molecular design can be preceded. It should be noted that owing to the complex
chemical structures of MJLCPs, some of the abovementioned factors may be
intertwined and may not be separated.
Chemical Structure-Phase Behavior Relationships
Backbone Flexibility
Most MJLCPs are based on polyacrylates/polymethacrylates and polystyrenes, and
they all have a polyethylene backbone. When the backbone is changed to more
flexible polysiloxane, the phase structures of MJLCPs are not exactly the same
(Zhang et al. 2010). MJLCPs with the more rigid polyacetylene backbone tend to
form smectic phases compared to their polystyrene analogs (Chen et al. 2006a).
Although polynorbornene appears to be a more rigid main chain than polyethylene,
the length of the repeating unit of polyethylene is only half those of polynorbornene,
and the side-chain density in the polynorbornene-based MJLCP is only half that in
the polyethylene-based MJLCP. Therefore, the overall rigidity and the LC-forming
ability of a polynorbornene-based MJLCP are determined by the side chain attached.
In the MJLCPs synthesized with a polynorbornene backbone, polymers with side
chains having longer alkyl tails are liquid crystalline (Zhu et al. 2014b). In addition,
they have lower isotropization temperatures and become isotropic upon heating.
Molecular Shape of Side Chain
Following the change of the linear rigid side-chain core in polymer 9 to a bent-type
core, all the resultant polymers 10 form Col h instead of SmA phases (Chai et al.
2007; Xu et al. 2009). And MJLCPs having dendrons in the side chains also show
columnar phases (Jin et al. 2010). Similarly, with the introduction of a triphenylene
(Tp) moiety in the side chain, the shape of a polyacetylene-based MJLCP changes to
columnar and exhibits a Col h phase instead of smectic phases for many MJLCPs
with a polyacetylene backbone (Yu et al. 2013).
Rigidity of Side Chain
Polymer 11 in Chart 1 having cyclic side groups of appropriate sizes is liquid
crystalline (Mei et al. 2010, 2012). The liquid crystallinity is attributed to the rigidity
of the cyclic pendants and the whole polymer chain.
Aliphatic Tail of Side Chain
The effect of alkyl tails on the phase behaviors of MJLCPs is frequently observed.
For example, the phase structure of polymer 3 in Chart 1 shows a strong dependence
on the length of side-chain aliphatic tails (Fig. 7) (Yin et al. 2003). Polymers with
side chains having propyl/isopropyl to hexyl tails form highly ordered 2D Col h
phases. Polymers having tails of other lengths are either partially liquid crystalline or
even amorphous.
When the length of aliphatic tails is increased, the LC phase of the MJLCP 6 (polymerized from 2-vinyl monomers) in Chart 1 changes from SmA (n = 2, 4) to SmC
2 Mesogen-Jacketed Liquid Crystalline Polymers: Molecular Design and. . .
39
chemical structures of MJLCPs, some of the abovementioned factors may be
intertwined and may not be separated.
Chemical Structure-Phase Behavior Relationships
Backbone Flexibility
Most MJLCPs are based on polyacrylates/polymethacrylates and polystyrenes, and
they all have a polyethylene backbone. When the backbone is changed to more
flexible polysiloxane, the phase structures of MJLCPs are not exactly the same
(Zhang et al. 2010). MJLCPs with the more rigid polyacetylene backbone tend to
form smectic phases compared to their polystyrene analogs (Chen et al. 2006a).
Although polynorbornene appears to be a more rigid main chain than polyethylene,
the length of the repeating unit of polyethylene is only half those of polynorbornene,
and the side-chain density in the polynorbornene-based MJLCP is only half that in
the polyethylene-based MJLCP. Therefore, the overall rigidity and the LC-forming
ability of a polynorbornene-based MJLCP are determined by the side chain attached.
In the MJLCPs synthesized with a polynorbornene backbone, polymers with side
chains having longer alkyl tails are liquid crystalline (Zhu et al. 2014b). In addition,
they have lower isotropization temperatures and become isotropic upon heating.
Molecular Shape of Side Chain
Following the change of the linear rigid side-chain core in polymer 9 to a bent-type
core, all the resultant polymers 10 form Col h instead of SmA phases (Chai et al.
2007; Xu et al. 2009). And MJLCPs having dendrons in the side chains also show
columnar phases (Jin et al. 2010). Similarly, with the introduction of a triphenylene
(Tp) moiety in the side chain, the shape of a polyacetylene-based MJLCP changes to
columnar and exhibits a Col h phase instead of smectic phases for many MJLCPs
with a polyacetylene backbone (Yu et al. 2013).
Rigidity of Side Chain
Polymer 11 in Chart 1 having cyclic side groups of appropriate sizes is liquid
crystalline (Mei et al. 2010, 2012). The liquid crystallinity is attributed to the rigidity
of the cyclic pendants and the whole polymer chain.
Aliphatic Tail of Side Chain
The effect of alkyl tails on the phase behaviors of MJLCPs is frequently observed.
For example, the phase structure of polymer 3 in Chart 1 shows a strong dependence
on the length of side-chain aliphatic tails (Fig. 7) (Yin et al. 2003). Polymers with
side chains having propyl/isopropyl to hexyl tails form highly ordered 2D Col h
phases. Polymers having tails of other lengths are either partially liquid crystalline or
even amorphous.
When the length of aliphatic tails is increased, the LC phase of the MJLCP 6 (polymerized from 2-vinyl monomers) in Chart 1 changes from SmA (n = 2, 4) to SmC
2 Mesogen-Jacketed Liquid Crystalline Polymers: Molecular Design and. . .
39