Gas permeation and microstructure of reduced graphene oxide/polyethyleneimine multilayer films created via recast and layer-by-layer deposition processes

Nowadays, graphene/polymer composite films with multilayer structure have attracted significant attention for gas barrier application. In this study, a series of reduced graphene oxide/polyethyleneimine (RGO/PEI) composite films were created via recast and layer-by-layer deposition processes. By using the recast process, the myriad PEI molecules in the precursor solution (the PEI : GO feeding ratio is 0.02 : 0.1, 0.05 : 0.1, 0.1 : 0.1, 0.3 : 0.1 and 0.5 : 0.1) ensure more effective reduction and surface modification of the graphene oxide (GO) sheets, while the undesirable free PEI molecules are eventually removed via a filtration process. Then, the RGO/PEI composite films were synthesized on PET substrate using a layer-by-layer assembly. The resulting films show a homogeneous and compact brick-wall structure with excellent gas barrier properties. Barriers against water vapor, nitrogen/oxygen, and carbon dioxide require different content of PEI in the composite film for optimal performance; the ideal values are 19.7, 23.8, and 24.1 wt%, respectively. These values are much lower compared with previously reported studies. Further, the permeability, free volumes, component ratio, morphology, and density of the RGO/PEI composite films have been carefully investigated and discussed. The results revealed that the mechanism behind the excellent gas barrier property of the RGO/PEI composite films is a synergistic effect created by the combination of the brick-wall structure, the small free volume holes, the suitable PEI content (ranging from 19.7 wt% to 24.1 wt%), the high density, and the hydrophobicity.


Layer-by-layer assembly technique for the preparation of RGO/PEI composite films.
A similar layer-by-layer assembly technique has been reported by the same research group [8,9], and a schematic can be found in Figure S1: Step 1: 20 mL of the Solution 3 (as mentioned in the Experimental of the manuscript) was used for each sample -just enough to produce one RGO/PEI composite film with a surface mass density of ∼ 1.0 × 10 −3 g cm −2 on a 10 cm × 10 cm PET substrate.
Step 2: 1 mL of the Solution 3 was carefully coated on a PET substrate using a glass coater.
The suspension was fully evaporated at 60 • C.
Step 3: Step 2 was repeated 20 times to prepare a film consisting of 20 layers of RGO/PEI composite.
The mechanism of the layer-by-layer assembly as well as the parallel arrangement of RGO sheets is that: For each layer, only a small amount of precursor solution was used, which is just enough to prepare one ultrathin layer of RGO/PEI composite (< 0.4 µm). When dispersed in an ultrathin layer, the RGO sheets can be forced to align parallel to the layer due to the space limitation. With the PEI molecules spread laterally in the layer with a high tendency, the strong interaction between PEI chains and RGO sheets ensures the formation of the RGO/PEI composite film with homogeneous brick-wall structure [9][10][11]. Similar technique has been reported by the same research group [9].
The reduction mechanism of GO by PEI.
The reduction mechanism of GO by PEI is shown here, which is referenced from a reported work by Hongyu Liu et.al [11]. The epoxy group can be reduced by the amine groups with a nucleophilic ring-opening reaction, followed by an elimination reaction. Another possible approach to reduce epoxy groups by amine groups was that the -NH

Water vapor transmission rate (WVTR) and gas (nitrogen, oxygen, and carbon dioxide) permeability measurement
A cup method was used to obtain the WVTR of films [12].
where ∆m is the mass difference between two consecutive measurements within a time of t, d is the thickness of the film, A is the effective area of the film used for water vapor transmission, and t is the time interval between two consecutive measurements.
Gas (Hydrogen, oxygen, carbon dioxide) permeability was measured by a differential pressure method on a Labthink VAC-V3 apparatus, monitoring the amounts of gas molecules that permeated across the membrane from one side to the other, under a constant environment condition (0% relative humidity and 25 • C). The operating pressure difference is 1 atm.

Positron annihilation lifetime measurements
The PEI/RGO composite were carefully scraped off the PET substrate, and the resulting powder-like PEI/RGO composites were mechanically pressed to discs. The positron annihilation experiments were conducted by using a fast-fast coincidence PALS with a time resolution function of 0.230 ns for the full width at half maximum (FWHM), and 1 million counts were collected for each spectrum. A 22 Na source (∼10 µCi) was firstly sandwiched by 2 PEI/RGO composite discs and then covered with an Al foil. The membranes with the source were placed in a testing chamber, at room temperature. The PATFIT program [13] as well as the LT program [14] were applied to analyse the positron lifetime spectra of the membranes, and the variances of the fits were in the range of 0.97∼1.20. Details of PALS measurements set up can be found in our recent paper [15]. with that of the electrons on the wall of free volume holes in the polymer [16][17][18][19][20][21][22][23]. For several decades, PALS has been widely used to measure atom-sized free volume holes in polymers. The average radii of hole free volumes in membranes, which are determined by the semiempirical relationship between τ o−Ps and mean free volume hole radius (R) in a spherical approximation given by the Tao-Eldrup model as [24,25], where R 0 =R+∆R, and ∆R=0.166 nm is the thickness of the homogeneous electron layer overlapping with the o-Ps wave function. The mean hole free volume V FV can be calculated from the following equation,