Effect of temperature on solubility and diffusivity
Effect of temperature was also tested for both penetrant solubilities and diffusivities in pure PDMS, ZIF-8, and Ni-ZIF-8 based MMMs. Figure 6(a, b), and Figure S15 (a, b) show the solution coefficients and solution selectivities of BD with respect to the feed temperatures over Ni-ZIF-8 MMMs, and ZIF-8 MMMs, respectively. It was noticed that the solution coefficient of both the penetrants was decreased with respect to the feed temperatures. The BD solution coefficient values decreased from 35.13 to 26.5 in 15% in Ni-ZIF-8 MMM, from 31.9 to 24.12 in 15% ZIF-8 MMM, and from 24.67 to 19 (cm3(STP).cm-3 membrane. bar-1) in PDMS from 40 to 50°C, respectively. These findings indicate the sorption domination in BD permeation across the membrane, as the BD permeance and ideal selectivity also decreased at elevated temperatures.
The diffusion coefficient values of BD and diffusion selectivity increase with respect to feed temperatures shown in Figure 6 (c, d), and Figure S15 (c, d) for Ni-ZIF-8 MMMs, and ZIF-8 MMMs, respectively. In rubbery polymers such as PDMS, the diffusion coefficient normally increased by the increase in temperature. BD diffusion coefficient increased 19% in pure PDMS, 16% in 15% ZIF-8 MMM, and 17% in 15% Ni-ZIF-8 MMM, respectively. On the other side, the diffusion selectivity also increased with respect to the increase in temperature. It was noted that the selectivity enhancement was slightly more (11%) in pure PDMS as compared with 15% ZIF-8 MMM (9%), and 15% Ni-ZIF-8 MMM (8.5%), which can be attributed to decreased BD sorption at elevated temperatures in ZIF-8 and Ni-ZIF-8.