Dissertation committee:
Ιωάννης Λελίδης, Επίκουρος Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Γεώργιος Νούνεσης, Ερευνητής Α', Εθνικό Κέντρο Έρευνας Φυσικών Επιστημών "Δημόκριτος"
Ιωάννης Ράπτης, Καθηγητής, Σχολή Εφαρμοσμένων Μαθηματικών και Φυσικών Επιστημών, Εθνικό Μετσόβιο Πολυτεχνείο
Δημοσθένης Σταμόπουλος, Αναπληρωτής Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Νικόλαος Στεφάνου, Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Δημήτριος Τσιούρβας, Ερευνητής Α', Εθνικό Κέντρο Έρευνας Φυσικών Επιστημών "Δημόκριτος"
Δημήτριος Φρατζεσκάκης, Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
Summary:
Nematic (N) liquid crystals (LCs) made of rod-like nanomolecules of anisotropic shape combine the fluidity of the ordinary fluids with the long-range orientational order, while smectics (Sm) possess also 1D translational order. Thermotropic LCs exhibit one or more mesophases between the crystalline solid and the isotropic liquid. Due to their long-range orientational ordering, NLCs exhibit orientationally dependent properties at the macroscopic level which combined with their fluid nature make them vulnerable to external electromagnetic fields and mechanical stresses. On the other hand, nanoparticles (NPs) of different types exhibit unique properties different from their macroscopic counterparts, such as quantum confinement effects and large surface-to-volume ratio and therefore they attract significant interest mainly oriented towards the formation of nanocomposite materials with groundbreaking properties. The combination of LCs and NPs conducts to novel soft nanomaterials which share the properties of their components and boost the potential of LCs in the field of applications. In this context, the investigation of the impact of nanoparticles on the liquid-crystalline order comprises the basis for the deep understanding of the general physical response of the nanocomposite anisotropic fluids. In this thesis we present a systematic experimental study which investigates the impact of nanoparticles upon the liquid-crystalline ordering while also deals with their effects on LC phase transitions. The study is parametric since we examine two LC components that differ with respect to their molecular polarity, NPs of different morphology, as well as mesophases of different symmetry. Crucial parameters of this investigation are the mass fraction of NPs in the host LC and the temperature. Initially, we present the CdSe/ZnS induced disorder in the LC compound 5OO8 characterized by low degree of molecular polarity, which is controlled by NPs mass fraction in the host material. We have determined the orientational order parameters of 5OO8-CdSe/ZnS mixtures of different concentrations as functions of the temperature in the N phase, by polarized micro-Raman spectroscopy using a well-known method for the first time applied in nanocomposite LCs. The degree of the N ordering has been also determined independently by birefringence measurements. The presence of NPs induces elastic deformations in 5OO8 as a result of the random homeotropic anchoring at LC-NP interfaces. When the NPs mass fraction enters a crossover their mean distances become quite small and the elastically-driven disorder is expanded over the macroscopic level. This phenomenon has been described as a transition between a “homogeneous” and a “distorted” N phase. In the “distorted” regime, the disorder is saturated and the N order parameter is dramatically decreased. The experimental observations are described by a simple mean field phenomenological model considering a NP induced interfacial “random-field” term acting as a perturbation in the classical interaction potential of LCs. According to this model, above the crossover condition NPs lead to an abrupt decrease of the N ordering for a typical strength of LC-NP interfacial interactions. The second research axis of this thesis orients around the investigation of the N ordering and the Sm structural properties of 5OO8, when this LC is combined with superhydrophobic perfluorinated silica (PFS) nanoplatelets. Moreover, the phase behavior of the nanocomposite systems of different NPs mass fractions has been investigated. It has been found that the PFS nanoplatelets conduct to a slight decrease of the degree of orientational ordering of 5OO8, even for quite significant mass fractions. Particularly, on the one hand PFS can be organized in the LC environment promoting the ordering, while on the other hand they are LC-phobic decreasing the ordering close to the LC-NP interfaces. The latter has been verified by wetting experiments. In parallel, LC undergoes elastic deformations to incorporate PFS in its body. It has been found that the isotropic-nematic (I-N) transition temperature (TIN) of the mixtures exhibits systematically a non-monotonic dependency on the NPs mass fraction. It has been for the first time verified experimentally that this behavior stems from the tendency of PFS to form small aggregates and therefore it is intimately related to the quality of the microstructure of the nanocomposite samples. That is, the more PFS are dispersed in LC volume without aggregating, the more the TIN decreases. Analogous behavior, exhibits the I-N coexistence width (TI+N), which increases non-monotonically with PFS mass fraction. Unexpectedly, it has been deduced by polarizing optical microscopy and calorimetric studies that the variations of TIN and TI+N are quantitatively correlated obeying to linear laws. In order to explain the aberrations of TIN and TI+N from monotony, as well as their linear variations with different slopes at the different concentration regimes, we have proposed that the interfacial interactions of LC with nanoplatelets that lead to linear variations of TIN and TI+N depend on the surface-to-volume ratio of NPs which decreases when NPs organize themselves to form bigger particles (in the simplest case nanocylinders). Despite the fact that NPs decrease the orientational ordering, they were found to further stabilize SmA phase. The significant enhancement of SmA width is speculated to be related with the NPs organization between the Sm layers. Nanoplatelets promote their own translational ordering which can be coupled with the N one, stabilizing the SmA over wider temperature windows even for relatively low PFS concentrations. The third and final part of the thesis is devoted in the investigation of the impact of CdSe/ZnS quantum dots on the orientational order and the structural properties of the LC 8CB whose molecules exhibit higher degree of polarity compared to 5OO8. It has been systematically observed that NPs can be adequately dispersed in 8CB over a concentration regime below a “critical threshold” (crossover) above which, depending on the stimulations receiving the system, different scenarios can be realized. On crossing the crossover, if NPs dispersion is still fostered, LC is quite dense in NPs and therefore the transition between a “homogeneous” and a “distorted” N phase, also observed in case of 5OO8 with the same NPs, is realized. However, in this regime, there is an increased probability the adequate conditions to be cultivated in order the NPs to get organized in fractal-like structures. In such a case, LC is subject to spatial confinement effects. The NPs structures trap LC and therefore exhibit optical activity. They can be observed by polarizing optical and fluorescence microscopy, but their presence is verified by X-ray diffraction experiments. In the case that NPs are dispersed in 8CB, the variations of TIN and TI+N are quantitatively correlated, verifying the results also observed in 5OO8-PFS mixtures. In the contrary, when NPs form microscopically observed structures the linear correlation saturates. The different physical behaviour exhibiting the nanocomposite LCs in which NPs are organized in fractal-like structures is reflected in the different way that I-N transition is realized. In particular, TIN exhibits hysteresis for the material trapped in the structures and for that in the free space. The presentation of results concerning nanocomposite LC system in which NPs organization leads to confinement effects, exceeds the initial goals of this thesis, however, we present some preliminary results in order to trigger the interest for future research in this direction. By tuning quantum dots organization in LCs we could pave the way for novel soft nanomaterials for outstanding photonic applications.
Keywords:
Liquid Crystals, Nanoparticles, Nematic phase, Smectic phases, Orientational order parameter, Birefringence, Isotropic-Nematic phase transition
