Nematic (N) LC phase is characterized by the absence of long range translational
order; however, all the molecules are parallel to the director. N phase is fluid-like in
nature with high molecular mobility (Fig. 3b) (de Gennes 1975). Smectic (Sm) LC
phase possesses higher degree of order compared to N. Sm phases are layered and
possess definite interlayer spacing. Based on the molecular arrangement within these
layers, different types of Sm phases are present. In Smectic A (SmA) LC phase, the
mesogens within a layer do not possess positional order but their molecular axis is
parallel to the layer normal. In Smectic C (SmC) phase, the molecules within layers
are tilted with respect to the layer normal. SmC is optically biaxial and is considered
as an important phase from application point of view in technology, because the
molecules can be rotated about an axis using external field. Chiral SmC (SmC*)
phase is a SmC phase with chiral mesogens and possess a helical twist from layer to
layer. The polarizing direction varies along the twist whereas the tilt angle with
respect to the normal remains constant.
Other types of mesogens with nontraditional shapes have also been reported. For
example, bent-core or banana LCs (BCLCs) are so named because there is a bend in
the shape of the mesogen as shown in Fig. 3c. Niori et al. (1996) were the first to
discover unique polar order formation in BCLCs and since then a variety of
mesophases were discovered in these materials and were named as B1–B8
depending upon the sequence in which they were discovered. The advancements
in the field of synthesis and characterization of phase structures of BCLCs was
recently published in a review article by Reddy and Tschierske (Coleman et al. 2003;
Heppke and Moro 1998; Link et al. 1997; Pelzl et al. 1999; Reddy and Tschierske
2005; Ros et al. 2005; Tschierske and Dantlgraber 2003). Like calamatic LCs,
BCLCs also undergo self-assembly and form different LC phases but the uniqueness
of BCLCs compared to linear mesogens is the additional complexity they impart to
the LC phase due to the unique bent shape (Reddy and Tschierske 2005). For
example, BCLCs possess a dipole moment in the direction perpendicular to the
molecular axis and their unique shape facilitates their efficient packing. When these
molecules, possessing a dipole moment, are packed into SmA layers, polarization of
the layers can be achieved (denoted as SmAP where P stands for polar order) as
shown in Fig. 3c. If the direction of polarization alternates between layers, it leads to
anti-ferroelectric (AF) behavior whereas uniform polarization between layers leads
to ferroelectric (F) behavior in the material. These two states can be switched by
applying external field such as electric field. When the bent core molecules are tilted
with respect to the Sm layer normal, SmCP (SmC denotes the mesogens are tilted
and P denotes polar order) LC phases are obtained. Research in the field of SmCP
LC phases has gained a lot of momentum in the recent years due to the wide variety
of orientations and ordering achievable in these systems.
Liquid Crystalline Polymers
Polymeric liquid crystals or liquid crystalline polymers (LCPs) are achieved by
incorporating mesogens into polymer chains. Within an LCP, the rigid mesogen
180
K. K. Tenneti et al.
order; however, all the molecules are parallel to the director. N phase is fluid-like in
nature with high molecular mobility (Fig. 3b) (de Gennes 1975). Smectic (Sm) LC
phase possesses higher degree of order compared to N. Sm phases are layered and
possess definite interlayer spacing. Based on the molecular arrangement within these
layers, different types of Sm phases are present. In Smectic A (SmA) LC phase, the
mesogens within a layer do not possess positional order but their molecular axis is
parallel to the layer normal. In Smectic C (SmC) phase, the molecules within layers
are tilted with respect to the layer normal. SmC is optically biaxial and is considered
as an important phase from application point of view in technology, because the
molecules can be rotated about an axis using external field. Chiral SmC (SmC*)
phase is a SmC phase with chiral mesogens and possess a helical twist from layer to
layer. The polarizing direction varies along the twist whereas the tilt angle with
respect to the normal remains constant.
Other types of mesogens with nontraditional shapes have also been reported. For
example, bent-core or banana LCs (BCLCs) are so named because there is a bend in
the shape of the mesogen as shown in Fig. 3c. Niori et al. (1996) were the first to
discover unique polar order formation in BCLCs and since then a variety of
mesophases were discovered in these materials and were named as B1–B8
depending upon the sequence in which they were discovered. The advancements
in the field of synthesis and characterization of phase structures of BCLCs was
recently published in a review article by Reddy and Tschierske (Coleman et al. 2003;
Heppke and Moro 1998; Link et al. 1997; Pelzl et al. 1999; Reddy and Tschierske
2005; Ros et al. 2005; Tschierske and Dantlgraber 2003). Like calamatic LCs,
BCLCs also undergo self-assembly and form different LC phases but the uniqueness
of BCLCs compared to linear mesogens is the additional complexity they impart to
the LC phase due to the unique bent shape (Reddy and Tschierske 2005). For
example, BCLCs possess a dipole moment in the direction perpendicular to the
molecular axis and their unique shape facilitates their efficient packing. When these
molecules, possessing a dipole moment, are packed into SmA layers, polarization of
the layers can be achieved (denoted as SmAP where P stands for polar order) as
shown in Fig. 3c. If the direction of polarization alternates between layers, it leads to
anti-ferroelectric (AF) behavior whereas uniform polarization between layers leads
to ferroelectric (F) behavior in the material. These two states can be switched by
applying external field such as electric field. When the bent core molecules are tilted
with respect to the Sm layer normal, SmCP (SmC denotes the mesogens are tilted
and P denotes polar order) LC phases are obtained. Research in the field of SmCP
LC phases has gained a lot of momentum in the recent years due to the wide variety
of orientations and ordering achievable in these systems.
Liquid Crystalline Polymers
Polymeric liquid crystals or liquid crystalline polymers (LCPs) are achieved by
incorporating mesogens into polymer chains. Within an LCP, the rigid mesogen
180
K. K. Tenneti et al.