supported by polarized IR and X-ray diffraction study on the crystal-Col h transition
of ultrahigh molecular weight PE under strain (Tashiro et al. 1996). Polarized IR
results indicate that in the crystalline phase, C-H bonds are nearly perfectly perpendicular to the chain axis and the alignment direction. In contrast, the degree of
alignment of C-H bonds in the Col h phase is much lower and reduces quickly to
nearly isotropic upon heating. At the same time, the degree of chain orientation, as
measured by X-ray diffraction, shows little difference between the Col h phase and
the crystalline phase. The two seems conflicting observations which support that in
the Col h phase, chains are locally disordered with a significant number of gauche
bonds and low orientational order of C-H bonds, while the overall straight shape of
chains is preserved. Each straight chain with internal conformational disorder
constitutes a liquid cylinder. With a circular cross section, these liquid cylinders
pack into a 2D hexagonal lattice, forming a Col h phase.
The overall straight shape limits how gauche bonds can be introduced into an
originally all-trans PE chain. For instance, if a chain contains two gauche bonds, a
GTGÃ kink preserves the overall straight chain, while a double gauche form (GG) bends
the chain to a large extent. Interestingly, markedly increased GTGÃ but few GG bonds
have been observed at the crystal-Col h transition of irradiated PE (Ungar 1986).
Poly(tetrafluoroethylene) (PTFE)
It is generally accepted that the phase I of polytetrafluoroethylene (PTFE), observed
between 30
C to 300
C under ambient pressure, is a Col h phase, although recently
reported electron diffraction result makes such an assignment somewhat questionable (Clark 2006). In this phase, PTFE chains are found in 15/7 helical conformation.
Rapid uncoiling and disordering motion makes PTFE chains effectively liquid
cylinders that pack laterally into a Col h phase.
Polytrifluoroethylene and Related Copolymers
A Col h phase has also been observed in polytrifluoroethylene (Lovinger and Cais 1984)
and vinylidene fluoride/trifluoroethylene copolymers (Lovinger et al. 1983). In polytrifluoroethylene, the clearing point of the Col h phase depends on the thermal history and
the degree of isoregicity. It was reasoned that an irregular succession of TG, TG’, or TT
sequences gives polytrifluoroethylene chains a disordered cylindrical conformation that
lays the foundation of the Col h phase (Lovinger and Cais 1984). Similarly, disordered
arrangement of TG, TG’, and TT forms with a tendency of having longer sequences of
two, three, or four such bonds in vinylidene fluoride/trifluoroethylene copolymer chains
which render them liquid cylinders assembling into a Col h phase, with the temperature
range of the phase depending on the copolymer composition (Lovinger et al. 1983).
Poly(1,4-trans)-1,3-butadiene
The Col h phase of poly (1,4-trans)-1,3-butadiene is probably the first reported
thermotropic Col h phase in polymers (Natta et al. 1956). Under ambient pressure,
the Col h phase can be observed between 76
C and 146
C (Corradini 1969). At
crystal-Col h transition, the c axis shrinks by 4%, while the inter-chain distance
increases by 7.5%. The transition entropic of crystal-Col h is twice as large as that
120
S. Jin
of ultrahigh molecular weight PE under strain (Tashiro et al. 1996). Polarized IR
results indicate that in the crystalline phase, C-H bonds are nearly perfectly perpendicular to the chain axis and the alignment direction. In contrast, the degree of
alignment of C-H bonds in the Col h phase is much lower and reduces quickly to
nearly isotropic upon heating. At the same time, the degree of chain orientation, as
measured by X-ray diffraction, shows little difference between the Col h phase and
the crystalline phase. The two seems conflicting observations which support that in
the Col h phase, chains are locally disordered with a significant number of gauche
bonds and low orientational order of C-H bonds, while the overall straight shape of
chains is preserved. Each straight chain with internal conformational disorder
constitutes a liquid cylinder. With a circular cross section, these liquid cylinders
pack into a 2D hexagonal lattice, forming a Col h phase.
The overall straight shape limits how gauche bonds can be introduced into an
originally all-trans PE chain. For instance, if a chain contains two gauche bonds, a
GTGÃ kink preserves the overall straight chain, while a double gauche form (GG) bends
the chain to a large extent. Interestingly, markedly increased GTGÃ but few GG bonds
have been observed at the crystal-Col h transition of irradiated PE (Ungar 1986).
Poly(tetrafluoroethylene) (PTFE)
It is generally accepted that the phase I of polytetrafluoroethylene (PTFE), observed
between 30
C to 300
C under ambient pressure, is a Col h phase, although recently
reported electron diffraction result makes such an assignment somewhat questionable (Clark 2006). In this phase, PTFE chains are found in 15/7 helical conformation.
Rapid uncoiling and disordering motion makes PTFE chains effectively liquid
cylinders that pack laterally into a Col h phase.
Polytrifluoroethylene and Related Copolymers
A Col h phase has also been observed in polytrifluoroethylene (Lovinger and Cais 1984)
and vinylidene fluoride/trifluoroethylene copolymers (Lovinger et al. 1983). In polytrifluoroethylene, the clearing point of the Col h phase depends on the thermal history and
the degree of isoregicity. It was reasoned that an irregular succession of TG, TG’, or TT
sequences gives polytrifluoroethylene chains a disordered cylindrical conformation that
lays the foundation of the Col h phase (Lovinger and Cais 1984). Similarly, disordered
arrangement of TG, TG’, and TT forms with a tendency of having longer sequences of
two, three, or four such bonds in vinylidene fluoride/trifluoroethylene copolymer chains
which render them liquid cylinders assembling into a Col h phase, with the temperature
range of the phase depending on the copolymer composition (Lovinger et al. 1983).
Poly(1,4-trans)-1,3-butadiene
The Col h phase of poly (1,4-trans)-1,3-butadiene is probably the first reported
thermotropic Col h phase in polymers (Natta et al. 1956). Under ambient pressure,
the Col h phase can be observed between 76
C and 146
C (Corradini 1969). At
crystal-Col h transition, the c axis shrinks by 4%, while the inter-chain distance
increases by 7.5%. The transition entropic of crystal-Col h is twice as large as that
120
S. Jin