Martin Tress

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.

The glass transition of thin polymer films in relation to the interfacial dynamics

As opposed to measurements on the glass transition of a polymer in the bulk, measurements of thin polymer layers reflect--due to the alterations of the glassy dynamics at the confining interfaces--several contributions acting together to give the net response of a polymer film. This fundamental difference is exemplified in detail for the particular case of broadband dielectric spectroscopy, an experimental tool extensively employed to investigate the glassy dynamics of polymers under condition of geometrical confinement.

Segmental and chain dynamics in nanometric layers of poly(cis-1,4-isoprene) as studied by broadband dielectric spectroscopy and temperature-modulated calorimetry

Segmental and chain dynamics in nanometric (7–400 nm) layers of poly (cis-1,4-isoprene) (PI) are analyzed by Broadband Dielectric Spectroscopy (BDS) and temperature-modulated AC calorimetry. While for the segmental mode, taking place at the length scale of 2–3 polymer segments and corresponding to the dynamic glass transition, no dependence on the layer thickness and molecular weight is found, the normal mode, reflecting fluctuations of the end-to-end vector of the chain, shows pronounced effects: (i) it strongly varies in its relaxation strength with the layer thickness; (ii) for polymers having a molecular weight Mw comparable to M*, the critical molecular weight marking the onset of reptation dynamics, the mean spectral position does not change with the thickness, (iii) in contrast, polymers with Mw > M* are found to be severely influenced in their relaxation strength and the mean spectral position of the normal mode relaxation, and (iv) it is proven that the concentration of the polymer solution out of which the layers are prepared by spincoating has a hitherto unrecognized impact on the chain dynamics in (one-dimensional) nanometric confinement. These results prove that the dynamic glass transition in thin layers of PI is not influenced by nanometric confinement, while the chain dynamics are altered in a manifold of ways due to interactions with the surface of the underlying substrate.

Fine-tuning of the dielectric properties of polysiloxanes by chemical modification

A series of novel polysiloxanes is presented, the glass transition temperatures and dielectric properties of which are systematically fine-tuned by utilizing thiol-ene post-polymerization reactions. The pendant vinyl groups of a high molecular weight polymethylvinylsiloxane P1 were exhaustively reacted with the thiol compounds 1-butanethiol (2) and 3-mercaptopropionitrile 3 both separately to give polymers P2 and P3, respectively, as well as in various ratios x, y so as to create materials P2xP3y with greatly differing contents of the polar nitrile group (y). All modifications proceeded quantitatively as was confirmed by 1H NMR spectroscopy. Because of the presence of the polarizable thioether and nitrile groups, the resulting siloxane polymers exhibit permittivity ranging from e′ = 4.7 to 18.4 for P2 and P3, respectively. The e′ values of all polymers P2xP3y carrying more than one kind of thiolether group lie within this range. Additionally, broadband dielectric spectroscopy measurements of P2, P21P31 and P3 have been conducted in the temperature range from −150 °C to 60 °C and the frequency range from 0.1 Hz to 1 MHz. Due to their high permittivity, polymers P2, P2xP3y and P3 are attractive candidates for dielectric elastomer actuators and flexible electronics.

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.

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 of Polymers at Nanometric Length Scales: From Thin Layers to Isolated Coils

The (dynamic) glass transition of polymers in nanometer thin layers is both a prevailing but as well a highly controversial topic. In the current review the literature for the most studied case of polystyrene (as freestanding films or as deposited and suspended layers) will be discussed. Based on this, the extraordinary impact of sample preparation is immediately evident and outlined in detail. Recent results are presented on nanometric thin (≥5 nm) layers of polystyrene (PS) having widely varying molecular weights and polymethylmethacrylate (PMMA) deposited on different substrates. For the dielectric measurements two sample geometries are employed: the conventional technique using evaporated electrodes and a recently developed approach taking advantage of silica nanostructures as spacers. All applied methods deliver the concurring result that deviations from glassy dynamics and from the glass transition of the bulk never exceed margins of ±3 K independent of the layer thickness, the molecular weight of the polymer under study and the underlying substrate. Novel experiments are described on thin layers of polyisoprene, a type A polymer, having relaxation processes on two different length scales, the segmental and the normal mode. A further exciting perspective is the measurement of the dynamics of isolated polymer coils, for which first results will be presented.

Molecular Dynamics in 1- and 2-D Confinement as Studied for the Case of Poly(Cis-1,4-Isoprene)

Broadband Dielectric Spectroscopy (BDS) is used to probe the molecular dynamics of Type A polymer, poly(cis-1,4-isoprene), when confined in the 1-dimensional (1D) exploring space of thin layers and the 2-dimensional (2D) constraining geometry of unidirectional anodic aluminum oxide (AAO) nanopores. For both cases, it was observed that the structural relaxation remains bulk-like in its mean relaxation rate, although the distribution of its relaxation times is broadened in 2D confinement. Furthermore, the fluctuation of the end-to-end vector is interrupted, with the 1D case being relatively less pronounced. By this clear-cut comparison, it is demonstrated that the effects of confinement on molecular dynamics depend, inter alia, on the dimensionality of the restricting space.

Glass Transition of Polymers with Different Architectures in the Confinement of Nanoscopic Films

The dynamic properties of nanoscopic polymeric films can significantly differ from the well-known bulk properties. In general, with decreasing film thickness the surface to volume ratio increases tremendously and interfacial interactions are expected to dominate the molecular dynamics of geometrically confined polymers. On the one hand, attractive interfacial interactions can inhibit cooperative dynamics and lead to a rise in \({T_g}\). On the other hand, repulsive interactions may depress \({T_g}\). However, the order of magnitude of the \({T_g}\) aberration in nanoscopic films is quite controversially discussed and some scientists even have doubt in the existence of confinement effects for films exceeding 10 nm in thickness. In the last few years, several factors were identified which may mimic confinement effects such as plasticizer effects due to solvent residues, degradation or oxidation processes and crosslinking. In this chapter we try to give a review about the determination and complexity of the glass transition of polymers in nanoscopic films and the unique role of temperature-dependent ellipsometry with its advantages but also methodical challenges therein.