Effects of Bulky Copolymers on Ion Transport in an Anhydrous Proton-Conducting Electrolyte

Abstract

Anhydrous proton-conducting electrolytes composed of a mixture of MePEG 7 SO 3 H ͓where PEG is poly͑ethylene glycol͔͒ acid and a sol-gel-based MePEG 3 copolymer were prepared. These copolymers were prepared by mixing the MePEG 3 ͑CH 2 ͒ 3 Si͑OCH 2 CH 3 ͒ 3 monomer and different bulky comonomers: phenyltriethoxysilane, diphenyldimethoxysilane, and isobutyltrimethoxysilane. The MePEG 3 copolymers showed smaller volume fractions of PEG and nearly the same fractional free volume as the parent MePEG 3 polymer. End-group analysis showed that the incorporation of bulky group decreased the degree of condensation in the sol-gel polymer. This loss of condensation lead to an increase in the fluidity of the MePEG 3 copolymers, which in turn resulted in larger ionic conductivities in the copolymers compared with the parent MePEG 3 polymer.Proton-conducting polymer electrolytes are important components for fuel cells and electrochromic displays. In this paper, we focus on the synthesis and investigation of structural effects of our polymer electrolyte on anhydrous proton conductivity in our poly͑ethylene glycol͒ ͑PEG͒-based electrolytes. To study the effects of polymer structure on the ionic conductivity in an electrolyte, we need to quantitatively define structural properties. In previous papers, our group used the parameter volume fraction of PEG ͑V f,PEG ͒ to discuss the concentration of coordinating polyether functionalities in the polymer electrolyte. 1,2 Several other groups have used parameters such as the fractional free volume ͑FFV͒ 3-15 and viscosity [16][17][18][19][20][21][22][23] to relate structural changes to changes in ion transport properties.In this paper, we seek to make a direct comparison between the structure of our polymer electrolyte and the ion transport properties in the polymer electrolyte. To do these experiments, we have developed a method for systematically altering the V f,PEG , FFV, and viscosity in our PEG-based polymer electrolytes by copolymerizing our MePEG 3 monomer with monomers containing bulky groups. These copolymerized bulky groups serve to impede the rearrangement of the main polymer backbone, which results in an overall less dense polymer. 14 Long and Swager used this technique to increase the free volume ͑FV͒ of a polymer by incorporating bulky triptycene subunits to the polymer backbone. 14 We expected that the copolymerization of bulky comonomers would increase the FFV of our MePEG 3 polymers. Furthermore, we expected that the increase in the polymer electrolyte's FFV results in an increase in ionic conductivity due to the increased diffusion coefficients of the ionic species. 5 The effects of bulky functional groups have been extensively studied with regard to gas permeation of polymer membranes. In general, bulky groups incorporated into polymers increase the FFV of the polymer, thereby increasing the gas permeability of the polymer. 24 In many of these studies, bulky groups such as CF 3 − ͓or C͑CF 3 ͒ 2 − ͔, 25,26 SiMe 3 − , 25 SO 2 − , 27 phenyl, t-butyl, 28 or even isopropyl ...

Volume Fraction of Ether Is a Significant Factor in Controlling Conductivity in Proton Conducting Polyether Based Polymer Sol–Gel Electrolytes

“…In general, the MePPG 3 polymer has greater FFV than the MePEG 7 polymer, and the copolymerization of MePPG 3 with the MePEG 7 polymer leads to copolymers with greater FFV as compared to the MePEG 7 polymer. While the differences in FFV seen in Table appear to be small, we have found that these small differences in FFV can have a large effect on the ionic conductivity. − , …”

“… a The effective MW for copolymers represents the MW of one “repeat unit” of the polymer. One repeat unit of MePEG 7 polymer is defined as MePEG 7 OCH 2 CH 2 CH 2 SiO 3/2 . b V m for polymers and copolymers represents the weighted V m calculated using the effective MW. c V w for polymers and copolymers represents the weighted V w calculated by the Bondi group contribution method. d Fractional free volume (FFV) is calculated according to eq . e The calculation of the volume fraction of ether ( V f,ether ) has been reported. ,, f The 75:25 ratio indicates that this copolymer is 75% mole fraction MePEG 7 polymer and 25% mole fraction MePPG 3 polymer (Table S1, Supporting Information). …”

“…Synthesis of MePEG 7 SO 3 H Acid (MePEG 7 (CH 2 ) 3 SO 3 H) was prepared as previously described. ,,− ,, …”

Ab Initio Molecular Dynamics Simulations of an Excess Proton in a Triethylene Glycol–Water Solution: Solvation Structure, Mechanism, and Kinetics

“…Polyethylene glycol (PEG) (also known as poly­(ethylene oxide) (PEO) in its long chain form) is a water-soluble polymer that has been widely studied for its interesting properties in aqueous environments. These properties include interactions of its ethylene oxide (EO) chains in aqueous solutions, − the ability to adsorb on surfaces and interfaces, and proton conductivity in polymer electrolytes. − The interaction of water with the PEG chain backbone has been widely studied to date, both by experiment ,, and simulation ,− due to the interesting ability of PEG to form bridged hydrogen bonds (HBs) from EO monomer units to water molecules to other EO monomer units within the same chain . These properties give PEG a wide variety of applications such as biphasic (two-phase) liquid–liquid catalysis, electrochemical energy conversion as a proton exchange membrane additive, − , and in drug delivery via modification of therapeutic molecules (known as PEGylation). − …”

“…6−18 The interaction of water with the PEG chain backbone has been widely studied to date, both by experiment 2,3,19 and simulation 4,20−26 due to the interesting ability of PEG to form bridged hydrogen bonds (HBs) from EO monomer units to water molecules to other EO monomer units within the same chain. 4 These properties give PEG a wide variety of applications such as biphasic (twophase) liquid−liquid catalysis, 27 electrochemical energy conversion as a proton exchange membrane additive, [8][9][10][11][12][13][14]28 and in drug delivery via modification of therapeutic molecules (known as PEGylation). 29−31 The conductivity of various ions with PEG in aqueous solution has also been of research interest in past years.…”