Chemical Cross-Linking of Highly Sulfonated Polystyrene Electrospun Fibers

Abstract

Electrospun nanofibers of highly sulfonated polystyrene (IEC ∼ 4.8 mequiv/g) might be good candidates for use in ion exchange applications because of their ability to form continuous, highly conducting pathways for cation transport. However, such fibers suffer in terms of dimensional stability due to high water uptake at elevated humidity. To improve the stability of the electrospun nanofiber mats in water, the polyelectrolyte was coelectrospun in the presence of a high molecular weight poly­(ethylene oxide) (PEO). Addition of PEO to the spinning dope helped improve the spinnability of the polyelectrolyte and subsequent heat treatment improved the stability of the fiber mats in water. Increasing PEO concentration in the fiber mats reduced the solubility of the heat-treated fiber mats in water; suggesting that the fibers were cross-linked by or in the presence of the PEO. To understand the chemistry of this apparent cross-linking reaction, the electrospun fiber mats were heated in a solid state 1H NMR rotor and changes in the spectrum were monitored as the reaction progressed. The hypothesis is that at higher temperatures, the two polymers react with each other to form sulfonic acid esters that form water-resistant cross-links. An alternative hypothesis of a free-radical cross-linking reaction was not supported by the observations.

Simultaneous enhancement of the efficiency and stability of organic solar cells using PEDOT:PSS grafted with a PEGME buffer layer

“…The water molecules formed in this reaction will immediately evaporate at the annealing temperature of 150°C. Some ethers of PEGME may be cleaved by strong acidic PSS and high temperature of 150°C during the annealing process because ether linked compounds can be cleaved by strong acid at high temperature [27,33]. This will be further discussed in more detail in TGA data of Fig.…”

“…The condensation reaction between the -SO 3 H groups in sulfonated polymers and the -OH groups in polyols to afford sulfonic acid esters is well known [25][26][27]. We also reported a complex including a PEDOT:PSS grafted with PEGME copolymer with excellent water, chemical and weather resistance by the condensation reaction [29].…”

“…It may be related to the water loss during the further esterification of unreacted sulfonic acid groups in PSS and hydroxyl groups in PEGME. Another possible mechanism is that, as ether-linked compounds can be cleaved by strong acid at high temperature [27,33], PEGME can be cleaved with a presence of unreacted acid groups at the temperature above 150°C. That is, unlike the pristine PEGME, decomposition of PEGME is accelerated in acidic conditions.…”

“…Mikhailenko et al cross-linked sulfonated poly(ether ether ketone) (SPEEK) with polyatomic alcohols through a condensation reaction with the sulfonic acid groups [26]. Furthermore, Subramanian et al suggested cross-linking sulfonated polystyrene (SPS) electrospun fibers in the presence of poly(ethylene oxide) (PEO) via a condensation reaction between the -OH groups in PEO and the -SO 3 H groups in SPS [27]. Since the condensation reactions can proceed in a typical annealing condition of PEDOT:PSS layer (>120°C), no further processing except mixing is required for fabrication of OSCs with the modified PEDOT:PSS buffer layer.…”

A nonionic surfactant simultaneously enhancing wetting property and electrical conductivity of PEDOT:PSS for vacuum-free organic solar cells

“…Here, for the temperature dependence of the films prepared from PH1000 solution added by PEG-TmDD, we propose the hypothesis that the PEG-TmDD decomposes into EG and TmDD during the thermal annealing, as shown in the following, where the PEG has been reported to decompose during thermal annealing under the acidic condition [47,48]:…”