Materials
TMC, melamine and N-Methyl-2-pyrrolidone (NMP) were open and stored in a N 2 -environment glovebox. All other reagents, including acetone, isopropyl alcohol (IPA), TFA, deuterated TFA, polystyrene (280 kDa), anisole, chloroform, hexane, dimethyl formamide and DMSO were obtained from commercial vendors and used as received. CD-flat holey carbon (8/2-hole pattern) grids used for TEM were purchased from Electron Microscopy Sciences.
Synthesis of 2DPA-1 powder
For a typical 2DPA-1 powder with a high r value, we first mix 1 mmol of TMC with 1 mmol of melamine in a 40-ml glass vial. Using a magnetic stir bar, stir the contents in a N 2 -environment glovebox with 18 ml of NMP followed by 2 ml of pyridine. Seal the glass vial with a solvent-resistant cap, and leave the reaction mixture to stir between 350 rpm and 400 rpm at room temperature inside the glovebox for 24 h. The r value can be manipulated by altering the duration the reaction mixture is left stirring. After the desired stir time has passed, add 20 ml of IPA into the reaction vial and let the vial continue stirring for 30 min to fully quench the reaction. Remove the vial from the glovebox, add 20 ml of deionized water, and filter the reaction mixture using a nylon membrane (Cytiva nylon membrane filters, 0.45-μm pore size, diameter 47 mm, product no. 7404-004) and then wash the solid residue repeatedly with IPA. Collect the IPA-rinsed solid residue from the filter paper and place in a clean 20-ml glass vial. Add 10 ml of deionized water and 10 ml of acetone (1:1 volume ratio) into the vial, and stir the solution with a magnetic stir bar for at least 8 h at room temperature. This helps dissolve any remaining impurities. After 8 h, filter the mixture again with a plastic filter funnel (Chemglass Life Sciences filter funnel, disposable, 110 ml, 10 μm polyethylene frit, product no. OP-6602-14), and wash the solid residue several times with acetone. Collect the solid from the filter and dry in a vacuum oven at 100 °C for 3 h. The resulting powder should possess a yellow to light orange hue.
Characterization methods for bulk 2DPA-1 and thin films
Characterizing 2DPA-1 samples with nuclear magnetic resonance (NMR) spectroscopy followed previously established protocol38. In brief, we first suspended 2DPA-1 powders in TFA at a concentration of 10 mg ml−1. We sonicated the solution in a Branson ultrasonic cleaner for 15 min before adding into a Wilmad NMR tube (5 mm diameter, economy). All the 1H NMR measurements were performed at room temperature on an Avance III HD 500 NMR Bruker spectrometer with D1 setting of 1 s. Characterization by means of 13C NMR is reported elsewhere38.
TEM imaging was conducted using S/TEM (Titan Themis Z G3 Cs-Corrected S/TEM) in the TEM mode with an accelerating voltage of 200 kV. Images were taken at magnifications ranging from ×11,000 to ×4,000,000 using the fast 4k × 4k complementary metal-oxide-semiconductor camera at MIT.nano. For TEM sample preparation of nanoplatelets, a diluted solution of 2DPA-1 dissolved in DMSO was drop-cast onto a single-graphene-layer TEM grid. For TEM sample preparation of nanofilms, the as-synthesized 2DPA-1 platelets were initially dissolved in TFA at a concentration of 10 mg ml−1 and then spin-coated onto a TEM grid at 1,000 rpm for 1 min. Both sample types were placed in a vacuum oven at 100 °C overnight to remove TFA and DMSO before TEM analysis.
The chemical stability of 2DPA-1 is imparted by the irreversible nature of its amide bonds having a high dissociation energy. Transamidation, a potential side reaction common to many polyaramids45,46, is not observed for 2DPA-1 when exposed to TFA for 9 h (ref. 38). Marginal changes in the chemical structure occur after sitting in TFA for 24 h, indicating that some transamidation can occur if the sample remains in highly acidic conditions for an extended period, but this is beyond the exposure that 2DPA-1 would experience before film processing.
One-dimensional polyaramids are notoriously hygroscopic47. Using 1H NMR, we find no notable effect of water on the chemical or molecular structure of 2DPA-1 after 539 d (about 1.5 years) of exposure (Supplementary Fig. 1). The data depict a slight increase in the r value from 5.15 to 5.45 after long-term exposure. The aromatic region and end group region also showed sharp peaks before water storage. Both observations indicate the presence of small particles before water storage, which may have disappeared because of particle aggregation or sedimentation. These data support the chemical stability of this material in protic polar solvents and the lack of hygroscopic degradation observed by the sustained gas barrier properties over 1,000 d in ambient humidity conditions.
Spectroscopy of bulk 2DPA-1 and thin films
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