Siloxane head groups and spacers in calamitic liquid crystals
Siloxane head groups and spacers in calamitic liquid crystals
Organo-siloxanes are a compromise between a low molar mass liquid crystal and a liquid crystalline side chain polysiloxane polymer. Like polymers the compounds segregate in mesogen rich layers and siloxane rich layers, but like low molar mass compounds the viscosity is in the magnitude of classic LMM liquid crystals. The siloxane group can be used as a head group, a linear linking group and a cyclic linking group. Up to four mesogens can be connected.
In the present work small siloxane groups were grafted onto a variety of mesogens in a variety of sizes and shapes. As a steroidal mesogen cholestanol derivatives, as aromatic cores laterally substituted biphenyl benzoates were synthesised and characterised.
All liquid crystals investigated were synthesised via established procedures. Olefinic terminated precursor compounds were hydrosilylated with smaller siloxane groups. Structure and phase properties were determined via optical microscopy, differential scanning microscopy, X-ray diffraction and dilatometry. The helielectric compounds were also characterised electro-optically, i.e. tilt angle, spontaneous polarisation and optical response time.
To investigate the role of the siloxane in steroidal liquid crystals three different series of cholestanol derivatives were synthesised and characterised. The mesophase behaviour depended strongly on the spacer length, the number of mesogens attached to the siloxane group and the siloxane length. Mono- and bimesogenic compounds exhibited an odd-even effect on the melting transition, whereas the clearing temperatures increased with an increase in spacer length. All LMM organo-siloxanes are liquid crystalline at ambient temperature. The analogous side chain polysiloxanes exhibited significantly higher transition temperatures and viscosity. A series of androstane derivatives revealed the importance of a short branched alkyl tail for the occurrence of a broad mesophase.
Biphenyl benzoates are known to exhibit ferroelectric mesophases. In several series of organo-siloxanes the role of the spacer, the siloxane moiety and the laterally attached halogen was investigated. The minimum spacer length for ferroelectric phases is 8 methylene units. For 11 methylene units unusual temperature independent phase properties are found.
University of Southampton
1997
Kloess, Petra Sabine
(1997)
Siloxane head groups and spacers in calamitic liquid crystals.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
Organo-siloxanes are a compromise between a low molar mass liquid crystal and a liquid crystalline side chain polysiloxane polymer. Like polymers the compounds segregate in mesogen rich layers and siloxane rich layers, but like low molar mass compounds the viscosity is in the magnitude of classic LMM liquid crystals. The siloxane group can be used as a head group, a linear linking group and a cyclic linking group. Up to four mesogens can be connected.
In the present work small siloxane groups were grafted onto a variety of mesogens in a variety of sizes and shapes. As a steroidal mesogen cholestanol derivatives, as aromatic cores laterally substituted biphenyl benzoates were synthesised and characterised.
All liquid crystals investigated were synthesised via established procedures. Olefinic terminated precursor compounds were hydrosilylated with smaller siloxane groups. Structure and phase properties were determined via optical microscopy, differential scanning microscopy, X-ray diffraction and dilatometry. The helielectric compounds were also characterised electro-optically, i.e. tilt angle, spontaneous polarisation and optical response time.
To investigate the role of the siloxane in steroidal liquid crystals three different series of cholestanol derivatives were synthesised and characterised. The mesophase behaviour depended strongly on the spacer length, the number of mesogens attached to the siloxane group and the siloxane length. Mono- and bimesogenic compounds exhibited an odd-even effect on the melting transition, whereas the clearing temperatures increased with an increase in spacer length. All LMM organo-siloxanes are liquid crystalline at ambient temperature. The analogous side chain polysiloxanes exhibited significantly higher transition temperatures and viscosity. A series of androstane derivatives revealed the importance of a short branched alkyl tail for the occurrence of a broad mesophase.
Biphenyl benzoates are known to exhibit ferroelectric mesophases. In several series of organo-siloxanes the role of the spacer, the siloxane moiety and the laterally attached halogen was investigated. The minimum spacer length for ferroelectric phases is 8 methylene units. For 11 methylene units unusual temperature independent phase properties are found.
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Published date: 1997
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Local EPrints ID: 460187
URI: http://eprints.soton.ac.uk/id/eprint/460187
PURE UUID: dbddab2a-95d4-4729-add2-608fe5f875ce
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Date deposited: 04 Jul 2022 18:08
Last modified: 04 Jul 2022 18:08
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Author:
Petra Sabine Kloess
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