CC BY
26
UDC 541.64:542.953.2
I. Ronova, A. Alentiev, M. Bruma, G. Zaikov
INFLUENCE OF SWELLING IN SC-CO2 OF THIN POLYIMIDE FILMS ON THEIR MICROSTRUCTURE TO OBTAIN POLYMER DIELECTRICS FOR MICROELECTRONICS AND HEAT-RESISTANT GAS SEPARATION MEMBRANES. PART 3
Keywords: polyimide films, swelling in supercritical CO2, free volume, gas transport parameters, dielectric constant.
In this article the swelling with supercritical carbon dioxide (sc-CO2) of thin films of polyimides having various structures was investigated. Transport parameters such as permeability and selectivity coefficients measured before and after treatment with sc-CO2 increased from 16% to 168% andfrom 5% to 49%, respectively.
Ключевые слова: полиимидные пленки, набухание в сверхкритическом СО2, свободный объем, газотранспортные характеристики, диэлектрическая постоянная.
В статье исследовано набухание тонких полиимидных пленок различной структуры в сверхкритическом диоксиде углерода (sc-СО2). Наблюдалось увеличение таких транспортных параметров, как коэффициенты проницаемости и селективности до и после воздействия сверхкритического СО2 с 16% до 168% и с 5% до 49% соответственно.
1 Introduction
Various classes of polymers have been synthesized and studied for use as gas separation membranes. Among these, polyimides areinan important position since they exhibit extraordinary high gas selectivity as well as excellent thermal and mechanical stability, and film-forming ability [1, 2]. Besides, there is a broad possibility of varying the chemical structure of the repeating unit aiming to change the physical properties of polyimides, including their transport characteristics [3-6].
Here, we present an investigation of the swelling process with sc-CO2 of thin films of some polyimides which were synthesized in m-cresol or in carbox-ylic acid medium such as benzoic acid or salicylic acid. The synthesis and general characterization of these polyimides were described previously [7, 8]. Some correlations are shown between the conformational rigidity parameters, such as free volume and characteristic ratio, and the transport characteristics.
2 Calculation methods
2.1 Calculation of conformational parameters
The correlation between physical properties of polymers and conformational rigidity of their chains shows that the contribution of conformational rigidity to their properties is significant [9]. The conformational rigidity of a polymer can be estimated using different parameters, such as statistical Kuhn segment (Afr) and characteristic ratio (C.). Kuhn segment was calculated as under the assumption of free rotation by using the equation (1) [10].
Afr = lim
Ur2 >^
nL
(1)
where <R > is mean square distance between the ends of the chain calculated for all possible conformations, n is the number of repeating units, lo is the contour length of a repeating unit, and L = nlo is the contour length of the chain, a parameter which does not depend on the chain conformation.
All the values of Kuhn segment were calculated with Monte Carlo method and the geometry of the
repeating unit was assigned by using quantum-chemical method AMI [11].
We also used another parameter of conformational rigidity named characteristic ratio C„ which shows the number of repeating units in Kuhn segment, as shown by equation (2).
C= l (2)
'0
For some of the studied polymers, we also calculated the Kuhn segment values taking into consideration the hindrance of rotation, according to the method previously described [11, 12]. Most of these polymers did not have any hindrance of rotation, or their hindrance was too low, and it was neglected. Previously it was shown that the values of conformational parameters calculated under the assumption of free rotation in the absence of voluminous substituents are practically equal to the values found experimentally from hydrodynamic data [13].
2.2 Calculation of free, occupied, accessible and fractional accessible volume
In order to correlate the geometry of the repeating units of polymers with transport properties, the following parameters were calculated: van der Waals volume (Vw), free volume (Vf), occupied volume (Vocc), accessible volume (Vacs), fractional accessible volume (FAV).
The occupied volume (Vocc) of a repeating unit is given by equation (3) as being the sum of the Van der Waals volume (Vw) of the repeating unit and the volume of space around this unit that is not accessible for a given type of molecule of gas, which is named "dead volume" (Vdead). It is evident that the occupied volume of a repeating unit depends on the size of the gas molecule.
Vocc=(Vw+Vdead) (3)
The accessible volume of a polymer (Vacs) is given by equation (4), where NA = 6.02^1023 is Avo-gadro's number, p is the polymer density, and Mo is the molecular weight of the repeating unit.
I _Na*Vqcc (4)
P MD
However, more often is used the so-called fractional accessible volume (FAV), without any dimensions, which gives a better concordance with the coefficients of diffusion and of permeability, that is given by equation (5) [14, 15].
FAV = VaCs-p (5)
To calculate the Van der Waals and the occupied volume of the repeating unit, we used the quantum chemical method AMI to refine the structure of the monomer unit [11]. The model of the repeating unit is a set of intersecting spheres whose coordinates of centers coincide with the coordinates of atoms and the radii are equal to the Van der Waals radii of the corresponding atoms.
Van der Waals volume (Vw) of the repeating unit is the volume of the body of these overlapping spheres. The values of Van der Waals radii were taken from the reference [16]. The model of the repeating unit was placed in a box with the parameters equal to the maximum size of repeating unit. By using the Monte Carlo method we designated the number of random points m that fall into repeating unit and the total number of tests M. Their ratio is multiplied by the volume of the box, as seen in equation (6)
Vw = (m/M)Vb ox (6)
Then we calculated the dead volume. Since the molecules of O2, N2 and CO2 have ellipsoidal shape, we calculated the dead volume of the two spheres with radii corresponding to the major and to the minor axes of the ellipsoid. A number of 106 spheres with the radius of the gas was generated for each atom of the repeating unit. The result was a system consisting of a repeating unit, surrounded by overlapping spheres of gas. Then, the system was placed in the "box", similar to the one used in the determination of Vw, and random points were generated in the volume of the box [17, 18]. Thus, without making any assumptions about packing of the polymer chains in the glassy state, we could quickly calculate the Van der Waals volume, and the occupied and the accessible volumes.
The free volume (Vf) was calculated with the equation (7):
(7)
P Mo (7)
The value Vf, thus calculated, shows the volume which is not occupied by the macromolecules in 1 cm3 of polymer film.
3 Experimental methods
3.1 Preparation of polymer films
The polyimides were synthesized by polycondensation reaction of an aromatic diamine with an aromatic dianhydride by traditional method using meta-cresol or benzoic acid and salicylic acid as solvent [7, 19], at high temperature to allow the complete imidization process and to exclude the cross-linking. The polycondensation reaction was run with equimolar quantities of diamine and dianhydride, at room temperature for 3 h, and then at 200°C for another 7 h. After cooling down to room temperature, the resulting viscous solution was poured in methanol to precipitate the pol-
ymer. The fibrous precipitate was washed with methanol and dried in vacuum oven at 100°C. These polymers showed good solubility in common solvents having low boiling point, such as chloroform and tetrahydrofuran, which are very convenient for film preparation.
The films, having the thickness usually in the range of 20-40 ^m, were prepared by using solutions of polymers in chloroform, having the concentration of 15 %, which were cast onto cellophane film and heated gently to evaporate the solvent. The films were carefully taken out of the substrate. To remove the residual m-cresol, the films were further extracted with methanol in Soxhlett apparatus, followed by heating in vacuum at 70°C for 3 days.
3.2 Measurement of glass transition temperature
The glass transition temperature (Tg) of the polymers was measured by differential scanning calorimetry, with a DSC-822e (Mettler-Toledo) apparatus, by using samples of polymer films. The samples were heated at the rate of 10°C/min under nitrogen to above 300°C. Heat flow versus temperature scans from the second heating run was plotted and used for reporting the Tg. The middle point of the inflection curve resulting from the second heating run was assigned as the Tg of the respective polymers. The precision of this method is ±7-10°C.
3.3 Measurement of density
The density of polyimide films was measured by using the hydrostatic weighing method. The study was performed with an equipment for density measurement and an electronic analytic balance Ohaus AP 250D from Ohaus Corp US, with a precision of 10-5g, which was connected to a computer. With this equipment we measured the change of sample weight (density) during the experiment, with a precision of 0.001 g/cm3 in the value of density. Ethanol and isopropanol were taken as liquids with known density. The studied polyheteroarylenes did not absorb and did not dissolve in these solvents, which for these polymers had low diffusion coefficients. The characteristic diffusion times were in the domain of 104 - 105 s, even for the most thin films studied here, which leads to higher times, of 1-2 order of magnitude, than that of the density measurement. This is why the sorption of solvent and the swelling of the film must have only insignificant influence on the value of the measured density. All measurements were performed at 23°C. The density was calculated with the equation (8):
Ps = Pi *Wa / (W3 - W) (8)
where ps is density of the sample, Wa is the weight of the sample in air, Wl is the weight of the sample in liquid, p is the density of liquid. The error of the density measurements was 0.3 - 0.5 %.
3.4 Measurement of dielectric constant
For each polymer in this series, dielectric permittivity of polyimide films was measured by using Alpha High Resolution Dielectric Analyzer from Novocontrol-Germany, in the domain of frequencies from 10-3 to 106 Hz, and it was approximated at the frequency equal to zero to obtain the value of dielectric constant (s0).
3.5 Method of treatment with supercritical carbon dioxide (sc-CO2)
The experimental set-up and the method of treatment with sc-CO2 were described in previous papers [20, 21]. This experimental set-up is composed of a generator which can provide CO2 up to 35 MPa pressure (High Pressure Equipment Company, USA). A system of valves ensures the CO2 access to the reaction cell with the volume of 30 cm3. The pressure generator and the reaction cell are provided with manometers to allow a control of the pressure. The temperature control allows a precision higher than ±0.2°C. The cell is designed for experiments at pressures up to 50 MPa and temperatures up to 120°C. CO2 desorption curves were obtained using the gravimetric technique. Sample mass was measured with an Ohaus AP 250 D electronic balance interfaced with a computer.
The following experimental technique was applied: The polymer film was weighed and placed into the cell. The films had the form of a disk with 15 mm diameter and thickness in the range from several to tens of microns. The cell was purged with CO2 to remove the air and water vapors, and it was sealed. Then it was heated to the temperatures shown in this paper, the pressure was increased to the values also shown in this paper, and it was kept a certain time necessary to attain the equilibrium degree of swelling. Then the cell was open, the sample was taken out and it was put on Ohaus AP250D electronic balance (precision of 10-5 g) for less than 10 s, and the CO2 desorption was fixed with the computer. To determine the mass degree of swelling with sc-CO2, we recorded gravimetrically the CO2 desorption from polymer.
3.6 Measurement of transport parameters
The transport parameters at 25 ± 3°C for He, O2, N2 and CO2 were measured using a mass spectrometric technique [22, 23] and barometric techniques on a Balzers QMG 420 quadrupole mass spectrometer (Liechtenstein) MKS Barotron [24], respectively. The upstream pressure was 0.8-0.95 at, and the downstream pressure was about 10-3 mm Hg for spectrometric method, while for barometric technique that pressure was in the range of 0.1-1 mm Hg; therefore, the reverse diffusion of penetrating gas was negligible. The permeability coefficients P were estimated using the formula: P = Js l/Dp, where Js (cm3 (STP)/cm2 9 s) is the flux of the penetrant gas through 1 cm2 of the film; Dp (cm Hg) is the pressure drop on the film; l (cm) is the film thickness. The diffusion coefficient D was determined by using the Daynes-Barrer (time lag) method: D = l2/6h, where h (s) is the time lag. The solubility coefficient S was estimated as the ratio: S = P/D
4 Results and Discussion
4.1 Influence of microstructure on the transport properties
To investigate the change of microstructure of polyimide thin films under the action of supercritical carbon dioxide and its influence on the transport properties we selected the polymers 3-6 (Table 1). For this study we prepared thicker films than in the investigation of dielectric properties (20-40 |im). After treatment with sc-CO2, all these four polymers
exhibited a lower density and a higher free volume compared with the untreated samples. The highest increase of free volume was observed in case of polymer 3 containing meta-substituted phenylene rings in the diamine segment [25, 26]. After swelling with sc-CO2, the free volume of polymer matrix increases with 4-16 % (Table 2). The highest increase of free volume was observed in case of polymer 3, being 16 %. The glass transition temperature of these polymers did not change (in the range of the precision of DSC measurement). It means that the Tg of these polymers is predominantly determined by the chemical structure of the polymer chain itself. Tables 3, 4, 5 and 6 present the permeability and diffusion coefficients through polymer films (membranes), before and after treatment with sc-CO2. The dependences of permeability (a) and diffusion (b) on the fractional accessible volume (FAV) for each polymer in the system one polymer—different gases are given in Figs. 1, 2, 3, 4 (before sc-CO2 treatment, the right-side lines being described by equation y = A + Bx). The value of the Van der Waals radius of Helium is 1.22-1.80 A, according to literature data [27-29].
Table 1 - Repeating unit and conformational param-
Polymer l (Ä) Ah (Ä) C F (Wt %) Vw (Ä3) AVfr* (%)
1 32.02 29.11 0.904 25.59 721.703 9.93
2 31.87 20.87 0.655 25.59 721.703 58.44
3 41.86 20.28 0.484 11.36 854.488 257.64
4 41.86 21.71 0.495 20.52 877.824 132.79
5 42.03 27.17 0.596 11.36 854.488 48.37
6 42.03 27.75 0.655 20.52 877.824 64.67
Repeating unit
C-
-k II
O
\ AV ' CH3 ---
mV
at a pressure of200 bar and a temperature of 60oC; F -fluorine
1
2
3
4
5
6
Table 2 - Change of free volume (Vf) after swelling at pressure of 120 bar and temperature of 40 °C of pol-yimides
Poly mer Before swelling in sc-CO, AVf (cm3/g)
P (g/cm3) V{ (cm3/g) Tg (°C)
1 1.389 0^070 167 0.0330
, 1.431 0.„30 181 0.0084
3 1.341 0^3,7 ,07 0.0113
4 1.4,8 0.„45 ,04 0.0171
Poly mer After swelling in sc-CO, AVf (%)
P (g/cm3) V{ (cm3/g) Tg (°C)
1 1.3,8 0^400 169 16
, 1.414 0^314 180 3.8
3 1.3,1 0^440 ,06 4.9
4 1.394 0^416 ,04 7.6
Table 3 - Change of transport parameters by swelling in supercritical carbon dioxide (sc-CO2 ) of polymer film 1. (He* - when the Van der Waals radius of Helium atom was taken equal to1.68 A)
Gas
He
He
O,
N2
CO,
Before swelling in sc-CO,
V y occ (A3) V y acs (cm3/g) 1/ FAV P (Barrer) D^108 (cm3/s)
896.194 0.1819 3.957 7.57 373
913.653 0.1714 4.,00 7.57 373
9,4.118 0.165, 4.359 0.497 1.19
9,8.4,4 0.16,6 4.4,8 0.08,4 0.3,
917.956 0.1688 4.,64 ,.06 0.,9
Gas
After swelling in sc-CO2
Vacs acs 1/ P AP D^108 AD
(cm3/g) FAV (Bar rer) (%) (cm3 /s) (%)
He* 0.,150 3.50, ,0.3 168., 3,0 -14.,
He 0.,045 3.68, ,0.3 168., 3,0 -14.,
O, 0.198, 3.799 1., 141.4 0.87 -,6.9
N, 0.1956 3.849 0.14 69.9 0.17 -46.0
CO, 0.,019 3.7,9 5.4 16,.1 0.,7 -7.85
Table 4 - Change of transport parameters by swelling in supercritical carbon dioxide (sc-CO2 ) of polymer film 2. (He* - when the Van der Waals radius of Helium atom was taken equal to1.68 A)
Gas
He
He
O,
N,
CO,
Before swelling in sc-CO,
Vocc occ (A3) Vacs acs (cm3/g) 1/ FAV P (Bar rer) D^108 (cm3/s)
9,9.4,1 0.1951 3.58, 16.8 869
948.30, 0.1848 3.781 16.8 869
959.58, 0.1805 3.910 1.03 ,.61
965.011 0.1758 3.965 0.169 0.65
95,.70, 0.18,4 3.831 4.13 0.64
Gas
After swelling in sc-CO,
Vacs acs 1/ P AP D^10 AD
(cm3/g) FAV (Bar rer) (%) 8 (cm3 /s) (%)
He* 0.,035 3.476 35.8 111.3 600 -31.0
He 0.193, 3.660 35.8 111.3 600 -31.0
O, 0.1889 3.743 ,.7 16,. 1 1.7 -34.9
N, 0.184, 3.840 0.4 136.7 0.40 -38.6
CO, 0.1908 3.706 10.0 14,. 1 0.43 -33.1
Table 5 - Change of transport parameters by swelling in supercritical carbon dioxide (sc-CO2 ) of polymer film 3. (He* - when the Van der Waals radius of Helium atom was taken equal to1.68 A)
Gas Before swelling in sc-CO,
Vocc occ (A3) Vacs acs (cm3 /g) 1/ FAV P (Barrer ) D^108 (cm3/s)
He* 9,1.75, 0.19,3 3.877 13.7 740
He 940.301 0.181, 4.115 13.7 740
O, 951.951 0.174, 4.,80 1.09 ,.44
N, 956.140 0.1717 4.343 0.185 0.67
CO, 945.71, 0.1780 4.193 5.18 0.67
Gas After swelling in sc-CO,
Vacs acs (cm3 /g) 1/ FAV P (Bar rer) AP (%) D^10 8 (cm3 /s) AD (%)
He* 0.,036 3.717 ,5., 83.9 ,00 -73.0
He 0.19,5 3.93, ,5., 83.9 ,00 -73.0
O, 0.1855 4.081 ,.1 9,.7 1.6 -34.4
N, 0.1830 4.137 0.4 116., 0.40 -40.3
CO, 0.189, 4.000 8.6 66.0 0.47 -,9.8
Table 6 - Change of transport parameters by swelling in supercritical carbon dioxide (sc-CO2 ) of polymer film 4. (He* - when the Van der Waals radius of Helium atom was taken equal to1.68 A)
Gas Before swelling in sc-CO,
Vocc occ Vacs acs 1/ P D^108
(A3) (cm3 FAV (Barr (cm3/s)
/g) er)
He* 9,9.658 0.1964 3.566 13.6 5,1
He 948.954 0.1859 3.765 13.6 5,1
O, 960.356 0.1798 3.896 1.03 ,.67
N, 964.170 0.1777 3.951 0.189 0.71
CO, 953.666 0.1834 3.81, 4.7, 0.71
Gas
After swelling in sc-CO,
Vacs acs 1/ P AP D^108 AD
(cm3/g) FAV (Bar rer) (%) (cm3/s) (%)
He* 0.,135 3.360 30.9 1,7., 1008 93.5
He 0.,030 3.533 30.9 1,7., 1008 93.5
O, 0.1968 3.644 ,.4 133.0 1.8 -3.,6
N, 0.1948 3.683 0.5 164.6 0.50 -,9.6
CO, 0.,005 3.578 11.1 135., 0.66 -7.,
In our calculations, we used the value of 1.,, A for the Van der Waals radius of Helium. It came out that the point corresponding to the value of 1.,, A clearly goes out of general dependences both of permeability coefficient and of diffusion coefficient for all four polymers. That point remains on the general dependence only in case when the Van der Waals radius of Helium is taken equal to 1.68 A. Similar results were obtained for Helium previously [30]. The value of 1.68 A obtained by us for the Van der Waals radius of Helium is close to the effective diameter (1.78 A) calculated by other authors [31].
In Tables 3-6, it can be seen that after swelling with sc-CO,, the permeability coefficients increased for different gases and different polymers from 66 to 168 %. For polymer 3, the increase of permeability coefficients of He, O,, and CO, is higher than that of N,. The
diffusion coefficients (D) decreased from 7.2 to 93% with exception He of polymer 6.
10 t
i -
TO S3
CL
0.1 -
He* \*He
y=47,(K)-l2.42\, R=98.97% y=35.77-8.31\, R=99.42%
—I-1-1-1-1-1-1-1-1-1-1-1
3.4 3,6 3.8 4.0 4.2 4.4 4.6 1/FAV
jn 10-
"s
u
S 10.1 ■:
He
■He »He
y=7d. 17-20-02*, K='W.76% y=60.74-13.S7\, R^)').26%
—I—I—I—I—I—p—I—p—I—I—I—I—I—I—I—I—I—I—I—I—I
3.5 3.6 3.7 3.8 3,9 4.0 4,1 4,2 4,3 4.4 4.5 1/FAV
b
Fig. 1 - Dependence of permeability coefficients P (a) and diffusion coefficients D (b) on the fractional accessible volume (FAV) for polymer 3. (He* - when the Van der Waals radius of Helium atom was taken equal to 1.68 Â)
He*
10
.. 1
0.1
y=41.08-HW2i, R=99.20% >=40.39-10.36*, R=90.28%
—I-1-1-1-1-1-1-1-1-1-1
3.5 3.6 3.7 3.B 3.9 4.0
1/FAV
100^
10 -
1
0,1
Hei
Polymer 2 ■ before • after
y=65.83-I7,33x, R=92.ff5% v=68.82-17,44R=99.36%
—I-1-1-1-1-1-1-1-1-1-1
3,5 3.6 3.7 3.8 3.9 4.0 1/FAV
b
Fig. 2 - Dependence of permeability coefficients P (a) and diffusion coefficients D (b) on the fractional accessible volume (FAV) for polymer 4. (He* - when the Van der Waals radius of Helium atom was taken equal to 1.68 Â)
(0 ■Q
0."
10T
1 -
0.1 T
He*
y=35.16-8.S7l, R =99.01 % y=34,52-8.09R=9S.(iO%
100-
£ 10-o
1 -.
0.1
0.01
—I—I—I—I—I—1—1—I—I—I—I—I—I—I—I
3,7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 1/FAV
He*
y =54.78-13.35i, R=99.86% y=59.1M3.69x, R-99.53%
i—I—,—I—I—I—.—,—,—I—I—I—.—I—,—I—I—I
3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 1/FAV
b
Fig. 3 - Dependence of permeability coefficients P (a) and diffusion coefficients D (b) on the fractional accessible volume (FAV) for polymer 5. (He* - when the Van der Waals radius of Helium atom was taken equal to 1.68 Â)
a
a
a
1
Ol.
0.1 -
He
y=42.54-11.6(K, R=99.19% v=37.47-9.64*. R=99.41%
—I-■-1-■-1-■-1->-1-■-1-■-1
3,4 3.5 3.S 3,7 3.8 3.9 4.0 1/FAV
a
1000
100-a
m £ 10-,
a
y—
□
1 -3
0.1
0.01
y=82.9l-22.63\, R=99.54% >=62.(19-15.78*, R=99.49%
-i-1-1-1-1-1-1-1-1-1-1-1-1-1
3.4 3.5 3.6 3.7 3,8 3.9 4.0
1/FAV
b
Fig. 4 - Dependence of permeability coefficients P (a) and diffusion coefficients D (b) on the fractional accessible volume (FAV) for polymer 6. (He* - when the Van der Waals radius of Helium atom was taken equal to 1.68 Â)
The change of D should be evidently observed for He, although the error of D(He) is quite high due to the short time lag (~0.5 s). And only for D(He) in polymer 6 increasing after treating in sc-CO2 was observed. It is interesting that for O2 and N2, the diffusion decreases for all polymers in a similar way, with 27-35 and 37-46 %, respectively. The minimum change of diffusion coefficient of CO2 was observed in case of polymers 3 and 4, by comparison with polymers 4 and 5. For polymers 3 and 6, the decrease of diffusion coefficient D for CO2 is of 7 %, while for polymers 4 and 5 it decreased with 30 %. It can be seen that the increase of free volume of polymers 3 and 6 (Tables 3 and 6) is significantly higher than that of polymers 4 and 5 (Tables 4 and 5). This behavior when diffusion coefficient decreases as the free volume increases is not conventional. According to so-called ''hole-wall'' model [32], diffusion selectivity is a measure of density or ordering of chain packing in ''holes.'' For instance, O2/N2 diffusion selectivity is increased for swollen polymers 3 (from 3.8 to 5.1), 4 (from 4.0 to 4.9), 5 (from 3.6 to 4.3), 6 (from 3.8 to 3.9). Thus, swelling of the polymers in sc-CO2 results in densification of macromolecular chain packing in ''walls'' and the ''walls'' become more selective to gas transport. Similar effects were observed
for the behavior of gas transport parameters in ''strain aged'' polymer film 3 [26]. Here, increasing of permselectivity of gas pairs was observed while free volume in the polymers and ordering of polymer 3 chain packing in ''walls'' increased. However, in our case (in contrary to [26]), increasing of selectivity is coupled with significant growing of free volume, and ''hole'' sizes, respectively, that results in growing of solubility coefficients (Table 7). It seems that the increase of free volume due to swelling in sc-CO2 is not associated with disturbed packing of macromolecular chains in polymer matrix between the microcavities, and it determines that the ordering of macromolecular chains which leads to the decrease of diffusion coefficients.
Table 7 - Solubility (S^102, cm3 (STP)/cm3cmHg) of gases in polymer films before and after swelling in sc-CO2
Gas Polymer 1 Polymer 2
before after AS (%) before after AS (%)
He 0.21 0.63 200 0.20 0.60 200
O2 4.2 14 233 3.9 15 285
n2 2.6 8.2 215 2.6 11.0 323
CO2 70.5 200 184 65.0 230 254
Gas Polymer 3 Polymer 4
before after AS (%) before after AS (%)
He 0.19 1.3 584 0.26 0.31 19
O2 4.5 13 189 3.9 14 259
n2 2.8 9.7 246 2.65 11.0 315
CO2 77.0 180 134 66.0 170 158
Figures 1, 2, 3, and 4, left-side lines, show the dependence of permeability coefficients (a) and diffusion coefficients (b) on fractional accessible volume (FAV) in the system one polymer—different gases, after swelling with sc-CO2. Table 8 presents the slope of these dependences (B) before and after swelling with sc-CO2. This slope can be considered as general selectivity of a polymer to the studied gases. In Table 8, it can be seen that with regard to permeability coefficients, the selectivity increased for all polymers from 4 to 49 %. With regard to diffusion coefficients, the selectivity increased for polymers 3 and 6, but it decreased for polymers 4 and 5. For these polymers 4 and 5, the increase of free volume is significantly lower than in case of polymers 3 and 6 (Table 1).
To understand the reason of this behavior, we examine the solubility coefficients of these gases. The solubility coefficients S of each gas increases by swelling with sc-CO2, that is by increasing of free volume (Table 7). For O2, the solubility coefficients of all polymers increased with 190-285 %, for CO2 they increased with 134-254 %, and for N2 they increased with 215-323 %, which shows that they increase in a similar way for these three gases through all four polymers. The increase of solubility coefficients for Helium, of 19-584 %, makes an exception; the marginal values were found for polymers 5 and 6, as it was the case of diffusion coefficients of these two polymers, which can also be
connected with the errors in measuring of diffusion coefficient of this gas. Since the solubility of gases in swollen polymers always increases, it can be concluded that the volume of microcavities increases which determines the increase of free volume and, therefore, of the permeability coefficients.
Table 8 - Coefficients B in the dependence P(1/FAV) and D(1/FAV)
Polymer P (Barrer)
B before B after AB, %
1 8.31 12.42 49.4
2 10.36 10.82 4.44
3 8.09 8.57 5.93
4 9.64 11.60 20.33
Polymer ^•108 (cm3/s)
B before B after AB, %
1 13.87 20.02 44.34
2 17.44 17.33 -0.63
3 13.69 13.35 -2.48
4 15.78 22.63 43.41
The significant difference in the change of transport parameters of gases after treatment with sc-CO2 between polymer 3 and polymers 4-6 is connected with the individual selection of the swelling conditions for each polymer: temperature, pressure, and speed of sc-CO2 diffusion out of the polymer matrix. We used identical conditions of sc-CO2 treatment for all the studied polymers.
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© I Ronova - Leading Researcher, Doctor of Chemistry, Nesmeyanov Institute of Organoelement Compounds, Moscow Russia, A. Alentiev - Doctor of Chemistry, Professor, Topchiev Institute of Petrochemical Synthesis, Moscow, Russia, M. Bruma - Head of Laboratory, Doctor of Chemistry, "Petru Poni" Institute of Macromolecular Chemistry, Iasi, Romania, G. Zaikov - Doctor of Chemistry, Professor of Plastics Tecnology Department, Kazan National Research Technological Univercity, ov_stoyanov@mail.ru.
© И. Ронова - ведущий научный сотрудник, доктор химических наук, Институт элементоорганических соединений им. А.Н.Несмеянова РАН, Москва, Россия, А. Алентьев - доктор химических наук, профессор, Институт нефтехимического синтеза им. А.В.Топчиева РАН, Москва, Россия, М. Брума - заведующий лабораторией, доктор химии, Институт макромолеку-лярной химии им. Петру Пони, Яссы, Румыния, Г. Заиков - доктор химических наук, профессор кафедры Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия, ov_stoyanov@mail.ru.
