crystallinity at lower crystallinity ranges. Particular polymers show an almost
constant or slight increase in gas permeability and diffusivity at a lower crystallinity range. The decrease in gas transport properties (i.e., permeability, solubility,
and diffusivity) is observed in rubbery PE and in glassy PET and PEN. Interestingly, these values for liquid crystalline polymers remain unchanged for the range
of crystallinity in this analysis. Gas permeability and diffusivity in PS increase
with increasing crystallinity, whereas the gas solubility decreases. Although gas
permeability and diffusivity for PLA slightly increase and then begins to decrease,
solubility decreases with increasing crystallinity. Based on these results, gas
transport properties do not always decrease at lower crystallinity values, whereas
gas transport properties decrease at higher crystallinity values. These results
suggest that the crystalline structure can form discontinuously in crystalline and
liquid crystalline polymersat lower crystallinities. When the crystallinity was
increased, the crystal structure gradually grows similar to a phase transition from
the discontinuous domains. One possibility is that there may be a continuous space
for gas diffusion around the interface between the crystalline and amorphous
regions, which may be created because of the stress of polymer chain arrangements
around the interface regions. The gas diffusion process is enhanced because the
space is larger than the size of the gas molecule. Hence, gas permeability increases
along with this increase in gas diffusion at lower crystallinities. Furthermore, gas
permeability and diffusivity begin to decrease after the phase transition of the
crystal structures. At a higher crystallinity, the crystal structure may effectively
prevent the diffusion of gas molecules. Therefore, the crystal structure can be
changed from discontinuous to continuous crystal domains after phase transition.
Clearly, gas transport properties can be strongly affected by crystal structure
parameters, such as crystal size and distribution, for crystalline and liquid crystalline polymers.
Relationship Between Experimentation and Extrapolation
in Crystalline and Liquid Crystalline Polymers
The purely amorphous gas transport parameters, P a , D a , and S a , in crystalline
polymers were extrapolated using the best fit lines (X c ! 0) in Figs. 2, 3, and 4
(Table 2). Different polymers show large differences in the gas transport parameters
in their purely amorphous form. These results indicate that gas transport properties
strongly depend on the chemical structure of the polymer. According to the solutiondiffusion model in polymeric dense membranes, CO 2 and O 2 permeability, P a , in this
analysis was consistent with solubility, S a , and diffusivity, D a . By contrast, N 2
permeability, P a , and the product of the solubility, S a , and diffusivity, D a , have
large deviations based on the limited experimental data.
The order of the gas permeability and solubility are as follows: CO 2 > O 2 > N 2 .
This order is consistent with gas condensability, such as in the gas critical temperature: 304 K (CO 2 ), 155 K (O 2 ), and 126 K (N 2 ) (Poling et al. 2000). Meanwhile, the
order of the gas diffusivity is as follows: O 2 > N 2 > CO 2 . This order is also
19 Gas Permeation and Barrier Properties of Liquid Crystalline Polymers
535
constant or slight increase in gas permeability and diffusivity at a lower crystallinity range. The decrease in gas transport properties (i.e., permeability, solubility,
and diffusivity) is observed in rubbery PE and in glassy PET and PEN. Interestingly, these values for liquid crystalline polymers remain unchanged for the range
of crystallinity in this analysis. Gas permeability and diffusivity in PS increase
with increasing crystallinity, whereas the gas solubility decreases. Although gas
permeability and diffusivity for PLA slightly increase and then begins to decrease,
solubility decreases with increasing crystallinity. Based on these results, gas
transport properties do not always decrease at lower crystallinity values, whereas
gas transport properties decrease at higher crystallinity values. These results
suggest that the crystalline structure can form discontinuously in crystalline and
liquid crystalline polymersat lower crystallinities. When the crystallinity was
increased, the crystal structure gradually grows similar to a phase transition from
the discontinuous domains. One possibility is that there may be a continuous space
for gas diffusion around the interface between the crystalline and amorphous
regions, which may be created because of the stress of polymer chain arrangements
around the interface regions. The gas diffusion process is enhanced because the
space is larger than the size of the gas molecule. Hence, gas permeability increases
along with this increase in gas diffusion at lower crystallinities. Furthermore, gas
permeability and diffusivity begin to decrease after the phase transition of the
crystal structures. At a higher crystallinity, the crystal structure may effectively
prevent the diffusion of gas molecules. Therefore, the crystal structure can be
changed from discontinuous to continuous crystal domains after phase transition.
Clearly, gas transport properties can be strongly affected by crystal structure
parameters, such as crystal size and distribution, for crystalline and liquid crystalline polymers.
Relationship Between Experimentation and Extrapolation
in Crystalline and Liquid Crystalline Polymers
The purely amorphous gas transport parameters, P a , D a , and S a , in crystalline
polymers were extrapolated using the best fit lines (X c ! 0) in Figs. 2, 3, and 4
(Table 2). Different polymers show large differences in the gas transport parameters
in their purely amorphous form. These results indicate that gas transport properties
strongly depend on the chemical structure of the polymer. According to the solutiondiffusion model in polymeric dense membranes, CO 2 and O 2 permeability, P a , in this
analysis was consistent with solubility, S a , and diffusivity, D a . By contrast, N 2
permeability, P a , and the product of the solubility, S a , and diffusivity, D a , have
large deviations based on the limited experimental data.
The order of the gas permeability and solubility are as follows: CO 2 > O 2 > N 2 .
This order is consistent with gas condensability, such as in the gas critical temperature: 304 K (CO 2 ), 155 K (O 2 ), and 126 K (N 2 ) (Poling et al. 2000). Meanwhile, the
order of the gas diffusivity is as follows: O 2 > N 2 > CO 2 . This order is also
19 Gas Permeation and Barrier Properties of Liquid Crystalline Polymers
535