Cylindrical-Chain Polymers
In these polymers, each polymer chain (in very rare cases, a bundle of chains) adopts an
overall straight, liquid (mobile and locally disordered) cylindrical conformation. Such a
liquid cylindrical conformation could result from the intrinsic conformational preference
of the polymer main-chain, entropy-driven self-compacting of the chain, the main-chain
being rigidified by steric-demanding side-groups or simply having rigid-rod chains.
When in a columnar mesophase, such cylindrical chains have very large mobility along
the column axis direction, which facilitates lamellar thickening and the growth of
extended chain crystals if crystallization occurs in a columnar mesophase.
Flexible Main-Chain Polymers
The liquid cylinders in this type of polymers form as the result of intrinsic conformational preference of the flexible main-chain.
Polyethylene (PE)
PE, the simplest polymer, under a pressure above 3.6 kbar, forms a hexagonal columnar
(Col h ) phase between the lower-temperature orthorhombic crystalline phase and the
isotropic liquid phase (Bassett et al. 1974; Yamamoto 1979). The Col h phase of PE has
also been observed in highly extended PE fibers under strain above its normal
melting point (Tashiro et al. 1996) or at much lower temperature (even at room
temperature) by introducing chemical defects, either via irradiation (Ungar and
Keller 1980) or adding small number of methyl branches (Sworen et al. 2003).
At the crystal-Col h transition, a significant lateral lattice expansion and contraction along the chain axis direction occurs. For instance, under the pressure of 6 kbar,
the average cross-sectional area of a PE chain in the crystalline phase (but in the
vicinity of the crystal-Col h transition) is 18.2 Å
2 . However, in the Col h phase, it
suddenly increases to 20.6 Å
2
, while the periodicity along the chain axis direction
reduced from 2.53 Å to 2.45 Å (Bassett et al. 1974). These observations suggest that
a significant number of gauche (G) conformers are introduced into the original
all-anti (T) chains at the crystal-Col h transition, which was confirmed by simultaneous Raman and X-ray diffraction study on PE at 4 kbar (Kurelec et al. 2000) and
(CH 2 CH 2 ) n
(CF 2 CF 2 ) n
(CF 2 CHF) n
(CF 2 CHF) x (CF 2 CH 2 ) y
n
CH 2 CH 2 n
Chart 1 Structures of
flexible main-chain columnar
phase-forming polymers
5 Columnar Phase-Forming Polymers
119
In these polymers, each polymer chain (in very rare cases, a bundle of chains) adopts an
overall straight, liquid (mobile and locally disordered) cylindrical conformation. Such a
liquid cylindrical conformation could result from the intrinsic conformational preference
of the polymer main-chain, entropy-driven self-compacting of the chain, the main-chain
being rigidified by steric-demanding side-groups or simply having rigid-rod chains.
When in a columnar mesophase, such cylindrical chains have very large mobility along
the column axis direction, which facilitates lamellar thickening and the growth of
extended chain crystals if crystallization occurs in a columnar mesophase.
Flexible Main-Chain Polymers
The liquid cylinders in this type of polymers form as the result of intrinsic conformational preference of the flexible main-chain.
Polyethylene (PE)
PE, the simplest polymer, under a pressure above 3.6 kbar, forms a hexagonal columnar
(Col h ) phase between the lower-temperature orthorhombic crystalline phase and the
isotropic liquid phase (Bassett et al. 1974; Yamamoto 1979). The Col h phase of PE has
also been observed in highly extended PE fibers under strain above its normal
melting point (Tashiro et al. 1996) or at much lower temperature (even at room
temperature) by introducing chemical defects, either via irradiation (Ungar and
Keller 1980) or adding small number of methyl branches (Sworen et al. 2003).
At the crystal-Col h transition, a significant lateral lattice expansion and contraction along the chain axis direction occurs. For instance, under the pressure of 6 kbar,
the average cross-sectional area of a PE chain in the crystalline phase (but in the
vicinity of the crystal-Col h transition) is 18.2 Å
2 . However, in the Col h phase, it
suddenly increases to 20.6 Å
2
, while the periodicity along the chain axis direction
reduced from 2.53 Å to 2.45 Å (Bassett et al. 1974). These observations suggest that
a significant number of gauche (G) conformers are introduced into the original
all-anti (T) chains at the crystal-Col h transition, which was confirmed by simultaneous Raman and X-ray diffraction study on PE at 4 kbar (Kurelec et al. 2000) and
(CH 2 CH 2 ) n
(CF 2 CF 2 ) n
(CF 2 CHF) n
(CF 2 CHF) x (CF 2 CH 2 ) y
n
CH 2 CH 2 n
Chart 1 Structures of
flexible main-chain columnar
phase-forming polymers
5 Columnar Phase-Forming Polymers
119