Helena Jasna Mencer

Polydispersity index and molecular weight distributions of polymers

The polydispersity index as a measure of the width of molecular weight distributions (MWD) is theoretically very important. However, considering MWDs of real polymers, its importance is partly lost. Consequently, several problems arise. The most common one is the misinterpretation of polydispersity index as a single and unique measure of the MWD width. In this work, the origin and solutions of the problem are clearly described via several theoretical distribution functions, i.e. those which are most widely applied for the description of polymer MWDs. The conclusions are confirmed by the analysis of MWDs of real samples, namely styrene-acrylonitrile copolymers. In addition, a complete procedure for the MWD data analysis is presented. Such a procedure is necessary for the correct interpretation and comparison of different, experimentally obtained, molecular weight distributions of polymers.

Impact strength of reactively extruded polystyrene/ethylene-propylene-diene rubber blends

Abstract The impact strength of blends consisting of polystyrene (PS) and ethylene-propylene-diene rubber (EPDM) could be increased by adding a poly(styrene/ethylene-propylene) (SEP) compatibilizer and an organic peroxide during reactive extrusion. The increased impact strength could be related to an enhanced adhesion between the dispersed EPDM phase and the PS matrix, as a consequence of radical graftlink or co-crosslink reactions between the rubbery part (EP) of the compatibilizer and the dispersed EPDM rubber. The morphology of the blend proved to be dependent upon the type and amount of peroxide used, as well as on the residence time (feeding position) of the peroxide in the extruder. The dependence of the mechanical properties of the blends on the type and concentration of peroxide used, is discussed in relation to the distribution of the peroxide between the PS and the EPDM phases.

Swelling kinetics of polymer-solvent systems

Abstract Swelling of poly(ethylene-co-vinyl acetate) (ESCORENE UL 00218, EXXON Chemicals, Belgium) in solvents of different types has been carried out by the volumetric method at three temperatures. The swelling kinetics are related to the swelling rate constants. A new variable, α′, is introduced and defined for comparing kinetic parameters in various polymer-solvent systems. A kinetic model of a reversible first order process was taken as a theoretical model because of the best fitting of experimental data to this model. The solvents are considered from the kinetic (swelling rate constants as indicators) and thermodynamic (equilibrium swelling degree as indicator) points of view. Poly(ethylene-co-vinyl acetate) interacts with solvents mainly by dispersive forces, but the kinetics of swelling may also be affected by other factors, such as the viscosity and molar volume of the solvent.

Dilute solution properties of cellulose diacetate in mixed solvents

In this paper the dilute solution properties of cellulose diacetate (CDA) in single solvents (acetone (A), N,N-dimethylformamide (DMF)) and in mixed solvents (acetone with DMF, N-methyl formamide (NMF) and formamide (F), respectively) were investigated by the low-angle laser light scattering method, as well as by the dilute solution viscometry method. The main experimental results comprised apparent weight average molar mass values, M2∗ and limiting viscosity number values, [η]H. The results were interpreted in terms of specific interactions of solution components. Basic solvents (acetone and DMF) primarily solvate hydroxyl groups of CDA and both the intensity and the direction of preferential solvation are determined by the preferred heteroassociation of solvent molecules (dipole interactions). Acidic solvents (formamide, NMF) primarily solvate acetyl groups of CDA. In this case the preferential solvation behaviour is determined by the autoassociation properties of acidic solvents (H-bonding). The autoassociaton of acidic solvent molecules is responsible for the intramolecular and intermolecular bridging of CDA-molecules as well. This phenomenon gives rise to the structuring of more concentrated CDA-solutions, which is of key importance when technical CDA-membranes are to be prepared. Within the framework of this paper it is proved that preliminary studies of the structuring of CDA-solutions may be performed in dilute solutions. On the basis of preliminary studies a suitable range of solvent compositions may be chosen and necessary but rather complex investigations in mid-concentration range may then be performed only in that, quite restricted composition range.

Estimation of the compatibility of poly(2,6-dimethyl-1,4-phenylene oxide) and poly(fluorostyrene-co-bromostyrene) from dilute solution viscosity measurements

Abstract The viscosity behaviour of dilute benzene solutions of poly(fluorostyrene-co-bromostyrene) (A) and poly(2,6-dimethyl-1,4-phenylene oxide) (B) and of their mixtures has been studied at 25°. The viscosity interaction coefficients, kAB, have been calculated from the Huggins slope coefficients of polymer mixtures. The results of the estimation of the compatibility of polymer pairs, according to Krigbaum and Wall, have been compared to DSC findings.

Prediction of copolymer miscibility by the viscometric method

Abstract The theoretical aspects of the polymer miscibility study by the viscometric method are reviewed. The miscibility criteria of Bohmer as well as of Opalicki and Mencer are modified by incorporating the Catsiff and Hewett definition of ideal behaviour instead of that of Krigbaum and Wall, and thus novel miscibility criterion variables are defined. Furthermore, a viscometric study was used to estimate the miscibility limit of copolymers of styrene and acrylonitrile differing in composition. Problems concerning the transfer of results and conclusions from diluted ternary polymer solutions (polymer 1 + polymer 2 + solvent) to the solid polymer blends (polymer 1 + polymer 2) are recognized and discussed.

An electron spin resonance study of molecular dynamics and heterogeneity in the styrene-acrylonitrile copolymers

Abstract The spin-probed copolymers of styrene and acrylonitrile differing in monomer content and chain structure were examined by the electron spin resonance (e.s.r.) methods in a wide temperature range. The measurements of spin–lattice relaxation time, T1DM, in slow motional region by the double modulation e.s.r. technique reveal two distinct relaxation regions in copolymers and pure polyacrylonitrile matrices. At very low temperatures (region I) a very distinct difference between T1DM values of pure polyacrylonitrile, alternating and statistical copolymers and polystyrene-rich copolymer is ascribed to the effect of packing density of polymer chains. The density modulates the electron spin–lattice relaxation rates and other vibrational modes contributing to the total relaxation rate. At higher temperatures (region II), where the skeleton of a spin probe begins to move, alternating and statistical copolymers of a similar composition show different relaxation rate. The data are reconciled on the basis of the sequence structure of copolymer chains. The difference in glass transition temperature between alternating and statistical copolymers as determined by the e.s.r. measurements reflects local motional heterogeneity as a consequence of chain microstructure. The presence of local motional heterogeneity of a probe near and above the glass transition results from the styrene-rich sequences contributing to the structural heterogeneity in statistical copolymer.

Gel permeation chromatography on chemically modified silica using different solvents

Abstract Gel permeation chromatographic separations of polystyrene, poly-2-vinylpyridine and polydextrans were carried out on porous silica gel chemically modified with an ether. Tetrahydrofuran, dimethylformamide and water were used as mobile phases. It was investigated to what extent a universal calibration plot could be applied to such systems. The non-universality of the “universal” calibration in applied systems was explained by the presence of some preferential interactions among polymer solvent and active sites on the gel. Chemically modified silica gel still exhibited adsorption properties.

Some physical characteristics of corning porous glass packing in gel permeation chromatographic separations

Abstract The nature of the interactions between the main gel permeation chromatography (GPC) components. viz., the solute, eluent and gel, was examined. Corning controlled porosity glass (CPG) was chosen as the column packing and the separation of low-molecular-mass organic molecules, polystyrene standards and narrow-molecular-mass-distribution poly-2-vinylpyridine samples was carried out. Pure eluents (methyl ethyl ketone and dimethylformamide) and mixed eluents (methyl ethyl ketone-ethanol, methyl ethyl ketone-acetone and dimethylformamide-methanol) were used. The results showed that the attractive physical forces between solutes and CPG also contribute to the mechanism of separation. These forces could be suppressed or avoided by changing the nature of the solvent.

Estimation of the three dimensional solubility parameter of copolymers of halogenated styrene

Abstract The three dimensional solubility parameter as well as the solubility volume of random copolymers of ortho(para)fluoro styrene and ortho(para)bromostyrene, having different copolymer composition, have been estimated. The theoretical models for the estimation have been sphere and ellipsoid. The results show the advantage of ellipsoid as a theoretical model.