Tunable gas selectivity of cellulose nanocrystals – Polyethylene glycol composite membranes

The kinetics of the CO2, D2 and CH4 transport through self-supporting cellulose nanocrystals (CNCs) films containing low-molecular weight poly(ethylene glycol) (PEG) additive as plasticizer agent was studied in single gas condition at 293 K and sub-atmospheric feed pressure. Neat CNC and PEG-CNC with 7 wt % PEG films act as impermeable barrier for CO2 and CH4 and exhibit D2 permeability values not larger than 0.05 barrer with penetrant diffusivity in the 10−10 cm2/s range. PEG addition at larger contents improves the permeability and diffusivity of all penetrant gases and gives to the PEG-CNC membrane a CO2 selective character. The CO2 permeability and ideal CO2/D2 selectivity of the PEG-CNC films are 0.42 ± 0.04 barrer and ∼3, respectively, at 14 wt % PEG content, 2.5 ± 0.2 barrer and ∼5 at 21 wt % PEG. The preferred CO2 transport exhibits a solution-selective character being the D2 diffusivity a factor ∼10 larger than the CO2 diffusivity. PEG addition, on the contrary, does not change the D2 selective character for the D2/CH4 gas couple. PEG well mixed with the gas-impermeable CNCs increases the PEG-CNC free volume and preferentially improves the CO2 solubility given the specific interactions between the quadrupolar CO2 penetrant and the dipolar ether groups of the PEG additive.