(606g) Effect of Block Copolymer Membrane Nanostructure on Ethanol-Water Transport

Block copolymers are recognized for exhibiting unique assets not present in homogeneous, single-component materials since they can combine more distinct monomer blocks with different physical, chemical, and mechanical properties [1]. The development of this field originated with the discovery of termination-free anionic polymerization, which made possible the sequential addition of monomers to living linear polymer chains [1,2]. Anionic polymerization is one of the most powerful techniques for controlling macromolecular architectures, molecular weight, composition, and functionality of block copolymers that can achieve a 100% conversion without chain transfer and/or chain termination [3,4], as reported by Fetter and Morton [2].

Thanks to a controlled polymerization synthesis, various combinations of monomer fractions can be made that provide different membrane morphology, grain size, and crystallite domains which allows the exploitation of their optimal mechanical resistance, transport ability, and functionalities [1,5,6].

In this concern, many types of research focused on polystyrene-b-poly(ethylene oxide) (SEO) in which the mechanical stability and resistance of the system is determined by the amorphous rigid phase of Polystyrene (PS), while Poly(ethylene oxide) (PEO) confers the crystallinity together with high connectivity and flexibility to the polymeric chain [7–9]. These block copolymers exhibit different morphologies, and thus different transport properties and species interactions, depending on the mass fraction of one block (PS) with respect to the other (PEO), as can be predicted from the phase diagram of a neutral diblock copolymer [10,11].

Several studies have successfully analyzed gas, vapor and ion transport through SEO nanostructured materials [8,12–14], however liquid permeability has not received proper attention although BCP membranes can be used as a competitive system for a variety of sustainable applications.

Now more than ever, the demand for energy increases, and the impact of fossil fuel combustion on climate change has glowed interest in cleaner energy sources [15]. In this concern, ethanol has received much attention for its exploitation as a biofuel since it is environmentally friendly and cheap, and it can serve as a solvent in numerous industrial processes, even though its usefulness is partially limited by the energy-intensive and price nature of the separation processes. In this concern, membrane technology can play a relevant role in binary water-ethanol mixture separation since it stands as a versatile and efficient method for separating mixtures by blocking less compatible molecules and selectively allowing ethanol molecules to permeate through the membrane.

In this work, PS-b-PEO diblock with different morphology were characterized for their use as membrane materials for ethanol-water transport. In particular, three different BCP structures have been characterized: lamellar (SEO l), cylindrical (SEO c) and spherical (SEO s), with PEO fraction moving from 0.44 to 0.32 and 0.08, respectively.

Lamellar, cylindrical and spherical BCPs have been tested for ethanol permeation at 30°C and with four different alcohol concentrations in the feed (1.7, 3.4, 5.1, and 8.6 M of EtOH) by using in-situ Fourier-transformed infrared spectroscopy (FTIR) that automatically collects the peaks spectra variation over time as much as ethanol is accumulated in the receptor cell.

The normalized permeability results, then calculated employing the Fick’s law based on an early time approximation, are regressed to the binary system and shown as a function of Molar ethanol concentration in Fig.1. Permeability increases exponentially with increasing solution concentration, ranging between 6x10^-9 to 7x10^-7 cm²/s according to the different morphologies, which corresponded to a typical behavior observed in polymeric materials, often associated with the changes of polymer free-volume upon sorption [16].

Moreover, DSC analysis has been used to measure the crystallinity (χ_C) of the different samples and how water and ethanol treatments affect the BCP structures. As reported in Fig. 2, the crystallinity increases following PEO fraction and, as expected, crystals, arranged in a lamellar structure, are larger and more stable with respect to cylindrical and spherical morphologies. Moreover, χ_C decreases rapidly after being immersed in water and ethanol, reaching values around 0.1 for SEO c to 0.03 for SEO l and SEO s, suggesting that the two solvents were able to partially dissolve the PEO structure, with water being slightly more effective in this concern with respect to ethanol.

These results demonstrated that spherical morphology membrane exhibited higher ethanol permeability respect to SEO l and SEO c, due to the lower PEO mass fraction that reduced the water transport and absorption. Moreover, by controlling the BCP morphology, it is possible to limit the PEO crystallite size and mitigate their dissolution in solvents, still maintaining the structural integrity of the material and by providing membrane resistance.

REFERENCES

1. Bates, F.S.; Fredrickson, G.H. Block Copolymer Thermodynamics: Theory and Experiment. https://doi.org/10.1146/annurev.pc.41.100190.002521 2003, 41, 525–557, doi:10.1146/ANNUREV.PC.41.100190.002521.
2. Morton, M.; Fetters, L.J. Anionic Polymerization of Vinyl Monomers. Rubber Chemistry and Technology 1975, 48, 359–409, doi:10.5254/1.3547458.
3. Anionic Polymerization: High Vacuum Techniques - Hadjichristidis - 2000 - Journal of Polymer Science Part A: Polymer Chemistry - Wiley Online Library Available online: https://onlinelibrary.wiley.com/doi/full/10.1002/ (accessed on 13 November 2023).
4. Lohse, D.J.; Hadjichristidis, N. Microphase Separation in Block Copolymers. Curr Opin Colloid Interface Sci 1997, 2, 171–176, doi:10.1016/S1359-0294(97)80023-4.
5. Noro, A.; Cho, D.; Takano, A.; Matsushita, Y. Effect of Molecular Weight Distribution on Microphase-Separated Structures from Block Copolymers. Macromolecules 2005, 38, 4371–4376, doi:10.1021/MA050040T.
6. Matsushita, Y.; Noro, A.; Iinuma, M.; Suzuki, J.; Ohtani, H.; Takano, A. Effect of Composition Distribution on Microphase-Separated Structure from Diblock Copolymers. Macromolecules 2003, 36, 8074–8077, doi:10.1021/MA0301496.
7. Oparaji, O.; Zuo, X.; Hallinan, D.T. Crystallite Dissolution in PEO-Based Polymers Induced by Water Sorption. Polymer (Guildf) 2016, 100, 206–218, doi:10.1016/J.POLYMER.2016.08.026.
8. Oparaji, O.; Minelli, M.; Zhu, C.; Schaible, E.; Hexemer, A.; Hallinan, D.T. Effect of Block Copolymer Morphology on Crystallization and Water Transport. Polymer (Guildf) 2017, 120, 209–216, doi:10.1016/J.POLYMER.2017.05.055.
9. Blatt, M.P.; Hallinan, D.T. Polymer Blend Electrolytes for Batteries and Beyond. Ind Eng Chem Res 2021, 60, 17303–17327, doi:10.1021/ACS.IECR.1C02938.
10. Matsen, M.W.; Schick, M. Stable and Unstable Phases of a Diblock Copolymer Melt. 1994, 72.
11. Tang, P.; Qiu, F.; Zhang, H.; Yang, Y. Morphology and Phase Diagram of Complex Block Copolymers: ABC Star Triblock Copolymers. Journal of Physical Chemistry B 2004, 108, 8434–8438, doi:10.1021/JP037911Q.
12. Goswami, M.; Iyiola, O.O.; Lu, W.; Hong, K.; Zolnierczuk, P.; Stingaciu, L.R.; Heller, W.T.; Taleb, O.; Sumpter, B.G.; Hallinan, D.T. Understanding Interfacial Block Copolymer Structure and Dynamics. Macromolecules 2023, 56, 762–771, doi:10.1021/ACS.MACROMOL.2C01814/SUPPL_FILE/MA2C01814_SI_001.PDF.
13. Minelli, M.; Giacinti Baschetti, M.; Hallinan, D.T.; Balsara, N.P. Study of Gas Permeabilities through Polystyrene-Block-Poly(Ethylene Oxide) Copolymers. J Memb Sci 2013, 432, 83–89, doi:10.1016/J.MEMSCI.2012.12.038.
14. Kim, K.; Hallinan, D.T. Lithium Salt Diffusion in Diblock Copolymer Electrolyte Using Fourier Transform Infrared Spectroscopy. Journal of Physical Chemistry B 2020, 124, 2040–2047, doi:10.1021/ACS.JPCB.9B11446/SUPPL_FILE/JP9B11446_SI_001.PDF.
15. Haelssig, J.B.; Tremblay, A.Y.; Thibault, J.; Huang, X.M. Membrane Dephlegmation: A Hybrid Membrane Separation Process for Efficient Ethanol Recovery. J Memb Sci 2011, 381, 226–236, doi:10.1016/J.MEMSCI.2011.07.035.
16. Hallinan; Daniel T; Jr Transport in Polymer Electrolyte Membranes Using Time-Resolved FTIR-ATR Spectroscopy. 2009.