In a polystyrene-b-polymethacrylate (PS-b-PMA)-based SC-LCBCP
system containing (S)-2-methylbutyl 4-(4-hydroxyphenylcarbonyloxy)-biphenyl4
0 -carboxylate mesogens with 10-carbon spacers (PDBPB), Zheng et al. reported
the phase behavior of a sample with f
LC ~ 0.59 where the BCP formed L phase and
within the L, the LC formed SmC* phase and these SmC* layers are oriented such
that their layer normal is parallel to the BCP layer normal (Zheng et al. 1998; Zheng
and Hammond 1996). This unique arrangement was attributed to the low T g of PMA
backbone. The authors claim that lower T g gives the polymer backbone flexibility to
backfold parallel to the interface and the longer spacers allow orientation of the
mesogens with the layer normal perpendicular to the IMDS. Hamley et al. (2005)
reported the phase structures of a PS-b-PMMA based block copolymer with chiral
cholesteryl mesogens with 12-carbon spacers where they also observed predominantly L morphologies over a large range of f. Interestingly, at highly asymmetric
volume fractions of the LC block ( f
LC
= 0.86), they observed PS C in LC matrix.
This indicates that the flexibility of polymer backbone in SC-LCBCP significantly
reduced the flat interface requirement and the domination of LCO over the MS is
subdued. The LC phase in this SC-LCBCP system was SmA in nature.
The competition between LC ordering and BCP microphase separation was
reported by a number of research groups. Highly asymmetric compositions make
it difficult to maintain Sm layer correlation between the mesogens due to the high
levels of interfacial curvature. Transformation of LC phase from Sm to N was
reported in samples where LC block formed S. Sanger et al. reported the influence
of N phase in inducing a structural transformation in a SC-LCBCP of PS-b-PB
where the PB was functionalized with a cyanobiphenyl-based mesogen connected to
the backbone using butyl alkyl spacer with f
PS
= 0.12 (Sanger et al. 1997). The
authors monitored the morphological transformation in the sample as a function of
annealing temperature using a combination of TEM and X-ray scattering. When the
samples were annealed at very high temperatures above the T I and quenched, they
displayed PS S domains in I phase. When the samples were slowly cooled from melt
into the N phase and then to RT, they exhibited PS C morphology. This transformation of the BCP morphology from BCC S to HCP C is induced by I➔N transition of
the LC matrix. The authors suggest that S domains create more distortion of the N
director so the transformation takes place through the coalescence of the PS spheres
such that the mesogens were oriented parallel to the C. Although the S BCP
morphology was more preferred in the isotropic state as it ensured the least free
energy state, as the N LC phase was formed, the elastic energy associated with the
LC molecules in S impose penalty on the system. In this case, the HCP structure
provides lower loss of elastic free energy. Although L structure would ensure
minimum elastic energy penalty, f
PS is too low to allow for a flatter interface. The
L to C transition was thermally reversible.
Ivanova et al. studied the phase behavior of N phase forming PS-b-PB based
LCBCP system where the LC block consists of cyanobiphenyl mesogens attached to
PB with short, butyl spacers (Ivanova et al. 2004). They investigated samples with
varying f
LC from ~0.3 to ~0.93 (total M w 77,000–125,000 g/mol) and developed a
phase diagram based on their observations at room temperature and a higher
202
K. K. Tenneti et al.
system containing (S)-2-methylbutyl 4-(4-hydroxyphenylcarbonyloxy)-biphenyl4
0 -carboxylate mesogens with 10-carbon spacers (PDBPB), Zheng et al. reported
the phase behavior of a sample with f
LC ~ 0.59 where the BCP formed L phase and
within the L, the LC formed SmC* phase and these SmC* layers are oriented such
that their layer normal is parallel to the BCP layer normal (Zheng et al. 1998; Zheng
and Hammond 1996). This unique arrangement was attributed to the low T g of PMA
backbone. The authors claim that lower T g gives the polymer backbone flexibility to
backfold parallel to the interface and the longer spacers allow orientation of the
mesogens with the layer normal perpendicular to the IMDS. Hamley et al. (2005)
reported the phase structures of a PS-b-PMMA based block copolymer with chiral
cholesteryl mesogens with 12-carbon spacers where they also observed predominantly L morphologies over a large range of f. Interestingly, at highly asymmetric
volume fractions of the LC block ( f
LC
= 0.86), they observed PS C in LC matrix.
This indicates that the flexibility of polymer backbone in SC-LCBCP significantly
reduced the flat interface requirement and the domination of LCO over the MS is
subdued. The LC phase in this SC-LCBCP system was SmA in nature.
The competition between LC ordering and BCP microphase separation was
reported by a number of research groups. Highly asymmetric compositions make
it difficult to maintain Sm layer correlation between the mesogens due to the high
levels of interfacial curvature. Transformation of LC phase from Sm to N was
reported in samples where LC block formed S. Sanger et al. reported the influence
of N phase in inducing a structural transformation in a SC-LCBCP of PS-b-PB
where the PB was functionalized with a cyanobiphenyl-based mesogen connected to
the backbone using butyl alkyl spacer with f
PS
= 0.12 (Sanger et al. 1997). The
authors monitored the morphological transformation in the sample as a function of
annealing temperature using a combination of TEM and X-ray scattering. When the
samples were annealed at very high temperatures above the T I and quenched, they
displayed PS S domains in I phase. When the samples were slowly cooled from melt
into the N phase and then to RT, they exhibited PS C morphology. This transformation of the BCP morphology from BCC S to HCP C is induced by I➔N transition of
the LC matrix. The authors suggest that S domains create more distortion of the N
director so the transformation takes place through the coalescence of the PS spheres
such that the mesogens were oriented parallel to the C. Although the S BCP
morphology was more preferred in the isotropic state as it ensured the least free
energy state, as the N LC phase was formed, the elastic energy associated with the
LC molecules in S impose penalty on the system. In this case, the HCP structure
provides lower loss of elastic free energy. Although L structure would ensure
minimum elastic energy penalty, f
PS is too low to allow for a flatter interface. The
L to C transition was thermally reversible.
Ivanova et al. studied the phase behavior of N phase forming PS-b-PB based
LCBCP system where the LC block consists of cyanobiphenyl mesogens attached to
PB with short, butyl spacers (Ivanova et al. 2004). They investigated samples with
varying f
LC from ~0.3 to ~0.93 (total M w 77,000–125,000 g/mol) and developed a
phase diagram based on their observations at room temperature and a higher
202
K. K. Tenneti et al.