LCBCPs with Covalent Interactions: RCBCPs
The thermodynamic phase behavior of coil-coil BCPs is characterized by the
parameters f and χN. However, in RCBCPs, along with f and χN, additional
parameters were required to account for the influence of the degree of asymmetry
between the two blocks on the free energy and the resulting microphase separated
structures (Matsen and Barrett 1998; Pryamitsyn and Ganesan 2004). Theoretical
calculations have shown that one of these parameters is the parameter ν which is the
ratio between the coil R g and the rod length. The term ν accounts for the difference in
scaling behavior of the two blocks because their aspect ratio varies differently with
M n . For a coil, an increase in the M n increases its interfacial area (A
coil ) at the
intermaterials dividing surface (IMDS), whereas in a rod an increase in M n increases
its length while A
rod remains constant. This induces instability at the interface and it
must be taken into account in mapping RCBCP phase behavior. The second additional parameter is μN (Maier-Saupe interaction parameter), and it is a measure of the
degree of orientational order that arises due to the rod-rod interactions. μ is the
strength of the orientational interactions that favor the alignment of rods and it varies
inversely with temperature (Pryamitsyn and Ganesan 2004).
A number of research groups reported unique phase behavior in RCBCPs based
on both theoretical calculations and experimental observations. Conventional BCP
morphologies such as S, C, G, and L were observed in low M n oligomeric rod
systems (Lee and Cho 2001; Lee et al. 2000, 2001, 2004; Lee and Oh 1996; Lee and
Yoo 2002). With an increase in M n and f
rod , the rod-rod interactions became stronger
resulting in the suppression of morphologies with curved interfaces (G, C, and S).
Structures with flat interfaces became more predominant. Using SCFT, morphologies such as striped and hockey-puck shapes were calculated in the coil-rich
samples whereas rod-rich samples exhibited zig-zag L and arrow-head structures
(Pryamitsyn and Ganesan 2004). Within these unique morphologies, depending
upon the orientation of the rod blocks, N, SmA, SmC and bilayered phases were
theoretically calculated using free energy calculations in the SSL (Chen et al. 1995,
1996; Semenov 1985, 1986; Thomas et al. 1997; Williams and Fredrickson 1992).
The WSL morphologies were predicted based on f
coil where samples with high f
coil
transformed to microphase separated structures (from an isotropic phase) while at
low f
coil , they transformed into N phase (Holyst and Schick 1992). Figure 5a, b
shows the phase diagram of RCBCPs developed using SCFT approach by
Pryamitsyn et al (Pryamitsyn and Ganesan 2004). Experimental observations corroborated theoretically calculated phase behavior. A variety of molecules that possess inherent rigidity were used as the rod block to synthesize RCBCPs. Both
polymers and oligomers were used as the rod block. The rod block, of most of the
RCBCP systems reported, consists of molecules that are either helical rods, mesogenic rods or conjugated rods.
Synthetic polypeptides adopt conformations that form α-helices and β-sheets that
are inherently rigid in nature. A unique double hexagonal structure (Fig. 5c) was
observed by Klok et al. who investigated the phase behavior of oligopeptide based
diblock oligomers of oligostyrene (St) (with degree of polymerization (DP) of 10)
7 Structure and Assembly of Liquid Crystalline Block Copolymers
183
The thermodynamic phase behavior of coil-coil BCPs is characterized by the
parameters f and χN. However, in RCBCPs, along with f and χN, additional
parameters were required to account for the influence of the degree of asymmetry
between the two blocks on the free energy and the resulting microphase separated
structures (Matsen and Barrett 1998; Pryamitsyn and Ganesan 2004). Theoretical
calculations have shown that one of these parameters is the parameter ν which is the
ratio between the coil R g and the rod length. The term ν accounts for the difference in
scaling behavior of the two blocks because their aspect ratio varies differently with
M n . For a coil, an increase in the M n increases its interfacial area (A
coil ) at the
intermaterials dividing surface (IMDS), whereas in a rod an increase in M n increases
its length while A
rod remains constant. This induces instability at the interface and it
must be taken into account in mapping RCBCP phase behavior. The second additional parameter is μN (Maier-Saupe interaction parameter), and it is a measure of the
degree of orientational order that arises due to the rod-rod interactions. μ is the
strength of the orientational interactions that favor the alignment of rods and it varies
inversely with temperature (Pryamitsyn and Ganesan 2004).
A number of research groups reported unique phase behavior in RCBCPs based
on both theoretical calculations and experimental observations. Conventional BCP
morphologies such as S, C, G, and L were observed in low M n oligomeric rod
systems (Lee and Cho 2001; Lee et al. 2000, 2001, 2004; Lee and Oh 1996; Lee and
Yoo 2002). With an increase in M n and f
rod , the rod-rod interactions became stronger
resulting in the suppression of morphologies with curved interfaces (G, C, and S).
Structures with flat interfaces became more predominant. Using SCFT, morphologies such as striped and hockey-puck shapes were calculated in the coil-rich
samples whereas rod-rich samples exhibited zig-zag L and arrow-head structures
(Pryamitsyn and Ganesan 2004). Within these unique morphologies, depending
upon the orientation of the rod blocks, N, SmA, SmC and bilayered phases were
theoretically calculated using free energy calculations in the SSL (Chen et al. 1995,
1996; Semenov 1985, 1986; Thomas et al. 1997; Williams and Fredrickson 1992).
The WSL morphologies were predicted based on f
coil where samples with high f
coil
transformed to microphase separated structures (from an isotropic phase) while at
low f
coil , they transformed into N phase (Holyst and Schick 1992). Figure 5a, b
shows the phase diagram of RCBCPs developed using SCFT approach by
Pryamitsyn et al (Pryamitsyn and Ganesan 2004). Experimental observations corroborated theoretically calculated phase behavior. A variety of molecules that possess inherent rigidity were used as the rod block to synthesize RCBCPs. Both
polymers and oligomers were used as the rod block. The rod block, of most of the
RCBCP systems reported, consists of molecules that are either helical rods, mesogenic rods or conjugated rods.
Synthetic polypeptides adopt conformations that form α-helices and β-sheets that
are inherently rigid in nature. A unique double hexagonal structure (Fig. 5c) was
observed by Klok et al. who investigated the phase behavior of oligopeptide based
diblock oligomers of oligostyrene (St) (with degree of polymerization (DP) of 10)
7 Structure and Assembly of Liquid Crystalline Block Copolymers
183