E. Bartsch

Re-entrant Glass Transition in a Colloid-Polymer Mixture with Depletion Attractions

Performing light scattering experiments we show that introducing short-ranged attraction to a colloid suspension of nearly hard spheres by addition of a free polymer produces new glass-transition phenomena. We observe a dramatic acceleration of the density fluctuations amounting to the melting of a colloidal glass. Upon increasing the strength of the attractions the system freezes into another nonergodic state sharing some qualitative features with gel states occurring at lower colloid packing fractions. This re-entrant glass transition is in qualitative agreement with recent theoretical predictions.

Dynamic light scattering study of concentrated microgel solutions as mesoscopic model of the glass transition in quasiatomic fluids

This paper presents a light scattering study of the dynamics of concentrated solutions of nearly monodisperse (σ≊0.16) spherical micronetwork particles consisting of highly cross‐linked polystyrene dissolved in carbon disulfide, i.e., a ‘‘good’’ solvent. Above volume fractions of φ=0.50 the intermediate scattering function, measured over a time window of 10−7 to 103 s using the ALV5000 correlator, decays in two steps and shows indications of nonergodic behavior for φ≥0.64. Such behavior is typical for glass forming systems and has recently been found close to the glass transition of a hard sphere colloidal system [W. van Megen and P. N. Pusey, Phys. Rev. A 43, 5429 (1991)]. Thus the introduced system can be used for modeling the glass transition of atoms on a mesoscopic scale. The traditional analysis of structural relaxation in terms of a Kohlrausch–Williams–Watts distribution yields a mean relaxation time which follows the empirical Mooney equation as a function of concentration and thus corresponds to Vogel–Fulcher–Tammann behavior. However, the necessity to add an unspecified ‘‘intermediate’’ process between the short and long time KWW decays demonstrates the limitations of this ‘‘pragmatic’’ approach. The mode coupling theory of the glass transition interprets the intermediate scattering function consistently over nearly seven decades in time, the intermediate region corresponding to the crossover from β to α relaxation (von Schweidler law). The critical volume fraction of 0.636 derived by this analysis corresponds to a value of 0.59 for an ideal monodisperse system which is well in accord with other experimental and computer simulation studies of the glass transition of atomic systems.

Multispeckle autocorrelation spectroscopy and its application to the investigation of ultraslow dynamical processes

We describe a modification of conventional dynamic light scattering (DLS) using a CCD camera as optical area detector. Scattered intensity autocorrelation functions are determined using both ensemble‐ and time‐averaging. Therefore, the new setup allows for much shorter measurement times compared to conventional DLS. Our apparatus has been checked by investigating a dilute dispersion of polystyrene (PS)–microgel lattices in glycerol. The results agree well with DLS measurements using the standard setup. Further, we present data for ultraslow dynamical processes in highly concentrated nonergodic suspensions of PS–microgel lattices, where the new technique, due to its shorter measurement time and much better statistical accuracy as compared to conventional DLS, is most useful.

Relaxation and phonons in viscous and glassy orthoterphenyl by neutron scattering

We present an extended set of incoherent neutron scattering measurements on the van der Waals liquido-terphenyl, obtained by time-of-flight and backscattering spectroscopy. In the supercooled liquid regime, data from three instruments are combined and analysed in terms of the selfcorrelationS(Q, t). In the time range 1...100 ps, the crossover from α-to β-relaxation is well described by the masterfunction of mode coupling theory, and fitted parameters are consistent with the previously established critical temperatureTc [Z. Phys. B83, 175 (1991)]. In the glassy regime, vibrations are harmonic and can be described by a density of states. Deviations at lowQ are quantitatively explained by a multiple scattering simulation. Throughout the article, experimental difficulties are discussed in some detail.

The glass transition dynamics of polymer micronetwork colloids. A mode coupling analysis

We studied the glass transition dynamics of polystyrene micronetwork colloids with an average cross-link density of 1:50 (inverse number of monomer units between cross-links) and a hydrodynamic radius of about 100 nm by dynamic light scattering. Special emphasis was put on extracting correct intermediate scattering functions in a system that might be termed as partially nonergodic. By using a charge-coupled device camera as a detector and averaging the intensity autocorrelation functions of 50 simultaneously monitored speckles the duration of the experiment could be significantly reduced as compared to the conventional “brute force’’ ensemble averaging. Despite some striking similarities to the behavior of hard sphere colloids the glass transition scenario in our system differs in several respects when analyzing the dynamics in the glass transition regime within the framework of mode coupling theory. Besides the existence of structural relaxation processes above φc we find indications that additional dynamic processes modify the β relaxation in the glassy phase. Our findings cannot be explained by the occurrence of hopping processes, but are rationalized via an increase of the particle compressibility and the surface friction on decreasing the cross-link density from its hard sphere limit.We studied the glass transition dynamics of polystyrene micronetwork colloids with an average cross-link density of 1:50 (inverse number of monomer units between cross-links) and a hydrodynamic radius of about 100 nm by dynamic light scattering. Special emphasis was put on extracting correct intermediate scattering functions in a system that might be termed as partially nonergodic. By using a charge-coupled device camera as a detector and averaging the intensity autocorrelation functions of 50 simultaneously monitored speckles the duration of the experiment could be significantly reduced as compared to the conventional “brute force’’ ensemble averaging. Despite some striking similarities to the behavior of hard sphere colloids the glass transition scenario in our system differs in several respects when analyzing the dynamics in the glass transition regime within the framework of mode coupling theory. Besides the existence of structural relaxation processes above φc we find indications that additional dyna...

Diffusional enhancement of holograms: phenanthrenequinone in polycarbonate

As a further development of the manufacture of holographic gratings by the diffusion of phenanthrenequinone in polymer glass, the traditionally employed poly(methyl methacrylate) is replaced by poly(bisphenol-A-carbonate). The post-exposure growth and decay of the gratings in the two materials are compared at different spatial periods and temperatures. The medium with polycarbonate demonstrates a fortunate combination of significantly improved stability of the gratings and their faster development. Our results indicate that the polycarbonate materials promise to be suitable media for holographic optical elements and data storage.

An alternative route to highly concentrated, freely flowing colloidal dispersions

Dense colloidal dispersions exhibit fluid states due to weak attractive interactions among particles even at particle volume fractions far above the colloidal glass transition. Here we demonstrate that this opens up a new route to manufacture highly concentrated, freely flowing dispersions. We have studied the rheological properties of two model dispersions in the dense, fluid state: PS-microgel particles suspended in an isorefractive organic solvent allowing for light scattering experiments and an aqueous polymer latex dispersion with short range repulsive interactions based on a commercial polymer latex system. Both systems essentially behave like hard spheres, their zero-shear viscosity diverges at a volume fraction ϕ = 0.58, linear viscoelastic behavior is well-described by the mode coupling theory and the absolute values of the plateau moduli are close to those reported for other hard sphere systems. Fluidization was achieved by introducing weak depletion attraction among particles via addition of non-adsorbing polymers to the continuous phase. Fluid states were observed up to ϕ ≈ 0.69 for the microgel and ϕ ≈ 0.644 for the aqueous system. At a given particle loading a minimum viscosity was achieved at polymer concentrations below the overlap concentration cp*. For the aqueous dispersion fluidization was observed for a broad range of polymer molecular weights Mw and the respective viscosity minimum did not vary systematically with Mw. The low viscosity values thus achieved for nearly monomodal systems could so far only be obtained for dispersions with broad multimodal particle size distribution, demonstrating the competitive nature of the new concept.

Colloidal polystyrene micronetwork spheres — a new mesoscopic model of the glass transition in simple liquids

The collective and single particle dynamics of highly concentrated polystyrene micronetwork spheres of 200 nm diameter as measured by photon correlation spectroscopy (PCS) and forced Rayleigh scattering (FRS), respectively, show the same qualitative features as known for molecular liquids or colloidal suspensions close to the glass transition. A two step decay of the density fluctuations is observed with PCS, which can be quantitatively interpreted by mode coupling theory with a critical volume fraction ϕc of 0.64. At the same volume fraction a glass transition is indicated independent of any theory through specific changes in the intensity fluctuation pattern. The long time self-diffusion coefficient, as determined by FRS, decreases over about three orders of magnitude in a small volume fraction interval between 0.55 and 0.68. However, traces of ergodicity restoring structure relaxation processes are found even beyond ϕc = ϕg.

Signatures of the glass transition in a van der Waals liquid seen by neutrons and NMR

Mode coupling theory (MCT) of the glass transition predicts the existence of a dynamic anomaly of supercooled liquids at a characteristic temperature Tc well above the caloric glass transition temperature Tg. A series of neutron scattering and NMR experiments on van der Waals liquids - a typical example of which is orthoterphenyl (OTP) - have been performed to test three predictions and to point out signatures of the glass transition. This contribution is meant to review our results in a concise manner: there is strong evidence for a Tc according to MCT from Debye-Waller factors, from line shape studies of the dynamic structure factor and from a comparison of self-diffusion and viscosity. NMR relaxation below Tc shows that there is at least one secondary process which so far is not accounted for by MCT and that the supercooled liquid becomes truely nonergodic closely above Tg on a time scale of a few seconds.

Comparative study of the Debye−Waller factor anomaly at the glass transition of isotopically substituted orthoterphenyls

Abstract We report on neutron-scattering measurements of the Debye—Waller factor of three isotopically substituted glassy orthoterphenyls: fully protonated, fully deuterated and selectively deuterated at the lateral phenyl rings. From the comparison of the observed anomaly in the glass transition region (β-process) found in the three systems, we can exclude any intramolecular phenyl-ring dynamics as the dominant motional mechanism for the β-process. The data thus support mode-coupling theory proposing a center-of-mass motion as precursor process of the glass transition.