Proceso y tensiones dentro de los 21 días del Paro Cívico

Ionomers neutralized with Br-, TFSI-, and F- show nearly identical X-ray

scattering in Figure 5.8, with three scattering features, two of which were previously observed in the non-ionic polymers in Figure 5.5. The high-angle peak, q = 15 nm-1,is

primarily the amorphous halo from the PEO side chains (peak II). The peak position (q ~ 3.7 nm-1) and intensity of the siloxane backbone-to-backbone spacing (peak I) are also

constant across at all ion contents and counterion types because these copolymers have the same PEO side chain (PEO3) and the various phosphonium side chains have similar

148

molar volumes. Electron densities for phosphonium salts are calculated to be 358, 377, and 419 e-/nm3 for F-, Br-, and TFSI-, respectively, based on densities approximated by

Ye and Shreeve.30 Because the electron densities are close to the density of PEO (371 e-

/nm3) relative to siloxane (310 e-/nm3), contrast between these polymers is nominally

independent of ion type. Peak III, which is intermediate between peak I and II, appears to be related to the presence of ion pairs in this ionomer, as peak III is strongest at highest ion contents (Figure 5.8).

Interestingly, there are no explicit scattering contributions from more extensive ionic aggregation at the 2-4 nm length scale. The suppression of ionic aggregates is attributed to the bulkiness of the phosphonium cations and charge shielding caused by its butyl segments. The absence of physical ionic cross-links is consistent with Tg remaining

low in all of these phosphonium ionomers. Considering the lack of explicit ionic aggregation, we expect that these ionomers will show ion pair-to-ion pair scattering between q = 3-5 nm-1, as estimated by assuming the ion pairs are randomly distributed in

the ionomer. Phosphonium bromide pairs, for example, will scatter ca. q ~ 3 nm-1 at 5

mol% phosphonium salt, and shift to ca.q ~ 5 nm-1 at 22 mol% phosphonium salt. Thus,

scattering peak III does not arise from inter-pair spacings, but peak III only exists in the ionomer form of these siloxanes, and does not change with ion type. The exact origin of this peak remains unclear.

149

Figure 5.8. X-ray scattering of PSPE-nA(3) at 25°C, where the anionic counterion A is

(a) Br-, (b) TFSI-, or (c) F-. Curves are shifted vertically for clarity.

Figure 5.9 compares X-ray scattering for PSPE-11A(3) for all three neutralizing anions at 25 °C and 125 °C. Thermal expansion causes the amorphous halo (peak II) to shift to slightly lower q at 125 °C. The scattering intensity of peak I increases in intensity

150

at elevated temperatures relative to the amorphous carbon halo for which no significant change in scattering intensity is expected. Upon cooling to 25°C the scattering patterns are fully recovered. Overall, the PSPE-11A(3) copolymers do not exhibit new features representative of a changed morphology across the temperature range of 25–125°C, which is important as we explore the transport properties as a function of temperature.

Figure 5.9. X-ray scattering of PSPE-11A(3), where A is Br-, TFSI-, or F-. Closed symbols (●) are data at 25 °C, and open symbols (○) are data at 125 °C. Curves are shifted vertically for clarity, and samples are thermally reversible.

151

5.3.5. Ionic Conductivity

The phosphonium single-ion conductors with different counterions and ion contents show a slight variation in ionic conductivity with ion content, Figure 5.10. It is well known that ion conduction in polymers is usually coupled to chain segmental motion31 and this seems universally true for all ionomers based on PEO, so Figure 5.10 is

normalized by Tg. However, when phosphonium salt concentration increases from 5 to

22 mol%, Tg barely changes (see Table 5.1). X-ray scattering data provide no evidence

for physical cross-linking via ionic aggregates at 25 or 125 °C, consistent with ionomer segmental dynamics being largely unaffected. Therefore, we observe only modest conductivity improvements at the highest ion contents, whereas ionomers usually show lower conductivity at high ion content because Tg usually increases strongly with ion

content.32 Consistently low Tgs and ionic conductivities that are relatively insensitive to

phosphonium composition demonstrate that the conductivity is dominated by segmental motion of the PEO side chains. Furthermore, conductivity varies smoothly with temperature, consistent with the absence of significant morphology changes across this temperature range (Figure 5.9).

Even though the Tg increases slightly with ion content, conductivity of the

phosphonium ionomers with F- mobile anions increase with ion content up to the highest

ion content studied (22 mol% phosphonium) because ion hopping distances are shortened by the higher ion content and Tg only mildly increased. The conductivity is as high as 10- 6

S/cm at room temperature making this a promising material for the electrolyte separator in a fluoride-ion battery.33, 34 Although not studied here, the conductivity of iodide salts

152

phosphonium ionomers also have potential use as single-ion conductors for dye- sensitized solar cells.

Figure 5.10. DC conductivity of phosphonium ionomers with different ion content as a function of temperature: (a) bromide counterion, (b) TFSI counterion, (c) fluoride counterion.

153

If the counterion is exchanged for a more bulky, charge delocalized mobile species, the ionomer exhibits a substantial increase in net counterion transport. Conductivities of phosphonium ionomers with the same 11 mol% ion content but different anion species are shown in Figure 5.11. The conductivities of those ionomers increase with increasing counterion size: F- < Br- < TFSI-. Since the morphologies of

these ionomers are comparable for all ion types, the differences in conductivity stem from the weaker ionic interactions associated with larger counterions. Ye, et al.9 studied

imidazolium-based polymerized ionic liquid and found that the conductivity of ionomers with TFSI- anions was greater than those of ionomers with PF6- or BF4-. They attributed

the difference to not only the size effect but also delocalized charge distribution and flexibility of the TFSI- anion.35 The electrode polarization analysis yields activation

energies (Ea) for the number density of simultaneous conductors for these counterions

summarized in Table 5.3. The low Ea for TFSI- containing phosphonium ionomer is

consistent with its highest conductivity, which might suggest that Ea is a key factor

determining conductivity in these low-Tg phosphonium ionomers in the absence of ionic

154

Figure 5.11. DC conductivities of phosphonium ionomers with different counterions having ion content n / (n + (1-n)) = 0.11. Note that the F- ionomer may have lost ionic groups during ion exchange, and thus might have a slightly lower conductivity than expected.

Table 5. 3. Conducting ion properties of different anions (A) in PSPE-11A(3)

Ionomer TFSI- Br- F-

Ea (kJ/mol) 9.4 14.2 18.3

Anion Size

(van der Waals radii, nm)36, 37 0.326 0.195 0.136 Ion Pair Energy

(gas phase, kJ/mol) 284 369 481

5.4. SUMMARY

Allyltributylphosphonium bromide (ATPB) was successfully synthesized under a solvent-free condition. These phosphonium salts and vinyl PEOx oligomers have been

155

Parent Br- ions were exchanged for different anions (F- or TFSI-). The ionomers with

TFSI-, Br- or F- counterions are stable at 120 oC in dry nitrogen or vacuum.

X-ray scattering indicates these phosphonium ionomers exhibit ion pairs rather than ionic aggregates and have very low Tgs that only increase weakly with ion content

and are insensitive to counter-anion. The very low Tgs are attributed to (1) the inherent

flexibility of the polysiloxane backbone, (2) the presence of ion-solvating PEO side chains that circumvent ionic aggregation, and (3) the electronic structure of the phosphonium cation that also limits ionic aggregation. X-ray scattering of the phosphonium ionomers also indicates that these comb copolymers demonstrate backbone-backbone contrast that is dependent on side chain length. The conductivities of

phosphonium ionomers are enhanced by increasing anion size. The ionomers with TFSI-

show the highest conductivity across the whole temperature range, owing to the largest size of TFSI- and weakest ionic interactions between TFSI- and the phosphonium cation attached to the polymer. Whereas conventional ionomers have Tg increase strongly with

ion content,50 our tetraalkylphosphonium ionomers with high ionic content of 22 mol%

only have Tg 11 K larger than that of their nonionic equivalent (PSPE-0(3) with Tg = -86 o

C). Tg barely changing with ion content is very rare, only previously reported in Weiss’

study of sulfonated polystyrene with a series of alkyl ammonium counterions.60

Tetrabutyl ammonium counterions, quite similar in size to our phosphoniums, exhibit similar insensitivity of Tg to ion content, while even longer tail ammonium counterions

actually act as plasticizers that lower Tg! This suggests a new direction for materials

156

5.5. REFERENCES

1. Kang, J. J.; Fang, S. B. Polymer Bulletin 2002, 49, 127-134.

2. Kang, J. J.; Li, W. Y.; Lin, Y. A.; Li, X. P.; Xiao, X. R.; Fang, S. B. Polym. Adv. Technol. 2004, 15, 61-64.

3. Sauvet, G.; Dupond, S.; Kazmierski, K.; Chojnowski, J. J. Appl. Polym. Sci. 2000,

75, 1005-1012.

4. Huang, Z. X.; Yu, Y. Z.; Huang, Y. J. Appl. Polym. Sci. 2002, 83, 3099-3104. 5. Ghassemi, H.; Riley, D. J.; Curtis, M.; Bonaplata, E.; McGrath, J. E. Appl Organomet Chem 1998, 12, 781-785.

6. Gu, S.; Cai, R.; Luo, T.; Chen, Z. W.; Sun, M. W.; Liu, Y.; He, G. H.; Yan, Y. S.

Angew. Chem.-Int. Edit. 2009, 48, 6499-6502.

7. Lu, S.; Pan, J.; Huang, A.; Zhuang, L.; Lu, J. Proceedings of the National Academy of Sciences of the United States of America 2008, 105, 20611-20614.

8. Tang, D.; Pan, J.; Lu, S.; Zhuang, L.; Lu, J. Science China-Chemistry 2010, 53, 357-364.

9. Ye, Y.; Elabd, Y. A. Polymer 2011, 52, 1309-1317.

10. Wang, S.-W.; Liu, W.; Colby, R. H. Chem. Mat. 2011, 23, 1862-1873.

11. Haav, K.; Saame, J.; Kuett, A.; Leito, I. European Journal of Organic Chemistry

2012, 2012, 2167-2172.

12. Cheng, S.; Zhang, M.; Wu, T.; Hemp, S. T.; Mather, B. D.; Moore, R. B.; Long,

T. E. Journal of Polymer Science Part a-Polymer Chemistry 2012, 50, 166-173.

13. Williams, S. R.; Wang, W.; Winey, K. I.; Long, T. E. Macromolecules 2008, 41, 9072-9079.

157

14. Cheng, S. J.; Beyer, F. L.; Mather, B. D.; Moore, R. B.; Long, T. E.

Macromolecules 2011, 44, 6509-6517.

15. Parent, J. S.; Penciu, A.; Guillen-Castellanos, S. A.; Liskova, A.; Whitney, R. A.

Macromolecules 2004, 37, 7477-7483.

16. Liang, S.; O’Reilly, M. V.; Choi, U. H.; Shiau, H.-S.; Bartels, J.; Chen, Q.; Runt, J.; Winey, K. I.; Colby, R. H. Macromolecules 2014, 47, 4428-4437.

17. Heiney, P. A. Commission on Powder Diffraction Newsletter 2005, 32, 9-11. 18. Chen, Q.; Liang, S.; Shiau, H.-s.; Colby, R. H. Acs Macro Letters 2013, 2, 970- 974.

19. Choi, U. H.; Mittal, A.; Price, T. L.; Gibson, H. W.; Runt, J.; Colby, R. H.

Macromolecules 2013, 46, 1175-1186.

20. Wang, W.; Liu, W.; Tudryn, G. J.; Colby, R. H.; Winey, K. I. Macromolecules

2010, 43, 4223-4229.

21. Galin, M.; Mathis, A. Macromolecules 1981, 14, 677-683. 22. Beiner, M.; Huth, H. Nat Mater 2003, 2, 595-599.

23. Arbe, A.; Genix, A. C.; Colmenero, J.; Richter, D.; Fouquet, P. Soft Matter 2008,

4, 1792-1795.

24. Floudas, G.; Štepánek, P. Macromolecules 1998, 31, 6951-6957.

25. Gerstl, C.; Brodeck, M.; Schneider, G. J.; Su, Y.; Allgaier, J.; Arbe, A.; Colmenero, J.; Richter, D. Macromolecules 2012, 45, 7293-7303.

26. Miller, R. L.; Boyer, R. F.; Heijboer, J. Journal of Polymer Science: Polymer Physics Edition 1984, 22, 2021-2041.

158

28. Beiner, M.; Kabisch, O.; Reichl, S.; Huth, H. Journal of Non-Crystalline Solids

2002, 307–310, 658-666.

29. Watanabe, J.; Ono, H.; Uematsu, I.; Abe, A. Macromolecules 1985, 18, 2141-

2148.

30. Ye, C. F.; Shreeve, J. M. J. Phys. Chem. A 2007, 111, 1456-1461. 31. Ratner, M. A.; Shriver, D. F. Chemical Reviews 1988, 88, 109-124. 32. Eisenberg, A.; Kim, J. S., Introduction to Ionomers. Wiley: 1998.

33. Yazami, R.; Hamwi, A. Fluorination of multi-layered carbon nanomaterials.

US7794880 B2, 2010.

34. Yazami, R. Fluoride Ion Electrochemical Cell. US20090029237 A1, 2009.

35. Arnaud, R.; Benrabah, D.; Sanchez, J. Y. Journal of Physical Chemistry 1996,

100, 10882-10891.

36. Ue, M.; Murakami, A.; Nakamura, S. Journal of the Electrochemical Society

2002, 149, A1385-A1388.

37. Jenkins, H. D. B.; Thakur, K. P. Journal of Chemical Education 1979, 56, 576- 577.

159