Segmental Dynamics of Ethylene Oxide ... - ACS Publications

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Segmental Dynamics of Ethylene Oxide-Containing Polymers with Diverse Backbone Chemistries Joshua Bartels,*,† Jing-Han Helen Wang,‡ Quan Chen,† James Runt,† and Ralph H. Colby† †

Department of Materials Science and Engineering and ‡Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: The dielectric response of seven nonionic ethylene oxide-containing polymers are investigated. Four different backbone chemistries are considered, including polymethacrylate, polyester ether, polyphosphazene, and polysiloxane. The chemistry of the backbone and the linking chemistry to incorporate ether oxygen (EO) groups dramatically affect polymer segmental dynamics. The Tg of the inorganic backbone polymers is ∼15 °C lower than that of the PEO homopolymer. Polysiloxanes exhibit the lowest Tg of −86 °C when attached with pendent −(CH2− CH2−O)4−CH3 groups. The strength of the alpha relaxation is the same for the hydrocarbon backbone polymers (Δε = 10), whereas the dielectric constants of inorganic polymers with short pendent groups is lower (Δε = 3). The difference in relaxation strengths is due to restricted motion of ether oxygens close to the backbone. This effect diminishes as the relative backbone concentration is decreased by increasing the pendent EO length. As pendent EO chain length is increased, the segmental relaxation broadens due to ether oxygens experiencing different local environments.

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INTRODUCTION It has been repeatedly stated in the ion-conducting polymer literature that poly(ethylene oxide) (PEO) is the preeminent polymer for solvating ions and promoting ion conduction, as was first observed by Fenton and Wright in 1973.1 The incorporation of polyethers or oligoethers in a highly ionic conductive polymer system appears to be unavoidable. PEO better solvates cations compared to its methylene oxide (−CH2−O−) and propylene oxide (−CH2−CH(CH3)−O−) analogues2,3 and supplies functional groups that are both chemically and electrochemically stable.4 For small cations (Li+, Na+, K+) a single PEO chain can adopt a low-energy conformation that surrounds the cation, similar to a crown ether.5,6 Ion conduction in solid polymers is coordinated with segmental motion of the amorphous phase, and the glass transition temperature (Tg) dominates the conductivity.7,8 The low Tg of PEO of −66 °C9 contributes to it being a favorable selection for ion conduction. The high crystallinity of the PEO homopolymer is its main disadvantage, and therefore ethylene oxide-containing functional groups are commonly added as oligoether pendent units on comb-branch polymers.10−14 Different types of linking chemistry provide a myriad of ways to connect ethylene oxide units to polymer backbones and allow for a diverse selection of backbone chemistries. Previously considered backbone chemistries include polymethacrylates,15−17 polysiloxanes,18−20 polyester ethers,21−23 and polyphosphazenes.24−27 Each architecture provides its own distinct advantages such as functional group versatility or controllable degree of polymerization. It is well-known that oligoether comb polysiloxanes and polyphosphazenes provide © XXXX American Chemical Society

higher segmental mobility (i.e., lower Tg) than the other backbones explored.18,28−30 The synthetic chemistry involved in creating each architecture is unique and few studies experimentally consider more than two polymer systems. The electrochemical stability of these backbone chemistries is important if they are to be used in batteries. A target of stability from 0 to 4−5 V is desired for battery applications;31 LiTFSIdoped PEO is found to have electrochemical stability similar to that of comparable organic solvents and is stable up to ∼4 V vs Li/Li+ (the stability of PEO is difficult to disambiguate from the stability of the anion).32−34 Variations of PEO have shown to have higher electrochemical stability, ranging from 4.3 to 5 V.35−39 Polysiloxanes and polyphosphazenes have displayed similar electrochemical stability when doped with LiTFSI (polyphosphazene) and LiCF3SO3 (polysiloxane), both being stable up to 5.0 V.40,41 In addition to this electrochemical stability, the polar nature of the inorganic backbone polymers helps protect them from attack by free radicals and imparts greater chemical and thermal stability.42 Polyphosphazenes are stable under basic conditions but undergo degradation under acidic conditions, whereas polysiloxanes are prone to attack by both acid and base and degrade in dilute basic solutions.43 Many ion conducting polymer systems (both polymer−salt mixtures and single-ion conducting ionomers) have been studied using the chemistries mentioned above, and therefore an important question is whether the chemistry connecting the Received: November 6, 2015 Revised: February 10, 2016

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5:1 ratio to AIBN, and their total content was 0.25 mol % of the total monomers. Before the reaction was initiated, the solution was dried by a procedure in the literature.57 Three freeze− pump−thaw cycles were applied to the reactant solution with liquid nitrogen under vacuum to remove oxygen and moisture from the reaction chamber. The temperature of the reaction was held at 80 °C for 9 h to achieve ∼70% conversion (DP = 280). After the reaction mixture was exposed to air, most of the chain ends were capped by the dodecyl trithiocarbonate group (RAFT agent) while a small portion of radical chain ends are terminated by recombination and disproportionation between polymer chains, or termination of the free radical site by solvent molecules or oxygen atoms, resulting in various possible end groups. PMMA5 and PMMA2 were synthesized by free radical polymerization in methanol solution. A dry glass reactor with a magnetic stir bar was charged with methanol, PEGnM, and AIBN (0.01 mol % of total vinyl monomers) as the initiator. The reaction vessel was dried by the same method as previously mentioned for PMMA9. The temperature of the reaction was maintained at 60 °C for 20 h in an argon atmosphere. The reactions were terminated by the same methods as PMMA9. Polymer Purification. All polymers were exhaustively dialyzed against deionized water to remove unreacted monomer and any lower molecular weight oligomers. The conductivity of the dialyzate was monitored, and dialysis was terminated after the conductivity returned to 0.2 μS/cm, the conductivity of freshly deionized water. Samples were dialyzed for 4−8 days filtering through 4−8 L of deionized water. The concentrated polymer solutions were first dried by a rotavap and then in a vacuum oven at 80 °C for a minimum of 24 h.

oligoether groups to the polymer backbone has a pronounced effect on the mobility of the ether oxygens.25,44−49 If ion conduction occurs principally through the segmental motion of EO units, then it is necessary to consider the effect of the linking chemistry on the relaxation of pendent EOs. In this work we consider seven different ethylene oxide-containing nonionic polymers and the effect that the backbone chemistry has on the dielectric relaxation of the dipoles. As pendent chain length increases, we expect pendent EOs to have additional environments for relaxation and therefore a broader segmental relaxation.50 Synthesis. Many of the polymers investigated in this study have been synthesized and studied in previous work by our group. The repeat unit chemistries of the polymers discussed herein are displayed in Figure 1. Short synthetic descriptions

Figure 1. Structures of the polymers investigated, where n is the number of ether oxygen (EO) units per monomer.

and publication references are given for each polymer synthesized previously, whereas the synthesis of the PMMAn polymers is discussed in detail as this has not been reported before by our research group. The polysiloxanes have the lowest degree of polymerization (DP = 52 as determined by 29 Si NMR) but have a sufficiently high molecular weight to prevent short chain effects from lowering the Tg.51 Polysiloxane PSXn and Polyester PEO600-0. Polyester ether samples were synthesized by step-growth addition condensation polymerization.22 Polysiloxane comb polymers were synthesized by functionalizing polymethylhydrosiloxane (Gelest) through hydrosilylation.52 The polysiloxanes and the EO-containing polyester were synthesized and studied by our group and were prepared according to previously published procedures.48,53,54 Polyphosphazene MEEP. Poly(bis(2-(2-methoxyethoxy)ethoxy)phosphazene) (MEEP) was prepared according to previously published procedures.55,56 Poly(methyl methacrylate)s PMMAn. Each polymer synthesis reaction was carried out under a dry argon atmosphere employing standard Schlenk line techniques. Poly(ethylene glycol) methyl ether methacrylates PEGnM, where n is the number of EO units after the ester bond (Sigma-Aldrich), were purified by a column designed to remove hydroquinone monomethyl ether (MEHQ) inhibitor (Sigma-Aldrich). 2,2′Azobis(isobutyronitrile) (AIBN) was recrystallized two times from dry methanol and stored at −18 °C before being used as the initiator. All solvents were used as received unless otherwise noted. PMMA9 was synthesized in solution by standard reversible addition−fragmentation chain transfer (RAFT) polymerization methods. Glassware was dried by heating under argon gas before being charged with PEO9M, 2-cyano-2-propyldodecyl trithiocarbonate (RAFT agent), AIBN, and 30 mL of dimethylformamide (DMF). The RAFT agent was kept in a

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EXPERIMENTAL METHODS

Thermal Analysis. Glass transition temperatures (Tg) were determined by differential scanning calorimetry (TA Instruments DSC Q10) using cooling and heating rates of 10 °C/min. Samples of 5−10 mg were dried in open pans at 80 °C under vacuum for 24 h before being hermetically sealed. Temperature ramps involved cooling to −100 °C and then heating to 150 °C. Samples were cycled 2−3 times to ensure reproducibility, with no signs of crystallization or melting, except for PMMA9 and PSX8 which crystallize on cooling (at −21 and −24 °C, respectively) and melt below ambient temperature. DSC data were collected upon cooling to match dielectric spectroscopy experimental conditions. Tg values were obtained from the cooling curves as the midpoint of the heat capacity transition, and a single Tg is observed for each polymer. Density. Density values were obtained by measuring the mass of samples that were molded into cylindrical geometries. Dried samples were placed on 7.9 mm diameter plates and loaded into a Rheometric Scientific Advanced Rheometric Expansion System (ARES)-LS1 rheometer. The top plate was moved down (accurate to 0.001 mm) until the sample edges were no longer concave. The tared rheometer plates were removed and weighed accurate to 0.0001 g. Density measurements were performed at 20 °C three times for each sample reported and are reproducible to within ±0.05 g/cm3. Prism Coupling. Refractive indices were acquired by prism coupling using a Metricon Model 2010/M prism coupler equipped with a 633 nm laser and a prism with an n range of 1.0−1.8. Samples were dried under vacuum and stored under Ar before being applied as ∼1 mm films directly to the prism under ambient conditions. Refractive indices were determined by the sharp drop in signal at the critical angle knee and are accurate to ±0.001.58−60 Dielectric Spectroscopy. Dielectric measurements were acquired using a Novocontrol Concept 40 dielectric spectrometer. Isothermal frequency sweeps with a range of 10−1−107 Hz were taken under a nitrogen atmosphere at constant temperature over a range from −80 B

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Macromolecules to 120 °C (controlled to 0.2 °C). Brass parallel plate electrodes were polished and cleaned with acetone before the samples were applied. After drying the sample at 80 °C under vacuum for 24 h on the bottom electrode, Teflon spacers (0.20−0.30 mm thick) were placed in the sample, and the second electrode (10−15 mm diameter) was placed on top. After being loaded in the spectrometer, samples were heated above 100 °C for ∼30 min to remove any small amount of water absorbed while loading the sample. Samples were determined to be dry when the dc conductivity remained stable for over 10 min.

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RESULTS AND DISCUSSION Density. The densities and glass transition temperatures (Tg) of the polymers studied are reported in Table 1. The Table 1. Physical Constants

MEEP PSX4 PSX8 PEO600-0 PMMA9 PMMA5 PMMA2

densitya (g/cm3)

DSC Tg (°C)

DRS Tgb (°C)

1.38 1.16 1.17 1.33 1.18 1.18 1.18

−80 −86 −72 −45 −62 −47 −25

−82 −85 −68 −49 −61 −31

EO/nm3

relative [EO]c

EO/mond

11.7 10.6 12.1 14.3 12.9 11.1 7.55

1.55 1.40 1.60 1.89 1.71 1.47 1

4 4 8 13 9 5 2

Density was measured at 20 °C. bDRS Tg is obtained from extrapolating the reciprocal peak frequency (rad/s) of the α relaxation to 100 s by fitting with the VFT equation. cRelative EO concentration is normalized to the concentration of PMMA2, the lowest EO concentration. dEther oxygens per monomer unit (EO/mon) does not take into consideration oxygens that are connected directly to the backbone or oxygens that are part of an ester bond.

a

variation in densities and numbers of ether oxygens per monomer (EO/mon) change the number density of EOs per unit volume. The relative concentration of EOs in the samples normalized to that of PMMA2 is shown to facilitate comparison. Thermal Characterization. Tg values depend on the pendent EO length and even more so on the backbone chemistry. Although the polymers all contain EO units similar to PEO, there is a difference of up to 60 °C in Tg among the polymers studied. Figure 2a displays the dependence of Tg on the backbone chemistry. As EO concentration increases, the inorganic backbone polymers exhibit an increase in Tg, whereas the opposite is observed for the PMMAn samples. The inorganic backbone polymers have similar Tgs, with each being lower than PEO (−66 °C). The flexible nature of the backbone allows for faster segmental motion and lowers Tg. The organic backbone polymers have Tg closer to PEO (PMMA9 = −62 °C and PEO600-0 = −45 °C) with the PEO600-0 polymer having a high Tg despite the high EO content due to the inflexible isophthalate linkages between PEO600 blocks.61 Although the PMMA9 polymer has a less flexible backbone, the PEO pendent chains are long and dominate Tg. Crystallization was observed in the PSX8 and PMMA9 samples with crystallization exotherms at −24 and −21 °C, respectively. The crystallization is much more pronounced in PSX8 even though the pendent group lengths are nearly the same. This is in agreement with observations in the literature which suggest the flexible nature of the polysiloxane backbone allows the pendent oligo-ethylene oxide to organize into crystals more easily.18 All other polymers

Figure 2. [a] Tg as a function of ether oxygen content; no clear correlation is observed across all backbone chemistries. The red dashed line corresponds to the Tg of neat high molecular weight PEO, and black lines are drawn to show trends for each polymer backbone type. [b] Static dielectric constant at 20 °C as a function of the DSC Tg of polyphosphazene (triangle), polysiloxanes (circles), polyester ether (diamond), and poly(methyl methacrylate)s (squares). The blue circle highlights the inorganic backbone polymers, and the red circle highlights the organic backbone polymers.

studied show no evidence of crystallization and are amorphous at all temperatures. Dielectric Response. The primary relaxation observed for each polymer is the α relaxation, reflecting segmental motion or the dynamic Tg. The peak frequency of the α process is obtained from fitting the dielectric loss ε″ at each temperature with the Havriliak−Negami equation.25,62 Peak frequencies are then plotted as a function of inverse temperature and fit by the VFT equation.50 Extrapolation of the relaxation time τα to 100 s gives the DRS determined glass transition temperature. Reported values of DRS Tg in Table 1 show excellent agreement with DSC Tg values. The DRS Tg of PMMA5 is not reported due to an insufficient number of temperatures where the peak of the α relaxation is observable, preventing the data from being reliably fit to the VFT equation. Figure 2a shows that EO concentration alone does not dominate the Tg: backbone chemistry dominates at low EO content. As the EO concentration is increased for the PMMA polymers, the Tg becomes closer to that of neat PEO (red C

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from DRS, below Tg. We employ the Onsager equation to determine the correlation factor g by using the literature value of the dipole moment of an EO unit for μ. Inorganic Backbone Polymers. In the case of the polyphosphazene and polysiloxanes investigated herein the EO dipoles dominate the dielectric response of the system. Using the literature value10,65 of 1.01 D for the dipole moment of an EO unit, a Kirkwood factor g < 1 is determined for each polymer at 20 °C (PSX4 g = 0.78, MEEP g = 0.90), suggesting some restricted rotation. The electronegativity differences in the backbone atoms of polysiloxane (Si and O) and of polyphosphazene (N and P) are 1.8 and 1.0, respectively. This electronegativity difference is expected to create small partial charges on the backbone that simulations suggest may interact with ions or other dipoles.66 As the pendent EO length increases, the volume fraction of the inorganic backbone decreases, and this interaction is expected to diminish. This is observed for PSX8 where Δε agrees well with the other organic backbone polymers at temperatures above −20 °C. The same effect would likely occur for a polyphosphazene with longer pendent EO groups. Figure 3 shows the strengths of the alpha relaxations normalized by the relative concentration of ether oxygens. The

dashed line). For the case of the inorganic backbones, an increase in the EO concentration results in an increase in Tg, while remaining below the Tg of neat PEO. PEO600-0 (purple diamond) has the highest EO content, but the rigid isophthalate linkages reduce segmental mobility, raising Tg. Figure 2b shows the measured static dielectric constant as a function of the polymers’ Tg. While no correlation is expected, it is desirable for ion-conducting polymer systems to have high mobility, low Tg, and a high dielectric constant. The polysiloxane samples, PSX4 and PSX8, have the most desirable characteristics as PSX4 has the lowest Tg and PSX8 has the highest dielectric constant: PSX8 has the best combination of these two parameters among the seven polymers. Ether oxygen concentration also plays an important role as this determines the number density of solvating species. The high-frequency dielectric constant ε∞ is determined independently by both dielectric spectroscopy and prism coupling (Table 2). The standard assumption is that the square of the refractive index, n, corresponds to ε∞ and should equal the observed high-frequency plateau in the permittivity ε′ at temperatures below Tg. Prism coupling is an optical method (λ = 633 nm) of measuring the refractive index where no orientational relaxations occur at such a high frequency so that n2 is strictly the electronic contribution to the dielectric constant.58−60 The small discrepancy between the two methods of determining ε∞ arises from a weak relaxation of strength Δε ∼ 1 that occurs at lower temperatures or higher frequencies than observable in the frequency and temperature range probed in our experiments. This relaxation is the so-called γ process observed in the literature at −130 °C that corresponds to local twisting motion of PEO segments in the glassy state.63 MEEP does not show this relaxation and has agreement between the two methods of determining ε∞ as noted previously.27 This local relaxation is not a part of the alpha relaxation and is not included in the following analysis, wherein the DRS ε∞ is utilized. Table 2. Dielectric Constants of Polymers optical ε∞ (n2)a

DRS ε∞b

εs at 20 °C

2.2 2.1 2.1 2.2 2.1 2.1 2.2

2.3 3.6 3.7 3.2 3.6 3.6 3.7

4.8 6.4 11.9 10.4 10.3 9.8 10.7

MEEP PSX4 PSX8 PEO600-0 PMMA9 PMMA5 PMMA2

Figure 3. Temperature dependence of the alpha relaxation strengths normalized by relative ether oxygen content (listed in Table 1, column 6).

Optical ε∞ values are obtained by 633 nm wavelength prism coupling.58 bDRS ε∞ values are obtained below Tg and at 106 Hz.

a

four polymers with the largest EO concentration are grouped together, with g ≈ 1. The other polymers are split into two responses: MEEP/PSX4 and PMMA2. MEEP and PSX4 have g < 1 perhaps from EOs interacting with positive P or Si in the backbone. PMMA2 shows the strongest response and has g > 1. The normalization does not include the contributions of the ester linkages, and therefore the PMMA2 relaxation, which is dominated by the ester, becomes stronger than the others when normalized by EO content. Comparing εs values in Table 2, it can be seen that the static dielectric constant for PMMA2 is comparable to the other organic backbone polymers. g ≈ 1 when PMMA2 is considered with a two-term Onsager equation to include both the ester contribution and the EO contribution to the response. The drop in the static dielectric constant of PSX8 around −20 °C corresponds to crystallization; this is surprisingly not observed in the PMMA9 dielectric response.

Alpha Relaxation. In order to thoroughly understand the dielectric response of these systems, the strength of the alpha relaxation shown in Figure 4 is considered through the Onsager equation:64 (εs − ε∞)(2εs + ε∞) 2

εs(ε∞ + 2)

=

νμ2 g 9ϵ0kT

(1)

where ε∞ is the high-frequency plateau in ε′, εs values are obtained from the clearly observable low-frequency plateau in ε′, μ is the dipole moment, ν is the number density of relaxing dipoles, and g is the Kirkwood−Fröhlich correlation factor.50 ε∞ values are taken at the lowest temperature data available D

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Macromolecules The static dielectric constant without the EO concentration normalization and the corresponding Onsager predictions are shown in the Supporting Information. Alpha Relaxation Breadth. Although the strengths of the relaxations are comparable for the organic backbone polymers, the breadth of the alpha process is very different for each. Figure 4 shows the alpha relaxations at Tg + 20 °C.

Figure 5. Width of the alpha relaxation depends on the number of bonds in each pendent group at Tg + 20 °C. a is the breadth parameter from Havriliak−Negami fitting in dielectric loss ε″ and is inversely related to relaxation breadth. A linear fit of the data (red line) shows a downward trend.

length of the pendent group increases, the relaxation broadens due to a wider variety of local environments for the EOs. In the case of the PMMA5 and PMMA9, long pendent EO groups lead to a broader relaxation. The alpha relaxation is even observed to have two components in Figure 4b. Both components are considered when determining Δε of these samples. We propose that the low-frequency relaxation corresponds to the ester bond and the EOs restricted by the slower motion of the polymer backbone. The faster highfrequency relaxation corresponds to the faster EO dipoles that are near the end of the side chain and are less restricted. A similar effect is observed in the literature where PEO dynamics are slowed when near an isophthalate group.67 The proximity of the slow relaxation in PMMA5 and PMMA9 to the electrode polarization prevents clear fitting in ε″ and derivative analysis of the dielectric storage data is used to remove the conductive contribution observed in ε″ according to published procedures.68,69 Figure 6 shows example spectra of the derivative loss plotted along with the dielectric loss, and the resulting peaks that are fit with the HN equation. When all pendent EOs are closely bound to the ester, in the case of PMMA2, the dielectric response displays a single slow process that agrees with the slow process observed in PMMA5 and PMMA9. This effect is not observed in the PSX8 sample because pendent dynamics are facile owing to the more flexible siloxane backbone and the lack of an ester linkage resulting in a single relaxation at high frequency. The temperature dependences of the observed relaxations are compared in Figure 7 by plotting the peak relaxation time versus the temperature normalized by the DSC determined Tg of each polymer. Each of the polymers shows similar behavior, with the exception of PMMA2. Likewise, the slow relaxation observed in PMMA5 and PMMA9 collapses with the PMMA2 relaxation. The excellent agreement of these relaxations further supports that the dipoles near the methacryl backbone are slowed due to restricted motion.

Figure 4. Dielectric loss ε″ as a function of frequency at Tg + 20 °C for [a] the inorganic backbone polymers and the polyester ether polymer and [b] the PMMA backbone polymers which show slow and fast dipolar responses.

The breadth varies greatly for each relaxation, affected by both linkage chemistry and side-chain length. The alpha relaxation observed in the dielectric loss is fit by the Havriliak−Negami empirical modification of the Debye function: * (ω) = εHN

Δε (1 + (iωτHN)a )b

(2)

where Δε is the strength of the relaxation, τHN is a relaxation time, a is a fitting parameter that is inversely related to the breadth of the relaxation, and b is the high-frequency asymmetry parameter.50 Figure 5 shows the breadth parameter a as a function of the length of the pendent group. As the E

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It may then be concluded that the polysiloxane chemistry is the most advantageous backbone chemistry as it allows for the highest segmental mobility (lowest Tg) while still having a narrow and strong segmental relaxation making εs as high as 12 at 20 °C. Short-chain EOs attached to flexible backbones prevent crystallization without hindering segmental motion. The PMMA backbone samples are less desirable as they show slower dynamics and a broader segmental relaxation without an advantage in the static dielectric constant or concentration of ether oxygens. Polysiloxanes and polyphosphazenes are of particular interest to the ion-conducting polymer field as the inorganic backbones offer lower Tg than conventional hydrocarbon backbones.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02423. Supporting Figures 1 and 2 (PDF)

Figure 6. Example spectra of the dielectric loss ε″ (solid lines) and derivative loss ε″derivative (open symbols) for PMMA9 at four temperatures. The derivative transformation reveals the slower relaxation which is fit with an HN function (solid black fit line).

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Division of Materials Research Polymers Program through Grants DMR-1404586 (RHC) and DMR-1505953 (JR). The authors thank Siwei Liang for preparation of the siloxane samples and Andrew Hess for preparation of the polyphosphazene sample.

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REFERENCES

(1) Fenton, D. E.; Wright, P. V. Complexes of Alkali Metal Ions with Poly(ethylene Oxide). Polymer 1973, 14, 589−590. (2) Armand, M. B.; Chabagno, J. M.; Duclot, M. J. Fast Ion Transport in Solids. In Fast Ion Transport in Solids; Vashishta, P., Mundy, J. N., Shenoy, G. K., Eds.; Elsevier: Amsterdam, 1979. (3) Armand, M. Polymer Electrolytes. Annu. Rev. Mater. Sci. 1986, 16, 245−261. (4) Gray, F.; Armand, M. Polymer Electrolytes. In Handbook of Battery Materials; Wiley-VCH Verlag GmbH & Co.: 2011; pp 627− 656. (5) Borodin, O.; Smith, G. D. Mechanism of Ion Transport in Amorphous Poly(ethylene oxide)/LiTFSI from Molecular Dynamics Simulations. Macromolecules 2006, 39, 1620−1629. (6) Shiau, H. S.; Liu, W.; Colby, R. H.; Janik, M. J. ClusterContinuum Quantum Mechanical Models to Guide the Choice of Anions for Li+ Conducting Ionomers. J. Chem. Phys. 2013, 139, 204905. (7) Benrabah, D.; Sanchez, J.; Deroo, D.; Armand, M. Synthesis and Electrochemical Characterization of New Bulky Lithium Salts. Solid State Ionics 1994, 70−71, 157−162. (8) Berthier, C.; Gorecki, W.; Minier, M.; Armand, M. B.; Chabagno, J. M.; Rigaud, P. Microscopic Investigations of Ionic Conductivity in Alkali Metal Salts-Poly(ethylene Oxide) Adducts. Solid State Ionics 1983, 11, 91−95. (9) Ibrahim, S.; Johan, M. R. Thermolysis and Conductivity Studies of Poly(ethylene Oxide) (PEO) Based Polymer Electrolytes Doped with Carbon Nanotube. Int. J. Electrochem. Sci. 2012, 7, 2596−2615.

Figure 7. Peak relaxation time from HN fitting of the relaxations observed in the dielectric loss (closed symbols) and derivative loss (open symbols). The temperatures are normalized by each polymer’s DSC determined Tg as reported in Table 1.

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SUMMARY When designing ion-conducting comb polymers, it is important to consider the backbone chemistry and the length of pendent oligoether employed. As the EO pendent length increases, the Tg of the polymer becomes dominated by the oligoether. This is an advantage for polymers with high Tg backbones but may be undesirable in the case of inorganic backbone polymers where Tg is below that of PEO. Long pendent EOs may also result in a broadening of the segmental relaxation. The chemistry of the polymer backbone can also strongly influence the strength of the alpha relaxation. Inorganic backbone polymers show weaker alpha relaxations because of restricted dipole movement and associations of short pendent groups. These apparent restrictions do not noticeably slow down the dynamics of segmental motion, as the Tg of these polymers remains below that of PEO. F

DOI: 10.1021/acs.macromol.5b02423 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02423 Macromolecules XXXX, XXX, XXX−XXX