Frequency dependent linear viscoelastic properties of ordered polystyrene latices

An experimental investigation of linear rheological properties of monodisperse ordered latex dispersions has been carried out. Measurements of the dynamic shear modulus and the dynamic viscosity in the frequency range between 0.04 Hz and 2500 Hz are reported. The volume fraction and the electrolyte concentration were variables in the experiment. The results show that a description of the rheological behaviour with a simple linear phenomenological equation is not possible. The measured dynamic moduli can only be described with a great number of relaxation times

Microstructures in phase-inversion membranes. Part I. Formation of macrovoids

A new mechanism for the formation of macrovoids in phase-inversion membranes is proposed. It is based on the observed difference in type of demixing of a thin film of a polymer solution when immersed in a nonsolvent bath: delayed or instantaneous demixing. The explanation for macrovoid formation assumes local conditions of delayed demixing in front of a certain layer of nuclei already formed, due to a change in the interfacial compositions at the advancing coagulation front, as compared to the original composition at the interface film-bath. Effects of variations in membrane formation conditions which can be calculated using the model of diffusive mass transport in thin films of polymeric solutions in combination with phase separation in phase-inversion membranes, completely support the mechanism as proposed

A correction procedure for the errors in single-crystal intensities due to the inhomogeneity of the primary X-ray beam

Graphite monochromators are known to give rise to non-homogeneous primary X-ray beams. When intensities of single crystals are measured the effective cross section of a non-spherical crystal in the X-ray beam depends on its orientation in the beam. Therefore, systematic errors in the measured integrated intensities are introduced by the inhomogeneity of the incoming beam. A correction for these errors can be made, knowing the intensity profile of the primary beam and the dimensions and orientation of the crystal in the beam. The correction can conveniently be applied with the absorption correction. Examples of the corrections are given for crystals with rational boundary planes. It is shown that the intensity of an X-ray reflection as a function of the rotation about the scattering vector ( rotation) can be calculated with fair accuracy. In some cases (large elongated crystals in an inhomogeneous beam) correction for absorption only may give results which are worse than those with no correction at all

Formation of membranes by means of immersion precipitation: Part II. the mechanism of formation of membranes prepared from the system cellulose acetate-acetone-water

Using equations and boundary conditions derived in Part I1, calculations have been performed on the ternary diffusion processes that occur in a cellulose acetate (CA) -acetone casting solution immersed into a water bath. The necessary concentration-dependent thermodynamic and hydrodynamic parameters have been derived from experimental data on the three limiting binary mixtures. Calculations show that immersion of the polymer solution into the coagulation bath results in an instantaneous increase of the polymer concentration at the surface of the solution. For a CA-acetone casting solution the thickness of this concentrated surface layer will increase until the onset of liquid-liquid demixing by means of nucleation and growth of the diluted phase that fixes the asymmetric polymer distribution in the film. The moment of onset of the demixing process depends on the thickness of the film. However, addition of a certain minimum amount of water to the casting solution results in an instantaneous onset of liquid-liquid demixing upon immersion, yielding a membrane with a very thin skin layer. The model calculations have been confirmed by light transmission measurements performed on immersed casting solutions

Demixing and gelation behavior of ternary cellulose acetate solutions

The demixing behavior on cooling of ternary systems of cellulose acetate/solvent/water has been examined for CA concentrations up to 40 wt% CA in several solvents. Cloud points have been measured as a function of cooling rate. The rapid process of liquid - liquid demixing can be discriminated from the slow process of aggregate formation by examining the dependence of the cloud point on the cooling rate and by structure analysis of quenched solutions with scanning electron microscopy. The appearance of aggregate formation depends strongly on the type of solvent. Slow cooling of ternary solutions in which acetone is the solvent leads to aggregate formation long before liquid - liquid demixing occurs. In addition, isothermal sol - gel transitions have been measured for quenched solutions at varying gelation times. It is concluded that gelation is not always preceded by aggregate formation

Formation of membranes by means of immersion precipitation : Part I. A model to describe mass transfer during immersion precipitation

Equations and boundary conditions are derived for the isothermal diffusion processes in the coagulation bath and in the polymer solution after immersion of a cast (ternary) polymer solution into a (binary) coagulation bath. The mass transfer is expressed in terms of thermodynamic driving forces and frictional coefficients. The frictional coefficients in the ternary system are assumed to be interrelated through the Onsager reciprocal relations and to be related to the measurable frictional coefficients defined in the three limiting binary composition ranges. In combination with knowledge about the demixing processes which can take place in the polymer solution (liquid-liquid phase separation or solid-like aggregate formation), this model makes it possible to calculate the polymer concentration profile in the immersed film at the moment of demixing of the polymer solution as a function of several process parameters. The calculated concentration profile and its relation to the asymmetric structure of the ultimate membrane are presented in Part II1

Preparation of asymmetric gas separation membranes with high selectivity by a dual-bath coagulation method

A new method for the preparation of gas separation membranes in a one-step procedure is presented, where common, non-volatile solvents can be used in the polymer solution. It concerns contacting of a polymer solution with two successive nonsolvent baths, whereby the first bath initiates the formation of a dense top layer and the second bath gives the actual polymer precipitation. Membranes made by this method will have high gas selectivity and do not need any additional coating. The new technique was used to make polyethersulfone (PES) hollow fibres from solutions consisting of 35% (w/w) polymer and 10% glycerol in N-methylpyrrolidone (NMP). High selectivities were obtained when using glycerol or 1-pentanol as the first nonsolvent and water as the second one. For a feed gas of 25 vol.% of CO2 in methane the intrinsic selectivity of PES [alpha (CO2/CH4) = 50] was easily obtained, without the necessity of an additinal coating step. By a step-wise, liquid exchange removal of residual fluids in the fibres, an improvement in flux could be obtained. This was accompanied by a somewhat lower selectivity compared to that of directly air-dried fibres