Measurement of helium and carbon dioxide permeability through polyamidoimide films

Measurement of helium and carbon dioxide permeability through polyamidoimide films K.D. Petrenko, L.I. Zamulina, Institute of Macromolecular Kiev, USSR Received 20 December

A.V. Pedosenko

and V.P. Privalko

Chemistry, Academy of Sciences of the UkrSSR, 252160,

1989

The helium and carbon dioxide permeability through film samples of polyamidoimides (PAI) of two series at room temperature and a partial pressure drop of 0.1 MPa across the membrane has been studied. A correlation between the chemical structure of the studied PAI and their permeability has been ascertained: the helium permeability through PAI samples grows with increasing content of the diamine fragment, which reduces the chain stiffness; the selectivity of the studied PAI-based membrane samples for the He/CO2 gas pair declines as the carbon dioxide permeability increases and the diamine fragment of PAI becomes more complex.

Keywords: membranes; helium; carbon dioxide; permeability

Introduction Advances in gas separation technology in recent times call for the development of new polymeric materials. The existing polyimide membranes“’ exhibit the highest selectivity values in separation of gas mixtures containing hydrogen as the desired component with respect to gas mixture components, such as. methane, nitrogen, lower hydrocarbons, etc. At the same time the permeability of such polymers for hydrogen and helium is low. on the order of 0.1-10 Barrers at room temperature. Increasing the permeability of such materials obviously calls for data on the variation of transport characteristics depending on the chemical structure of polymers, and on the level of their structural organization. In addition, the need for the production of membranes in an asymmetric form’ places high demands upon the processing properties of polymers, such as, the actual film and tibre formation, multiple solubility, and stability of high-concentration solutions. This paper presents the results of studies of the permeability of helium, as a safe (from the experimental standpoint) analog of a desired component of gas mixtures. hydrogen, through PAI film samples (according to Reference 4, we may assume a similarity of the parameters of helium and hydrogen transport through polymeric membranes) and of carbon dioxide. PAI are linear rigid-chain glassy polymers capable of a multiple dissolution in amide solvents: they differ from known PI’ by the existence of amide groups in the backbone and have a complete’ chemical cyclization. The present study was aimed at establishment of correlations between the permeability and some experimental and calculated PAI characteristics: glass transition temperature, density, free volume fraction, cohesive energy density, molar concen0950-42 14/90/020087-04 8 1990 Butterworth-Heinemann Ltd

tration of various fragments of the PAI chain, and length of the virtual bond of macromolecules.

Experimental Materials Polyamidoimides

of a structural

formula:

N-R-CONH-R,-NHco’

n

L

where R: -CH?R,: 0

(1st series. PAI A) (2nd series, PAI B) (PAf 1) (PAT 2) (PAT 3) (PAT 4)

~0~0~

(PAT 5)

+$J+

(PAT 6)

were synthesized by the procedures in References 6 and 7. The molecular mass ofthe PAI was within (2.3-3.8) X 104.

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UD. Petrenko et al.

Membranes 20-40 pm thick PAI films were prepared from 12-l 8% (mass) solutions of the polymers in N,N’-dimethylformamide on a centrifuge with the use of polyethylene substrates. The time of formation of the films in an air atmosphere at a temperature of 60-80°C was 2-3 h. After hardening, the PAI films were detached from the substrates and dried in a vacuum of - 1 mm Hg at 80°C for 6-8 h until a constant weight was obtained. Experimental

and calculation procedures

The helium and carbon dioxide permeability, K, through PAI film samples was measured on a chromatographic set with the use of heat conductivity detectors8. Argon and helium were employed, respectively, as the carrier gas; the partial pressure of the carrier gas at the membrane was assumed to be equal to 760 mm Hg. The error in determining the coefficient K was within 20%. The density, p. of the polymers was determined by hydrostatic weighing. The glass transition temperature, T!. of the studied PAI was determined by the DSC technique. The value of the fraction of the ‘geometric’ free volume of the polymer was determined by the method of increments’: PN.

1

Avi

Ii

j-=1--

where:

c

(1)

Av, is the occupied

(Van-der-Waals)

volume of

the repeating unit of the PAI; M is the molecular mass of the repeating unit; and NA is the Avogadro number. The PAI cohesive energy density (CED) was also calculated by the method of increments’: AET

c 6?=

’ NA

(2) c

Av,

where AE,? is the contribution of every atom of the repeating unit of the PAI and of the intermolecular interaction type to the cohesive energy. The length, (2). of the virtual bonds of PAI macromolecules was calculated according to”:

(I) = Z,l(n + 1) Table 1

where lo is the contour length of the repeating unit of the PAI and n is the number of flexible joints in the repeating unit of the PAL

Results and discussion Table I presents the values of the helium and carbon dioxide permeability coefficients for the studied PAI samples. As can be seen, they lie approximately in one and the same range of magnitudes, but at the same time the differences in the values extend beyond the limits of the K value measurement error. Growth in the CED of the polymers is accompanied by decline in the helium permeability, which is in good agreement with the literature data”. In addition, the CED growth is accompanied by an appropriate increase in the density of the polymer and an increase in its glass transition temperature. According to Reference 12, increase in the T, of polymers results in a growth of the fraction of excess free volume regions, responsible for the transport of penetrant gas molecules, and a consequent increase in permeability. This conception obviously contradicts the above-described behaviour, where the permeability is inversely proportional to the Tg of the polymer. Then the data on the K and Tp values in the studied PAI series, obtained by us, make it possible to check the adequacy of these approaches to the results of our experiments. The maximum helium permeability has turned out to be observed for PAI A samples with the least T,; this means that the conception suggested in Reference 12 is invalid in our case. Indeed, the observed increase in the length of the virtual bond of PAI macromolecules, (I) (which is in fact the measure of the thermodynamic stiffness of the chain), with increasing Tr. also does not correlate with the increase in parameter K expected according to Reference 12. The growth of the K values in the PAI A l-6 series can be, in our opinion. analysed within the scope of dependences lgKH’ =f(Cdiamine. C,,,), where Cd,,,,“, and C,,, are, respectively, the molar and the mass concentration of the diamine fragment and of the phenylene group of the repeating unit of PAT. As seen from Figure la, increase of Cdiaminr results in growth of the lgKHe value. The dianhydride fragment permeability values KY&,,, A = 0.55 Barrers and K~I’,,,,B = 1.66 Barrers can be obtained by extrapolation of dependences lgKH’ =f(Cdia,i,,) to zero, and the free volume fraction values fdianh,A = 0.430 and fdianh,B = 0.447 from Equation (1) by using the dependence p =f(Cdia,i,,) also extrapolated to the zero value. As can be seen, dianhydride fragments of PAI are comparable in the permeability with diamine fragments of the simplest

Some properties of the PAI samples studied

Polymer

P (kg mm3)

Ts (K)

f

6 X 1 0m4 (J m-3)o.5

PAI PAI PAI PAI PAI PAI

Al A2 A3 A4 A5 A6

1397 1277 1329 1362 1342 1338

565 538 505 535 520 493

0.313 0.337 0.317 0.314 0.312 0.313

23.2 22.2 22.4 22.3 21.7 21.7

PAI PAI PAI PAI

Bl 82 83 84

1397 1381 1394 1370

593 491 525 569

0.305 0.275 0.275 0.304

20.1 19.1 19.3 19.2

a 1 Barrer = 1 O-lo cm3 (STP) cm cmm2 s cmHg

88

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KHe (Barrers)a

Kc02 (Barrers)

8.65 7.51 7.60 7.49 6.92 5.46

1.2 1.2 2.7 3.6 3.8 3.3

0.77 0.44 1.38 0.74 2.29 0.68

19.96 12.60 12.73 12.58

2.5 2.7 3.4 5.3

0.72 1.04 1.60

(0 (A)

Measurement

of He and COz permeability: ECD.Petrenko et al.

‘diem 0.2

0.1

0.3

KHe , Borrer

0.4

1

a)

3

2

4

a) 64 00

5 4 QY

1 A

2.

2O

3.

ZE 3 3.

Y-

2 5O

‘0

b)

b)

0 2 1

5

.4

4

s5Bc.H AA 2. 1

0” 2

4

I

20

93

i 0.3

0.5

0.7

C Ph, wm3

2

1 Helium permeability through PAI as a function of: Figure 1 (a) molar concentration of diamine fragment of the polymer repeating unit; (b) mass concentration of phenylene groups in the polymer repeating unit

structure in both PAI series. However, the complication of the diamine fragment in the series results in a decline in the PAI stiffness, which shows up in a decrease of the parameters (I) and Tg (see Table I), i.e. leads to loosening of PAI, with the result that KHe increases. Substitution of the methylene group with the phenylene group in the dianhydride fragment in passing from series 1 to series 2 leads to an increase in the helium and carbon dioxide permeability for all PAI (with identical diamine fragments). As is also the case for separate PAI A and PAI B series, an empirical rule” holds: a lower permeability corresponds to a sample with a greater CED. For the PAI B series in this case a growth of (I), Tp, and density of samples and a decrease offare observed (see Table I). It follows that the conception” that ‘a greater f and, accordingly. K value correspond to a higher TL1’is complied with at a comparison of series 1 and 2. Thts. in our opinion, can be explained as follows. Since the PAI under study can be considered as peculiar block copolymers consisting of a rigid block (dianhydride fragment) and a flexible block (diamine fragment), then obviously the approach of Reference 12 will be more clearly evident at the comparison of the T,, f. and KHe values for the rigid ‘glass-like’ dianhydride fragments of PAI, while the analysis of the K”‘. Tg, andfvalues within the scope of one series demonstrates that the correlation KHe- TE -fdoes not hold because of a modifying action of flexible diamine blocks. On the basis that the helium permeability is as a rule determined by its diffusion component, which is in its turn related to the character of the packing of PAI macromolecule fragments. and also proceeding from the assumption of the responsibility of phenylene groups (their concentration. isomerism) for the transport in diamines” it is, in our opinion, expedient to examine the (Figure lb). The experimental dependence 1gK He =f(C,,)

pz

, Barrer

Figure 2 Selectivity of PAl-based membrane for He/CO* gas pair as a function of: (a) helium permeability; (b) carbon dioxide permeability

data obtained for both PAI series turn out to be adequately described by one straight line. This can be seemingly ascribed to the fact that it is the phenylene groups of repeating units of PAI which determine the character of the local packing of PAI macromolecule fragments and excess free volume elements. Table I presents also the data on the carbon dioxide permeability; calculated values of the selectivity of membranes, based on the studied PAI, for the heliumcarbon dioxide gas pair are presented in Figure 2 as a function of the CO> permeability. As can be seen, there is no correlation between the parameters K”’ and $J”~‘~‘~;at the same time an increase in the carbon dioxide permeability through the PAI samples is accompanied by a decrease in the selectivity of the membranes. It is interesting to note that a similar behaviour is exhibited by polyorganophosphazenes I5. Then, as also in the helium transport the CO7 permeability is mainly determined by its diffusion component. This is also supported by the fact that the PAI modification in both studied series is not accompanied by an increase in the content of polar groups, capable of specific interactions with polar carbon dioxide molecules, in the repeating unit of PAL

Conclusion The results of the measurements of the helium and carbon dioxide permeability through samples of PAI of two series lead to the following conclusions: l

complication of the diamine fragment in both PAI series leads to a loosening of the polymer and is accompanied by an increase in the helium permeability; l a proportional relationship between lgKHe and the

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of He and CO, permeability:

UD. Petrenko et al.

mass concentration of phenylene groups in the repeating unit of PAI has been found; dianhydride and diamine fragments of PAI of the two series have close permeability levels; within the scope of each of the two studied series, an increase ofKH’ of PAI is accompanied by a decrease of Tg; at the same time a comparative analysis of the data for the two series demonstrates that the regularityI KHe - Tg -f holds. A possible cause of this phenomenon is the heterogeneity of repeating units of PAI and specific features of distribution of the free volume in the polymer; selectivity of the studied samples of PAI-based membranes for the He/CO, gas pair decreases with increasing carbon dioxide permeability and increasing complication of the diamine fragment of PAL

4

5 6

7

8 9

IO II

References 1 Haraya,

2

3

90

K., Hakuta, K., Obata, Y., Shindo, N., Itoh, K., Wakabayashi, K. and Yoshitome, H. Development of gas separation membranes in Japanese ‘C, Chemistry’ Project Gas Sep & pUrif(1987) 1 3-10 Tanaka, K., Kita, H., Okamoto, K., Nakamura, A. and Kusuki, Y. The effect of morphology on gas permeability and pennselectivity in polyimide based on 3.3’.4.4’-biphenyltetracarboxylic dianhydride and 4.4’-oxydianiline Po!vmer J (1989) 21 127-35 L.oeb, S. and Sourirajan, S. US Patent 3 133 132 (1964)

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Dubjaga, V.P., Perepechkin, L.P. and Katalevskij, E.E. Polymeric Membranes Khimija, Moscow. USSR (1981) Ch 1 IO-38 Bessonov, MA., Koton, M.M., Kudjavtsev, V.V. and Lajus, LA. Poliimides: Class of Heat-resistant Polymers Naukas Leningrad, USSR (1983) Ch 1 IO-38 Strul, M., Neamtu, G., Mantaluta, E. and Zugravescu, I. Aromatic polyamideimides Rev Rouman Chem (1971) 16 941-950 Zamulina, LA, Demchenko, S.S., Privalko, V.P., Rosovitskij, V.F. and Khomenkova, K.K. Synthesis and properties of trimellitimido-N-acetic acid-based polyamidoimides Qsokomolek Soedin (I 985) 276 584-587 Rejtlinger, S.A. Permeability of Polymeric Mzrerials Khimijag Moscow. USSR (1974) Ch 1 I 238-269 Askadskij, AA. and Matveev, Ju.1. Chemical Structure and Physical Properties of Polymers Khimija Moscow. USSR (1983) Ch 4 101-151 Ch 7 215-245 Birshtejn, T.M. Flexibility of polymeric chains containing flat cyclical groups Qsokomolek Soedit! (1977) 19A 54-62 Tepljakov, V.V. and Durgap’jan, S.G. Oxygen enrichment of air with use of polymeric membranes. In: Membrune Proce.sses of Separation of Liquid and Gus Mixtures MKhTI. Moscow. USSR (1982) 122 108-I 17 Jampol’skij, Ju.P. and Shishatskij, S.M. Coefficients of gas diffusion in polymers and free volume at glass transition temperature Dokl AN SSSR (1989) 304 1191-I 195 Sakaguchi, Y., Tokai, M., Kawada, H. and Kato, Y. Separation of Hz and CO through poly(sulfone-amide) membranes. II. Highly permselective membranes containing bis(3-aminophenyl) sulfone as a diamine component Polvmer J (1988) 20 365-370 Kajiwara, M. Gas permeability of poly(organophosphazenes) JMurer Sci Serf (1988) 7 102-104