The permselectivity of polyorganosiloxanes containing ester functionalities

217

of Membrane Science, 56 (1991) 217-228 Elsevier Science Publishers B.V., Amsterdam

Journal

The permselectivity of polyorganosiloxanes ester functionalities A.J. Ashworth*,

B.J. Brisdon,

School of Chemistry

and School of Chemical

R. England’,

containing

B.S.R. Reddy** and I. Zafar***

Engineering’,

University

of Bath, Bath BA2 7AY

(Great Britain)

(Received June 29,1989; accepted in revised form August 27,199O)

Abstract The permeabilitiesof carbon dioxide, methane,oxygen and nitrogenin a rangeof polyorganosiloxanemembranescontaininga side-chainesterfunctionalityof O-21.4 mol% of Si atoms, have been determined by a continuous flow method over the temperature range 35-100°C. The ratio of the CO, and CH, permeabilities, or permselectivity, increases with increase in ester functionality. As measurement of the diffusivities at 35°C shows that there is little change in the relative diffusivity of CO, to CH, with increase in ester functionality, the greater permselectivity results from an increase in the relative solubility of CO, to CH,. This is confirmed by measurements of the relative solubility using a vacuum microbalance. Moreover, since the activation energy for CO, permeation is much lower than that for CH,, the permselectivity becomes greater as the temperature is lowered. In contrast, the ratio of the oxygen/nitrogen permeabilities decreases slightly with increase in ester functionality as a result of a decreasing relative solubility, the relative diffusivity again remaining virtually unchanged. Membrane preparation and structure; diffusion; gas and vapor permeation; gas separations; solubility and partitioning; polyorganosiloxane membranes

Keywords:

Introduction The separative ability of asymmetric and composite membranes used in gas separation processes is governed by the transport characteristics of a thin film of a polymer. Permeation through the polymer film is dependent on solution and diffusion of the gases in the polymer. The transport of a gas through such a membrane is therefore described by a permeability coefficient which can be expressed as the product of the diffusivity and the solubility coefficients of the gas in the membrane [ 11 as represented in eqn. (1): P=DxS

(1)

*To whom correspondence should be addressed. **On leave from the Central Leather Research Institute, Madras, India. ***Present address: Morgan Materials Technology Limited, Stourport-on-Severn, shire, Great Britain.

0376-7388/91/$03.50

0 1991-

Elsevier Science Publishers B.V.

Worcester-

218

An assessment of the permselectivity LYABof a membrane for a binary gas mixture (A + B ) may then be obtained from the ratio of the respective permeability coefficients (PA/P,), when the pressure of the permeants downstream from the membrane is much lower than their upstream pressure [ 11. Using the relation in eqn. (1) the permselectivity can be thought of in terms of a diffusivity (SA/Sn) [2] acor mobility selectivity (DA/DB ) and a solubility selectivity cording to eqn. (2 ) : CVAB=PA/PB=(DA/DB)(SA/S~)

(2)

In general, diffusion in a particular polymer is governed by the size and shape of the diffusing molecules. If the polymer is to provide selective permeation between two permeants of similar diameter it must have a greater attraction for one of the permeants, resulting in that permeant having a greater solubility in the polymer. Polysiloxanes combine the advantages of the synthetic variability of organic polymers with the thermal and oxidative stability of inorganic polymers. Unfortunately, although poly (dimethylsiloxane) membranes have a high permeability this is accompanied by a low selectivity. Stern et al. [3] found that substitution of increasingly bulkier functional groups in the side or backbone chains of silicone polymers resulted in a significant decrease in permeability for a given penetrant gas that was due mainly to a decrease in diffusivity, while the solubility decreased to a much lesser extent. Thus the polymer matrix was could becoming more like a “molecular sieve”, and a selectivity enhancement be achieved as a result of changes in relative diffusivity. However this enhancement is probably limited to mixtures of permeants of different size and shape. The aim of the research, of which this study forms a part, was to produce polyorganosiloxane membranes containing functional groups in the side chains which would interact preferentially with one of the components of a permeant gas mixture, thus providing a permselectivity based primarily on a difference in solubility. One of the silicone polymers studied by Stern et al. [3], poly (trifluoropropylmethylsiloxane), incorporating a trifluoropropyl group (CF,CH,CH,-) as a side chain in the polymer repeat unit, is of interest in this context. This polymer gave increased transport of carbon dioxide in preference to methane. Whereas the diffusivity selectivity, DCOn/DCH4, was approximately unity, the selectivity was greater than six and undoubtedly resulted from an interaction between the polar carbon dioxide and the electronegative fluoro groups. The removal of carbon dioxide from methane is of considerable current economic importance, and our first venture was to produce a range of polyorganosiloxanes with an ester functionality, reasoning that the ester group should provide a greater attraction for carbon dioxide than for methane, as carbon dioxide is known to interact with carbonyl groups present in ketonic and ester solvents [ 41. This report details the permeability measurements over the tem-

219

perature range of 35 to 100’ C of carbon dioxide, methane, and also oxygen and nitrogen, in a series of polyorganosiloxane memuranes functionalised to various degrees with ester groups and relates the permselectivities determined to relative diffusivities and relative solubilities. Experimental

Materials The preparation of the functionalised membranes involved two stages. First, a known quantity of the ester functionality was introduced into a linear poly(methylhydridosiloxane), Me,SiO [MeSi (H)O] ,SiMe,, (where n- 40), by the platinum catalysed addition under anhydrous conditions of the ally1 ester, CH,=CHCH,COOCH,. The synthesis and characterisation of typical examples siloxanes of the resulting linear ester functionalised Me,SiO[MeSi(H)O].{MeSi[ (CH,),COOMe]O},SiMe, with well defined (x+ y ) and (z: y) values have been reported elsewhere [ 51. The lower functionalised polymers with high (x:y) were fluids, and the higher functionalised polymers were gums which were readily miscible with non-polar organic solvents. Second, the formation of the membrane was achieved by catalysed crosslinking of the remaining Si-H units in the linear ester functionalised siloxane with an cu,w-dihydroxypoly (dimethylsiloxane ) , (disilanol), in the presence of small quantities of Si ( OMe) 4 as curing agent for any unreacted Si-H and SiOH moieties. The idealised composition of the resulting membrane is illustrated in Fig. 1, and the stoichiometry of the reaction was such that each SiH entity could react with the cross-linking agents to produce MeSiO, and SiO, centres. The reactants were dissolved in the minimum amount of toluene required to produce a clear fluid mixture, with a catalytic quantity of dibutyl tin-laurate to facilitate the coupling reactions. The cross-linking and solvent removal were carried out at ambient temperature in a press using a 50 ton force, with the

Me

I

Me, SiO-

_$i_O_

---SiMe,

I

(CH, 13

-O-Si-O-

C02Me

SiOMe

Fig. 1. Diagrammatic representation of membrane structure.

220

liquid polymer mixture held between sheets of cellulose acetate in order to produce unfilled membranes of 0.2-0.4 mm thickness. The molecular weights and the ratio of Si-H groups to ester groups (x :y ) in the linear functionalised polymers, together with the molecular weights of the disilanols, used in the preparation of a particular membrane are given in Table 1. Unfilled membranes of pure poly (dimethylsiloxane ), PDMS, were prepared using both the high and low molecular weight disilanols. Number average molecular weights (Mn) were determined at the Rubber and Plastics Research Association, Shrewsbury by gel permeation chromatography based on calibration with polystyrene using toluene as solvent. As the ultimate objective of this work is to develop functionalised membranes with enhanced permselectivities, priority has been given to the establishment of any direct relationship between the concentration of functionality and permselectivity. Membranes were examined using a DuPont 910/9900 differential scanning calorimeter (DSC) as illustrated in Fig. 2, but no attempts were made to determine further physical characteristics, such as cross-linking densities, of these early membranes. Thermograms were produced by precooling the calorimeter with liquid nitrogen, placing the membrane sample in the calorimeter to shock cool it to - 150’ C and then applying a heating rate of 20”C/min. Curve B for the 17.3 mol% functionalised membrane is very similar to curve A for the non-functionalised membrane, both showing a glass transition temperature around - 120” C. The large exotherm in the region of - 95°C and endotherm at about - 40’ C correspond to the crystallization initiated on warming followed by crystalline melting [ 61. Densities of the membranes reported in Table 2 were determined using the water displacement method. Air bubbles were avoided by immersing a fully outgassed membrane sample under water in a vacuum system. The thickness of the membranes was determined by averaging many measurements TABLE

1

Membrane Me&O

preparation [MeSi(H)O],{MeSi[

from linear the (CH,),COOMe]O},SiMe,,

functionalised ester and disilanols

Ester functionality in membrane

Linear polymer

(mol% Si atoms)

(x:y)

3.4 5.5 7.0 8.35 17.3

3O:lO 20:20 10:30 10:30 10:30

3680 (3570)" 4710 (4570)" 5630 (5570)" 5630 5630

21.4

10:30

5630

“Theoretical

ratio

Mn values based on Si 42 chain length.

polymers,

Disilanol Mn Mn 71000 71000 71000 71000 1350 1350

221 A

I3

0.61.0 0.4-

xi

z 5

5 0.2-

0.5

4

4 u.

L

!G

P 0.0

-0.

5

0

201

Temperature

-0.4

; -200

-100

0

PC)

Fig. 2. DSC thermograms for a heating rate of BO”C/min. Curve A for non-functionalised poly (dimethylsiloxane) membrane, Curve B for 17.3 mol% ester functionalised membrane. TABLE 2 Membrane densities at 25 ’ C Ester functionality (mol% Si atoms)

Density (g/cm3)

0

0.984

7.0 17.3 21.4

1.002 1.013 1.023

with an electronic micrometer. Good agreement was found between these determinations and thicknesses estimated from the membrane density, mass and area. Apparatus The permeabilities of the single gases in these membranes over the temperature range 35-100 oC and the diffusivities at 35’ C were determined at atmospheric pressure using the dynamic method in which the permeant is passed over one face of the membrane, and that permeating through the membrane is collected by a carrier gas stream and determined using a thermal conductivity

222

detector. The method developed by Pasternak et al. [ 71 has been used previously by one of us [8] and the apparatus employed here was identical ,xcept that a Pye 104 gas chromatograph thermal conductivity detector was used in place of the flame ionization detector. The detector was calibrated by the chromatographic method of employing a sample loop of known volume, and by using calibrant gas mixtures of carbon dioxide and methane in helium supplied by Electrochem Ltd. Stoke-on-Trent. The flow rates of the permeant gas and helium as carrier gas were 60 cm”-min-l. The permeability coefficient was determined from the steady state signal and the diffusion coefficient from the gradient of the signal before steady state is reached [ 7,8]. Diffusion coefficients measured previously by this method for poly (hexafluoropropylene-co-tetrafluoroethylene) and Teflon@ FEP membranes were found to agree with those determined by analysis of the kinetics of separate absorption experiments [ 71. However, with the siloxane membranes studied here the diffusion coefficients, which are an order of lo* larger than those for FEP, were found to vary with the carrier gas flow rate. At 35 oC with the 7 mol% ester functionalised membrane, carrier gas flow rates of 59, 101 and 137 cm”-min-’ gave diffusion coefficients for CO, of 12.0, 15.22 and 16.1 x 10W’ cm2-sec-1. Thus the diffusion coefficients determined at a constant carrier gas flow rate of 60 cm3-min-’ reported here provide a comparison of the diffusion in the different siloxane membranes studied, but should not be regarded as absolute values. The probable explanation of this behaviour can be related to the very high permeability of these siloxane membranes, as a consequence of which the boundary condition in the mathematical model [ 71 used to calculate the diffusion coefficient of a zero partial pressure of permeant at the downstream face of the membrane may not be strictly met. Under the normal experimental conditions of a carrier gas flow of 60 cm”-min-’ with a membrane area of 4.91 cm’, a membrane thickness of ca. 0.3 mm and a CO, pressure of 1 atm at the upstream face of the 7 mol% ester functionalised membrane, the partial pressure of permeant at the downstream face would be ca. 0.4 cmHg. The faster carrier gas flow rates would reduce the downstream partial pressure of the permeant giving an increased measured diffusion constant. Another possibility associated with the greater permeability of these membranes is that of interference in the transport process by the permeation of the helium carrier gas in opposition to the permeant, but this is not thought to have an appreciable influence [ 9 1. The true diffusion coefficients for the single gases can be obtained from the permeability measurements by using independent solubility determinations. The equilibrium sorption of CO, and CH, in some of the membranes was determined using a Sartorius Model 4102 electronic vacuum microbalance in conjunction with a Texas Instruments Bourdon pressure gauge allowing monitoring of weight and pressure changes to + 0.01 mg and + 0.01 mmHg respectively. The apparatus has been described previously [lo]. Absorption iso-

223

therms were determined on samples of ca. 1.8 g of a membrane tared with a similar weight of Pyrex rod. Corrections for buoyancy were made based on the densities of the membrane and the Pyrex. Results and discussion The permeabilities and diffusivities determined for COz, CH4, 0, and N, at 35 oC and atmospheric pressure are shown in Fig. 3 and 4. The permeabilities for CH4, 0, and N, show a slight decrease with increasing ester functionality, while that for CO, remains effectively constant within the experimental error ( - 7% ). The largest contribution to this error comes from the variation in thickness over the area of the membrane sample. The diffusivities show a more

0

10

5

15 ester

functionalitylmol

20 S Si

atoms

Fig. 3. Permeability coefficients at 35°C plotted against ester functionality carbon dioxide, (m) methane, (+) oxygen, (A) nitrogen. I

ii

I

I

, 0

5

I

I

/

,

15

10 ester

functiowdiiylmol

of the membrane:

(0 )

20 % Si atoms

Fig. 4. Apparent diffusion coefficients at 35°C plotted against ester functionality brane: (0 ) carbon dioxide, (m) methane, (+) oxygen, ( A ) nitrogen.

of the mem-

224

marked decrease with increasing ester functionality. Unfilled PDMS membranes prepared from both high and low molecular weight disilanols (Mn= 71000 and 1350 respectively), and hence having different cross-linking densities, were found to have very similar transport properties for CO:! and CH,. The results reported for a PDMS membrane with no ester functionality are for the membrane prepared from the high molecular weight disilanol but these values are also typical of PDMS membranes prepared with the low molecular weight precursor, so indicating little dependence on cross-link chainlength. The solubilities at 35 ‘C estimated according to eqn. ( 1) are shown in Fig. 5. It can be seen that while the solubilities for CH4, 0, and N, remain essentially constant, that for CO, shows a definite increase with increasing ester functionality. Permeation of all the gases in all the membranes gave linear Arrhenius plots over the temperature range studied. The activation energies for permeation determined from the measurements at a minimum of four temperatures in the range 35-100’ C are reported in Table 3. The activation energies for permeation for a particular permeant do not change greatly as the ester functionality is increased, but that for CO, can be seen to be much lower than that for the other permeants. The ratio of the permeabilities or permselectivity for the gas pairs COJCH, and 0,/N, are reported in Tables 4 and 5, together with the ratios of the diffusivities and solubilities, or mobility and solubility selectivities, calculated according to eqn. (1) and (2). In the case of CO, and CH, the selectivity of the membranes for CO2 over CH, increased with increased ester functionality, and can be seen to be a direct result of the improved solubility selectivity as the diffusivity selectivity has remained virtually unchanged. Addition of the ester groups in the polymer side chains has clearly increased the interaction with CO, in comparison with CH4. Whereas in the case of 0, and N, the selec-

0

5

10

15 ester

functionality/mol

20 % Si atoms

Fig. 5. Solubility coefficients estimated from the transport measurements at 35’ C plotted against ester functionality of the membrane: (0 ) carbon dioxide, ( n ) methane, (+ ) oxygen, ( A ) nitrogen.

225 TABLE 3 Activation energies for permeation Ester functionality (mol% Si atoms )

E, (kJ/mol)

0

3.4 5.5 7.0 8.35 17.3 21.4

co2

CH,

0,

N2

1.05 1.33 1.59 0.88 1.34 0.33 1.00

8.10 8.33 1.66 8.75 1.78 1.32 7.03

8.91 9.29 9.62 8.49 9.20 8.33 12.2

12.9 12.3 12.9 11.4 12.0 11.1 13.4

TABLE 4 Selectivities for CO,/CH, at 35°C Ester functionality (mol% Si atoms) 0

3.4 5.5 7.0 8.35 17.3 21.4

Permeability CO&H,

Diffusivity CO&H,

Solubility CWCH,

3.14 3.28 2.66 3.15 3.56 3.49 3.92

1.11 1.22 1.03 1.06 1.20 1.06 0.91

2.82 2.70 2.58 2.99 2.96 3.30 4.32

TABLE 5 Selectivities for 0,/N, Ester functionality (mol% Si atoms) 0

3.4 5.5 7.0 8.35 17.3 21.4

at 35 ’ C Permeability 0,/N,

Diffusivity 0,/N,

Solubility 0,/N,

2.17 2.01 2.11 1.91 1.97 1.90 1.84

1.06 1.26 1.07 1.05 1.07 1.08 1.06

2.06 1.61 1.97 1.82 1.84 1.75 1.73

tivity of the membranes for 0, over N2 has slightly decreased with greater ester functionality due to a small reduction in the solubility selectivity. An independent check on the solubilities was made by determining the equi-

librium sorption of CO, and CH, in some of the membranes at 30°C using a vacuum microbalance. The sorption isotherms determined were linear with the exception of that for the highest ester functionality of 21.4 mol% which is initially concave to the pressure axis. The isotherms determined for CO, and CH, with the 21.4 mol% functionalised membrane, together with the CO, isotherm for the 17.3 mol% membrane given for comparison, are shown in Fig. 6. The curvature found in the isotherm for the 21.4 mol% ester functionalised membrane was unexpected and is more typical of glassy polymers, where it is explained on the basis of dual mode sorption. The isotherm was redetermined and was perfectly reproducible. These membranes have been produced in order to give specific interactions between CO, and the ester groups, and it may be that a level of functionality has been reached where some clustering of the COz around these groups occurs. Alternatively there may be changes in the morphology of the highest functionalised membrane and this will be investigated if membranes of greater ester functionality than 21.4 mol% can be produced satisfactorily. The solubilities determined and reported in Table 6 are the Henry’s Law solubilities for the linear isotherms and the secant slope of the sorption isotherms [2] calculated at 1 atmosphere in the case of the 21.4 mol% functionalised membrane. The solubilities determined from the quotient of the permeability and diffusivity coefficients were a factor of two greater than those determined directly and undoubtedly reflect the assumptions made in their estimation by this method. However, as can be seen in Table 6, the agreement between the direct and indirect measurements of the CO, and CH, solubility

0

20

40

60

80

pressvekmtlg Fig. 6. Sorption isotherms at 30” C determined with a vacuum microbalance: (A ) carbon dioxide+ 21.4 mol% ester membrane; (V ) carbon dioxide+ 17.3 mol% ester membrane; (+) methane+ 21.4 mol% ester membrane.

227 TABLE 6 Comparison of solubilities at 30°C Ester functionality (mol% Si atoms)

0

7.0 17.3 21.4

Solubility x lo3 {cm3(STP)/cm3-cmHg}

Solubility ratio CWCH,

CO,

CH,

pbalance

dynamic

17.5 17.9 18.1 24.3

6.25 5.77 5.56 5.47

2.80 3.10 3.26 4.44

2.82 2.99 3.30 4.32

ratio is good and confirms the increase in solubility selectivity with increasing ester functionality. Moreover, as the activation energy for CO, permeation reported in Table 3 is much lower than for CH, permeation the permselectivity resulting from this solubility selectivity will become greater as the temperature decreases. Koros [2] has discussed the solubility selectivity of membranes for CO,/ CH, separation and pointed out that the solubility selectivities of polymers and chemically alike low molecular weight solvents are similar. Thus the high density of carbonyl groups in cellulose acetate imparts a character similar to that of methyl acetate and results in very similar relative solubilities for CO, to CH, of 11.4 and 11.5 respectively. It was also pointed out that there was a relation between the solubility selectivity for CO, and CH, and the mass density of carbonyl groups for various polymers and low molecular weight solvents. The mass density of carbonyl groups was calculated simply by using the mass density of the various media and the weight fraction of the carbonyl functionality in the low molecular weight solvent or polymer repeat unit containing the carbonyl functionality. The mass density of carbonyl groups in the 21.4 mol% ester functionalised membrane can be calculated to be 0.066 g-cme3. The maximum mass density of carbonyl groups that could be obtained if a membrane were produced from a polymer with an ester group attached to each Si atom would be about 0.2 g-cmP3. A higher mass density than that calculated for cellulose acetate of 0.335 g-cmP3 could only be achieved by attaching two ester groups to each Si atom. However, it would appear that a higher solubility selectivity for CO,/CH, than that found of 4.3 could be achieved if a polymer membrane with a higher degree of ester functionality could be produced, but that the maximum solubility selectivity expected for this gas pair would be about 11 as found for cellulose acetate. However a functionalised polyorganosiloxane would have the advantage of a greater permeability to CO, than cellulose acetate.

228

Acknowledgements

The authors gratefully acknowledge the support of the Science and Engineering Council for this work.

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