of Col h -i (isotropic liquid), signifying the LC phase nature of the phase. The large
entropy of the Col h phase is a result of the intra-chain conformational disorder from a
statistic succession of three nearly isoenergetic conformers: A
Æ trans A
Æ
T (A stands
for anticlined), A
Æ trans A
Ç
T, and G trans A
Æ
T (De Rosa et al. 1985).
Poly( p-xylylene)
A Col h phase has also been observed in poly( p-xylylene) between 287 and
437
C. Computation results have suggested that the increase of disorder of
polymer chains in its Col h phase over the underlying crystalline phase can be
satisfactorily accounted for by only considering the phenyl rotation. With the
large amplitude phenyl rotation, the polymer chains become essentially internally
disordered and mobile cylinders, the fundamental moieties of a Col h phase
(Miller et al. 1990).
Self-Compacting Polymers
The columnar phase of this type of polymers is mainly stabilized by the entropy gain
of polymer chains in the mesophase over both the more ordered (usually crystalline)
and less ordered (usually isotropic) neighboring phases and can be best explained by
the self-compacting chain model (Allegra and Meille 2004). In this model, the polymer
main-chain is considered as the entropy reservoir. Upon the mesophase formation
during heating, either from an underlying crystalline phase or an amorphous phase, the
polymer main-chain reduces its dimension along the chain axis direction (selfcompacting), maximizing its entropy. As the result of a self-compacting main-chain,
the side-groups pack tightly around the main-chain, making each polymer chain a
cylinder. Besides the conformational entropy gain of the main-chain, flexible sidechains, if they do exist, also have a larger conformational freedom when being pulled
together by a “self-compacted” flexible main-chain than that in the isotropic melt.
Consequently, the entropy gain in the mesophase is greater than the entropy loss from
the limited bending of the macromolecule imposed by the cylindrical shape. Since the
stabilization of the mesophase is largely entropic, the isotropization enthalpy for selfcompacting polymers is usually very small.
The most representative self-compacting polymers are poly(di-n-alkylsiloxanes) and
organopolyphosphazenes, with a very flexible main-chain adept at self-compacting.
Many of these polymers also carry flexible side-groups, usually n-alkyl chains.
Poly(di-n-alkylsiloxanes)
Poly(di-n-alkylsiloxanes) (PDmSiO, m being the number of carbon atoms in each nalkyl chain, as shown in Fig. 1) with m = 2–6 are capable of forming a Col h phase
(Molenberg et al. 1997). The mesophase stability strongly depends on m. The
isotropization temperature of the columnar phase increases rapidly with m before
levels off at m = 4 (Out et al. 1995a).
Many aspects of PDmSiO agree with the self-compacting chain model. For
instance, all PDmSiOs show very low enthalpy of isotropization, lower than
5 Columnar Phase-Forming Polymers
121
entropy of the Col h phase is a result of the intra-chain conformational disorder from a
statistic succession of three nearly isoenergetic conformers: A
Æ trans A
Æ
T (A stands
for anticlined), A
Æ trans A
Ç
T, and G trans A
Æ
T (De Rosa et al. 1985).
Poly( p-xylylene)
A Col h phase has also been observed in poly( p-xylylene) between 287 and
437
C. Computation results have suggested that the increase of disorder of
polymer chains in its Col h phase over the underlying crystalline phase can be
satisfactorily accounted for by only considering the phenyl rotation. With the
large amplitude phenyl rotation, the polymer chains become essentially internally
disordered and mobile cylinders, the fundamental moieties of a Col h phase
(Miller et al. 1990).
Self-Compacting Polymers
The columnar phase of this type of polymers is mainly stabilized by the entropy gain
of polymer chains in the mesophase over both the more ordered (usually crystalline)
and less ordered (usually isotropic) neighboring phases and can be best explained by
the self-compacting chain model (Allegra and Meille 2004). In this model, the polymer
main-chain is considered as the entropy reservoir. Upon the mesophase formation
during heating, either from an underlying crystalline phase or an amorphous phase, the
polymer main-chain reduces its dimension along the chain axis direction (selfcompacting), maximizing its entropy. As the result of a self-compacting main-chain,
the side-groups pack tightly around the main-chain, making each polymer chain a
cylinder. Besides the conformational entropy gain of the main-chain, flexible sidechains, if they do exist, also have a larger conformational freedom when being pulled
together by a “self-compacted” flexible main-chain than that in the isotropic melt.
Consequently, the entropy gain in the mesophase is greater than the entropy loss from
the limited bending of the macromolecule imposed by the cylindrical shape. Since the
stabilization of the mesophase is largely entropic, the isotropization enthalpy for selfcompacting polymers is usually very small.
The most representative self-compacting polymers are poly(di-n-alkylsiloxanes) and
organopolyphosphazenes, with a very flexible main-chain adept at self-compacting.
Many of these polymers also carry flexible side-groups, usually n-alkyl chains.
Poly(di-n-alkylsiloxanes)
Poly(di-n-alkylsiloxanes) (PDmSiO, m being the number of carbon atoms in each nalkyl chain, as shown in Fig. 1) with m = 2–6 are capable of forming a Col h phase
(Molenberg et al. 1997). The mesophase stability strongly depends on m. The
isotropization temperature of the columnar phase increases rapidly with m before
levels off at m = 4 (Out et al. 1995a).
Many aspects of PDmSiO agree with the self-compacting chain model. For
instance, all PDmSiOs show very low enthalpy of isotropization, lower than
5 Columnar Phase-Forming Polymers
121