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Composites containing 4-n-octyl-4?-cyanobiphenyl (8CB) either confined to nanopores of molecular sieves with very large pores or coating silica nanoparticles of aerosil type at high silica-to-8CB ratios are investigated by IR spectroscopy. Band shape analysis was performed in wavenumber regions in which the peaks due to CN stretching, CH stretching and CH out-of-plane vibrations appear. Some of molecules confined to molecular sieves show spectroscopic features characteristic to a bulk-like 8CB matter located in the centre of the pores or in the inter-grain space. Other features of the IR spectra are due to 8CB molecules located in the surface layer, mostly forming hydrogen bonds between their CN groups and surface OH groups. Another part of the 8CB molecules in the surface layer may also interact by ? electrons of the aromatic rings. Hydrogen bonding is less hindered for the molecules of the surface layers onto aerosil particles than inside pores of the molecular sieves. Comparison is also made with the case of composites based on molecular sieves with small pores.
Thermal analysis measurements were applied to study the behavior of composite systems containing a nematic liquid crystal (LC), 4-n-octyl-4?-cyanobiphenyl (8CB) and aerosil particles. The silica density ?s (g of silica per 1 cm3 of LC) was varied from 0.20 to 12 g/cm3 in order to approach the monomolecular structure of the surface layer. Thus, thermogravimetric (TG) measurements were performed to get information on the LC content of the samples. Moreover, they allowed an unusual application, to characterize the interaction of 8CB molecules with aerosil surface because the corresponding curves show multistep processes the temperatures of which depend on the silica density. Differential thermal analysis (DTA) also indicates several processes. Some of them, mostly due to desorption, are endothermal. Other processes are exothermal, and these are assigned to chemical transformations. The sample with silica density of 12 g/cm3 has no endothermal desorption peak at low temperatures, and has the highest shift for the DTA peaks. It was supposed that its surface layer has a monomolecular structure. For representative samples, TGDTA studies were supported by FTIR investigations. Thus conventional TGDTA techniques have provided new tools to characterize the nanocomposites. Differential scanning calorimetry gives the behavior of the phase transitions. Samples with low silica density exhibit a depression of the phase transition temperatures characteristic for the bulk 8CB while those with high silica densities do not show any phase transitions.
The molecular dynamics of a side-chain polymer liquid crystal with a poly(methyl acrylate) backbone and a (p-alkoxy-phenyl)-benzoate mesogenic group have been studied in the unaligned state as a function of temperature and pressure using dielectric spectroscopy. Polarizing optical microscopy, differential scanning calorimetry, and pressure-volume-temperature (PVT) measurements revealed three transition temperatures separating four phases (glass, smectic, nematic, and isotropic). Different dynamic processes have been identified reflecting librational modes (? process), local relaxation of the mesogenic group (? process), the segmental mode (? process) associated with the dynamic glass transition, and a slower process (? process) reflecting the side-chain dynamics within the liquid crystal order. Pressure exerts a stronger influence on the ? as compared to the ? process. Starting from the nematic phase, pressure was found to induce the nematic-to-smectic transformation. The associated dynamic changes were in excellent agreement with the PVT results implying that the dynamics are directly coupled to the thermodynamic state. Pressure was found to enhance the stability of the smectic order within the P-T phase diagram.
The glassy dynamics of poly(propylene glycol) (PPG) and poly(dimethyl siloxane) (PDMS) confined to a nanoporous host system revealed by dielectric spectroscopy, temperature-modulated DSC and neutron scattering is compared. For both systems the relaxation rates estimated from dielectric spectroscopy and temperature-modulated DSC agree quantitatively indicating that both experiments sense the glass transition. For PPG the segmental dynamics is determined by a counterbalance of adsorption and confinement effect. The former results form an interaction of the confined macromolecules with the internal surfaces. A confinement effect originates from an inherent length scale on which the underlying molecular motions take place. The increment of the specific-heat capacity at the glass transition vanishes at a finite length scale of 1.8 nm. Both results support the conception that a characteristic length scale is relevant for glassy dynamics. For PDMS only a confinement effect is observed which is much stronger than that for PPG. Down to a pore size of 7.5 nm, the temperature dependence of the relaxation times follows the Vogel-Fulcher-Tammann dependence. At a pore size of 5 nm this changes to an Arrhenius-like behaviour with a low activation energy. At the same pore size vanishes for PDMS. Quasielastic neutron scattering experiments reveal that also the diffusive character of the relevant molecular motions --found to be characteristic above the glass transition-- seems to disappear at this length scale. These results gives further strong support that the glass transition has to be characterised by an inherent length scale of the relevant molecular motions.
Dielectric spectroscopy in the frequency range from 10-2 to 106 Hz and in the temperature range from 190 to 440 K is employed to study the effect of azobenzene concentration on the dielectric relaxation processes of an amorphous comb-like copolymethacrylate. Four concentrations (x=29, 45.5, 54, and 74.5 mol%) of photochromic group 4-amino-azobenzene were investigated, where as comonomer a methacrylate unit having a derivative of benzanilide in the side group is used. Two prominent processes, the ?-relaxation at low temperatures which is related to rotational fluctuations of the mesogenic unit around its long axes and the dynamic glass transition (?-relaxation, segmental dynamics) at higher temperatures are observed for all azobenzene concentrations. In addition in between the ?- and the ?-relaxation a ??-process is observed for the polymers with the two lowest azobenzene concentrations, which seems to be related to the azobenzene unit. The dependence of the dielectric strength and the relaxation rate of the relaxation processes on the azobenzene concentration is discussed and interpreted in a simple phenomenological model, where also data obtained by semi empirical quantum chemical calculations are used.
Dielectric spectroscopy and temperature-modulated differential scanning calorimetry are employed to study the molecular dynamics of oligomeric poly(propylene glycol) (PPG) melts of different molecular weights confined to nanoporous glasses (pore sizes 2.5, 5.0, 7.5 and 20 nm). Moreover, the results obtained for the polymers are compared with those for the corresponding monomer. The experimental results are discussed in the framework of the interplay of confinement and adsorption effects. For large pore sizes (> 5 nm) a speeding up of the molecular dynamics is observed (the confinement effect), whereas for small pore sizes (< 5 nm) a slowing down of the segmental fluctuations is found (the adsorption effect). In addition, a minimal length scale for the glass transition of 1.6 nm is estimated for PPG confined in nanoporous glasses. This supports the idea that the molecular motions responsible for the glassy dynamics must be describable by a characteristic length scale.
Polarized Raman scattering investigations are carried out to investigate the conformations of the macromolecules inside the pores. These experiments show that the confined polymer chains are locally stretched. This effect increases with decreasing pore size.
A broadband dielectric study of the dispersions of silica spheres (Aerosil 380) in octylcyanobiphenyl (8CB) with densities between 0.083 and 10 g of silica per 1 cm3 of liquid crystal (LC) is reported. High values of silica densities were achieved for the first time to observe the behaviour of the LC monolayer on the surface of the silica particles. The relaxation characteristic of the bulk LC is noticed especially for the samples with low silica densities. Additionally, a slow relaxation was detected, even at temperatures for which the bulk LC is in the crystalline state. It was assigned to the surface layer of LC molecules. Remarkably, the temperature dependence of the relaxation rates for this slow process shows a behaviour typical for glass-forming liquids.
Dielectric spectroscopy and temperature modulated DSC are employed to study the molecular dynamics of oligomeric poly(propylene glycol) (PPG) melts of different molecular weights confined to nanoporous glasses (pore sizes 2.5, 5.0, 7.5 and 20 nm). Moreover the results obtained for the polymer are compared with the corresponding monomer. For large pore sizes an acceleration of the segmental dynamics compared to the bulk state is observed which is already known for low molecular-weight glass forming liquids. For smaller pore sizes the molecular dynamics is slower than in the bulk. The observed behavior is nearly independent of the molar mass of the polymer and of the treatment of internal glass surfaces. The experimental results are discussed in the frame of an interplay of confinement and adsorption effects. Moreover a length scale of about 1.6 nm is estimated as a minimal length scale for the cooperativity for the glass transition. In addition to the ?-relaxation the whole chain dynamics (normal mode relaxation) can be measured by dielectric spectroscopy because PPG has a dipole component parallel to the chain. For virgin internal surfaces the relaxation rate of the normal mode relaxation is shifted dramatically to lower relaxation rates. That can be explained by adsorption effects. For treated surfaces this effect is strongly reduced and it is concluded that also in this case the chain dynamics are influenced by geometric (confinement) effects.