Importance of Topology
In bottlebrush diblock copolymers, two different variables control polymer composition, the length of the polymer backbone and length of the side chains. Blocks
having symmetric brush lengths can be visualized as elongated soft cylinders, and
increasing the block backbone lengths affects only in further elongation of soft
cylinder, while it does not have pronounced effect on the cross-sectional area
between the blocks, which is very important parameter for molecular packing.
Repulsion of high-density side chains on each repeating unit of bottlebrush block
copolymers leads to stretching of the backbone to obtain higher sizes of lamellar
domains even at asymmetric backbone lengths of each block. Rzayev in his work
studied the effect of asymmetric topology on the similar length blocks, comprising of
longer side chain lengths on one block and shorter side chain lengths on the other
blocks, on the change in cross-sectional area to get curved cylindrical phases
(Rzayev 2009).
Current system LCBBC73–25 k, with LC side chains on one block and semicrystalline PEG side chains with 2000 gm/mole MW on other block, though cannot
be visualized as highly asymmetric bottlebrush because block with shorter length LC
side chains can be considered analogous to linear block. Because of the linear nature
of one block in LCBBC73–25 k polymer molecule, change in composition is
obtained by varying the block lengths (Φ LC = 0.73) unlike asymmetric bottlebrush
where similar block lengths are used with varying side chain lengths to obtain the
difference in volume fractions. However, in LCBBC73–25 k, large asymmetry
between PEG side chains (M n = 2000 g/mol) and cyanobiphenyl side chains can
be the driving force to obtain curved interphase cylindrical domains unlike
bottlebrushes.
As we have already studied the magnetic function of cyanobiphenyl LC to get
magnetic field-based self-assembly in hexagonally packed cylindrical system with
minor block forming the cylinders and major phase consisting of LC ordering
(Deshmukh et al. 2014; Gopinadhan et al. 2014), it would be insightful to study
the effect of PEG brushlike cylinder forming minor block as compared to linear PEG
minor block in the system aligned with magnetic field-based self-assembly. Comparison in the systems with brushlike minor block with the linear minor block
counterpart may shed light on the effect of brush architecture of minor block on
d-spacing, inter-cylinder spacing, and order-disorder temperatures of phase segregation. Brush systems tend to phase segregate with larger domain size due to
backbone stretching as compared to their linear counterparts. Larger aligned PEG
domains would be feasible to have more metal ion loading for anisotropic transport
as in case for the lithium-ion batteries.
While we have not synthesized the block copolymer system with linear PEG
forming minor block with side chain cyanobiphenyl LC forming the major phase,
block copolymers with similar architectures were studied by Majewski et al. to
obtain anisotropic ion transport in highly aligned hexagonally packed cylinders of
linear block PEG after magnetic field-based self-assembly (Majewski et al. 2010).
Magnetic functionality due to cyanobiphenyl mesogen is important because it
488
L. Mahajan et al.
In bottlebrush diblock copolymers, two different variables control polymer composition, the length of the polymer backbone and length of the side chains. Blocks
having symmetric brush lengths can be visualized as elongated soft cylinders, and
increasing the block backbone lengths affects only in further elongation of soft
cylinder, while it does not have pronounced effect on the cross-sectional area
between the blocks, which is very important parameter for molecular packing.
Repulsion of high-density side chains on each repeating unit of bottlebrush block
copolymers leads to stretching of the backbone to obtain higher sizes of lamellar
domains even at asymmetric backbone lengths of each block. Rzayev in his work
studied the effect of asymmetric topology on the similar length blocks, comprising of
longer side chain lengths on one block and shorter side chain lengths on the other
blocks, on the change in cross-sectional area to get curved cylindrical phases
(Rzayev 2009).
Current system LCBBC73–25 k, with LC side chains on one block and semicrystalline PEG side chains with 2000 gm/mole MW on other block, though cannot
be visualized as highly asymmetric bottlebrush because block with shorter length LC
side chains can be considered analogous to linear block. Because of the linear nature
of one block in LCBBC73–25 k polymer molecule, change in composition is
obtained by varying the block lengths (Φ LC = 0.73) unlike asymmetric bottlebrush
where similar block lengths are used with varying side chain lengths to obtain the
difference in volume fractions. However, in LCBBC73–25 k, large asymmetry
between PEG side chains (M n = 2000 g/mol) and cyanobiphenyl side chains can
be the driving force to obtain curved interphase cylindrical domains unlike
bottlebrushes.
As we have already studied the magnetic function of cyanobiphenyl LC to get
magnetic field-based self-assembly in hexagonally packed cylindrical system with
minor block forming the cylinders and major phase consisting of LC ordering
(Deshmukh et al. 2014; Gopinadhan et al. 2014), it would be insightful to study
the effect of PEG brushlike cylinder forming minor block as compared to linear PEG
minor block in the system aligned with magnetic field-based self-assembly. Comparison in the systems with brushlike minor block with the linear minor block
counterpart may shed light on the effect of brush architecture of minor block on
d-spacing, inter-cylinder spacing, and order-disorder temperatures of phase segregation. Brush systems tend to phase segregate with larger domain size due to
backbone stretching as compared to their linear counterparts. Larger aligned PEG
domains would be feasible to have more metal ion loading for anisotropic transport
as in case for the lithium-ion batteries.
While we have not synthesized the block copolymer system with linear PEG
forming minor block with side chain cyanobiphenyl LC forming the major phase,
block copolymers with similar architectures were studied by Majewski et al. to
obtain anisotropic ion transport in highly aligned hexagonally packed cylinders of
linear block PEG after magnetic field-based self-assembly (Majewski et al. 2010).
Magnetic functionality due to cyanobiphenyl mesogen is important because it
488
L. Mahajan et al.