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Suppressed Transition and Dynamic self-asembly of ionic superdiscs in cylindrical nanochannels
(2023)
Liquid crystalline mesophases in nanoconfinement exhibit intriguing phase transition behaviors and relaxation dynamics. Here, we investigate the molecular mobility and electrical conductivity of a columnar ionic liquid crystal confined in self-ordered nanoporous alumina oxide membranes of pore size ranging from 180 nm down to 25 nm. We use nano-broadband dielectric spectroscopy (BDS) and calorimetry to study the dynamics and phase behavior. Calorimetric investigation reveals a complete suppression of the columnar – isotropic transition, while the plastic crystalline – columnar transition temperature decreases with inverse pore size and deviates from the Gibbs – Thomson equation.
For the bulk case, BDS detects two relaxation modes in the crystalline phase, the γ relaxation and the α1 relaxation, and two relaxation modes in the columnar phase, the α2 and α3 relaxation. All relaxation modes slow down for the confined case compared to the bulk. However, a new relaxation mode reflecting the interfacial layer emerges for the 80 and 25 nm. We discuss the possible molecular origins of the different relaxation modes observed. For the bulk ILC, a clear jump of 4 orders of magnitude in the absolute values of DC conductivity occurs at the transition from the plastic crystalline to hexagonal columnar phase, for the confined ILC, this transition is smooth. DC conductivity is reduced for the confined case, except for the 25nm, where the values are similar to the bulk.
Suppressed Transition and Dynamic self-asembly of ionic superdiscs in cylindrical nanochannels
(2023)
Liquid crystalline mesophases in nanoconfinement exhibit intriguing phase transition behaviors and relaxation dynamics. Here, we investigate the molecular mobility and electrical conductivity of a columnar ionic liquid crystal confined in self-ordered nanoporous alumina oxide membranes of pore size ranging from 180 nm down to 25 nm. We use nano-broadband dielectric spectroscopy (BDS) and calorimetry to study the dynamics and phase behavior. Calorimetric investigation reveals a complete suppression of the columnar – isotropic transition, while the plastic crystalline – columnar transition temperature decreases with inverse pore size and deviates from the Gibbs – Thomson equation.
For the bulk case, BDS detects two relaxation modes in the crystalline phase, the γ relaxation and the α1 relaxation, and two relaxation modes in the columnar phase, the α2 and α3 relaxation. All relaxation modes slow down for the confined case compared to the bulk. However, a new relaxation mode reflecting the interfacial layer emerges for the 80 and 25 nm. We discuss the possible molecular origins of the different relaxation modes observed. For the bulk ILC, a clear jump of 4 orders of magnitude in the absolute values of DC conductivity occurs at the transition from the plastic crystalline to hexagonal columnar phase, for the confined ILC, this transition is smooth. DC conductivity is reduced for the confined case, except for the 25nm, where the values are similar to the bulk.
Ionic Liquid Crystals (ILCs) are materials that combine the properties of liquid crystals together with ionic conduction. It is known that liquid crystal mesophases in confinement exhibit anomalous dynamics and phase behavior. However, similar studies about factors that control the macroscopic properties of ILCs in confinement are limited. Here, Broadband Dielectric Spectroscopy (BDS), X-ray scattering, and calorimetry were applied to study the molecular dynamics, and phase behavior of a guanidinium based columnar ionic liquid crystal confined in self-ordered alumina oxide nanopores of pore sizes ranging from 180 nm down to 25 nm. It is aimed to understand how pore size and pore surface wettability (hydrophilic or hydrophobic) influence the molecular dynamics, and phase behavior for this system which are crucial for applications. The DSC measurements show: (i) the crystalline-liquid crystalline transition temperature has non-monotonic dependence on inverse pore diameter and (ii) the liquid crystalline-isotropic transition is completely suppressed for all the confined samples. This thermally suppressed transition was detected by BDS and X-ray scattering and is considered as a continuous phase transition instead of a discontinuous first order transition. BDS investigations reveal several relaxation processes for the bulk and confined scenarios. The relaxation modes are suppressed and become slower for the confined scenarios compared to the bulk. Possible molecular origins for these relaxation processes are discussed, and it is shown that the self-assembly of these ILCs is dynamic in nature.
ILC Meeting
(2021)
We summarized our recent findings about the molecular dynamics investigation of bulk and finite ionic liquid crystals. We investigate the molecular dynamics and electrical conductivity for a homologous series of linear shaped guanidinium based cyclic ILCs that vary in alkyl chain length, CLCRs (R = 8, 10, 12, 14, 16) by employing broadband dielectric spectroscopy (BDS), and calorimetry comprised of Fast Scanning Calorimetry (FSC) and temperature modulated FSC (TMFSC). Besides conductivity at high temperatures, the dielectric dispersion reveals two relaxation modes: the fast γ and the slow α1 relaxation. The former is assigned to the localized fluctuations while the latter is due to segmental dynamics of the alkyl chains. The γ mode slows down for long chain length CLCs (12,14,16) compared to their shorter analogues. Calorimetric investigation reveals one process, the α2 process, for CLC12,14 and 16 and two processes, α2 and α3, for CLC8 and 10. The α2 process of all CLCRs has a similar temperature dependence as the dielectric α1 relaxation, which indicates both BDS and FSC probe the segmental dynamics of alkyl side chains, as observed for the bent shaped cyclic ILCs. We interpret the α3 process of CLC8 and 10 as the segmental dynamics of the cation core. For all CLCRs, the absolute values of DC conductivity increase by 4 orders of magnitude at the transition from the plastic crystalline to hexagonal columnar phase. This increase is due to the change in the underlying conduction mechanism from delocalized electron hopping in the crystalline phase to one dimensional ion mobility in the columnar phase. The glassy dynamics shifts to higher temperatures with increasing alkyl chain length. Conversely, the DC conductivity drops by 3 orders of magnitude from CLC8 to CLC16.
Liquid crystalline mesophases in nanoconfinement exhibit intriguing orientational order and phase transition behaviors. Here, the plastic crystal to hexagonal columnar, and hexagonal columnar to isotropic transition temperatures are studied for the guanidinium-based ionic discotic liquid crystal confined in self- ordered nanoporous alumina membranes. The phase transition temperature of the plastic crystal to hexagonal columnar phase is reduced with inverse pore diameter. The hexagonal columnar to isotropic transition is suppressed completely in all pores and a possible explanation is given. The results are of technological relevance for the design of liquid crystal-based devices such as batteries and sensors with optimum tunable properties.
Discotic liquid crystals (DLCs) are a promising class of soft matter for electronic applications. This is due to their ability to organize and stack themselves into columns in a hexagonal columnar mesophase, a mesophase in between the plastic crystalline and isotropic phase, driven by the overlap of the π orbitals of their aromatic core. This leads to a high charge-carrier mobility along the column axis. Further, these columns could then be considered as “molecular nanowires”.
Spatial confinement of soft matter in nanoporous media influences its structure, thermodynamic properties, and mobility. Embedding liquid crystals and polymers into nanopores of anodic aluminum oxide (AAO) results in a 2D nanoconfinement of these materials. This confinement affects their properties, compared to the bulk, such as phase transition temperatures and enthalpies, molecular mobility, and architecture of the crystallization.
In this study, 2,3,6,7,10,11 hexakis[hexyloxy] triphenylene (HAT6), a triphenylene-based DLC, was confined into nanoporous AAO membranes. The structure, dynamics and the phase behavior of the confined HAT6 were investigated by broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). HAT6 was embedded into nanoporous AAO membranes by melt infiltration in the isotropic phase under argon atmosphere. The membranes have parallel aligned cylindrical nanopores, with different pore diameters in the range of 10-180 nm. Bulk HAT6 forms a hexagonal columnar phase; in between the isotropic phase above 371 K and the plastic crystalline phase below 340 K. Unlike the bulk, the confined HAT6 split the phase transitions in two or more, which might be interpret as different phase structures; close to the wall and at the pore center. Moreover, the phase transitions of the confined HAT6 shifted, with decreasing pore diameter, to lower temperatures. The dependencies of the phase transition temperatures on the pore size was well-described by the Landau-de Gennes model. Furthermore, pore surfaces of a series of membranes were chemically modified, resulting in a more hydrophobic pore surface than the unmodified ones. HAT6 was embedded into the modified membranes by the same aforementioned preparation. The influence of the changed host-guest-interaction, on the structure, dynamics, and the phase behavior of HAT6 confined in the modified membranes, was also investigated by BDS and DSC.
Discotic liquid crystals (DLCs) are a promising class of soft matter for electronic applications. This is due to their ability to organize and stack themselves into columns in a hexagonal columnar mesophase, driven by the overlapping of the π orbitals of their aromatic core. This leads to a high charge-carrier mobility along the column axis. Previous studies on DLCs showed that their properties, such as phase transition temperatures and enthalpies, are susceptible to nanoconfinement. In this study, 2,3,6,7,10,11 hexakis[hexyloxy] triphenylene (HAT6), a triphenylene based DLC, was confined into parallel aligned cylindrical nanopores of anodic aluminum oxide (AAO) membranes by melt infiltration in the isotropic phase under an argon atmosphere. Furthermore, the pore surfaces of a series of membranes were chemically modified, resulting in a more hydrophobic pore surface than the unmodified ones. The structure, dynamics, and the phase behavior of HAT6 confined into modified and unmodfied nanopores of AAO were investigated by broadband dielectric spectroscopy and differential scanning calorimetry. Results will be discussed in detail.
Discotic liquid crystals (DLCs) are self-assembled materials where self-assembly is driven by non-covalent intermolecular interactions. The corresponding molecules consist of a flat and rigid aromatic core substituted by flexible aliphatic side chains. While the former is responsible for the π-stacking, the latter gives rise to an increased solubility, processability, and rich thermotropic behavior. The disc-shaped molecules form columns that further assemble into two-dimensional arrays with a hexagonal mesophase. The alkyl chains fill the intracolumnar space giving rise to a nanophase separated state.
Two different homologous systems based on triphenylene derivatives were investigated as model systems for DCLs where the length of the aliphatic side chains is widely varied. Measurements were carried out in the bulk state and confined to the nanometre wide channels of anodic aluminum membranes (Pore diameter 20 nm, 40nm, 80 nm and 180 nm) by means of inelastic and quasielastic neutron scattering. These investigations were accompanied by calorimetry and dielectric relaxation spectroscopy. From the neutron scattering experiments the vibrational density of states (Boson Peak) from neutron time-of-flight spectroscopy, the molecular dynamics on a time scale of ca. 1 ns (elastic scans from neutron backscattering spectrometers) as well as quasielastic neutron scattering by a combination of time-of-flight spectroscopy and backscattering were considered. Structural information is retrieved by X-ray diffraction carried out on a synchrotron.
The dynamics of these unique soft matter materials are discussed in detail considering their structure (length of the aliphatic side chain), the effect of self-confinement (of the aliphatic chains in the intracolumnar space) and of outer confinement (influence of the pore size). Comparison is further made to pyrene based system.