Electro optical properties of liquid crystal nanocomposites

Abstract

The thesis presents the results of the study on electro-optical and elastic properties of smectogen octylcyanobiphenyl (8CB) liquid crystal doped with well dispersed multi-walled carbon nanotubes (MWCNTs). The study aims to uncover the effect of MWCNT doping on electro-optical properties of smectic liquid crystal 8CB. Nanoparticle doping has been investigated as a possible way to enhance the properties of liquid crystals. Such studies focused on doping nematic liquid crystals have shown that it is possible to increase dielectric anisotropy, lower threshold voltages and increase electrical response times. One of the promising nanoparticles is carbon nanotubes. Carbon nanotubes are exotic materials with very high shape anisotropy. They are basically one atom thick carbon sheets rolled into tubes. If the nanotubes are formed from many cocentric tubes they are called multi-walled carbon nanotubes. Due to their shape anisotropy, they also exhibit anisotropic mechanical and electronic qualities. What makes them especially intriguing to study as liquid crystal dopants is their geometry. Carbon nanotubes are known to produce liquid crystalline dispersions, and once they are dispersed in a liquid crystalline medium, they are expected to impart their anisotropic qualities on this already anisotropic media and enhance them. The study is carried out on four different samples: one set dispersed with pristine, used as is, MWCNTs of concentrations 0.007 wt.\% and 0.07 wt.\% (percent by weight), and another set dispersed with -COOH functionalized MWCNTs at same concentrations. So the scope of this thesis is investigating the effects of functionalized and nonfunctionalized MWCNT doping on 8CB liquid crystal host's dielectric anisotropy, threshold voltage, birefringence and response time in Freédericksz transition. Each of said properties were measured against temperature and voltage. The doping of MWCNTs is carried out by solvent dispersion method. The 8CB and MWCNTs are mixed in high purity toluene. The mixture is heated to 43 $^{\circ}$C, where 8CB is in isotropic liquid phase. The mixture is then sonicated and magnetically stirred. Once toluene is evaporated, the remaining 8CB - MWCNT dispersion, or so-called nanocomposite, is filled into sandwich type sample cells by capillary action. Differential scanning calorimetry measurements and polarizing microscopy imaging are also conducted to confirm the existence of nematic and smectic A phases and transition temperatures. Experimental datum for the temperature dependence of birefringence, dielectric anisotropy, threshold voltage, voltage-on and voltage-off response are presented. Birefringence data is obtained by the rotating analyzer method. Dielectric data is collected by capacitance measurements. Threshold voltages and response times are obtained by light transmission measurements through crossed polarizers during voltage application on their respective sample cells. The isotropic to nematic and nematic to smectic A phase transition temperatures are obtained from birefringence measurements. The splay elastic constants and rotational viscosities of functionalized and non-functionalized MWCNT dispersed 8CB nanocomposites are calculated from these dielectric anisotropies, threshold voltages and voltage-off response times. The nematic range, temperature interval between isotropic to nematic and nematic to smectic A transition is extended in all nanocomposite samples. Non-functionalized MWCNT nanocomposites have wider nematic ranges than functionalized ones. It is observed that dielectric anisotropies are lowered in all MWCNT dispersed samples, higher concentrations having lower dielectric anisotropies. While birefringence is lowered in pristine MWCNT nanocomposites and functionalized MWCNT 0.07 wt.\% nanocomposite, compared to 8CB, functionalized 0.007 wt.\% nanocomposite's birefringence is higher than pure 8CB's. These findings suggest that while nematic order is not enhanced with MWCNT dispersions at hand, but the exception, functionalized 0.007 wt.\%, points to a possibility of better incorporation of MWCNTs in liquid crystalline media by dispersion of functionalized MWCNTs at low concentrations. The threshold voltages, with the exception of non-functionalized 0.07 wt.\%, are increased. Calculation of splay elastic constants (K$_{11}$) shows that elasticity of the nanocomposites also differ from pure 8CB. The non-functionalized MWCNT nanocomposites have lower elastic constants in the nematic range, while funtionalized MWCNT nanocomposites have higher. Also, a crossover of elastic constants is observed, with respect to temperature, as f0.007 wt.\% nanocomposite's elastic constant increases at an higher rate than f0.07 wt.\%'s elastic constant. Voltage-on and voltage-off response times are higher in all nanocomposites. Functionalized 0.007 wt.\% nanocomposite has the highest voltage-on and voltage-off response times. On the other hand, functionalized 0.07wt.\% has the closest response times to 8CB. The calculation of rotational viscosity from response times and elastic constants reveal that rate of change of rotational viscosity with respect to temperature is highest in funtionalized 0.007 wt.\% nanocomposite and rotational viscosities of all nanocomposite samples are higher than pure 8CB. With non-functionalized samples, rate of change of rotational viscosity is higher with 0.007 wt.\%. Temperature dependencies of rotational viscosity of 0.07 wt.\% and pure 8CB are similar. The same is true for functionalized 0.07 wt.\%. The study reveals the temperature dependence of various electro-optical properties of 8CB dispersed with functionalized and non-functionalized MWCNTs. With drops in birefringence and dielectric anisotropy, accompanied by increasing response times and threshold voltages, no electro-optical enhancement is observed in studied samples. However, with effect of functionalization and at optimal concentration, it would be possible to enhance a selected electro-optical property, i.e. improving birefringence while keeping response times at a reasonable level for electrical switching. The results of the study also hint at a complex web of dependencies related to functionalization and concentration, which justifies further numerical and experimental studies on smectic liquid crystal - MWCNT nanocomposites.