F. Kremer

Broadband dielectric spectroscopy on side group liquid crystal polymers

Abstract Dielectric studies on three liquid-crystalline side group polymers were performed in the temperature range from 100 K to 450 K and in the frequency range from 10−1 Hz to 107 Hz. All samples exhibit an α relaxation with a WLF dependence of the activation energy as well as a β relaxation, which are comparable in the glassy state. Their mean relaxation times, their relaxation time distribution, in its asymmetry and half-width, and their activation energies are nearly unaffected by the spacer length, terminal group and/or main chain. These findings are supported by N.M.R. measurements. A third relaxation process occurring close to the clearing point is also observed and assigned to the cooperative rotation of the side group around the main chain.

Dielectric study on the miscibility of binary polymer blends

The α-relaxations of various polymer blends have been measured by broad-band dielectric relaxation spectroscopy (1 Hz to 1 MHz) over a wide temperature range (−50 to 220°C). In all blends under investigation the dipole moment of one polymer was negligible compared with that of its counterpart, which simplified the evaluation of blending effects on the α-relaxation. In mixtures of poly(vinyl methyl ether) (PVME) with polystyrene, which are well known to exhibit lower critical solution temperature behaviour, the α-relaxation of PVME could be measured in the one- and two-phase region. The results are compared with differential scanning calorimetric measurements of this system. The dielectric miscibility criterion ensured for the blend PVME/polystyrene was then used to study miscibility of other polymer blends. In the temperature range under investigation, mixtures ofPVME/poly( p -methylstyrene) andPVME/poly(styrene- block-p -methylstyrene) are incompatible, whereaspoly( o -chlorostyrene) blended with poly(methylphenylsiloxane) is either compatible or semicompatible.

Field dependent soft and goldstone mode in a ferroelectric liquid crystal as studied by dielectric spectroscopy

Abstract Broadband dielectric spectroscopy (frequency range 10 -1 –10 7 Hz and temperature range 100 to 450 K) was used to analyze in detail the frequency and temperature dependence of the soft and the Goldstone mode in a ferroelectric liquid crystal.

Molecular dynamics and the glass transition in a columnar liquid crystal formed by a chiral discotic mesogen

Abstract A new type of chiral discotic liquid crystal is described. It is based on a triphenylene core but carries only one asymmetric side chain per mesogen. This system displays a columnar liquid-crystalline mesophase with a helical superstructure and a pitch of 3·0 nm over a temperature range of 227 K. Upon cooling it forms a glassy state. By broadband dielectric and 2H NMR spectroscopy two motional processes are detected. The axial rotation of the discs around the column axis exhibits non-Arrhenius behaviour and is directly related to the glass transition. The second process is ascribed to localized side chain motions involving the ester linkages.

The temperature-dependence of the far-infrared spectra of l-alanine

Abstract Far-i.r. spectra in the region 20–600 cm−1 are reported for solid l -alanine as a function of temperature between 6 and 300 K. Understanding of the temperature-sensitive as well as the temperature-insensitive bands is aided by the normal coordinate analysis of l -alanine. Bands containing contributions from hydrogen bonds, skeletal deformation and NH3+ torsion are found to be highly sensitive to temperature changes. Although the hydrogen bonds are stronger at lower temperatures, no evidence of amide formation is detected even at 6 K. Thus the zwitterion form seems to be more stable than the amide form in the temperature range studied here.

Thermal and dielectric properties of glassy ionenes

Abstract Poly[(dialkylimino)alkylene] salt (‘ionenes’) −[−( CH 2 ) N − N ( R ) 2 −] n q.n.X where m = 6 or 10, R = CH3 or CH2CH3 and X = monovalent (q = 1) or bivalent (q = 0.5) counterions, form amorphous glassy films when quenched from the melt or cast from solution followed by rapid evaporation of the solvent. The glass transition temperature, Tg, was found to be between 295 and 350 K depending on the structure of the polymer and the type of counterion. The dielectric properties were studied in the frequency range 10–107 Hz and in the temperature range − 170

Frequency/temperature response of conductivity in poly(p-styrene sulphonate) salts of polypyrrole

Abstract Electrical conductivities of a series of polypyrrole (PPY) films with poly(p-styrene sulphonate) (PSS) counterions, prepared electrochemically from various water 1,4- dioxane solutions, were investigated at several frequencies ranging from 0 Hz to 10.3 GHz within the temperature range 10–300 K. The frequency response of conductivity was found to increase systematically with increasing amounts of 1,4-dioxane in the polymerization solutions. At 10.3 GHz, the temperature dependencies of conductivity for samples from solutions ranging from 0 to 40% 1,4-dioxane became similar. This behaviour is related to changes in the spatial distribution of counterion sites induced by addition of 1,4-dioxane to the polymerization solutions. Calculations from the a.c. conductivity results, according to the hopping model of Mott, require decay lengths of localized states less than 10 A to provide reasonable densities of localized states. We conclude on this basis that the entities responsible for charge transport in these systems are likely to be unpaired.

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 α-relaxation which is correlated to the dynamic glass transition of the polybutadiene, and a β-relaxation corresponding to a local relaxation of polybutadiene segments. In the polar functionalized systems, besides these two relaxations, an additional relaxation process (called α*) is observed, which occurs at lower frequencies than the α-process. While the β-relaxation remains unaffected by the functionalization the cooperativity of the α-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 α-relaxation is increased by free phenyl urazole units. The α* 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.

Segmental orientation and mobility of ferroelectric liquid crystal polymers

IR spectroscopy was used to study the orientation and mobility of different molecular segments in a side chain ferroelectric liquid crystalline polymer (FLCP) in the book-shelf geometry. It was directly shown that the tilt angles for the mesogenic units and the spacers are different. The data obtained allowed us to construct a detailed model of segmental orientation in the SC phase for this FLCP. This model is consistent with the ‘zigzag’ model for tilted smectic phases. The rotational bias of carbonyl bonds is also confirmed and a possible orientation function for the carbonyl group is discussed. Time-resolved step-scan FTIR spectroscopy enabled us to follow the intra- and inter-molecular response of the FLCP to an external electric field with a time resolution of 5 mus. It was detected that mesogenic moiety, spacer and backbone take part in the reorientation process. The time responses of different molecular segments are similar on the time scale of a few hundred microseconds.

COLLECTIVE AND MOLECULAR-DYNAMICS IN LOW MOLAR MASS AND POLYMERIC FERROELECTRIC LIQUID-CRYSTALS

Abstract By use of a novel sample preparation technique, it is possible to measure with one sample cell the collective and molecular dynamics (10−1 Hz-109 Hz) of macroscopically (in bookshelf geometry) oriented ferroelectric liquid crystals. Below 106 Hz, two ferroelectric modes, Goldstone- and soft-mode, are observed, being assigned to fluctuations of the phase and amplitude of the helical superstructure respectively. Between 106 and 109 Hz, one dielectric loss process exists, the β-relaxation which originates from the librational motion (hindered rotation) of the mesogen around its molecular long axis. This process does not split or broaden at the non-ferroelectric-ferroelectric phase transition and it has an Arrhenius type temperature dependence. In comparing a racemic mixture with a chiral sample, it performs a similar frequency and temperature dependence. The experimental findings for the β-relaxation are in qualitative contrast to the predictions of the (generalized) Landau expansion of the free ene...