(571g) Tuning the Gas Barrier Properties and Selectivity By Means of Graphene-Based Coatings on Polymeric Film

The peculiar properties of graphene and its derivatives are very attractive for several applications, and significant efforts are devoted to their implementation in various fields. Among the others, the very low gas permeability of the graphene sheet together with the characteristic 2-D structure are very attractive for the fabrication of barrier materials, as well as of gas separation membranes. However, the production of a large and defect-free graphene surface is not practically feasable, and consequently the assembling technique and the method used to couple graphene with e.g. polymeric materials are crucial. Clearly, the resulting material cannot be fully impermeable as a single graphene sheet, and gaseous species are forced to diffuse around graphene sheets. The organization can exploited for gas separation purposes, selectively blocking only some penetrants, as in molecular sieves.

In this work, two different assembly methods are employed to fabricate graphene oxide based coatings on polymer substrates.

First, a low oxygen graphene oxide (G, C/O â??8) solution obtained by electrochemical exfoliation was used to produce graphene coatings by vacuum-assisted filtration; the obtained graphene cakes were then transferred, by roll-to-roll or simple press, onto various polymeric substrates, suitable e.g. for packaging, such as polyethylene terephthalate (PET), polyproylene (PP) or polylactic acid (PLA). The coating was firmly attached to the polymer matrix, and the adhesion proved to be very effective resisting also over time, while the distribution of the graphene sheets and their assembly have been observed by Scanning Electron Microscopy measurements. This simple production technique was successfully used to produce low oxygen permeability multilayer films, showing a reduction of the O₂ transfer rate of more than 70% with respect to the bare substrate in case of PET and more than 90% in case of PP and PLA.

A second assembly method involved the use of the layer-by-layer (LBL) technique, to produce alternating ultra-thin multilayer structures to coat the polymeric matrixes. After its proper activation, the substrate is alternatively dipped into polyelectrolyte or graphene oxide water solution, exploiting electrostatic forces to obtain a molecular-thin layer with a high order degree. Highly oxidized graphene oxide (GO, C/O â??1) was used as a electronegative-behave species, together with the partially-positive polyethyleneimine (PEI) to built a well ordered, self assembling coating on top of various substrates. The SEM images revealed a quite flat top surface of the coating, in which the GO sheets can be clearly identified, while the analysis of the sample cross section suggested the existence of an alternated structure.

The highly ordered GO structure produced by this method was able to provide very effective barrier systems, leading to a 96% reduction of the O₂ transfer rate on PET, by means of a 25 bilayer translucent coating (whose thickness can be estimated as 200nm). The estimated O₂ permeability of the coating only resulted remarkably low, comparable to the best already-available gas barrier solutions.

The sieving ability of the obtained GO coating was also investigated by means of direct permeability test of CO₂ and smaller gas molecules, such as H₂ and He, allowing the determination of selectivity values. The peculiar assembly produced by the LbL technique, indeed, conferred to the coating a sieving ability based on the probe different molecular sizes, which is suitable for H₂/CO₂ separation, relevant in pre-combustion CO₂ capture applications. Indeed, the evaluation of the coating selectivity returned an astonishing value of 70, proving the validity of such approach. The same method is also applied to another polymer matrix of relevance in membrane field, Matrimid polyimide, characterized by moderate H₂/CO₂ separation performances, obtaining the same results.

Therefore, this technique allows to conveniently tailor the gas transport properties through very thin coating layers characterized by large sieving selectivity values, and moderate resistance towards the diffusion of small probes such as H₂.