Article pubs.acs.org/Macromolecules
Doubly-Charged Ionomers with Enhanced Microphase-Separation Keren Zhang, Gregory B. Fahs, Kevin J. Drummey, Robert B. Moore, and Timothy E. Long* Department of Chemistry, Macromolecules Innovation Institute, Virginia Tech, Blacksburg, Virginia 24061, United States S Supporting Information *
ABSTRACT: Two styrenic DABCO salt monomers allowed the synthesis of DABCO salt-containing random copolymers with two quaternized nitrogen cations on each ionic pendant group. Triethyl-(4-vinylbenzyl)ammonium chloride (VBTEACl) containing random copolymers served as singly charged controls. DABCO salt-containing copolymers with 20 mol % or lower ionic contents exhibited microphase-separated morphologies, agreeing with the multiplet-cluster model for random ionomers. Thermomechanical and morphological analyses revealed that doubly charged DABCO salts promoted more well-defined microphase-separation than singly charged analogues. Stronger ionic association of DABCO salts compared to trialkyl ammoniums resulted in superior thermomechanical and tensile properties of DABCO salt-containing ionomers. The doubly charged copolymers exhibited less water uptake per charge than the singly charged analogues. Anion exchange of the halides to more hydrophobic anions led to enhanced thermal stability, increased phase-mixing, and reduced water uptake for DABCO salt-containing copolymers and their singly charged controls. Alkyl substituent lengths on the DABCO salts affected water uptake of DABCO salt-containing copolymers. However, thermomechanical properties and thermal stability did not differ significantly between copolymers with hexyl and tetradecyl substituents.
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INTRODUCTION Industrial interests in ionomers surged from the 1950s with the commercialization of Dupont’s Hypalon (chlorosulfonated polyethylene) and Surlyn (poly(ethylene-co-methacrylate salt)).1 Today, ionomers dominate a large market share of elastomers, membranes, resins, coatings, adhesives, and biomedical materials.2,3 Ionic sites prove essential for the viscoelastic and mechanical performance of ionomers, and ionic interactions drive the formation of ionic aggregates, known as mutiplets, in a neutral polymer matrix.4 Areas around these mutiplets contain polymer chains with restricted mobility, which overlap and form an ionic phase with elevated glass transition temperature (Tg). Incorporation of ionic aggregates into a low Tg (lower than the application temperature) soft phase affords flexibility and elastomeric properties. In addition, ionomers with limited or in the absence of a flexible component serve as thermoplastics. Ionomers typically offer superior processability compared to covalently cross-linked polymers due to reversible ionic interactions. Other physical cross-linking mechanisms such as hydrogen bonding and chain entanglement also contribute to thermo-reversibility and often serve as a supplementary means for improving mechanical strength due to their weaker association strength than ionic interactions.2,5,6 Combining ionic interactions with segmented or block copolymer architectures affords a higher order of microphaseseparation and superior physical properties for ionomers, such as ion-containing polyurethanes.7,8 However, the preparation of segmented and block copolymers generally requires more complex synthetic routes. Thus, random ionomers represent preferential materials for processable thermoplastics and © XXXX American Chemical Society
elastomers if the ionic groups induce phase-separation and retain a physically cross-linked network with desirable mechanical performance.1,2 Furthermore, water uptake and ion transport properties of ionomers enable a wide variety of membrane applications.9,10 Mechanical integrity and ion/water transport properties of ionomers such as Nafion proves essential for their applications as proton exchange membrane (PEM) in fuel cells.11 Establishing structure−property relationships for novel random ionomers will assist the property optimization of these polymers and determine their potential commercial impact for various applications. Recently, an emerging design feature for novel ioncontaining polymers involves monomers carrying more than one ionic site.12−17 These multiply charged monomers led to superior properties than their singly charged controls. Li et al. synthesized a polyelectrolyte with bis(trialkyl ammonium) acrylate, displaying a lower critical aggregation value and larger aggregate sizes than its singly charged analogue at similar charge concentration.16 Laschewsky and others reviewed various zwitterion-containing polymers with unique properties.12,18−20 Many other researchers explored the potential of multiply charged polyelectrolytes for water treatment, gene delivery, and antimicrobial material.14,21−24 However, earlier studies focused on multiply charged polyelectrolytes and their solution properties while multiply charged ionomers and their solid-state properties remained relatively unexplored. MoreReceived: April 20, 2016 Revised: July 22, 2016
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DOI: 10.1021/acs.macromol.6b00832 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Chemical structures of poly(VBDC6BrCl-co-nBA), poly(VBDC14BrCl-co-nBA), and poly(VBTEACl-co-nBA) random copolymers.
Scheme 1. Anion Exchange of Poly(VBDC6BrCl-co-nBA) to Poly(VBDC6BF4-co-nBA) or Poly(VBDC6Tf2N-co-nBA) Using NaBF4 or LiTf2N, Respectively
Analytical Methods. Polymers were dissolved in methanol or acetone and casted into PTFE Petri dishes. The solutions were maintained at room temperature for 24 h to allow solvent evaporation, placed in vacuo for 24 h at room temperature, and finally at 40 °C in vacuo for 24 h. All film samples remained in a desiccator prior to any experiment. Dynamic mechanical analysis (DMA) utilized a TA Instruments Q800 Dynamic Mechanical Analyzer in tension mode at a frequency of 1 Hz, oscillatory amplitude of 15 μm, and a static force of 0.01 N. The temperature ramp was 3 °C/min. The peak maxima of tan δ curves determined T g values. A TA Instruments Q5000 thermogravimetric sorption analyzer (TGA-SA) probed the water sorption behavior of ammonium-containing copolymer samples using a relative humidity (RH) sweep procedure from 0−95% RH with 5% increase in each step. Each RH step proceeded until the sample weight equilibrated (
