4M	four-phase Maxwell model
α	permeability selectivity, -
αS	solubility selectivity, -
αD	diffusion selectivity, -
φ	volume fraction, -
θ	time-lag, s
AR	aspect ratio, -
Barrer	permeability unit, 10−10 cm3 (STP) cm cmHg−1·s−1·cm−2
BET	Brunauer, Emmett and Teller method for the determination of the surface area
C	concentration, mol m−3 or cm3(STP) cm−3
CHA	framework type code of zeolite chabasite
Cytop ®	poly(perfluoro-4-vinyloxy-1-butene)
D	diffusivity, m2/s
dk	kinetic diameter, Å
FEM	finite element numerical modelling
FFV	fractional free volume, -
Hyflon® AD	co-polymers of tetrafluoroethylene and perfluoro-4-methoxy-1,3-dioxole
J	flux, mol/(m2·s) or cm3(STP)/(cm3·s)
l	membrane thickness, μm
MD	molecular dynamics
MFI	framework type code of zeolite ZSM-5
MMM	mixed matrix membrane
MOF	metal organic framework
Nafion®	copolymers of tetrafluoroethylene and sulfonic acid terminated perfluorovinyl ethers
NELF	non-equilibrium lattice fluid
NMR	nuclear magnetic resonance
p	pressure, Pa
P	permeability, Barrer or Nm3·m·m−2·Pa−1·s−1
PF	permeability of the dispersed phase, Barrer or Nm3·m·m−2·Pa−1·s−1
PM	permeability of the continuous phase, Barrer or Nm3·m·m−2·Pa−1·s−1
PMMM	permeability of the MMM, Barrer or Nm3·m·m−2·Pa−1·s−1
PALS	positron annihilation lifetime spectroscopy
PFG-NMR	pulsed field gradient NMR
S	solubility, mol·m−3·Pa−1 or cm3(STP)·cm−3·Pa−1
Tc	critical temperature, K
Tg	glass transition temperature, K
PFP	perfluorinated polymer
PIM	polymer of intrinsic microporosity
SAPO-34	silicon aluminium phosphate of chasbasite (CHA) structure topology.
SEM	scanning electron microscopy
Silicalite-1	pure SiO2 zeolite of ZSM-5 (MFI) structure topology
Teflon®	polytetrafluoroethylene
Teflon® AF	co-polymer of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxole
STP	standard temperature and pressure