How Do Intermolecular Interactions Affect Swelling of Polyketones with

Article pubs.acs.org/JPCC

How Do Intermolecular Interactions Affect Swelling of Polyketones with a Differing Number of Carbonyl Groups? An In Situ ATR-FTIR Spectroscopic Study of CO2 Sorption in Polymers Andrew V. Ewing,† Anton A. Gabrienko,†,‡ Sergey V. Semikolenov,‡ Konstantin A. Dubkov,‡ and Sergei G. Kazarian*,† †

Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia

‡

ABSTRACT: ATR-FTIR spectroscopy was used in situ to study nine unsaturated polyketones derived from cis-1,4polybutadiene rubber, each containing a different concentration of carbonyl groups, under high-pressure CO2 conditions (up to 100 bar). The study was aimed to systematically determine the relationship between the concentration of carbonyl groups in the polyketones and their ability to absorb CO2 and swell. A linear relationship between increasing carbonyl concentration and the overall degree of swelling and CO2 sorption was observed for polyketones with a concentration of carbonyl groups below a specific value based on quantitative analysis from the ATR-FTIR spectra. However, polyketones, which had the highest concentration of carbonyl groups, did not follow this correlation. Instead, there was evidence of intermolecular interactions between the carbonyl groups in the polymer chains, which decreases the total CO2 sorption capacity and inhibited swelling. The effect of the different molecular weights of polymer was also studied with respect to polyketone swelling and CO2 sorption. No correlation was observed when comparing polymers with different molecular weights but contained a similar concentration of carbonyl groups. Hence, the main physical properties that affect the overall swelling and CO2 sorption into polyketone samples were quantitatively determined using ATR-FTIR spectroscopy. polymer;17,18 as well as in organic synthesis, catalysis, and coordination chemistry.7,19,20 Polymer processing has benefitted from the use of scCO2, where the longer term objectives are for this approach to effectively replace harmful organic solvents currently used in such applications. The sorption of CO2 into a polymer is fundamentally important for processing applications.21 Sorption is affected by the fact that CO2 is a Lewis acid and thus has the ability to weakly interact with Lewis base functional groups in a polymeric chain.22−24 This has significant effects on a polymer under high-pressure CO2 conditions, for example, reducing the hardness of glassy and semicrystalline polymers by lowering the glass-transition temperature.25,26 This is known as plasticization and, temporarily, means there are fewer interchain interactions and hence a higher mobility within the polymer.27 As a result, subjecting such polymers to high-pressure and supercritical

1. INTRODUCTION There is great potential to exploit supercritical fluids as alternatives to organic solvents because of the characteristic physical and chemical properties that they possess.1−3 One of the major assets of supercritical fluid technology is the ability to selectively “tune” its properties as a solvent by changing its density.4 In particular, supercritical carbon dioxide (scCO2) is attractive to a range of industrially relevant processes because of the drive toward “greener” processing routes. 5−7 The motivation to employ scCO2 in industrial processes stems from the fact that CO2 has low toxicity, flammability, and zero ozone depletion levels. Additionally, because of the gaseous nature of CO2 under ambient temperature and pressure, it is easy to remove after use, thus avoiding contamination in the final product. Supercritical processing has been applied to the extraction of residual solvents, unreacted starting material, and unwanted side products;8,9 separation, crystallization,10 and blending of polymers;11,12 for the swelling and sorption,13−16 which is relevant to the impregnation of new materials into the © 2014 American Chemical Society

Received: October 9, 2014 Revised: November 16, 2014 Published: November 17, 2014 431

dx.doi.org/10.1021/jp510208e | J. Phys. Chem. C 2015, 119, 431−440

The Journal of Physical Chemistry C

Article

Table 1. Physical Properties of Unsaturated Polyketone Samples sample

conversion of CC bonds/XCC (%)

concentration of CO groups/mol·g−1

density/g·cm−3

Mn × 10−3

Mw × 10−3

Mw/Mn

1S 2S 3S 4S 5S 6S 7S 8S 9S

3.5 7.7 20.7 25.4 32.5 40.0 48.0 65.8 31.4

0.000625 0.001375 0.003625 0.004313 0.0055 0.006563 0.007813 0.01019 0.005312

0.825 0.860 0.963 0.950 0.978 0.995 1.01 1.037 0.985

34 16 5.3 4.1 2.2 ∼2 1.1

87 42 14 10 5.1 ∼4.8 2.5

2.6 2.6 2.6 2.4 2.3 2.3 2.3

∼1.3

∼3.0

2.3

FTIR sampling modes, transmission,54 and attenuated total reflection (ATR)53,55 has been demonstrated. In this study, in situ ATR-FTIR spectroscopy was used to study the effects of varying concentrations of carbonyl groups on swelling of different polymeric materials and CO2 sorption. Typically, polymers will swell as CO2 is physically sorbed into the bulk of the sample. Polymers containing many Lewis basic sites, such as carbonyl groups, are reported to specifically interact with CO2 at high pressures and can result in swelling of the polymer.35,55 It is expected that as the concentration of the Lewis basic groups in polymers increases the overall polymer swelling and CO2 sorption will also increase. ATR-FTIR spectroscopy has previously revealed some very interesting phenomena about these systems, such as specific interactions between CO2 molecules and carbonyl groups resulted in the splitting of the band assigned to the degenerate bending mode of CO2,55 which was demonstrated for different types of polymers containing carbonyl groups. In this work, the swelling behavior of same type of polymers but with differing molecular weights and concentration of carbonyl groups were systematically studied. In situ ATR-FTIR spectroscopy provided a correlation about the extent of polymer swelling and the concentration of CO2 under high-pressure conditions to reveal the influence of the number of carbonyl groups within the polymeric chains.

CO2 provides unique opportunities for processing that otherwise would not be possible using traditional organic solvent methodologies. However, for the true potential of this processing technology to be realized a broader understanding of the CO2 behavior with the polymer functional groups is needed.27 Much focus has been given with respect to the significance of different functional groups in the polymeric chains, for example, hydrocarbon tails28 and ether,29 amine,30 or carbonyl21,31−33 groups. The positioning of the polar groups; that is, steric hindrance effects, number of electron-donating moieties, and morphology of the polymer are all shown to affect the sorption of CO2 into a sample.34−36 Thus, the interaction between CO2 and specific functional groups in different polymeric systems plays an important role in CO2 sorption and polymer swelling and solubility in highpressure and supercritical CO2.21,37,38 Physical effects such as a reduction in the interfacial tension, viscosity, permeability, and glass-transition temperature can occur to polymers subjected to a high-pressure CO2 environment.39 Understanding and controlling the solubility of polymers is potentially a major driving force toward more facile processing routes, that is, allowing the formation of new products from mixing, blending, and modifying CO2-soluble raw materials, which otherwise may be unusable. However, the role of these specific interactions has not been studied in a systematic manner yet by, for example, changing the structure or composition of polymer samples. In this work, a range of unsaturated polyketones were studied, where the number of carbonyl groups and molecular weight of polyketones were systematically varied. The objective was to establish a correlation between the physical properties of the polyketones and the amount of CO2 sorbed into the sample during swelling. Theoretical40−42 and experimental43−47 approaches have been designed to study the behavior of CO2 sorbed into polymeric materials. However, one of the most informative methods is to use Fourier transform infrared (FTIR) spectroscopy.17 This methodology has been developed and applied to a range of systems, including drug loading into polymers,48 determining the CO2 uptake into polymeric sponges,49,50 modifying the morphology of polymers51 and functionalizing natural biomaterials.52 In situ FTIR spectroscopy reveals chemically specific information about the interactions between CO2 and functional groups within the polymer as well as physical effects occurring to the polymer under high-pressure conditions. Because FTIR spectroscopy is a quantitative approach, it means that the degree of swelling can be calculated.53 Specifically designed optical cells are required to experimentally acquire information about the interactions between polymers and scCO2 using FTIR spectroscopy. The design of infrared cells for the collection of data in the different

2. EXPERIMENTAL SECTION Polymer Synthesis. The polymers studied for this work were unsaturated polyketones synthesized by oxidation of cis1,4-polybutadiene rubber with a SKD trademark produced by Voronezh Synthetic Rubber Plant (Voronezh, Russian Federation). The oxidation reaction was performed in a high-pressure Parr reactor with a 2000 cm3 capacity. The reactor was loaded with 100−200 g of rubber, 800−900 mL of benzene (solvent), and 2 mol of N2O similar to the methods previously described.56,57 The amount of N2O was in excess, ca. 2.5 times greater than the amount of CC bonds in the loaded rubber. To produce the different polymers, we varied the reaction times (6−12 h), temperatures (180−230 °C), and pressures (3−6 MPa). After termination of the reaction, gas chromatography was used to measure the conversion of N2O oxidant thus to calculate the conversion of the carbon double bonds and amount of oxygen introduced into the polymer (Table 1). On the basis of the 1H and 13C NMR (Bruker AVANCE-7) spectra of polymers (in 1,4-dioxane) measured at 400.13 and 100.61 MHz, respectively, quantitative analysis of polymer CO groups (ketones and aldehydes) was performed.57 The molecular weights and molecular weight distribution (Table 1) of the polymers were determined by high-temperature gel permeation chromatography (GPC).56 432



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Figure 1. Schematic of in situ ATR-FTIR spectroscopic study of polymer swelling induced by high-pressure CO2 in a miniature specially designed cell attached to the ATR accessory.

ATR-FTIR Spectroscopy. ATR-FTIR spectra were collected using a Tensor 27 FTIR spectrometer (Bruker, Germany) with a mercury cadmium telluride single-element detector. The ATR accessories (Specac, U.K.) were positioned in the sample compartment, connected to a dry air supply for purging, and aligned for measurements. Two accessories were used for this investigation, diamond (Golden Gate), and germanium ATR accessories (Specac, U.K.). A spectral resolution of 4 cm−1 and 128 coadded scans were used for all measurements of the polymer samples throughout this investigation. Thin films of the viscous liquid polymers were placed to cover the whole ATR-crystal during the experiments. High-Pressure Setup. The high-pressure cell was sealed around the ATR-crystal, containing the polymer on the measuring surface, using a Teflon O-ring and clamped using the ATR accessory mechanism (Figure 1). High-pressure CO2 (99.9% pure, purchased from BOC) was introduced using a syringe pump (HiP), and the pressure was regulated using a pressure gauge. Each polymer was studied three times under the high-pressure CO2 conditions. The data points in the plotted graphs show the average values for the swelling and CO2 sorption, calculated from the spectra of the different measurements. The accuracy was determined by plotting the standard deviation based on the calculated swelling and CO2 sorption values.

S=

V + ΔV c0 ΔV = −1= −1 V V c

A = ε ·c·de , u S=

(1) (2)

A0 de , u · −1 A de0, u

(3)

⎛ ⎤⎞ ⎡ ⎛ n ⎞2 ⎛ n ⎞2 ⎜n ⎟ = ⎜ 2 cos θ ⎢3sin 2 θ − 2⎜ 2 ⎟ + ⎜ 2 ⎟ sin 2 θ ⎥⎟ ⎥ ⎢⎣ ⎜ n1 n n λ ⎝ 1⎠ ⎝ 1⎠ ⎦⎟⎠ ⎝

de , u

⎛ ⎛ ⎛ n 2 ⎞2 ⎤ ⎛ n 2 ⎞ 2 ⎞⎡⎛ ⎛ n 2 ⎞2 ⎞ 2 ⎜ ⎜ ⎢ ⎟ ⎟ ⎜ /⎜2π ⎜1 − ⎜ ⎟ ⎟ ⎜1 + ⎜ ⎟ ⎟sin θ − ⎜ ⎟ ⎥ ⎜ ⎝ ⎝ n1 ⎠ ⎥⎦ ⎝ n1 ⎠ ⎠⎢⎣⎝ ⎝ n1 ⎠ ⎠ ⎝ 1/2 ⎞ ⎛ ⎛ ⎞2 ⎞ ⎜sin 2θ − ⎜ n2 ⎟ ⎟ ⎟ ⎜ ⎝ n1 ⎠ ⎟⎠ ⎟⎟ ⎝ ⎠

(4)

Determining the Angles of Incidence and Refractive Indices. Distilled water was used to calculate angles of incidence for diamond (n1 = 2.419) and germanium (n1 = 4.01) ATR-crystals. For that, the absorbance of the bending band (v2) of water at 1643.5 cm−1 was measured from the corresponding spectra recorded using these ATR-crystals. Knowing the water molar absorptivity62 for this band (ε1643.5 = 21.8 M−1cm−1) and concentration (c = 55.34 M) at 25 °C means the effective thicknesses can be calculated and was shown to be 1.47 and 0.31 μm for diamond and germanium crystals, respectively. Furthermore, taking into account that the refractive index of water (n2) is 1.333, the solution of eq 4 gives the following values for angels of incidence: 45.3° for diamond and 44.7° for germanium. Because the lenses in the ATRaccessory are used to focus infrared light onto these ATRcrystals, these angles are effective averages for the corresponding angles of incidence. Thus, the refractive indices of the polyketones can be determined. This was done by recording ATR-FTIR spectra of all polymer samples (1S−9S) at 25 °C and atmospheric pressure (ambient conditions) using both diamond and germanium ATR-crystals. Obviously, the concentration of a polymer sample (c0) is constant under ambient conditions and does not depend on the crystal used for spectral measurement (eq 5). Hence, eq 6 can be derived from eqs 2 and 5. Conversely, the absorbance of the spectral bands of polymer will vary as a result of different effective thicknesses from the two ATR-crystals used. This means that the absorbance ratio (eq 6) can be measured from the absorbance of the spectral bands of the polymer, giving the possibility to calculate the

3. RESULTS AND DISCUSSION In general, swelling is defined as a ratio of an additional volume that the material occupies after being swollen, for instance, by high-pressure CO2, and its initial volume (eq 1). Hence, swelling is proportional to the concentrations of a material: initially (c0) and after being swollen (c). Determining the swelling by ATR-FTIR spectroscopy is based on the comparison of selected spectral band intensities from a sample spectra that are recorded before (A0) and during CO2 exposure (A). The procedure and methodology of using ATR-FTIR spectroscopy for swelling measurements were developed and described by Flichy et al.58 This spectroscopic approach has previously been reported to be consistent with alternative standard optical50 and gravimetric59 techniques; the polymers in this investigation were not studied using these methods. According to the Beer−Lambert law (eq 2), absorbance of a spectral band (A) is proportional to the molar absorptivity (ε), concentration (c), and effective thickness for unpolarized light (de,u). Thus, swelling can be measured from ATR-FTIR spectroscopic data by using eq 3. However, the effective thickness (eq 4) is governed by the wavelength of incidence light (λ), angle of incidence (θ), and refractive indices of ATRcrystal (n1) and sample (n2).60,61 So, these parameters should be carefully determined prior to swelling measurements. 433



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As a result, the measured parameters previously described, namely, the angles of incidence and changing refractive indices during CO2 exposure, are essential to correctly estimate the swelling of the polyketone samples and CO2 concentration. Dependence of Polyketones Swelling on Carbonyl Groups Concentration. After determining the correct parameters relevant to the measurements, the swelling of nine polyketone samples with different concentrations of carbonyl (CO) groups were studied by ATR-FTIR spectroscopy in situ under high-pressure CO2 conditions. Figure 3

refractive indices of the studied polyketones at ambient conditions (n02) by numerical solution of eq 4 for both ATRcrystals to satisfy an equality from eq 6. Determination of the refractive indices for all of the polyketone samples under ambient conditions proved to give a very consistent value of 1.462 ± 0.003. 0 cd0 = cGe

Ado 0 AGe

=

(5)

ddo 0 dGe

⎛ A0 d ⎞ ⎛ A0 d ⎞ ⎜⎜ · e0, u ⎟⎟ = ⎜⎜ · e0, u ⎟⎟ ⎝ A de , u ⎠d ⎝ A de , u ⎠Ge

(6)

(7)

Another issue related to the swelling phenomenon in highpressure CO2 conditions is the possibility of change in a polymer refractive index during exposure.58 Indeed, this can be easily verified by comparing the degree of polymer swelling in both ATR-FTIR spectra recorded using the diamond and germanium ATR-crystals, assuming there is no change in the polymer refractive index and using eq 3. Figure 2 shows the

Figure 3. ATR-FTIR spectra of polyketone 7S during CO2 exposure from ambient pressure to 100 bar obtained at 25 °C on the diamond ATR-crystal. The band corresponding to ν(CO) vibration at 1710 cm−1 is presented.

shows the typical response of polymeric spectral bands (for instance, the band at 1710 cm−1) in the ATR-FTIR spectra of a swelling polymer under high-pressure CO2 conditions. As the pressure increases, the concentration of CO2 becomes higher due to sorption into the polyketone, and thus the polymer volume increases due to swelling. In ATR-FTIR spectra, this reflects as a decrease in the absorbance of the polymer bands (Figure 3), which is governed by the Beer−Lambert law (eq 2) and thus proportional to a concentration or number of molecules in the measured volume. Thus, the expansion of sample volume or swelling can be measured using eq 3. A carbonyl (CO) stretching band at 1710 cm−1 was used to calculate swelling of the polyketone samples 1S−9S. The change of refractive index of the polymers during CO2 exposure was taken into account for corresponding calculations. The results are presented in Figures 4 and 5. Figure 4 shows swelling of four polyketones, specifically 1S, 6S, 7S, and 8S, across the full range of studied pressures (ambient−100 bar). Gradual increase in CO2 pressure results in higher swelling values. Apparently, the maximum swelling for the studied samples has not been achieved even at 100 bar. However, it can be observed from Figure 4 that the four polyketone samples behave differently with respect to swelling measured. Thus, polyketones 6S and 7S show a higher degree of swelling (17.4 and 17.6%, respectively) compared with those values obtained for samples 8S and 1S (14.6 and 10.1% respectively), with the latter being least swollen by CO2. So, there is an interest to correlate the concentration of carbonyl groups with the swelling of the corresponding polyketones. The comparison of swelling at 100 bar for all polyketones is presented in Figure 5. This shows the relationship between the

Figure 2. CO2-induced swelling of polyketone 6S (obtained at 25 °C from the values of absorbance of the ν(CO) band at 1710 cm−1) assuming no change in refractive index on diamond (blue squares and solid line) and germanium (red circles and solid line) crystals and when taking into account refractive index change on diamond (blue squares and dashed line) and germanium (red circles and dashed line) crystals. The values recorded for the refractive index of polyketone 6S are represented by the green line and circles. The black arrows point to the axes corresponding to the curves.

swelling measurement results from this investigation for polyketone 6S. (The procedure of swelling measurement will be described later.) Evidently, the swelling values are expected to be the same when measured using different ATR-crystals, so much so that the difference observed cannot be explained by anything but change to the refractive index of polyketone 6S. Assuming this and using eq 3, eq 7 can be derived. By knowing the absorbance of the band at 1710 cm−1, it is possible to calculate the refractive index of the polyketone at each individual CO2 pressure by numerical solution of eq 7. The dependence of the polymer refractive index on CO2 pressure is shown in Figure 2. Applying the corresponding refractive indices to recalculate the polymer swelling at each particular pressure means more precise results were obtained (dashed lines, Figure 2). It is seen now that the data obtained with the diamond and germanium crystals are in good accordance as one would expect if there were no change to the refractive indices. 434



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0.8%, which is in good accordance with a linear trendline found for samples 1S−6S. This value is similar to that of the sample 5S (16.7 ± 0.4%), which interestingly has a very similar content of carbonyl groups (5S = 5.5 × 10−3; 9S = 5.3 × 10−3) but almost twice as high molecular weight (5S = 2.2 × 103; 9S = 1.3 × 103). This is significant because the results imply that the varying molecular weight of polyketones has very little effect on the polymers CO2 sorption capacity. Apparently, this factor does not play a crucial role for the significant decrease in swelling degree observed for the samples 7S and 8S. So, these findings are rather unusual and need further explanation. Information about CO2 concentration and interactions with the functional groups in polyketones were further derived from ATR-FTIR spectroscopic data to answer these questions. Concentration of CO2 in Swollen Polyketones. ATRFTIR spectroscopy provides data not only to measure polyketone swelling but also to calculate CO2 concentration in said sample. For that, the CO2 antisymmetric stretching band (v3) at 2335 cm−1 can be used, which independently provides additional information about studied sample. Thus, such data can support our previous results related to the polyketone swelling because swelling is expected to correlate with CO2 concentration. ATR-FTIR spectral changes within the regions associated with the CO2 antisymmetric stretching (v3) and bending (v2) vibrational bands depending on CO2 pressure are presented in Figure 6. The higher the pressure of CO2 that the polymer sample was exposed to resulted in a higher concentration of CO2 due to the equilibrium between sorbed and gaseous states of CO2. This is displayed as an increase in the absorbance of CO2 spectral bands, which is proportional to the concentration.

Figure 4. Dependence of the polyketone samples 1S, 6S, 7S, 8S swelling on exposure to different CO2 pressures, from ambient to 100 bar measured at 25 °C. The band of the ν(CO) vibration at 1710 cm−1 was used for the calculations, assuming changes to the refractive index under high-pressure conditions.

Figure 5. Correlation between the varying concentration of carbonyl groups and CO2 induced swelling of the polyketone samples 1S−9S (measured at 25 °C and 100 bar from a CO stretching band at 1710 cm−1 assuming the changes in refractive index). Some marks are highlighted in red to differentiate them from others.

concentration of carbonyl groups and the maximum swelling calculated from the measured samples. It is well-established that there are specific intermolecular interactions between CO2 molecules and electron-donating functional groups, which explains the facilitation of the CO2 sorption into polymer materials.54 One can reasonably expect that an increase in the concentration of the carbonyl groups would lead to an increase in swelling. However, the presented data show that this proposition is only partially true and a linear trendline has been added to the plot in Figure 5 to emphasize our finding. Indeed, the polymers 1S−6S follow the expected trend: higher concentration of carbonyl groups leads to higher swelling. Moreover, the collected data seems to match a linear correlation. The amount of swelling in polyketone 7S (17.6 ± 0.4%) and 8S (14.6 ± 0.1%) does not conform to this rule, showing relatively low swelling compared with 6S (17.4 ± 0.7%), despite the fact that these polymers have higher content of carbonyl groups (Table 1). The possible influence of polymer molecular weight on swelling should be also verified. Actually, the studied samples have relatively large variation in molecular weights, and this factor has to be taken into account for reliable data interpretation. For that, the polyketone 9S was also studied. The results obtained show that 9S has a swelling of 16.2 ±

Figure 6. ATR-FTIR spectra of CO2 sorbed by polyketone 7S during CO2 exposure from ambient pressure to 100 bar obtained at 25 °C on the diamond crystal. The CO2 antisymmetric stretching (v3) and bending (v2) bands at 2335 and 655 cm−1, respectively, are presented. The band at 680 cm−1 is a result of the C−H out-of-plane vibrations of polyketone. 435



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spectroscopy as a result of splitting of the CO2 bending mode v2 band. For instance, the ATR-FTIR spectrum of CO2 molecules interacting with carbonyl groups of PMMA film contains one antisymmetric stretching band (v3) at 2338 cm−1, one hot-band at 2326 cm−1 ((v3 + v2) − v2),64 and two bending bands at 662 and 654 cm−1 from out-of-plane and in-plane v2 modes, respectively.55 The ATR-FTIR spectra presented in Figure 6 do not correspond to CO2 interacting with carbonyl groups but show predominantly the bands at 2335, 2325, and 655 cm−1, which belong to CO2 physically sorbed by the polyketone. In principle, a spectrum of swollen polymer could contain both sets of bands related to physically sorbed and interacting CO2. However, we did not observe the bands of interacting CO2 molecules for the studied samples, even for the polyketone samples with high carbonyl content when exposed to low CO2 pressures. This is likely due to the relatively high content of physically sorbed CO2. Indeed, it is seen in Figure 7 that the overall amount of CO2 sorbed in the polyketones exposed to 100 bar is approximately one order of magnitude higher than the concentration of carbonyl groups. So, assuming that one carbonyl group can interact with one or two CO2 molecules (according to the CO···CO2 complex structures, Figure 8), we can suppose that under these conditions the studied polyketones contain mostly physically sorbed CO2 with the interacting CO2 molecules present in a much lower quantity. In this case, the bands of physically sorbed CO2 should prevail in the measured ATR-FTIR spectra. However, the bands of physically sorbed CO2 can be removed from the spectra of swollen polyketone by careful spectral subtraction. This means that only the bands associated with interacting CO2 are revealed. Because sample 1S has the lowest concentration of carbonyl groups, its ATR-FTIR spectrum contains mostly the bands of physically sorbed CO2 with minor contribution of the bands of interacting CO2. Hence, the spectrum of polyketone 1S swollen at 100 bar can be subtracted from the spectra of other polyketones recorded at 100 bar pressures as a method to remove overlapping bands of noninteracting CO2. It should be noted that precise selection of the subtraction coefficients for all pairs of spectra is needed to obtain good quality and representative spectra of interacting CO2. The resulting ATR-FTIR spectrum for polyketone 5S, after spectral subtraction of polyketone 1S, is shown in Figure 9. This ATR-FTIR spectrum is typical for CO2 under Lewis acid−base interaction with carbonyl groups55 because the removal of double degeneracy for bending mode of CO2 is clearly observed. Similar spectral features associated with CO2 bonded with carbonyl groups were obtained by subtraction for samples 2S−9S. These spectra contain the following bands corresponding to the vibrations of CO2 molecules interacting with carbonyl groups: 2337 cm−1 for stretching v3 mode, 2326 cm−1 for (v3 + v2) − v2 hot-band, and 662 and 650 cm−1 for out-of-plane and in-plane (nondegenerate) bending v2 modes, respectively. Therefore, the presence of interactions between the CO2 molecules and carbonyl groups in the polyketone has been confirmed. The possibility exists to find out how the sorbed CO2 concentration depends on the concentration of carbonyl groups based on having recorded ATR-FTIR spectra of interacting CO2 for all studied polyketones. By using the absorbance of the band at 2337 cm−1, the amount of interacting CO2 can be calculated similarly to how it was done for overall CO2 concentration in the previous section. It should be noted

Thus, the Beer−Lambert law (eq 2) can be applied to calculate CO2 concentration assuming the molar absorptivity of sorbed CO2 corresponding to v3 band is equal to the molar absorptivity of CO2 dissolved in the solvents at high pressures (1.0 × 106 cm2·mol−1).63 This gives the concentration of CO2 in mol·cm−3 units, which can be recalculated to mol·g−1 units (cw) by using eq 8, where S is swelling and ρ is a density of the polymer. Figure 7 shows the relationship of the concentration of CO2

Figure 7. Correlation between the varying concentration of carbonyl groups in the polyketones and the concentration of sorbed CO2 by the polyketone samples 1S−9S (measured at 25 °C and 100 bar and calculated from the v3 band of CO2 at 2335 cm−1). Some marks are highlighted in red to differentiate them from others.

that was sorbed by the polyketone samples with respect to their concentration of carbonyl groups. The overall amount of sorbed CO2 is observed to be dependent on the concentration of carbonyl groups in a similar way as swelling does (Figure 5). So, this independent result is consistent with our results on polymer swelling, thus supporting our observation of decreased swelling for the samples prepared with a relatively high concentration of carbonyl groups. cw =

cV ·(1 + S) ρ

(8)

CO2 Interaction with Carbonyl (CO) Groups of Polyketones. Unfortunately, the data described in the previous sections do not provide a comprehensive explanation for the decrease of swelling or amount of sorbed CO2 in the polyketones 7S and 8S. However, further information about CO2 molecules interacting with polyketones, particularly with carbonyl functional groups, can be derived from obtained ATRFTIR spectra. The most likely geometries when CO2 interacts with carbonyl groups in the polymer chain are the formation of a “bent” T-shaped complex and 1:2 complex, as shown in Figure 8.55 The formation of such complexes removes the degeneracy of the v2 mode and therefore can be revealed by ATR-FTIR

Figure 8. Possible schematic structures of interacting complex. Left: “bent” T-shape configuration, between a carbonyl group and CO2 molecules.55 Right: a proposed 1:2 structure of interactions in systems studied here (one of CO2 molecules may be out of plane). 436



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one carbonyl group interacts with two CO2 molecules65 as it follows from Figure 10 (equation from trendline). For instance, according to the polymers characteristics, the difference of carbonyl content between samples 1S and 4S is ca. 0.0037 mol· g−1. Polymer 4S contains a larger amount of interacting CO2 (ca. 0.0073 mol·g−1) with respect to the polymer 1S. Thus, the additional number of carbonyl groups inserted by deeper oxidation of CC bonds leads to double the amount of CO2 molecules specifically interacting with these functional groups. Hence, for the first time, experimental quantitative data have been obtained and presented that allow us to propose the most plausible explanation that carbonyl (CO) groups form a 1:2 complex with CO2 molecules via intermolecular Lewis acid−base interactions (Figure 8). Another observation, based on these results, can help explain the reasons for the decrease in swelling for polyketones 7S and 8S. As it is seen in Figure 10, these samples show lower concentration of interacting CO2 than it is predicted by the linear correlation. This can be interpreted that there are a certain number of carbonyl groups that are not available for interaction with CO2 molecules. Hence, CO2 can be used to distinguish between various types of carbonyl functionalities in a polymer. It can be estimated from the current data that approximately 86 and 77% of all carbonyl groups presented in the polyketones 7S and 8S are available to form a complex with CO2 molecules. Thus, excluding unavailable carbonyl groups, we can establish that polyketone 7S has a similar number of CO groups to that of the polyketone 6S. Intriguingly, as a result of this, these two polymers show almost equal degrees of swelling and CO2 total concentration (Figures 5 and 7). However, this effect is more drastic in the case of the polymer 8S. This polyketone has a degree of swelling and overall CO2 concentration comparable to that of the samples 3S and 4S. At the same time, the amount of available carbonyl groups is almost twice as high for 8S with respect to two other samples. Hence, the sorption capacity, which is mostly related to the number of physically sorbed but not specifically interacting CO2, of the polyketone 8S is lower than expected based on the concentration of available carbonyl groups. So, there is a factor that prevents carbonyl groups interacting with CO2 molecules and thus decreases the sorption capacity of polymers with relatively high oxygen content. One plausible explanation could be steric hindrance brought about by the increased carbonyl group content in the polymers. However, we believe that intermolecular interactions between polymer molecules due to relatively large number of carbonyl groups can have such drastic effect. So, when the concentration of carbonyl groups is high enough, the number of sites possible for intermolecular dipole−dipole interactions (CO···CO) between polymer molecules reaches a crucial point. In the case of 8S, the intermolecular interaction becomes relatively strong, inhibiting the access of the CO2 molecules into the space between polymer molecules. This has an adverse effect for CO2 sorption and thus influences the final sorption capacity for CO2 in the polymer. Thus, ATR-FTIR spectroscopy provides important quantitative information about the behavior of polymers containing different concentrations of carbonyl groups under high-pressure CO2 conditions. It is now clear that there is an optimal concentration of carbonyl groups in the polyketone samples that provides maximum swelling or CO2 sorption capacity. The higher concentration of carbonyl groups causes a reduction in the sorption properties of a material due to stronger

Figure 9. Regions of ATR-FTIR spectrum of CO2 interacting with carbonyl groups of polyketone 5S at 100 bar. The spectrum is a result of spectral subtraction of polyketone 1S from 5S. CO2 stretching band (v3) at 2337 cm−1, hot-band ((v3 + v2) − v2) at 2336 cm−1 and two (nondegenerate) bending bands (v2) at 662 and 650 cm−1 for out-ofplane and in-plane modes, respectively.

that the spectra, after subtraction, present the difference in the concentrations of interacting CO2 between polyketone 1S and the other polyketones (2S−9S). So, the measured values for the concentration of CO2 have to be correlated with respect to the difference in the concentration of carbonyl groups between sample 1S and other polyketones. This corresponding correlation is presented in Figure 10. The data clarify a few issues related to polyketone samples interacting with CO2. The samples 2S−6S and 9S show a linear dependence between the amount of CO2 molecules interacting with the polyketone carbonyl groups and the concentration of these functional groups. Moreover, it is plausible to suggest that

Figure 10. Correlation between the concentrations of carbonyl groups and interacting CO2 for the polyketone samples 2S−9S. The spectra were measured at 25 °C and 100 bar, and the concentrations of interacting CO2 were assessed from the absorbance of the v3 band at 2337 cm−1. Some marks are highlighted in red to differentiate them from others. 437



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intermolecular dipole−dipole interactions between polymer molecules which inhibits CO2 sorption.

Notes

4. CONCLUSIONS In situ ATR-FTIR spectroscopy has been used to study the effect of high-pressure CO2 on the swelling of polyketone samples containing a differing number of carbonyl groups. This study demonstrates the ability of in situ ATR-FTIR spectroscopy to provide reliable quantitative information revealing the physical effects occurring to polymeric samples under highpressure conditions: specifically the degree of swelling, CO2 sorption, and interactions between the functional groups in polymers and CO2. First, changes to the refractive index of the polyketones subjected to high-pressure conditions have been calculated from spectra measured using a diamond and germanium ATRFTIR accessory. This allowed the required information to be obtained, needed when calculating the degree of polymeric swelling and CO2 sorption. Second, the effect of the varying concentration of carbonyl groups in polyketones on the overall swelling of the materials was investigated. To establish a relationship between the concentration of carbonyl groups in polyketones and polymer swelling, we studied nine polyketones derived from cis-1,4polybutadiene rubber, each with different number of carbonyl groups. There was a linear correlation between the overall amount of swelling and the concentration of carbonyl groups. However, this correlation was not valid for the polyketones with the highest concentration of carbonyl groups. A very similar correlation about the amount CO2 sorbed in swollen polyketones was determined by in situ ATR-FTIR spectroscopy. This indicates that the amount of sorbed CO2 depends on the concentration of carbonyl groups in the measured polymer. Calculating the amount of only the interacting CO2 in the swollen polyketones helped explain these dependencies. In the polyketone samples containing a high number of carbonyl groups, there would be more CO sites capable of interacting with one another via dipole−dipole intermolecular interactions. Thus, the ability of CO2 to absorb into the sample would be inhibited and the overall swelling would decrease. This explanation was based on the results obtained. Finally, the effect of other physical properties, namely, the polymer molecular weight, was studied where no difference in the polymer swelling was observed. Thus, it can be concluded that the concentration of carbonyl groups in polymeric samples determines the overall swelling and CO2 sorption into such materials. This work demonstrates the potential to use ATR-FTIR spectroscopy as an approach to further investigate similar polymeric systems in high-pressure CO2 environments. The great potential to exploit the use of high-pressure and supercritical CO2 for a range of polymer processing applications means there is a need to learn about the dependency on other Lewis base functional groups with respect to polymer swelling and CO2 sorption. Furthermore, the in situ ATR-FTIR spectroscopic approach is not limited to only high-pressure CO2 applications and can be extended to study relevant systems in other supercritical and high-pressure environments.

ACKNOWLEDGMENTS We thank Dr. M. A. Matsko for the GPC study of the polyketone samples.

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The authors declare no competing financial interest.

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