Gas Barrier and Molecular Permeability in Vat-Printed Photopolymers: Diffusion, Sorption, Network State, Defects and Qualification

Vat photopolymerisation can fabricate both gas-permeable and gas-barrier polymer structures, but the underlying transport mechanisms and measurement quantities are often conflated. For dense homogeneous polymers, permeability reflects the combined ability of a penetrant to sorb into and diffuse through the network. Printed parts introduce additional pathways and state variables: conversion gradients, post-cure, residual species, water uptake, layer interfaces, shrinkage, voids, thickness and surface sealing. Direct VPP literature demonstrates highly gas-permeable printed PDMS-like networks, DLP-printed mixed-matrix membranes for H2/CO2 separation, and DLP-printed ionic-liquid-containing membranes with gas-specific permeability. Other VPP studies demonstrate tunable pressure-driven permeability through intentionally porous phase-separated membranes; those data are not equivalent to molecular gas permeability in dense barriers. This review defines the distinctions between gas transmission rate, permeance, permeability, leakage and porous flow; maps chemistry, network state and defect structure to expected transport behaviour; and proposes a minimum reporting and qualification framework. ISO 15105-1:2007, the developing ISO/DIS 15105-1 and ISO 15105-2:2025, together with ASTM D1434-23, provide method anchors for films and sheeting but do not by themselves validate a complete three-dimensional VPP article. 3Dresyn NPGB1 and 3D-ADD SAP1/SAP2 are included only as controlled manufacturer implementation examples spanning opposite transport-design directions.