Synthesis and NMR studies of the polymer membranes based on poly(4-vinylbenzylboronic acid) and phosphoric acid
Graphical abstract
Introduction
The demand for proton conducting anhydrous membranes has been increased over the last decade due to their use in fuel cells, as well as in electrochromic displays, sensors and super capacitors [1], [2], [3], [4]. Although there are several approaches for anhydrous systems, previous concept was mainly based on the doping of the polymers bearing basic sites with inorganic acids (e.g. H3PO4, H2SO4). Pure H3PO4 itself is a good proton conductor because of its extensive self-ionization and low pKa. Structure diffusion was proposed as the proton transport mechanism in fused phosphoric acid, where the transference number of proton is close to unity (∼0.975) [5]. Phosphoric acid interacts with polymers through hydrogen bonds and facilitates the formation of homogeneous blends. In general, phosphoric acid in polymer electrolytes acts as proton solvent and as well as plasticizer increasing the cooperative segmental motions of the polymer chains. Several homogeneous polymer electrolytes such as poly(ethyleneoxide)·xH3PO4 [6], poly(vinylalcohol)·xH3PO4 [7], linear (L) or branched (B) polyethyleneimine·xH3PO4 [8], [9], poly(acrylamide)·xH3PO4 [10], poly(vinylpyrrolidone)·xpolyphosphoric acid [11] were reported. In addition, extensive research on more stable systems such as phosphoric acid doped polybenzimidazole (PBI) has been done and physicochemical characterizations as well as fuel cell performances were reported [4], [12], [13], [14], [15].
In these polymer electrolytes the structural diffusion is predominant where the proton diffusion is mainly controlled by proton transport within non-aqueous phase, i.e., proton exchange between H4PO4+ and H2PO4− [4]. In addition to these, heterogeneous systems based on phosphoric acid incorporated Nafion yielded a super acid under anhydrous conditions where the charged species are generated yielding excess H4PO4+ and an immobilized –SO3− anion [16]. The self-ionization of the phosphoric acid is reduced, yielding a lower concentration of the H2PO4− ion. Then the proton transport through structure diffusion is reduced. Different from Nafion/H3PO4 system, it would be useful to develop an alternative host matrix where the channels can be filled with phosphoric acid and diffusion can occur over the acidic phase. Moreover, the absence of strong acidic units in the host material such as –SO3H may enhance the structure diffusion between H4PO4+ and H2PO4−.
To have such membranes, it would be interesting to use thermally stable boronic acid functional polymers such as poly(4-vinylbenzylboronic acid) as host matrix. The insertion of phosphoric acid may cause phosphoric acid functional polymers by forming a chemical linkage, i.e., B–O–P at higher temperatures. Previously it was reported that the homopolymers and copolymers with boron show higher thermal stability than homologous polymers without boron [17]. In addition, boronphosphate (BPO4) was used as an important additive in sulfonated polymers, i.e., SPEEK which contributed positively to the mechanical properties as well as proton conductivity of the polymer electrolyte membranes [18], [39]. An interesting feature of BPO4 is that it retains water up to 300 °C [19]. The adsorbed water is present in a partially dissociated form because the surface is also covered with hydroxyl groups of various types, i.e., free B–OH, P–OH, geminal P–OH and H-bonded OH groups associated with both B and P. BPO4 acts as a proton conductor probably due to the presence of the above-mentioned protogenic groups on the surface [20]. The conductivity of BPO4 increased with atomic ratio of B/P and reached to a maximum value when B/P ratio is 0.80 [21].
This paper reports on a new host matrix, poly(4-vinylbenzylboronic acid), P4VBBA which was produced by free radical solution polymerization of 4-vinylbenzylboronic acid (Fig. 1). Then P4VBBA was doped with phosphoric acid at several stoichiometric ratios. The membranes' properties were investigated by FT-IR, 11B MAS NMR, 31P MAS NMR, TG and DSC. The proton conductivity of anhydrous electrolytes was measured with a dielectric-impedance analyzer.
Section snippets
Materials
4-Vinylbenzeneboronic acid (>98%) was provided from Alfa Aesar. The initiator α,α′-Azodiisobutyramidin dihydrochloride (AIBADC) was supplied from Aldrich. Orthophosphoric acid (>99%) and DMF were purchased from Merck.
Sample preparation
Poly(4-vinylbenzeneboronic acid), P4VBBA, was produced by free radical polymerization of 4-vinylbenzeneboronic acid in DMF using AIBADC (0.5 mol %). The reaction mixture was purged with nitrogen and the polymerization reaction was performed at 70 °C for 4 h (Fig. 1). After
FT-IR study
The FT-IR spectra of homopolymer P4VBBA and acid doped P4VBBA·xH3PO4 are shown in Fig. 2. The OH stretching of –B(OH)2 group gives a broad peak at around 3380 cm−1. The aromatic CH stretching is located at 3030 cm−1 and the absorption bands at 2930 cm−1 and 2850 cm−1 are assigned to the symmetric and antisymmetric stretching vibrations of aliphatic CH units. Two strong peaks at 1660 cm−1 and 1606 cm−1 are attributed to CC stretching bands of p-disubstituted benzene ring. The strong CH out-of plane
Conclusions
In this work boron containing host matrix poly(4-vinylbenzylboronic acid), P4VBBA was synthesized through free-radical homopolymerization of 4-Vinylbenzeneboronic acid. Then proton conducting composite membranes, P4VBBA·xH3PO4, were produced after doping of phosphoric acid at different molar compositions (x = 1 and x = 2) with respect to the polymer repeat unit. The materials were characterized by NMR and FT-IR studies. TGA measurements demonstrated that the composite materials show almost no
Acknowledgements
Support from TÜBİTAK under contract number 105M345 is acknowledged. This work was partially supported by the BMBF under the contract number GIN-SF-049. MRH acknowledges financial support by the Carlsberg Foundation.
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