Dynamics in ferro- and antiferroelectric phases of a liquid crystal with fluorinated molecules as studied by dielectric spectroscopy

For 1-[3-fluoro-4-(1-methylheptyloxycarbonyl)phenyl]-2-[4-2,2,3,3,4,4,4-heptafluorobutoxybutoxy)biphenyl-4-yl]ethane (1F7), built of chiral molecules, results of dielectric measurements of liquid-crystalline and solid phases are presented. Rich polymorphism of liquid-crystalline (SmC*, SmC*_A and SmI*_A) phases as well as of solid (Cr1 and Cr2) phases were observed down to –130°C. At a frequency range from 0.1 Hz to 3 MHz, the relaxation processes were detected in ferroelectric SmC*, antiferroelectric SmC*_A and highly ordered SmI*_A smectic phases. The mechanism of complex dynamics (moleculear and collective) was identified with the help of the bias field. Vitrification of conformationally disordered crystal phase Cr2 was found in accordance with calorimetric observations.     

Dynamics of glassy and liquid m-toluidine investigated by high-resolution dielectric spectroscopy

The glass-former m-toluidine displays the characteristic properties of a fragile supercooled liquid, which suggest the existence of a slow secondary relaxation process. In view of the recently realized importance of such a secondary relaxation feature, we have conducted a dielectric search for the secondary process in viscous and glassy m-toluidine. Based on high-resolution experiments on the distilled liquid, a secondary process can be identified which has the properties typical of a Johari-Goldstein beta relaxation. As a result, the previous hypothesis that the methyl group might be responsible for suppressing the secondary dynamics in glassy m-toluidine no longer holds.     

Molecular and collective modes in ferroelectric liquid crystals studied by dielectric spectroscopy

The complex dielectric permittivity has been measured for a ferroelectric liquid crystal in the range 102-109Hz. Six different relaxations have been obtained and characterized: soft mode (SmA* and SmC* phases), Goldstone mode (SmC* phase), rotation around molecular long axis, rotation around molecular short axis, ferroelectric domain mode (SmC* phase) and an internal motion associated with a polar group. Strengths and frequencies of these modes have been obtained for the different phases for different bias fields. Using these results together with spontaneous polarization and molecular tilt measurements we have also obtained the rotational viscosities associated with the soft mode and the Goldstone mode. We explain the results in the light of the so-called Landau extended model, concluding that the biquadratic coupling between polarization and tilt is quite important with regard to the bilinear coupling. This fact has been used to explain the noticeable increase of the activation energy of the frequency of the mode related to the rotation around the molecular long axis at the SmA*-SmC* phase transition.     

Polymer liquid dynamics studied via dielectric normal mode spectroscopy

Dielectric spectroscopy was applied to study dynamics of cis-polyisoprenes (PI), used as type-A probe, in blends with polybutadiene (PB) and in block copolymers with polystyrene (PS) of SI- and SIS-type. For dilute high-molecular weight (M) PI/low- M PB blends we identified Rouse mode with M²-dependent relaxation time τ, while for low- M Pi/high- M PB blends, we identified pure reptation mode with M³ -dependent τ. In between τ ∞ M^α with the exponent α varying from 2 to 3 as M_B was increased, as suggested by Graessley with constraint release via tube renewal mechanism. For the blends with the MW ratio M_I/M_B = 2.5, we found bulk polymer behaviour with τ ∞ M^3.5, in which competition between pure reptation and tube renewal appear to be essential and the contribution of contour length fluctuation may be ruled out. For SI-diblock copolymers between Tg(I) < T < Tg(S) we observed normal modes of I-block chains tethered on rigid S-domains. The mode distribution as judged from the dielectric loss ε” curves was dependent on the domain morphology, reflecting restricted motions of crowded I-tethered chains. For SIS-triblock copolymers normal modes became appreciable, even below their critical solution temperature, in the range of T > Tg(S), exhibiting broadening due presumably to their micro-phase-separated structure. The relaxation mechanisms for such end-capped I-chains in SIS-triblock copolymers could be junction hopping in those with isolated S-domains but chain rotating in those with S continuous morphologies.     

Dynamics of water in partially crystallized polymer/water mixtures studied by dielectric spectroscopy

The dielectric relaxation process of water was investigated for polymer/water mixtures containing poly(vinyl methyl ether), poly(ethyleneimine), poly(vinyl alcohol), and poly(vinylpyrrolidone) with a polymer concentration of up to 40 wt % at frequencies between 10 MHz and 10 GHz in subzero temperatures down to -55 degrees C. These polymer/water mixtures have a crystallization temperature TC of water at -10 to -2 degrees C. Below TC, part of the water crystallized and another part of the water, uncrystallized water (UCW), remained in a liquid state with the polymer in an uncrystallized phase. The dielectric relaxation process of UCW was observed, and reliable dielectric relaxation parameters of UCW were obtained at temperatures of -26 to -2 degrees C. At TC, the relaxation strength, relaxation time, and relaxation time distribution change abruptly, and their subsequent changes with decreasing temperature are larger than those above TC. The relaxation strength of UCW decreases, and the relaxation time and dynamic heterogeneity (distribution of relaxation time) increase with decreasing temperature. These large temperature dependences below TC can be explained by the increase in polymer concentration in the uncrystallized phase C(p,UCP) with decreasing temperature. C(p,UCP) is independent of the initial polymer concentration. In contrast to the relaxation times above TC, which vary with the chemical structure of the polymer and its concentration, the relaxation times of UCW are independent of both of them. This indicates that the factor determining whether the water forms ice crystals or stays as UCW is the mobility of the water molecules.     

Molecular dynamics of a biodegradable biomimetic ionomer studied by broadband dielectric spectroscopy

Broadband dielectric spectroscopy was used to investigate the bulk molecular dynamics of a recently developed biodegradable biomimetic ionomer potentially useful for biomedical applications. Isothermal dielectric spectra were gathered for a phosphoryl choline (PC)-functionalized poly(trimethylene carbonate) (PTMC) ionomer and unfunctionalized PTMC at temperatures ranging from 2 to 60 degrees C over a broad frequency range of 10(-3) to 10(6) Hz. Four relaxations were clearly identified, two of which were shown to stem from the PTMC polymer backbone. A detailed analysis showed that the formation of zwitterionic aggregates was responsible for the material's bulk functionality and that bulk conduction processes may provide useful information for assessing the PC ionomer as a candidate for drug delivery applications. Finally, it was concluded that absorbed water concentrates around the aggregates, resulting in an increased mobility of the PC end-groups.     

Liquid structure of the urea-water system studied by dielectric spectroscopy

Dielectric spectroscopy measurements for aqueous urea solutions were performed at 298 K through a concentration range from 0.5 to 9.0 M with frequencies between 200 MHz and 40 GHz. Observed dielectric spectra were well represented by the superposition of two Debye type relaxation processes attributable to the bulk-water clusters and the urea-water coclusters. Our quantitative analysis of the spectra shows that the number of hydration water molecules is approximately two per urea molecule for the lower concentration region below 5.0 M, while the previous molecular dynamics studies predicted approximately six water molecules. It was also indicated by those studies, however, that there are two types of hydration water molecule in urea solution, which are strongly and weakly associated to the urea molecule, respectively. Only the strongly associated water was distinguishable in our analysis, while the weakly associated water exhibited the same dynamic feature as bulk water. This implies that urea retains the weakly associated water in the tetrahedral structure and, thus, is not a strong structure breaker of water. We also verified the model of liquid water where water consists of two states: the icelike-ordered and dense-disordered phases. Our dielectric data did not agree with the theoretical prediction based on the two-phase model. The present work supports the argument that urea molecules can easily replace near-neighbor water in the hydrogen-bonding network and do not require the presence of the disordered phase of water to dissolve into water.     

Dynamics in supercooled ionic organic liquids and mode coupling theory analysis

Optically heterodyne-detected optical Kerr effect experiments are applied to study the orientational dynamics of the supercooled ionic organic liquids N-propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (PMPIm) and 1-ethyl-3-methylimidazolium tosylate (EMImTOS). The orientational dynamics are complex with relaxation involving several power law decays followed by a final exponential decay. A mode coupling theory (MCT) schematic model, the Sj?gren model, was able to reproduce the PMPIm data very successfully over a wide range of times from 1 ps to hundreds of ns for all temperatures studied. Over the temperature range from room temperature down to the critical temperature Tc of 231 K, the OHD-OKE signal of PMPIm is characterized by the intermediate power law t(-1.00+/-0.04) at short times, a von Schweidler power law t(-0.51+/-0.03) at intermediate times, and a highly temperature-dependent exponential (alpha relaxation) at long times. This form of the decay is identical to the form observed previously for a large number of organic van der Waals liquids. MCT analysis indicates that the theory can explain the experimental data very well for a range of temperatures above Tc, but as might be expected, there are some deviations from the theoretical modeling at temperatures close to Tc. For EMImTOS, the orientational dynamics were studied on the ps time scale in the deeply supercooled region near its glass transition temperature. The orientational relaxation of EMImTOS clearly displays the feature associated with the boson peak at approximately 2 ps, which is the first time domain evidence of the boson peak in ionic organic liquids. Overall, all the dynamical features observed earlier for organic van der Waals liquids using the same experimental technique are also observed for organic ionic liquids.     

Collective and non-collective excitations in antiferroelectric and ferrielectric liquid crystals studied by dielectric relaxation spectroscopy and electro-optic measurements

The dynamics of different molecular modes in four antiferroelectric liquid crystal substances have been studied by a combination of spectroscopic methods.The fastest motion is the reorientation around the molecular long axis, here found in the low GHz range by time domain spectroscopy. The reorientation around the short axis has a characteristic frequency of about 10kHz and is detected by frequency domain spectroscopy in the homeotropic configuration. As for the collective excitations, the Goldstone and soft modes, characteristic of the ferroelectric phase, have counterparts in the antiferroelectric phase which appear very different. There are two characteristic peaks in the spectrum, one at high frequency, about 100kHz, the other at low frequency, about 10 kHz. The latter has often been mistaken for short axis reorientation and both have been attributed to soft modes. By combining different experimental techniques and different geometries it can be shown that neither is a soft mode, but both are collective modes of different character: the high frequency mode corresponds to fluctuations where molecules in neighbouring layers are moving in opposite phase, the low frequency mode to phase fluctuations in the helicoidal superstructure. In materials exhibiting a C* phase in addition to the C*a or C* gamma phases, an additional strong peak appears in at least one lower-lying phase adjacent to the C* phase. We show that this peak, which we call a hereditary peak, has nothing to do with the antiferroelectric or ferrielectric order, but is just the Goldstone peak from a coexisting C* phase. In the same way, a Goldstone mode peak from the C* gamma phase may appear in the underlying C* a phase. In a general way, narrow phases like C* gamma, being bounded by first order transitions on both sides (C* a -C* gamma -C*) are likely to show non-characteristic (hereditary) peaks from both adjacent phases.     

Relation between solvent and protein dynamics as studied by dielectric spectroscopy

We present results obtained by dielectric spectroscopy in wide frequency (10(-2)-10(9) Hz) and temperature ranges on human hemoglobin in the three different solvents water, glycerol, and methanol, at a solvent level of 0.8 g of solvent/g of protein. In this broad frequency region, there are motions on several time-scales in the measured temperature range (110-370 K for water, 170-410 K for glycerol, and 110-310 K for methanol). For all samples, the dielectric data shows at least four relaxation processes, with frequency dependences that are well described by the Havriliak-Negami or Cole-Cole functions. The fastest and most pronounced process in the dielectric spectra of hemoglobin in glycerol and methanol solutions is similar to the alpha-relaxation of the corresponding bulk solvent (but shifted to slower dynamics due to surface interactions). For water solutions, however, this process corresponds to earlier results obtained for water confined in various systems and it is most likely due to a local beta-relaxation. The slowing down of the glycerol and methanol relaxations and the good agreement with earlier results on confined water show that this process is affected by the interaction with the protein surface. The second fastest process is attributed to motions of polar side groups on the protein, with a possible contribution from tightly bound solvent molecules. This process is shifted to slower dynamics with increasing solvent viscosity, and it shows a crossover in its temperature dependence from Arrhenius behavior at low temperatures to non-Arrhenius behavior at higher temperatures where there seems to be an onset of cooperativity effects. The origins of the two slowest relaxation processes (visible at high temperatures and low frequencies), which show saddlelike temperature dependences for the solvents water and methanol, are most likely due to motions of the polypeptide backbone and an even more global motion in the protein molecule.     

Anion configuration at the air/liquid interface of ionic liquid [bmim]OTf studied by sum-frequency generation spectroscopy

The air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), is investigated using infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra clearly show low-frequency modes [CF 3-symmetric stretching (ss) mode and SO 3-symmetric stretching (ss) mode] of the OTf anion, demonstrating the existence of anions polar oriented at the interface. The amplitude of the CF 3-ss peak of the OTf anion has the opposite sign with respect to that of the SO 3-ss peak, indicating that OTf anions at the surface have polar ordering where the nonpolar CF 3 group points away from the bulk into the air, whereas the SO 3 group points toward the bulk liquid. The line width of the SFG peak from the submerged SO 3 group is appreciably narrower than that from IR absorption, suggesting the environment of the surface OTf anions is much more homogeneous than that of the bulk. The vibrational calculations also suggest that the anions and the cations form a more specific aggregated configuration at the surface as compared to the bulk.     

Dynamics and deformation behaviour in non-hydrogen bonded model urethanes under heat and stress as studied by dielectric and infrared spectroscopy

A model polyether-urethane with monodisperse N-alkyl polyurethane hard segment has been investigated by differential scanning calorimetry, dielectric spectroscopy and infrared spectroscopy. The temperature dependent changes in the enthalpic and dielectric properties have been correlated with specific molecular motions and transition processes in the hard and soft phase. The orientation of the hard and soft segments under uniaxial stress changes with the strain and mechanisms for the orientation and deformation behaviour have been proposed. Effects of the thermal history of the sample on the properties have been discussed as well.     

Molecular motions in amorphous ibuprofen as studied by broadband dielectric spectroscopy

The molecular mobility of amorphous ibuprofen has been investigated by broadband dielectric relaxation spectroscopy (DRS) covering a temperature range of more than 200 K. Four different relaxation processes, labeled as alpha, beta, gamma, and D, were detected and characterized, and a complete relaxation map was given for the first time. The gamma-process has activation energy E a = 31 kJ.mol (-1), typical for local mobility. The weak beta-relaxation, observed in the glassy state as well as in the supercooled state was identified as the genuine Johari-Goldstein process. The temperature dependence of the relaxation time of the alpha-process (dynamic glass transition) does not obey a single VFTH law. Instead two VFTH regimes are observed separated by a crossover temperature, T B = 265 K. From the low temperature VFTH regime, a T g (diel) (tau =100 s) = 226 K was estimated, and a fragility or steepness index m = 93, was calculated showing that ibuprofen is a fragile glass former. The D-process has a Debye-like relaxation function but the temperature dependence of relaxation time also follows the VFTH behavior, with a Vogel temperature and a pre-exponential factor which seem to indicate that its dynamics is governed by the alpha-process. It has similar features as the Debye-type process observed in a variety of associating liquids, related to hydrogen bonding dynamics. The strong tendency of ibuprofen to form hydrogen bonded aggregates such as dimers and trimers either cyclic or linear which seems to control in particular the molecular mobility of ibuprofen was confirmed by IR spectroscopy, electrospray ionization mass spectrometry, and MD simulations.     

Interfacial tension between a liquid crystalline polyesteramide and polypropylene obtained by dielectric spectroscopy

Dielectric spectroscopy is an unexplored technique in the elucidation of the morphology of polymer blends. Especially the appearance of interfacial polarization can reveal important information about the microstructure of a polymer blend. A model system of liquid crystalline polymer fibers lined up in a thermoplastic matrix was investigated. After heating above the melting temperature of both phases, the fibers developed distortions which grew with time. Dielectric spectroscopy was used to follow the change in shape of the distorted fibers. The use of only two frequencies made it possible to increase the number of relevant data points in the initial stages of the fiber breakup process. From these measurements it was possible to calculate the growth rate and hence the interfacial tension between the two polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 815–825, 1998     

Hydrogen bonds in liquid water studied by photoelectron spectroscopy

The authors report on photoelectron emission spectroscopy measurements of the oxygen 1s orbital of liquid water, using a liquid microjet in ultrahigh vacuum. By suitably changing the soft x-ray photon energy, within 600-1200 eV, the electron probing depth can be considerably altered as to either predominantly access the surface or predominantly bulk water molecules. The absolute probing depth in liquid water was inferred from the evolution of the O1s signal and from comparison with aqueous salt solution. The presence of two distinctive components in the core-level photoelectron spectrum, with significantly different binding energies, is revealed. The dominant contribution, at a vertical binding energy of 538.1 eV, was found in bulk and surface sensitive spectra. A weaker component at 536.6 eV binding energy appears to be present only in bulk water. Hartree-Fock calculations of O1s binding energies in different geometric arrangements of the water network are presented to rationalize the experimental distribution of O1s electron binding energies.     

The molecular dynamics of thermoreversible networks as studied by broadband dielectric spectroscopy

Polybutadienes modified by a small number of 4-phenyl-1,2,4-triazoline-3,5-dione form thermoreversible networks via hydrogen bonding between the polar stickers. The molecular dynamics of systems with different contents of polar stickers are investigated by broadband dielectric spectroscopy in the frequency regime of 10^–1–10⁹ Hz. Unmodified polybutadiene shows two relaxation processes, the -relaxation which is correlated to the dynamic glass transition of the polybutadiene, and a -relaxation corresponding to a local relaxation of polybutadiene segments. In the polar functionalized systems, besides these two relaxations, an additional relaxation process (called ^*) is observed, which occurs at lower frequencies than the -process. While the -relaxation remains unaffected by the functionalization the cooperativity of the -relaxation increases by the formation of reversible junctions and slows down considerably. This indicates a decreased mobility of the polymer matrix. At the same time the dipole moment of relaxing units contributing to the -relaxation is increased by free phenyl urazole units. The ^* is assigned to the local complex dynamics resulting from the dissociation and formation of dimeric contacts. Hence, for this dynamic process, the absolute value of the dipole moment fluctuates with time and causes a dielectric absorption. This interpretation is in agreement with the hindered reptation model of Leibler, Rubinstein and Colby and simultaneous measurements of infrared dichroism and birefringence.     

Nucleation from a supercooled binary mixture studied by crossed polarizers

Polarized light passing through a supercooled binary liquid mixture sample is analyzed during the moment of the nucleation of the crystal phase to determine whether the stable equilibrium crystal is nucleated, or whether a transient phase of different composition or broken-symmetry is formed. This experiment is performed for the particular case of heterogeneous nucleation of a supercooled clathrate-forming liquid mixture, tetrahydrofuran (THF)/water, compared with ice nucleating from pure supercooled water. The new experimental results are consistent with the hypothesis that the equilibrium clathrate hydrate crystal is nucleated directly, with no transient phase detected on the time scale of these experiments.     

Broadband dielectric spectroscopy on a discotic liquid crystalline polymer

Broadband dielectric spectroscopy is employed for the first time to study the molecular dynamics in discotic liquid crystalline polymers. One dielectric relaxation process is found which is strongly broadened and asymmetric. It is assigned a local hindered rotation of the ester groups attaching the spacer to the discotic mesogen. This assignment is supported by NMR measurements on the identical substance.Dedicated to Prof. E. W. Fischer on the occasion of his 60th birthday.     

Speciation of uranyl complexes in ionic liquids by optical spectroscopy

Uranyl complexes dissolved in room-temperature ionic liquids have diagnostic absorption and emission spectra which reflect the molecular symmetry and geometry. In particular, the characteristic vibrational fine structure of the absorption spectra allows identification of the molecular symmetry of a uranyl complex. The concept of speciation of uranyl complexes is illustrated for the hydrated uranyl ion, the tetrachloro complex [UO2Cl4]2-, the trinitrato complex [UO2(NO3)3]-, the triacetato complex [UO2(CH3COO)3]-, and the crown ether complex [UO2(18-crown-6)]2+ in imidazolium and pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids. The competition between 18-crown-6 and small inorganic ligands for coordination to the uranyl ion was investigated. The crystal structures of the hydrolysis product [(UO2)2(mu2-OH)2(H2O)6] [UO2Br4](18-crown-6)4 and imidazolium salt [C6mim]2[UO2Br4] are described.     

A supercooled imidazolium iodide ionic liquid as a low-viscosity electrolyte for dye-sensitized solar cells

A series of allyl-functionalized imidazolium salts are reported, including 1-allyl-3-ethylimidazolium iodide and 1-allyl-3-propylimidazolium iodide, which have melting points close to room temperature and show typical properties of supercooled fluids if heated above their melting points. Their viscosities in the liquid state are considerably lower than the benchmark ionic liquid used in solar cells, viz., 1-propyl-3-methylimidazolium iodide. Electrolytes containing these new liquids provide excellent efficiencies and good stability in dye-sensitized solar cells when subjected to an accelerated-light soaking test at 60 degrees C. The structures of three of the new salts have been established in the solid state by single-crystal X-ray analysis.