Wilhelm Kossack

Glassy Dynamics in Condensed Isolated Polymer Chains

Polymer Dynamics While free surfaces should allow polymer chains to move faster than in the bulk, the presence of a substrate might slow down the motion if there is an attraction between the two. Tress et al. (p. 1371; see the Perspective by Russell) used dielectric spectroscopy to study “polymer islands” deposited on a substrate from dilute solution, where some islands contained just a few or only one polymer chain. The confinement of the polymer chain to small-surface geometries had virtually no influence on the dynamics of the polymers, aside from the segments in direct contact with the substrate. The glass transition of isolated polymer chains is mainly bulk-like, with altered dynamics only for segments at the substrate. [Also see Perspective by Russell] In the course of miniaturization down to the nanometer scale, much remains unknown concerning how and to what extent the properties of materials are changed. To learn more about the dynamics of condensed isolated polymer chains, we used broadband dielectric spectroscopy and a capacitor with nanostructured electrodes separated by 35 nanometers. We measured the dynamic glass transition of poly(2-vinylpyridine) and found it to be bulk-like; only segments closer than 0.5 nanometer to the substrate were weakly slowed. Our approach paves the way for numerous experiments on the dynamics of isolated molecules.

Molecular Order and Dynamics of Tris(2-ethylhexyl)phosphate Confined in Uni-Directional Nanopores

Abstract Infrared Transition Moment Orientational Analysis (IR–TMOA) and Broadband Dielectric Spectroscopy (BDS) are combined to study molecular order and dynamics of the glass-forming liquid Tris(2-ethylhexy)phosphate (TEHP) confined in uni-directional nanopores with diameters of 4, 8, and 10.4 nm. The former method enables one to determine the molecular order parameter of specific IR transition moments. It is observed that the central P=O moiety of TEHP has a weak orientational effect (molecular order parameter Sz = −0.1 ± 0.04) due the nanoporous confinement, in contrast to the terminal C–H groups. BDS traces the dynamic glass transition of the guest molecules in a broad spectral range and at widely varying temperature. An enhancement of the mobility takes place when approaching the glass transition temperature and becomes more pronounced with decreasing pore diameter. This is attributed to a slight reduction of the density of the confined liquid caused by the 2-dimensional geometrical constraint.

Confinement for More Space: A Larger Free Volume and Enhanced Glassy Dynamics of 2-Ethyl-1-hexanol in Nanopores

Broadband dielectric spectroscopy and positron annihilation lifetime spectroscopy are employed to study the molecular dynamics and effective free volume of 2-ethyl-1-hexanol (2E1H) in the bulk state and when confined in unidirectional nanopores with average diameters of 4, 6, and 8 nm. Enhanced α-relaxations with decreasing pore diameters closer to the calorimetric glass-transition temperature (T(g)) correlate with the increase in the effective free volume. This indicates that the glassy dynamics of 2D constrained 2E1H is mainly controlled by density variation.

The kinetics of mutarotation in L-fucose as monitored by dielectric and infrared spectroscopy

Fourier Transform Infrared Spectroscopy and Broadband Dielectric Spectroscopy are combined to trace kinetics of mutarotation in L-fucose. After quenching molten samples down to temperatures between T = 313 K and 328 K, the concentrations of two anomeric species change according to a simple exponential time dependence, as seen by an increase in absorbance of specific IR-vibrations. In contrast, the dielectric spectra reveal a slowing down of the structural (α-) relaxation process according to a stretched exponential time dependence (stretching exponent of 1.5 ± 0.2). The rates of change in the IR absorption for α- and β-fucopyranose are (at T = 313 K) nearly one decade faster than that of the intermolecular interactions as measured by the shift of the α-relaxation. This reflects the fact that the α-relaxation monitors the equilibration at a mesoscopic length scale, resulting from fluctuations in the anomeric composition.

Glassy dynamics and physical aging in fucose saccharides as studied by infrared- and broadband dielectric spectroscopy

Fourier Transform Infra Red (FTIR) and Broadband Dielectric Spectroscopy (BDS) are combined to study both the intra- and inter-molecular dynamics of two isomers of glass forming fucose, far below and above the calorimetric glass transition temperature, T(g). It is shown that the various IR-active vibrations exhibit in their spectral position and oscillator strength quite different temperature dependencies, proving their specific signature in the course of densification and glass formation. The coupling between intra- and inter molecular dynamics is exemplified by distinct changes in IR active ring vibrations far above the calorimetric glass transition temperature at about 1.16T(g), where the dynamic glass transition (α relaxation) and the secondary β relaxation merge. For physically annealed samples it is demonstrated that upon aging the different moieties show characteristic features as well, proving the necessity of atomistic descriptions beyond coarse-grained models.

Molecular dynamics of itraconazole confined in thin supported layers

Broadband Dielectric Spectroscopy (BDS) is used to study the molecular dynamics of thin layers of itraconazole – an active pharmaceutical ingredient with rod-like structure and whose Differential Scanning Calorimetry (DSC) scans reveal liquid crystalline-like phase transitions. It is found that (i) the structural relaxation process remains bulk like, within the limits of experimental accuracy, in its mean relaxation rate, while (ii) its shape is governed by two competing events: interfacial interactions, and crystalline ordering. Additionally, (iii) the dynamics of the δ-relaxation – assigned to the flip–flop rotation of the molecule about its short axis – deviates from bulk behaviour as the glass transition is approached for the confined material. These observations are rationalized within the framework of molecular dynamics as currently understood.

Molecular dynamics and morphology of confined 4-heptyl-4′-isothiocyanatobiphenyl liquid crystals

Molecular dynamics and orientational order of 4-heptyl-4′-isothiocyanatobiphenyl (7BT) in non-intersecting nano-pores of mean diameters from 4 nm to 10.5 nm are studied by a combination of Broadband Dielectric and Fourier-Transform Infrared Spectroscopy. The smectic E phase observed in bulk 7BT is replaced by short-range molecular order imposed by the surface potential within the pores. In contrast to bulk dielectric properties of 7BT, geometrical confinement leads to modification of the molecular dynamics. Two dielectric relaxation processes exhibiting Arrhenius-like thermal activation are detected for molecules in nanopores of mean diameters from 6 nm to 10.5 nm. The slower process is assigned to molecular reorientation around the short axis (δ-relaxation) whereas the faster dipolar relaxation process (β-process) is attributed to librational motion of the molecules close to the pore-walls. Infrared Transition Moment Orientational Analysis reveals different molecular arrangement in pores of diameters 10.5 nm compared to the molecules in 4 nm and 6 nm pores.

The interplay between inter- and intra-molecular dynamics in a series of alkylcitrates

The inter- and intra-molecular dynamics in a series of glass-forming alkylcitrates is studied by a combination of Broadband Dielectric Spectroscopy (BDS), Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR), Fourier-Transform Infrared (FTIR) spectroscopy and Differential Scanning Calorimetry (DSC). Analyzing the temperature dependencies of specific IR absorption bands in terms of their spectral position and the corresponding oscillator strengths enables one to unravel the intramolecular dynamics of specific molecular moieties and to compare them with the (primarily dielectrically) determined intermolecular dynamics. With decreasing temperature, the IR band positions of carbonyls (part of the core units) and H-bonded moieties of citrates show a red shift with a kink at the calorimetric glass transition temperature (Tg) while other moieties, whose dynamics are decoupled from those of the core units, exhibit a blue shift with nominal changes at Tg. The oscillator strength of all units in citrates depicts stronger temperature dependencies above Tg and in some, the ester linkage and H-bonded units show a change of slope at a temperature where structural and faster secondary relaxations merge. By that, a wealth of novel information is obtained proving the fundamental importance of intramolecular mobility in the process of glass formation, beyond coarse-grained descriptions.

Influence of the remanent polarisation on the liquid crystal alignment in composite films of ferroelectric poly(vinylidene fluoride-trifluoroethylene) and a cyanobiphenyl-based liquid crystal

ABSTRACT Polymer-dispersed liquid crystals (PDLCs) of ferroelectric poly(vinylidene fluoride-trifluoroethylene) and nematic 4-cyano-4ʹ-n-hexylbiphenyl (6CB) or 4-cyano-4ʹ-n-pentylbiphenyl (5CB) were prepared to study the effect of the remanent polarisation of the polymer on the liquid crystal alignment. We measured the macroscopic alignment of the liquid crystal molecules in the thickness direction by means of Infrared Transition-Moment Orientational Analysis. Electrical poling at 100 V/µm caused an increased order parameter up to 0.15. After subsequent annealing above the nematic-to-isotropic phase-transition temperature, the order parameter was reduced to 0.02. Nevertheless, the order parameter was still higher than for non-poled film indicating a slight orientation in thickness direction. Both values are lower than those expected from model calculations. In agreement with dielectric measurements, we attribute this result to the shielding effect of mobile charge carriers within the liquid crystal inclusions. GRAPHICAL ABSTRACT

Molecular Dynamics of Condensed (Semi-) Isolated Polymer Chains

While structure and conformation of condensed (semi-) isolated low molecular weight and polymeric molecules is well explored, nothing is known about their molecular dynamics as measured in a broad spectral range (1 Hz–1 MHz) and at widely varying temperature (200–400 K). This is achieved in this study by employing broadband dielectric spectroscopy (BDS) with nanostructured electrode arrangements, enabling electrode separations down to 35 nm. The approach offers, additionally, to determine in parallel the structure and conformation of the identical sample by scanning force microscopy (AFM). For the case of high molecular weight poly (2-vinylpyridine), it is demonstrated that even condensed (semi-) isolated polymer chains perform glassy dynamics, well comparable to the bulk state with the characteristic Vogel-Fulcher-Tammann temperature dependence; only \({\sim }10{-}15\) % of the mobile segments are weakly slowed down. The results are in full accord with the understanding that the length scale of the dynamic glass transition is \({\sim }2-3\) polymer segments. Complementarily, AFM and infrared (IR) spectroscopy are employed to examine the chain conformation and to analyze the interfacial interactions, respectively. The former yields a condensed coil conformation, the average volume of which agrees within limits of \(\pm \)10 % with that calculated for a single chain assuming bulk density. A fraction of \({\sim }30\) % of the segments is found to be in a 0.4-nm-layer being directly in contact with the solid substrate. From IR investigations it is deduced that only half of them are influenced by the substrate presumably due to Coulombic interactions.