Aleksandra Wolinska-Grabczyk

Synthesis, characterization, and pervaporation properties of segmented poly(urethane‐urea)s

Poly(urethane-urea)s (PUUs) from 2,4-tolylene diisocyanate (2,4-TDI), poly(oxytetramethylene)diols (PTMO) or poly(butylene adipate)diol (PBA), and various diamines were synthesized and characterized by Fourier transform infrared spectroscopy, gel permeation chromatography, differential scanning calorimetry, and density measurements. Transport properties of the dense PUU-based membranes were investigated in the pervaporation of benzene–cyclohexane mixtures. It was shown that the pervaporation characteristics of the prepared membranes depend on the structure and length of the PUU segments. The PBA-based PUUs exhibit good pervaporation performance along with a very good durability in separation of the azeotropic benzene–cyclohexane mixture. They are characterized by the flux value of 25.5 (kg μm m−2 h−1) and the separation factor of 5.8 at 25°C, which is a reasonable compromise between the both transport parameters. The PTMO-based PUUs display high permeation flux and low selectivity in separation of the benzene-rich mixtures. At the feed composition of 5% benzene in cyclohexane, their selectivity and flux are in the range of 3.2 to 11.7 and 0.4 to 40.3, respectively, depending on the length of the hard and soft segments. The chemical constitution of the hard segments resulting from the chain extender used does not affect the selectivity of the PUU membranes. It enables, however, the permeability of the membranes to be tailored.

Preparation and characterization of new polyimide films containing zeolite L and/or silica

New polyimide (PI) films containing zeolite L and/or silica, having a thickness of tens of micrometers, were prepared using a PI matrix with pendant carboxylic groups. The influence of various ratios of inorganic filler on thermal stability, dielectric behavior, and gas transport properties of the films was studied with respect to their structure. The surface morphology was investigated by scanning electron microscopy. The films were flexible, tough, and showed good thermal properties with a glass transition temperature in the range of 187–218°C. The dielectric spectroscopy revealed γ and β subglass transitions having the corresponding relaxation activation energy in the range of 44.7–51.4 kJ mol−1 and 65.1–121.6 kJ mol−1, respectively. At temperatures above 200°C, a conductivity relaxation process was evidenced. Gas permeation tests using small molecules (O2, N2 and CO2), at 6 atm and 30°C, indicated that the PI films containing higher amount of zeolite showed the highest permeability for all gases.

Surface morphology of the polyurethane-based pervaporation membranes studied by atomic force microscopy. II. Structure-transport properties behavior

This second part of our research on the morphology of the polyurethane-based pervaporation membranes studied by atomic force microscopy (AFM) presents analysis of the structural characteristics, reported in the first part of the paper, with regard to transport properties of the polyurethanes determined from sorption experiments. The correlation has been found between the sorption equilibrium values, as well as the diffusivity of the permeating solvent, and morphology of the membranes studied. This can be related to the restricting role of the hard segment domains in suppressing the membrane swelling, as well as in making the movement of the permeating molecules more tortuous. AFM was also utilized to trace the morphology alteration expected to take place in the membranes exposed to solvents. The results have shown that this phenomenon can be observed for the polyurethanes with developed micron-sized structures after treating with solvent of a high swelling power. In that case, microstructural transformation from circular structures into more perfectly ordered pseudo-lamellae have been found to occur. The solvent sorption/desorption process was found not to affect the membrane initial morphology.

Surface Morphology of the Polyurethane-Based Pervaporation Membranes Studied by Atomic Force Microscopy. I. Effect of the Polyurethane Molecular Structure

Atomic force microscopy (AFM) was employed to study the surface morphologies of the polyurethane-based pervaporation membranes and to evaluate the effect of the polyurethane (PU) molecular structure. Segmented poly(tetramethylene oxide)-based PUs varying in the length of both segments as well as in the structure of the hard segments were tested. Small spherical or extended hard domains, on the size range of tens of nm, and their large, interconnected associates for a higher hard segment volume fraction, have been observed at relatively flat surface of the diamine-based PUs. The surface of the diol-based PUs appears to be generally much coarser and shows globular or circular structures of a few micron size which are thought to be composed of the hard segment lamellae. The morphology variations observed by AFM were found to correlate well with the small-angle X-ray scattering (SAXS) data revealing the necessity of the periodic arrangement in space of the segmental level domains for the formation of a higher level of the domain morphology.

Depth-influenced structure through permeating polymer membrane using SAXS synchrotron method

Abstract Depth-dependent structure of the PU membrane under working conditions performed on a model sample representing a membrane section in the scale 10:1 has been studied. The X-ray scattering in a polyurethane sample, covered by deposited aluminium on both sides, was measured along the sample depth using the synchrotron SAXS beamline. The model sample was saturated with benzene at its bottom, while from its top the solvent evaporated freely. Assuming the lamellar structure of the polyurethane we found that the average inter-lamellar distance increases monotonically from the value measured for the dry polymer to the value obtained for the saturated one. The relation of our result with the existing models of permeation is discussed. Further analysis of the lamellae structure indicated that the increase of the inter-lamellar distance refers only to its amorphous, a less dense part. Furthermore, we found that the change of the inter-lamellar distance is due to gradual formation of the solvent-sensible structure.

Dielectric and gas transport properties of highly fluorinated polyimides blends

The physicochemical properties of highly fluorinated polyimide blends were studied in order to highlight the advantages of employing a high content of fluorine atoms to tailor the properties of polyimide materials. Thus, dynamo-mechanical analysis was used to evidence the physical phenomena taking place during heating. The trend of storage modulus, loss modulus and loss factor tangent with temperature during and after the α relaxation was explored. The dielectric spectroscopy was carried out to investigate the dielectric behaviour at different temperatures and frequencies and to evaluate the values of dielectric constant, dielectric loss and electrical resistance. The dielectric spectroscopy data were corroborated with the dynamo-mechanical analysis to highlight the sub-glass transitions encountered in these blend films. Dielectric loss diagrams showed sub-glass transitions in the form of γ and β relaxations mainly due to the segmental mobility of the polymer chain components. The gas separation performance of some of the investigated blends was assesed, and their ability to achieve simultaneously higher gas permeability and higher selectivity was demonstrated.

TEMPERATURE EFFECTED STRUCTURAL TRANSITIONS IN POLYURETHANES SATURATED WITH SOLVENTS STUDIED BY SAXS SYNCHROTRON METHOD

ABSTRACT A set of segmented polyurethanes (PU) differing in the hard-segment structure was saturated with solvents and after the equilibrium saturation was reached, put to temperature-dependent SAXS investigations. The time-resolved mode of SAXS measurements with a linear increase of temperature from −70°C to +70°C, i.e., within the temperature range between Tg of soft and hard segments, was applied. The order-order transition leading to a greater degree of order was found at higher temperatures for almost all systems investigated. Some of the PUs exhibit two kinds of microphase separated domains. The results obtained are discussed with respect to the mean-field theory of copolymers and Koberstein and Stein model for hard microdomain structure in PUs, and correlated with temperature dependence of membrane permeability in pervaporation process.

Sorption Behavior of Permeants in the Polyurethane‐Based Pervaporation Membranes Studied by DSC

Differential scanning calorimetry (DSC) was used to study sorption behavior of various liquid permeants in the pervaporation membranes formed of homologous series of polyether‐, polyester‐, and polydiene‐based polyurethane elastomers. It has been found that there are three major types of liquid permeant within the membrane: (1) non‐crystallizable liquid, (2) crystallizable‐bound liquid, and (3) crystallizable‐bulk liquid. The kind and number of these forms and their relative contribution appear to be a characteristic feature of a particular system and depend on the specific interactions between permeant and membrane material and between permeant molecules themselves. For weak polymer‐permeant interactions compared to those concerning permeant, crystallizable‐bulk liquid (3) has only been detected within the membrane. For moderate polymer/permeant interactions, represented by the sorption equilibrium values falling around 30%, the permeant in two different forms, (1) and (3), of various composition has been found to exist. For strong polymer/permeant interactions, all types of permeant have been observed including a form (2) classified as crystallizable‐bound liquid. This type of liquid permeant, showing complex melting endotherm of significantly depressed Tm values, has been identified for the first time for permeants without hydrogen‐bonding ability. The observed phenomena are considered as general behavior of membranes under pervaporation conditions and are expected to have an effect upon permeation.

Correlation between Cohesive Energy Density, Fractional Free Volume, and Gas Transport Properties of Poly(ethylene-co-vinyl acetate) Materials

The transport properties of the poly(ethylene-co-vinyl acetate) (EVA) materials to He, N2, O2, and CO2 are correlated with two polymer molecular structure parameters, that is, cohesive energy density (CED) and fractional free volume (FFV), determined by the group contribution method. In our preceding paper, the attempt was made to approximate EVA permeability using a linear function of 1/FFV as predicted by the free volume theory. However, the deviations from this relationship appeared to be significant. In this paper, it is shown that permeation of gas molecules is controlled not only by free volume but also by the polymer cohesive energy. Moreover, the behavior of CO2 was found to differ significantly from that of other gases. In this instance, the correlation is much better when diffusivity instead of permeability is taken into account in a modified transport model.