RMs are strongly affected by the epoxy-to-curing agent ratio as well as the curing
mechanism (Pei et al. 2013). The anisotropic networks have many potential applications such as advanced composites, microelectronics, and mechanical actuators.
The films prepared from the cyclopolymerization reaction of nonconjugated diene
groups tend to exhibit higher thermal stabilities and lower melting points compared
with other LC constituents. Acrylate or methacrylate groups are readily polymerized
by the free radical mechanism because of a fast propagation rate. The networks
exhibit the remarkable chemical, optical, and mechanical properties, which account
for the wide uses of acrylate-based resins in industries. Polymerization mechanism
and kinetics of acrylates system have been thoroughly investigated (Warner and
Terentjev 2007).
In general, the polymerization of RM is initiated with heat or light. In the case of
UV irradiation, RM can be polymerized either by cationic or radical initiators
depending on the reactive moieties. Cationic polymerization is widely used to cure
the multifunctional vinyl and epoxide groups, while radical polymerization is
employed for acrylate systems (Crivello and Reichmanis 2014). Recently, the
photopolymerization is preferred over the thermally induced polymerization because
of the free selection of the proper mesophase and the precise adjustment of the
properties. The conversion to polymers takes place instantaneously with low energy
consumption. The homogeneous mixtures of RM and initiator is macroscopically
oriented first and then irradiated with UV light to obtain a well-defined LC network
structures. The LC networks reveal the similar optical properties of the monomeric
LC state prior to the polymerization.
Anisotropic LC Networks
Basic Principles and Characteristics
To obtain the anisotropic LC networks, the polymerization should be performed at a
temperature where the system exhibits LC phases. Although the polymerization of
monofunctional RM yields a linear polymer with a high degree of order on a
microscopic scale, the macroscopic order is not sustainable because of the molecular
shrinkage or the phase transition during a polymerization process. The RMs
containing more than two functional groups are suitable to create the well-defined
and stable three-dimensional LC networks. Order parameter of the networks
representing the average alignment of the molecules with respect to the director
can be determined by measuring birefringence, infrared dichroic ratio, and X-ray
diffraction (Chatani et al. 2014).
LC networks are largely divided into thermosets and elastomers depending on the
cross-linking density (Fig. 3). In the LC thermosets, phase transition and response to
the external stimuli are restricted due to the dense network structures. Therefore, the
molecular order should be properly adjusted before polymerization to meet the
required mechanical and optical properties. Lightly cross-linked LC elastomers
(LCE) exhibit the elastic and anisotropic properties simultaneously (Xie and
Zhang 2005). Low cross-linking density may allow the LC molecules to retain
4 Anisotropic Liquid Crystal Networks from Reactive Mesogens
99
mechanism (Pei et al. 2013). The anisotropic networks have many potential applications such as advanced composites, microelectronics, and mechanical actuators.
The films prepared from the cyclopolymerization reaction of nonconjugated diene
groups tend to exhibit higher thermal stabilities and lower melting points compared
with other LC constituents. Acrylate or methacrylate groups are readily polymerized
by the free radical mechanism because of a fast propagation rate. The networks
exhibit the remarkable chemical, optical, and mechanical properties, which account
for the wide uses of acrylate-based resins in industries. Polymerization mechanism
and kinetics of acrylates system have been thoroughly investigated (Warner and
Terentjev 2007).
In general, the polymerization of RM is initiated with heat or light. In the case of
UV irradiation, RM can be polymerized either by cationic or radical initiators
depending on the reactive moieties. Cationic polymerization is widely used to cure
the multifunctional vinyl and epoxide groups, while radical polymerization is
employed for acrylate systems (Crivello and Reichmanis 2014). Recently, the
photopolymerization is preferred over the thermally induced polymerization because
of the free selection of the proper mesophase and the precise adjustment of the
properties. The conversion to polymers takes place instantaneously with low energy
consumption. The homogeneous mixtures of RM and initiator is macroscopically
oriented first and then irradiated with UV light to obtain a well-defined LC network
structures. The LC networks reveal the similar optical properties of the monomeric
LC state prior to the polymerization.
Anisotropic LC Networks
Basic Principles and Characteristics
To obtain the anisotropic LC networks, the polymerization should be performed at a
temperature where the system exhibits LC phases. Although the polymerization of
monofunctional RM yields a linear polymer with a high degree of order on a
microscopic scale, the macroscopic order is not sustainable because of the molecular
shrinkage or the phase transition during a polymerization process. The RMs
containing more than two functional groups are suitable to create the well-defined
and stable three-dimensional LC networks. Order parameter of the networks
representing the average alignment of the molecules with respect to the director
can be determined by measuring birefringence, infrared dichroic ratio, and X-ray
diffraction (Chatani et al. 2014).
LC networks are largely divided into thermosets and elastomers depending on the
cross-linking density (Fig. 3). In the LC thermosets, phase transition and response to
the external stimuli are restricted due to the dense network structures. Therefore, the
molecular order should be properly adjusted before polymerization to meet the
required mechanical and optical properties. Lightly cross-linked LC elastomers
(LCE) exhibit the elastic and anisotropic properties simultaneously (Xie and
Zhang 2005). Low cross-linking density may allow the LC molecules to retain
4 Anisotropic Liquid Crystal Networks from Reactive Mesogens
99