Poly(N-vinyl triazole-b-N-vinyl imidazole) Grafted on MWCNTs as Nanofillers to Improve Proton Conducting Membranes

Abstract

Carbon nanotubes (CNTs) are of particular interest because of their ability to enhance the mechanical strength in the material; however, processing difficulties of CNTs often restrict utilization up to its full potential. To resolve this, in the current study, we have developed a simple and efficient method to functionalize the surface of a multiwalled carbon nanotube (MWCNT) with precise functional polymer chains, which were covalently graf ted on the surface of the MWCNT, and delved into an application to demonstrate this material as an efficient nanofiller in developing a proton conducting membrane (PEM) from polybenzimidazole (PBI). At first, the MWCNT surface was converted to a polymerizable surface by attaching a trithiocarbonate based chain transfer agent (CTA). Then, a N-heterocyclic block copolymer, namely poly-N-vinyl-1,2,4-triazole-b-poly-N-vinyl imidazole (pNVT-b-pNVI), was grown from the CTA anchored surface with a one-pot surface initiated reversible addition−fragmentation chain transfer (SI-RAFT) technique. Graf ting of block copolymer chain was confirmed by GPC, NMR, TGA, TEM, FESEM, and EDX studies. To the best of our knowledge, this will be the first report of a growing block copolymer structure graf ted covalently on the surface of the MWCNT. The novelty of the work was further enhanced by incorporating pNVT-b-pNVI-g-MWCNT as a nanofiller into the oxypolybenzimidazole (OPBI) membrane to obtain homogeneous nanocomposite membranes with excellent thermomechanical and tensile properties, thermal stability, superior proton conduction when doped with phosphoric acid (PA), and PA holding capacity. The nanocomposite membrane with 2.5 wt % nanofiller loading displayed a tensile stress of 1.8 MPa and a strain of 176% at break. The basic N-heterocyclic rings dangling from the block copolymer chains graf ted on the MWCNT surface allowed formation of strong H-bonding, acid−base interaction with PA, which is responsible for high acid uptake and superior PA retention, and also exhibited proton conductivity as high as 0.164 S cm −1 at 180 °C, which is a 2.6-fold increment when compared with a pristine OPBI membrane. This significant increase in conductivity is attributed to the proton conducting nanochannel pathway generated along the polymer-g-MWCNT surface.

Polymer-Grafted Graphene Oxide/Polybenzimidazole Nanocomposites for Efficient Proton-Conducting Membranes

“…GPC of all the polymer- g -GO samples (GOP1-GOP6) was performed after chemical treatment of the polymer-grafted GO samples with N-hexylamine in the presence of sodium dithionite (see Scheme S5). The resulting chemical environment produced by these reagents causes breaking of the trithiocarbonate bond and linking between the grafted homopolymer chains with the GO surface and liberates the free polymer chains in the solution mixture , The bare polymer chains were then purified and then run through a GPC using HPLC water as the eluent solvent to measure the molecular weight. The GPC chromatograms are shown in Figure C, and molecular weights and polydispersity index are tabulated in Table .…”

“…The characterizations of the synthesized polymer- g -GO materials are thoroughly performed by FT-IR analysis, thermogravimetric analysis (TGA), gel permeation chromatography (GPC), Brunauer–Emmett–Teller (BET) N 2 sorption analysis, Raman spectroscopy, field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (FESEM-EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), dynamic vapor sorption (DVS), and zeta potential measurement and discussed in detail in the result and discussion (see experimental details in the Supporting Information). The synthesis of OPBI was adapted from the literature following a modified procedure , and described in detail in the Supporting Information. Experimental details of the characterizations of composite membranes are also given in the Supporting Information.…”

Proton-Conducting Polymeric Membranes Based on 1,2,4-Triazole

“…These copolymer membranes display high proton conductivity at elevated temperatures and in anhydrous conditions. There are a number of studies on the development of proton-conducting membranes based on 1-vinyl-1,2,4-triazole copolymers ( Table 3 ) [ 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 ].…”

“…Recently, the first report has appeared on the growing structure of a block copolymer covalently grafted to the surface of a multiwalled carbon nanotube (MWCNT) [ 79 ]. At first, a trithiocarbonate-based chain transfer agent was attached to the surface of the MWCNT.…”

“…As a result, a unique homogeneous nanocomposite membrane was obtained, which showed a tensile stress of 1.8 MPa, a strain of 176% at break, and a high proton conductivity of 0.164 S/cm at 180 °C, after doping with phosphoric acid. This significant increase in conductivity is attributed to the proton-conducting nanochannel pathway generated along the block copolymer-g-MWCNT surface [ 79 ].…”