CC BY
20
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 2
Keywords: polyimide films, swelling in supercritical CO2, free volume, porous morphology, dielectric constant.
We have shown that the swelling with sc-CO2 of polyimide films is directly connected with conformational rigidity and with fluorine content in the repeating unit of the respective polymers. The best swelling takes place when the value of characteristic ratio is 0.4 - 0.8. Also, higher the fluorine content of the repeating unit, more significantly decreases the dielectric constant of the polymer films. The formation of nano-foams by swelling with sc-CO2 may lead to a dielectric constant value even below 1.5.
Ключевые слова: полиимидные пленки, набухание в сверхкритическом СО2, свободный объем, пористая морфология, диэлектрическая постоянная.
Показано, что набухание полиимидных пленок в сверхкритическом СО2 непосредственно связано с конформа-ционной жесткостью и содержанием фтора в повторяющемся звене соответствующих полимеров. Степень набухания выше при значениях характеристического отношения 0,4 - 0,8. Кроме того, чем выше содержание фтора в повторяющемся звене, тем гораздо значительнее уменьшается диэлектрическая постоянная полимерных пленок. Образование нанопен в процессе набухания в сверхкритическом СО2 может даже привести к величине диэлектрической постоянной ниже 1,5.
1 Introduction
The dielectric constant of polyimides depends mainly on the free volume of polymer matrix and on the polarizability and hydrophobicity of macromolecular chains. One way to increase the free volume of polymer matrix is by treating with supercritical carbon dioxide (sc-CO2) [1, 2]. The degree of swelling with sc-CO2 depends on conformational rigidity of macromolecular chains, glass transition temperature and on the presence of fluorinated units in the polymer structure. Previously, it was shown that a large number of polymers of various structures did not swell or swelled insignificantly in sc-CO2. This behavior was explained by the fact that those studied polymers had been synthesized in N-methylpyrrolidone as solvent which facilitated the formation of cross-links between the macromolecular chains and thus the penetration of CO2 molecules was hindered [3, 4].
Here, we present an investigation of the swelling process with sc-CO2 of thin films of some polyi-mides which were synthesized in m-cresol or in carbox-ylic acid medium such as benzoic acid or salicylic acid. We have shown that the swelling with sc-CO2 of polyi-mide films is directly connected with conformational rigidity and with fluorine content in the repeating unit of the respective polymers. The best swelling takes place when the value of characteristic ratio is 0.4 - 0.8. Also, higher the fluorine content of the repeating unit, more significantly decreases the dielectric constant of the polymer films. The formation of nano-foams by swelling with sc-CO2 may lead to a dielectric constant value even below 1.5.
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 [5]. 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) [6].
A- =
lim
<R2 >^
rnn
(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 [7].
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 [7, 8]. 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 [9].
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 Avoga-dro's number, p is the polymer density, and Mo is the molecular weight of the repeating unit.
= 1 (4)
p M0
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) [10, 11].
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 [7]. 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 [12]. 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 [13, 14]. 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):
= 1 (7)
P M0
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 [15, 16], 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 polymer. 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 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 / (Wa - W) (8)
where ps is density of the sample, Wa is the weight of the sample in air, W 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 (so).
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 [17, 18]. 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 С. The cell is designed for experiments at pressures up to 50 MPa and temperatures up to 120°С. 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 Investigation of the films microstructure before and after swelling in sc-CO2
The dimension of pores and their distribution is very important from the point of view of mechanical and dielectric properties of porous materials. The morphology of the surface of polymer films was studied by
using atomic force microscopy (AFM) in tapping mode, in air, at room temperature. The polymer films were cast on cellophane. The topographic images of the surface of polymer were obtained by using scanning probe microscope FemtoScan produced by Advanced Technologies Center, Russia. The analysis and processing of AFM images were performed with a program FemtoScan Online [19]. We used cantilevers of type fpN20 from F.V. Lukin Research Institute of Physical Problems, Russia. The medium resonance frequency of cantilever was 420 kHz. The radius of the curvature of the tip was less than 25 nm. The depth and diameter of pores were measured from the profiles of topographic images.
The investigation of the cross-sections of films before and after treatment with sc-CO2 was performed by using transmission electron microscopy (TEM) with LEO 912AB OMEGA apparatus from Karl Zeiss, Germany. The spatial resolution was less than 0.5 nm. We prepared cross-sections of polymer films with the aid of microtome Ultracut (Reichert-jung, Germany), having the thickness of 0.1 |im and we examined them immediately after their preparation. The cross-sections were placed on formvar coated copper grids.
4 Results and Discussion
4.1 Lowering the dielectric constant
The influence of the increase in free volume during swelling in supercritical carbon dioxide on the dielectric constant was examined in two series of polyimides: the first series (tab. 1) and the second series (tab. 2). Two polymers of the first series (1 and 6) are present in the second series (3 and 1). The 3 polymer of the second series was swelling in sc CO2 in more stringent conditions. Polymer 1 of the second series was synthesized in another solvent. All polymers of the second series were synthesized in meta-cresol [20].
Table 1 - Repeating unit, glass transition temperature and conformational parameters of the first series of polyimides ___
Poly Tg F lo Afr(Ah)
mer
1 2 3 4 5 6
1 180 11.36 41.86 20.28 0.484
2 188 - 37.39 18.31 0.489
3 225 - 27.13 20.93 0.771
4 234 16 27.13 21.71 0.800
5 205 - 42.05 24.81 0.590
6 234 25.59 32.20 29.11 0.904
_Repeating unit_
1 - synthesis in meta-cresol
2 - synthesis in NMP and in salicylic acid
3 - synthesis in benzoic acid
End table 1
4 - synthesis in benzoic acid
5 - synthesis in benzoic acid
6 - synthesis in benzoic acid
C,
<J
C'
II
O
F - Fluorine
Table 2 - Repeating unit and conformational parameters of the second series of polyimides
Polymer l (Á) Ah (Á) Cw F (Wt %) Vw (Á3) 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.
N n
C.
M J
•N J
C'
O
O
Ap1
r'
O
C
■<J
C . O
I
O
■CC
O
at a pressure of200 bar and a temperature of 60oC; F -fluorine
At the beginning we consider the behavior of the first series of polymers. The polymer 1 of this series was synthesized in meta-cresol, while the polymer 2 in NMP and salicylic acid. The other four studied polymers (Table 1) were synthesized in benzoic acid. To see and understand how the physico-chemical characteristics of polyimides change under the action of sc-CO2 we analyzed them before treating with sc-CO2. Table 1 pre-
sents the chemical structure of the repeating unit of the first series of polyimides, the solvent used for their synthesis, glass transition temperature, Kuhn segment value and characteristic ratio Cw. The dependence of Tg on characteristic ratio is linear (Figure 1) and it has a normal aspect: with increasing of the conformational rigidity, the probability of conformational transitions leading to the melting of polymer matrix decreases. Since all the points are situated on a line having a high correlation coefficient (99.46%), it means that all the studied polymers have enough high molecular weight and they form coils. Besides, the imidization process was complete in all those polymers and non-cyclized units were absent [21, 22]. The dependence of Tg on free volume confirms this explanation (Figure 2). With increasing of the conformational rigidity, the free volume of polymer matrix decreases (Figure 3), the packing of polymers becomes stronger. With increasing of the conformation-al rigidity, the dimensions of linear parts of the polymer chain increase, and as a consequence, the packing of chains is tighter while the free volume of polymer is smaller. On the other hand, while decreasing of free volume, the possibility of conformational transitions decreases and thus, the glass transition temperature increases [23].
mo-i
230220210 200 -190-1B0-
y=119,74+140,9 Ox, R=99.4&%
—I—■—I—'—I—'—I—■—I—'—I—■—I—■—I—'—1
0,40 0,45 0,50 0,55 0.50 0,55 0,70 0,75 0,80 0,85
Fig. 1 - Dependence of glass transition temperature (Tg) on characteristic ratio (C„) for the first series of polyimides
V=367.85-604.62!(, R=98.86%
O
240
230 220 "210200 190
1,0-1 1 ■\=446.52-1201,77x, R=95.97%
1—'—I—1—I—1—I—*—I—1—I—1—I—1—I—1—r-
0,16 0T1B 0.20 0,22 0,24 0,26 0,28 0,30 0,32 0,34 0,36
Fig
. 2 - Dependence of glass transition temperature (Tg) on free volume (Vf) for the first series of polyimides
Although in Figure 1 all points corresponding to the studied polymers are situated on one line, in Figures 2 and 3 the corresponding points are divided in two dependences. One dependence contains polymers 1, 3
O
1
2
3
4
5
N
6
and 5 of the first series which do not contain fluorine atoms in dianhydride segment; their Tg increases sharply with decreasing of free volume. The second dependence contains polymers 4 and 6 of the first series which do contain fluorine atoms in dianhydride segment, and polymer 2 which was synthesized in salicylic acid.
Fig. 3 - Dependence of free volume (Vf) on characteristic ratio (C„) for the first series of polyimides
The conditions of swelling in sc-CO2 are shown in Table 3.
Table 3 - Conditions of sc-CO2 treatment, density, change of free volume (AVfr) and dielectric constant (eo) of the first series of polyimides
Poly mer Parameters of treatment Density, g/cm3 So
Before treat ment After treatment
1 150 bar 40 °C 1.368 1.281
150 bar 65 °C 1.170
2 NMP Initial 1.246
200 bar 40 °C 1.266
200 bar 50 °C 1.224
200 bar 60 °C 1.215
200 bar 80 °C 1.210
2 Initial 1.199
120 bar, 40 °C 1.186
120 bar,60 °C 1.113 3.42
200 bar,40 °C 1.156 3.48
200 bar,60 °C 0.995 2.76
3 Initial 1.379 3.19
250 bar, 60oC 1.239 3.15
250 bar, 80°C 1.155 2.78
250 bar, 100°C 0.889 2.45
4 Initial 1.432 2.80
200 bar, 80°C 1.375 2.74
350 bar, 80°C 1.328 2.56
500 bar, 80°C 1.318 2.46
5 Initial 1.314 3.28
250 bar, 60oC 1.099 3.12
250 bar, 80°C 1.014 2.93
250 bar, 100°C 0.967 2.87
6 Initial 1.436 2.99
250 bar, 80oC 1.395 2.96
250 bar, 100°C 1.388 2.89
350 bar, 100°C 1.137 2.77
Poly mer Para meters of treat ment Free volume Vf (cm3/g) Increase of free volume (AVfr,)
Before treat ment After treat ment
1 150 bar 40 °C 0.2178 0.2681 0,0503, 23.1%
150 bar 65 °C 0.3415 0,1237, 56..8%
2 NMP Initial 0.2717
200 bar 40 °C 0.2590 -0.0127
200 bar 50 °C 0.2861 0.0144, 5.3%
200 bar 60 °C 0.2922 0.0205, 7.5%
200 bar 80 °C 0.2956 0.0239, 8.8%
2 Initial 0.2963
120 bar, 40 °C 0.3054 0.0091, 3.0%
120 bar, 60 °C 0.3608 0.0644, 21.8%
200 bar, 40 °C 0.3274 0.0310, 10.5%
200 bar, 60 °C 0.4683 0.171, 58.0%
3 Initial 0.1833
250 bar, 60oC 0.2751 0.0918, 45.9%
250 bar, 80°C 0.3312 0.1479, 80.7%
250 bar, 100°C 0.5906 0.4073, 209.0%
4 Initial 0.2154
200 bar, 80°C 0.2443 0.0289, 13.4%
350 bar, 80°C 0.2698 0.0544, 25.3%
500 bar, 80°C 0.2753 0.0599, 27.8%
5 Initial 0.2068
250 bar, 60oC 0.3554 0.1486, 41.8%
250 bar, 80°C 0.4320 0.2252, 119.2%
250 bar, 100°C 0.4798 0.2912, 142.0%
6 Initial 0.2285
250 bar, 80oC 0.2490 0.0205, 9.0%
250 bar, 100°C 0.2526 0.0241, 10.5%
350 bar, 100°C 0.4116 0.1831, 80.1%
The polymer films 5 and 6 of the first series of polyimides were also treated with sc-CO2 at 250-350 bar and 60-100°C. Their densities were measured in ethanol. The free volume increased with 7% up to 140%. The dielectric constant values of those polymers are shown in Table 3. The values of dielectric constant
are in the range of 2.45-3.48, being comparable or even lower than the value of Kapton polyimide film measured under the same conditions (3.13-3.24) [24].
As seen in table 3, polymer 3 (ULTEM) of the first series (based on m-phenylene-diamine and isopropylidene-diphenoxy-bis(phthalic anhydride)) was the most sensitive to treatment with sc-CO2. In relatively mild conditions of treatment with sc-CO2 (250 bar, 100°C), its free volume doubled: the increase was 222.2% and its dielectric constant became significantly lower (2.45) compared with the value before swelling (3.19). The analogue polymer 4 of the first series which contains hexafluoroisopropylidene units between imide rings exhibited a lower dielectric constant (2.80) even before treating with sc-CO2, due to the presence of fluorine substituents. This value decreased by treating with sc-CO2 to almost the same value (2.46) as for polymer 3 of the first series. Even if polymer 3 of the first series did not contain fluorine substituents, which is very important for electronic applications, it did swell quite well with sc-CO2 and it has the advantage of being commercially available (under the name of ULTEM).
In polymer 4 of the first series the content of fluorine is 16%. The swelling with sc-CO2 was performed under the same temperature (80°C) and in a large domain of pressure parameters, from 200 bar to 500 bar. Regardless the severe conditions of sc-CO2 treatment (500 bar), the increase of free volume was maximum 27.8%. It means that the presence of fluorine atoms decreases the dielectric constant, but it prevail the increase of free volume. The study of those two polymers, 3 and 4 of the first series, having identical structure except fluorine atoms in polymer 4 shows that their sc-CO2 swelling process is more sensitive to temperature than to pressure.
Polymer 5 of the first series underwent sc-CO2 treatment under the same conditions as polymer 3 of the first series: constant pressure of 250 bar, and various temperatures, of 60°C, 80°C, and 100°C. Its characteristic ratio is lower (0.59) than that (0.771) of polymer 3 and its glass transition temperature (205°C) is also lower than that (225°C) of polymer 3. Its free volume increased less than that of polymer 3, from 0.2068 to 0.4798, which means an increase of 132% compared to 222.2% increase of polymer 3. The dielectric constant decreased but not very significantly, from 3.28 to 2.87 (Table 3).
In case of polymer 6 of the first series, the content of fluorine is 24% and the treatment with sc-CO2 was performed at 80°C and 100°C, and at 250 bar and 350 bar. The increase of free volume by treatment at 250 bar and 80°C 100°C was insignificant, of only 910.5%. Under more severe conditions, 350 bar and 100°C, the increase of free volume was 80% and dielectric constant decreased from 2.99 to 2.77. The film of this polymer 6 of the first series having a thickness of 20 |im practically did not swell with sc-CO2 , while a thicker film, of 40 |im, did swell and the results are those shown in Table 3.
The low dielectric constant of polymers 4 and 6 of the first series before treatment with sc-CO2 are due to the presence of hexafluoroisopropylidene groups which disturb the packing of macromolecular chains in
glassy state and thus they determine the increase of free volume. Thus the amount of polarizable groups per volume unit is lower, which also leads to the decrease of dielectric constant [18]. All these results on dielectric constant values before and after treatment with sc-CO2 are illustrated in Figure 4.
■ Polymer 2 y=5.29-5.36x,R=98.82%
Free volume, Vh, cm!/g
Fig. 4 - Variation of dielectric constant by treatment with sc-CO2 for the first series of polyimides
As seen in Figure 4, the decrease of dielectric constant with increase of free volume is not identical in these polymers. The equations which were introduced in Figure 4 for the linear dependence of dielectric constant on free volume show that the correlation coefficients are high enough and the lines have different inclinations. For polymers 2 and 4 of the first series the inclination of the lines is stronger, which indicates the easy reconstruction of packing of chains in polymer matrix under the action of sc-CO2 and rapid decrease of dielectric constant with small increase of free volume. The increase of pressure and temperature during the swelling process with sc-CO2 in these cases could give even lower values of dielectric constant. The polymers 3, 5 and 6 of the first series exhibit a rapid increase of free volume during swelling with sc-CO2, but the decrease of dielectric constant with the increase of free volume takes place slower. The difference in the behavior of these two groups of polymers seems to be connected with the distribution of dipoles in the repeating unit. That corresponds well with dividing these polymers in two groups when discussing the dependence of glass transition temperature on free volume of the polymers before swelling with sc-CO2.
We have calculated the Kuhn segment under the assumption of free rotation (Afr), characteristic ratio (CM) and Van der Waals volume (Vw). These polymers were synthesized in да-cresol and the films were prepared from their solutions in chloroform. Table 4 shows the values of glass transition temperature of the second series of polyimides. These values are significantly lower (in the range of 200-290°С) than those of traditional aromatic polyimides which are well above 300°С. These lower glass transition temperatures could be due to the plasticizing of solid polymers with m-cresol. In polymer films, as it was mentioned above, the residual m-cresol was removed by extraction with methanol and the films were further heated in vacuum at 70°С. The values of Tg measured afterwards are given in table 4. For the first three polymers the values of Tg are also known
from literature [26-28] and they are somewhat higher than the values measured by us after removal of m-cresol and methanol. It is known that Tg depends on the free volume of polymer matrix [23].
Now, we examine the dependence of Tg values measured after removal of solvents on the characteristic ratio (Figure 5). This dependence is linear, with a high correlation coefficient. It means that the cyclization to polyimide structure was complete, except for polymer 3 in this series. However, we can not make a clear conclusion about cyclization degree of polymer 3 since the cause of loss of points for polymer 3 (Table 2) could be exposed after removal of meta-cresol microcavities which lower its glass transition temperature Tg.
Fig. 5 - Dependence of glass transition temperature (Tg) on characteristic ratio (Cw) for the second series of polyimides
Therefore, the study was undertaken on cross-section of polymer films, by using transmission electron microscopy (TEM). A typical TEM image is shown in figure 6. The thickness of the film was 80 ^m before swelling with sc-CO2 and, as can be seen in figure 6, it contained pores at 10 ^m under the surface having the dimensions of 18 nm - 3 ^m; it shows that even before treatment with sc-CO2 the polymer matrix was porous.
Table 5 presents the parameters of swelling process, increasing of free volume and of dielectric constant of the second series of polymers.
Fig. 6 - TEM image of a section of polymer film 3 before swelling with sc-CO2 for the second series of polyimides
Table 5 - Increase of free volume and decrease of dielectric constant after swelling in sc-C02 at a pressure of 200 bar and various temperatures (T)for the second series of polyimides
Table 4 - Glass transition temperature Tg (°C) before and after extraction of m-cresol
Polymer Before extraction of m cresol Tg value from literature After extraction of m-cresol Polymer Before extraction of m-cresol After extraction of m-cresol
1 196.9 234 [26] 251 4 111 181
2 131 224 [27] 211 5 130 207
3 102 180 [28] 164 6 133 204
Sample (Test) T (0C) Before swelling in sc-C02 AVf (%)
P (g/cm3) Vf (cm3/g)
1 (1) 1.458 0.1976
1 (2) 60 1.478 0.1883 9.93
1 (3) 40 1.477 0.1891 19.25
1 (4) 80 1.483 0.1864 15.77
2 (1) 1.497 0.1799
2 (2) 60 1.480 0.1879 58.44
2 (3) 40 1.518 0.1708 92.38
2 (4) 80 1.501 0.1769 37.37
3 (1) 1.381 0.2111
3 (2) 60 1.401 0.2009 257.64
3 (3) 40 1.389 0.2070 157.87
3 (4) 80 1.376 0.2138 212.07
4 (1) 1.465 0.2030
4 (2) 60 1.467 0.2022 132.79
4 (3) 40 1.476 0.1980 179.04
4 (4) 80 1.384 0.2400 95.17
5 (1) 1.268 0.2620
5 (2) 60 1.290 0.2485 48.37
5 (3) 40 1.284 0.2517 28.96
5 (4) 80 1.290 0.2480 28.71
6 (1) 1.431 0.2193
6 (2) 60 1.459 0.2058 64.67
6 (3) 40 1.462 0.2047 64.48
6 (4) 80 1.452 0.2094 68.72
Sample (Test) T (0C) After swelling in sc-C02 So
P (g/cm3) Vf (cm3/g)
1 (1) 3.00
1 (2) 60 1.439 0.2070 2.98
1 (3) 40 1.401 0.2255 2.92
1 (4) 80 1.421 0.2158 2.94
2 (1) 2.54
2 (2) 60 1.273 0.2977 2.04
2 (3) 40 1.225 0.3286 1.96
End table 5
2 (4) 80 1.368 0.2430 2.16
3 (1) 1.94
3 (2) 60 0.812 0.7185 1.62
3 (3) 40 0.955 0.5338 1.76
3 (4) 80 0.847 0.6672 1.63
4 (1) 2.46
4 (2) 60 1.052 0.4707 1.88
4 (3) 40 0.969 0.5525 1.58
4 (4) 80 1.055 0.4684 2.00
5 (1) 2.43
5 (2) 60 1.116 0.3687 2.23
5 (3) 40 1.174 0.3246 2.35
5 (4) 80 1.182 0.3192 2.27
6 (1) 1.89
6 (2) 60 1.152 0.3389 1.58
6 (3) 40 1.2240 0.3375 1.54
6 (4) 80 1.201 0.3533 1.45
The swelling was performed for three samples of each polymer under constant pressure, at three temperature values, followed by rapid decompression of CO2 pressure. As seen in table 5, the swelling at 200 bar and 60°C depends on both conformational rigidity and on the content of fluorine in the repeating unit. For example, polymers 1 and 2 of the second series have the same content of fluorine, but while the characteristic ratio decreases from 0.904 to 0.655 (Table 2), the free volume increases almost 6 times (table 5). In the case of polymers 3 and 4 of the second series, having almost identical conformational rigidity, the increase of fluorine content from 11.36 % to 20.52 % leads to the decrease of free volume almost twice. Now we compare the behavior of polymer 1 from the first series with polymer 3 from the second series. Both polymers were synthesized in rn-cresol. Polymer 1 was treated with sc-CO2 at a pressure of 150 bar, temperature of 65°C, with slow pressure-release, while polymer 3 was treated with sc-CO2 at a pressure of 200 bar, temperature of 65°C, with rapid pressure-release. The difference in sc-CO2 treatment conditions led to an increase of 5 times in the free volume of polymer 3 in the second series. This behavior may lead to the conclusion that by rapid release of CO2 pressure, the flexible polymer (CM = 0.484) can form a foam. To confirm this conclusion, we have investigated the surface of polymer film 3 by using atom force microscopy (AFM) (Figure 7).
Before swelling (Figure 7a), the sample showed a smooth flat surface with root mean square roughness (RMS) of ~10 nm. On the surface, a low quantity of pores was seen (0.003 - 0.3 pores/^m2) having the depth of 6-32 nm. After swelling with sc-CO2 the RMS value increased significantly, to 30 nm, on the surface appeared zones with high pores concentration, up to 8 pore/ ^m2, with depth up to 270 nm, and undisclosed pores in the form of hills appeared having the length up to 24 nm and diameter of 1300 nm (Figure 7b). But, through-holes did not appear on all the studied films. In addition, we have investigated these samples after swelling by using transmission electron microscopy (TEM). A representative image is shown in figure 8. The thickness of the sample was 130 ^m. Figure 8
shows that the pores appeared at 5 ^m below the surface and their dimension was 100 nm to 10 ^m. Thus, we have shown that when the pressure of the CO2 is released rapidly after swelling, the flexible polymers may form foams.
b
Fig. 7 - AFM image of a section of polymer film 3 of the second series of polyimides before (a) and after (b) swelling with sc-CO2 at 200 bar and 60°C with rapid release of CO2 pressure
Fig. 8 - TEM image of a section of polymer film 3 of the second series of polyimides after swelling with sc-CO2 at 200 bar and 60°C, with rapid release of CO2 pressure
For each polymer film the dielectric permittivity was measured in the domain of frequencies from 10-3 to 106 Hz and it was approximated for the frequency value of zero to obtain the dielectric constant (s0). Figure 9 presents the dependence of dielectric constant on free volume. As can be seen, the polymers behave differently when increasing the free volume.
a
Vf, cmJig
Fig. 9 - Dependence of dielectric constant (eo) on free volume (Vf) for the second series of polyimides
For same of them the dielectric constant decreases significantly when the free volume modifies only a little; for the other polymer films the dielectric constant varies insignificantly. All the dependences are described by the equation y = A-Bx with a high correlation coefficient, R. Table 6 presents the values of B in these equations.
Table 6 - Change of free volume (Vf) after swelling at pressure of 120 bar and temperature of 40 °C of the second series of polyimides
Poly mer Before swelling in sc-CO2 AVf (cm3/g)
P (g/cm3) V{ (cm3/g) Tg (°C)
1 1.389 0.2070 167 0.0330
2 1.431 0.2230 181 0.0084
3 1.341 0.2327 207 0.0113
4 1.428 0.2245 204 0.0171
Poly mer After swelling in sc-CO2 AVf (%)
P (g/cm3) V{ (cm3/g) Tg (°C)
1 1.328 0.2400 169 16
2 1.414 0.2314 180 3.8
3 1.321 0.2440 206 4.9
4 1.394 0.2416 204 7.6
It can be seen that the inclination at rapid decrease of dielectric constant (high value of B) is strongly related with the content of fluorine in the repeating unit (polymers 1, 2 and 6 of the second series). Higher the fluorine content, more significantly decreases the dielectric constant with the increase of free volume. Thus, on one hand the presence of fluorine atoms and the high rigidity hinder the swelling with sc-CO2, and on the other hand in the case of polyimides containing fluorine the dielectric constant is more sensitive to the increase of free volume.
In this article we have shown that the swelling with sc-CO2 of polyimide films is directly connected with conformational rigidity and with fluorine content in the repeating unit of the respective polymers. The best swelling takes place when the value of characteris-
tic ratio is 0.4 - 0.8. Also, higher the fluorine content of the repeating unit, more significantly decreases the dielectric constant of the polymer films. The formation of nano-foams by swelling with sc-CO2 may lead to a dielectric constant value even below 1.5.
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© 1 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.
