Transparent, free-standing, flexible and selective CO2 adsorbent films fabricated from homopolymer/metal salt hybrid gels

Abstract

This article details our efforts to prepare free-standing macroscopic films by optimized coordinative crosslinking of poly(4-vinylpyridine) (P4VP) chains using transition metal ions. We encountered homogenous gels (at <1% w/v polymer concentration) en route to the fabrication of such films. The gels and films were obtained through coordinative crosslinking of P4VP chains by Ni(II) ions while the use of other metal ions (viz. Co(II), Cu(II) or Zn(II)) for this purpose resulted in either heterogeneous gels or gelatinous precipitates. Based on the gelation kinetics and the mechanical strength of the resulting films, the most optimum coordination ratio was observed to be 1 : 1 molar ratio of 4VP : Ni(II) in the methanol–water mixed solvent system. Highly transparent, flexible, free-standing films of any dimension and shape could be fabricated through controlled evaporation of the solvent from the gels. The microscopic features of these films were quite similar to those of xerogels. Magnetic susceptibility measurements performed on the ground films showed ferromagnetic interactions between adjacent Ni(II) centres in the temperature range of 75 to 10 K. These coordinatively crosslinked pliable films exhibited selective, although moderate, adsorption of environmentally relevant CO₂ at room temperature. Through variation of the metal content in the films, we further demonstrated that the metal centres present in the film are intimately involved in the adsorption of CO₂. We believe such coordinatively crosslinked polymer films can be a potential alternative to metal–organic frameworks (MOFs) in gas adsorption applications, with the added benefit of flexibility and macroscopic dimensions.