polymerization is conducted in an aligned monomeric state, the higher degree of
macroscopic orientation is easily accomplished over the large area in comparison to
the thermoplastic LCP systems. In addition, the polymerization shrinkage is low due
to the presence of order, typically less than 7% in volume (Satsangi et al. 2005). The
macroscopic orientation of LC networks is usually achieved either by the mechanical
stretching of a weakly cross-linked network in conjunction with a cross-linking
reaction or by the polymerization in the aligned state. Mass production is possible
due to the versatile accessibility of conventional coating techniques. This chapter
primarily deals with the fundamental characteristics and recent research interests of
anisotropic LC networks, elastomers, and gels fabricated using various
programmed RMs.
Reactive Mesogens
RMs have been used as precursors to form the anisotropic LC networks (Dierking
2000). They are mainly composed of three parts: (i) the rigid rod- or disk-shaped
mesogenic groups; (ii) the reactive end groups involving acrylate, vinyl, epoxide,
and diene derivatives; and (iii) the flexible spacers located between the mesogens
and reactive groups (Fig. 2). Phase behavior of RM shows the strong dependence on
its constituents, similarly to that observed in nonreactive low molecular weight LC
(LMWLC). The effects of mesogenic units and spacer lengths on the phase behaviors and corresponding optical properties have been well overviewed in literatures
(Seki 2014). Polymerization of RM containing more than two polymerizable reactive groups leads to the densely cross-linked networks in which the molecular
alignment is immobilized. Phase transition occurs at higher temperatures by increasing the cross-linking density. The cross-linking reaction may also lead to the
formation of a different mesophase (Hsieh et al. 1998).
RMs functionalized with vinyl reactive groups reveal relatively broad mesophase
temperature ranges compared to other RMs. The properties of epoxide-terminated
Fig. 2 Schematic illustration
of mono- and difunctional
RMs
98
D.-Y. Kim et al.
macroscopic orientation is easily accomplished over the large area in comparison to
the thermoplastic LCP systems. In addition, the polymerization shrinkage is low due
to the presence of order, typically less than 7% in volume (Satsangi et al. 2005). The
macroscopic orientation of LC networks is usually achieved either by the mechanical
stretching of a weakly cross-linked network in conjunction with a cross-linking
reaction or by the polymerization in the aligned state. Mass production is possible
due to the versatile accessibility of conventional coating techniques. This chapter
primarily deals with the fundamental characteristics and recent research interests of
anisotropic LC networks, elastomers, and gels fabricated using various
programmed RMs.
Reactive Mesogens
RMs have been used as precursors to form the anisotropic LC networks (Dierking
2000). They are mainly composed of three parts: (i) the rigid rod- or disk-shaped
mesogenic groups; (ii) the reactive end groups involving acrylate, vinyl, epoxide,
and diene derivatives; and (iii) the flexible spacers located between the mesogens
and reactive groups (Fig. 2). Phase behavior of RM shows the strong dependence on
its constituents, similarly to that observed in nonreactive low molecular weight LC
(LMWLC). The effects of mesogenic units and spacer lengths on the phase behaviors and corresponding optical properties have been well overviewed in literatures
(Seki 2014). Polymerization of RM containing more than two polymerizable reactive groups leads to the densely cross-linked networks in which the molecular
alignment is immobilized. Phase transition occurs at higher temperatures by increasing the cross-linking density. The cross-linking reaction may also lead to the
formation of a different mesophase (Hsieh et al. 1998).
RMs functionalized with vinyl reactive groups reveal relatively broad mesophase
temperature ranges compared to other RMs. The properties of epoxide-terminated
Fig. 2 Schematic illustration
of mono- and difunctional
RMs
98
D.-Y. Kim et al.