Leveraging Molecular Architecture To Design New, All-Polymer Solid

Mar 20, 2018 - The present strategy offers tremendous potential for the design of all-polymer electrolytes with optimized mechanical properties and io...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Leveraging Molecular Architecture To Design New, All-Polymer Solid Electrolytes with Simultaneous Enhancement in Modulus and Ionic Conductivity Emmanouil Glynos,*,† Paraskevi Petropoulou,† Emmanouil Mygiakis,∥ Alkmini D. Nega,∥ Wenyang Pan,⊥ Lampros Papoutsakis,†,‡ Emmanuel P. Giannelis,⊥ Georgios Sakellariou,*,∥ and Spiros H. Anastasiadis*,†,‡ †

Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, P.O. Box 1385, 711 10 Heraklion, Crete, Greece ‡ Department of Chemistry, University of Crete, P.O. Box 2208, 710 03 Heraklion, Crete, Greece ∥ Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografrou, 15 771 Athens, Greece ⊥ Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States ABSTRACT: The primary challenge regarding solid polymer electrolytes (SPEs) is the development of materials with enhanced mechanical modulus without sacrificing ionic conductivity. Here, we demonstrate that when stiff/rigid polymer nanoparticles that are thermodynamically miscible with a polymer are utilized in a blend with a liquid electrolyte, the elastic modulus and the ionic conductivity of the resulting SPEs increase compared to the linear polymer blend analogues. In particular, when poly(methyl methacrylate), PMMA, nanoparticles, composed of high functionality starshaped PMMA, were added to low molecular weight poly(ethylene oxide), PEO, doped with bis(trifluoromethane)sulfonamide (LiTFSI), the resulting SPEs exhibit 2 orders of magnitude higher conductivity and 1 order of magnitude higher mechanical strength compared to their linear PMMA blend analogues. In addition, the former remain solidlike over an extended temperature range. Key to their performance is the morphology that stems from the ability of the PMMA nanoparticles to disperse within the liquid electrolyte host, allowing for the formation of a highly interconnected network of pure liquid electrolyte that leads to high ionic conductivity (comparable to that of the neat PEO electrolyte). The present strategy offers tremendous potential for the design of all-polymer electrolytes with optimized mechanical properties and ionic conductivity over a wide temperature window for advanced electrochemical devices.

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INTRODUCTION Solid polymer electrolytes (SPEs) have been the topic of great interest mostly because of their applications in a wide variety of solid-state electrochemical devices, such as high energy lithium−metal batteries, fuel cells, dye sensitized solar cells, electrochromic devices, and water desalination systems.1−4 Despite the many advantages SPEs possess over classic aprotic electrolyte solutions, e.g., they are inherently safer, nonflammable, nonvolatile, and nontoxic, with a wider electrochemical stability and superior mechanical integrity, their low ionic conductivity at room temperature prohibits their realization in future emerging applications. The primary challenge regarding SPEs is the decoupling of the mechanical modulus with ionic conductivity; this coupling places undesirable limits on the charge/discharge rate in a battery cell or on the fast coloration−bleaching response time in electrochromic devices. This limit stems from the coupling of the ion motion with the local segmental motion of the ion-conducting polymer.5,6 As a result, in most solid polymer electrolytes, an improvement of © XXXX American Chemical Society

mechanical properties comes with the cost of greatly reduced ionic conductivity and vice versa. The first observation of ionic conductivity in complexes of lithium salts with linear poly(ethylene oxide), PEO, was reported in 1973,7 while its application in electrochemical devices, such as a lithium-ion battery, was demonstrated several years later. Since then, PEO has been the subject of extensive studies.2,4,8−10 Nevertheless, PEO has certain drawbacks: it forms crystalline regions that interfere with ion transport, since ions move through amorphous regions. Moreover, in homogeneous polymer materials ion motion/transport is coupled with segmental dynamics,2 and any attempt to improve conductivity via faster polymer motions results in a decrease in stiffness. This coupling has been a major drawback for the use of polymer electrolytes in several applications where solid state, Received: November 10, 2017 Revised: March 2, 2018

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DOI: 10.1021/acs.macromol.7b02394 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules mechanically robust electrolytes are necessary, such as in lithium metal batteries or in electrochromic devices for “smart windows”. An interesting line of investigation is the incorporation of a mechanically rigid component to a liquid electrolyte that will impart the desired mechanical strength without sacrificing ionic conductivity.2,11−15 To this end, a low molecular weight PEO mixed with lithium salt exhibits high ionic conductivity at room temperature, σ ∼ 10−3 S/cm, but lacks mechanical integrity with a storage modulus of G′ ∼ 10 Pa, limiting its application in a solid-state material.12−14 Common approaches to mechanically reinforcing liquid PEO electrolytes involve the addition of a second stiff/glassy and insulating polymer phase or the incorporation of inorganic nanofillers;16−18 such approaches offer the additional advantage of significantly reducing the degree of crystallinity of PEO in the nanocomposites.19,20 To this end, in the case of all polymer electrolytes, the two most promising lines of investigation involve the use of PEO-based block copolymers (BCPs)2,21,22 or the addition of a second polymer that should be thermodynamically miscible with PEO for the fabrication of polymer blend electrolytes.23,24 The ability of linear BCP’s to order in various morphologies25 by self-assembly has been explored utilizing copolymers, in which one block is soft and ion-conducting while the other is insulating and stiff/glassy. One commonly studied system is polystyrene-block-poly(ethylene oxide), PS-b-PEO, in the presence of a lithium salt.26−34 At a certain temperature T, the conductivity in nanostructured BCP materials, σ(T), may be expressed as a fraction of the conductivity of the conductive phase, σc(T), as σ(T) = αϕcσc(T), where ϕc is the volume fraction of the conductive phase and α accounts for the morphology and interconnectivity of the conductive phase. For nonoriented ordered BCP’s, α is 1/3 for cylindrical and 2/3 for lamellar morphologies, since, on average, only one-third and two-thirds of the grains, respectively, will contribute to the ion transport in a specific direction, while α = 1 for bicontinuous gyroid and spherical morphologies with the conducting block to be the majority block, as it has been described by the Sax− Ottino model.35 Nevertheless, the Sax−Ottino model does not account for grain boundary defects and bending/torsion of the conductive channels, and the conductivity of linear BCP systems is lower than the theoretical values. It is important to point out that the ideal morphology with the highest possible σ(T) is the spherical morphology with the conducting phase to form the interconnecting three-dimensional continuum matrix. In such morphology, α = 1 while grain boundaries are alleviated. Nevertheless, such polymeric materials have poor mechanical properties as their behavior is dominated by the soft conducting phase;36 they were of no direct interest and were abandoned many years ago. The research in this field over the years has been concentrated on copolymers wherein the insulating glassy block is the majority. For the most well studied BCP system of PS-b-PEO blended with an ionic liquid or a lithium salt, a PS/PEO molar ratio around unity provides a good balance between mechanical strength and ionic conductivity. Despite the desirable mechanical properties of linear BCP’s (G′ ∼ 108 Pa) over a wide range of temperatures, the conductivity is still ∼10−6 S/cm at room temperature (2 orders of magnitude lower than what is required for any practical application), and only at temperatures higher than ∼80 °C, σ(T) ∼ 10−4 S/cm in the best performing linear BCP/ Li salt electrolytes.22

In the case of polymer blend electrolytes, the polymers should be thermodynamically miscible and have complementary properties; i.e., one polymer should have high ionic conductivity (liquid, with a low glass transition temperature, Tg), and the other should possess excellent mechanical properties, i.e., it should exhibit a high Tg. A well-known example is the incorporation of poly(methyl methacrylate), PMMA, into PEO; the enthalpic interactions between these polymers are weakly attractive and display a small negative interaction parameter, χ.14,37−41 This blend is dynamically asymmetric (i.e., it shows a strong dynamic contrast between the two components) due to the large difference in the component Tg’s: ∼−75 and 110 °C for PEO and PMMA, respectively. To promote mechanical strength, the blend should remain glassy/rigid at T < Tg of the blend.42 At temperatures above the melting temperature (Tm) of PEO, these blends form a single-phase material with amorphous heterogeneities only in the range of a few nanometers (self-concentration effects associated with the connectivity of monomers along the chains).43 In these dynamically asymmetric systems, PEO chains suffer a dramatic retardation in mobility and hindered relaxations, even for a 20 wt % PMMA blend.44 Such and coworkers14 reported the room temperature ion conductivity of liquid, low molecular weight PEO (Tm ∼ 0 °C)/linear PMMA blends doped with LiCF3SO3 to be ∼10−5 S/cm for linear PMMA weight fractions of 30%, and it decreased to σ ∼ 10−6 S/cm for blends containing 50 wt % linear PMMA. Ion transport was reported to be coupled to the segmental relaxation of these single-phase, homogeneous systems. However, no direct ionic conductivity−mechanical property relationship was reported. Herein, we report the use of PMMA multiarm star “nanoparticles”, PMMA-NP, as additives to liquid PEO electrolytes to synthesize all-polymer solid electrolytes that exhibit a significant increase in both their mechanical strength and ionic conductivity compared to the corresponding linear PMMA/PEO blends. The new approach can be distinguished from other studies on polymer blend electrolytes and offers the following design advantages: star-shaped polymers of high functionality (f, number of arms) composed of high-Tg arms may be considered as hard colloidal nanoparticles at temperatures below Tg and impart mechanical reinforcement,23,24 while the attractive enthalpic interactions of the PMMA chains with the liquid PEO host enable the formation of all-polymer nanostructured materials with highly interconnected ionconducting channels to promote high ionic conductivity. In Figure 1a, the role of macromolecular architecture of the PMMA component on the polymer blend electrolyte morphology is highlighted. For linear PMMA, LPMMA, the material could be considered homogeneous/single-phase over length scales larger than a few to 10 nm (i.e., larger than the relevant length scale of self-concentration effects that is the polymer Kuhn length). In contrast, when PMMA “nanoparticles” are used, a nanostructured polymer composite material is formed, with highly interconnected ion conducting channels of neat liquid PEO. As a result, the PMMA “nanoparticle”/PEO blends exhibit ionic conductivities relative close to that of the neat PEO electrolyte at room temperature, even for blends containing 53 wt % PMMA-NP, which is approximately 2 orders of magnitude higher than that of the corresponding linear PMMA/PEO blends. B

DOI: 10.1021/acs.macromol.7b02394 Macromolecules XXXX, XXX, XXX−XXX

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Characterization. Morphology. The SPE morphology was investigated using a high-resolution transmission electron microscope (HR-TEM) equipped with a field emission gun. The TEM specimens had a thickness 100−110 nm and were prepared by spin-coating the polymer solution on a TEM grid. The samples were, subsequently, placed under vacuum at approximately 120 °C for 24 h. Before carrying out the TEM measurements, the samples were stained with RuO4 for 5 min; RuO4 preferentially stains the PEO phase. Electrochemical Characterization. A Novocontrol N40 broadband dielectric spectrometer (BDS) was employed for the measurement of the ionic conductivity of the SPEs as a function of temperature. The polymer electrolyte samples were heated until melting and were, then, pressed between the electrodes in the glovebox under a controlled argon environment; short-circuiting was prevented with a donutshaped Teflon ring. Prior to the BDS measurements, the samples were equilibrated at 130 °C in the BDS sample chamber under nitrogen. Subsequently, the samples were measured from 20 to 100 °C over the frequency range of 10−107 Hz. The dc conductivity was estimated from the plateau value of the high-frequency region using the real part of conductivity versus frequency plot and without invoking any model as described by Jonscher.46 The samples were measured on subsequent cooling and heating to ensure the reproducibility of the measurements and equilibrium properties. Mechanical Properties. The samples for the rheological measurements were prepared by shaping the SPEs into 8 mm discoid specimens by compression molding under vacuum for about 5 min. The samples were subsequently left to cool down to room temperature and were placed in an 8 mm diameter Teflon mold and annealed in an oven under vacuum at temperatures ∼140 °C for 2 days. The mechanical properties were evaluated in an ARES 100 FRTN1 straincontrolled rheometer (TA Instruments) equipped with an ARES convection oven in a parallel plate geometry (∼8 mm in diameter). Prior to rheological measurements, the samples were placed on the lower rheometer plate, the temperature was increased to approximately 80−160 °C (depending on the sample), and the samples were left at this temperature for 20 min; the upper plate was, then, brought in contact and the gap thickness was adjusted to about 1 mm. The viscoelastic behavior was investigated by small-amplitude oscillatory shear measurements in the 10−1 < ω < 102 rad/s frequency range. Prior to the frequency sweep tests, the linear strain range was determined by strain-sweep experiments at the maximum frequency, i.e., at ω = 100 rad/s. Frequency sweep tests were also performed at ω = 10 and 1 rad/s. Differential Scanning Calorimetry (DSC). The thermal properties of the solid polymer electrolytes were measured with a PL-DSC (Polymer Laboratories) differential scanning calorimeter. The range of temperatures covered was between −100 and 140 °C with a heating/ cooling rate of 10 °C/min, whereas two heating/cooling cycles were performed in all cases. In all cases, the glass transitions were obtained from the second heating in order to eliminate thermal history. During the measurements, the samples were maintained under constant nitrogen flow.

Figure 1. (a) Schematics to represent the organization of entangled linear PMMA (left, red chains) and PMMA nanoparticles (right, red star-shaped particles) within a linear PEO electrolyte (dark background). (b) TEM micrographs of a 45 wt % PMMA-NP/linear PEO0.55K blend; the PMMA nanoparticles appear bright (negative staining) after staining with RuO4 for 5 min, since RuO4 preferentially stains the PEO phase. The inset in (b) shows a photograph of the freestanding 45 wt % PMMA-NP/linear PEO electrolyte; its transparency is due to the good dispersion of the PMMA-NP within the PEO matrix.

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EXPERIMENTAL SECTION

Materials. Synthesis of PMMA Nanoparticles. The PMMA nanoparticles, PMMA-NPs, were synthesized by anionic polymerization in large scale through an easy and well-established methodology.45 Initially, highly cross-linked anionic cores were synthesized after the reaction of “living” very short polystyrene chains (Mn = 1000 g/mol) with divinylbenzene (DVB), which were subsequently used as initiating sites for the polymerization of the PMMA arms. The molecular weight, Mw, of the PMMA arms was approximately 20 kg/ mol and the functionality (number of arms) of the nanoparticles ∼100 as were determined by combination of static light scattering in THF at 25 °C and 1H NMR spectroscopy. A linear PMMA homopolymer (Mw = 100 kg/mol and polydispersity index ∼1.08), which was purchased from Sigma-Aldrich, was utilized. Its characteristics were provided by Sigma-Aldrich. Poly(ethylene glycol) monomethyl ether, PEO, was purchased from Sigma-Aldrich with average molecular weight Mn = 550 g/mol. The PEO was dried under vacuum at ∼100 °C for more than 24 h before stored in the glovebox with sub-ppm oxygen and water levels (MBraun). Lithium bis(trifluoromethane)sulfonamide, LiTFSI salt, purchased from Sigma-Aldrich, was received under argon, opened in the glovebox, and then dried at 130 °C and inside the dry atmosphere of the glovebox for 2 days. Preparation of Polymer Electrolytes. The desired amounts of the polymer components (PEO, linear PMMA, or PMMA-NP) and lithium bis(trifluoromethane)sulfonamide, LiTFSI, were dissolved in dry tetrahydrofuran (THF), stirred until homogeneous, and then initially dried at room temperature inside an argon-filled glovebox. The samples were subsequently placed on a hot plate within the glovebox at a temperature of 130 °C to ensure complete removal of the solvent and of the sample processing history.

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RESULTS AND DISCUSSION Figure 1b shows the transmission electron microscopy (TEM) images for specimens containing 30 wt % PMMA multiarm star nanoparticles. A homogeneous dispersion of PMMA nanoparticles (PMMA-NP) within the oligomeric PEO can be seen, with PMMA particle diameters DPMMA‑NP ∼ 90 nm. The favorable enthalpic interactions between the PMMA arms and the liquid PEO host drive the dispersion of the polymer particles within the host. It is important to point out that in the TEM images presented in Figure 1b the samples were labeled with RuO4, which preferentially stains the PEO phase within the staining time employed, i.e., 5 min; thus, PMMA nanoparticles appear white/gray, and the PEO phase appears dark.47,48 The TEM data confirm the effectiveness of utilizing PMMA multiarm star nanoparticles for the generation of allC

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G″ > G′ with G′ ∼ ω2 and G″ ∼ ω) and the rubbery behavior (frequency regime where G′ > G″). The crossover, where G′/ G″ = 1, occurs at lower frequencies with increasing the linear PMMA concentration in the blend. The macromolecular architecture of PMMA has a dramatic effect on the degree of mechanical reinforcement of the liquid PEO electrolytes (Figure 2); when PMMA-NP is used the storage modulus is higher than the loss one, G′ > G″, for the entire frequency range with a rather weak frequency dependence of both G′ and G″, typical of an apparent solidlike behavior. Note that for the PMMA-NP/PEO samples the values of the storage moduli G′, which describe the sample strength, are much higher than the corresponding values for the linear PMMA/PEO blends over the entire frequency range investigated. This effect becomes most pronounced at lower frequencies (left portion in Figure 2). The storage and loss moduli, G′ and G″, at T = 30 °C are plotted in Figure 3a for a frequency of ω = 10 rad/s as a

polymer PMMA/PEO-based composite materials with highly interconnected ion-conducting phase. It is important to point that the samples were optically transparent (inset of Figure 1b), indicative of the good dispersion of the PMMA nanoparticles within the PEO homopolymer matrix. The effect of macromolecular architecture of PMMA on the degree of mechanical reinforcement of liquid PEO electrolytes was evaluated by small-amplitude oscillatory shear measurements (Figure 2). At T = 30 °C, the linear PMMA/PEO blends

Figure 3. (a) Storage modulus, G′ (filled symbols), and loss modulus, G″ (open symbols), at T = 30 °C for varying concentration of linear PMMA (blue circles) and PMMA-NP’s (red stars). (b) Storage modulus, G′ (filled symbols), and loss modulus, G″ (open symbols), as a function of temperature obtained from the frequency sweeps in the linear regime for ω = 10 rad/s for the 53 wt % blends of linear PMMA (blue circles) and PMMA-NP (red stars).

Figure 2. Storage modulus, G′ (filled symbols), and loss modulus, G″ (open symbols), versus frequency for the LPMMA-100K/PEO (blue circles) and PMMA-NP/PEO (red stars) blends of different compositions (weight fractions): (a) 30, (b) 45, and (c) 53 wt % in LPMMA or PMMA-NP’s.

function of blend composition (wt %) for the PMMA-NP/PEO and LPMMA/PEO blends. It is clear that for every composition investigated the mechanical reinforcement is significantly stronger for the PMMA nanoparticles compared to that of the linear PMMA blends (Figure 3a). At temperatures around room temperature and for the 30 wt % concentration, the LPMMA/PEO blends exhibit a liquidlike behavior (G′ < G″).

show evidence of entangled polymer behavior for the blend containing 30 wt % PMMA, which becomes much more pronounced for the 53 wt % blends; the behavior for the 45 wt % falls in between. A crossover between the storage and the loss moduli, G′ and G″, respectively, is observed, in the region between the terminal behavior (low-frequency regime where D

DOI: 10.1021/acs.macromol.7b02394 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules In contrast, the 30 wt % PMMA-NP/PEO blend shows a solidlike behavior (G′ > G″). Furthermore, for the sample containing 53 wt % PMMA, the value of G′ is ∼70 kPa for the LPMMA blends while the corresponding value of G′ for the PMMA-NP blend is ∼500 kPa (Figure 3a). This value corresponds to an order of magnitude increase in the elastic modulus for the PMMA-NP/PEO blends compared to the corresponding linear LPMMA/PEO mixtures. The mechanical response of the PMMA-NP/PEO blends has a significantly weaker dependence on temperature (Figure 3b); at T ∼ 80 °C and for a composition of 53 wt %, the values of the G′ of the PMMA-NP/PEO blend are more than 2 orders of magnitude larger than the G′ of the linear LPMMA/PEO blend. Most importantly, the 53 wt % PMMA-NP blends retain an apparent solidlike behavior with G′ > G″ for temperatures up to 100 °C. In contrast, a transition from a solidlike behavior (G′ > G″) to a liquidlike behavior to (G′ < G″) is observed at T ∼ 70 °C for the 53 wt % linear PMMA blends. As expected, the crossover temperature, at which G′/G″ = 1, appears at a lower temperature with decreasing concentration of the linear PMMA in the blends. Similar conclusions regarding the significant effect of PMMA-nanoparticle additives on the mechanical reinforcement of SPEs can be drawn, as well, when the values of G′ and G″ at a frequency of ω = 1 rad/s are considered. Figure 4 illustrates the ionic conductivities of electrolytes based on both LPMMA/PEO and PMMA-NP/PEO blends (with the addition of LiTFSI at [EO]/[Li+] = 8) as a function of temperature. The values are referenced to the conductivity of neat PEO similarly mixed with the Li salt (black solid squares in Figure 4a). The ionic conductivities of the PMMA-NP/PEO solid polymer electrolytes are significantly higher than those of the LPMMA/PEO blends despite the significant enhancement of the mechanical strength of the PMMA-NP/PEO electrolytes compared to that of the linear- PMMA/PEO blends. In addition, the conductivities remain relatively close to those of the pure PEO electrolyte. In particular, the room temperature ionic conductivities of the PMMA nanoparticle blends are more than 1 and 2 orders of magnitude higher than those of the linear PMMA blends for the 45 and 53 wt % compositions, respectively. This is remarkable since the PMMA-NP/PEO blends exhibit an elastic modulus 1 order of magnitude higher than that of the linear LPMMA/PEO blends as well. This important result highlights the significance of the present approach: when dispersed, high-Tg polymer nanoparticles are introduced in a liquid electrolyte, the conductivity is minimally decreased, while the effect on modulus is dramatic compared to those of the corresponding linear polymer blends. It is important to point out that the 53 wt % PMMA-nanoparticle blends exhibit an ionic conductivity that is only 5 times lower than that of the pure liquid PEO electrolyte, while their G′ is more than 5 orders of magnitude higher. The results suggest that while the sample shows an apparent solidlike behavior its conductivity remains well within the practical-for-applications conductivity range for lithium-ion batteries (gray area in Figure 4a). The conductivity, σ(T), may be expressed as a fraction of the conductivity of the conductive phase, σcond(T), as σ (T ) =

Figure 4. (a) The dc conductivity as a function of temperature for the pure PEO (black squares), the 45 and 53 wt % linear LPMMA/PEO blends (filled and open blue circles, respectively), and the 45 and 53 wt % PMMA-NP/PEO blends (filled and open red stars, respectively); the gray region highlights the σ > 10−4 S/cm regime. (b, c) Temperature dependence of ionic conductivity for the solid polymer blend electrolytes along with the expected reduction of conductivity for tortuous ion-conducting channels in a heterogeneous electrolyte with 1.5 ≤ τ ≤ 3 for (b) 45% linear LPMMA/PEO and 45 wt % PMMA-NP/PEO blends and (c) 53% linear LPMMA/PEO and 53 wt % PMMA-NP/PEO blend. The solid lines correspond to τ = 1.5 and the dashed lines to τ = 3.

volume fraction of the conductive phase. This equation was employed to generate the solid and dashed curves in Figures 4b and 4c, utilizing the ϕc of the PEO component in the blends and the σcond(T) measured for pure liquid PEO (black squares in Figure 4a); the assumed values of τ are between 1.5 and 3, which have been shown to describe the diffusion of small molecules in one phase of a disordered and continuous network of morphologies.49−51 It is clear that the conductivity data for the 45 wt % PMMA-NP/PEO blends can be described by a value τ = 1.5 (Figure 4b), indicating that the conducting matrix is continuous over macroscopic distances. The ionic con-

ϕσ (T ) c cond τ

(1)

where τ is the tortuosity factor, which describes the distance that an ion must travel relative to a straight path, and ϕc is the E

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PMMA-NP are compared to those with linear PMMA. Once more, the observed behavior highlights the importance of the present approach: without changing the chemistry of the monomers but by simply modifying the macromolecular architecture of the PMMA additives, and in particular by introducing PMMA nanoparticles in the liquid PEO polyelectrolyte, the morphology of the blends is transformed from a homogeneous polymer (shown in Figure 1a, left) to a nanostructured system (shown in Figure 1a, right), the contacts between PEO and PMMA segments dramatically decrease, and the resulting continuum liquid PEO matrix of lower TgPEO and faster segmental dynamics promote the high ionic conductivity reported in Figure 4. A direct comparison of the storage modulus, G′, versus ionic conductivity, σ, for the 43 and 53 wt % blends of linear PMMA/PEO and PMMA-NP/PEO blends over the temperature range 20−100 °C is provided in Figure 6. There is a

ductivities of the 53 wt % PMMA-NP/PEO blends require values of τ in the range τ = 1.5−3 (Figure 4c). Moreover, it is clear that the conductivity data for the linear PMMA blends are well out of that regime. This behavior can be explained in terms of the significant slowdown of the PEO segmental dynamics in the single-phase materials formed with the linear PMMA. As it was shown by Zawada et al.,44 the PEO segmental dynamics in blends with PMMA are significantly slower than those for pure PEO, even for low PMMA weight fractions; for a 20 wt % PMMA blend, the PEO segmental relaxation times increased by an order of magnitude whereas they increased by about 2 orders of magnitude for the 40 wt % PMMA blend. It is important to point out that, in principle, a correlation of structural dynamics with conductivity could be extracted for the BDS measurements, as it has been demonstrated in the literature.20,52−54 Nevertheless, because of the high ionic conductivity of the PMMA-NP/PEO electrolytes (σ > 10−4 S/cm) developed in the present paper, a structural relaxation process from either the dielectric permittivity or the electric modulus spectra cannot be clearly obtained. The correlation of structural dynamics with conductivity in the present systems is beyond the scope of the current paper since it would require the preparation of samples with lower amounts of LiTFSI and, thus, much lower conductivity. It should be noted, however, that the aim of the current work was to achieve high conductivities, and indeed, we report the synthesis of solid polymer electrolytes with conductivities, σ, higher than 10−4 S/ cm, which can be suitable for several applications including lithium-ion batteries and electrochromic windows. Figure 5 shows the glass transition temperature of the PEO component (i.e., of the conducting phase), TgPEO, as a function

Figure 6. The dc conductivity versus storage modulus for the 45 wt % (open symbols) and 53 wt % (filled symbols) blends for linear PMMA/PEO (blue circles) and PMMA-NP/PEO (red stars) solid polymer electrolyte blends. The lines were drawn as a guide to the eye.

significant simultaneous improvement in both the ionic conductivity and the modulus, when PMMA nanoparticles are used as additives to liquid PEO electrolytes compared to those based on linear PMMA. For the linear PMMA/PEO blends, there is transition in the σ versus G′ dependence, from σ ∼ (G′)−0.5 to σ ∼ (G′)−2.4 with increasing G′. This transition occurs when the mechanical response of the polymer electrolyte changes from a solidlike (G′ > G″) to a liquidlike (G′ < G″) behavior; this change occurs at a temperature around the effective glass transition temperature of the high-Tg PMMA component in the blend.55 At temperatures lower than this temperature, the blend exhibits a solidlike behavior (G′ > G″). In that regime, the ionic conductivity is strongly coupled with the storage modulus G′ as σ ∼ (G′)−2.4 (in the right region of Figure 6). Although ionic conductivity relies primarily on the segmental motion of PEO, the macroscopic ion transport is significantly slowed down by the presence of slow PMMA chains. For temperatures above the effective glass transition temperature of the PMMA chains, the electrolyte becomes liquidlike with G″ > G′ and the ionic conductivity becomes relatively decoupled of G′ as shown by the very weak dependence as σ ∼ (G′)−0.5. In contrast to the linear PMMA blends, the PMMA nanoparticle blends show a strong decoupling between conductivity, σ, and mechanical strength, G′, i.e., σ ∼ (G′)−0.5, even though they exhibit a solidlike behavior with G′ > G″. This unique characteristic is attributed

Figure 5. Weight fraction dependence of the glass transition temperature determined by DSC of the PEO component in linearPMMA/PEO (blue circles) and PMMA-NP/PEO (red stars) solid polymer electrolyte blends.

of the weight fraction of PMMA for the linear PMMA/PEO and the PMMA-NP/PEO blends. In both cases, the Tg increases with increasing PMMA content due to the miscibility of the PEO segments with the high-Tg PMMA component. The trend of these results compares well with previous studies on the glass transition behavior of PMMA/PEO blends.42,55 Nevertheless, there is a noticeable effect of the PMMA architecture on the TgPEO; the increase of TgPEO with the PMMA weight fraction shows a weaker dependence for the PMMA-NP/PEO blends. This may be attributed to the significantly fewer contacts of the faster, low-Tg PEO segments with the slow, high-Tg PMMA segments when blends with F

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ance solid polymer electrolytes with enhanced adhesion to the electrodes without reduced clarity, which is of paramount importance for electrochromic devices in the “smart” window industry.

to the morphology of the PMMA-NP/PEO blends. The dispersion of the PMMA nanoparticles allows for the formation of highly interconnected ion-conducting matrix offering significant decoupling of the mechanical properties and the ionic conductivity, even at temperatures where the blends are solidlike. The present system provides significant advantages over the well-studied all-polymer nanostructured polymer electrolytes composed of block copolymers (BCPs). As discussed in the Introduction, the ideal morphology for a BCP electrolyte should be the spherical morphology with the conducting phase to be the majority, where the interconnecting three-dimensional continuous matrix, which is free of grains, would provide the highest possible σ(T). Nevertheless, those materials exhibit poor mechanical properties, since their behavior is dominated by the soft conducting phase.36 In the arena of linear BCPs there are no options for increasing the mechanical strength without significantly reducing ionic conductivity; by increasing the volume fraction of the mechanically rigid block, a transition from the spherical morphology to the lamellar one occurs that decreases the interconnectivity of the conducting phase, while the formation of grain boundaries significantly decreases the ionic conductivity of the material.21 The system proposed in the current work overcomes these restrictions, allowing for the formation of nanostructured materials with isotropic and highly interconnected conduction pathways independent of the volume fraction of the stiff/insulating phase. The mechanical properties of the resulting solid polymer electrolytes improved noticeably by more than 5 orders of magnitude compared to the pure PEO electrolyte, while the ionic conductivity was σ > 10−4 S/cm, less than 1 order of magnitude lower compared to that of the pure PEO, even at volume fractions of the PMMA phase higher than 50%. We recently showed that highly interconnected structures, in which both the ion conducting and the mechanical reinforcing phases are both interconnected, could be obtained by the addition of nanostructured polymer particles, based on miktoarm stars of PEO and PS arms, (PS)n(PEO)n, where n = 30 is the number of arms, in a low molecular weight PEO.13 In that case, the final morphology was the result of two competing interactions: (i) intermolecular attractions between the PS cores, which tend to aggregate since they are immiscible with the oligomeric PEO, and (ii) the athermal interactions between the longer PEO arms and the oligomeric PEO host. Despite their good mechanical strength (G′ ∼ 0.1 GPa) as a result of the interconnected insulating/rigid phase, the conductivity dropped to σ ≈ 5 × 10−5 S/cm at 44 wt %, (PS)30(PEO)30. Using PMMA-NP, the resulting polymer electrolytes maintained σ ≈ 3 × 10−4 S/cm even at a nanoparticle loading of 53 wt % due to the homogeneous dispersion state of PMMA particles within the oligomeric PEO. It should be pointed out that in contrast to the (PS)30(PEO)30/ PEO membranes, the PMMA-NP/PEO electrolytes were optically transparent due to the good dispersion of PMMANP within the PEO matrix; while this is not studied explicitly herein, a similar approach may have important applications in systems where optical transparency is required such as in electrochromic devices. In those cases, other improvements in the properties can be taken advantage of due to the macromolecular architecture. For example, as macromolecular architecture was shown to have a major effect on enhancing the wettability of polymers on surfaces,56,57 complex macromolecular PMMA architectures hold promise for high perform-

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SUMMARY In summary, we have investigated the effect of polymer architecture on the viscoelastic and conductivity properties of solid polymer composite electrolytes composed of PMMA and low molecular weight PEO. By incorporating PMMA nanoparticles composed of high functionality star-shaped PMMA to low molecular weight liquid PEO, the ionic conductivity and the modulus increased synergistically. The enhancement was significant: more than 2 orders of magnitude in conductivity and 1 order of magnitude in mechanical strength. Key to this phenomenon is the blend morphology: for linear PMMA/PEO blends a homogeneous, single-phase material is obtained, whereas for PMMA-nanoparticle/PEO blends, a nanostructured composite blend is formed with highly interconnected conducting PEO regions, as the result of a homogeneous dispersion of PMMA nanoparticles within the PEO electrolyte. The morphology of the PMMA-nanoparticle/PEO blends leads to dramatically fewer contacts between PEO and PMMA segments, when compared to the linear PMMA/PEO blends, resulting in faster PEO segmental dynamics and thus promoting high ionic conductivity. These differences in morphology result in a significant decoupling of conductivity from the mechanical behavior for PMMA-NP/PEO blends.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]. *E-mail [email protected]. *E-mail [email protected]. ORCID

Emmanouil Glynos: 0000-0002-0623-8402 Georgios Sakellariou: 0000-0003-2329-8084 Spiros H. Anastasiadis: 0000-0003-0936-1614 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was financially supported by the Stavros Niarchos Foundation within the framework of the project ARCHERS (“Advancing Young Researchers’ Human Capital in Cutting Edge Technologies in the Preservation of Cultural Heritage and the Tackling of Societal Challenges”).

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DOI: 10.1021/acs.macromol.7b02394 Macromolecules XXXX, XXX, XXX−XXX