Emmanuel Urandu Mapesa

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.

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.

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.

Impact of inter- and intramolecular interactions on the physical stability of indomethacin dispersed in acetylated saccharides.

Differential scanning calorimetry (DSC), broadband dielectric (BDS), and Fourier transform infrared (FTIR) spectroscopies as well as theoretical computations were applied to investigate inter- and intramolecular interactions between the active pharmaceutical ingredient (API) indomethacin (IMC) and a series of acetylated saccharides. It was found that solid dispersions formed by modified glucose and IMC are the least physically stable of all studied samples. Dielectric measurements showed that this finding is related to neither the global nor local mobility, as the two were fairly similar. On the other hand, combined studies with the use of density functional theory (DFT) and FTIR methods indicated that, in contrast to acetylated glucose, modified disaccharides (maltose and sucrose) interact strongly with indomethacin. As a result, internal H-bonds between IMC molecules become very weak or are eventually broken. Simultaneously, strong H-bonds between the matrix and API are formed. This observation was used to explain the physical stability of the investigated solid dispersions. Finally, solubility measurements revealed that the solubility of IMC can be enhanced by the use of acetylated carbohydrates, although the observed improvement is marginal due to strong interactions.

Structure and Dynamics of Asymmetric Poly(styrene-b-1,4-isoprene) Diblock Copolymer under 1D and 2D Nanoconfinement

The impact of 1- and 2-dimensional (2D) confinement on the structure and dynamics of poly(styrene-b-1,4-isoprene) P(S-b-I) diblock copolymer is investigated by a combination of Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Grazing-Incidence Small-Angle X-ray Scattering (GISAXS), and Broadband Dielectric Spectroscopy (BDS). 1D confinement is achieved by spin coating the P(S-b-I) to form nanometric thin films on silicon substrates, while in the 2D confinement, the copolymer is infiltrated into cylindrical anodized aluminum oxide (AAO) nanopores. After dissolving the AAO matrix having mean pore diameter of 150 nm, the SEM images of the exposed P(S-b-I) show straight nanorods. For the thin films, GISAXS and AFM reveal hexagonally packed cylinders of PS in a PI matrix. Three dielectrically active relaxation modes assigned to the two segmental modes of the styrene and isoprene blocks and the normal mode of the latter are studied selectively by BDS. The dynamic glass transition, related to the segmental modes of the styrene and isoprene blocks, is independent of the dimensionality and the finite sizes (down to 18 nm) of confinement, but the normal mode is influenced by both factors with 2D geometrical constraints exerting greater impact. This reflects the considerable difference in the length scales on which the two kinds of fluctuations take place.

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.

Broadband Dielectric Spectroscopy in nano-(bio)-physics

Broadband Dielectric Spectroscopy as one of the major tools in molecular physics benefits from the extraordinary advantage that its sensitivity increases with decreasing thickness of a sample capacitor and hence with a decreasing amount of sample material. This enables one for instance to carry out broadband spectroscopic measurements on quasi-isolated polymer coils in nano-structured capacitor arrangements, having thicknesses as small as 10 nm. It is demonstrated that for polymers like atactic poly-(methyl methacrylate) (PMMA) or poly-2-vinyl-pyridine (P2VP) the dynamic glass transition can be measured for (averaged) sample thickness as small as ~ 2 nm in a wide spectral range (10 mHz to 10 MHz) and at temperatures between 150 K to 350 K. No shift of the mean relaxation rate and no broadening of the relaxation time distribution function are found compared to the bulk liquid. - Electrode polarization is a ubiquitous phenomenon which takes place at the interface between a metal and an ionic conductor. A quantitative theory will be presented, which enables one to deduce from its characteristic frequency, temperature and concentration dependencies - by use of a novel formula - the bulk conductivity of the ion conducting liquid under study. It is shown that the electrical relaxation processes take place within a nanometric layer at the (ionic conductor/metal) interface which can be analysed in detail.