CC BY
31
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 1
Keywords: polyimide films, swelling in supercritical CO2, free volume, porous morphology, dielectric constant.
Here, the swelling with supercritical carbon dioxide (sc-CO2) of thin films ofpolyimides having various structures has been investigated. It was shown that the degree of swelling is significantly influenced by the solvent which was used for the synthesis of those polyimides, by the solvent which was used for the preparation of thin films and by the conformational rigidity of the polymers. Also, the change of pore size on the surface of the polymer films has been examined by AFM-method.
Ключевые слова: полиимидные пленки, набухание в сверхкритическом СО2, свободный объем, пористая морфология,
диэлектрическая постоянная.
В статье исследовано набухание тонких пленок полиимидов различной структуры в сверхкритическом диоксиде углерода (sc-CO2). Показано, что степень набухания в значительной степени зависит от растворителя, который используют для синтеза этих полиимидов и получения из них тонких пленок, и конформационной жесткости полимеров. Кроме того, методом AFM исследовано изменение размера пор на поверхности полимерных пленок.
1 Introduction
Some requirements to use polyimides for interlayer and intermetal dielectrics in advanced microelectronic applications are: high thermal stability, high glass transition temperature, good mechanical properties, low dielectric constant, low coefficient of thermal expansion and low moisture absorption. 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 polyimides which were synthesized in m-cresol or in carboxylic acid medium such as benzoic acid or salicylic acid. Influence on the free volume of the medium of synthesis and of the solvent was investigated. Also we examined the change of pore size on the surface of the polymer films
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].
Л =
lim
<R2 > ^
rnn
(1)
where <R2> 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 AM1 [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 =■
Vr
L
(2)
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 (Voce), 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 Avogadro's number, p is the polymer density, and Mo is the molecular weight of the repeating unit.
1 na.V°
(4)
P Ma
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 = VacsT
(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)Vbo
(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 were 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):
l/,= 1 -»A
P
Mn
(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 [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 = Pl • Wa / (Wa - Wl) (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, pi 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 (eo).
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 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 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 Influence on the free volume of the medium of synthesis and of the solvent from which the polymer films were cast
To explain the influence of solvent which was used for the synthesis of polymers and for the preparation of solutions for casting thin films, we examined polymer 2 of the series of polyimides (Table 1). First, we took polymer 2 which was synthesized in NMP [20] and the films were cast from three solvents: NMP, chloroform and tetrahydrofuran.
The treatment with sc-CO2 was performed at 200 bar and at temperatures of 40, 60 and 80°C. As seen in Table 2, the density of polymer films cast from different solvents, before sc-CO2 treatment, differ in the second digit after the comma, being 1.222, 1.246 and 1.212 g/cm3, respectively. This can be explained by the affinity of the solvent in relation with that polymer. The solvatation interaction between macromolecules and solvent molecules influences significantly the ability of polymer chains to undergo conformational transitions; it affects their equilibrium flexibility.
Table 1 - Repeating unit, glass transition temperature and conformational parameters of polyimides
Polymer Tg F lo Afr(Ah)
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
0 CH3 0
2- Synthesis in NMP and in salicylic acid
3 - Synthesis in benzoic acid
4 - Synthesis in benzoic acid
5 - Synthesis in benzoic acid
6 - Synthesis in benzoic acid
, , CF3 ,
zyW V^
N r \=/L
O
F - Fluorine
Table 2 - Change of the degree of swelling with increase of temperature, at 200 bar, of polyimide 2 of the series synthesized in NMP (the films were prepared by casting from various solvents: tetrahydrofuran, chloroform or NMP)
Solvent Tetra hydro furan CHCl3 NMP
Before treat ment Pc (g/cm3) 1.222 1.246 1.212
Vfc (cm3/g) 0.2875 0.2717 0.2942
T = 400C Pi 3 (g/cm3) 1.238 1.266 1.234
Vf1 (cm3/g) 0.2769 0.2590 0.2795
AVfr - 0.0106 - 0.0127 - 0.0147
T = 600C Pi (g/cm3) 1.211 1.215 1.214
Vfi (cm3/g) 0.2951 0.2922 0.2929
AVfr 0.0076; 2.6% 0.0205; 7.5% - 0.0048
T = 800C Pi (g/cm3) 1.204 1.210 1.210
Vfi (cm3/g) 0.2997 0.2956 0.2956
AVfr 0.0122; 4.3% 0.0239; 8.8% 0.0014; 0.48%
AVf - change of free volume; Vfo - free volume before treatment; - free volume after sc-CO2 treatment
In case of solvents like NMP which are able to form enough strong solvate shells around macromolecules, the possibility of conformational transitions will decrease significantly, and therefore it causes a decrease of flexibility equilibrium. In such solvents, the dimensions of macromolecular coils increase which leads to the modification of their hydrodynamic properties, while the quantity of kinetically independent parts, which serve as the segments of macromolecules, is reduced. This leads to a change of the characteristics of polymer solutions. This is why after removal of solvent (NMP), the resulting film is loose, less dense, and therefore its measured density is lower. In case of solvents like chloroform in which the interactions between macromolecular chains prevent the solvatation, the macromolecules will move in a relatively dense coil. The density of the films prepared from such solvents will be higher.
As seen in Table 2 and Fig. 1, at 40°C, the removal of residual solvent takes place from film which was cast from chloroform solution; tetrahydrofuran solvent is removed at 40°C and 50°C, then at 60°C the film starts to swell, while NMP solvent remains in the polymer film even at 60°C, and at 80°C the degree of swelling is so low as it can be neglected.
Fig. 1 - Relative change of free volume (AVf) with temperature (T) for polyimide film 2
The highest degree of swelling of polymer 2 synthesized in NMP was attained when the film was cast form chloroform solution, but it did not exceed 8.8% (Table 3). The structure of this polyimide 2 is similar to the structure of polyimide 1 (Table 1) which was synthesized in m-cresol.
n
Table 3 - Conditions of sc-CO2 treatment, density, change of free volume (AVfr) and dielectric constant (so) of the series of polyimides
Poly Parameters of Density, g/cm3
mer treatment Before After
treat treatment
ment
1 150 bar 40 0C 1.368 1.281
150 bar 65 0C 1.170
2 NMP Initial 1.246
200 bar 40 0C 1.266
200 bar 50 0C 1.224
200 bar 60 0C 1.215
200 bar 80 0C 1.210
2 Initial 1.199
120 bar, 40 0C 1.186
120 bar,60 0C 1.113
200 bar,40 0C 1.156
200 bar,60 0C 0.995
3 Initial 1.379
250 bar, 60oC 1.239
250 bar, 800C 1.155
250 bar, 1000C 0.889
4 Initial 1.432
200 bar, 800C 1.375
350 bar, 800C 1.328
500 bar, 800C 1.318
5 Initial 1.314
250 bar, 60oC 1.099
250 bar, 800C 1.014
250 bar, 1000C 0.967
6 Initial 1.436
250 bar, 80oC 1.395
250 bar, 1000C 1.388
350 bar, 1000C 1.137
So
3.42
3.48
2.76
3.19
3.15
2.78
2.45
2.80
2.74
2.56
2.46
3.28
3.12
2.93
2.87
2.99
2.96
2.89
2.77
Poly Para Free volume Vf Increase of
mer meters of (cm3/g) free volume
treat Before After (AVft,)
ment treat ment treat ment
1 150 bar 40 0C 0.2178 0.2681 0,0503, 23.1%
150 bar 0.3415 0,1237,
65 0C 56..8%
2 Initial 0.2717
NMP 200 bar 40 0C 0.2590 -0.0127
200 bar 0.2861 0.0144,
50 0C 5.3%
200 bar 0.2922 0.0205,
60 0C 7.5%
200 bar 0.2956 0.0239,
80 0C 8.8%
2 Initial 0.2963
120 bar, 0.3054 0.0091,
40 0C 3.0%
120 bar, 0.3608 0.0644, 21.8%
60 0C
200 bar, 0.3274 0.0310, 10.5%
40 0C
200 bar, 0.4683 0.171,
60 0C 58.0%
3 Initial 0.1833
250 bar, 0.2751 0.0918, 45.9%
60oC
250 bar, 0.3312 0.1479, 80.7%
800C
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%
As seen in Table 3, polymer 1 swelled in sc-CO2 quite well, which leads to the conclusion that the insignificant swelling of polymer 2 (synthesized in NMP) even in more severe conditions than for polymer 1 is most probably connected with the solvent used for its synthesis. Therefore, we synthesized polymer 2 in salicylic acid [15] and we prepared the film from chloroform solution. The treatment with sc-CO2 was performed at 120 bar, 200 bar, and at 40°C and 60°C. As seen in Table 3, polymer 2 synthesized in salicylic acid swelled quite well in sc-CO2, even if we had to use more severe conditions than for polymer 1. Thus, we have proved that the solvent used for the synthesis of polyimides is very important when treating the film with sc-CO2. The question is now in which way the NMP solvent can prevent the swelling of our polymers with sc-CO2. Previously, when studying the Fourier transform infrared spectra at high temperature of polyamidic acids which are intermediary products in the synthesis of polyimides, it was shown that during the synthesis or during the preparation of thin films from polyamidic acid solutions in NMP followed by prolonged heating up to 200°C to remove that solvent, the following two concurrent processes were possible [21]:
• formation of cross-links between polyamidic acids chains
NHimm»
NH«m«»
• formation of complexes between amidic groups and NMP
C^P
The formation of cross-links in polymers containing residual NMP prevails their swelling with sc-CO2. The formation of complexes with amidic groups prevail the free rotation around N-phenyl bonds and leads to the decrease of the number of conformers, and therefore it facilitates the formation of inter-chain anhydride cross-links with high yield. The low degree of swelling of 3.8-7% of those polyimides is also connected with the possible formation of anhydride bridges during the synthesis of those polyimides in NMP, also leading to cross-links between chains. However, in case of polyimides, such cross-links are formed not so often as in case of polyamidic acids [22].
Another argument to the formation of crosslinks is the higher density of polymer film 2 synthesized in NMP than the density of polymer film 2 synthesized in salicylic acid (Table 3). In both cases, the films were cast from chloroform solution of polymer. In the first case, the presence of cross-links made the polymer more rigid, and therefore its packing was denser. Thus, the study of polymer 2 synthesized in two different solvents, NMP and salicylic acid, proved the conclusion made earlier on the basis of infrared spectra regarding the formation of cross-links during imidization of polyamidic acids in NMP.
4.2 Effect of solvent used for density measurement
The density of polymer film 2 of the series of polyimides (Table 1) which was synthesized in NMP, before and after treatment with sc-CO2, was measured in two solvents: ethanol and isopropanol. By using the value of density we calculated the free volume with equation (7) before and after sc-CO2 treatment. As shown in Table 4, the free volume values, calculated by using the density values measured in those two solvents are different from each other.
Table 4 - Change of the swelling degree with the increase of temperature of sc-CO2 treatment of polyimide 2 of the series of polyimides, synthesized in NMP, whose film was cast from chloroform. (The treatment with sc-CO2 was run at a pressure of 200 bar. The densities were measured in ethanol and in isopropanol)
T (0C) Ethanol
P (g/cm3) Vfr (cm3/g) _V3 * (cm3/g; % of increase)
Before treat ment 1.246 0.2717
40 1.266 0.2590 -0.0127
50 1.224 0.2861 0.0144; 5.3%
60 1.215 0.2922 0.0205; 7.5%
80 1.210 0.2956 0.0239; 8.8%
T (0C) Isopropanol
P (g/cm3) Vfr (cm3/g) _Vf * (cm3/g; % of increase)
Before treat ment 1.267 0.2584
40 1.250 0.2692 0.0108; 4.2%
50 1.238 0.2769 0.018; 2.5%
60
80 1.235 0.2788 0.0204; 7.9%
* - _Vf = Vft - Vfo, _Vf - change offree volume; Vfo - free volume before treatment; Vft - free volume after sc-CO2 treatment
Then we calculated the change of free volume by comparison with the free volume of the sample before sc-CO2 treatment which was measured in the same solvent: AVf = Vft - Vfo, where Vfo is the free volume of the sample before treatment and Vft is the free volume of the film after treatment with sc-CO2 at a given temperature. Figure 1 presents the change of free volume with the temperature of sc-CO2 treatment, for the three solvents. Before treatment, the film sample contains some amount of residual NMP remaining in polymer after its synthesis in this solvent synthesized in NMP and cast from three solvents; the density was measured in ethanol.
During sc-CO2 treatment, this solvent was removed from polymer matrix. This is why in figure 2 the first value of the change of free volume when measured in ethanol was negative. The NMP solvent was extracted by CO2 from polymer matrix and the density of polymer film increased.
0,025-
0,020 -
0,015-
0,010-
Ui
E 0,005-
o
3" 0,000-
-0,005-
-0,010-
-0,015-
ethil alcohol i so propyl alcohol
40
50
60 T,°C
70
Fig. 2 - Variation of free volume (AVf) of polymer film 1 (Table 1) (synthesized in NMP) by swelling with sc-CO2 at various temperatures, when the density was measured in isopropanol or in ethyl alcohol
When measuring the density in isopropanol the free volume of polymer included the volume of pores on the surface of film where the molecules of isopropanol could not enter due to their higher volume compared with that of ethanol. Figure 2 shows that the change of free volume values by swelling with sc-CO2 at 40°C and 50°C, calculated by using the density values measured in isopropanol, is higher than in the case when density
was measured in ethanol. It means that on the surface of polymer film there were some small pores in which ethanol molecules having the Van der Waals diameter of 4.16 A could enter, while isopropanol molecules having a diameter of 6.72 A could not enter. The measured free volume in case of isopropanol is equal to the volume inside the polymer film plus the volume of pores on its surface. By treating the polymer film 2 of the series of polyimides (synthesized in NMP) with sc-CO2 at 50°C the difference between free volume values measured in ethanol and isopropanol first increases and then, at temperatures above 50°C, it decreases and it becomes negative. We presume that the pores on the film surface become smaller at low temperature of sc-CO2 treatment, then their sizes increase at higher temperatures.
4.3 Study of the surface of polyimide films
We examined the change of pore size on the surface of the polymer films (polymer 2, Table 5). The pores were formed in the films during their preparation. For each sample about 100 pores were measured. The data were summarized in Table 5. Figure 3 shows the topographic images of the films before and after sc-CO2 treatment. Information regarding the roughness of the surface can be found on a color scale: black color corresponds to minimum, white color corresponds to maximum.
Table 5 - Change of pore size with temperature of swelling of the polymer 2 in the series of polyimides
nm ! i 0 0 §
T, (°C) H*, (nm) % of pores with H <20 % of pores with H >20 D*, (nm) 0 0 <1 D th 1 s re o p f o % % of pores with D from to 200 nm % of pores with D>200 :
Before
swel 10.3 91 9 109 57 37 6
ling
40 5.8 97 3 94 66 32 2
60 9.1 90 10 152 21 63 16
80 12.4 84 16 161 10 73 17
* - H - average pore depth, D - average pore diameter
The surfaces of polymer films have a globular structure, with diameter of globules of 10-30 nm. The root mean square roughness of the defect-free regions of the surface is 0.4 nm. On the surface of the polymer films before and after sc-CO2 treatment we found an amount of pores of 1-2 pores/^m2. The dimension of pores varies in a large domain. The height has changed from a few Angstroms to 100 nm. The diameter ranges from 20 nm to 600 nm. The depth can be underestimated for narrow and deep pores because the probe can only touch the walls of the pores, but not the bottom. Besides pores, some particles can be observed on these images and their formation can be explained by
partial degradation of the polymer surface during treatment with sc-CO2. By sc-CO2 treatment at 40°C the number of pores having small diameter and depth increases, which leads to the increase of total free volume (free volume inside the film plus the volume of the pores) of the polymer when the density was measured in isopropanol. When the polymer film was treated with sc-CO2 above 40°C, the number of small pores decreased significantly. It is understandable why in figure 2 at the beginning, at temperatures of 40°C and 50°C, the free volume value based on measurement in isopropanol is higher than that measured in ethanol, and later at 80°C it becomes smaller. It means that the total volume of pores measured in isopropanol decreases because the depth and the diameter of pores become larger (Table 5) [23].
Fig. 3 - Topographic images (AFM) of the surface of polymer film 1 (Table 1) synthesized in NMP: (a) before treatment with sc-CO2; (b) after treatment with sc-CO2 at 40°C
The surface structure of polymer films 5 and 6 of the series of polyimides (Table 1) before and after sc-CO2 treatment was examined by AFM (Figures 4 and 5). The films exhibited isotropic globular structure with globules heights of up to 1 nm relative to the surface. The polyimide films before treatment contained a small amount of surface pores (up to 15 pores per 100 |im2) with depth of up to 30 nm.
The sc-CO2 treatment of polymer film 5 leads to the increase of root means square roughness (RMS) due to the appearance of some hard particles on the surface having the height of up to 350 nm. We presume that such hard particles may appear due to partial degradation of the surface during treatment with sc-CO2 (possibly mechanical degradation). For polymer film 5 the RMS value increases with 5 nm up to 19-33 nm after sc-CO2 at 250 bar and at a temperature of 60-100°C.
i ffcjsl» mm
p'
i—-,-
0 700 1400 2100 2800
Fig. 4 - AFM image of the surface of polymer film 2 (Table 1): (a) before treatment with sc-CO2; (b) after treatment with sc-CO2 at 250 bar and 80°C
0 1000 2000 3000 4000 5000 6000 7000
Fig. 5 - AFM image of the polymer film 3 (Table 1): (a) before treatment with sc-CO2; (b) after treatment with sc-CO2 at 350 bar, 100°C
When the treatment was run at 80°C a regular pore system appeared on one surface of the film with depth of 2.5 ± 0.8 nm and diameter of 119 ± 34 nm (Figure 5b). When the temperature of sc-CO2 treatment
increased to 100°C, a small number of pores were observed on the surface (up to 10 pores per 100 |im2) with depth of up to 35 nm and diameter of 100 nm up to 600 nm. The sc-CO2 treatment of polymer film 6 of the third series of polyimides did not lead to the appearance of supplementary pores on the surface. The RMS changed insignificantly. It increased from 3 nm to 6 nm and 4 nm after sc-CO2 treatment at 350 bar and 80°C and 100°C, respectively.
The structure inside polymer films was studied by TEM technique. The TEM technique showed that polymer films 5 and 6 became opaque after sc-CO2 treatment. This means that there were defects in the films with dimensions comparable to the wavelength of visible light. Nano- and micro-pores were formed in the core of the films. Figures 6a and 6b present cross-sections of polyimide film 3 before and after sc-CO2 treatment that were examined by TEM.
Fig. 6 - TEM image of the cross-sections of polymer film 3 (Table 1). The cross-sections are displayed on copper net having the dimension of 50|m: (a) before swelling with sc-CO2; (b) after swelling with sc-CO2 at 250 bar and 100°C
It can be seen that pores of various diameters from 10 nm to 10 |im appear after sc- CO2 treatment. It means that both nano-pores and micro-pores appear after sc-CO2 treatment. In the first place, this is connected with the pressure of sc-CO2. The formation of pores was observed only inside of the film. Near the surfaces of the films, pores were not detected. For different samples the regions without pores extended to 5-30 |im from the surface.
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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, chembio@sky.chph.ras.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 макромолекулярной химии им. Петру Пони, Яссы, Румыния, Г. Заиков - доктор химических наук, профессор кафедры Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия, chembio@sky.chph.ras.ru.
