Intermolecular Interactions: New Way to Govern Transport Properties

May 19, 2010 - E-mail: [email protected]., †. A.V. Topchiev Institute of Petrochemical Synthesis. , ‡. Institute of Polymer Research. This article ...
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Ind. Eng. Chem. Res. 2010, 49, 12031–12037

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Intermolecular Interactions: New Way to Govern Transport Properties of Membrane Materials Yuri Yampolskii,*,† Alexandre Alentiev,† Galina Bondarenko,† Yulia Kostina,† and Matthias Heuchel‡ A.V. TopchieV Institute of Petrochemical Synthesis, 119991, Leninsky Pr. 29, Moscow, Russia, and GKSS Research Center, Institute of Polymer Research, Kantstr. 55, D-14513 Germany

Investigation of the gas permeation parameters of several polyetherimides and polymers of intrinsic microporosity showed that their properties (permeability and permselecitivity) are sensitive to the presence of low molecular mass compounds (chloroform, lower alcohols, water) that are capable of forming hydrogen bonds. Using the FTIR method and quantum chemical calculations, it was shown that such bonds are formed and they induce structural (conformation) changes in the polymer matrices. Other types of weak interactions, dipole-dipole interactions, are responsible for unusual sorption behavior of acetone in amorphous Teflon AF2400. Therefore, intermolecular interactions can influence transport and sorption properties of glassy membrane materials. Introduction One of the most important findings in membrane science over the last decades was the firm establishment of the relationships between the chemical structure of materials for gas separation and their gas permeation parameters. A big contribution in this field was made by D. R. Paul and his school. Thus, a summary of the results for various polymers with backbones containing aromatic fragments was made 15 years ago.1 It was shown that similar changes in the values of the permeability and diffusion coefficients can be observed due to the introduction of the same groups into main chains and side groups of polymers that belong to different chemical classes (polycarbonates, polysulfones, polyimides, etc.). Demonstration of such effects can be found in the literature for various connector groups (-O-; -CH2-; -C(CH3)2-; -C(CF3)2-) or substituents (CH3, CF3, CH(CH3)2; Si(CH3)2).2,3 These variations of the design of the membrane material affect chain stiffness and, via it, packing density of macromolecules, as is manifested in intersegmental distances (d-spacing) sensed by wide-angle X-ray scattering (WAXS) in amorphous glassy polymers1 and free volume as found, e.g., by the Bondi method or measured using sophisticated physical techniques such as positron annihilation lifetime spectroscopy.4 Demonstration of these, let us call them “first-order effects”, led to development of different schemes for prediction of the gas permeation parameters of polymers on the basis of their chemical structure.5-8 The reliability of such predictions produced an impression that an accurate selection of chemical structure of membrane materials would facilitate a directed movement of the data points. Indeed, it is easy to find literature examples of when an evolution of the data points on Robeson diagrams9 allows a simultaneous improvement of permeability and permselectivity for a series of structurally related polymers. However, this approach does not allow surmounting the upper bounds in such diagrams. In such predictions, possible interactions between polymer matrix and penetrants (solutes) are commonly neglected. Meanwhile, there is much evidence that such interactions can

affect the observed transport parameters. The most common and often observed effect is the influence of so-called residual solvent on transport parameters of membranes.10,11 However, these effects well-known to all experimenters can be different in different systems both in a quantitative and even qualitative manner, and no generalization has been proposed. Recently we demonstrated for different systems that hydrogen bonds or even weaker dipole-dipole interactions between polymer chains and low molecular mass compounds can strongly influence the permeation and sorption parameters. In this article, we give a brief review of the results that demonstrated a role of intermolecular interactions in transport and sorption parameters. Its discussion is based on novel results and those partly published elsewhere.12,13 Hydrogen Bonding in the Systems PolyetherimidesChloroform. Some time ago, rather attractive gas permeation parameters were found14 for a polyetherimide: P(O2) ) 0.84 barrer, R(O2/N2) ) 12 with the data point above the Robeson upper bound.9 First, it was ascribed to the presence of CF3 groups in the structure of the polyetherimide (PEI); however, after a study of another polymer whose structure included CH3 groups instead CF3, other explanations were needed. So a big group of various PEIs was studied. The structures of some of them are shown in Table 1. These polymers have rather flexible chains due to the presence of ether bonds: their glass transition temperatures are in the range Table 1. Structure of the Polyetherimides

* To whom correspondence should be addressed. E-mail: [email protected]. † A.V. Topchiev Institute of Petrochemical Synthesis. ‡ Institute of Polymer Research. 10.1021/ie100097a  2010 American Chemical Society Published on Web 05/19/2010

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Table 2. Permeability Coefficients P of PEIs with Different Film Treatment at 35°Ca P, Barrer no.

H2

He

O2

N2

CO2

CH4

CO

I (stand) I (sa) II (stand) II (sa) III (stand) III (sa) IV (stand) IV (sa)

8.87 6.67 6.96 5.72 3.48 8.81 4.76 3.36

10.50 8.45 7.85 6.41 3.47 11.3 7.38 5.89

0.81 0.68 0.56 0.29 0.28 0.39 0.30 0.35

0.17 0.08 0.11 0.05 0.106 0.028 0.105 0.076

2.76 2.16 1.75 1.29 1.02 1.05 0.59 1.09

0.148 0.056

0.27 0.14

0.044 0.086 0.023 0.034

0.13 0.07 0.07

a

The designation “stand” means standard treatment, “sa” means strain aging. Table 3. Separation Factors of PEIs with Different Film Treatmenta R ) P1/P2 no.

H2/N2

He/N2

H2/CH4

O2/N2

CO2/N2

I (stand) I (sa) II (stand) II (sa) III (stand) III (sa) IV (stand) IV (sa)

51 82 63 114 33 312 45 44

61 105 71 128 33 404 70 77

60 119

4.7 8.4 5.0 5.8 2.6 14.0 2.8 4.6

16 27 16 26 10 37 5.6 14

129 41 383 140

CO2/CH4 19 38 29 12 46 17.5

a The designation “stand” means standard treatment, “sa” means strain aging.

170-180 °C which is relatively low for polyimides. The films were cast in most cases from chloroform solutions. It was found that the gas permeation parameters depend on the protocol of film preparation. Two methods for film pretreatment and removal of residual solvents were used. In the first “standard” method, the free films were dried for a week in ambient atmosphere and then kept in vacuum until a “constant” weight is achieved. It was observed that during this process the films experienced some contraction. In the second method, the residual solvent was allowed to slowly evaporate in the ambient conditions for 2-3 months when the films were kept fixed by the perimeter either with O-ring or in the cylinder where the casting process was performed. Thus, a contraction of the films was prevented. This protocol of films preparation was called “strain aging”. The permeability of the films of PEI undergoing different treatment was determined using the mass-spectrometric method (Balzers QMG-420 instrument). Measurements were carried out at room temperature (22 °C) with upstream pressure in the range 100-400 mmHg and downstream pressure about 10-3 mm Hg. The Daynes-Barrer method was employed. In the experiment, the time dependence of the intensities of the ions with m/e ) 32 and 28 was controlled for pure oxygen and nitrogen, respectively. It was found that the gas permeation parameters of PEIs strongly depended on the protocol of film preparation (Tables 2 and 3). A transition from standard protocol to strain aging results in most cases in some decrease in permeability, however, different for different penetrants. However, it is nearly always accompanied by increases in permselectivity, in some cases very substantial increases (Table 3). This transition is accompanied by increases in film density; simultaneously, permeability decreases and separation factors (e.g., R(O2/N2)) increase (Figure 1). The most extensive studies were carried out for PEI I. Figure 2 shows the permeability-permselecivity diagram for O2/N2 pair in different samples of this polymer. In addition to the two film

Figure 1. Effects of film density on permeability coefficients of nitrogen (a) and separation factors for an O2/N2 pair (b) at 20 °C in PEI I (adapted from ref 30).

Figure 2. Separation factors R(O2/N2) and permeability coefficients P(O2) of PEI I after different film treatment (the dashed line is the Robeson upper bound of 1991).

treatment methods mentioned earlier, some samples were subjected to annealing close to Tg of this polymer (200 °C). It is obvious that strain aging, in a rather reproducible way, leads to great increases in permselectivity and only slightly affects permeability. These effects are observed only, if chloroform (protic solvent) was used in film casting. If another good but aprotic solvent (THF) was used, very small increase in permselectivity is observed. Annealing results in more significant decreases in P(O2) values and only modest growth of the separation factor. It should be noted that according X-ray spectroscopy tests15 after the standard treatment the polymer contains substantial quantity of chloroform (8-10%). After strain aging, some solvent is also retained in the film, and only annealing removes virtually the whole residual solvent. A general conclusion that can be made after the study of this PEI (and other PEIs as well) is that the chemical structure of a polymer does not define in all the cases the position of the data points in the Robeson diagram. Figure 3 shows the same data points as does Figure 2 but now superimposed over the points

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Table 4. Energy and Structure Characteristics of the Conformers at the Ph-O-Ph′ Site and Their Complexes with Chloroform

Figure 3. Comparison of the values of R(O2/N2) and P(O2) of PEI I and other polymers (the dashed line is the Robeson upper bound of 1991).

of various other polymers. It can be seen that the points that belong to the structure of PEI I can be found in all parts of the diagram: inside “the cloud” of the points, below it, and even above upper bound.9 This unexpected result required some elucidation. For this purpose, FT spectroscopy and quantum chemical calculation were employed. The experimental details were described earlier.12 Spectra of PEI samples containing different quantities of residual solvents were measured using a spectrometer IFS66v/s and treated using Soft Spectra and OPUS programs. Because of requirements of spectral analysis, the experiments were performed with relatively thin films (about 5 µm), so removal of residual solvent was much more thorough than in gas permeation experiments, where the film thickness was much larger (30-100 µm). Optimization of the geometry of the PEI models calculation of the complexes were employed using the semiempirical AM1 method, however in some cases more the rigorous DFT method (B1LYP/6-31G, Gaussian-98, serial no. PC91419457W-1423R) was used. Conformation analysis with accounting for the energy of inner rotation was carried out using the molecular mechanics method with force field MM2. It was found that chloroform exists in the PEI films in the free form (764 cm-1) that disappears from the sample at the Tb of CHCl3 and a bound form (758 cm-1) that disappears at Tg of polymers. Removal of strongly bound residual chloroform was accompanied by changes in the IR spectrum of the polymer: a new band at 1262 cm-1 appears, besides there are changes in positions and relative intensities of other bands in the region of absorption of C-O-C/ bonds. This can be considered as evidence of changes in conformations of Ph-O-Ph/ connector groups due to heating and desorption of chloroform. It can be added that nothing similar happens in the films cast from THF solutions: no indication of a two-sorbed state, easy removal of the aprotic solvent at low temperature, no changes in the polymer spectra. It was assumed that hydrogen bonding of chloroform and some sites in the PEI macromolecules are responsible for this behavior and conformation changes in the PEIs. Three sites with the PEI macromolecules can be suspected as participants in hydrogen bonds: N atoms and carbonyls of imide groups and ether bonds of PEI. The calculation indicated that the interaction of the acid proton of chloroform with sOs groups of PEI is more energetically favorable (interaction energy is -32 kJ/mol) than for sN< and Od groups. In addition, analysis of the spectra of polymer samples after removal of CHCl3 indicated that the position and relative intensity of the bands that belong to imide

ring did not change. Hence, hydrogen bonding takes place between dissolved chloroform molecules and ether sites in PEI. For conformation analysis of the ether connector group, a simple model H2N-Ph-O-Ph-CH3 was chosen. The calculations showed that in the absence of chloroform polymer chains comprise 2 conformers with very similar energies (Table 4), so interconversion of these conformers proceeds virtually with no energy barriers. In the presence of chloroform, formation of the complex with conformation 1 is energetically much more preferable than that of conformer 2. If removal of the residual chloroform proceeds in the regime of strain aging, when inner rotation is restricted, the film is enriched with conformer 1. When solvent removal proceeds in a free-standing film (standard regime), then the conformation set within the sample will be different. So, observed variations in the transport parameters and density of the PEI films can be explained by differences of the conformation set that appear in different protocols of film preparation. Some additional information was obtained using the molecular dynamics method. The procedure of calculation was the same as described earlier.16 For three PEIs, size distribution was computed for so-called Rmax approximation. In the terms accepted in the present paper, the calculations were performed for an annealed film (neat PEIs) and for the film subjected to standard treatment which contained 10% of CHCl3. The results are shown in Figure 4. It can be seen that appearance of chloroform in the polymer matrix increases free volume and shifts it to larger sizes of microcavities. This result is consistent with the changes induced by annealing, that is, by substantial reduction of permeability (see Figure 2). Thus, independent methods (experimental spectral, quantum chemical calculations based on consideration of individual chains or simple models and molecular dynamics that consider polymer matrix and are based on classical mechanics) are in agreement with the data of gas permeation properties of PEIs containing different quantities of residual solvent (chloroform) capable to form hydrogen bonds with PEIs. Although IR spectroscopy and quantum chemical calculations revealed several peculiarities of the systems containing PEIs and a residual solvent (CHCl3) capable to form hydrogen bonds, they give no explicit answers how hydrogen bonding affects the unusual transport properties of strain aged PEI. It should be emphasized though that the behavior of strain aged PEIs differ basically and in several aspects from extensively studied common aging process in polymeric membranes.17 It is known that aging is more typical for thin films or asymmetric membranes, while our experiments were performed with thick films (25-40 µ). The following differences can be also noted: (1) In typical aging experiments, permeability coefficients P for all tested gases decrease in time while separation

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Hence, it is more probable to suppose that it was size distribution and not R, which changed during the strain aging process. It can be added however that the PALS study revealed some differences between PEI samples containing much residual solvent and experienced annealing: the former exhibited much larger τ3 (R) values. Normally, increases in temperature result in a monotonous increase in τ3 (R) values in glassy polymers.19 Meanwhile, heating of PEIs showed some minima in the temperature range where removal of bound residual solvent starts. Second heating of the samples did not show such unusual behavior confirming the role of removal of chloroform from the sample. Decreases of lifetimes τ3 (at room temperature) after heating are consistent with observed reduced permeability of annealed PEI samples. Hydrogen Bonding in PIM. Polymers with intrinsic microporosity (PIM) have interesting gas permeation propertiess relatively high permeability coefficients and separation factors, especially for gas pairs including CO2, that locate the data points above upper bounds on Robeson diagrams.20-22 The structure of the more extensively studied member of this group, so-called PIM-1, is shown below:

Figure 4. Size distribution of free volume elements in PEIs after standard treatment and annealing of the films based on MD simulation: PEI I (a), PEI II (b), PEI III (c). (black bars) PEI + chloroform. (open bars) Neat PEI.

factors increase, whereas in our experiments a decrease of the P values proceeds selectively: permeability coefficients of the “fast” gas (O2) remain virtually constant while P(N2) decrease leading to a growth of separation factors R(O2/N2). It can be added that similar trends of selectivity are observed for other gas pairs, though the corresponding R values are below Robeson’s upper bounds (Table 3). (2) On the other hand, aging of free PEI films results in “normal” behavior similar to that observed for other systems:17 a reduction of the P values and some increase in R. These parameters approach in time those observed after annealing of the sample at the temperatures close to Tg. (3) The authors are not aware of reported experiments where the effects of the nature of a residual solvent were demonstrated, which is the case of studied PEIs. Therefore, the observed increases in selectivity take place only if the film is kept in a strained state in the process of aging and the residual solvent is capable to form hydrogen bonds with the polymer. One can assume also that strain aging is accompanied with some changes of free volume or its size distribution. In this regard, it is worthwhile to consider the data of studies of PEI I and II using positron annihilation lifetime spectroscopy (PALS).18 It was found that positronium lifetimes τ3 (or the average size of free volume element (FVE) R) were very similar for the samples underwent standard treatment or strain aged.

The molecular structure of PIM-1 contains sites of contortion (spiro-centers) with dihedral angles of 90° within an assumed rigid ladder polymer structure:20

Another interesting feature of these polymers is exceptional sensitivity of the observed gas permeation parameters to the protocol of film formation. It was shown21 that contacts of the films with lower alcohols (MeOH and EtOH) result in significant increase in observed permeability. On the other hand, water treatment of freshly prepared films leads to significant decrease of the permeability coefficients. These differences amount of 1 order of magnitude. Such unusual behavior demanded some explanations that were provided by use of spectral studies and quantum chemical calculations. The experiments were carried out using IFS-66/ vs instrument with a high temperature Bruker cell. The quantum chemical calculations were performed using a nonempirical Gaussian method and semiempirical AM-1 method with Mopac software.

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Figure 5. IR spectra of PIM-1 containing sorbed water (1) and ethanol (2).

Structural calculations indicated that dioxane fragments in the polymer chains can exist in three conformations: the most energetically favored flat form and “bath” and “chair” forms which do not differ much from the flat form: ∆E ) 14.8 and 11.4 kJ/mol, respectively. The presence of such conformations that differ insignificantly by energy implies a possibility of low scale mobility of the ladder-type backbone chains, which was partly confirmed by molecular dynamics calculations.23 IR spectra in the range 1250-1350 cm-1 characteristic for C-O-C bonds of the dioxane rings revealed some changes in the band widths and positions after contacts of the films with water and lower alcohols (especially strong for water). This was interpreted as an evidence of conformation changes in the polymer. Sorption of water and alcohols into PIM-1 film results in changes that indicate intermolecular interactions between the polymer and solute molecules. Thus, the appearance of the band characteristic for the associates O-HO-Et (a shoulder at 3340 cm-1) can be seen (Figure 5). In addition, the spectra show the bands that belong to absorption of OH-groups of associated alcohols and water (3400-3600 cm-1). It is interesting that there are some differences in the absorption of OH-bands in the spectra of the films absorbed ethanol and water: in the case of the alcohol the bands of monomer form (3670 cm-1) are more intensive, whereas sorption of water results in more intensive form characteristic for water associates (wide band at 3560 cm-1). Hence, the alcohol is present mainly in the monomer form and water as the associates. Quantum chemical calculations were performed for mutual orientations of two independent chains of PIM-1. It should be admitted that such calculations could give only rough estimates of microcavities formed by two adjacent chains, because only molecular dynamics gives reliable values of free microcavities being in agreement with direct probing of free volume (first and foremost, positron annihilation lifetime spectroscopy or PALS). Optimization of our calculations indicated that the formation of the microcavities having inner diameters of 10.6 Å is energetically more favorable. It should be emphasized that this value is in reasonable agreement with the size distribution of free volume in PIM-1 obtained by means of molecular dynamics23 (4-11 Å) and the value determined by PALS method24,25 (9.4 Å). The calculations performed for the associates showed that energetically most favorable water associate (∆E ) 77,1 kJ/mol) consisting of 5 H2O molecules has a diameter of 8.50 Å; that is, it can be accommodated into such a microcavity. On the other hand, the most favorable associate of ethanol (tetramer) is too large (>10 Å), so its accommodation is problematic based on steric hindrance. Water associates are capable to form sufficiently strong bonds with oxygen atoms

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of both chains that lead to a reduction of the size of the microcavity to 9.6-8.7 Å. Single molecules of ethanol form relatively weaker hydrogen bonds, and this process is accompanied by increase of the size of microcavity to 12.5 Å. Weakly bonded alcohol can be easily desorbed at room temperature leaving larger holes inside the polymer, while water associates require heating above 100 °C for their removal. In fact, only above 160 °C full disappearance of water band was observed in IR spectra. In addition, namely such a sequence of the sizes of microcavities was found in different states of PIM-1 by means of PALS.21 Thus, some evidence on the role of hydrogen bonding was demonstrated for PIM-1. Apparently, the picture is not that clear as for polyetherimides, and further efforts, maybe using atomistic modeling, are desirable. Nonetheless, some differences between the two groups of membrane materials are obvious now: for PIM-1, the effects are much stronger for permeability and rather weak for permselectivity. The reasons for it can be related to the much stiffer nature of PIM-1 chains and its supposedly opened pore structure. PIM-1 does not show a glass transition, i.e. its Tg is above its decomposition temperature or above 350 °C. In addition, it should have a developed “pore structure”. According to low temperature N2 adsorption, its inner surface area is very large (700 m2/g).20 If the pores were not interconnected, diffusion limitations of nitrogen at 77 K would prevent “probing” the whole surface area. And virtual absence of “closed pores” implies an absence of high separation factors, because selectivity in gas permeation in polymers is usually provided by the processes within “the walls” of the pores or microcavities. Dipole-Dipole Interaction in Sorption in a Perfluorinated Polymer.13 An investigation of organic vapor sorption in amorphous Teflon AF2400 (copolymer of 87% of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole and 13% of tetrafluoroethylene) revealed some unusual shape of sorption isotherm in the case of acetone.26 Different forms of sorption isotherms were observed for various solutes. Thus, hydrocarbons and their derivatives with low polarity showed a common shape of sorption isotherms concave to the pressure axis, which is in agreement with the dual mode sorption model (DMS). For alcohols (MeOH and EtOH), the sorption isotherms were concave to the concentration axis. This rather unexpected behavior more typical for sorption in rubbers was explained by cluster formation in the strongly hydrophobic matrix of the perfluorinted polymer. The most unusual behavior was observed for acetone: it showed a completely linear isotherm up to very great activities p/ps ) 0.8. Such sorption isotherms are characteristic for light gases (e.g., hydrogen and helium) in glassy polymers. In the case of an active organic vapor (acetone) and great concentration of the solute in the polymer (up to 25 cm3(STP)/cm3 polymer), it may imply some specific interactions with the polymer and high concentration of the sites responsible for them (absence of saturation typical for DMS). Since no hydrogen atoms are present in the structure of AF2400, hydrogen bonding was excluded, and dipole-dipole interactions were considered as the next candidate for such interactions. The relatively large dipole moment of acetone (2.6 D), the highest among the solutes studied, made such an assumption possible. Using IR spectroscopy, it was shown that in the polymer film after contact with acetone, beside acetone’s own band that belongs to the CdO vibration (1725 cm-1), an additional band appears at 1687 cm-1. This band is present in the spectrum much longer, after the former band completely disappears. In the spectrum of the polymer numerous changes can be observed in the range 1100-1300 cm-1: the positions of band maxima and

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relative intensity are changed in the presence of sorbed acetone. Such changes can be observed 1-1.5 days after the contact of the film with acetone. All this unambiguously indicates the presence of some relatively strong interactions of the polymer with acetone.

Table 5. Permeability Coefficients of PIM-1 (Barrer) at 35°C for Three States of PIM-120 state

O2

N2

CO2

as cast (from CHCl3) contact with H2O soaked in MeOHa

580 150 1610

180 45 500

4390 1550 12600

Theoretical assignment of the individual bands in the spectrum of AF2400 allowed us to ascribe these changes to various stretch vibrations of perfluorodioxole ring and CF3 groups. Therefore, sorbed acetone causes the variation of the vibrations of the whole ring. In order to elucidate the nature of the interactions of acetone with the ring, the calculation of energy, electronic, and structural characteristics was performed with the optimization of the geometry of the complex.

in the dense part of the polymer, in other words, in the walls of free volume elements.

On the surface of the potential energy of the system, a minimum was found; it corresponds to the complex having an energy 10.5 kJ/mol less than the total energy of acetone and the ring. Although this value is noticeably smaller than the typical energy of hydrogen bonding, it is larger than the characteristic energy of van de Waals and dipole-dipole interactions in other systems.27 The dipole moment of the complex is 3.4 D, that is, larger than the dipole moment of acetone and individual ring (0.2 D). The distance between interacting partners (“bond length”) is in the range 2.6-3.2 Å; there is a substantial shift of electronic density into the region between participants of this interaction, and both reactants (acetone and the ring) donate electrons into this region. These results explain why there is no evidence for saturation in the sorption isotherm of acetone: the concentration of acetone sorbed even at the highest activity p/ps is 7 × 1020 molecules/cm3 of the polymer, while “the concentration” of perfluorodioxole rings is one order highers4 × 1021 1/cm3.

Acknowledgment

It should be taken into account that sorption of acetone in AF2400 does not involve simple filling of microcavities which is the case for other solutes that are not capable of participating in dipole-dipole interactions with perfluorodioxole rings: in this case, saturation of the isotherms should be observed at sufficiently large solute concentrations. The amount of acetone absorbed in AF2400 can be compared with the hole number density N in the polymer. This value has been determined directly using PALS. According to Dlubek et al.28 N ) 4.4 ( 0.02 1/cm3. This value can be also roughly estimated via the fractional free volume (FFV) of the polymer. The FFV of AF2400 is 32%.29 This means that in 1 cm3 of the polymer, the free volume amounts to 3.2 × 1023 Å3. The radius of FVE in AF2400 is29 5.95 Å; hence, the elementary volume is 882 Å3. By dividing these two values, one obtains 3.6 × 1020 1/cm3, rather similar to the direct estimation by Dlubek et al.28 Both values have the same order as the concentration of sorbed acetone, though are somewhat smaller. The absence of saturation in the sorption isotherm of acetone indicates that hole filling mechanism provides only a part of the total sorption process and the interaction of acetone with the rings can also take place

a

Similar results were obtained for EtOH.

Conclusions Thus, intermolecular interactions were demonstrated for several polymers of different chemical structure. However, there are some common features in all the systems studied. In all cases, some structural changes in polymers caused by interactions with low molecular mass substances were observed. Such interactions induce long-term effects. So a possibility appears to explain well-known effects of “residual solvents” by specific physicochemical mechanisms. The sites in the polymer chains that take part in these interactions are rather common in glassy polymers; in particular, -O- bridges can be found, e.g. in polycarbonates and polysulfones, so it will make sense to look for similar effects in other systems. And apparently, it is desirable to demonstrate the absence of such effects in other polymers that do not include possible sites for interactions. And at last, the presence of such effects expands the arsenal of a researcher to control the transport properties of membrane materials.

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ReceiVed for reView January 15, 2010 ReVised manuscript receiVed April 1, 2010 Accepted April 15, 2010 IE100097A