Synthesis, Characterization, and Gas Permeation Properties

t-Butylcarbamates of Cellulose Derivatives
of polymers is the fact that the CO2 permselectivity was almost retained despite a
considerable increase in permeability (Table 4). There was a fourfold increase in the
CO2 permeability of 3 upon carbamoylation (3, 4.6; 3a, 19) but its PCO2/PN2 selectivity
underwent a relatively slight change (3, 32.8; 3a, 28.8). Moreover, the carbamate
derivative of 4 exhibited a 22 times enhancement in PCO2 (4, 1.7; 4a, 38), whereas its
CO2/N2 permselectivity experienced a change from 38.6 to 29.2 only. The increased
CO2 permeability without any significant loss of permselectivity is probably a
consequence of the increased CO2 diffusivity and retained CO2/N2 solubility
selectivity of these polymeric membranes; the latter being attributable to the presence
of polar carbamate linkages.
Gas Diffusivity and Solubility. Gas permeability (P), which is the
steady-state, pressure- and thickness-normalized gas flux through a membrane, can be
expressed as the product of gas solubility (S) in the upstream face of the membrane
and effective average gas diffusion (D) through the membrane, strictly in rubbery and
approximately in glassy polymers: 1a–c,16
P = S × D
A detailed investigation of the gas permeation characteristics was carried out
by determining the gas diffusion coefficients (D) and gas solubility coefficients (S) of
the polymers. The plots of S versus D values of polymers 2, 2a, 4, and 4a for O2 and
CO2 are illustrated in Figures 9 and 10, respectively. The gas permeability
enhancement, as a consequence of the incorporation of polar carbamate linkages
having spherical and bulky alkyl termini, has been revealed to stem from the
amplification of the gas diffusion coefficients of the polymer membranes.
The D values of all the gases increased upon carbamoylation (1a–4a); for
instance, 2 displayed the DO2 value of 6.9, and that of 2a was 9.1, in the units of 10 7
cm 2 s -1 . The gas diffusion coefficients, delineated as a complex interplay between
fractional free volume and local mobility, tend to increase if accompanied by an
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