Dynamics of Disordered Materials in Confined Geometries

This thesis is concerned with effects of geometrical confinement on dynamics in disordered materials in the nanoscopic range. Of particular interest is the dynamics around the liquid to glass transition. Two systems have been investigated: thin polymer films and polymer gels. The dynamics was examined over a broad range in time, 10-12 - 103 s, using photon correlation spectroscopy, dielectric spectroscopy and quasi-eleastic neutron scattering. The large scale structure of the gels was investigated using small angle neutron scattering. The effect of polymer chain confinement was examined for polystyrene films that are thinner than the equilibrium end-to-end distance of a single polymer chain. The experiments are the first study describing the form of the structural relaxations in thin free-standing polymer films. It shows that the relaxation becomes faster for thinner films, but the shape of the relaxation function remains similar to that of the bulk polymer. Cooperativity and confinement effects were also investigated in polymer gels systems consisting of propylene carbonate (PC) stabilized with poly(methyl methacrylate). The experiments reveal that the main structural relaxation of PC confined in the gel can be described by a simple cooperative model. An onset temperature for cooperative relaxations is identified to be located approximately 60 K above the glass transition temperature. Salt-doped polymer gel electrolytes are promising for application in electrochemical devices since they combine high ionic conductivity with mechanical stability. Experiments aiming at determining the molecular basis for the ion conduction mechanism in such systems are presented. A complex relaxational behavior was observed with at least three relaxation processes. It is concluded that there is a close relation between the diffusion of the solvent and the ionic conductivity

Glass transition relaxations in thin suspended polymer films

Using dielectric spectroscopy, we probe the structural relaxation processes in thin freely suspended polystyrene films. For thin films we observe a relaxation scenario distinctly different from that of bulk polystyrene. The main glass transition relaxation becomes faster with decreasing film thickness, indicating a reduction in the glass transition temperature. For thin films we observe an additional process with an Arrhenius temperature dependence. This process is unique for thin freely suspended films and is not present in bulk polystyrene

Dynamics of Disordered Materials in Confined Geometries

This thesis is concerned with effects of geometrical confinement on dynamics in disordered materials in the nanoscopic range. Of particular interest is the dynamics around the liquid to glass transition. Two systems have been investigated: thin polymer films and polymer gels. The dynamics was examined over a broad range in time, 10-12 - 103 s, using photon correlation spectroscopy, dielectric spectroscopy and quasi-eleastic neutron scattering. The large scale structure of the gels was investigated using small angle neutron scattering. The effect of polymer chain confinement was examined for polystyrene films that are thinner than the equilibrium end-to-end distance of a single polymer chain. The experiments are the first study describing the form of the structural relaxations in thin free-standing polymer films. It shows that the relaxation becomes faster for thinner films, but the shape of the relaxation function remains similar to that of the bulk polymer. Cooperativity and confinement effects were also investigated in polymer gels systems consisting of propylene carbonate (PC) stabilized with poly(methyl methacrylate). The experiments reveal that the main structural relaxation of PC confined in the gel can be described by a simple cooperative model. An onset temperature for cooperative relaxations is identified to be located approximately 60 K above the glass transition temperature. Salt-doped polymer gel electrolytes are promising for application in electrochemical devices since they combine high ionic conductivity with mechanical stability. Experiments aiming at determining the molecular basis for the ion conduction mechanism in such systems are presented. A complex relaxational behavior was observed with at least three relaxation processes. It is concluded that there is a close relation between the diffusion of the solvent and the ionic conductivity

Dielectric relaxation of Sorbitol revisited

International audienceWe have performed broad-band dielectric spectroscopy to study the merging of the alpha- and beta-relaxations in Sorbitol. To investigate this merging in detail the data were analysed both directly in frequency domain and after transforming the data to time domain. At the liquid-glass transition temperature, Tg, the standard analysis give a clear kink in the temperature dependence of the relaxation time of the beta-relaxation. However, using a more sophisticated analysis we show that the data can be equally well described by extrapolating the low temperature Arrhenius behaviour of the beta-relaxation to the merging region of the alpha- and beta-relaxations. The Sorbitol data is thus compatible with a scenario with no anomalous temperature behaviour in the merging region

Secondary relaxation in confined and bulk propylene carbonate

Dielectric loss spectra of propylene carbonate in bulk and dissolved in a polymer matrix are investigated with focus on the currently debated excess wing. The spatial restrictions caused by the polymer matrix slow down the main structural (α) relaxation, while the excess wing continuously transforms towards a shoulder with increasing spatial restrictions. We show that the excess wing in a consistent manner can be described by a secondary relaxation with only a weak dependence of the concentration of polymer. Thus the results imply that the excess wing is due to a secondary relaxation submerged in the α-relaxation

A unified picture of static and dynamic length scales in polymer solutions

After a thorough study of static and dynamic properties of polystyrene solution, and taking into account the local solvent viscosity, an incontrovertible support for the applicability of blob model from semidilute to high concentrated solutions, is presented. The study demonstrates the functionability of the blob model in predicting III/IV crossovers as a consequence of rodlike structure of the partial chain. The study predicts the theoritical picture of the length scales valid for highly concentrated solutions, with local solvent viscosity taken into account. The present study is also applicable to various macromolecular systems, including DNA and actin filament over wide range of polymer concentrations

The effect of molecular composition of xylan extracted from birch on its assembly onto bleached soft wood kraft pulp

The effect of the molecular structure of xylan on its assembly onto softwood kraft pulps was investigated. Various xylan-rich fractions were isolated from birch wood chips by different mild treatments using water (H 2 O), acetic acid (HAc) and sulfur dioxide (SO 2 ). The isolation involved prehydrolysis followed by alkaline extraction, with only the time, temperature and medium in the prehydrolysis step varied. After separation, the fractions were neutralised (pH 7) and some material agglomerated and became insoluble. Dynamic light scattering measurements revealed differences in aggregate size among the different fractions. The fractions that agglomerated to the greatest extent contained less glucuronic acid and a higher amount of lignin. Such fractions adsorbed to a significantly higher extent (25.5% compared to 5.0% for the lowest soluble fraction) onto bleached softwood kraft pulps in experiments performed in autoclaves. The adsorption was carried out with fixed process parameters (100\ub0C, pH 10, 120 min). The adsorption of various xylan fractions resulted in different surface morphology on cellulose microfibrils, as observed by AFM. Pulps treated with xylans that were soluble at pH 7 showed small differences from the control sample, which was treated in an autoclave without the addition of xylan. Pulps treated with xylans that agglomerated at pH 7 showed a greater difference in the amount of nanosized aggregates covering the surface. ESCA analysis of the chemical surface composition indicated that samples containing more lignin showed a greater shift towards carbon-carbon bonds. A novel sequence for a pulping process is suggested. Copyright \ua9 by Walter de Gruyter

Molecular Weight Dependence of Network Length Scales in Polymer Solutions

The generality of the blob concept using solutions of styrene chains with different molecular weights was investigated. It was found that the correlation lengths and their concentration dependence dramatically change for nondilute solutions with the decrease in chains of molecular weight. It was observed that these changes lead to new structures of the chain networks. The results show that chain networks also change with the decrease in the number of Kuhn segments