Gel-vapor deposition of ZIF-8 films on polymeric substrates: Influence of process parameters on microstructure

Producing zeolitic imidazolate framework-8 (ZIF-8) thin films on flexible polymers requires balancing precursor conversion, phase purity, intergranular packing, and substrate stability. Here, gel-vapor deposition was used to grow ZIF-8 films on poly(vinylidene fluoride) (PVDF) and polyethersulfone (PES), and the effects of gelation temperature, ligand-vapor exposure, and deposition cycling were systematically evaluated. Processing at 150 °C promoted phase-pure ZIF-8 formation but caused curling and embrittlement, whereas treatment at 120–130 °C preserved flexibility but produced mixed ZIF-8/ZIF-L phases and batch variability. Staged gelation at 120 and 150 °C for 1 h each, followed by 2-methylimidazole vapor treatment, yielded reproducible, phase-pure coatings while retaining flexibility. Thermal analyses and time-resolved ex situ XRD showed that the heating schedule governed desolvation, solid retention, and non-monotonic precursor restructuring, with staged treatment separating desolvation from densification and increasing solid retention. Quantitative fluorescence confocal optical microscopy, porometry, and C 3 H 6 /C 3 H 8 transport revealed more extensive dye-accessible pathways in PES-supported films. Contact-angle surface-free-energy measurements showed similar precursor wetting on PVDF and PES and convergence of surface properties after conversion, suggesting that defect evolution reflected interfacial transformation and polymer thermomechanical response. Repeated sol-gel-vapor cycling produced the densest intergranular packing, with mean-flow pore diameters of 15.7 nm on PVDF and 23.3 nm on PES and C 3 H 6 /C 3 H 8 selectivities of 3.3 and 2.49, respectively. However, solid-state NMR indicated greater retention of precursor-associated carbon environments after repeated cycling, while XRD detected residual ZnO in the cycled PVDF sample. Thus, improved packing did not necessarily indicate more complete precursor conversion. These findings define a practical processing window and establish how thermal history, deposition sequence, and polymer support collectively govern flexible ZIF-8 film formation.