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11.4 Functional Silicones in Liquid Crystalline Materials
The LCs represent a state of matter discovered in 1888 by Reinitzer and Lehmann,
defined as an intermediary state between ordinary liquids and three-dimensional
(3D) solids (Demus et al. 1998). LCs are used intensively on LCD screens (displays), but other applications also include the scaffolds for biosystems and nanoparticle (NP) assemblies, as well as actuators and sensors (Ohm et al. 2010; Carlton
et al. 2013). LCs gained a privileged place among the organo-siloxane compounds,
as a consequence of the hallmark of the siloxane segments on the overall properties
and mesomorphic behavior. Siloxane moieties are used as flexible segments into
main chain or side chain LCPs and as elastomeric matrix in LC elastomers (LCEs).
Usually, a significant decrease in T g values is obtained, as well as a decrease in the
other transition temperatures, due to the low rotational energy of the Si-O bond
(Aguilera et al. 1983). The role of siloxane segments has been demonstrated as
compared to aliphatic ones, thus reducing transition temperatures in LCs, due to
their flexibility and bulkiness, which is used in order to obtain high mobility of the
mesophases and mesogenic properties at moderate temperatures. Another characteristic of the siloxane-containing LCs is the microphase segregation which generally occurs, due to the incompatibility of the siloxane moieties with most of the
organic structures. Siloxane backbone materials tend to show weaker anisotropy
due to the flexibility of the backbone which opposes the perfect confinement
between the layers (Demus et al. 1998).
Siloxane-containing LCs, their chemistry, physical properties and applications
have been previously reviewed (Teyssier and Boileau 2000; Racles 2008; Zhang
et al. 2018). The search for better performance in areas such as actuators and sensors, medical science, microelectronics, optoelectronics, etc. maintains the interest
of the scientific community for this area. Keeping this in view, the functional siloxanes are briefly described by selected examples of structures and application-related
properties, emphasizing the role of the siloxane units in LC materials.
Like any other functional moiety, the binding of mesogenic groups to the (poly)
siloxane backbone can be carried out in three main routes: hydrosilylation, thiol-ene
addition and chemical modification of non-mesogenic functional groups (by esterification, etherification, free radical polymerization etc.). The LC behavior (temperatures, nature of mesophases, phase transitions) of polysiloxanes substituted with
mesogenic groups depends largely on their structure and is influenced by the microphase separation in copolymers (Rössle et al. 2005). In addition, the mechanical
properties of the LCEs obtained therefrom, especially the smectic layer compression, are also a consequence of the chain microstructure. For example, the phase
behavior of side chain LC polysiloxanes containing cholesteryl groups depends on
the length of the mesogenic core, the flexible spacer and the copolymer composition. Interestingly, copolymers with both cholesteryl and biphenyl mesogenic
groups have shown cholesteric phases, while the homopolymers with cholesteryl
groups have exhibited smectic phases (Hu et al. 2004). Recently, Katsuki et al.
(2019) investigated siloxane-based monomeric and dimeric LCs which carry
C. Racles et al.
11.4 Functional Silicones in Liquid Crystalline Materials
The LCs represent a state of matter discovered in 1888 by Reinitzer and Lehmann,
defined as an intermediary state between ordinary liquids and three-dimensional
(3D) solids (Demus et al. 1998). LCs are used intensively on LCD screens (displays), but other applications also include the scaffolds for biosystems and nanoparticle (NP) assemblies, as well as actuators and sensors (Ohm et al. 2010; Carlton
et al. 2013). LCs gained a privileged place among the organo-siloxane compounds,
as a consequence of the hallmark of the siloxane segments on the overall properties
and mesomorphic behavior. Siloxane moieties are used as flexible segments into
main chain or side chain LCPs and as elastomeric matrix in LC elastomers (LCEs).
Usually, a significant decrease in T g values is obtained, as well as a decrease in the
other transition temperatures, due to the low rotational energy of the Si-O bond
(Aguilera et al. 1983). The role of siloxane segments has been demonstrated as
compared to aliphatic ones, thus reducing transition temperatures in LCs, due to
their flexibility and bulkiness, which is used in order to obtain high mobility of the
mesophases and mesogenic properties at moderate temperatures. Another characteristic of the siloxane-containing LCs is the microphase segregation which generally occurs, due to the incompatibility of the siloxane moieties with most of the
organic structures. Siloxane backbone materials tend to show weaker anisotropy
due to the flexibility of the backbone which opposes the perfect confinement
between the layers (Demus et al. 1998).
Siloxane-containing LCs, their chemistry, physical properties and applications
have been previously reviewed (Teyssier and Boileau 2000; Racles 2008; Zhang
et al. 2018). The search for better performance in areas such as actuators and sensors, medical science, microelectronics, optoelectronics, etc. maintains the interest
of the scientific community for this area. Keeping this in view, the functional siloxanes are briefly described by selected examples of structures and application-related
properties, emphasizing the role of the siloxane units in LC materials.
Like any other functional moiety, the binding of mesogenic groups to the (poly)
siloxane backbone can be carried out in three main routes: hydrosilylation, thiol-ene
addition and chemical modification of non-mesogenic functional groups (by esterification, etherification, free radical polymerization etc.). The LC behavior (temperatures, nature of mesophases, phase transitions) of polysiloxanes substituted with
mesogenic groups depends largely on their structure and is influenced by the microphase separation in copolymers (Rössle et al. 2005). In addition, the mechanical
properties of the LCEs obtained therefrom, especially the smectic layer compression, are also a consequence of the chain microstructure. For example, the phase
behavior of side chain LC polysiloxanes containing cholesteryl groups depends on
the length of the mesogenic core, the flexible spacer and the copolymer composition. Interestingly, copolymers with both cholesteryl and biphenyl mesogenic
groups have shown cholesteric phases, while the homopolymers with cholesteryl
groups have exhibited smectic phases (Hu et al. 2004). Recently, Katsuki et al.
(2019) investigated siloxane-based monomeric and dimeric LCs which carry
C. Racles et al.