LCBCPs with Covalent Interactions: Mesogen Jacketed Liquid
Crystalline Block Copolymers
As previously discussed, by laterally linking the “waist” of LC mesogens directly to
polymer backbones (without spacers), MJLCPs can be achieved (Gopalan and Ober
2001; Pragliola et al. 1999; Wan et al. 1998; Zhang et al. 1999; Zhou et al. 1989). In
most of the MJLCP systems, rigid columns of the mesogens are formed due to the
strong interaction between the side chain mesogens and the polymer backbone.
These rigid columns pack together forming columnar nematic (Φ N )/hexagonal
(Φ H ) phase, and it is these supramolecular columns, rather than the individual
mesogens, that possess the orientational order. The rigid nature enables the MJLCPs
to serve as rods and form a new type of rod-coil block copolymers. Morphology and
rheological behavior of rod-coil poly(styrene)-block-poly(2,5-bis-(4-butyl-benzoyl)
oxystyrene) (PS-b-PBBOS) has been reported by Ober et al. (Gopalan et al. 2003a,
b). A series of MJLCP-based rod-coil block copolymers, poly(styrene-block-(2,5-bis
[4-methoxyphenyl]oxycarbonyl)styrene) (PS-b-PMPCS) have been synthesized and
their solution self-assembly behavior has been investigated (Tu et al. 2000; Wang
et al. 2005; Yi et al. 2004). Symmetric PS-b-PMPCS with relatively low M w forms
simple lamellar phase with the supramolecular PMPCS rods aligning parallel to the
lamellar normal. Each LC layer consists of approximately two layers of PMPCS and
a bilayer Sm A phase was thus proposed (Li et al. 2004).
The phase structure of asymmetric PS-b-PMPCS BCP is much more complicated (Tenneti et al. 2005). For example, PS 171 -b-PMPCS 32 , with f
PMPCS ~ 0.37,
a perforated layer (PL) structure was observed. Figure 10 shows the TEM
micrographs of the samples that were microtomed normal to y, x, and
z directions defined in Fig. 11 (Tenneti et al. 2005). From these images, it is
interesting to observe that the dark areas are not continuous layers. This discontinuity of the dark layers in both xz and yz planes suggests the formation of a PL
structure. Due to the large volume fraction of PS ( f
PS ~0.63), PS punctuates the
PMPCS layers and forms the perforation. The in-plane symmetry of the PL phase
can be unambiguously determined by viewing thin section of the sample along
z direction and it has been shown that a tetragonal perforated layer (TPL)
structure is formed in PS 171 -b-PMPCS 32 rod-coil BCP sample, although most
of the reported PL structures in BCPs have hexagonal symmetry (HPL) (Forster
et al. 1994; Fredrickson 1991; Hamley and Bates 1994; Laradji et al. 1997; Qi
and Wang 1997; Zhu et al. 2001). Based on the TPL model, PS molecules
punctuate and form isolated “islands” in the PMPCS layer, and these in-plane
isolated domains obey a square lattice symmetry with a = b = 29.6 nm. Therefore, the lattice of the perforation can be estimated to be a = b = 29.6 nm and
c = 42.7 nm. Figure 11 shows the schematic representation of the hierarchical
structure of the TPL structure of PS 171 -b-PMPCS 32 : strong interaction between
PMPCS mesogens and the backbone leads to a near extended conformation of the
PMPCS, which form the rods. This rod-coil system self assembles into the TPL
structure where PS punctuates the PMPCS layers. The PMPCS rods are parallel
to the layer normal of the BCP structures.
194
K. K. Tenneti et al.
Crystalline Block Copolymers
As previously discussed, by laterally linking the “waist” of LC mesogens directly to
polymer backbones (without spacers), MJLCPs can be achieved (Gopalan and Ober
2001; Pragliola et al. 1999; Wan et al. 1998; Zhang et al. 1999; Zhou et al. 1989). In
most of the MJLCP systems, rigid columns of the mesogens are formed due to the
strong interaction between the side chain mesogens and the polymer backbone.
These rigid columns pack together forming columnar nematic (Φ N )/hexagonal
(Φ H ) phase, and it is these supramolecular columns, rather than the individual
mesogens, that possess the orientational order. The rigid nature enables the MJLCPs
to serve as rods and form a new type of rod-coil block copolymers. Morphology and
rheological behavior of rod-coil poly(styrene)-block-poly(2,5-bis-(4-butyl-benzoyl)
oxystyrene) (PS-b-PBBOS) has been reported by Ober et al. (Gopalan et al. 2003a,
b). A series of MJLCP-based rod-coil block copolymers, poly(styrene-block-(2,5-bis
[4-methoxyphenyl]oxycarbonyl)styrene) (PS-b-PMPCS) have been synthesized and
their solution self-assembly behavior has been investigated (Tu et al. 2000; Wang
et al. 2005; Yi et al. 2004). Symmetric PS-b-PMPCS with relatively low M w forms
simple lamellar phase with the supramolecular PMPCS rods aligning parallel to the
lamellar normal. Each LC layer consists of approximately two layers of PMPCS and
a bilayer Sm A phase was thus proposed (Li et al. 2004).
The phase structure of asymmetric PS-b-PMPCS BCP is much more complicated (Tenneti et al. 2005). For example, PS 171 -b-PMPCS 32 , with f
PMPCS ~ 0.37,
a perforated layer (PL) structure was observed. Figure 10 shows the TEM
micrographs of the samples that were microtomed normal to y, x, and
z directions defined in Fig. 11 (Tenneti et al. 2005). From these images, it is
interesting to observe that the dark areas are not continuous layers. This discontinuity of the dark layers in both xz and yz planes suggests the formation of a PL
structure. Due to the large volume fraction of PS ( f
PS ~0.63), PS punctuates the
PMPCS layers and forms the perforation. The in-plane symmetry of the PL phase
can be unambiguously determined by viewing thin section of the sample along
z direction and it has been shown that a tetragonal perforated layer (TPL)
structure is formed in PS 171 -b-PMPCS 32 rod-coil BCP sample, although most
of the reported PL structures in BCPs have hexagonal symmetry (HPL) (Forster
et al. 1994; Fredrickson 1991; Hamley and Bates 1994; Laradji et al. 1997; Qi
and Wang 1997; Zhu et al. 2001). Based on the TPL model, PS molecules
punctuate and form isolated “islands” in the PMPCS layer, and these in-plane
isolated domains obey a square lattice symmetry with a = b = 29.6 nm. Therefore, the lattice of the perforation can be estimated to be a = b = 29.6 nm and
c = 42.7 nm. Figure 11 shows the schematic representation of the hierarchical
structure of the TPL structure of PS 171 -b-PMPCS 32 : strong interaction between
PMPCS mesogens and the backbone leads to a near extended conformation of the
PMPCS, which form the rods. This rod-coil system self assembles into the TPL
structure where PS punctuates the PMPCS layers. The PMPCS rods are parallel
to the layer normal of the BCP structures.
194
K. K. Tenneti et al.