repeating units would be ~0.29 nm, which is longer than corresponding period of
0.25 nm in the crystalline poly(dipropylsiloxane) (α form) (Gearba et al. 2006). This
indicates the polysiloxane main-chain in the Col h phase of this polymer is highly
stretched. In contrast, the threshold MW of poly(diethylsiloxane), a self-compacting
polymer, is 2.8 Â 10
4 Da (DP = 275) (Molenberg and Möller 1997).
Since the rigidifying effect of mesogen-jacketed LC polymers originates from the
steric effect of the mesogens, the mesophase-forming ability of the side-groups
themselves is not necessary for a polymer to form a columnar LC phase, as long as
their steric effect and the coupling with the backbone are strong enough. For instance,
monomer M3 (Chart 5) does not form a mesophase. However, the corresponding
polymer P3 features a high T g (173 to 208
C, depending on the MW) and a
Col h phase when the MW exceeds 0.53 Â 10
4 (DP = 22) (Zhang et al. 2014).
The LC phase retains after cooling to room temperature. Such a small threshold
DP supports a highly rigidified backbone in the columnar LC phase.
Even a single benzene ring, when functionalized appropriately, may exert enough
rigidifying effect to form a Col h phase. PmVT (Chart 5) exhibits a Col h phase with
m = 10–13. For P12VT, the threshold MW to show its Col h phase is 1.07 Â 10
4 Da
(DP = 30). The persistent length of ~ 40 nm underscores the high rigidity of the
polymer (Mei et al. 2010). The only difference between P12VT and PD6VT (Yin
et al. 2003) (Chart 3) is that the former has a cyclic side-chain, while the latter is
equipped with two linear side-chains. However, the threshold DP for the latter to be
LC is greater than 200. Moreover, the main driving force toward the columnar LC
phase in the latter polymer is entropic, evidenced by the appearance of a reentry
isotropic phase upon cooling the columnar phase. At least a part of entropy gain in
the Col h phase of PD6VT comes from the large number of conformations accessible
to n-hexyl chains. In a sharp contrast, being a part of a ring, the available conformations of polymethylene segments are greatly reduced in P12VT. This not only
makes them much more rigid, dramatically decreasing their entropy contribution to
the mesophase, but also substantially increases the overall steric demand of the entire
side-group so that the whole polymer chain is rigidified into an extended cylinder,
the basic unit of a Col h phase.
Polyvinyltrimethylsilane (PVTMS, Chart 5), without any rigid groups, shows a
T g of 120
C and a Col h phase with a clearing point above the thermal degradation
temperature (Antipov et al. 1992). When the mobile Col h phase is cooled below T g , a
glassy mesophase forms. Cylindrical polymer chains arising from rigidifying effect
of highly bulky trimethylsilyl groups are most likely responsible for the formation of
the mesophase, and this is supported by the rather high glass transition temperature
of the polymer.
A dendritic side-group, especially a high generation one, can potentially generate a large steric demand and therefore is capable of stretching dendron-jacketed
polymer chains into cylinders and then packing into a columnar phase. Just like
other columnar phase-forming polymers with a rigidified main-chain, a threshold
DP is expected for a dendron-jacketed polymer to form a columnar LC phase. For
example, DP-dependent phase behavior has been observed in polystyrene (12G2AG-PS) and polymethacrylate (12G2-AG-PMA) jacketed by second-generation
130
S. Jin
0.25 nm in the crystalline poly(dipropylsiloxane) (α form) (Gearba et al. 2006). This
indicates the polysiloxane main-chain in the Col h phase of this polymer is highly
stretched. In contrast, the threshold MW of poly(diethylsiloxane), a self-compacting
polymer, is 2.8 Â 10
4 Da (DP = 275) (Molenberg and Möller 1997).
Since the rigidifying effect of mesogen-jacketed LC polymers originates from the
steric effect of the mesogens, the mesophase-forming ability of the side-groups
themselves is not necessary for a polymer to form a columnar LC phase, as long as
their steric effect and the coupling with the backbone are strong enough. For instance,
monomer M3 (Chart 5) does not form a mesophase. However, the corresponding
polymer P3 features a high T g (173 to 208
C, depending on the MW) and a
Col h phase when the MW exceeds 0.53 Â 10
4 (DP = 22) (Zhang et al. 2014).
The LC phase retains after cooling to room temperature. Such a small threshold
DP supports a highly rigidified backbone in the columnar LC phase.
Even a single benzene ring, when functionalized appropriately, may exert enough
rigidifying effect to form a Col h phase. PmVT (Chart 5) exhibits a Col h phase with
m = 10–13. For P12VT, the threshold MW to show its Col h phase is 1.07 Â 10
4 Da
(DP = 30). The persistent length of ~ 40 nm underscores the high rigidity of the
polymer (Mei et al. 2010). The only difference between P12VT and PD6VT (Yin
et al. 2003) (Chart 3) is that the former has a cyclic side-chain, while the latter is
equipped with two linear side-chains. However, the threshold DP for the latter to be
LC is greater than 200. Moreover, the main driving force toward the columnar LC
phase in the latter polymer is entropic, evidenced by the appearance of a reentry
isotropic phase upon cooling the columnar phase. At least a part of entropy gain in
the Col h phase of PD6VT comes from the large number of conformations accessible
to n-hexyl chains. In a sharp contrast, being a part of a ring, the available conformations of polymethylene segments are greatly reduced in P12VT. This not only
makes them much more rigid, dramatically decreasing their entropy contribution to
the mesophase, but also substantially increases the overall steric demand of the entire
side-group so that the whole polymer chain is rigidified into an extended cylinder,
the basic unit of a Col h phase.
Polyvinyltrimethylsilane (PVTMS, Chart 5), without any rigid groups, shows a
T g of 120
C and a Col h phase with a clearing point above the thermal degradation
temperature (Antipov et al. 1992). When the mobile Col h phase is cooled below T g , a
glassy mesophase forms. Cylindrical polymer chains arising from rigidifying effect
of highly bulky trimethylsilyl groups are most likely responsible for the formation of
the mesophase, and this is supported by the rather high glass transition temperature
of the polymer.
A dendritic side-group, especially a high generation one, can potentially generate a large steric demand and therefore is capable of stretching dendron-jacketed
polymer chains into cylinders and then packing into a columnar phase. Just like
other columnar phase-forming polymers with a rigidified main-chain, a threshold
DP is expected for a dendron-jacketed polymer to form a columnar LC phase. For
example, DP-dependent phase behavior has been observed in polystyrene (12G2AG-PS) and polymethacrylate (12G2-AG-PMA) jacketed by second-generation
130
S. Jin