Article pubs.acs.org/Macromolecules
Molecular Mobility and Cation Conduction in Polyether−Ester− Sulfonate Copolymer Ionomers Gregory J. Tudryn,† Michael V. O’Reilly,‡ Shichen Dou,† Daniel R. King,† Karen I. Winey,‡ James Runt,† and Ralph H. Colby†,* †
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272, United States S Supporting Information *
ABSTRACT: Poly(ethylene oxide) [PEO] ionomers are candidate materials for electrolytes in energy storage devices due to the ability of ether oxygen atoms to solvate cations. Copolyester ionomers are synthesized via condensation of sulfonated phthalates with glycol mixtures of PEO and poly(tetramethylene oxide) [PTMO] to create random copolymer ionomers with nearly identical ion content and systematically varying solvation ability. Variation of the PEO/PTMO composition leads to changes in Tg, dielectric constant and ionic aggregation; each with consequences for ion transport. Dielectric spectroscopy is used to determine number density of conducting ions, their mobility, and extent of aggregation. Conductivity and mobility display Vogel temperature dependence and increase with PEO content; despite the lower Tg of PTMO. Conducting ion densities show Arrhenius temperature dependence and are nearly identical for all copolymer ionomers that contain PEO. SAXS confirms the extent of aggregation, corroborates the temperature response from dielectric measurements, and reveals microphase separation into a PTMO-rich microphase and a PEO-rich microphase that contains the majority of the ions. The trade-off between ion-solvation and low Tg in this study provides fundamental understanding of ionic aggregation and ion transport in polymer single-ion conductors.
1. INTRODUCTION The increasing demand for electronic devices integrated into everyday life has driven the need for improved fundamental understanding of ion transport and materials design for higher performance and more robust electrolytes.1−3 The electrolyte’s role in conducting ions can be described as a two step process in liquids: solvation of ionic species via polar solvent interactions and conveyance of the conducting ionic moieties to and from the electrodes.1−3 Conduction in polymer electrolytes is similar in concept, albeit slower due to strong coupling of ion motion with segmental dynamics, 1−6 particularly for cations, causing higher frictional resistance to ion motion than predicted by the Stokes−Einstein relation.7 Nevertheless, demand for polymer electrolytes is high due to the potential advantages, i.e., ease of thin film coating, flexibility, lower toxicity, and mitigation of catastrophic lithium dendrite formation.8 Achieving these advantages requires improvements in conductivity, and therefore advancement in the fundamental knowledge of ion transport. Here we systematically vary the polymer’s ability to solvate cations at nearly fixed ion content, and observe the effect on ion transport in sulfonate ionomers with Li+ or Na+ counterions. Use of single-ion conductors (ionomers) by covalently tethering anions to the polymer minimizes contributions from anion migration, thus increasing Li+ and Na+ transference numbers to unity, and allows direct © 2012 American Chemical Society
application of the Poisson−Boltzmann equation for cation transport dynamics. Previously,9 we found for PEO-based ionomers with various ion contents that glass transition temperature (Tg) is a dominating factor. Higher molar mass PEO spacers between sulfonate sites increase Li+ counterion conductivity and mobility, despite lower stoichiometric ion content, due to lower Tg which provides faster segmental dynamics. Similarly, Sun and Angell also reported that conductivity of ionomers at a single temperature decreases with increasing ion content due to the effect of ionic groups acting as physical cross-links, raising Tg.10 Tg effects were demonstrated by plotting conductivity against T − Tg or Tg/T for PEO ionomers as a function of varied PEO segment length between ionic sites,9,11,12 or by plotting molar conductance against α-relaxation frequency for PEO copolymer ionomers.9 Polymers containing other polyethers such as poly(propylene oxide) (PPO)13 and poly(tetramethylene oxide) (PTMO)14,15 have also attracted attention as electrolytes. Shilov14 reported on carboxylate−polyurethanes prepared from PTMO oligoethers, which exhibited low conductivity due to poor solvation of ionic groups in PTMO-rich soft segments. Polizos Received: October 10, 2011 Revised: April 5, 2012 Published: April 23, 2012 3962
dx.doi.org/10.1021/ma202273j | Macromolecules 2012, 45, 3962−3973
Macromolecules
Article
(q = 0.07−17 nm−1), differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS). 2.1. Synthesis and Ion Exchange. Poly(ethylene glycol) (PEG600, 99%), poly(tetramethylene glycol) (PTMG650, 99%), triphenyl phosphite (TPP, 97%), titanium(IV) isopropoxide (99.999%), lithium chloride (99+ %), dimethyl isophthalate (DMI, 99%), and dimethyl 5-sulfoisophthalate sodium salt (DM5SIS, 98%) were supplied by Aldrich. The synthesis of the polyester copolymer ionomers, Table 1, was performed by a two-step melt polycondensation. Monomers were dried under vacuum for a minimum of 72 h at 353 K over 4 Å molecular sieves prior to synthesis. A three-neck glass reactor was heated dry and assembled with a mechanical stirrer. Following an argon purge (three times), the reactor was charged with oligomeric diol and esters (diols:diesters = 1:0.88 molar ratio) and titanium(IV) isopropoxide catalyst (0.05 wt %), then maintained under an argon atmosphere. A batch of approximately 60 g was mechanically stirred while maintaining a reaction temperature of 483 K for 4 h, then 503 K for 2 h. The methanol condensate was removed using a liquid nitrogen cold trap. Second, diesters (12 mol % of diols) and triphenyl phosphite (0.05 wt % of total reagents) were added after cooling to 453 K, then reheated and maintained at 523 K for 2−3 h. The total molar ratio of diols/diesters was controlled at 1:1. Vacuum (99.9% using inductively coupled plasma to measure atomic emission of sulfur and residual sodium spectra on a Perkin-Elmer Optima 5300. Water was removed using a rotary-evaporator, then annealing samples at 353−363 K at
