(Desper and Schneider 1976; Kojima and Magill 1989; Kojima et al. 1992) as well as
para-substituted benzyl (Desper et al. 1977), methoxy, methylthio (Gomez et al.
1991), and ethyl (Gomez et al. 1990) counterparts, exhibits a Col h phase. The
intercolumn spacing in the mesophase is linearly related to the size of R (Kojima
and Magill 1989).
Very interestingly, a number of organopolyphosphazenes exhibit an abnormal
phase sequence in which a columnar mesophase cools into an isotropic phase, i.e., a
reentry isotropic phase. For instance, the combination of polarized light microscopy
and X-ray results revealed an isotropic-columnar-isotropic phase sequence during
both heating and cooling for poly[bis(butoxy)phosphazene] and poly[bis(pentoxy)
phosphazene] (Papkov et al. 1992). When poly[bis(ethylphenoxy)phosphazene)] is
quenched from the liquid phase to a temperature below the crystal-mesophase
transition, the mesophase does not form, and crystallization directly occurs from
the isotropic phase (Gomez et al. 1990). Since the nucleation barrier of the crystallization is highly unlikely to be smaller than that of the mesophase formation, such
an observation implies that at a temperature below crystal-mesophase transition, the
less ordered isotropic phase is more stable than the more ordered mesophase. In
other words, the isotropic phase is a reentry phase. The existence of such a reentry
isotropic phase below the mesophase directly supports entropy as the major driving
force for the columnar mesophase formation and consistent with the “selfcompacting” chain model, although possible phase separation between the inorganic
polyphosphazene backbone and the organic side-groups may also contribute to the
phase formation. It is worth noting that, in some cases, the 2D order of cylindrical
polyphosphazene chains persists to a significant degree even in the isotropic liquid
phase, as indicated by an unusually sharp inter-chain X-ray diffraction peak (Kojima
and Magill 1989).
Other Self-Compacting Columnar Phase-Forming Polymers
As one of the key driving forces for self-compacting chains to form a mesophase is
the main-chain entropy gain, a high-mobility main-chain is instrumental to the
mesophase formation. This explains why such a phase formation is most commonly
observed with polymers with a highly mobile main-chain such as polysiloxanes and
polyphosphazenes. However, polymers with less mobile backbones could also form
the same type of mesophase, as long as the temperature is high enough for the
backbone to be mobile. One of such examples is poly[di(alkyl)vinylterephthalates],
as shown in Chart 3 . Although the backbone of this series of polymers is much less
mobile than polysiloxanes and polyphosphazenes, a Col h phase was observed for
m = 3–6, 8, 10 (Yin et al. 2003). Similar to polysiloxanes, MW dependence is
observed for the PD3VT. The polymer with a MW of 2.6 Â 10
4 (DP = 94) did not
show any mesophase, while higher MW materials (0.8–2.2 Â 10
5 ) exhibit a Col h
phase. For PD6VT, PD8VT, and PD10VT, a reentry amorphous phase has been
observed below the mesophase, signifying the entropy-driven nature of the mesophase. Spectroscopic study on PD4VT demonstrated that the n-alkyl chain gauche
fraction remains constant in the lower-temperature amorphous phase but increases
substantially in the higher-temperature mesophase, suggesting the entropy gain of
124
S. Jin
para-substituted benzyl (Desper et al. 1977), methoxy, methylthio (Gomez et al.
1991), and ethyl (Gomez et al. 1990) counterparts, exhibits a Col h phase. The
intercolumn spacing in the mesophase is linearly related to the size of R (Kojima
and Magill 1989).
Very interestingly, a number of organopolyphosphazenes exhibit an abnormal
phase sequence in which a columnar mesophase cools into an isotropic phase, i.e., a
reentry isotropic phase. For instance, the combination of polarized light microscopy
and X-ray results revealed an isotropic-columnar-isotropic phase sequence during
both heating and cooling for poly[bis(butoxy)phosphazene] and poly[bis(pentoxy)
phosphazene] (Papkov et al. 1992). When poly[bis(ethylphenoxy)phosphazene)] is
quenched from the liquid phase to a temperature below the crystal-mesophase
transition, the mesophase does not form, and crystallization directly occurs from
the isotropic phase (Gomez et al. 1990). Since the nucleation barrier of the crystallization is highly unlikely to be smaller than that of the mesophase formation, such
an observation implies that at a temperature below crystal-mesophase transition, the
less ordered isotropic phase is more stable than the more ordered mesophase. In
other words, the isotropic phase is a reentry phase. The existence of such a reentry
isotropic phase below the mesophase directly supports entropy as the major driving
force for the columnar mesophase formation and consistent with the “selfcompacting” chain model, although possible phase separation between the inorganic
polyphosphazene backbone and the organic side-groups may also contribute to the
phase formation. It is worth noting that, in some cases, the 2D order of cylindrical
polyphosphazene chains persists to a significant degree even in the isotropic liquid
phase, as indicated by an unusually sharp inter-chain X-ray diffraction peak (Kojima
and Magill 1989).
Other Self-Compacting Columnar Phase-Forming Polymers
As one of the key driving forces for self-compacting chains to form a mesophase is
the main-chain entropy gain, a high-mobility main-chain is instrumental to the
mesophase formation. This explains why such a phase formation is most commonly
observed with polymers with a highly mobile main-chain such as polysiloxanes and
polyphosphazenes. However, polymers with less mobile backbones could also form
the same type of mesophase, as long as the temperature is high enough for the
backbone to be mobile. One of such examples is poly[di(alkyl)vinylterephthalates],
as shown in Chart 3 . Although the backbone of this series of polymers is much less
mobile than polysiloxanes and polyphosphazenes, a Col h phase was observed for
m = 3–6, 8, 10 (Yin et al. 2003). Similar to polysiloxanes, MW dependence is
observed for the PD3VT. The polymer with a MW of 2.6 Â 10
4 (DP = 94) did not
show any mesophase, while higher MW materials (0.8–2.2 Â 10
5 ) exhibit a Col h
phase. For PD6VT, PD8VT, and PD10VT, a reentry amorphous phase has been
observed below the mesophase, signifying the entropy-driven nature of the mesophase. Spectroscopic study on PD4VT demonstrated that the n-alkyl chain gauche
fraction remains constant in the lower-temperature amorphous phase but increases
substantially in the higher-temperature mesophase, suggesting the entropy gain of
124
S. Jin