The Effect of Conformation Order on Gas Separation Properties Of

polymers

Article The Eﬀect of Conformation Order on Gas Separation Properties of Polyetherimide Ultem Films

Julia Kostina 1,*, Sergey Legkov 1, Alexander Kolbeshin 1, Roman Nikiforov 1, Denis Bezgin 1, Alexander Yu. Nikolaev 2 and Alexander Yu. Alentiev 1,*

1 A.V. Topchiev Institute of Petrochemical Synthesis Russian Academy of Sciences, 119991 Moscow, Russia; [email protected] (S.L.); [email protected] (A.K.); [email protected] (R.N.); [email protected] (D.B.) 2 A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, 119334 Moscow, Russia; [email protected] * Correspondence: [email protected] (J.K.); [email protected] (A.Y.A.)

Received: 16 June 2020; Accepted: 7 July 2020; Published: 16 July 2020

Abstract: Changes of the spectral characteristics of absorption bands depending on the films’ treatment method were registered for polyetherimide Ultem films. The possibility of selection of structural criteria (the ratio of the functional groups absorption bands intensities) showing all conformational changes in elementary unit with metrological processing of the results is shown. It is demonstrated that film formation from chloroform solution leads to elementary unit fragments, Ph–O–Ph0, which have an effect on macromolecule conformation and result in increasing of space between fragments of macromolecules (local polymer matrix packing loosening). Desorption of residual chloroform from films by ethanol or supercritical CO2 leads to a change of conformers set in Im–Ph–Im0 units. Quantum chemical modeling showed the possibility of convergence of these fragments in neighboring macromolecules, and consequently of interchain π–π interaction (local densification of chain packing of the polymer matrix). After annealing at a temperature higher than glass transition temperature, the polyetherimide film exhibits the most disordered (amorphous) state at all of the fragments. It is demonstrated that the results, obtained by the combination of theoretical and experimental vibrational spectroscopy methods, are in good agreement with data of chain packing ordering found by analysis of gas separation parameters.

Keywords: gas separation; polyetherimide; FTIR spectroscopy; conformational order; quantum chemical calculations; non-covalent interactions; residual solvent

1. Introduction One of the key problems of the application of glassy amorphous polymers is a dependence of sample properties and its history. This is due to the thermodynamic nonequilibrium of the glassy state. Therefore, glassy amorphous polymers are used in different stationary states which depend on preparation technique and operation mode. Especially, significant effects occur for thin films and coatings. For example, glassy amorphous polymers are applied as materials for thin continuous layers (several tens up to several hundred nanometers) of nonporous gas separation membranes. Therefore, specifically for membrane material science, the stability of gas separation properties in time and the effect of preparation technique and operation mode of the polymer membrane (film) on its gas separation characteristics are very important issues [1]. As a rule, polymer membranes are made out of polymer solutions, thus gas separation properties are sensitive to solvent type and polymer solution concentration [2–5], presence of residual solvent, and method of its removal [6–9]. Thermodynamic nonequilibrium state of glassy amorphous polymers leads to a change in nonequilibrium free volume and consequently gas separation properties of polymer membranes during exploitation due to physical

Polymers 2020, 12, 1578; doi:10.3390/polym12071578 www.mdpi.com/journal/polymers Polymers 2020, 12, 1578 2 of 15 aging [1,9,10] or plasticization with a separated component [1,11]. On the other hand, this is why gas separation characteristics might be considered as a nonequilibrium level assessment criterion for polymer film, prepared by one method or another [12]. As there are no direct methods of measurement for chain packing ordering for X-ray amorphous polymers, a combination of analysis of gas diffusion coefficients [12] with the results of other methods can be promising for the determination of a trend for a change of chain packing ordering and the effect of this trend on physicochemical and transport properties. One of these methods is vibrational spectroscopy, which provides evaluation of conformational structure change on atomic level and molecular level. An additional factor supporting the selection of vibrational spectroscopy as independent method of assessment of tendency of treatment technique effect on conformational structure of X-ray amorphous polymers is a well-developed theory of vibrational spectroscopy. It allows one to use both quantum chemistry methods to interpret experimental data and alternatively to substantiate reliability of theoretical calculations by results generated by test samples IR spectra analysis. Using a combination of experimental and theoretical methods of vibrational spectroscopy, and based on the results of an analysis of changes of test samples spectra, it is now also possible to resolve the inverse problem of vibrational spectroscopy for polymers, i.e., to register changes of elementary unit structure based on the model justified by experimental spectral data. This approach can be successfully applied to calculate interactions, including noncovalent interactions. Comparison of theoretical vibrational spectra with experimental ones can allow one to make adequate conclusions and to find relationship between structural (particularly, conformational) changes in macromolecule and polymeric object properties altogether. Previously, in [8,13–16], it has been shown that interaction between functional groups of polymer with a solvent with formation of hydrogen bonds leads to reorientation of these groups over the polymer backbone, changing its conformational parameters [13–16]. If hydrogen bond formation leads to an increase in the quantity of elementary unit conformers of the same geometry, it is appropriate to suppose ordering of conformational composition of elementary units of macromolecule, which results in chain packing ordering. In turn, polymer chain packing ordering may lead to modification of free volume elements distribution in the matrix [15]. In these cases, the modes ratio in bimodal free volume elements (FVE) size distribution changes with an increase of specific mode concentration. Such modification of FVE size distribution is accompanied by a sharp increase in gas separation selectivity with maintaining [13,15] of gas permeability (sometimes these data exceed upper bound [15]) of permeability-selectivity diagram [17], which makes polymer material perspective for membrane gas separation. In [15], the relationship of gas separation parameters with conformational chain ordering when the polymer is treated with non-solvents (alcohols) also forming complexes with hydrogen bonds was also considered. Gas separation selectivity growth with gas permeability increase has also been observed for polyetherimides (PEI) films swelling in supercritical CO2 (sc-CO2) with further slow depressurization (0.008–0.025 MPa/min) [18–20]. It is well known that polymer treatment with sc-CO2 facilitates desorption of a residual solvent [21] independently of its capability to form hydrogen bonds with polymer functional groups. However, according to the authors of [12], such film treatment can lead to increase of chain packing ordering in polyetherimides. Nevertheless, there has been no confirmation of changes of conformational chain ordering after amorphous polymer film treatment with sc-CO2 obtained by other methods. The aim of this work is to verify the hypothesis on the effect of different film formation and treatment conditions on conformational order of macromolecules. Quantitative criteria of amorphous polymer chain packing ordering are proposed for PEI Ultem (Scheme1) based on analysis of structural characteristics from IR spectra of polymeric films at di fferent formation and treatment conditions and diffusion coefficients of non-condensable gases (H2, He, N2, O2, CO2, and CH4). Formation of hydrogen bonds has been noted earlier for this polymer [13,15], and quantitation of chain packing ordering change has been performed using gas separation characteristics analysis [12] at different film formation and treatment conditions. Polymers 2020, 12, 1578 3 of 15 Polymers 2020, 12, x FOR PEER REVIEW 3 of 15

O CH3 O

O C O N N

CH3 n O O

SchemeScheme 1.1. ElementaryElementary unitunit ofof UltemUltem structurestructure ((TTgg == 215 °C).◦C).

2. Materials and Methods 2. Materials and Methods 2.1. Film Formation 2.1. Film Formation All Ultem PEI films (ULTEM® 1000 Resin, SABIC Innovative Plastics, Riyadh, Saudi Arabia) were All Ultem PEI films (ULTEM® 1000 Resin, SABIC Innovative Plastics, Riyadh, Saudi Arabia) formed from a 5% solution in chloroform (ChP). were formed from a 5% solution in chloroform (ChP). For gas separation properties investigation, films of 30–35 µm thickness were prepared by casting For gas separation properties investigation, films of 30–35 µm thickness were prepared by over a cellophane support from 5% solution in chloroform with subsequent drying for 2–3 days at casting over a cellophane support from 5% solution in chloroform with subsequent drying for 2–3 ambient temperature and then keeping them in vacuum at 1–2 mmHg until constant weight is achieved. days at ambient temperature and then keeping them in vacuum at 1–2 mmHg until constant weight Films for investigation by vibrational spectroscopy with a thickness of 5–7 µm were formed from is achieved. a solution of chloroform on optical glass made of silicon and dried at room temperature to constant Films for investigation by vibrational spectroscopy with a thickness of 5–7 µm were formed from weight. At least 2 films were formed for each state. a solution of chloroform on optical glass made of silicon and dried at room temperature to constant 2.2.weight. Film At Treatment least 2 films Methods were formed for each state.

2.2. FilmFor gasTreatment separation Methods experiments: - As-castFor gas filmseparation formation experiments: by solvent evaporation (see above); 12% to 13% of residual chloroform is fixed in films in these conditions [22]; - As-cast film formation by solvent evaporation (see above); 12% to 13% of residual chloroform is - Annealing at 10–20 C above the glass transition temperature [12,22]; fixed in films in these◦ conditions [22]; -C H OH treatment—swelling in ethanol for one day with further evacuation [12,19]; - Annealing2 5 at 10–20 °C above the glass transition temperature [12,22]; - sc-CO treatment—swelling in sc-CO for 4 h at 120 C and 450 atm. - C2H5OH2 treatment—swelling in ethanol2 for one day◦ with further evacuation [12,19]; - Forsc-CO IR2 spectroscopy,treatment—swelling film treatment in sc-CO on2 for Si optical4 h at 120° glasses C and repeated 450 atm. film treatment modes for gas separationFor IR films.spectroscopy, film treatment on Si optical glasses repeated film treatment modes for gas separation films. 2.3. Investigation Methods 2.3. InvestigationFTIR spectra Methods of films on Si optical glasses were recorded on an IFS 66 v/s vacuum Fourier transform 1 1 spectrometerFTIR spectra (“Bruker”, of films Bremen, on Si Germany, optical glasses range: 400–4000were recorded cm− , resolutionon an IFS 1 66 cm v/s− ). vacuum For metrological Fourier processing of the results, the IR spectra of each film were recorded at least twice, moving the sample in transform spectrometer (“Bruker”, Bremen, Germany, range: 400–4000 cm–1, resolution 1 cm−1). For themetrological cell compartment. processing of the results, the IR spectra of each film were recorded at least twice, moving Polymer films density (ρ) was determined at room temperature 24 2 C by hydrostatic weighing the sample in the cell compartment. ± ◦ in isopropanol.Polymer films Fractional density free (ρ volume) was determined (FFV) was determined at room temperature by Bondi method: 24 ± 2 FFV °C =by1–1.3 hydrostatic Vw/Vsp, ρ whereweighing Vw inis isopropanol. Van der Waals Fractional volume offree elementary volume (FFV) unit, was Vsp determined= M/ is a specific by Bondi occupied method: volume FFV = 1– of polymer, and M-polymer elementary unit molecular weight. 1.3 Vw/Vsp, where Vw is Van der Waals volume of elementary unit, Vsp = M/ρ is a specific occupied volumeDiff ofusion polymer, coeffi cientsand M-polymer (D) of N2,O elementary2, CO2, and unit CH molecular4 in films wereweight. obtained by integral barometric method on MKS Barotron assembly. Diffusion coefficients (D) of N2, O2, CO2, and CH4 in films were obtained by integral barometric For metrological processing of the results, the average value of the relative optical density for the method on MKS Barotron assembly. P 2 n (x x) For metrological processing of the results, the average value of the relative2 opticali =density1 i for selected pairs of absorption bands x (x = A1/A2), the variance of the mean V(x) = s (x) = − , the selected pairs of absorption bands ⎯x (x = A1/A2), the variance of the mean () =n 1() = s(x) − the∑ relative( ̅) standard deviation from the mean (reproducibility) sr(x) = and the confidence() interval , the relative standard deviation from the mean (reproducibility)x () = and the ∆A were calculated. confidenceThe significance interval ∆ ofA thewere di ffcalculated.erences in the results for the series of films subjected to various treatment methodsThe wassignificance evaluated of usingthe differences the modified in the Student’s results test:for the the series average of standardfilms subjected deviation to various of two treatment methods was evaluated usings the modified Student’s test: the average standard deviation + x x2 q f1 V1 f2 V2 1 n1 n2 measurement series was found s(x) = ⎯ and test statistics ξ = |− | ∙· , where of two measurement series was found s(x) = + and test statistics ξ = ( ) n1+n2 , where f1 f2 s x()

f1 и f2—are the degrees of freedom for two series of measurement and n1 и n2—are the number of

parallel measurements from which и were calculated; confidence probability P = 0.90.

Polymers 2020, 12, 1578 4 of 15

f 1 и f 2—are the degrees of freedom for two series of measurement and n1 иn2—are the number of parallel measurements from which x1 и x2 were calculated; confidence probability P = 0.90. Changes in structural characteristics were considered significant according to the condition for assessing the significance of the results for homogeneous dispersions, ξ > t (P, f), where t is the Student’s coefficient for confidence probability P and the number of degrees of freedom f. For ξ < t (P, f ) the changes were considered insignificant and it was assumed that the treatment method does not affect the conformational order in macromolecules or the sensitivity of the method is not sufficient for reliable recording of such effect.

3. Results and Discussion

3.1. Packing Ordering by Gas Separation Data

Similarly to slow depressurization Ultem films treatment with sc-CO2 with further fast depressurization [12,18] significantly decreases density and increases free volume (Table1). However, similarly to slow depressurization case [12,18], free volume increase is not accompanied by diffusion coefficients increase: D is shown to decrease for “as cast” films (Table1) and to practically remain the same for annealed films. At the same time, sc-CO2 treatment leads to significant diffusion selectivity increase in all cases (Table1), which indicates chain packing ordering increase [ 12]. A similar effect, although less pronounced, is caused by film treatment with ethanol. In [12], quantitative characteristics of packing ordering determined through parameters of linear regressions of gas diffusion coefficients and gas-kinetic diameters [23] are proposed (Table1).

log D = a b d2 (1) − · These characteristics are relative (compared to the closest to equilibrium state annealed sample) cohesion energy changes ∆Ecoh (inter-chain contacts in dense polymer matrix part quantity), entropy change of the basic state ∆S0 (∆S0 increase is responsible for a chain packing ordering) and, finally, experimental value of diffusion selectivity αD (higher diffusion selectivity means that chain fragment packing in dense polymer matrix part is denser, more ordered).

Table 1. Density, fractional free volume, diffusion coefficients, diffusion selectivity, and relative changes in the parameters a and b, changes of entropy of the basic state and cohesion energy for Ultem films.

8 2 d 0 ρ, FFV, D 10 , cm /S A ∆a, ∆b, ∆S , ∆Ecoh, Sample, Treatment 3 · Ref g/cm % % % J/mol K kJ/mol O2 CO2 O2/N2 CO2/CH4 · as cast 1.294 11.2 1.5 0.33 4.4 3.6 40 24 32 50 [12,19] as cast + C2H5OH 1.282 12.0 0.99 0.26 4.5 7.2 64 48 51 100 [12,19] as cast + sc-CO2 * 1.276 12.4 0.46 0.11 4.1 7.6 65 55 52 115 [12,18] as cast + sc-CO2 ** 1.268 13.0 0.69 0.17 6.3 6.1 54 43 43 90 This work annealing 1.288 11.6 0.42 0.10 4.2 2.4 0 0 0 0 [12,22] annealing + sc-CO2 * 1.278 12.3 0.42 0.11 5.7 13.0 85 74 68 155 [12,18] annealing + sc-CO2 ** 1.268 13.0 0.67 0.13 5.2 4.3 47 37 37 78 This work * Depressurization rate of 0.025 MPa/min, ** Depressurization rate of 45 MPa/min.

According to these criteria, the most disordered is the packing of chains in the annealed sample. (Table1). The unannealed “as-cast” sample shows higher level of order, which was previously explained by elementary unit conformational composition order resulting from non-covalent bonding of oxygen (O) atom in phenyl-ether fragment with chloroform [13–16]. Ethanol and sc-CO2 treatment leads to further ordering. In this case, sc-CO2 treatment eﬀect on ordering depends on depressurization rate. sc-CO2 treatment with slow depressurization leads to the greatest ordering. Film samples prepared by sc-CO2 treatment with fast depressurization have lower density and higher free volume, but its ordering levels are comparable to the samples prepared by ethanol treatment (Table1). Polymers 2020, 12, 1578 5 of 15

3.2. Conformation Order of Films According to Quantum Chemical Modeling Data The previously considered interaction of polymer functional groups with a solvent, and as a result formation of hydrogen bonds, can lead to a reorientation of polymer functional groups about a polymer backbone changing its conformational parameters and increasing the content of a conformer of a single geometry (conformation order) [13–16]. Relevant effects of such conformation order are possible only with a particular value of comparator parameter for kinetic chain rigidity. The authors suggested using the proportion of phenyl-ether junctions per elementary unit (k), which was calculated as the ratio of the number of ether O atoms to the number of phenyl rings in the PI backbone as such parameter. For Ultem k = 0.4, whereas meta-substituted aromatic ring is located between the imide cycles, and aromatic cycles, and isopropylidene groups form a tetrahedron with which the para-substituted phenyl rings are connected. For such PI, despite the possibility of non-covalent interactions of Ph–O–Ph0 fragment with the solvent, conformation order in the elementary unit either cannot be achieved at all or is determined by other structural parameters. Therefore, the next task was to find a structural parameter (criterion) that (a) can be reliably determined from experimental spectral data, (b) uniquely characterizes changes of the conformational set of the elementary unit, and (c) can be compared with the measured or calculated physicochemical characteristics of a polymer object. Specific inter-chain interactions leading to the creation of ordered regions and in the extreme—to a semicrystalline structure—for PI, which has only para-substituted aromatic cycles in the elementary unit at Ph–O–Ph0 fragments (Kapton) are described previously. A method for selecting an optimal conformation of the macromolecule in the crystalline region was developed for it, and variants of polymer chains packing were calculated [24,25]. In these works, it was shown that the para-substituted aromatic rings at the Ph–O–Ph0 nodes in Kapton are “hinges” that are characterized by a variety of conformers. It is noted that for any conformations of such fragments defined only by the angle of rotation, the length of the monomeric unit does not change. In this case, imide cycles form two types of conformations: “propeller” and “roof-like”, and the energy of these states is such that the transitions of one conformation to another are impossible, both exist simultaneously. This structure of PI leads to the possibility of intermolecular π–π interactions and formation of a denser packing of polymer chains. The possibility of π–π interaction for fragmentary models of Ultem containing an aromatic ring between imide cycles (Im–Ph–Im0 fragment) was evaluated by calculation of geometric, energy, and electronic parameters (HF/6-31G(d, p)) and bringing the calculations to the theoretical vibrational spectra (B3LYP/6-31G(d, p)). Quantum chemical calculations have shown an almost equal probability of the existence of two conformers of Im–Ph–Im0 fragment (Table2) with di fferent values of geometric parameters. In other words, similar to the “propeller” and “roof-like” conformation in Kapton, in Ultem, too, both Im–Ph–Im0 conformations exist simultaneously, and inter-chain π–π interaction between these polyheteroarylene fragments bound by a meta-substituted aromatic ring becomes possible. Calculations of the geometric parameters of two fragments located relative to each other in a “roof-like” conformation (model III) confirmed our assumption: the interaction energy (∆E ~ 16.4 kJ/mol, Table2), the distance between the atoms and the change in charges on the atoms − correspond to π–π interaction between Im–Ph–Im0 fragments. For example, the change in charge on the oxygen atoms of the imide cycle in absolute value is |∆q| 0.068–0.100, on nitrogen atoms ≈ ≈ 0.225–0.259; the change in bond lengths is ~2%. Polymers 2020, 12, x FOR PEER REVIEW 6 of 15 Polymers 2020, 12, x FOR PEER REVIEW 6 of 15 Polymers 20202020,, 1212, ,x 1578 FOR PEER REVIEW 6 of 15 6 of 15 Table 2. Geometric and energy characteristics of two Im–Ph–Im’ fragment conformers. Table 2. Geometric and energy characteristics of two Im–Ph–Im’ fragment conformers. Δ TableTable 2. Geometric 2. Geometric and− and energyEmin energy, characteristics characteristicsDihedral of two Angle of Im–Ph–Im’ two Im–Ph–Im fragment0 fragment conformers. conformers. Full Mulliken Atomic Charges Conformer Structure −ΔEmin, Dihedral Angle Bond Length, Å Full Mulliken Atomic Charges Conformer Structure kJ/molΔ N(3)-C(6)-N(7)-O(9), Deg. Bond Length, Å C N O − Emin∆, E , DihedralDihedral Angle Angle Full MullikenFull Mulliken Atomic AtomicCharges Charges ConformerConformer StructureStructure kJ/mol minN(3)-C(6)-N(7)-O(9), Deg. Bond Length,Bond Length, Å Å C N O kJ/mol−kJ /molN(3)-C(6)-N(7)-O(9),N(3)-C(6)-N(7)-O(9), Deg. Deg. C N O O(1)–C(2) 1.213 CNO O(1)–C(2)C(2)–N(3) 1.2131.425 (2) 0.584 (1) 0.477 O(1)–C(2) 1.213 (3) 0.667 I 0 28.6 C(2)–N(3)C(8)–O(9)O(1)–C(2) 1.4251.213 1.213(2)(8) 0.5840.586 (1)(9) 0.4770.471 C(2)–N(3) 1.425 (2) 0.584 (3) 0.667 (1) 0.477 I 0 28.6 C(8)–O(9)C(2)–N(3) 1.213 1.425(8) 0.586(2) 0.584(7) 0.667 (9) 0.471 (1) 0.477 C(8)–N(7) 1.418 (10) 0.584 (7)(3) 0.667(3) 0.667(11) 0.477 I I 0 028.6 28.6 C(8)–O(9)C(8)–O(9) 1.213 1.213(8) 0.586(8) 0.586 (9) 0.471 (9) 0.471 C(10)–O(11)C(8)–N(7) 1.418 1.213 (10) 0.584 (7) 0.667(7) 0.667(11) 0.477 C(8)–N(7)C(8)–N(7) 1.418 1.418(10) 0.584(10) 0.584 (11) 0.477(11) 0.477 C(10)–O(11) 1.213 C(10)–O(11)C(10)–O(11) 1.213 1.213

O(1′)–C(2′) 1.214 O(1′)–C(2′) 1.214 C(2′)–N(3O(1 ′)–C(2) 1.425) 1.214(2′) 0.581 (1′) 0.478 C(2O(1′′)–N(3)–C(20′) 1.2141.4250 (2′) 0.581 (3′) 0.673 (1′) 0.478 II 3.44 * −34.2 C(8′)–O(9C(20′)–N(3) 1.2120) 1.425(8′) 0.582(20 ) 0.581(3′) 0.673 (9′) 0.459(1 0) 0.478 II 3.44 * −34.2 C(2C(8′)–N(3)–O(9′) 1.2121.425 (8(2′) 0.5820.581 (7′) 0.667(3 0) 0.673(9(1′) 0.4590.478 II 3.44 * 34.2 C(8′)–N(7C(80′)–O(9) 1.4180) 1.212(10′) 0.579(80) 0.582(3′) 0.673 (11′) 0.478(9 0) 0.459 II 3.44 * −34.2 − C(8′)–O(9′) 1.212 (8′) 0.582 (7′) 0.667(7 0) 0.667(9′) 0.459 C(10C(8′′)–N(7)–O(11C(80′)–N(7)′ )1.418 1.2140) 1.418 (10′) 0.579(100 ) 0.579(7′) 0.667 (11′) 0.478(11 0) 0.478 C(10C(8′)–N(7)–O(11C(100′)–O(11)′ )1.418 1.2140 ) 1.214(10′) 0.579 (11′) 0.478 C(10′)–O(11′) 1.214

O(1)–C(2) 1.189 O(1)–C(2) 1.189 O(1)–C(2)C(2)–N(3) 1.1891.327 (2) 0.866 (1) 0.577 O(1)–C(2)C(2)–N(3) 1.189 1.327 (2) 0.866 (1) 0.577 19.4 19.4 C(2)–N(3) 1.327 (2) 0.866 (3) 0.932(3) 0.932(1) 0.577 IIIIII 16.4 **16.4 ** 19.4 C(8)–O(9)C(8)–O(9) 1.185 1.185 (8) 0.856(8) 0.856(3) 0.932 (9) 0.539 (9) 0.539 21.7 21.7 C(2)–N(3) 1.327 (2) 0.866 (7) 0.922(7) 0.922(1) 0.577 III 16.4 ** 19.4 C(8C(8)–O(9)′)–N(7C(8 ′)–N(7) 1.185 1.393) 1.393(10)(8) 0.856 0.882(10) 0.882(3) 0.932 (11)(9) 0.539 0.578(11) 0.578 III 16.4 ** 21.7 C(8)–O(9)0 1.1850 (8) 0.856 (7) 0.922 (9) 0.539 21.7 C(10)–O(11)C(8′)–N(7C(10)–O(11)′) 1.393 1.189 1.189(10) 0.882 (7) 0.922 (11) 0.578 C(10)–O(11)C(8′)–N(7′) 1.393 1.189 (10) 0.882 (11) 0.578 C(10)–O(11) 1.189 * Calculated Calculated asas the didifferenceﬀerence betweenbetween energy energy minima minima for for conformers. conformers. ** ** Calculated Calculated as as the the di differenceﬀerence between between the the total total energy energy of o thef the model model and and the the sum sum of of the the total total energies of *energiesthe Calculated conformers. of the as conformers.the difference between energy minima for conformers. ** Calculated as the difference between the total energy of the model and the sum of the total energies* Calculated of the as conformers.the difference between energy minima for conformers. ** Calculated as the difference between the total energy of the model and the sum of the total energies of the conformers.

Polymers 2020, 12, x FOR PEER REVIEW 7 of 15

Calculations of the geometric parameters of two fragments located relative to each other in a “roof-like” conformation (model III) confirmed our assumption: the interaction energy (∆E ~−16.4 kJ/mol, Table 2), the distance between the atoms and the change in charges on the atoms correspond to π–π interaction between Im–Ph–Im’ fragments. For example, the change in charge on the oxygen ⏐ ⏐ ≈ ≈ atomsPolymers of2020 the, imide12, 1578 cycle in absolute value is ∆q 0.068–0.100, on nitrogen atoms 0.225–0.259;7 the of 15 change in bond lengths is ~2% Considering that the only meta-substituted aromatic ring is located between imide fragments in the elementaryConsidering unit that of PI, the π only–π interactionmeta-substituted or its absence aromatic will, ring in is the located first place, between affect imide polarization fragments of in C=Othe elementaryand C-N bonds unit in of PI,imideπ– πcycle,interaction and, consequently, or its absence the will, change in the in first optical place, density affect polarizationof absorption of bandsC=O andof νC=O C-N, νC–N bonds stretching in imide vibrations, cycle, and, and consequently, δNCO, δCNC deformational the change in oscillations. optical density In addition, of absorption the effectbands should of νC =beO, noticeableνC–N stretching on the vibrations, deformational and oscillationsδNCO, δCNC ofdeformational protons (out of oscillations. the plane of In the addition, ring) inthe the e ffmetaect-substituted should be noticeable benzene ring on the in pa deformationalrticularly. To oscillationsaccurately assign of protons the absorption (out of the bands plane of of the the spectrumring) in the to metathe -substitutedvibration types benzene for models ring in particularly.I and II (Table To accurately2) theoretical assign vibrational the absorption spectra bands were of calculated.the spectrum Their to analysis the vibration showed types that for the models I and II (Table2) theoretical vibrational spectra were calculated. Their analysis showed that the - change of position of the meta-substituted aromatic ring relative to the imide cycles leads to an - insignificantchange of positionchange in of the the frequencymeta-substituted of the corresponding aromatic ring vibration relative to in the the imide theoretical cycles vibrational leads to an spectruminsignificant (2–3 cm change−1), while in the the frequency spectra of of both the corresponding models are not vibration identical; in the theoretical vibrational 1 - onlyspectrum 730 and (2–3 780 cmcm−−1 ),frequencies while the spectra refer to of δCCH both and models δCNC vibrations, are not identical; respectively. - only 730 and 780 cm 1 frequencies refer to δ and δ vibrations, respectively. In the experimental Ultem− spectrum these freqCCHuenciesCNC correspond to absorption bands with maximaIn at the 745 experimental and 777 cm Ultem−1, which, spectrum given the these limitations frequencies of the correspond model into comparison absorption with bands the with PI 1 macromolecule,maxima at 745 can and be 777 considered cm− , which, a satisfactory given the result limitations (Figure of 1). the model in comparison with the PI macromolecule, can be considered a satisfactory result (Figure1).

745

777

782 780 729 3 727 3 2 2

1 1 -1 800 750 ν, см 1100 1000 900 800 700 600 ν, см-1

(a) (b)

FigureFigure 1. 1. FragmentsFragments of of theoretical vibrationalvibrational spectra spectra of of models models I and I and II of II Im–Ph–Im of Im–Ph–Im’0 unit inunit the in region the 1 regionof 1050–500 of 1050–500 cm− (cma) and–1 (a) the and absorption the absorption of the characteristicof the characte frequenciesristic frequencies of the modelsof the models (b), curves (b), 1 curvesand 2 ,1 and and a 2 fragment, and a fragment of the IR of spectrum the IR sp ofectrum the Ultem of the ﬁlm Ultem (curve film 3 ).(curve 3).

π π SummingSumming up up the the analysis analysis of of the the results results of of theo theoreticalretical calculations, calculations, it itcan can be be noted noted that that the the π–π– interactioninteraction or or its its absence absence in in Ultem Ultem is is possible possible only only in in Im–Ph–Im’ Im–Ph–Im fragment,0 fragment, which which should should be be reflected reflected inin all all the the absorption absorption bands bands of of the the respective respective functi functionalonal groups, groups, though though will will not not lead lead to to a asignificant significant changechange in in the the positions positions of of the the maximum maximum of of the the ab absorptionsorption bands bands for for corresponding corresponding vibrations, vibrations, but but onlyonly to to change change of of bonds bonds polarization, polarization, i.e., i.e., to to the the change change in in intensity. intensity. 3.3. FTIR Spectroscopy 3.3. FTIR Spectroscopy Thin ( 7 µm) Ultem films were cast from chloroform solution (as cast films) on an optical Si-glass. ≤ These films were (a) treated with ethanol, (b) treated with supercritical fluid (sc-CO2), (c) annealed, and (d) annealed and treated with sc-CO2. Minimum two films were formed for each treatment type. For further statistical processing IR-spectra of each film were registered in transmission mode not less than twice. In addition to main absorption bands of Ultem, absorption bands of chloroform (with maxima at 1 667, 754 and with “shoulder curve” at 3023 cm− ) are registered in IR-spectra of as cast film (Figure2). Polymers 2020, 12, x FOR PEER REVIEW 8 of 15

Thin (≤ 7 µm) Ultem films were cast from chloroform solution (as cast films) on an optical Si- glass. These films were (a) treated with ethanol, (b) treated with supercritical fluid (sc-CO2), (c) annealed, and (d) annealed and treated with sc-CO2. Minimum two films were formed for each treatment type. For further statistical processing IR-spectra of each film were registered in transmission mode not less than twice. Polymers 2020, 12, 1578 8 of 15 In addition to main absorption bands of Ultem, absorption bands of chloroform (with maxima at 667, 754 and with “shoulder curve” at 3023 cm–1) are registered in IR-spectra of as cast film (Figure It2). should It should be notedbe noted that that positions positions of these of these absorption absorption bands bands maximums maximums diﬀ erdiffer from from its positions its positions in the in 1–1 IRthe spectrum IR spectrum of chloroform of chloroform (curve (curve3, Figure 3, Figure2: 670, 2: 670, 760, 760, and and 3018 3018 cm cm− ),), which which shows shows a a presencepresence ofof noncovalentnoncovalent bondsbonds betweenbetween chloroformchloroform protonproton andand PIPI elementaryelementary unitunit atomatom (or(or atoms).atoms).

3023 3018 2970

3070

3200 3100 3000 2900 ν, cm-1

(a)

1720 3

1360 1276 1266

1600 1780 850 670 667

2 1 1800 1500 1200 900 ν, cm-1

(b)

Figure 2. IR-spectra for films of Ultem before (curve 1) and after ethanol treatment (curve 2) at CH-stretching vibrations absorption region (a) and at “fingerprinting” region (b) in comparison with CHCl spectrum (curve 3). 3 Only polyimide absorption bands are registered in the IR spectra of Ultem films after all treatment methods. Figure2 shows example of IR spectrum fragment for Ultem film after its ethanol treatment. Let us observe what happens to the conformational composition of the polyimide elementary unit. Polymers 2020, 12, x FOR PEER REVIEW 9 of 15

Figure 2. IR-spectra for films of Ultem before (curve 1) and after ethanol treatment (curve 2) at CH- stretching vibrations absorption region (a) and at “fingerprinting” region (b) in comparison with

CHCl3 spectrum (curve 3).

Only polyimide absorption bands are registered in the IR spectra of Ultem films after all Polymerstreatment2020 methods., 12, 1578 Figure 2 shows example of IR spectrum fragment for Ultem film after its ethanol9 of 15 treatment. Let us observe what happens to the conformational composition of the polyimide elementary unit. ComparisonComparison ofof thethe IRIR spectraspectra ofof filmsfilms pre-pre- andand post-treatmentpost-treatment diddid notnot revealreveal anyany changeschanges inin thethe positionspositions ofof thethe absorptionabsorption bandsbands maxima,maxima, butbut didifferenfferencesces inin thethe relativerelative intensityintensity ofof onlyonly somesome ofof thethe absorptionabsorption bandsbands of of Ultem Ultem functional functional groups groups were were registered. registered. For For clarity, clarity, the the spectra spectra are are located located on the on samethe same baseline. baseline. In order In order to correctly to correctly compare compare these th changes,ese changes, one needs one needs to normalize to normalize the spectrum the spectrum to an “internalto an “internal standard”. standard”. ItIt cancan be noted that that the the absorption absorption bands bands of of some some types types of of vibrations vibrations are are completely completely identical identical in inthe the IR IR spectra spectra of of films films pre- pre- and and post-treatment; post-treatment; for for clarity, clarity, all all the the spectra spectra are are placed on thethe samesame baseline.baseline. Given that the symmetry of the parapara-substituted-substituted phenyl rings in UltemUltem ensuresensures thatthat theirtheir spectralspectral characteristicscharacteristics (maximum (maximum position, position, half-width, half-width, and and intensity) intensity) remain remain unchanged unchanged regardless regardless of theof noncovalentthe noncovalent interaction interaction of neighboring of neighboring atoms, theatoms, absorption the absorption band of deformational band of deformational oscillations 1 ofoscillations neighboring of neighboring H atoms during H atomspara during-substitution para-substitution in the aromatic in the ringaromatic (850 cmring− (850) does cm not−1) does change not itschange intensity its intensity for all of for film all treatment of film treatment methods, method and it cans, and be usedit can as be an used internal as an standard internal (Figure standard3). Changes(Figure 3). in Changes relative intensityin relative were intensity observed were for observed absorption for bandsabsorption of symmetrical bands of symmetrical C=O stretching C=O 1 1 vibrationsstretching andvibrations CN stretching and CN vibrations stretchi inng imidevibrations cycle (1720in imide cm− cycleand 1360 (1720 cm −cm, respectively);–1 and 1360 cm C-O–1, 1 vibrationsrespectively); in phenyl-ether C-O vibrations fragment in phenyl-ether (1300–1040 cmfragment− ); basic (1300–1040 vibrations cm of C–1=); C-bondsbasic vibrations in aromatic of ringsC=C- 1 δ δ withbonds maxima in aromatic at 1600, rings 1500, with and maxima 1480 cm at− 1600,; and 1500, deformational and 1480 oscillationscm–1; and deformational of CCH and oscillationsCNC (777 and of 745 cm 1, respectively) [26]. δCCH and− δCNC (777 and 745 cm–1, respectively) [26].

FigureFigure 3.3. Fragments of the IR spectra of Ultem films films in the region of absorption of stretching vibrations ofof imideimide andand aromaticaromatic cyclescycles ((aa)) andand thethe regionregion ofof absorptionabsorption ofof vibrationsvibrations ofof C-OC-O bonds,bonds, normalizednormalized 1 toto thethe absorptionabsorption bandband atat 850850 cmcm−−1 ((b): 1—a filmfilm formed fromfrom CHClCHCl33 (as cast);cast); 22—as-cast—as-cast filmfilm afterafter

ethanolethanol treatment;treatment ;3 3—as-cast—as-cast filmfilm afterafter scsc-CO-CO22 treatment;treatment ; 44—as-cast—as-cast filmfilm afterafter stepwisestepwise annealing;annealing; 55—as-cast—as-cast filmfilm afterafter stepwisestepwise annealingannealing andand scsc-CO-CO22 treatment.treatment.

ConformationalConformational changes of C–O–CC–O–C bonds due toto thethe formationformation oror rupturerupture ofof hydrogenhydrogen bondsbonds withwith thethe chloroformchloroform fromfrom which which the the films films were were formed formed will will necessarily necessarily a ffaffectect the the polarization polarization of of C-H C- bondsH bonds in in the the methyl methyl group. group. It It would would not not lead lead to to change change in in geometric geometric parameters parameters ofof isopropylideneisopropylidene groupgroup (with(with absorptionabsorption bandsbands maximummaximum shift), but it will aaffectffect absorptionabsorption bands intensity, whichwhich isis 1 1 indeedindeed seenseen inin spectraspectra atat 29702970 cmcm−–1 (Figure(Figure 22)) andand 13801380 cmcm–1− (Figures(Figures 22 andand 33).). ItIt cancan be concluded that that all all of of the the treatment treatment methods methods of of Ultem Ultem films films (annealing, (annealing, ethanol, ethanol, and and scsc-CO treatment) may lead to conformational changes of macromolecules due to bond polarization CO2 treatment)2 may lead to conformational changes of macromolecules due to bond polarization change and conformational changes of elementary unit, and it explains difference in film IR-spectra after treatment. Evaluation of these changes in Ultem structure and their accurate correlation with film macroparameters changes the required selection of absorption bands for which variations of spectral characteristics after treatment exceed single deviations from average values in parallel determinations (sr). Polymers 2020, 12, x FOR PEER REVIEW 10 of 15 change and conformational changes of elementary unit, and it explains difference in film IR-spectra after treatment. Evaluation of these changes in Ultem structure and their accurate correlation with film macroparameters changes the required selection of absorption bands for which variations of spectral characteristics after treatment exceed single deviations from average values in parallel determinations (sr). Polymers 2020, 12, 1578 10 of 15 Moreover, changes in the structural criterion for various types of film processing should be significant. To select the structural criterion, the reproducibility of the optical density (A) measurementMoreover, results changes was evaluated in the structural for 11 initial criterion films for and various for two types parallel of film measurements processingshould of each be typesignificant. of film treatment. To select the structural criterion, the reproducibility of the optical density (A) measurement resultsFor wasvalues evaluated of A for for which 11 initial the value films and of the for relative two parallel standard measurements deviation of from each the type mean of film⎯sr treatment.did not exceedFor 0.04 values and the of Aconfidence for which interval the value ∆A of was the relative≤ 0.07, standardthe significance deviation of changes from the in mean the srelativer did not opticalexceed density 0.04 and (A1/A the2) confidenceusing the modified interval Student’s∆A was t-test0.07, fo ther all significance films was evaluated. of changes Changes in the relativewere ≤ consideredoptical density significant (A1/A if,2) according using the modifiedto the condition Student’s for t-test evaluating for all filmsthe significance was evaluated. of the Changes results werefor homogeneousconsidered significant variances, if,test according statistics toξ > the t (P condition, f), where for t is evaluating the Student the coefficient significance for the of the confidence results for probabilityhomogeneous P and variances, the number test of statistics degreesξ of> tfreedom(P, f ), where f. Fort isξ the< t (P Student, f), the coe changesfficient were for the considered confidence insignificantprobability andP and this the ratio number A1/A2 ofwas degrees excluded of freedom from considerationf. For ξ < t (Pas, af) structural, the changes criterion. were considered insignificantIn Figure and4, significant this ratio A changes1/A2 was in excluded some structural from consideration criterion asfor a different structural methods criterion. of film treatmentIn Figure relative4, significant to the initial changes film: in someannealing, structural annealing criterion + forsc-CO di fferent2 (Figure methods 4a,b) of and film treatmentethanol treatment,relative toand the sc-CO initial2 treatment film: annealing, without annealing annealing+ (Figuresc-CO2 4c,d)(Figure are4 shown.a,b) and ethanol treatment, and sc-CO 2 treatment without annealing (Figure4c,d) are shown.

A1/A2 A1/A2

1360/850 2.0 745/850 1.0

777/850 1276/850

1.5 0.5 1266/850 777/745 исходнаяas castпл (CHCl3)annealing Отжиг Отжиг+ск-СО2 asисходная cast пл (CHCl3)annealing Отжигannealing Отжиг+ск-СО2 + ск-СО annealing + ск-СО2 2 (CHCl3) (CHCl ) 3

(a) (b)

A1/A2 A1/A2

1360/850 2.5 1.2 745/850

2.0 0.9 1276/850 777/850

1266/850 0.6 1.5 777/745

исходная пл (CHCl3) исходная пл+С2H5OH исходная пл+ск-СО2 as cast as cast as cast + ск-СО исходная пл (CHCl3) исходная пл+С2H5OH исходная пл+ск-СО2 2 as cast as cast + as cast + ск-СО2 (CHCl ) + C H OH 3 2 5 (CHCl3) C2H5OH

Figure 4. Changes in the relative intensities of the absorption bands of PI II functional groups (a,c):

CO vibrations in benzimide and Ph–O–h0 groups; (b,d)—vibrations in the Im–Ph–Im0 site) depending on the processing of its films: (a,b)—as cast film, as-cast film after stepwise annealing, as-cast film

after stepwise annealing and sc-CO2 treatment; (c,d)—as-cast film; as cast film after ethanol treatment; as-cast film after sc-CO2 treatment. Polymers 2020, 12, 1578 11 of 15

The intensities of the absorption bands corresponding to the stretching vibrations and deformational oscillations of all CO-groups change in case of annealing of the films, which leads to desorption of chloroform, rupture of the hydrogen bonds “chloroform–oxygen” between the residual solvent and the polymer, and conformational disordering in the Ultem elementary unit. 1 The intensity of the absorption band decreases at 1276 cm− and the intensity of the absorption 1 band increases at 1266 cm− , which is consistent with the data in [14]. The change in the intensity of absorption bands related to the vibrations of the imide cycle, 1 including those involving C=O groups (1720, 1360 cm− ), is insignificant (Figure4a). sc-CO2 treatment of the annealed film leads to the decrease in the intensity of all these absorption bands. On the contrary, the intensity of the absorption bands related to the vibrations of the imide and aromatic cycles in Im–Ph–Im0 unit demonstrates significant changes for annealed film, and insignificant changes for annealed film treated by sc-CO2 (Figure4b). Ethanol treatment of as-cast film, accompanied by desorption of chloroform, but not leading to disordering in Ph–O–Ph0 fragment, on the contrary, results in sharp increase of the intensity of the absorption bands of the corresponding vibrations (Figure4c), while sc-CO2 treatment of as-cast film leads to insignificant changes compared to the values for as cast film. For the absorption bands of the Im–Ph–Im0 unit both ethanol and sc-CO2 treatment methods lead to significant changes in the intensity relative to as-cast film. It is interesting to note that for the absorption bands that 1 characterize the deformational oscillations of δCNC in the imide cycle (745 cm− ) and δCCH in the 1 aromatic ring between the imide cycles (777 cm− ), the dependences are antibate (Figure4d). This demonstrates that Ultem film treatment affects Ph–p–Ph0 and Im–Ph–Im0 units differently and indicates the possibility of at least two fundamentally different conformational changes in pH–O–Ph0 and Im–Ph–Im0 fragments in the Ultem macromolecule depending on the treatment conditions of its films. Competing processes also explain the significant change in the sr value that characterizes the reproducibility of measurement results, depending on the conditions of films treatment: the worst results reproducibility for all absorption bands was recorded for annealed films. They are characterized by the greatest conformational disordering. In contrast to the previously studied polyimides [13], for Ultem the probability of hydrogen bonds is realized not only with the etheric O atoms, but also with the O atoms of the imide cycle, as evidenced by the change in the ratio of the νs/νas intensities of the stretching vibrations of C=O and the behavior 1 of the absorption bands at 1480 and 1360 cm− , related to δAr and δOCN, respectively. 1 The decrease in the intensity of the absorption band at 1276 cm− (A1276/A850) and the increase in 1 the intensity of the absorption band at 1266 cm− (A1266/A850) are corresponding to conformational disordering at the Ph–O–Ph0 site of elementary unit PI II (chloroform desorption, H-bonds destruction between the residual solvent and the polymer). The ratio A1276/A1266 characterizes the ratio of conformers, that is, conformation order. The decrease in the intensity of the absorption bands related to vibrations of the aromatic cycle in the Im–Ph–Im0 fragment indicates the decrease in the interplanar angle due to the intermolecular π–π interaction. The ratio of intensities A777/A745 characterizes the position of the aromatic ring between the imide cycles relative to the main PI chain, i.e., intermolecular π–π interaction.

3.4. The Influence of the Conformation of Macromolecules on the Parameters of Packing Ordering Suggested above as a structural criterion, the ratios of absorption band intensities of some functional groups in Ultem for samples subjected to various treatment methods can be compared with the quantitative characteristics of the packing ordering based on the analysis of gas diffusion coefficients [12] for film samples prepared under the same conditions (Table1). The results are shown in Table3. Polymers 2020, 12, 1578 12 of 15

Table 3. Quantitative characteristics of packing ordering for Ultem films and for various treatment methods based on the analysis of diffusion coefficients and structural criteria of Ultem samples prepared under equivalent conditions.

ρ, Ordering Parameters Structural Criteria Sample, Treatment 3 g/cm D 0 α (CO2/CH4) ∆E , kJ/mol ∆S ,J/mol K A1276/A1266 A777/A745 coh · as cast 1.298 3.6 50 32 1.27 0.48 as cast + C2H5OH 1.282 7.2 100 51 1.21 0.58 as cast + sc-CO2 * 1.268 6.1 90 43 1.21 0.59 annealing 1.288 2.4 0 0 1.18 0.61 annealing + sc-CO2 * 1.268 4.3 78 37 1.15 0.62 * Depressurization rate of 45 MPa/min.

It can be noted that when films are annealed (“as cast annealing”), the selectivity of diffusion → decreases by ~30%, which is accompanied by an increase of the value of A777/A745 (changes in the Im–Ph–Im0 units due to π–π interaction) by almost 30% of the value for the as cast film. Simultaneously, the ratio A1276/A1266 (change of conformational composition at the Ph–O–Ph0 fragment, disordering and increase of the content of the second conformer) decreases by approximately 10% and correlates 0 with change in the values of ∆S and ∆Ecoh. When the annealed film is sc-CO2-treated (annealing annealing+sc-CO2), the diffusion selectivity D 0 → α (CO2/CH4), as well as the values of ∆S and ∆Ecoh grow symbiotically with the growth of the ratio A777/A745, and the parameter A1276/A1266 decreases by ~10%. In other words, when the annealed film is sc-CO2-treated, the crucial role in changing of the ordering parameters is played by π–π interactions, i.e., local compaction of the film in Im–Ph–Im0 units. Conformational disordering in the Ph–O–Ph0 units is accompanied by a decrease in the film density, i.e., local disordering in these fragments determines the loosening of the packing as a whole. Ethanol or sc-CO2 treatment of as-cast film, as in the previous case, increases the probability of π–π interaction, which is accompanied by an increase in the value of the structural parameter 0 A777/A745 relative to the initial value by ~20%; the cohesion energy, the value of ∆S , and the selectivity of diffusion of CO2/CH4 pair of gases also increase, significantly more than for the annealed film. In this case the conformation order is maintained (the value of the parameter A1276/A1266is slightly reduced within the measurement error). Thus, noncovalent interactions with chloroform during the formation of a film from a solution set the conformational ordering in the Ph–O–Ph0 units for the Ultem macromolecule. Ethanol and sc-CO2 treatment does not violate this ordering, but contributes to additional ordering in the Im–Ph–Im0 fragments due to π–π interaction, which leads to a sharp D 0 increase in the ordering parameters (α , ∆S and ∆Ecoh). Analysis of these changes leads to a single explanation of these facts in total. Films treatment may lead both to some conformation ordering, especially due to hydrogen bonds with chloroform and ethanol, and to disordering, though not necessarily leading to intermolecular interaction. In Ph–O–Ph0 fragment the ratio A1276/A1266 can be considered as a structural criterion, which characterizes the ratio of conformers in this fragment, i.e., conformational ordering. Intermolecular π–π interaction is only possible in Ultem between Im–Ph–Im0 fragments. As a structural criterion for evaluation of such interactions, we can use the intensity ratio A777/A745, which characterize the position of the aromatic ring between the imide cycles relative to the PI backbone. Experimentally observed changes in the intensity of absorption bands of only particular functional groups in IR spectra, depending on the film treatment method, can only have place if Im–Ph–Im0 fragments drawing closer simultaneously with Ph–O–Ph0 fragments distancing from each other (Figure5). The process which will prevail depending on the Ultem film treatment type will determine the effect of conformational changes on the film properties. Polymers 2020, 12, 1578 13 of 15 Polymers 2020, 12, x FOR PEER REVIEW 13 of 15

O N O

N O O O

O O O O O N O N N O O O O N O O

O O N O

O N

Figure 5. Potential relative relative position of of PI fragments responsible for the presence of π–π interaction between Im–Ph–Im’ Im–Ph–Im0 fragments and an increase increase of inter-chain distan distancece between Ph–O–Ph’ Ph–O–Ph0 fragments.

Several important conclusions can be drawn from th thee results of this work. Treatment Treatment of films films of amorphous glassyglassy polymers polymers containing containing functional functional groups groups that causethat thecause existence the existence of various of geometric various geometricconformers conformers in the elementary in the elementary unit affects unit the affect conformationals the conformational structure structure of the elementary of the elementary unit, the unit,conformation the conformation of the macromolecule of the macromolecule as a whole, as and a whole, the packing and ofthe polymer packing chains of polymer in the film.chains Changes in the film.of the Changes conformational of the conformational composition composition of a macromolecule of a macromolecule due to noncovalent due to noncovalent interactions interactions of various ofnature various are notnature only are reflected not only in the reflected vibrational in the spectra vibrational of a polymer spectra object, of a butpolymer also correlate object, but with also the correlatephysicochemical with the characteristics physicochemical of this characteristics object. of this object. Practically, this eeffectffect is expressed in the possibility of explaining the seemingly illogical changes in gas transport characteristics when treating Ultem filmsfilms withwith ethanolethanol andand scsc-CO-CO22 [[12]:12]: the the film film density decreases and the free free volume volume increases, increases, bu butt the the diffusion diffusion coefficients coefficients decrease, which cannot be explained in the framework of the the free free volume volume theory. theory. Taking Taking into account the results of quantum chemical calculations and comparative analysis of changes in structural criteria and quantitative characteristics ofof packingpacking ordering, ordering, it canit can be statedbe stated that inthat Ultem, in Ultem, depending depending on the treatmenton the treatment method, method,both processes both ofprocesses ordering ofof the ordering conformational of the structureconformational in the Ph–O–Ph structure0 unit, in accompaniedthe Ph–O–Ph’ by aunit, local accompaniedincrease of the by distance a local between increase macromolecules of the distance in thesebetween fragments macromolecules (loosening ofin thethese package), fragments and (looseningintermolecular of πthe–π interactionpackage), overand Im–Ph–Imintermolecular0 fragments, π–π accompaniedinteraction over by a localIm–Ph–Im’ packing compactionfragments, accompanied(Figure5) occur. by a local packing compaction (Figure 5) occur. These results demonstrate that the IR spectroscopy method can be used as a tool for describing subtle structural changeschanges thatthat are are important important in in the th studye study of of the the structure–property structure–property relationship, relationship, and and the theparameters parameters obtained obtained from from the analysis the analysis of spectra of sp canectra correlate can correlate with the with macroparameters the macroparameters of polymer of polymerobjects, explaining,objects, explaining, in particular, in particular, changes changes in the physicochemical in the physicochemical characteristics characteristics of polymer of polymer films. films. Therefore, one should not only be guided by proper interpretation of absorption bands, but also have statistical data of spectral measurements interpretation.

Polymers 2020, 12, 1578 14 of 15

Therefore, one should not only be guided by proper interpretation of absorption bands, but also have statistical data of spectral measurements interpretation.

4. Conclusions The combination of experimental and theoretical methods of vibrational spectroscopy allows resolving of inverse problems applied to polymers to adequately identify changes of macromolecules conformational composition depending on the films treatment. The possibility of structural criterion (ratio of functional groups absorption bands intensities) selection to adequately reflect all of the conformational changes has been demonstrated and statistical processing of the measurement results confirmed accuracy of the data obtained. For amorphous glassy polyetherimide Ultem conformational ordering of macromolecules changes depending on film treatment procedure takes place, which is verified by data of Ultem films gas separation parameters study. Moreover, the presence of residual chloroform in films favors conformational ordering of macromolecules to remain unchanged in fragments Ph–O–Ph0. The desorption of residual chloroform with ethanol or sc-CO2 leads to an increase in conformational ordering in Im–Ph–Im0 units. This can result in approximation of these chain fragments of surrounding macromolecules and in inter-chain π–π interaction (local densification of packing). The annealed Ultem film shows the most disordered (amorphous) state at all of the fragments of macromolecules. Results of vibrational spectroscopy are well in agreement with the data of analysis of films gas separation parameters. It can be suggested that plasticization of polymer with ethanol or sc-CO2 leads to the formation of conformationally ordered parts in the polymer matrix, which maintain in a film and ensures high diffusion selectivity.

Author Contributions: Conceptualization, A.Y.A. and J.K.; methodology, A.Y.A. and J.K.; investigation, J.K., S.L., A.K., R.N., D.B. and A.Y.N.; writing—original draft preparation, J.K.; writing—review and editing, A.Y.A. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Russian Science Foundation (Grant No. 19-19-00614). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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