B. A. Wolf

Asymmetric polysulfone and polyethersulfone membranes: effects of thermodynamic conditions during formation on their performance

Abstract Membranes of polysulfone (PSU) and polyethersulfone (PES) were prepared from ternary and quaternary mixtures containing N , N -dimethylformamide (DMF) as solvent, water as non-solvent, and acetone (AC) as additive. The conditions for phase inversion and the desired phase separation mechanisms were selected on the basis of the phase behavior determined for the solvent/non-solvent/polymer systems. The influences of the composition of the casting solution, of the support, and of film thickness on structure and permeation properties of the membranes were analyzed by scanning electron microscopy (SEM) plus flux and separation experiments. The mechanisms of phase inversion that should prevail under the different conditions according to the measured phase diagrams were corroborated by means of light scattering experiments.

Effect of block copolymers on the interfacial tension between two ‘immiscible’ homopolymers

The effect of block copolymers on the interfacial tension σ was investigated for poly(ethylene oxide) (PEO)/poly(dimethylsiloxane) (PDMS) and for PDMS/polystyrene (PS). With PEO/PDMS the additives were the triblock copolymers PDMS-block-PEO-block-PDMS [P(DMS-EO-DMS)] and PEO-block-PDMS-block-PEO [P(EO-DMS-EO)]. For the former additive the number of monomeric units of the end blocks varied between 4 and 32 and that of the middle block between 23 and 77; these numbers are 70 and 52, respectively, for the latter. In the case of the system PS/PDMS, the diblock copolymer PS-block-PDMS [P(S-DMS)], consisting of 430 S and 68 DMS units, was studied. The effects turned out to be largest for the system PDMS/PEO/P(DMS-EO-DMS); it was therefore studied in greater detail within the temperature range of 70–150°C. Upon addition of increasing amounts of copolymer, σ falls rapidly to ∼ 10% of its initial value and levels off as the critical micelle concentration (< 0.5 wt% in the PEO phase at 100°C) is surpassed. Similarly, at a given concentration of the additive, σ approaches a limiting value as the number of monomeric units in the PDMS block is increased above 15. In contrast to the value of σ of the pure blend, which is practically independent of temperature, that of the ternary system increases markedly with temperature. The results are compared with the predictions of Vilgis and Noolandi.

Solubility of polymers

Polymers differ from low molecular weight compounds in a number of ways; among them are (i) the comparatively low thermal expansivities and compressibilities of their melts at higher temperatures, (ii) the frequent occurence of solidification to a glass instead of a crystal at low temperatures, and (iii) the fact that even chemically uniform polymer samples normally consist of a large number of species differing in molecular mass and architecture. The consequence of these peculiarities of polymers with respect to their solubility are discussed for the example of linear chain molecules. Firstly, the survey will review the experimental methods (some of which were specially developed for polymer solutions) for the determination of solubility limits and critical conditions. Then the general phenomena observed with systems containing high molecular weight material will be worked out and discussed theoretically. The main independent variables are: temperature, pressure, molar mass of the polymer and shear rate (for solubility in flowing systems). In addition, isotopic effects on the solubility will also be considered.

Complex miscibility behaviour for polymer blends in flow

Abstract Experimental observations of the effect of shear flow on the miscibility of binary polymer blends are compared to calculations based on a generalized Gibbs energy of mixing Gγ˙. This mixing free energy characterizes the steady state established at shear rateγ˙, as the sum of G z , the equilibrium Gibbs energy and E s , the energy the system stores while flowing.

Demixing of unsheared and sheared solutions of polystyrene in tert-butylacetate and the pressure influence on their flow behaviour

SummaryFor the system tert-butyl acetate/polystyrene (M= 670000) the phase separation behaviour (upper critical solution temperatures, upper critical solution pressures) is investigated by means of visual and turbidimetric cloud-point measurements and by viscometry. For near-critical concentrations the visually determined cloud-points are found at higher, for low concentrations at lower temperatures as compared with the turbidimetrically determined binodal temperature,Tbin Plots of the invers of the turbidity measured atTbin as a function of the polymer concentration,w2, yield a minimum from which the critical composition can be determined. The viscometrically obtained demixing points (break-down of the viscosity at normal and at elevated pressures) show that the polymer solutions are stabilized at moderate concentrations. Irrespective of the preselected temperatures and of the present shear-rates (90-640 s−1, the demixing pressures are reduced by almost 20 bar. At sufficiently dilute or concentrated solutions the viscometric demixing points approach the binodal points. The above influences of the shear-field are explained by the rupture of intermolecular segment/segment contacts, which will be most efficient in the neighbourhood of the critical concentration for the formation of entanglements. In the case of the homogeneous solutions, the viscosity coefficientn increases in a fairly exponential manner by a factor of 3-4 per 1000 bar within the range ofp,T,w2 and shear-rate under investigation (1–1000 bar, -25 to f20 °C, 4–10 wt-% and 25–640 s−1). In plots of logn or ofV* (volume of activation) vs. concentration, extra effects show up when the critical conditions are approached; they result in a “hump” in logn vs.wn2 and in a minimum in V* vs.w2.ZusammenfassungFür das System tert-Butylacetat/Polystyrol (M = 670000) wird die Phasentrennung (obere kritische Entmischungstemperatur, oberer kritischer Entmischungsdruck) mit Hilfe von visuellen und turbidimetrischen Trübungsmessungen sowie Viskositätsmessungen untersucht. Im Vergleich zur turbidimetrisch bestimmten Binodaltemperatur,Tbin, liegen die visuell bestimmten Trübungstemperaturen für annähernd kritische Konzentrationen bei höheten, für niedrige Konzentrationen bei niedrigeren Werten. Auftragungen der reziproken Turbidität bei Tbin als Funktion der Polymerkonzentration,w2, zeigen ein Minimum, aus dem die kritische Konzentration bestimmt werden kann. Die viskosimetrisch erhaltenen Entmischungspunkte (Zusammenbruch der Viskosität bei Normaldruck und bei erhöhtem Druck) zeigen, daß die Polymerlösungen bei mittleren Konzentrationen stabilisiert werden. Unabhängig Von Temperatur und Schergefälle (90-640 s−1) liegen die Entmischungsdrücke um ca. 20 bar tiefer. Bei verdünnten oder konzentrierten Lösungen verringert sich die Differenz zwischen viskosimetrischen Entmischungspunkten und Binodalpunkten. Die Einflüsse des Schergefälles werden durch das Zerreißen von intermolekularen Segment/Segment-Kontakten erklärt. Dieser Effekt des Zerreißens von Kontakten ist besonders ausgeprägt in dem Konzentrationsbereich, in dem die Moleküle beginnen sich zu übetlappen. Im untersuchtenp,T,ω2 und Schergef←lebereich (1 – 1000 bar, -25 bis +20°C, 4–10 Gew.% und 25–640 s−1) nimmt die Viskosität der homogenen Lösungen annähernd exponentiell um einen Faktor 3-4 pro 1000 bar zu. In Auftragungen von log ν oder von V+ (Fließaktivierungsvolumen) gegen die Konzentration treten bei Annäherung an die kritischen Bedingungen Zusatzeffekte auf, die zu einem „Buckel” in den log ν (ω2)-Kurven und zu einem Minimum in V* (ω2) führen.

Calculation of the phase separation behavior of sheared polymer blends

The influence of shear on the phase diagrams of model blends of polymer A end polymer B, exhibiting phase separation upon heating, was calculated on the basis of the generelized Gibbs energy of mixing G γ . , characterizing the study state established at shear rate γ . , as the sum of G z , the equilibrium Gibbs energy, end E a , the energy the system can store while flowing. The results obtained for the two model systems A150/B200 and A75/B200) the figures denote the molar masses in kg/mol) are qualitatively identical. With increasing shear drate the homogeneous region expands first, then shrinks below its equilibrium value within a certain γ . range

Solvent quality as reflected in concentration‐ and temperature‐dependent Flory–Huggins interaction parameters

Flory-Huggins interaction parameters (χ) between poly(dimethylsiloxane) (weight-average molecular weight = 152 kg/mol) and various solvents (methyl ethyl ketone, toluene and n-octane) were determined as a function of composition and temperature with vapor-pressure measurements. These data, complemented by independent information for dilute and very concentrated solutions, serve as the basis for a discussion of solvent quality via different theoretical relations. Regardless of polymer concentration, the χ values fall from methyl ethyl ketone via toluene to n-octane, the ketone being the worst solvent and the hydrocarbon being the best solvent. The variation of χ with composition and temperature is complex. Within the range of moderate polymer concentrations, the influences of composition decrease with increasing solvent quality. Additional effects become noticeable at the ends of the composition scale. The enthalpy parts (χ H ) and entropy parts (χ S ) of the Flory-Huggins interaction parameter, obtained from χ(T), vary considerably with composition and change their sign in some cases; these constituents of the Flory-Huggins interaction parameter do not permit a direct assessment of solvent quality. A clear-cut picture is, however, regained with a comparison of the interdependence of χ S and χ H . The elimination of explicit concentration influences re-establishes the order in the solvent quality setup via χ

Ionic Polymers Based on Dextran: Hydrodynamic Properties in Aqueous Solution and Solvent Mixtures

Hydrodynamic properties of a series of ionic polysaccharides with different charge density but the same molecular weight have been evaluated in salt-free aqueous solution and aqueous/organic solvent mixtures by means of capillary viscometry. The polyelectrolytes investigated contain quaternary ammonium salt groups, N-ethyl-N,N-dimethyl-2-hydroxypropylammonium chloride, attached to a dextran backbone. The experimental viscometric data have been plotted in terms of the Wolf method. The results show that the experimental data fit well with this model and allow the calculation of intrinsic viscosities and other hydrodynamic parameters, which provide new information about the dependence of the polyion conformation on its polyion charge density as well as on solvent composition.

Making Floryr–Huggins Practical: Thermodynamics of Polymer-Containing Mixtures

The theoretical part of this article demonstrates how the original Flory–Huggins theory can be extended to describe the thermodynamic behavior of polymer-containing mixtures quantitatively. This progress is achieved by accounting for two features of macromolecules that the original approach ignores: the effects of chain connectivity in the case of dilute solutions, and the ability of polymer coils to change their spatial extension in response to alterations in their molecular environment. In the general case, this approach leads to composition-dependent interaction parameters, which can for most binary systems be described by means of two physically meaningful parameters; systems involving strongly interacting components, for instance via hydrogen bonds, may require up to four parameters. The general applicability of these equations is illustrated in a comprehensive section dedicated to the modeling of experimental findings. This part encompasses all types of phase equilibria, deals with binary systems (polymer solutions and polymer blends), and includes ternary mixtures; it covers linear and branched homopolymers as well as random and block copolymers. Particular emphasis is placed on the modeling of hitherto incomprehensible experimental observations reported in the literature.

Pressure dependence of the miscibility of poly(vinyl methyl ether) and polystyrene: Theoretical representation

The present calculations were performed on the basis of the Sanchez-Lacombe-Balasz lattice fluid theory. The two system specific parameters e12* and δe* required for that purpose have been obtained from the spinodal temperatures measured (SANS) for mixtures of poly(vinyl methyl ether) (PVME) and deuterated polystyrenes (d-PS) by Schwahn and coworkers. The experimental data reported for atmospheric pressure and six representatives of the present system are well described theoretically, where e12* does not depend on molar mass and δe* decreases only slightly as the chain length of d-PS is raised. The measured pressure influences on the spinodal conditions correspond to an approximately linear reduction of δe* with increasing P; this observation should reflect the volume changes associated with the formation of specific interactions. According to the present calculations the critical composition shifts markedly towards pure PVME as P is raised. Since experimental data are commonly expressed in terms of the F...