From monomeric to polymeric ferroelectric liquid crystals A comparative study of ferroelectric properties

Abstract

Two new ferroelectric oligosiloxanes, a cyclic tetramer and a twin, have been synthesized. By a comparative study with their corresponding monomer and side chain polysiloxanes, the influence of oligo-and polymerization on the liquid crystalline and ferroelectric properties have been investigated. Polymerization leads to a stabilization of LC phases through increase of the clearing temperatures and suppression of crystallization. Oligomerization also leads to mesophase broadening, but, due to the low degree of polymerization, the effect is inferior to the linear polysiloxanes. The low viscosity of the oligosiloxanes ensures response times in the microsecond region, thus being comparable with their monomer and conventional LMWFLCs.It is Pound that polymerization increases the spontaneous polarization P,. This is attributed to the density increase after polymerization, enhancing the inter-mesogenic interactions. The collective and local dynamics of the OFLCs are influenced differently with respect to their molecular structures. Each oligomer is already a good model for its corresponding polymer concerning the soft mode dynamics. For the local p-relaxation a similar temperature dependence of the relaxation times z for the cyclic tetramer and for the side chain polysiloxanes is observed. The long axial rotation of the twin, having a very efficient decoupling, is significantly faster, thus resembling the monomer.

Synthesis of new stereoregular 2,4,6,8-tetraphenylcyclotetrasiloxanes with mesogenic groups and the influence of spatial isomerism on the phase state of individual isomers and their mixtures

“…Unfortunately, none of the published works discusses the influence of the spatial structure of stereoisomers or the composition of a mixture of isomers of methylcyclosiloxanes on the type of packing in the LC state. 3- 10 We earlier 11 attempted to study this influence for cyclohexasiloxane. It was found that methylcyclosiloxanes enriched in the trans isomer form different LC textures on cooling of the isotropic melt.…”

Cyclic Co-oligomeric Ferroelectric Liquid Crystals with Fast Switching Properties, Wide SmC*Temperature Range and de Vries-type SmA*-SmC*Transition

“…In particular, oligomeric siloxanes possessing pendant mesogenic groups attracted lots of interest, [21][22][23][24][25][26][27][28][29][30][31][32][33] since the siloxane moieties readily give rise to a nano-phase-separated structure as is well known for liquid crystalline polysiloxanes. 11,12 As a result, the temperature range of the smectic phase becomes enlarged, whereas the switching times stay relatively short, due to their oligomeric nature.…”

“…Although these problems can be minimized with polymeric ferroelectric liquid crystals (PFLCs), they limit the switching time and present serious difficulties in the fabrication process due to very high viscosities of polymeric materials. [4][5][6][9][10][11][12] Another problem associated with the use of FLCs are the "Chevron instabilities", 13 which result as a consequence of the smectic layer shrinkage at the smectic A -smectic C* transition. A possible solution for this problem can be liquid crystalline materials with a de Vries-type phase transition, in which the transition happens with a negligible layer shrinkage.…”

Self‐Assembly of Linear and Cyclic Siloxane‐Containing Mesogens: Investigation of Layered Structures in Bulk and Thin Films

“…[24][25][26] Most attention has been paid to cyclic and linear oligosiloxane linkers. Besides synthesis of the compounds, [27][28][29] the different structures and the phase behavior have been studied experimentally [30][31] and by theoretical molecular modeling. [32] Furthermore, effects of magnetic and electric fields, [33] as well as the electrical and electro-optical behavior, [34][35] have been investigated.…”

“…Flash chromatography was performed with an Analogix Flash Chromatography System from Varian equipped with silica gel columns with a particle size of 0.035-0.050 mm under variable-wavelength UV detection and application of appropriate solvent gradients. 1 H, 13 C, and 29 Si NMR spectra were recorded on a Bruker ARX 300 spectrometer at ambient temperature. The 1 H and 13 C spectra were referenced to the residual solvent signals of the deuterated solvents (Deutero); chemical shifts d are reported in parts per million (ppm).…”