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Preliminary investigation of gas transport mechanism in a H+ irradiated polyimide-ceramic composite membrane
Nuclear Instruments and Methods in Physics Research B 152 (1999) 325±334
Preliminary investigation of gas transport mechanism in a H irradiated polyimide-ceramic composite membrane Xinglong Xu, M.R. Coleman
*
Chemical and Environmental Engineering, University of Toledo, 3048 Nitschke Hall, Toledo, OH 43606-3390, USA Received 15 September 1998; received in revised form 18 December 1998
Abstract Recent research by our group indicated that ion beam irradiation can simultaneously increase the gas permeability and permselectivity of polymeric membrane materials. The temperature dependence of the gas permeation properties of a H ion irradiated polyimide-ceramic composite membrane was investigated to address issues of changes in the gas transport mechanism in irradiated polymers. As was seen for glassy polymers, the temperature dependence of the permeation properties of the irradiated membrane followed an Arrhenius type relationship. Both the activation energy (Ep ) for gas permeation and the pre-exponential factor (P0 ) of the irradiated polymer were greater than the values of the unmodi®ed bulk polymer. Large increases in the pre-exponential factor of the irradiated sample for small size gas molecules (He, O2 and CO2 ) combined with the dominant contribution of the pre-exponential factor to the permselectivity for several gas pairs (He/CH4 , O2 /N2 , and CO2 /CH4 ) implied that the irradiated sample had a dierent permeation mechanism than the bulk material. Ó 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 61.80 Jh; 61.82 Pv Keywords: Gas separation; Ion irradiation; Gas transport mechanism; Activation energy for permeation; Microstructure
1. Introduction Though selective gas permeation of polymeric materials has been known since 1831, polymer based gas separation membranes have only found widespread commercial application since the early 1980s [1,2]. While the majority of commercial gas
* Corresponding author. Tel.: +1-419-530-8091; fax: +1-419530-8086; e-mail: [email protected]
separation membranes are polymeric in nature, there is considerable interest in development of ceramic and carbon molecular sieving membranes for applications in challenging environments (e.g., high temperature) [3±5]. Development of materials having both high permeabilities and permselectivities combined with mechanical stability is an important issue for the extension of membranes to a variety of systems. Two approaches have been widely investigated for the development of high performance membrane materials: (i) synthesis of
0168-583X/99/$ ± see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 0 6 7 - 1
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glassy polymers with structures tailored using established structure-property relationships, and (ii) modi®cation of polymeric permselective layers using diverse post-synthesis techniques (e.g., pyrolysis and surface ¯uorination) [3±8]. Selective alteration of gas transport properties through chemical modi®cation using established structure-property principles has resulted in an improvement in the gas separations properties of polymeric materials [9±11]. However, an analysis of the extensive literature data for several binary gas mixtures (i.e., O2 /N2 , H2 /CH4 , CO2 /CH4 , etc.) by Robeson [12], revealed an upper bound for the permeability versus permselectivity for these mixtures for most of the reported polymeric membrane materials. Virtually no values existed for polymers above this linear upper bound on the trade-o curve. In comparing the oxygen and nitrogen transport properties of polymers, zeolites and carbon molecular sieves, Singh and Koros [10], found that the ``upper bound'' line for polymers drawn by Robeson still applied in 1996. For similar permeabilities, the molecular sieving materials exhibited much greater selectivities than the polymers and were, therefore, well above this upper bound. Carbon molecular sieving (CMS) membrane formed from the pyrolysis of aromatic polymers have been shown to exhibit very attractive permeation properties [13]. However, the CMS membranes formed from the pyrolysis of polymer asymmetric membranes are typically very fragile. The improved permselective properties of the sieving materials relative to glassy polymers have been attributed to dierences in the entropy selectivities [13] between these classes of materials. Ion beam irradiation has been shown to result in an evolution in the chemical structure from the virgin polymer to a graphite-like material with increasing ion beam ¯uence. Intensive energy deposition of the incident ion can result in the following chemical processes within the thin polymer surface layer: (i) degradation of the polymer chain with formation of small volatile molecules and free radials which leave defects in the polymer matrix, (ii) crosslinking between the polymer chains, (iii) formation of new chemical bonds such as double bonds, and (iv) oxidation or other chemical reactions in the presence of chemical atmosphere
during the irradiation process [14]. Modi®cation of polymeric permselective layers using ion beam irradiation is being investigated by our group as a method to develop materials which combine the attractive transport properties of carbon molecular sieves with the mechanical stability of polymers. An investigation of the eect of ion beam irradiation on iodine diusion into polymers indicated that ion beam irradiation could modify the microstructure and diusion properties of the polymer surface layer [15,16]. While irradiation at low ¯uence resulted in an enhancement of the iodine diusion rate into the modi®ed layer and prevented iodine diusion beyond this layer, irradiation at high ¯uence led to the formation of a layer within the polymer surface which did not allow iodine diusion into the polymer. A study of the evolution of the permeation properties of irradiated Kaptonâ polyimide thick free-standing ®lms con®rmed the observation that ion beam irradiation can modify the microstructure of polyimide ®lms. Low ¯uence ion irradiation resulted in increases in the permeabilities of both methane and hydrogen with a corresponding decrease in the H2 /CH4 selectivity within the dense ®lm. Following high ¯uence irradiation, there was a signi®cant increase in both the hydrogen permeability and the permselectivity (H2 /CH4 ) [15,17]. Atomic force microscopy (AFM) studies of the surface of free-standing 6FDA±pMDA polyimide ®lms (25±50 lm thick) following N irradiation revealed a dramatic evolution in the morphology of the polymer surface layer as a function of ion ¯uence [18]. Detailed roughness and bearing analyses of the AFM images showed that the freestanding virgin polyimide ®lms had deep surface valleys. Even at a low ¯uence, ion beam irradiation altered the microstructure of the surface layer and formed a modi®ed surface layer in which the initial deep surface valleys were eliminated. AFM analysis also indicated that low ¯uence irradiation induced the formation of large microvoids in the surface layer of the polymer, and high ¯uence irradiation resulted in the formation of a large number of small size microvoids in the surface. The results of these surface morphology studies agree well with the ion beam irradiation eects on
X. Xu, M.R. Coleman / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 325±334
iodine diusion and gas permeation properties of free standing polyimide ®lms. For example, the large increase in permeability of both H2 and CH4 following low dose irradiation is consistent with an increase in the openness of the polymer matrix (i.e. the formation of large microvoids). However, the formation of a high concentration of small microvoids may result in an overall increase in the permeability of small gases (i.e., H2 ) with a corresponding increase in the permselectivity of small penetrant molecules relative to larger molecules (i.e., H2 /CH4 ). The evolution in the gas transport properties of polyimide-ceramic composite membranes following irradiation with a H ion beam was investigated by our group [19]. In this study, the thin permselective polymer layer of the composite membranes was irradiated throughout the entire thickness to allow a direct comparison of its gas permeation properties with those of unmodi®ed polymer. Signi®cant increases in the permeabilities of the gases having smaller kinetic molecular diameter (i.e., He and O2 ) and the permselectivities of several important gas pairs including He/ CH4 , O2 /N2 and He/N2 were observed. The data points of the ion beam irradiated membranes for each gas pair were located above the upper bound limit for polymers on the plot of permeability versus permselectivity. A hypothesis was proposed to explain the permeation results in terms of the impact of ion irradiation on the free volume, rigidity, and structure of the polymer matrix. Irradiation induced degradation of the polymer has been shown to result in the introduction of defects in the polymer matrix with a corresponding increase in free volume [19]. This would be expected to result in an increase in the permeability. Simultaneously, there is an increase in the rigidity of the polymer structure due to ion irradiation induced crosslinking of the polymer chains, which may lead to an increase in permselectivity. These combined modi®cations of the polymer microstructure could result in an increase in both the permeability of small molecules and permselectivity. The permeability of a penetrant through a polymer membrane is de®ned as the thickness and pressure normalized ¯ux. Permeation of gases
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through polymers is through a solution-diusion process in which a gas molecule dissolves into the feed side, diuses through the membrane because of the concentration gradient and re-dissolves into low pressure permeate side. The permeability coecient (P) can, therefore, be expressed as a product of an average diusion coecient (D) and a solubility coecient (S) [20] P DS:
1
The ideal selectivity, aA=B , of a membrane for a component gas A relative to a component gas B can be expressed as the ratio of the pure gas permeabilities of the two component in the membrane material. DA SA : 2 aA=B PA =PB DB SB Permeation in polymers is an activated process in which the temperature dependence of the permeability coecient, P, diusion coecient, D, and solubility, S, can be described using the following Arrhenius type expressions [21,22]: ÿEp ; 3 P P0 exp RT D D0 exp S S0 exp
ÿEd ; RT
ÿHs ; RT
4 5
where P0 , D0 , and S0 are the pre-exponential factors, Ep is the activation energy of permeation, Ed the activation energy for diusion, and Hs the heat of sorption. By combining Eqs. (1)±(5), the permeability coecient can be expressed as ÿEp ÿEp D0 S0 exp P P0 exp RT RT ÿ Ed Hs ; 6 D0 S0 exp RT P 0 D 0 S0 ;
7
Ep E d H s :
8
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From the transition state theory [23], the pre-exponential factor of diusion can be expressed as kT S 9 D0 ek2 exp d ; R h where k is the average diusive jump length in the diusion medium, Sd the activation entropy of diusion, k the Boltzmann constant and h is Planck's constant. A detailed study of the temperature dependence of gas sorption and transport properties combined with an analysis of the physical properties of membrane materials can provide insight into the transport mechanism of a penetrant within the materials. For example, Costello et al. [24] reported a typical investigation of the temperature dependence of transport properties of the polyimides 6FDA±6FpDA and 6FDA±6FmDA. Ion beam induced modi®cation of the polymer microstructure is expected to result in a modi®cation in the transport mechanism relative to the virgin polymer which may be re¯ected in a variation in the temperature dependence of the transport properties. Therefore, the temperature dependence of the permeation properties were determined for an irradiated polyimide ®lm to probe the eect of irradiation on the mechanism of gas transport. Due to the limitation of the energy range available with the implantor and the large quantity of material required for measurement of sorption isotherms with the currently available equipment, the current work was limited to the temperature dependence of gas permeation properties. The temperature dependence of the permeation properties of a polyimide-ceramic composite membrane which was irradiated with 2:5 1015 / cm2 H at 180 keV will be reported in this paper. 2. Experimental 2.1. Preparation of 6FDA±6FpDA-ceramic composite membranes The ¯uorine containing polyimide, 6FDA6FpDA, was used in this study and is shown in Fig. 1. The 6FDA±6FpDA was kindly supplied by Professor William Koros of the University of
Fig. 1. Structure of the polyimide isomers, Hexa¯uoro Dianhydride 4,40 -Hexa¯uoro Diamine (6FDA±6FpDA).
Texas at Austin. The polymer-ceramic composite membranes used in this work consisted of a polymeric permselective layer with a thickness of approximately 1.2 lm deposited on a porous ceramic membrane support using a preparation method which has been described in detail elsewhere [25]. Anaporeâ ceramic membranes purchased from Whatman Inc. with an average pore size of 0.02 lm were used as the porous substrates for the composite membranes. A dilute solution of 6FDA±6FpDA in methylene chloride was prepared in a dry environment to avoid humidity which could cause the polymer to precipitate during the ®lm formation process. A prede®ned volume of polymer solution, which was determined based upon the desired thickness of the polymer layer, was deposited on the ceramic support and covered to control the evaporation rate of the solvent. The composite membranes were allowed to dry in air for at least 12 h and were then dried in a vacuum oven at 100 C for more than one hour to remove any residual solvent. A weight dierence method which included an estimation of the surface area of the polymer coverage, was used to estimate the thickness of the selective polymer layer of the composite membrane. Cross section imaging using scanning electronic microscopy (SEM) con®rmed the thickness results of the weight dierence method [19]. 2.2. Ion beam irradiation process Ion implantation was performed using a IM200 implantor at the Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Sciences. A small ion beam current density ( 6 1 lA/cm2 ) was used to avoid overheating of the sample. The mean projected range (Rp ) of the ir-
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radiating ions and energy loss in the samples were estimated using SRIM Code [26]. The Rp of the H ion at 180 keV in 6FDA±6FpDA was 2.09 lm and the longitudinal straggling (DRp ) was about 0.13 lm so that the total thickness modi®ed by ion irradiation Rm which is de®ned as Rp DRp , would be 2.2 lm for H irradiation of 6FDA±6FpDA. The thickness of the permselective layer of the composite membrane used in this work was just 1.2 lm, so that modi®cation of the entire thickness of the polymeric permselective layer was assured. 2.3. Gas permeation measurements The pure gas permeances of O2 , N2 , He, CO2 and CH4 were measured for the membrane at 35 C prior to irradiation and at temperatures ranging from 30 C to 60 C following the irradiation. A Millipore test cell in a temperature controlled box was used for the permeance measurements [25]. For each permeance measurement, the feed gas was applied to the upstream side of the membrane at a pressure of 50 psig and the permeate side was held at atmospheric pressure. Once steady-state for the permeance was reached (>20 h), the volumetric ¯ow rate of the permeate stream was determined using a soap-bubble ¯owmeter. The permeances were determined from the volumetric ¯owrate, temperature, and prede®ned membrane area. 3. Results and discussion Previous studies [15±17] of ion beam modi®cation of polymers indicated that with increasing irradiation ¯uence, ion irradiation results in a gradual change in the microstructure and mor-
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phology of polymers. In the very low dose range, there was an increase in both concentration and size of ``packing defects'' (regions of localized high free volume) with increasing irradiation ¯uence. In the high dose range, there was a gradual closing of the open structure with irradiation ¯uence due to increasing crosslinking. Irradiation of 6FDA± 6FpDA-ceramic composite membranes with H at ¯uences up to 2:0 1015 /cm2 resulted in signi®cant increases in the permeability of small molecules with corresponding increases in permselectivities [19]. In fact, the irradiated materials exhibited permeation properties which were well above the trade-o curve of typical glassy polymers. In this work, an ion ¯uence of 2:5 1015 H /cm2 at 180 keV was used to modify a 6FDA± 6FpDA-ceramic composite membrane in order to extend the irradiation range. The permeances and permselectivities for several gas pairs in the 6FDA±6FpDA-ceramic composite membrane were measured at 50 psig and at 35 C prior to and following H ion irradiation at 2:5 1015 /cm2 and are listed in Table 1. The permeabilities of each of the gases through the polymer selective layer were estimated using the measured permeance values combined with an average thickness determined using a weight difference method. The estimated permeabilities of both the virgin and irradiated polymer-ceramic composite membrane are compared in Table 2 with the bulk values reported in the literature for the 6FDA±6FpDA. The permselectivities in the polymer-ceramic composite prior to irradiation were lower than the permselectivities of the bulk material so that the composite membrane was not completely defect-free. As was seen for previous irradiated samples, irradiation resulted in a signi®cant increase in the permeability of each of the
Table 1 Comparison of the permeancesa for ®ve gases (N2 , O2 , CH4 , He and CO2 ) and the three relative permselectivities of the 6FDA± 6FpDA-ceramic composite membrane at 35 C before and after the irradiation
Before irradiation After irradiation Ratio (Aft/Bef) a
Pt (He)
Pt (CO2 )
Pt (O2 )
Pt (N2 )
Pt (CH4 )
a(O2 /N2 )
a(He/CH4 )
a(CO2 /CH4 )
130.5 286.78 2.2
39.07 59.11 1.51
12.64 18.69 1.48
3.79 2.83 0.75
3.02 1.45 0.48
3.34 6.61 1.98
43.12 198.1 4.59
12.94 40.9 3.16
Permeance is reported in GPU: 1 GPU 1 10ÿ6 cm3 /(cm2 cm Hg s).
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Table 2 Comparison of the permeabilitiesa for ®ve gases (N2 , O2 , CH4 , He and CO2 ) and the three relative permselectivities of the 6FDA± 6FpDA-ceramic composite membrane at 35 C before and after the irradiation (the permeabilities were estimated based on the permeance and an estimated average thickness of the polymer layer of the composite membranes, the implantation was performed with 2:5 1015 cm2 H ion at 180 keV, the relative data of the initial bulk 6FDA±6FpDA are also displayed in the table)
Before irradiation After irradiation Ratio (Aft/Bef) Bulk materialb
P(He)
P(CO2 )
P(O2 )
P(N2 )
P(CH4 )
a(O2 /N2 )
a(He/CH4 )
a(CO2 /CH4 )
158 347 2.2 137c
47.27 71.52 1.51 63.9c
15.29 22.61 1.48 16.3d
4.59 3.42 0.75 3.47d
3.65 1.75 0.48 1.6c
3.34 6.61 1.98 4.7
43.12 198.1 4.59 85.6
12.94 40.9 3.16 40
a
Permeability is reported in barrer: 1 barrer 1 10ÿ10 cm3 cm/(cm2 cm Hg s). Bulk data from Ref. [11]. c Upstream pressure: 10 atm. d Upstream pressure: 2 atm. b
smaller gases (He, O2 , CO2 ) combined with a decrease in the permeability of the larger gases (CH4 , N2 ). This resulted in large increases in permselectivities for several industrial interesting gas pairs (i.e., O2 /N2 , H2 /CH4 , and CO2 /CH4 ). The permeabilities in the H irradiated composite membrane were measured over temperatures ranging from 30 C to 60 C. A plot of the permeabilities vs. inverse temperature in the H irradiated 6FDA±6FpDA-ceramic composite membrane is shown in Fig. 2. The activation energies and the pre-exponential factors for each of the gases were determined by applying the Arrhenius relationship in Eq. (3) to the temperature dependent permeation data for the irradiated sample. As shown in Fig. 2, the Arrhenius relationship correlated well with the permeabilities for each of the gases studied. The activation energies and the pre-exponential factors are compared in Table 3 with the values for the virgin bulk polymer reported by Costello et al. [24]. Large dierences in both the activation energies and the pre-exponential factors between the irradiated sample and the initial bulk polymer were observed. The ratios of the activation energies and preexponential factors of the irradiated materials over those of initial bulk materials are given in Table 4. The dierence in the activation energy term in Eq. (3) determined at 35 C between the irradiated membrane and the bulk material is also given in Table 4 for each of the gases. The data was presented in the form of ratios to allow a comparison of the relative importance of these factors to the
Fig. 2. Gas permeability vs. inverse temperature in the 6FDA± 6FpDA-Ceramic composite membrane irradiated with 2:5 1015 H /cm2 at 180 keV: (a) Nitrogen; (b) Oxygen; (c) Helium; (d) Methane; (e) Carbon dioxide.
modi®cation of permeation properties of the irradiated materials. While there was an increase in the pre-exponential factor for each of the gases, the larger increases were seen for the smaller
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Table 3 Comparison of the activation energies and the pre-exponential factors of the gas permeation for the ®ve gases (N2 , O2 , CH4 , He and CO2 ) between the H irradiated sample and the initial bulk material (the data of the initial bulk material are from Ref. [24] (Barrer)
Bulk 6FDA±6FpDA Ep (kcal/mole)
Bulk 6FDA±6FpDA P0
Irradiated 6FDA± 6FpDA Ep (kcal/mole)
Irradiated 6FDA± 6FpDA P0 (Barrer)
He CO2 O2 N2 CH4
2.0 0.72 2.0 3.2 4.3
3400 210 420 570 1700
3.5 3.8 3.5 4.12 4.8
103 500 35 600 7380 3130 4400
Table 4 Ratios of the activation energy and the pre-exponential factor for gas permeation of the H ion irradiated sample over that of the initial bulk material Ratio (P0 He CO2 O2 N2 CH4
30.4 170 17.6 5.49 2.59
irradiated /
P0
initial )
Ratio (Ep 1.75 5.28 1.77 1.32 1.11
penetrants (i.e., He, CO2 and O2 ). The value of pre-exponential factors increased from by 2.5 times for CH4 to 170 times for CO2 following ion irradiation. With the exception of CO2 , each of the gases exhibited between 10% and 80% increases in activation energy of permeation following irradiation. However, there was a ®ve-fold increase in the activation energy of the carbon dioxide in the irradiated polymer. The signi®cant changes in both the activation energy (Ep ) and the pre-exponential factor (P0 ) imply that there has been a modi®cation in the mechanism of gas transport irradiation. This agrees well with the results of prior studies of the impact of ion irradiation on the morphology and properties of polymers. Extensive studies of the eect of crosslinking of rubbery polymers on gas permeation indicate that there was a decrease in the diusion coecient with increasing degree of crosslinking [22]. For these polymers, an increase in the energy of activation for diusion and pre-exponential factor was also seen with increasing crosslinking density. Interestingly, the impact of crosslinking on the relative contribution from the exponential term
irradiated /
Ep
initial )
Exp(ÿ(Ep (35° C)
irradiated
ÿ Ep
initial )/RT)
0.086 0.007 0.086 0.222 0.442
exp ÿEd =RT in Eq. (4) to the diusivity was greater than the impact on the pre-exponential factor, i.e., D0 . As a result, there was an overall decrease in the diusion coecient with increasing crosslinking density that would be expected for the more rigid crosslinked polymer. Similarly, the increase in activation energies for gas permeation following irradiation indicated that activated gas permeation was more dicult than in the bulk material. This could be a result of crosslinking of the polymer chains induced by the bombarding ions and the formation of a rigid polymer network which would require greater energy to open suitable gaps for diusion. Therefore, the increase in permeability of the smaller gases following irradiation may be due to the introduction of additional packing defects within the polymer matrix, the increase in permselectivity may be due to the increased rigidity of the polymer following the formation of irradiation induced crosslinking. As shown in Table 4, the signi®cant increase in the permeability of the small gases was due to the large increases in the pre-exponential factor for permeation (P0 ). By combining the Eqs. (6) and
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(9), P0 can be expressed in terms of transition state theory as S 2 kT 10 exp d ; P0 S0 ek R h where k is the average diusive jump length in the diusion medium, Sd the activation entropy of diusion, k the Boltzmann constant and h is Planck's constant. An analysis of the relative contribution of the activation energies and the preexponential factors to the permselectivities can provide additional information regarding the eect of ion irradiation on the gas transport properties of the polyimide. For the O2 /N2 gas pair, the difference in the kinetic diameters of the two molecules is very small, so that the average diusive jump length in the polymeric materials, k, can be considered similar for these gases [10]. The contribution to the permselectivity from the pre-exponential factor for a gas pair can be represented by the ratio of pre-exponential factors for each gas as shown in Eq. (11) P0A S0A SdA ÿ SdB : 11 exp P0B S0B R The contributions to the permselectivities from the activation energies and the pre-exponential factors for the three industrially important gas pairs are shown in Table 5. For each of these gas pairs, irradiation resulted in a large increase in the contribution of the pre-exponential factor to the permselectivity combined with a signi®cant decrease in the contribution to the permselectivity from the activation energy. The irradiated sample still exhibited permselectivities that were greater than the bulk material (Table 1). The most signi®cant variation in the pre-exponential factor and
the activation energy contribution to permselectivity occurred for the CO2 /CH4 gas pair. The contribution from the pre-exponential factor term increased by 67 times while the contribution to the selectivity from the energy activation term decreased to 0.014 time the initial bulk value. The dramatic changes in the relative contributions to permselectivity imply a fundamental modi®cation of the transport mechanism and the microstructure of the materials which is worth to further study. A detailed investigation of the impact of irradiation on the sorption and diusion of gases in glassy polymers is currently under way. The Arrhenius equation can be written in a generic form as ÿE ; 12 k A exp RT where E is the activation energy, A is pre-exponential factor or frequency factor. In the collision and transition-state theories of chemical reactions, the numerical constants A and E were shown to represent quantities indicative of the fundamental process of chemical reactions; i.e., E represents the energy of activation, and A represents the frequency at which atoms or molecules collide in a way that leads to reaction. From a mathematical point of view, irrespective of the value of the activation energy, the value of k in the Arrhenius equation is limited by the pre-exponential factor A. In the ®eld of membrane based gas separations, several studies have discussed the impact of chemical structure on the activation energy E, but less has been reported about the pre-exponential factor. One reason for the greater focus on the activation energy may be that the activation en-
Table 5 The contribution of permselectivities from the permeation activation energy and the pre-exponential factor evaluated at 35 C for the three gas pairs: O2 /N2 , CO2 /CH4 and H2 /CH4 (the corresponding initial bulk data is also displayed in the table for comparison) Exp(ÿEp /RT) (35 C) term contribution
P0 term contribution
a(He/CH4 ) a(O2 /N2 ) a(CO2 /CH4 )
Irradiated
Initial
Ratio (Ir/In)
Irradiated
Initial
Ratio (Ir/In)
23.54 2.36 8.11
2.00 0.74 0.12
11.77 3.2 67.58
8.06 2.77 5.01
42.95 7.11 348.16
0.19 0.39 0.014
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ergy plays a greater role in determining the eect of chemical structure on temperature dependent transport properties of polymers than does the preexponential factor. When we extend our focus from polymers to a larger range of materials, including modi®ed polymers (pyrolyzed polymers and ion beam irradiated polymers), carbon molecular sieving materials, and zeolites, the pre-exponential factor plays a more important role in the permeation process than it does within a class of materials (i.e., polymers). As discussed earlier, the pre-exponential factor contains information about the relationship between the microstructure of the material and the properties of a given gas, including the activation entropy of diusion. Considering Eq. (3), even with very small values of the activation energy for permeation, the value of the permeability will never be greater than P0 . Therefore, for equivalent activation energies Ep , a larger pre-exponential factor will result in a large permeability and P0 represents the potential maximum permeability. For the ion beam irradiated sample, the potential maximum permeabilities for the more permeable gas become suciently large that signi®cant increases in the activation energy for permeation do not prevent increases in the permeabilities for the more permeable gases. A study of the temperature dependence of sorption, diusion, and permeation of several gases in the ion implanted samples over a large range of ion ¯uences, combined with a study of free volume distribution using positron annihilation spectroscopy with variable energy positron is currently underway. This study is intended to clarify the ion beam irradiation induced microstructural evolution in the polymers and the resulting eects on the gas transport mechanism in the modi®ed materials. 4. Conclusion The temperature dependence of the gas transport properties of the 6FDA±6FpDA-ceramic composite membrane irradiated with 2:5 1015 H /cm2 at 180 keV was discussed in this paper. The results were compared with the values for the bulk polymer reported by Costello et al. Large increases in the pre-exponential factor of the more
333
permeable gases following irradiation balanced the increase in activation energy, thereby, resulting in simultaneous increases in permeabilities of the small molecules and permselectivities. Unlike the current generation of polymers, the pre-exponential factor plays a key role in the modi®cation in the gas permeation properties. These results combined with prior studies of irradiation eects on polymer morphology imply that there is an evolution in both the microstructure and mechanism of transport in the polymer following irradiation. Acknowledgements The authors would like to thank Professor W.J. Koros of the University of Texas at Austin for supplying the 6FDA±6FpDA. The support of the National Science Foundation through the Presidential Faculty Fellows Program (CTS-9553267) in funding this project is gratefully acknowledged. The authors would also like thank Mr. Zexin Lin of the Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Sciences for the ion implantation. References [1] J.K. Mitchell, Philadelphia J. Med. Sci. 13 (1931) 36. [2] D.R. Paul, J.W. Barlow, H. Keskkula, Mark±Bikales± Overberger±Menges: Encyclopedia of Polymer Science and Engineering, vol. 12, Wiley, New York, 1988. [3] C.W. Jones, W.J. Koros, Carbon 32 (1994) 1419. [4] C.W. Jones, W.J. Koros, Carbon 32 (1994) 1427. [5] V.C. Geiszler, W.J. Koros, 35 (1996) 2999. [6] S. Wang, M. Zeng, Z. Wang, 31 (1996) 2299. [7] A.S. Damie, S.K. Gangwal, J.J. Spivey, J. Longanbach, V.K. Venkataraman, Key Engrg. Mater. 61/62 (1991) 273. [8] J.D Le Roux, D.R. Paul, J. Kampa, R.J. Lagow, J. Membrane Sci. 94 (1994) 121. [9] M.R. Coleman, W.J. Koros, J. Membrane Sci. 50 (1990) 285. [10] M.R. Coleman, W.J. Koros, J. Polym. Sci. Polym. Phys. 32 (1994) 1915. [11] W.J. Koros, M.R. Coleman, D.R.B. Walker, Ann. Rev. Mater. Sci. 22 (1992) 47. [12] L.M. Robeson, J. Membrane Sci. 62 (1991) 165. [13] A. Singh, W.J. Koros, Ind. Engrg. Chem. Res. 35 (1996) 1231. [14] X.L. Xu, Contribution a l'Etude des Eets d'Irradiation Induits dans Les Polymeres par des Faisceaux d'Ions,
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[15] [16] [17] [18] [19]
X. Xu, M.R. Coleman / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 325±334 Dissertation of Doctorat d'Etat et Sciences at University of Lyon, France, 87±08, 1987. J. Davenas, X.L. Xu, Nucl. Instr. and Meth. B 71 (1992) 33. X.L. Xu, J.Y. Dolveck, G. Boiteux, M. Escoubes, M. Monchanin, J.P. Dupin, J. Davenas, Mater. Res. Soc. Symp. Proc. Ser. 354 (1995) 351. X.L. Xu, J.Y. Dolveck, G. Boiteux, M. Escoubes, M. Monchanin, J.P. Dupin, J. Davenas, J. Appl. Polym. Sci. 55 (1995) 99. X.L. Xu, M.R. Coleman, J. Appl. Polym. Sci. 66 (1997) 459. X.L. Xu, U. Myler, P.J. Simpson, M.R. Coleman, the ACS symposium series, in press.
[20] J. Crank, The Mathematics of Diusion, Clarendon Press, Oxford, 1975. [21] V.T. Stannett, in: J. Crank, G.S. Park, (Eds.), Diusion in Polymer, ch. 2, Academic Press, New York, 1968. [22] G.J. van Amerongen, Rubb. Chem. Tech. 37 (1964) 1065. [23] S. Glasstone, K.J. Laidler, H. Eyring, The Theory of Rate Processes, 1st ed., McGraw-Hill, New York, 1941. [24] L.M. Costello, W.J. Koros, J. of, Polym. Sci: Part B: Polym. Phys. 33 (1995) 135. [25] M.E. Rezac, W.J. Koros, J. Appl. Polym. Sci. 46 (1992) 1927. [26] J.F. Ziegler, J.P. Biersack, U. Littmark, in: J.F. Ziegler (Ed.), The Stopping and Range of Ions in Solids, vol. 1, Pergamon Press, New York, 1985.
Preliminary investigation of gas transport mechanism in a H irradiated polyimide-ceramic composite membrane Xinglong Xu, M.R. Coleman
*
Chemical and Environmental Engineering, University of Toledo, 3048 Nitschke Hall, Toledo, OH 43606-3390, USA Received 15 September 1998; received in revised form 18 December 1998
Abstract Recent research by our group indicated that ion beam irradiation can simultaneously increase the gas permeability and permselectivity of polymeric membrane materials. The temperature dependence of the gas permeation properties of a H ion irradiated polyimide-ceramic composite membrane was investigated to address issues of changes in the gas transport mechanism in irradiated polymers. As was seen for glassy polymers, the temperature dependence of the permeation properties of the irradiated membrane followed an Arrhenius type relationship. Both the activation energy (Ep ) for gas permeation and the pre-exponential factor (P0 ) of the irradiated polymer were greater than the values of the unmodi®ed bulk polymer. Large increases in the pre-exponential factor of the irradiated sample for small size gas molecules (He, O2 and CO2 ) combined with the dominant contribution of the pre-exponential factor to the permselectivity for several gas pairs (He/CH4 , O2 /N2 , and CO2 /CH4 ) implied that the irradiated sample had a dierent permeation mechanism than the bulk material. Ó 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 61.80 Jh; 61.82 Pv Keywords: Gas separation; Ion irradiation; Gas transport mechanism; Activation energy for permeation; Microstructure
1. Introduction Though selective gas permeation of polymeric materials has been known since 1831, polymer based gas separation membranes have only found widespread commercial application since the early 1980s [1,2]. While the majority of commercial gas
* Corresponding author. Tel.: +1-419-530-8091; fax: +1-419530-8086; e-mail: [email protected]
separation membranes are polymeric in nature, there is considerable interest in development of ceramic and carbon molecular sieving membranes for applications in challenging environments (e.g., high temperature) [3±5]. Development of materials having both high permeabilities and permselectivities combined with mechanical stability is an important issue for the extension of membranes to a variety of systems. Two approaches have been widely investigated for the development of high performance membrane materials: (i) synthesis of
0168-583X/99/$ ± see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 0 6 7 - 1
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glassy polymers with structures tailored using established structure-property relationships, and (ii) modi®cation of polymeric permselective layers using diverse post-synthesis techniques (e.g., pyrolysis and surface ¯uorination) [3±8]. Selective alteration of gas transport properties through chemical modi®cation using established structure-property principles has resulted in an improvement in the gas separations properties of polymeric materials [9±11]. However, an analysis of the extensive literature data for several binary gas mixtures (i.e., O2 /N2 , H2 /CH4 , CO2 /CH4 , etc.) by Robeson [12], revealed an upper bound for the permeability versus permselectivity for these mixtures for most of the reported polymeric membrane materials. Virtually no values existed for polymers above this linear upper bound on the trade-o curve. In comparing the oxygen and nitrogen transport properties of polymers, zeolites and carbon molecular sieves, Singh and Koros [10], found that the ``upper bound'' line for polymers drawn by Robeson still applied in 1996. For similar permeabilities, the molecular sieving materials exhibited much greater selectivities than the polymers and were, therefore, well above this upper bound. Carbon molecular sieving (CMS) membrane formed from the pyrolysis of aromatic polymers have been shown to exhibit very attractive permeation properties [13]. However, the CMS membranes formed from the pyrolysis of polymer asymmetric membranes are typically very fragile. The improved permselective properties of the sieving materials relative to glassy polymers have been attributed to dierences in the entropy selectivities [13] between these classes of materials. Ion beam irradiation has been shown to result in an evolution in the chemical structure from the virgin polymer to a graphite-like material with increasing ion beam ¯uence. Intensive energy deposition of the incident ion can result in the following chemical processes within the thin polymer surface layer: (i) degradation of the polymer chain with formation of small volatile molecules and free radials which leave defects in the polymer matrix, (ii) crosslinking between the polymer chains, (iii) formation of new chemical bonds such as double bonds, and (iv) oxidation or other chemical reactions in the presence of chemical atmosphere
during the irradiation process [14]. Modi®cation of polymeric permselective layers using ion beam irradiation is being investigated by our group as a method to develop materials which combine the attractive transport properties of carbon molecular sieves with the mechanical stability of polymers. An investigation of the eect of ion beam irradiation on iodine diusion into polymers indicated that ion beam irradiation could modify the microstructure and diusion properties of the polymer surface layer [15,16]. While irradiation at low ¯uence resulted in an enhancement of the iodine diusion rate into the modi®ed layer and prevented iodine diusion beyond this layer, irradiation at high ¯uence led to the formation of a layer within the polymer surface which did not allow iodine diusion into the polymer. A study of the evolution of the permeation properties of irradiated Kaptonâ polyimide thick free-standing ®lms con®rmed the observation that ion beam irradiation can modify the microstructure of polyimide ®lms. Low ¯uence ion irradiation resulted in increases in the permeabilities of both methane and hydrogen with a corresponding decrease in the H2 /CH4 selectivity within the dense ®lm. Following high ¯uence irradiation, there was a signi®cant increase in both the hydrogen permeability and the permselectivity (H2 /CH4 ) [15,17]. Atomic force microscopy (AFM) studies of the surface of free-standing 6FDA±pMDA polyimide ®lms (25±50 lm thick) following N irradiation revealed a dramatic evolution in the morphology of the polymer surface layer as a function of ion ¯uence [18]. Detailed roughness and bearing analyses of the AFM images showed that the freestanding virgin polyimide ®lms had deep surface valleys. Even at a low ¯uence, ion beam irradiation altered the microstructure of the surface layer and formed a modi®ed surface layer in which the initial deep surface valleys were eliminated. AFM analysis also indicated that low ¯uence irradiation induced the formation of large microvoids in the surface layer of the polymer, and high ¯uence irradiation resulted in the formation of a large number of small size microvoids in the surface. The results of these surface morphology studies agree well with the ion beam irradiation eects on
X. Xu, M.R. Coleman / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 325±334
iodine diusion and gas permeation properties of free standing polyimide ®lms. For example, the large increase in permeability of both H2 and CH4 following low dose irradiation is consistent with an increase in the openness of the polymer matrix (i.e. the formation of large microvoids). However, the formation of a high concentration of small microvoids may result in an overall increase in the permeability of small gases (i.e., H2 ) with a corresponding increase in the permselectivity of small penetrant molecules relative to larger molecules (i.e., H2 /CH4 ). The evolution in the gas transport properties of polyimide-ceramic composite membranes following irradiation with a H ion beam was investigated by our group [19]. In this study, the thin permselective polymer layer of the composite membranes was irradiated throughout the entire thickness to allow a direct comparison of its gas permeation properties with those of unmodi®ed polymer. Signi®cant increases in the permeabilities of the gases having smaller kinetic molecular diameter (i.e., He and O2 ) and the permselectivities of several important gas pairs including He/ CH4 , O2 /N2 and He/N2 were observed. The data points of the ion beam irradiated membranes for each gas pair were located above the upper bound limit for polymers on the plot of permeability versus permselectivity. A hypothesis was proposed to explain the permeation results in terms of the impact of ion irradiation on the free volume, rigidity, and structure of the polymer matrix. Irradiation induced degradation of the polymer has been shown to result in the introduction of defects in the polymer matrix with a corresponding increase in free volume [19]. This would be expected to result in an increase in the permeability. Simultaneously, there is an increase in the rigidity of the polymer structure due to ion irradiation induced crosslinking of the polymer chains, which may lead to an increase in permselectivity. These combined modi®cations of the polymer microstructure could result in an increase in both the permeability of small molecules and permselectivity. The permeability of a penetrant through a polymer membrane is de®ned as the thickness and pressure normalized ¯ux. Permeation of gases
327
through polymers is through a solution-diusion process in which a gas molecule dissolves into the feed side, diuses through the membrane because of the concentration gradient and re-dissolves into low pressure permeate side. The permeability coecient (P) can, therefore, be expressed as a product of an average diusion coecient (D) and a solubility coecient (S) [20] P DS:
1
The ideal selectivity, aA=B , of a membrane for a component gas A relative to a component gas B can be expressed as the ratio of the pure gas permeabilities of the two component in the membrane material. DA SA : 2 aA=B PA =PB DB SB Permeation in polymers is an activated process in which the temperature dependence of the permeability coecient, P, diusion coecient, D, and solubility, S, can be described using the following Arrhenius type expressions [21,22]: ÿEp ; 3 P P0 exp RT D D0 exp S S0 exp
ÿEd ; RT
ÿHs ; RT
4 5
where P0 , D0 , and S0 are the pre-exponential factors, Ep is the activation energy of permeation, Ed the activation energy for diusion, and Hs the heat of sorption. By combining Eqs. (1)±(5), the permeability coecient can be expressed as ÿEp ÿEp D0 S0 exp P P0 exp RT RT ÿ Ed Hs ; 6 D0 S0 exp RT P 0 D 0 S0 ;
7
Ep E d H s :
8
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From the transition state theory [23], the pre-exponential factor of diusion can be expressed as kT S 9 D0 ek2 exp d ; R h where k is the average diusive jump length in the diusion medium, Sd the activation entropy of diusion, k the Boltzmann constant and h is Planck's constant. A detailed study of the temperature dependence of gas sorption and transport properties combined with an analysis of the physical properties of membrane materials can provide insight into the transport mechanism of a penetrant within the materials. For example, Costello et al. [24] reported a typical investigation of the temperature dependence of transport properties of the polyimides 6FDA±6FpDA and 6FDA±6FmDA. Ion beam induced modi®cation of the polymer microstructure is expected to result in a modi®cation in the transport mechanism relative to the virgin polymer which may be re¯ected in a variation in the temperature dependence of the transport properties. Therefore, the temperature dependence of the permeation properties were determined for an irradiated polyimide ®lm to probe the eect of irradiation on the mechanism of gas transport. Due to the limitation of the energy range available with the implantor and the large quantity of material required for measurement of sorption isotherms with the currently available equipment, the current work was limited to the temperature dependence of gas permeation properties. The temperature dependence of the permeation properties of a polyimide-ceramic composite membrane which was irradiated with 2:5 1015 / cm2 H at 180 keV will be reported in this paper. 2. Experimental 2.1. Preparation of 6FDA±6FpDA-ceramic composite membranes The ¯uorine containing polyimide, 6FDA6FpDA, was used in this study and is shown in Fig. 1. The 6FDA±6FpDA was kindly supplied by Professor William Koros of the University of
Fig. 1. Structure of the polyimide isomers, Hexa¯uoro Dianhydride 4,40 -Hexa¯uoro Diamine (6FDA±6FpDA).
Texas at Austin. The polymer-ceramic composite membranes used in this work consisted of a polymeric permselective layer with a thickness of approximately 1.2 lm deposited on a porous ceramic membrane support using a preparation method which has been described in detail elsewhere [25]. Anaporeâ ceramic membranes purchased from Whatman Inc. with an average pore size of 0.02 lm were used as the porous substrates for the composite membranes. A dilute solution of 6FDA±6FpDA in methylene chloride was prepared in a dry environment to avoid humidity which could cause the polymer to precipitate during the ®lm formation process. A prede®ned volume of polymer solution, which was determined based upon the desired thickness of the polymer layer, was deposited on the ceramic support and covered to control the evaporation rate of the solvent. The composite membranes were allowed to dry in air for at least 12 h and were then dried in a vacuum oven at 100 C for more than one hour to remove any residual solvent. A weight dierence method which included an estimation of the surface area of the polymer coverage, was used to estimate the thickness of the selective polymer layer of the composite membrane. Cross section imaging using scanning electronic microscopy (SEM) con®rmed the thickness results of the weight dierence method [19]. 2.2. Ion beam irradiation process Ion implantation was performed using a IM200 implantor at the Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Sciences. A small ion beam current density ( 6 1 lA/cm2 ) was used to avoid overheating of the sample. The mean projected range (Rp ) of the ir-
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radiating ions and energy loss in the samples were estimated using SRIM Code [26]. The Rp of the H ion at 180 keV in 6FDA±6FpDA was 2.09 lm and the longitudinal straggling (DRp ) was about 0.13 lm so that the total thickness modi®ed by ion irradiation Rm which is de®ned as Rp DRp , would be 2.2 lm for H irradiation of 6FDA±6FpDA. The thickness of the permselective layer of the composite membrane used in this work was just 1.2 lm, so that modi®cation of the entire thickness of the polymeric permselective layer was assured. 2.3. Gas permeation measurements The pure gas permeances of O2 , N2 , He, CO2 and CH4 were measured for the membrane at 35 C prior to irradiation and at temperatures ranging from 30 C to 60 C following the irradiation. A Millipore test cell in a temperature controlled box was used for the permeance measurements [25]. For each permeance measurement, the feed gas was applied to the upstream side of the membrane at a pressure of 50 psig and the permeate side was held at atmospheric pressure. Once steady-state for the permeance was reached (>20 h), the volumetric ¯ow rate of the permeate stream was determined using a soap-bubble ¯owmeter. The permeances were determined from the volumetric ¯owrate, temperature, and prede®ned membrane area. 3. Results and discussion Previous studies [15±17] of ion beam modi®cation of polymers indicated that with increasing irradiation ¯uence, ion irradiation results in a gradual change in the microstructure and mor-
329
phology of polymers. In the very low dose range, there was an increase in both concentration and size of ``packing defects'' (regions of localized high free volume) with increasing irradiation ¯uence. In the high dose range, there was a gradual closing of the open structure with irradiation ¯uence due to increasing crosslinking. Irradiation of 6FDA± 6FpDA-ceramic composite membranes with H at ¯uences up to 2:0 1015 /cm2 resulted in signi®cant increases in the permeability of small molecules with corresponding increases in permselectivities [19]. In fact, the irradiated materials exhibited permeation properties which were well above the trade-o curve of typical glassy polymers. In this work, an ion ¯uence of 2:5 1015 H /cm2 at 180 keV was used to modify a 6FDA± 6FpDA-ceramic composite membrane in order to extend the irradiation range. The permeances and permselectivities for several gas pairs in the 6FDA±6FpDA-ceramic composite membrane were measured at 50 psig and at 35 C prior to and following H ion irradiation at 2:5 1015 /cm2 and are listed in Table 1. The permeabilities of each of the gases through the polymer selective layer were estimated using the measured permeance values combined with an average thickness determined using a weight difference method. The estimated permeabilities of both the virgin and irradiated polymer-ceramic composite membrane are compared in Table 2 with the bulk values reported in the literature for the 6FDA±6FpDA. The permselectivities in the polymer-ceramic composite prior to irradiation were lower than the permselectivities of the bulk material so that the composite membrane was not completely defect-free. As was seen for previous irradiated samples, irradiation resulted in a signi®cant increase in the permeability of each of the
Table 1 Comparison of the permeancesa for ®ve gases (N2 , O2 , CH4 , He and CO2 ) and the three relative permselectivities of the 6FDA± 6FpDA-ceramic composite membrane at 35 C before and after the irradiation
Before irradiation After irradiation Ratio (Aft/Bef) a
Pt (He)
Pt (CO2 )
Pt (O2 )
Pt (N2 )
Pt (CH4 )
a(O2 /N2 )
a(He/CH4 )
a(CO2 /CH4 )
130.5 286.78 2.2
39.07 59.11 1.51
12.64 18.69 1.48
3.79 2.83 0.75
3.02 1.45 0.48
3.34 6.61 1.98
43.12 198.1 4.59
12.94 40.9 3.16
Permeance is reported in GPU: 1 GPU 1 10ÿ6 cm3 /(cm2 cm Hg s).
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Table 2 Comparison of the permeabilitiesa for ®ve gases (N2 , O2 , CH4 , He and CO2 ) and the three relative permselectivities of the 6FDA± 6FpDA-ceramic composite membrane at 35 C before and after the irradiation (the permeabilities were estimated based on the permeance and an estimated average thickness of the polymer layer of the composite membranes, the implantation was performed with 2:5 1015 cm2 H ion at 180 keV, the relative data of the initial bulk 6FDA±6FpDA are also displayed in the table)
Before irradiation After irradiation Ratio (Aft/Bef) Bulk materialb
P(He)
P(CO2 )
P(O2 )
P(N2 )
P(CH4 )
a(O2 /N2 )
a(He/CH4 )
a(CO2 /CH4 )
158 347 2.2 137c
47.27 71.52 1.51 63.9c
15.29 22.61 1.48 16.3d
4.59 3.42 0.75 3.47d
3.65 1.75 0.48 1.6c
3.34 6.61 1.98 4.7
43.12 198.1 4.59 85.6
12.94 40.9 3.16 40
a
Permeability is reported in barrer: 1 barrer 1 10ÿ10 cm3 cm/(cm2 cm Hg s). Bulk data from Ref. [11]. c Upstream pressure: 10 atm. d Upstream pressure: 2 atm. b
smaller gases (He, O2 , CO2 ) combined with a decrease in the permeability of the larger gases (CH4 , N2 ). This resulted in large increases in permselectivities for several industrial interesting gas pairs (i.e., O2 /N2 , H2 /CH4 , and CO2 /CH4 ). The permeabilities in the H irradiated composite membrane were measured over temperatures ranging from 30 C to 60 C. A plot of the permeabilities vs. inverse temperature in the H irradiated 6FDA±6FpDA-ceramic composite membrane is shown in Fig. 2. The activation energies and the pre-exponential factors for each of the gases were determined by applying the Arrhenius relationship in Eq. (3) to the temperature dependent permeation data for the irradiated sample. As shown in Fig. 2, the Arrhenius relationship correlated well with the permeabilities for each of the gases studied. The activation energies and the pre-exponential factors are compared in Table 3 with the values for the virgin bulk polymer reported by Costello et al. [24]. Large dierences in both the activation energies and the pre-exponential factors between the irradiated sample and the initial bulk polymer were observed. The ratios of the activation energies and preexponential factors of the irradiated materials over those of initial bulk materials are given in Table 4. The dierence in the activation energy term in Eq. (3) determined at 35 C between the irradiated membrane and the bulk material is also given in Table 4 for each of the gases. The data was presented in the form of ratios to allow a comparison of the relative importance of these factors to the
Fig. 2. Gas permeability vs. inverse temperature in the 6FDA± 6FpDA-Ceramic composite membrane irradiated with 2:5 1015 H /cm2 at 180 keV: (a) Nitrogen; (b) Oxygen; (c) Helium; (d) Methane; (e) Carbon dioxide.
modi®cation of permeation properties of the irradiated materials. While there was an increase in the pre-exponential factor for each of the gases, the larger increases were seen for the smaller
X. Xu, M.R. Coleman / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 325±334
331
Table 3 Comparison of the activation energies and the pre-exponential factors of the gas permeation for the ®ve gases (N2 , O2 , CH4 , He and CO2 ) between the H irradiated sample and the initial bulk material (the data of the initial bulk material are from Ref. [24] (Barrer)
Bulk 6FDA±6FpDA Ep (kcal/mole)
Bulk 6FDA±6FpDA P0
Irradiated 6FDA± 6FpDA Ep (kcal/mole)
Irradiated 6FDA± 6FpDA P0 (Barrer)
He CO2 O2 N2 CH4
2.0 0.72 2.0 3.2 4.3
3400 210 420 570 1700
3.5 3.8 3.5 4.12 4.8
103 500 35 600 7380 3130 4400
Table 4 Ratios of the activation energy and the pre-exponential factor for gas permeation of the H ion irradiated sample over that of the initial bulk material Ratio (P0 He CO2 O2 N2 CH4
30.4 170 17.6 5.49 2.59
irradiated /
P0
initial )
Ratio (Ep 1.75 5.28 1.77 1.32 1.11
penetrants (i.e., He, CO2 and O2 ). The value of pre-exponential factors increased from by 2.5 times for CH4 to 170 times for CO2 following ion irradiation. With the exception of CO2 , each of the gases exhibited between 10% and 80% increases in activation energy of permeation following irradiation. However, there was a ®ve-fold increase in the activation energy of the carbon dioxide in the irradiated polymer. The signi®cant changes in both the activation energy (Ep ) and the pre-exponential factor (P0 ) imply that there has been a modi®cation in the mechanism of gas transport irradiation. This agrees well with the results of prior studies of the impact of ion irradiation on the morphology and properties of polymers. Extensive studies of the eect of crosslinking of rubbery polymers on gas permeation indicate that there was a decrease in the diusion coecient with increasing degree of crosslinking [22]. For these polymers, an increase in the energy of activation for diusion and pre-exponential factor was also seen with increasing crosslinking density. Interestingly, the impact of crosslinking on the relative contribution from the exponential term
irradiated /
Ep
initial )
Exp(ÿ(Ep (35° C)
irradiated
ÿ Ep
initial )/RT)
0.086 0.007 0.086 0.222 0.442
exp ÿEd =RT in Eq. (4) to the diusivity was greater than the impact on the pre-exponential factor, i.e., D0 . As a result, there was an overall decrease in the diusion coecient with increasing crosslinking density that would be expected for the more rigid crosslinked polymer. Similarly, the increase in activation energies for gas permeation following irradiation indicated that activated gas permeation was more dicult than in the bulk material. This could be a result of crosslinking of the polymer chains induced by the bombarding ions and the formation of a rigid polymer network which would require greater energy to open suitable gaps for diusion. Therefore, the increase in permeability of the smaller gases following irradiation may be due to the introduction of additional packing defects within the polymer matrix, the increase in permselectivity may be due to the increased rigidity of the polymer following the formation of irradiation induced crosslinking. As shown in Table 4, the signi®cant increase in the permeability of the small gases was due to the large increases in the pre-exponential factor for permeation (P0 ). By combining the Eqs. (6) and
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(9), P0 can be expressed in terms of transition state theory as S 2 kT 10 exp d ; P0 S0 ek R h where k is the average diusive jump length in the diusion medium, Sd the activation entropy of diusion, k the Boltzmann constant and h is Planck's constant. An analysis of the relative contribution of the activation energies and the preexponential factors to the permselectivities can provide additional information regarding the eect of ion irradiation on the gas transport properties of the polyimide. For the O2 /N2 gas pair, the difference in the kinetic diameters of the two molecules is very small, so that the average diusive jump length in the polymeric materials, k, can be considered similar for these gases [10]. The contribution to the permselectivity from the pre-exponential factor for a gas pair can be represented by the ratio of pre-exponential factors for each gas as shown in Eq. (11) P0A S0A SdA ÿ SdB : 11 exp P0B S0B R The contributions to the permselectivities from the activation energies and the pre-exponential factors for the three industrially important gas pairs are shown in Table 5. For each of these gas pairs, irradiation resulted in a large increase in the contribution of the pre-exponential factor to the permselectivity combined with a signi®cant decrease in the contribution to the permselectivity from the activation energy. The irradiated sample still exhibited permselectivities that were greater than the bulk material (Table 1). The most signi®cant variation in the pre-exponential factor and
the activation energy contribution to permselectivity occurred for the CO2 /CH4 gas pair. The contribution from the pre-exponential factor term increased by 67 times while the contribution to the selectivity from the energy activation term decreased to 0.014 time the initial bulk value. The dramatic changes in the relative contributions to permselectivity imply a fundamental modi®cation of the transport mechanism and the microstructure of the materials which is worth to further study. A detailed investigation of the impact of irradiation on the sorption and diusion of gases in glassy polymers is currently under way. The Arrhenius equation can be written in a generic form as ÿE ; 12 k A exp RT where E is the activation energy, A is pre-exponential factor or frequency factor. In the collision and transition-state theories of chemical reactions, the numerical constants A and E were shown to represent quantities indicative of the fundamental process of chemical reactions; i.e., E represents the energy of activation, and A represents the frequency at which atoms or molecules collide in a way that leads to reaction. From a mathematical point of view, irrespective of the value of the activation energy, the value of k in the Arrhenius equation is limited by the pre-exponential factor A. In the ®eld of membrane based gas separations, several studies have discussed the impact of chemical structure on the activation energy E, but less has been reported about the pre-exponential factor. One reason for the greater focus on the activation energy may be that the activation en-
Table 5 The contribution of permselectivities from the permeation activation energy and the pre-exponential factor evaluated at 35 C for the three gas pairs: O2 /N2 , CO2 /CH4 and H2 /CH4 (the corresponding initial bulk data is also displayed in the table for comparison) Exp(ÿEp /RT) (35 C) term contribution
P0 term contribution
a(He/CH4 ) a(O2 /N2 ) a(CO2 /CH4 )
Irradiated
Initial
Ratio (Ir/In)
Irradiated
Initial
Ratio (Ir/In)
23.54 2.36 8.11
2.00 0.74 0.12
11.77 3.2 67.58
8.06 2.77 5.01
42.95 7.11 348.16
0.19 0.39 0.014
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ergy plays a greater role in determining the eect of chemical structure on temperature dependent transport properties of polymers than does the preexponential factor. When we extend our focus from polymers to a larger range of materials, including modi®ed polymers (pyrolyzed polymers and ion beam irradiated polymers), carbon molecular sieving materials, and zeolites, the pre-exponential factor plays a more important role in the permeation process than it does within a class of materials (i.e., polymers). As discussed earlier, the pre-exponential factor contains information about the relationship between the microstructure of the material and the properties of a given gas, including the activation entropy of diusion. Considering Eq. (3), even with very small values of the activation energy for permeation, the value of the permeability will never be greater than P0 . Therefore, for equivalent activation energies Ep , a larger pre-exponential factor will result in a large permeability and P0 represents the potential maximum permeability. For the ion beam irradiated sample, the potential maximum permeabilities for the more permeable gas become suciently large that signi®cant increases in the activation energy for permeation do not prevent increases in the permeabilities for the more permeable gases. A study of the temperature dependence of sorption, diusion, and permeation of several gases in the ion implanted samples over a large range of ion ¯uences, combined with a study of free volume distribution using positron annihilation spectroscopy with variable energy positron is currently underway. This study is intended to clarify the ion beam irradiation induced microstructural evolution in the polymers and the resulting eects on the gas transport mechanism in the modi®ed materials. 4. Conclusion The temperature dependence of the gas transport properties of the 6FDA±6FpDA-ceramic composite membrane irradiated with 2:5 1015 H /cm2 at 180 keV was discussed in this paper. The results were compared with the values for the bulk polymer reported by Costello et al. Large increases in the pre-exponential factor of the more
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permeable gases following irradiation balanced the increase in activation energy, thereby, resulting in simultaneous increases in permeabilities of the small molecules and permselectivities. Unlike the current generation of polymers, the pre-exponential factor plays a key role in the modi®cation in the gas permeation properties. These results combined with prior studies of irradiation eects on polymer morphology imply that there is an evolution in both the microstructure and mechanism of transport in the polymer following irradiation. Acknowledgements The authors would like to thank Professor W.J. Koros of the University of Texas at Austin for supplying the 6FDA±6FpDA. The support of the National Science Foundation through the Presidential Faculty Fellows Program (CTS-9553267) in funding this project is gratefully acknowledged. The authors would also like thank Mr. Zexin Lin of the Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Sciences for the ion implantation. References [1] J.K. Mitchell, Philadelphia J. Med. Sci. 13 (1931) 36. [2] D.R. Paul, J.W. Barlow, H. Keskkula, Mark±Bikales± Overberger±Menges: Encyclopedia of Polymer Science and Engineering, vol. 12, Wiley, New York, 1988. [3] C.W. Jones, W.J. Koros, Carbon 32 (1994) 1419. [4] C.W. Jones, W.J. Koros, Carbon 32 (1994) 1427. [5] V.C. Geiszler, W.J. Koros, 35 (1996) 2999. [6] S. Wang, M. Zeng, Z. Wang, 31 (1996) 2299. [7] A.S. Damie, S.K. Gangwal, J.J. Spivey, J. Longanbach, V.K. Venkataraman, Key Engrg. Mater. 61/62 (1991) 273. [8] J.D Le Roux, D.R. Paul, J. Kampa, R.J. Lagow, J. Membrane Sci. 94 (1994) 121. [9] M.R. Coleman, W.J. Koros, J. Membrane Sci. 50 (1990) 285. [10] M.R. Coleman, W.J. Koros, J. Polym. Sci. Polym. Phys. 32 (1994) 1915. [11] W.J. Koros, M.R. Coleman, D.R.B. Walker, Ann. Rev. Mater. Sci. 22 (1992) 47. [12] L.M. Robeson, J. Membrane Sci. 62 (1991) 165. [13] A. Singh, W.J. Koros, Ind. Engrg. Chem. Res. 35 (1996) 1231. [14] X.L. Xu, Contribution a l'Etude des Eets d'Irradiation Induits dans Les Polymeres par des Faisceaux d'Ions,
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X. Xu, M.R. Coleman / Nucl. Instr. and Meth. in Phys. Res. B 152 (1999) 325±334 Dissertation of Doctorat d'Etat et Sciences at University of Lyon, France, 87±08, 1987. J. Davenas, X.L. Xu, Nucl. Instr. and Meth. B 71 (1992) 33. X.L. Xu, J.Y. Dolveck, G. Boiteux, M. Escoubes, M. Monchanin, J.P. Dupin, J. Davenas, Mater. Res. Soc. Symp. Proc. Ser. 354 (1995) 351. X.L. Xu, J.Y. Dolveck, G. Boiteux, M. Escoubes, M. Monchanin, J.P. Dupin, J. Davenas, J. Appl. Polym. Sci. 55 (1995) 99. X.L. Xu, M.R. Coleman, J. Appl. Polym. Sci. 66 (1997) 459. X.L. Xu, U. Myler, P.J. Simpson, M.R. Coleman, the ACS symposium series, in press.
[20] J. Crank, The Mathematics of Diusion, Clarendon Press, Oxford, 1975. [21] V.T. Stannett, in: J. Crank, G.S. Park, (Eds.), Diusion in Polymer, ch. 2, Academic Press, New York, 1968. [22] G.J. van Amerongen, Rubb. Chem. Tech. 37 (1964) 1065. [23] S. Glasstone, K.J. Laidler, H. Eyring, The Theory of Rate Processes, 1st ed., McGraw-Hill, New York, 1941. [24] L.M. Costello, W.J. Koros, J. of, Polym. Sci: Part B: Polym. Phys. 33 (1995) 135. [25] M.E. Rezac, W.J. Koros, J. Appl. Polym. Sci. 46 (1992) 1927. [26] J.F. Ziegler, J.P. Biersack, U. Littmark, in: J.F. Ziegler (Ed.), The Stopping and Range of Ions in Solids, vol. 1, Pergamon Press, New York, 1985.