increase in temperature, all the samples (except the sample with highest f
coil
which is isotropic throughout the temperature range) transformed into N phase
that further transformed into an isotropic phase. Using a combination of X-ray
and light scattering, optical and electron microscopy results, the authors developed an experimental phase diagram for the weakly segregated system, which is
shown in Fig. 9c (Olsen and Segalman 2005, 2006). The width of the N phase
region in the phase diagram increases with increase in the f
rod . Within the PPV
domains, the rods could be arranged either in monolayers or tilted bilayers.
However, they did not observe either the hockey puck morphology at high f
coil
or the zig-zag structures at high f
rod . The authors claim that a higher aspect ratio is
required for the formation of puck-shaped morphologies. Accordingly, greater
aspect ratio leads to greater elastic energy of bending which in turn induces
greater coil stretching and the puck formation is a way of reducing this chain
stretching. The high temperature phase behavior of these systems follows the
trend: low f
coil L ➔ N ➔ Isotropic and high f
coil L ➔ Isotropic.
The same research group also reported the phase behavior of PPV-b-PI system
with higher M w PPV (5600 g/mol) with increased geometrical asymmetry
between the rod and coil blocks (Olsen and Segalman 2007). The increase in
M w resulted in moderately segregated BCP system compared to their previous
weakly segregated system (M n PPV = 3545 g/mol). Keeping the PPV length
constant, the authors reported the phase structures of a series of sample with
varying f
coil (0.31–0.91). At moderate f
coil , the authors observed L phases,
however, increasing the f
coil to 0.81 led to the formation of PPV nanodomains
packed in a hexagonal lattice. PPV rods inside these domains do not possess any
preferred orientation with respect to the interface. These nanodomains had
rectangular interfaces (~32 nm wide and ~12 nm thick) corresponding to a
packing of ~1000 rods per cluster. The packing of the rods improved as the f
rod
increased. The sample with f
coil
= 0.81 is considered as the boundary between the
transition from L to hexagonally close packed aggregate structure (H). Faster
quenching from melt resulted in the retention of these H aggregates, which were
stable at low temperatures, whereas slower annealing below the ODT resulted in
OOT into L phase. Since entropic factors of coil stretching prefer H phase
compared to L phase, the OOT into L phases suggested that in these samples,
the rod orientation dominates the BCP microphase separation leading to L phase
formation. The H phase is present only at higher f
coil . The authors proposed that
within the L phases, the rods are oriented perpendicular to the L interface.
Heating the samples resulted in N phase before isotropization in all the samples,
irrespective of the f
coil . This is different from their observation of absence of N
phase at high f
coil in weakly segregated samples and is attributed to the increase in
the rod-rod interactions due to higher M w . Based on their experimental observations, Segalman et al. developed a phase diagram for the PPV-b-PI system in the
moderate segregation limit as shown in Fig. 9d. L phases dominated the phase
diagram at low and moderate f
coil , and at high f
coil , the H phase was observed.
Increase in PPV M w led to the formation of N phase in all the samples irrespective
of the f
coil .
7 Structure and Assembly of Liquid Crystalline Block Copolymers
193
coil
which is isotropic throughout the temperature range) transformed into N phase
that further transformed into an isotropic phase. Using a combination of X-ray
and light scattering, optical and electron microscopy results, the authors developed an experimental phase diagram for the weakly segregated system, which is
shown in Fig. 9c (Olsen and Segalman 2005, 2006). The width of the N phase
region in the phase diagram increases with increase in the f
rod . Within the PPV
domains, the rods could be arranged either in monolayers or tilted bilayers.
However, they did not observe either the hockey puck morphology at high f
coil
or the zig-zag structures at high f
rod . The authors claim that a higher aspect ratio is
required for the formation of puck-shaped morphologies. Accordingly, greater
aspect ratio leads to greater elastic energy of bending which in turn induces
greater coil stretching and the puck formation is a way of reducing this chain
stretching. The high temperature phase behavior of these systems follows the
trend: low f
coil L ➔ N ➔ Isotropic and high f
coil L ➔ Isotropic.
The same research group also reported the phase behavior of PPV-b-PI system
with higher M w PPV (5600 g/mol) with increased geometrical asymmetry
between the rod and coil blocks (Olsen and Segalman 2007). The increase in
M w resulted in moderately segregated BCP system compared to their previous
weakly segregated system (M n PPV = 3545 g/mol). Keeping the PPV length
constant, the authors reported the phase structures of a series of sample with
varying f
coil (0.31–0.91). At moderate f
coil , the authors observed L phases,
however, increasing the f
coil to 0.81 led to the formation of PPV nanodomains
packed in a hexagonal lattice. PPV rods inside these domains do not possess any
preferred orientation with respect to the interface. These nanodomains had
rectangular interfaces (~32 nm wide and ~12 nm thick) corresponding to a
packing of ~1000 rods per cluster. The packing of the rods improved as the f
rod
increased. The sample with f
coil
= 0.81 is considered as the boundary between the
transition from L to hexagonally close packed aggregate structure (H). Faster
quenching from melt resulted in the retention of these H aggregates, which were
stable at low temperatures, whereas slower annealing below the ODT resulted in
OOT into L phase. Since entropic factors of coil stretching prefer H phase
compared to L phase, the OOT into L phases suggested that in these samples,
the rod orientation dominates the BCP microphase separation leading to L phase
formation. The H phase is present only at higher f
coil . The authors proposed that
within the L phases, the rods are oriented perpendicular to the L interface.
Heating the samples resulted in N phase before isotropization in all the samples,
irrespective of the f
coil . This is different from their observation of absence of N
phase at high f
coil in weakly segregated samples and is attributed to the increase in
the rod-rod interactions due to higher M w . Based on their experimental observations, Segalman et al. developed a phase diagram for the PPV-b-PI system in the
moderate segregation limit as shown in Fig. 9d. L phases dominated the phase
diagram at low and moderate f
coil , and at high f
coil , the H phase was observed.
Increase in PPV M w led to the formation of N phase in all the samples irrespective
of the f
coil .
7 Structure and Assembly of Liquid Crystalline Block Copolymers
193