Phasenverhalten von Polyethylen in Mischsystemen

Die vorliegende Arbeit beschäftigt sich mit dem Phasenverhalten von Polyethylen (PE) in nicht-reaktiven und in reaktiven Systemen. Von drei eng verteilten Polyethylenen (Mw = 6,4, 82 bzw. 380 kg/mol) in n-Hexan sowie für das System 2,2-Dimethylbutan / PE 82 wurde die Entmischung in Abhängigkeit von der Zusammensetzung, dem Druck und der Temperatur experimentell bestimmt. Die Modellierung der Trübungskurven erfolgte nach der Theorie von Sanchez und Lacombe. Dieser Ansatz beschreibt die Ergebnisse qualitativ und kann in einem engen Temperatur- und Druckbereich für gegebenes Molekulargewicht die kritische Temperatur und den kritischen Druck quantitativ vorhersagen. Durch Extrapolation der kritischen Temperatur der verschiedenen Lösungen von PE in n-Hexan auf unendliches Molekulargewicht nach Shultz-Flory wurde im Druckbereich von 20 bis 100 bar und im Temperaturbereich von 130 bis 200 °C eine Grenzlinie bestimmt. Diese Linie trennt unabhängig vom Molekulargewicht des Polymers und der Zusammensetzung der Mischung das Zweiphasengebiet vom homogenen Bereich. Im Fall des Mischlösungsmittels n-Hexan / 2,2-Dimethylbutan wurde für eine annähernd kritische Polymerkonzentration die Abhängigkeit der Entmischungsbedingungen von der Zusammensetzung untersucht. Durch einfache Erweiterung der Sanchez-Lacombe-Theorie und Einführen eines Fitparameters konnte das ternäre System beschrieben werden. An einer breit verteilten PE-Probe wurden Experimente zur Fraktionierung von PE in n-Hexan durchgeführt. Die Analyse der in den koexistenten Phasen enthaltenen Polymere lieferte Informationen über die Konzentration und die Molekulargewichtsverteilung des PE in diesen Phasen sowie die kritische Zusammensetzung der Mischung. Von verschiedenen PE-Lösungen (Mw = 0,5 kg/mol) wurde die polymerisationsinduzierte Phasenseparation in Isobornylmethacrylat mit und ohne Vernetzer untersucht. Mit 15 Gew.-% PE und in Abwesenheit von Vernetzer findet die Entmischung erst bei hohen Umsätzen statt. Die Charakterisierung der resultierenden Proben zeigte, dass sich etwas mehr als 5 Gew.-% PE im Polyisobornylmethacrylat lösen. Die Glasübergangstemperaturen der Polymermischungen steigen mit steigender Vernetzer- und sinkender Polyethylenkonzentration. Bei Proben mit 15 Gew.-% PE zeigte sich folgendes: 5 Gew.-% Vernetzer führen zu großen PE-Bereichen (150 - 200 nm) in der Matrix und der Kristallinitätsgrad ist gering. Bei der Polymermischung mit 10 Gew.-% Vernetzer bilden sich sehr kleine Polyethylenkristalle (< 80 nm) und der Kristallinitätsgrad ist hoch. Ohne Vernetzer hängt der Kristallinitätsgrad - wie bei reinem PE - von der Abkühlrate ab, mit Vernetzer ist er von ihr unabhängig.

Thermodynamics of aqueous biopolymer solutions and fractionation of Dextran

Flory-Huggins interaction parameters and thermal diffusion coefficients were measured for aqueous biopolymer solutions. Dextran (a water soluble polysaccharide) and bovine serum albumin (BSA, a water soluble protein) were used for this study. The former polymer is representative for chain macromolecules and the latter is for globular macromolecules. The interaction parameters for the systems water/dextran and water/BSA were determined as a function of composition by means of vapor pressure measurements, using a combination of headspace sampling and gas chromatography (HS-GC). A new theoretical approach, accounting for chain connectivity and conformational variability, describes the observed dependencies quantitatively for the system water/dextran and qualitatively for the system water/BSA. The phase diagrams of the ternary systems water/methanol/dextran and water/dextran/BSA were determined via cloud point measurements and modeled by means of the direct minimization of the Gibbs energy using the information on the binary subsystems as input parameters. The thermal diffusion of dextran was studied for aqueous solutions in the temperature range 15 < T < 55 oC. The effects of the addition of urea were also studied. In the absence of urea, the Soret coefficient ST changes its sign as T is varied; it is positive for T > 45.0 oC, but negative for T < 45.0 oC. The positive sign of ST means that the dextran molecules migrate towards the cold side of the fluid; this behavior is typical for polymer solutions. While a negative sign indicates the macromolecules move toward the hot side; this behavior has so far not been observed with any other binary aqueous polymer solutions. The addition of urea to the aqueous solution of dextran increases ST and reduces the inversion temperature. For 2 M urea, the change in the sign of ST is observed at T = 29.7 oC. At higher temperature ST is always positive in the studied temperature range. To rationalize these observations it is assumed that the addition of urea opens hydrogen bonds, similar to that induced by an increase in temperature. For a future extension of the thermodynamic studies to the effects of poly-dispersity, dextran was fractionated by means of a recently developed technique called Continuous Spin Fractionation (CSF). The solvent/precipitant/polymer system used for the thermodynamic studies served as the basis for the fractionation of dextran The starting polymer had a weight average molar mass Mw = 11.1 kg/mol and a molecular non-uniformity U= Mw / Mn -1= 1.0. Seventy grams of dextran were fractionated using water as the solvent and methanol as the precipitant. Five fractionation steps yielded four samples with Mw values between 4.36 and 18.2 kg/mol and U values ranging from 0.28 to 0.48.

Measurement and modeling of cloud condensation nuclei in continental air

Atmospheric aerosol particles serving as cloud condensation nuclei (CCN) are key elements of the hydrological cycle and climate. Knowledge of the spatial and temporal distribution of CCN in the atmosphere is essential to understand and describe the effects of aerosols in meteorological models. In this study, CCN properties were measured in polluted and pristine air of different continental regions, and the results were parameterized for efficient prediction of CCN concentrations.The continuous-flow CCN counter used for size-resolved measurements of CCN efficiency spectra (activation curves) was calibrated with ammonium sulfate and sodium chloride aerosols for a wide range of water vapor supersaturations (S=0.068% to 1.27%). A comprehensive uncertainty analysis showed that the instrument calibration depends strongly on the applied particle generation techniques, Köhler model calculations, and water activity parameterizations (relative deviations in S up to 25%). Laboratory experiments and a comparison with other CCN instruments confirmed the high accuracy and precision of the calibration and measurement procedures developed and applied in this study.The mean CCN number concentrations (NCCN,S) observed in polluted mega-city air and biomass burning smoke (Beijing and Pearl River Delta, China) ranged from 1000 cm−3 at S=0.068% to 16 000 cm−3 at S=1.27%, which is about two orders of magnitude higher than in pristine air at remote continental sites (Swiss Alps, Amazonian rainforest). Effective average hygroscopicity parameters, κ, describing the influence of chemical composition on the CCN activity of aerosol particles were derived from the measurement data. They varied in the range of 0.3±0.2, were size-dependent, and could be parameterized as a function of organic and inorganic aerosol mass fraction. At low S (≤0.27%), substantial portions of externally mixed CCN-inactive particles with much lower hygroscopicity were observed in polluted air (fresh soot particles with κ≈0.01). Thus, the aerosol particle mixing state needs to be known for highly accurate predictions of NCCN,S. Nevertheless, the observed CCN number concentrations could be efficiently approximated using measured aerosol particle number size distributions and a simple κ-Köhler model with a single proxy for the effective average particle hygroscopicity. The relative deviations between observations and model predictions were on average less than 20% when a constant average value of κ=0.3 was used in conjunction with variable size distribution data. With a constant average size distribution, however, the deviations increased up to 100% and more. The measurement and model results demonstrate that the aerosol particle number and size are the major predictors for the variability of the CCN concentration in continental boundary layer air, followed by particle composition and hygroscopicity as relatively minor modulators. Depending on the required and applicable level of detail, the measurement results and parameterizations presented in this study can be directly implemented in detailed process models as well as in large-scale atmospheric and climate models for efficient description of the CCN activity of atmospheric aerosols.