Thermally Rearranged Nanofibrous Membranes for Separation in Ion and Liquid

Abstract

In this thesis, thermally rearranged nanofibrous membranes (TR-NFMs) were for the first time developed and introduced for various applications. TR-NFMs were fabricated via electrospinning technique and their potentials were demonstrated for use in various applications, such as mechanical barrier, membrane contactor, and liquid separation applications. Although the nanofibrous membranes in literature have been already well-developed and widely used for biomaterials and sensor materials due to distinct advantages (e.g. their high surface area and porous structure), the physical, chemical and thermal properties of them were not sufficient for use in other applications. Therefore, for the use in other applications, we have developed new kinds of nanofibrous membranes having highly porous structure, mechanically robustness, remarkable chemical and thermal stabilities. Herein, to develop the new kinds of nanofibrous membranes, we have studied covering from a brief review of the electrospinning technique to fabrication and modification of the nanofibrous membranes for various applications. In Chapter 1, the key parameters of electrospinning to modify morphology and physical properties of the nanofibrous membranes were introduced and classified into three categories; (1) polymer properties, (2) process parameters, and (3) ambient conditions. Based on the study of the key parameters, the physical and chemical properties of TR-NFMs were controlled and developed to satisfy requirements for each application. Furthermore, other nanofibrous membranes were also developed with the similar procedure to optimize fabrication conditions as described in this chapter. Therefore, from Chapter 1, the key parameters of electrospinning and the approach to obtain well-tailored nanofibrous membranes were introduced as well as TR-NFM and other nanofibrous membranes were wellfabricated by controlling the key parameter to obtain intended properties for each application. In this thesis, we focused on the use of TR-NFMs for various applications due to their extraordinary physical, chemical, and thermal properties compared with other nanofibrous membranes. For the first application, TR-NFMs were used as a physical barrier. In Chapter 2, TR-NFMs were suggested to use as a lithium ion battery separator (LIBS) for high power density application, such as electric vehicle and energy storage system. Though the conventional LIBS in laptop and mobile phone have been successfully commercialized and widely used until now, their thermal and dimensional stabilities and compatibility with electrolyte are not enough to operate in high power density lithium-ion battery systems. To secure the safety of huge battery system and to validate the fast charge/discharge performance of the battery system, LIBS should be improved in terms of thermal and dimensional stabilities at high temperature and compatibility with electrolyte. The developed TR-NFMs exhibited excellent chemical, thermal, and dimensional stabilities due to the intrinsic properties of the thermally rearranged polymer. The electrolyte compatibility of TR-NFMs was also dramatically improved compared with conventional LIBS, resulting in accelerating the transport of lithium ions through the membranes. As a result, the cycle retention and C-rate capability, as well as thermal and chemical stabilities of TR-NFMs exhibited better than conventional LIBS, demonstrating that TR-NFMs can be considered as one of the most promising candidates for the lithium-ion battery system in high power density applications. This research was supported by LG Chem. Ltd. The two papers were published in “Chem. Commun. 2014 51 2068-2071” and “J. Power Sources 2016 305 259-266” and one domestic patent was applied (KR: 10-2013-0162260). In the previous chapter, the mechanical and thermal properties of TRNFMs were spotlighted, on the other hand, in Chapter 3, the surface properties of TR-NFMs were studied for use in membrane distillation (MD) and membrane crystallization (MCr) applications. As TR-NFMs also exhibited excellent hydrophobicity, rough surface, and highly porous and interconnected structure, the anti-wetting property and MD performance of TR-NFMs were dramatically improved compared with conventional membranes and other nanofibrous membranes reported in the literature. Moreover, the surface properties of TR-NFMs can be controlled inducing the heterogeneous nucleation of inorganic salts in an unsaturated solution on the surface of TR-NFMs with lower energy consumption compared with conventional processes. To control the surface properties of TRNFMs for MD and MCr applications, we developed two types of nascent nanofibrous membranes and two types of nanocomposite membranes using two types of nanoparticles, all of which were prepared by the thermally rearranged polymer. By solving the troublesome points such as membrane wetting and severe temperature polarization phenomena, the developed membranes exhibited excellent water flux (80 L m-2 hr-1 , ΔT = 60 oC) and long-term stability (stable for 180 hr of duration time) as well as outstanding salt rejection (> 99 % against non-volatile salts). Furthermore, the heterogeneous nucleation on the surface of TR-NFMs was controlled to obtain uniform inorganic crystals at fast growth rate under an unsaturated solution. It can be demonstrated that TR-NFMs can be also considered as the most promising candidate in membrane contactor applications. This research collaborated with Professor Enrico Drioli and one paper was published in “Energy Environ. Sci. 2017 9 878-884”. The patents were filed and applied (KR: 10-1571393, PCT: PCT/KR2014/008235, US: 14/917,759, and CN: 201480049776.0). Lastly, TR-NFMs were employed as a porous substrate for thin film composite (TFC) membranes. Since the TFC membranes were developed by Cadotte et al. via interfacial polymerization in the mid-1960s, the TFC membranes have been widely used for liquid separation applications due to their excellent water flux and salt rejection. Although the TFC membranes have been successfully commercialized for reverse osmosis (RO), the TFC membranes have few limitations for use in other liquid separation applications such as severe internal concentration polarization, aging, and limited thermal and chemical stabilities. Therefore, in Chapter 4, TR-NFMs were utilized as a substrate by modifying the hydrophobic characteristic to be hydrophilic using the dopamine coating. The thin, highly porous, and hydrophilic TR-NFMs can not only meet the requirements of the substrate for the TFC membranes but also reduce the mass transfer resistance and withstand harsh operating conditions. Herein, the potential of the developed thin film composite membrane (TR-TFC) consisting of an ultrathin polyamide layer on the top of TR-NFMs was investigated in organic solvent nanofiltration (OSN) and pressure retarded osmosis (PRO) applications. The developed TR-TFC exhibited excellent DMF permeance (8 ~ 12 L m-2 hr-1 bar-1) and tight rejection (molecular weight cut off, MWCO is 600 g mol-1). Note that the DMF permeance of TR-TFC is fifty times higher than that of PEEK membrane, which was reported as an only membrane to use in DMF solvent system without severe aging even at high temperature. TR-TFC can also operate in DMF solvent system at high temperature (at 90 oC) without any compromise in rejection. Furthermore, as TRTFC has a thin and highly porous structure to reduce the mass transfer resistance inside the porous substrate, the internal concentration polarization (ICP) inside TRTFC was dramatically reduced. In PRO applications, the ICP adversely affects reducing the effective osmotic pressure and consequently decreasing water flux and power density. The developed TR-TFC showed a highly porous structure and was five - ten times thinner than conventional RO and forward osmosis (FO) membranes, bringing high water flux and high power density. In conclusion, it can be also demonstrated that TR-NFM is a fascinating substrate for use in OSN and PRO applications and the potential of TR-TFC was also well validated. The research for OSN collaborated with Professor Andrew G. Livingston and one paper is prepared. The patents were applied (KR: 10-2016-0023238, 10-2016-0023239, 10-2017-0021377, and 10-2017-0024638, and PCT: PCT/KR2017/001669 and PCT/KR2017/001938). In Chapter 5, the further applications of the developed nanofibrous membranes were suggested and the modification of the developed nanofibrous membranes for each application with brief approaches was also introduced as a future study.