than that of amorphous PLA membranes. One possible explanation based on this
result is that a continuous space for gas diffusion may exist around the interface
between the crystalline and amorphous regions (Sawada et al. 2010). In the same
way, permeability in PS is also independent of its crystallinity (Hodge et al. 2001;
Prodpran et al. 2002). Syndiotactic PS (s-PS) is known to have several crystalline
structures (i.e., α, β, γ, and δ crystalline). These structures not only affect the
physical properties of the polymer but their gas transport properties as well (Hodge
et al. 2001; Prodpran et al. 2002). This behavior is similar to PLA membranes. In
general, the gas permeability of the β-form crystalline structure is lower than that
of the α-form because the β-form structure has more tightly packed polymer chain
segments. The density of the β-form is higher than that of the amorphous polymers,
whereas the density of the α-form is lower than that of the amorphous polymers.
The gas permeability and solubility of the β-form decrease with increasing crystallinity, and its diffusivity remains constant with increasing crystallinity. The gas
permeability and diffusivity of the α-form structure increase and the solubility
decreases with increasing crystallinity. Based on these results, porous microcrystalline structures with nanochannels may be adopted to increase gas diffusivity,
thereby increasing gas permeability (Hodge et al. 2001; Prodpran et al. 2002).
Furthermore, s-PS with molecular cavities of different shapes and sizes were
obtained from an α-form crystal (Tsujita et al. 2005). Therefore, each crystalline
polymer has an intrinsic crystalline phase with different sizes and distributions.
These phases affect not only their physical and thermal properties but also their gas
transport properties. The density of crystalline PMP is slightly lower than that of its
amorphous counterpart (Griffith and Ranby 1960; Ranby et al. 1962). This result
was considered based on the characteristic crystalline structure of PMP. The bulky
pendant structure in PMP inhibits close polymer segments from packing and
results in a low-density crystal with gaps of almost 4 Å between chain segments
(Puleo et al. 1989). Therefore, a small molecule can pass though this characteristic
crystalline structure in PMP. By comparison, the properties of liquid crystalline polymers are different from those of amorphous and crystalline polymers in terms of
fluidity and orientation. These kinds of polymers have three types of phases, namely,
nematic, cholesteric, and smectic phases. As shown in Fig. 1, glassy liquid crystalline
polymers, such as poly( p-hydroxybenzoic acid-co-6-hydroxy-2-naphthioic acid)
(HBA/HNA) and poly( p-phenylene terephthalamide) (PPTA), show remarkably
high gas barrier properties. For example, the O 2 permeabilities of HBA/HNA copolymer and PPTA are lower than that of PET or PVC but are similar to that of
polyacrylonitrile (Weinkauf and Paul 1991; Weinkauf et al. 1992). These high gas
barrier properties are attributed to the efficient packing of the molecules in the solid
state. The gas barrier property of the HBA/HNA copolymer also increases with
increasing HNA content. The napthyl unit restricts the rotation mobility of the main
chain and packed polymer chain segments; thus, high gas barrier properties were
developed. Interestingly, in spite of the low crystallinity (13–26%) of the HBA/HNA
copolymer, it has a high gas barrier property (Weinkauf and Paul 1992b). However, the
solubility of the liquid crystalline polymer in organic solvents, such as acetone, is quite
low (Freeman and Hill 1998).
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S. Ando et al.
result is that a continuous space for gas diffusion may exist around the interface
between the crystalline and amorphous regions (Sawada et al. 2010). In the same
way, permeability in PS is also independent of its crystallinity (Hodge et al. 2001;
Prodpran et al. 2002). Syndiotactic PS (s-PS) is known to have several crystalline
structures (i.e., α, β, γ, and δ crystalline). These structures not only affect the
physical properties of the polymer but their gas transport properties as well (Hodge
et al. 2001; Prodpran et al. 2002). This behavior is similar to PLA membranes. In
general, the gas permeability of the β-form crystalline structure is lower than that
of the α-form because the β-form structure has more tightly packed polymer chain
segments. The density of the β-form is higher than that of the amorphous polymers,
whereas the density of the α-form is lower than that of the amorphous polymers.
The gas permeability and solubility of the β-form decrease with increasing crystallinity, and its diffusivity remains constant with increasing crystallinity. The gas
permeability and diffusivity of the α-form structure increase and the solubility
decreases with increasing crystallinity. Based on these results, porous microcrystalline structures with nanochannels may be adopted to increase gas diffusivity,
thereby increasing gas permeability (Hodge et al. 2001; Prodpran et al. 2002).
Furthermore, s-PS with molecular cavities of different shapes and sizes were
obtained from an α-form crystal (Tsujita et al. 2005). Therefore, each crystalline
polymer has an intrinsic crystalline phase with different sizes and distributions.
These phases affect not only their physical and thermal properties but also their gas
transport properties. The density of crystalline PMP is slightly lower than that of its
amorphous counterpart (Griffith and Ranby 1960; Ranby et al. 1962). This result
was considered based on the characteristic crystalline structure of PMP. The bulky
pendant structure in PMP inhibits close polymer segments from packing and
results in a low-density crystal with gaps of almost 4 Å between chain segments
(Puleo et al. 1989). Therefore, a small molecule can pass though this characteristic
crystalline structure in PMP. By comparison, the properties of liquid crystalline polymers are different from those of amorphous and crystalline polymers in terms of
fluidity and orientation. These kinds of polymers have three types of phases, namely,
nematic, cholesteric, and smectic phases. As shown in Fig. 1, glassy liquid crystalline
polymers, such as poly( p-hydroxybenzoic acid-co-6-hydroxy-2-naphthioic acid)
(HBA/HNA) and poly( p-phenylene terephthalamide) (PPTA), show remarkably
high gas barrier properties. For example, the O 2 permeabilities of HBA/HNA copolymer and PPTA are lower than that of PET or PVC but are similar to that of
polyacrylonitrile (Weinkauf and Paul 1991; Weinkauf et al. 1992). These high gas
barrier properties are attributed to the efficient packing of the molecules in the solid
state. The gas barrier property of the HBA/HNA copolymer also increases with
increasing HNA content. The napthyl unit restricts the rotation mobility of the main
chain and packed polymer chain segments; thus, high gas barrier properties were
developed. Interestingly, in spite of the low crystallinity (13–26%) of the HBA/HNA
copolymer, it has a high gas barrier property (Weinkauf and Paul 1992b). However, the
solubility of the liquid crystalline polymer in organic solvents, such as acetone, is quite
low (Freeman and Hill 1998).
526
S. Ando et al.