Article pubs.acs.org/Macromolecules
Precision Ionomers: Synthesis and Thermal/Mechanical Characterization Brian S. Aitken,† C. Francisco Buitrago,‡ Jason D. Heffley,† Minjae Lee,§ Harry W. Gibson,§ Karen I. Winey,‡,⊥ and Kenneth B. Wagener*,† †
George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States ‡ Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States § Department of Chemistry, Virginia Polytechnic Institute & State University, Blacksburg, Virginia 24061, United States ⊥ Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: Six perfectly regioregular polyethylene (PE)-based ionomers containing 1-methylimidazolium bromide groups on exactly every 9th, 15th, or 21st carbon (precision ionomers) and two regiorandom analogues have been synthesized and characterized via dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). Because these materials were synthesized by a postpolymerization functionalization route, their number-average molecular weights (Mns) and polydispersity indices (PDIs) could be accurately calculated based on measurements of the preionized polymers; Mns range from 36 to 53 kDa with PDIs all close to 2. Thermal gravimetric analysis (TGA) indicates stability up 250 °C, and DSC measurements indicate that crystallinity is a function of the polymer backbone spacer length. Tms range from ∼80 to 106 °C, with longer spacer lengths inducing semicrystallinity. DSC measured glass transition temperatures (Tgs) range from −1.6 to 26.8 °C and appear to be dependent on both spacer length and crystallinity. DMA data loosely mirror the DSC results, but with transitions occurring at lower temperatures that we attribute to differences in the thermal history and/or the different heating ramp rates used.
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INTRODUCTION Ionomers comprise a class of polymers containing a relatively low concentration of pendant ionic groups. At a global production rate of about 300 million pounds/year, they are of great commercial importance and find use in a range of applications as ion transport membranes, electromechanical devices, thermoplastic elastomers, adhesives, and other uses.1 Moreover, while polyanions are by far the more common derivatives, recently there has been considerable interest in polycations, which are the focus of this paper, due to their potential applicability in anion exchange membrane fuel cells2 and mechanical actuators.3 Ionic polymers are often prepared via polymerization of acryloyl- or vinyl-functionalized ionic liquids or by ionization of electrically neutral polymers.1−17 Countless studies have been and continue to be conducted in efforts to understand and control ionomer morphology. With some exceptions, such as regularly sequenced polyurethane,18−20 polysiloxane,21,22 and poly(ethylene oxide)23,24 based ionomers, most current synthetic approaches yield a random (or pseudorandom) distribution of ionic groups along a polymer backbone (type A of Figure 1); thus, the impact of perfect regioregularity on ionomer morphology and performance in various applications remains largely unexplored due to a lack of synthetic methodology.1,25 © 2012 American Chemical Society
Figure 1. General architectures of regiorandom ionomers (A) and precision ionomers (B).
We recently reported the synthesis of an ionomer and an ionene (in which the ionic group is in the main chain of the polymer) by acyclic diene metathesis polymerization (ADMET) of α,ω-diene-functionalized ionic liquids.26 This method, for the first time, provided access to a polyolefin-based precision ionomer (type B of Figure 1); an imidazolium hexafluorophosphate group was located on each and every 21st carbon along a linear polyolefin backbone. Synthesis of these materials by ADMET proved quite challenging due to Received: October 13, 2011 Revised: November 28, 2011 Published: January 6, 2012 681
dx.doi.org/10.1021/ma202304s | Macromolecules 2012, 45, 681−687
Macromolecules
Article
4H), 3.45 (d, 2H), 4.91−5.10 (m, 4H), 5.65−5.90 (m, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 26.1, 32.5, 34.1, 39.50, 39.55, 114.8, 138.8. 9-(Bromomethyl)deptadeca-1,16-diene (5) (x = 6). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.18−1.50 (m, 20H), 1.61 (p, 1H), 2.06 (q, 4H), 3.45 (d, 2H), 4.90−5.05 (m, 4H), 5.73−5.90 (m, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 26.7, 29.1, 29.3, 29.8, 32.8, 34.0, 39.7, 39.8, 114.4, 139.3. Polymerization of Monomers 4−6. A typical synthesis of P1− P4 is given. 3.00 g (7.25 mmol) of 6 was added to a custom-built flatbottomed mechanically stirred reactor designed to continuously spread the polymerization matrix as a thin film. The vessel was evacuated overnight to remove dissolved oxygen and then purged with argon, and 59 mg (72 μmol, 1 mol %) of Grubbs' first-generation catalyst was carefully added under a strong flow of argon. A cycle of evacuation and refilling with argon was repeated three times before a final evacuation. The reaction mixture was then heated to 40 °C and stirred at 350 rpm for 24 h under vacuum, at which point the vessel was cooled to room temperature, refilled with argon, and 10 mL of dry degassed toluene was added to dissolve the polymer (P3). After dissolution, 1 mL of dry degassed ethyl vinyl ether (EVE) was added, and stirring was continued for 1 h at room temperature under argon. After 1 h the EVE was evaporated in vacuo, and the polymer solution was left to stir open to air for 24 h to facilitate decomposition of the ethereal ruthenium alkylidene into ruthenium oxide, as indicated by a change in color from pale red-brown to black. The solution was concentrated to ∼5 mL by passing air over its surface under gentle heating, and finally it was passed through a short plug of silica (approximately 2 cm in diameter × 6 cm tall) using toluene as eluent to remove the ruthenium species. The column fractions were concentrated to ∼10 mL of clear, very pale yellow solution, which was precipitated into 400 mL of methanol to yield a bright white tacky gum. The gum was dried under vacuum overnight and then coalesced into a clear colorless gum (2.54 g, 91%) over a period of a few days. P1 (x = 3). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.24−1.50 (m, 8H), 1.58−1.70 (m, 1H), 2.0−2.18 (m, 4H), 3.44 (d, 2H), 5.29−5.40 (m, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 25.9, 26.9, 29.6, 32.3, 32.4, 33.9, 39.5, 39.6, 129.8, 130.4. GPC (THF, light scattering detector, PS standards): Mn = 26 kDa, PDI = 2.0. P2 (x = 6). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.16−1.50 (m, 20H), 1.55−1.63 (m, 1H), 2.0−2.12 (m, 4H), 3.45 (d, 2H), 5.31−5.40 (m, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 26.7, 27.4, 29.1−29.3 (broad overlapping signals), 29.8, 33.8, 34.0, 39.7, 39.8, 129.5, 130.0. GPC (THF, light scattering detector, PS standards): Mn = 31 kDa, PDI = 1.9. P3 (x = 9). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.20−1.44 (m, 32H), 1.55−1.64 (m, 1H), 1.90−2.05 (m, 4H), 3.43 (d, 2H), 5.33− 5.42 (m, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 26.8, 27.7, 29.1−29.8, (broad overlapping signals), 29.9, 33.8, 34.0, 39.7, 39.9, 129.5, 130.0. GPC (THF, light scattering detector, PS standards): Mn = 40 kDa, PDI = 2.0. P4 (Random Copolymer Analogous to P3). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.20−1.40 (m, 20H) 1.54−1.64 (m, 1H), 1.90− 2.11 (m, 10H), 3.43 (d, 2H), 5.30−5.45 (m, 5H). 13C NMR (75 MHz, CDCl3), δ (ppm): 26.0, 26.4, 27.7, 27.8, 29.0−29.9 (broad overlapping signals), 33.5, 33.6, 39.70, 39.75, 39.80, 39.85, 39.9, 129.5, 129.9, 130.2, 130.3, 130.5, 131.0. GPC (THF, light scattering detector, PS standards): Mn = 33 kDa, PDI = 2.0. Hydrogenation Conditions for Synthesis of P5−P8. A typical procedure for hydrogenation of P1−P4 is given. One gram of P3 was dissolved in 50 mL of dry degassed toluene and sparged with argon for 30 min to remove oxygen. The solution was transferred to a Parr bomb hydrogenation apparatus charged with 30 mg of Wilkinson’s catalyst. The bomb was charged to 400 psi with hydrogen gas, and the gas was released via a needle valve. The sequence of pressurization and release of hydrogen was repeated three times to remove oxygen. The bomb was then charged to 800 psi, heated to 50 °C, and left to stir for 48 h, at which point a small sample was removed for NMR. After collecting 1H NMR (acquisition time of 1 h) to ensure complete loss of residual olefin signals and indicate reaction completion, the reactor
difficulties in achieving the very high monomer purity required for the reaction and the extremely high viscosities encountered after oligomerization. Later attempts to prepare a library of ionomers containing functionality spaced at shorter intervals for a structure property study were hampered by the same issues as were discussed in the original report; however, difficulties were exacerbated further by the greater ion content. Additionally, due to the incompatibility of ionic liquids with hydrocarbons, we found it impossible to produce regiorandom materials by copolymerization with linear hydrocarbon dienes, which is the usual method in our laboratory.27 Thus, no comparisons between regioregular and regiorandom analogues were possible. Furthermore, because the polymers were produced by ADMET, thereby containing one alkene per repeat unit, we intended to enhance crystallinity by saturating the double bonds. However, the polymers were soluble only in DMSO, and quantitative hydrogenation using Wilkinson’s catalyst proved impossible. Finally, as is often the case for ionomers, measurement of their molecular weights proved impossible by any means due to aggregation even in hot electrolyte solutions. In the present report, we have overcome all of the aforementioned difficulties by preparing regioregular primary bromide-functionalized polymers and their regiorandom analogues via ADMET. This was followed by hydrogenation of the alkene residuals in the soluble neutral polymers and finally quantitative quaternization using excess 1-methylimidazole. The set of eight ionomers was characterized via NMR, DSC, TGA, DMA, and molecular weights were calculated based on GPC measurements of the precursor primary bromide functional polymers.
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EXPERIMENTAL SECTION
Materials and Instrumentation. All materials were purchased from Aldrich and used as received, unless noted otherwise. Dry solvents were obtained from an MBraun solvent purification system. The 1,9-decadiene was dried and purified via a potassium mirror distillation and stored over activated molecular sieves. Grubbs' firstgeneration ruthenium catalyst, bis(tricyclohexylphosphine)benzylidine ruthenium(IV) dichloride, was a generous gift from Materia, Inc. Proton and 13C NMR spectra were acquired on a Varian Mercury 300 MHz spectrometer. Differential scanning calorimetry (DSC) data were obtained on a TA Instruments Q1000 from −60 to 220 °C at a typical scan rate of 10 °C/min under a He purge. Dynamic mechanical analysis (DMA) was performed with a Rheometrics Solids Analyzer (RSAII) at a frequency of 0.16 Hz and a maximum strain set to 3% under nitrogen purge. A static load of 100 gmf was applied initially and was then adjusted to 125% of each measured dynamic load. This adjusted static load was necessary to prevent sample buckling. Mechanical data were collected starting at −50.0 °C and every 20 s thereafter, while the temperature increased at a rate of 3.0 °C/min. The DMA samples were films pressed from a mold at ∼10 °C higher than the maximum thermal transition reported by DSC. The film dimensions based on the mold were approximately 53 mm long, 6 mm wide, and 0.3 mm thick; exact dimensions for each polymer deviated slightly but were accurately measured and used in DMA calculations. Gel permeation chromatography (GPC) was performed at 40 °C using a Waters Associates GPCV2000 liquid chromatography system with an internal differential refractive index detector and two Waters Styragel HR-5E columns (10 μm PD, 7.8 mm i.d., 300 mm length) using HPLC grade THF as the mobile phase at a flow rate of 1.0 mL/min. Monomer Synthesis. Synthesis of 1−3 and 6 has been previously described by our group.28,29 Synthesis of 4 and 5 from 1 and 2 was carried out using a procedure identical to that used in the preparation of 6 from 3.29 6-(Bromomethyl)undeca-1,10-diene (4) (x = 3). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.30−1.45 (m, 8H), 1.63 (p, 1H), 2.05 (q, 682
dx.doi.org/10.1021/ma202304s | Macromolecules 2012, 45, 681−687
Macromolecules
Article
Figure 2. Synthesis of six precision ionomers and two regiorandom analogues. was opened and the mixture was stirred under a slow stream of pressurized air for 24 h to facilitate decomposition of Wilkinson’s catalyst into rhodium oxide. The concentrated solution of P8 was passed through a short plug of silica and ultimately precipitated into methanol in a manner similar to that used for the removal of ruthenium from P1−P4. After precipitation and drying in vacuo, 963 mg (96%) of white gum was obtained. P5 (Random Copolymer Analogous to P8). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.10−1.40 (m, 40H), 1.57−1.68 (m, 1H), 3.44 (d, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 25.9, 26.7, 29.0−29.6 (broad overlapping signals), 33.9, 34.0, 39.6, 39.7, 39.9. GPC (THF, light scattering detector, PS standards): Mn = 35 kDa, PDI = 2.0. P6 (x = 3). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.20−1.50 (m, 16H), 1.60−1.70 (m, 1H), 3.45 (d, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 25.8, 29.6, 29.9, 33.9, 39.5, 39.6. GPC (THF, light scattering detector, PS standards): Mn = 28 kDa, PDI = 2.1. P7 (x = 6). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.14−1.49 (m, 28H), 1.57−1.63 (m, 1H), 3.44 (d, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 26.6, 29.1−29.6 (broad overlapping signals), 34.0, 39.7, 39.8. GPC (THF, light scattering detector, PS standards): Mn = 35 kDa, PDI = 2.0. P8 (x = 9). 1H NMR (300 MHz, CDCl3), δ (ppm): 1.18−1.44 (m, 40H), 1.55−1.65 (m, 1H), 3.43 (d, 2H). 13C NMR (75 MHz, CDCl3), δ (ppm): 26.7, 29.1−29.6 (broad overlapping signals), 34.0, 39.7, 39.9. GPC (THF, light scattering detector, PS standards): Mn = 44 kDa, PDI = 2.0. Quaternization of 1° Br Polymers To Produce Ionomers P9− P16. Typical quaternization reaction conditions for synthesis of P9− P16 are given. To a 50 mL round-bottom flask charged with 1 g of P3 dissolved in 15 mL of anhydrous THF was added 2 mL of 1methylimidazole. The flask was equipped with a Vigreux column, and the entire reaction vessel was purged with argon for 30 min. The mixture was then heated at reflux for 4 h, the THF was evaporated in vacuo, and 15 mL of dry DMSO was added. The reaction was heated at 80 °C for 40 h, at which time the DMSO and excess 1methylimidazole were removed by vacuum distillation. The polymer was then rigorously dried by heating at 80 °C under high vacuum (
