Block Copolymer Ionomers

Synthesis of Block Copolymer Ionomers (BCPI)

The cationic polymerization systems to produce styrene/diene block co-polymers (BCP’s) have been described in the literature as early as 1956. 10 The first cationic polymerization of isobutylene (IB) monomers was observed by Kennedy and Faust in 1986. 11 More recently, Storey and coworkers have achieved living cationic polymerizations of isobutylene and styrene (See Figure 1). 12 These polymerizations utilize either dicumyl or

Figure 1: Architecture of Block Copolymers

tricumyl chloride as initiators for two and three arm star polymers, respectively. Titanium tetrachloride (TiCl4) was used as the co-initiator in a 60/40 v/v hexane/methyl chloride solution. In this reaction solution, pyridine is also present. Low polydispersities were observed. 12 Storey et al. has also obtained living polymerization of isobutylene using a monofunctional initiating system. This initiating system consists of TiCl4 and pyridine in a 60/40 v/v hexane/methyl chloride solvent system at -80°C. 13 These living PIB-BCPs react with divinylbenzene to produce a core-last multi-arm star polymer. This polymer was coined as ‘gel core multi-arm star block copolymer.’ Termination of polymerization is typically induced by the addition of methanol to the living system. Synthesis of this type of multi-arm star polymer was also demonstrated by Faust et al. 14 Also, Storey and coworkers have recently produced PS-PIB multi-arm star block copolymers 15 using a similar approach.

Block copolymers of PS and polydienes (PD) were converted into BCPIs (See Figure 2). This was conducted by Storey and coworkers who anionically polymerized a three arm star BCP consisting of polybutadiene (PB) inner blocks and oligomeric PS outer blocks. 16 Others have

Figure 2: Architecture of Block Copolymer Ionomers

converted Kratonâ materials into BCPIs, first by hydrogenating the polydiene blocks and, subsequently, sulfonating the styrenic blocks. 17 Sulfonation of PS-PIB BCPs is performed by utilizing sulfuric acid and acetic anhydride in a solution of BCP and methylene chloride 18 forming an in situ acetyl sulfate as the sulfonating reagent. Sulfonated PS swells approximately 10% with water and other protic solvents. 19 The extent of sulfonation is then determined by titration.

The inherent saturation of the PIB results in an improved thermal and oxidative stability over the analogous hydrogenated PB. It also eliminates the possibility of a sulfonation reaction taking place on an elastomeric soft block.

Block Copolymer Phase Behavior

BCP of PS and PB exhibit microphase separation of the dissimilar blocks. Depending upon the relative block volume fraction, spheres, cylinders, or lamellae of PS in a PB matrix are possible. 18 More recently, a bicontinuous morphology has been observed in PS-polyisoprene (PI) block copolymers. This newly discovered morphology has been coined the ‘ordered bicontinuous double diamond’ (OBDD). 20-22 Thomas and coworkers have observed this bicontinuous structure in PS-PI star block copolymers with PI as the major component. Similarly, Hasegawa and coworkers have observed the inverse of OBDD in the linear block copolymers with PS as the major component. 23 Figure 1 shows the range of microstrucures possible in PS-PI block copolymers. 24 However, these morphologies are considered as applying to PS-PD block copolymer systems.

Figure 3: Possible phase separated morphologies in PD-PS block copolymer systems.
fA refers to the volume fraction of the A block, which in this case is polystyrene.

The morphology will affect the bulk properties of BCP. The properties affected include the mechanical strength and toughness of the polymer as well as the gas permeation properties. For instance, Thomas and coworkers have conducted gas diffusion and solubility studies on various Kratonâ, PS-PB block copolymer, materials exhibiting a variety of morpholgies. 25 They have conducted studies of carbon dioxide transport through PS-PB block copolymers of varying composition and microphase separated morphologies. Models of diffusion and the solubility of gases in the polymers developed by Sax and Ottino were used to predict the diffusional properties of the carbon dioxide in various membranes. 25 These rather simple models were successful for the various phase separated morphologies investigated, such as body centered cubic spheres, hexagonally packed cylinders, lamellae, and OBDD. These studies conducted by Thomas et al. show that the gas transport properties are influenced, not only by the volume fraction of the respective blocks, but also by the morphology and connectivity of different phases. The morphology of the PS-PIB block copolymers must be taken into account when tailoring the permselectivity of these materials. It has been shown that microphase separated block copolymers will exhibit order on a local level. However, on a bulk or global scale, the block copolymers exhibit anisotropy. 25 The particular morphology of the block copolymers have significant effect on the bulk mechanical and permeation properties.

REFERENCES

10. Szwarc, M. Journal of the American Chemical Society 78, 2656.
11. Faust, R.; Kennedy, J.P. Journal of Polymer Science, Polymer Chemistry Edition 25, 1847, 1987.
12. Storey, R.F. Journal of Materials Science 1992, A29 (11), 1017.
13. Storey, R.F.; Shoemake, K.A.; Chisholm, B.J. submitted for publication.
14. Faust, R.; McKenna, S.T.; Wang, I. Macromolecules 1995, 28, 4681.
15. Storey, R.F.; Shoemake, K.A. American Chemical Society Polymer Preprint 1994, 35 (2), 578.
16. Storey, R.F; Nelson, M.E. Macromolecules 1991, 24, 2920.
17. Eisenberg, A.; Nishida, M. Macromolecules 1996, 29, 1507.
18. Lee, Y., Ph.D. Dissertation, University of Southern Mississippi, Hattiesburg, MS, 1991, pp. 84-85.
19. Jiang, M., Ph.D. Dissertation, University of Calgary, Calgary, Alberta, 1995, pp. 65, 156.
20. Kinning, D.J.; Thomas, E.L.; Alward, D.B.; Fetters, L.J.; Handlin, D.L. Jr. Macromolecules 1986, 19, 1288.
21. Thomas, E.L.; Alward, D.B.; Kinning, D.J.; Martin, D.C.; Handlin, D.L. Jr.; Fetters, L.J. Macromolecules 1986, 19, 2197.
22. Herman, D.S.; Kinning, D.J.; Thomas, E.L.; Fetters, L.J. Macromolecules 1987, 20, 2940.
23. Hasegawa, H.; Tanaka, H.; Yamasaki, K.; Hashimoto, T. Macromolecules 1987, 20, 1651.
24. Bates, F.S. Washington Science 1991, 251, 898.
25. Kinning, D.J.; Thomas, E.L.; Ottino, J.M. Macromolecules 1987, 20, 1129.

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