overall conductivities of block copolymer electrolytes. One possible factor was the continuity of conducting channels across the grain boundary between block copolymer domains.52−54 For example, Jinnai et al. found that, for a HEX-forming sample, the tortuosity and continuity of the cylindrical domains were strongly affected by the grain orientation angle.53 When the grain orientation angle was ∼90°, most of the cylindrical domains bent at the grain boundary in order to not intersect each other (i.e., the domains were discontinuous). Furthermore, reduction of cross-sectional area of conducting channels (i.e., decrease in conductivity) could be found in the LAM-forming samples.52,55 This concern of domain continuity addressed the importance of long-range order of copolymer microdomains as well as the advantages of 3-D conducting pathways (e.g., the continuity of the matrix of HEX is not affected [or is less affected] by the grain orientation angle). Additionally, sample preparation for a conductivity study can be very critical, as most of hot-pressing and solvent-based methods involve either shear or selectively swelling blocks, leading to preferential domain orientations that are typically unfavorable to conduction in the desired direction.

I(q*, β)q*2 π

∫0 I(q*, β)q*2 sin β dβ

(1)

F2 was then calculated using eq 2 and eq 3: F2 = 1 − 3⟨cos2 β⟩

⟨cos2 β⟩ =

∫0

(2)

π

cos βP(β) sin β dβ

(3)

F2 was typically used for morphologies such as HEX and LAM, where a two-spot pattern is seen in the ⊥ direction. In our case, F2 for the HEX-forming SO75 and SO75P24 samples were 0.73 and 0.64, respectively, and F2 for LAM-forming SO70P24 sample was 0.70. These values were comparable to those reported using reciprocating shear or capillary rheometer alignment techniques.28 Using the same method, F2 calculated for HPL-forming SO70 and bSO73P24 samples were 0.42 and 0.46, respectively. These lower values of F2 for a 3-D structure such as HPL, in comparison to the HEX-forming and LAMforming specimens, were supported by the off-meridional Bragg reflections noted in our data.33,37,47 The ionic conductivity profiles of hot-pressed (i.e., nonflow aligned) samples (Figure 5) showed that the HEX-forming SO75P24 and HPL-forming bSO73P24 samples exhibited much higher conductivities than the LAM-forming SO70P24 specimen. Even after normalization to account for PEO volume fraction and PEO homopolymer conductivity, samples with 3-D conducting pathways (SO75P24 and bSO73P24) displayed much higher normalized conductivities than the specimen with 2-D conducting pathways (SO70P24) (∼0.75 and ∼0.55 vs ∼0.14) (see Figure 6). When comparing the ionic conductivity of different morphologies, we also must take into account two other effects: molecular weight and sample preparation. In our system, changing f PEO from 0.70 to 0.75 increased the PEO molecular weight from 32 to 41 kg/mol. We expect that this increase leads to an approximate 30% increase in normalized conductivity, as this relative increase was noted in a literature for a LiTFSI-doped PS−PEO system over a similar molecular weight range.48 Therefore, the normalized conductivity of SO70P24 may increase to ∼0.19 if its PEO molecular weight was identical to that of SO75P24. However, this value is still much lower than the bSO73P24 and SO75P24 samples. As mentioned previously, Weber et al. found that samples prepared by solvent-casting and melt-pressing can result in very different conductivities.22 Although our hot-pressing procedure was designed to create as little shear stress as possible, the sample preparation may still result in some degree of domain orientation. To examine this effect, we compared ionic conductivities of the samples in different directions, throughplane (σ 0° , σ 180° ) and in-plane (σ 90° ). The in-plane conductivities of SO70P24 were 2.5 times larger than the corresponding through-plane conductivities at temperatures above 60 °C. This increase indicated that the domain orientations were not random after hot-pressing, and thus the

V. CONCLUSIONS In this work we demonstrated the advantages of block copolymer systems with 3-D conducting pathways for electrolyte membranes by examining the conductivity profiles of 4695

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LiClO4-doped PS−PEO samples with various morphologies. To analyze the morphologies of block copolymer electrolytes, a flow alignment technique was applied to align the domains of the sample. Three morphologies, lamellae (LAM), hexagonally perforated lamellae (HPL), and hexagonally packed cylinders (HEX), were obtained by slightly changing f PEO from 0.70 to 0.75. Additionally, for a LAM-forming sample, we found that the moderate shear created during hot-pressing can orient domains against the conducting direction decreasing the conductivity as much as 2.5 times. However, for a hot-pressed HEX-forming sample, conductivity was not affected by the domain orientations due to the 3-D conducting pathways of the PEO matrix. Our results showed that the normalized conductivity increased dramatically upon a morphology change from LAM (2-D conducting pathways) to HPL/HEX (3-D conducting pathways) even when the PEO molecular weight and nonuniform domain orientation effects were considered. These results for samples larger than 40 kg/mol differed from those reported for low molecular weight PS−PEO systems.

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ASSOCIATED CONTENT

S Supporting Information *

Illustration of the hot-pressing setup for the conductivity samples, illustration of cutting and rotating conductivity samples for measuring σ90° and σ180°, and 1-D SAXS profiles of flow-aligned and hot-pressed samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge AFOSR-PECASE (FA9550-09-1-0706) for financial support. Use of 1H NMR instrumentation for polymer characterization was supported by NSF CRIF: MU, CHE 0840401. We acknowledge the University of Delaware W. M. Keck Electron Microscopy Facility for use of their TEM and cryo-microtome facilities.

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REFERENCES

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