JournalofMembrane Science 89 (1994) 131-141 ELSEVIER

The effect of film thickness on oxygen sorption and transport and water-saturated Kapton@ polyimide

G. Mensitieri, M.A. Del Nobile, T. Monetta, L. Nicodemo, F. Bellucci*

in dry

Department of Materials and Production Engineering, University of Naples ‘Federico II’, ‘Piazzale Tecchio 80, 80125 Naples, Italy

(Received September 28, 1993; accepted in revised form November 30, 1993)

Abstract

Sorption and transport of oxygen in dry and water saturated Kapton@ polyimide films of different thickness were investigated at 25 ’ C with pure oxygen using: (i ) a manometric permeation apparatus (gas-membrane-gas configuration), (ii) a high pressure gas sorption equipment, and (iii) an electrochemical technique based on the oxygen Clark electrode (liquid-membrane-liquid configuration), respectively. A sharp increase of permeability, solubility, and diffusivity coefficient as thickness increased in the range 13-50 pm was observed for the oxygen transport through both dry and water-saturated Kapton. The variation of the oxygen transport parameters in the range of thicknesses investigated were small in the case of dry polyimide compared to the case of the water-satu- rated polymer. An interpretation is presented in which the oxygen transport parameters in both dry and water- saturated conditions are analyzed in terms of ‘dual-sorption-dual-mobility’ model and related to the different polymer morphologies. such as crystallinity level, crystallinity orientation and excess free volume content, char- acterizing samples of different thickness.

Key words: Oxygen sorption; Oxygen transport; Kaptona; Polyimide; Water

1. Introduetlon

The permeation process of small molecules through polymers, as originally proposed by Graham [ 1 ] , can be generally modeled as a ‘so- lution-diffusion’ mechanism, governed by both a thermodynamic and a kinetic factor. Hence, both gas solubility and gas mobility have to be examined in order to elucidate the permeability behavior. Due to the non-equilibrium nature of the polymeric matrix, permeation of gases and

*Corresponding author.

Elsevier Science B.V. ?XDI0376-7388(93)E0209-3

vapors in glassy polymers is a rather complex phenomenon [ 2,3].

A model for pure gas sorption in glassy poly- mers that successfully interprets the experimen- tal behavior, is the ‘dual-sorption’ model [2,4- 7 1. According to this model the existence of two populations of sorbate is postulated, one dis- solved in the bulk of the polymer (for which Henry’s sorption law holds), and the other ad- sorbed on the surface of non-equilibrium micro- voids (excess free volume) frozen into the poly- mer structure (for which Langmuir’s sorption law holds ) . Subsequently, a modified version of this

132 G. Mensitieri et al. /Journal ofMembrane Science 89 (1994) 131-141

model was introduced to describe the sorption of tivity [ 371) coupled with a relatively high gas gas mixtures in glassy polymers [ 8 1. permeability.

Diffusion of gaseous molecules into glassy polymers has been adequately interpreted in terms of the ‘dual-mobility’ model [ 2,9, lo]. Ac- cording to this model, adsorbed and absorbed gas molecules have a different mobility. With suita- ble modifications, the ‘dual-mobility’ model was extended to interpret the diffusion of gas mix- tures through glassy polymers [ 111.

As reported in the literature [ 2,12,13 1, both the kinetic and the thermodynamic factor are strongly affected by the glassy polymer morphol- ogy, such as crystallinity level, form and orien- tation of the crystallites as well as the amount of excess free volume of the amorphous phase.

DuPont Kapton@ polyimide [ poly (4,4’-oxy- diphenyl pyromellitimide ) , PMDA-ODA ] is a glassy polymer with wide paracrystalline do- mains. A heterogeneous structure has been re- ported [ 141 consisting of an amorphous and a more ordered region. Molecular aggregation in the ordered domains is intermediate between an ordinary crystalline and an amorphous struc- ture. Elevated glass transition temperature, ex- cellent thermal stability, and good mechanical and insulating properties make this commercial polymer an important material in aerospace and microelectronic applications where it may func- tion as a dielectric [ 15 1. In such applications, device reliability often depends on the rate of ar- rival of ionic contaminants, water, and oxygen at the packaging/metallic circuitry interface, i.e., on the transport properties of the polymeric pack- aging material [ 16-23 1.

As far as the transport properties are con- cerned, water vapor sorption and diffusion through Kapton and various commercially available polyimides, has been investigated by different authors [ 24-27 1. Polyimides have been extensively investigated also as suitable mem- branes for gas mixtures separation, and sorption and transport of pure gas and gas mixtures have been examined and widely reported in the liter- ature [28-361. It has been evidenced that this class of polymers is characterized by a good gas selectivity due to chain stiffness (mobility selec-

Water transport in Kapton polyimide has been described in terms of the ‘dual-sorption’ model by Yang and coworkers [ 26,271. At high temper- atures and high water activities chemical reac- tions take place, probably related to a residual amount of uncyclized polyamic acid. However, the occurrence of these reactions can be ne- glected at 25 O C and for short exposure times to water. Moreover, water diffusion is reported [ 261 to be dependent on concentration. In fact, according to the ‘dual-mobility’ model, water diffusivity increases with concentration at low activities, due to the progressive saturation of adsorption sites, while, at high activities, pene- trant clustering depresses the diffusion coeffi- cient that consequently shows a maximum if re- ported as function of concentration. Water diffusivity in the 50 pm film has been found to be roughly three times larger than in the case of the 7.5 pm film, probably due, according to the authors [ 271, to differences in polymer mor- phology. In fact, paracrystalline aggregates are reported to exist in Kapton [ 141 and their ori- entation change with sample thickness. Thinner films ( 7.5 pm thick) are characterized by a higher chain alignment in the plane of the film and con- sequently tortuosity effects are more relevant compared to 50 pm films, causing the difference in diffusivity [ 27 1. A systematic analysis of these morphological differences as function of Kapton thickness has been recently reported [ 38 1. The degree of order and the orientation of ordered regions have been shown to vary considerably with thickness. As consequence, the gas trans- port parameters are expected to have different values depending on the film thickness.

No investigations have been reported in the literature, at authors’ knowledge, dealing with sorption and transport of oxygen trough water- saturated Kapton polyimide. The purpose of the work described herein is, thus to provide an interpretation of the sorption and transport properties of oxygen in dry and water-saturated Kapton polyimide as function of film thickness. The polyimide samples thicknesses adopted var- ied between 7.5 and 125 ,um.

G. Mensitieri et al. /Journal ofMembraneScience 89 (1994) 131-141 133

2. Theoretical background

This paper reports an investigation on sorp- tion and transport of oxygen in dry and water- saturated glassy polyimide and interprets the re- sults in terms of the ‘dual-sorption-dual-mobil- ity’ model. It seems, therefore, useful to include a short description of the theoretical aspects this model relies upon.

Sorption of gases and vapors that do not affect the polymeric structure has been described using the well known ‘dual-sorption’ model [ 2 1. Ac- cording to this model, the total amount of pene- trant sorbed by the polymer, C, can be expressed as follows:

Cgbp C=K,p+ - l+bp

where K,, is the Henry’s law constant, b is the Langmuir’s law affinity constant for the interac- tion between the gas or vapor and polymer free surface present in the frozen microvoids, and C;, represents the Langmuir adsorption capacity per unit volume of polymer.

A different mobility for the molecules ab- sorbed according to the Henry’s law and those adsorbed according to the Langmuir isotherm can be postulated. Accordingly, two diffusivity constants are introduced (‘dual-mobility’ model [ lo] ), i.e., DD and &, & being the diffusivity of dissolved molecules and DH being the diffu- sivity of adsorbed molecules.

In a permeation experiment, setting the down- stream pressure and the initial penetrant con- centration into the polymer film equal to zero, the following relationship between the gas permeability coefficient, P, and the upstream pressure (po) can be obtained:

P=k,D,( 1+ ggJ=DA”s(Pd

where:

(2)

COJO)

DA”= l POWPO) s D(pW (3)

0

d$ D

K_ Gb -K

(4)

(5)

In a gas permeation experiment an important parameter is the ‘time lag’. Paul and Koros [ lo] (adopting the method proposed by Frisch [ 391 as applied by Paul [ 401 to obtain time asymp- totic expression for the integral amount of sol- vent that has permeated the membrane) ob- tained a complex expression for the time lag, 13, as function of F, K, &, b, p. and membrane thickness (L). In the ideal case of a polymer-gas system obeying the 1st and 2nd Fick’s laws, the constant diffusion coefficient can be obtained by performing a permeation experiment in which the same initial and boundary conditions used to obtain Eq. (2) are imposed. In this case, it can be shown [ 411 that

(6)

In order to distinguish D from DA”, in the fol- lowing the diffusion coefficient obtained from Eq. (6) will be referred to as &,

The diffusion coefficient that could be deter- mined by the time-lag measurement in a per- meation experiment performed on a glassy poly- mer (Do) is a complex function of DH and &.

Nevertheless in the limiting case of bpo+O, and for any value of CH, it can be shown that DAV and Do coincide, i.e.:

D _D _ b&+&&b DD+~H Av- 9- k,+Chb = l+K (7)

For gas mixtures the permeability of one com- ponent is affected by the presence of the other due to the competition for adsorption sites on the surface of the microvoids. Koros [ 81 obtained the following expression for the concentration of component 1 in the Langmuir mode (C,, ), i.e.:

c

HI = G,b;p,

1 +b;p,

where

(8)

134 G. Mensitieri et al. /Journal ojh4embraneScience 89 (1994) 131-141

b:= b1 1 +hpz

(9)

and CL, is the Langmuir capacity of the glassy polymer for pure component 1. Eq. (8 ) was ob- tained assuming that the effective molar density of component 1 in the Langmuir mode in the presence of component 2 is equal to the effective molar density of component 1 in the Langmuir mode in the absence of component 2. The above observations led to the following practical con- sequence. When a gas diffuses in a polymer whose adsorption sites are saturated by another com- ponent. an effective reduction of solubility can be observed due to the reduction of the affinity constant (b; <b, ). Chem et al. [ 42,431 inter- preted on this basis the permeation of carbon dioxide/water and carbon dioxide/methane mixtures through Kapton@ H films. They re- ported in both cases detectable decrements of CO2 permeability. In fact, according to the ‘dual- sorption-dual-mobility’ model, permeability monotonically increases with b:

CiikD,DD,DH = (1 +bp)2’o

In the case of semicrystalline polymers, the analysis of gas transport can be still performed on the basis of the ‘dual-sorption-dual-mobility’ model. In fact, crystalline regions act towards the penetrant as an impermeable dispersed phase. Due to the presence of crystalline impervious re- gions, the diffusive path length increases. This effect can be taken into account by introducing a ‘tortuosity factor’, 7, which is greatly influenced by dimensions, size dispersion, shape and ori- entation of crystalline and paracrystalline re- gions as well as crystallinity level.

3. Experimental

3.1. Materials

02+2 H20+4 ee4 OH- (10)

The anode (CE) of the electrochemical cell is made of silver and is separated from the cathode by an insulating material. Furthermore the an- ode is short-circuited with the reference elec- trode (REF), thus acting as both counter and reference electrode. Both the anode and cathode are in contact with an aqueous 3 wt% KC1 solu- tion. Contact of the cathode with the solution is obtained through the membrane which covers the cathode. The membrane, sealed to the electro- lyte solution container, is made of Kapton polyi- mide of the desired thickness. The measure- ments were performed according to the following procedure.

Kapton polyimide films used in this investi- The current flowing through the cathode at the gation were kindly provided by DuPont de Nem- steady state, I,, is related to the permeability ours, Electronic Department, Wilmington, DE coefficient, P, through:

with thicknesses ranging from 8 to 127 ,um. The repeat unit of Kapton polyimide is:

3.2. Apparatus and methods

Two different permeation apparatuses were used in this work. These apparatuses, based on the liquid-membrane-liquid and gas-mem- brane-gas configuration. are schematically de- scribed below. Further details can be found else- where [ 441. Moreover, a high pressure gas sorption apparatus was used to detect the poly- mer oxygen sorption isotherms.

3.3. The liquid-membrane-liquid permeation apparatus

A schematic of the experimental set-up and of the oxygen electrode (WE) employed in this pa- per is shown in Fig. 1. The electrode has a golden cathode on which oxygen is reduced according to the reaction:

G. Mensitieri et al. /Journal ofMembrane Science 89 (1994) 131-141 135

(4

(b)

P, POTENTIOSTAT

A: AMMETER

R, RECORDER

E: ELECTRODE

G: G15 SUPPLIER

S : MAGNETIC STIRRER

T: THERMOSTATIC BATH

WE. WORKING ELECTRODE

CE COUNTER ELECTRODE

REF: REFERENCE ELECTRODE

Fig. 1. Schematic representation of the oxygen electrode (a), and of the experimental set-up (b ) .

I 4FPA(C,-C,)

co= L (11)

where F is the Faraday’s constant, C, and C, are the concentration at the two membrane sides. In particular, C, is the concentration of oxygen in distilled water in which the electrode is im- mersed, and C, is the concentration of oxygen in the solution at the membrane side faced to the cathode and hence the oxygen concentration in the electrolyte in contact with the cathode. In Eq. ( 11) it has been assumed that the resistances of the external layer of water and of the very thin layer of electrolyte solution confined between the membrane and the cathode are negligible com- pared to the polymer film resistance.

To determine P it is necessary, therefore. to measure the total current flowing through the cell, I, and the concentration at the cathodic inter- face, C,. This concentration, however, can be re-

duced to zero if the cathode operates under the condition of limiting diffusion current. This condition can be achieved by applying a suitable potential difference between the anode and the cathode of the electrochemical cell. As reported in the literature [ 441, the value of - 600 mV is a reliable value for the potential to apply be- tween the cathode and the anode to obtain a lim- iting diffusion regime. Higher values of E should be avoided because the current depends on the applied potential, while lower values should also be avoided due to possible secondary side reac- tions (water reduction). Thus Eq. ( 11) becomes:

Z 4FPAC,

L&o = L (12)

where IL,_ is the limiting current at steady state. To achieve the steady-state condition, the oxy- gen electrode is immersed in distilled water (well mixed by a magnetic stirrer) and argon was bub- bled through the solution to strip oxygen and other dissolved gases. Due to the argon strip- ping, the oxygen content decreases and, conse- quently, the current IL,oo also decreases with time approaching to zero according to Eq. ( 12). A typical transient is shown in Fig. 2 for a 25 ,um thick Kapton film. When no further decrease of the current is observed, oxygen is bubbled through the solution and the transient is re-

0.41”. .I . . .. I. .. .I . ,. .,

_._ 0 3000 6000 9000 12000 15000

1, s

Fig. 2. Current I, in M, versus the time t, in s, for a cathode covered with a 25 m thick Kapton film at a potential of - 600 mV versus the anode and at 25 ‘C. The results were obtained at a stirring rate of 1250 rpm.

136 G. Mensitieri et al. /Journal ofA4embraneScience 89 (1994) 131-141

corded again until a plateau in the current versus time plot is observed. This plateau indicates that a new steady state condition was attained. From the value of the current at steady state, IL+, the value of P can be obtained by means of Eq. ( 12 ) . It must be pointed out that due to the OH- pro- duced according to Eq. ( lo), the cathode must be cleaned and the electrolytic solution must be renewed after each run to avoid any further ef- fect owing to the use of unbuffered solutions.

3.4. The gas-membrane-gas permeation apparatus

Permeability tests were also performed in a gas-membrane-gas configuration (dry condi- tions) . A technique based on the detection of the pressure increase at the downstream side of the polymer film pressurized at the upstream side was used. The polymer sheet was located in a gas- tight permeation cell separating the upstream and downstream chambers. Before the permeation test, each sample was allowed to desorb moisture eventually absorbed in the material during stor- age and both chambers were degassed for several days at N 10m4 torr. Prior to each test the up- stream side was then pressurized with oxygen, while the downstream side was still at the degass- ing pressure. The value of the upstream gas pres- sure was measured by means of a pressure trans- ducer with a full scale range of 10 atm (Transinstruments BHG4240 with an accuracy of 0.04% F.S. ). A very sensitive pressure trans- ducer with a full scale range of 10 torr (M.K.S. Baratron 221A, with an accuracy of 0.5% of the reading) was used to monitor the pressure in- crease on the downstream side. On the down- stream side the pressure never exceeded 1 / 1000 of the upstream pressure during all the permea- tion tests. This was obtained through a proper adjustment of the downstream chamber volume as the film under investigation was changed. The exact value of this volume was measured by means of gas expansion techniques. The permea- tion apparatus was positioned in an air circulat- ing box, whose temperature controller was able to keep the apparatus at the set temperature with an accuracy of + 0.1 “C.

The volume (cm3 [ STP] ) of gas permeated through the polymer as function of time was evaluated from the downstream pressure values using the ideal gas law. In each experiment enough time was allowed to ensure the attain- ment of the steady-state permeation. The perme- ability was computed from the slope of the linear steady-state part of the curve representing the permeated gas volume as function of time. The ‘time lag’ was determined from the intercept of the steady-state permeation curve on the time abscissa. As already discussed, this parameter is related to the gas diffusivity.

The tests were performed at an upstream pres- sure of 1 atm and at a temperature of 25 o C. These experimental conditions are the same as those adopted in the case of experiments performed in the liquid-membrane-liquid configuration.

3.5. High pressure gas sorption apparatus

Oxygen sorption isotherms at 25 “C in dry Kapton films with different thicknesses were performed using a dual volume-pressure decay apparatus in the pressure range O-45 atm. The design of the apparatus and experimental proce- dure were similar to those described by Koros and Paul [ 45 1. The amount of sorbed oxygen was evaluated from the gas pressure decrease. In fact, the sorption of oxygen molecules into the poly- mer causes a pressure decay that is proportional to the amount of sorbed gas. The boundary con- ditions change continuously during the experi- ment until sorption equilibrium was attained. The apparatus was immersed in a water bath having a temperature control accuracy of t 0.05 “C.

4. Results and discussion

Sorption and transport of water through Kap- ton polyimide is well documented in the litera- ture [ 24-271. Less documented is the oxygen sorption and transport through polyimide espe- cially in presence of water. As already pointed out, polymer morphology as well as the presence of a second interactive low molecular species can

G. Mensitieri et al. /Journal ofMembraneScience 89 (1994) 131-141 137

remarkably affect the permeation of gas through glassy polymer films. In the following an inter- pretation of the changes in the sorption and transport of oxygen in dry and water saturated Kapton films of different thickness will be presented.

According to the data reported by Sroog in his comprehensive review on polyimides [ 38 1, the crystallinity level in Kapton HN polyimide in- creases with the thickness. In addition, the crys- tal orientation changes also with the thickness resulting in a more parallel orientation to the sample surface for thinner samples. On the base of these observations it cannot be established if the tortuosity z increases or decreases with thick- ness. However, Yang et al. [ 271 found an in- crease in the water diffusion coefficient with the thickness and attributed this effect to a decrease of the ‘tortuosity factor’. Another factor that in- fluences oxygen mobility in Kapton is the amount of excess free volume. As will be shown in the following, oxygen sorption tests performed at different pressures evidenced an increase in mi- crovoids content with thickness.

In Fig. 3 are reported the permeability data as function of the sample thickness. Oxygen perme- ability showed a marked dependence on the film thickness both in the dry and water saturated polymer films. In particular a sharp variation of the permeability value was observed in the 8-50 pm thickness range. This effect is likely to be re- lated to the different morphologies of the Kap- ton films with different thicknesses.

0 20 40 60 80 100 120 140

Thickness [Fm]

Fig. 3. Oxygen permeability versus Kapton thickness for pure oxygen ( 0 ) and for water dissolved oxygen (0 ). Lines do not represent any model but are drawn only to assist reading.

Oxygen permeability in dry conditions was de- tected to be substantially greater than the perme- ability measured in the case of the water-satu- rated polymer. This effect is related to the reported ‘antiplasticizing’ effect of water mole- cules that reduces the oxygen permeability due to partially expelling of oxygen gas from the Langmuir adsorption sites. In fact, as discussed in the introduction, the presence of water de- presses the value of b causing a decrease of oxy- gen permeability. The presence of water has been reported to affect not only the solubility but also the mobility of a second penetrant as reported in the case of carbon dioxide permeation through Kapton [42]. However, this further contribu- tion to permeability reduction can be neglected in the case of a less interacting penetrant as oxygen.

The shape of permeability versus thickness curves can be analyzed on the base of solubility and diffusion coefficients. In the following the sorption and transport of oxygen in the gas- membrane-gas and in the liquid-membrane- liquid configuration, will be examined separately.

4. I. Gas-membrane-gas configuration

Transport of oxygen through Kapton in a gas- membrane-gas configuration was interpreted in terms of oxygen sorption capacity and of an av- erage diffusion coefficient, DA”. To evaluate ox- ygen solubility as function of pressure [S,,(p) 1, the gas sorption isotherms for the different sam- ples were determined. DAV was subsequently evaluated from Eq. (2) as the ratio P/S,,, where P and S,,, were both measured at 1 atm. In Fig. 4 are reported the isotherms determined for samples with thicknesses in the 13- 125 ,um range showing that oxygen solubility increases with thickness. Since the chemical structure of the samples is identical and the crystallinity level increases with thickness, this increment can only be attributed to an increase of the ‘excess free volume’. In fact b should not change with thick- ness since it depends only upon polymer-gas in- teractions while KD should decrease due to the increase in crystallinity. Hence the gas solubility

138 G. Mensitieriet al. /JournalofMembrane.Scrence89(1994) 131-141

0 v I 1 I I I

0 IO 20 30 30 so Pressure (atm)

1.0 IOU - 0 20 40 60 x0 IO0 I20 140

Thickne\\ / pm]

Fig. 4. Oxygen sorption isotherms at 25°C in Kapton sam- Fig. 6. Pure oxygen average diffusivity (DA,,) versus thick- ples with different thicknesses. Lines do not represent any ness of Kapton samples at 25°C. Lines do not represent any model but are drawn only to assist reading. model but are drawn only to assist reading.

t E 0.32 3 -5 0.28 i z 0.24

0.2

r

0 20 40 60 80 IO0 120 140

Thickness [urn]

Fig. 5. Pure oxygen solubility versus thrckness of Kapton samples at 1 atm and 25’ C. Lines do not represent any model but are drawn only to assist reading.

increment is likely to be ascribed to an increase of CL. Although a quantitative determination of these parameters was attempted, the confidence interval of the determined values was too large (due to the unavailability of data at pressures higher than 45 atm) to let us confirm this hypothesis.

Sorption data were used to estimate the aver- age diffusivity from the permeability measure- ments. In Figs. 5 and 6 are reported the solubility at 1 atm, S,o,( 1 ), and DAv as function of the sample thickness, respectively. As can be seen from these figures, a sharp increase of both S,,,( 1) and DAv was observed at thicknesses in the range 20-40 ,um. This behavior can be eluci- dated on the basis of the following morphologi- cal considerations. In fact, as reported above, the solubility coefficient (S,,,) increases by in-

creasing film thickness because the excess free volume effect overwhelms the effect of increas- ing crystallinity level. The increase of excess free volume, according to the ‘dual-sorption-dual- mobility’ model, causes a depression of Dav if D,< DD. In fact, in this case ( aDA,/ ach )KD,l,,DD.DH ~0, as can be inferred from the following relationship:

(13)

An analogous effect should be related to the increase of crystallinity that brings about a depression of D,, related to an increase of the diffusion path tortuosity. Nevertheless, despite the increase of both excess free volume and crys- tallinity level, the average diffusivity coefficient increases with thickness. This can be ascribed to a decrease of the tortuosity factor related to the crystal orientation that counteracts the effect of increased crystallinity and Langmuir capacity values.

The sigmoidal shape detected for both D, and S TOT as a function of the sample thickness is ob- viously reflected by a similar behavior of oxygen permeability (reported in Fig. 3) which is ac- tually the product of these two quantities.

4.2. Liquid-membrane-liquid configuration

A similar analysis of the permeation process was performed in the case of the water-saturated

G. Mensitieri et al. /Journal ofMembrane Science 89 (1994) 131-141 139

polymer film. Also in this case the oxygen permeability was split in two contributions, de- termining both diffusivity and solubility. In this case the diffusivity was evaluated by means of the ‘time-lag’ method (De) and the sorption ca- pacity was consequently obtained from Eq. (2 ) as the ratio P/De. Even if this method of evalua- tion of the transport parameters is generally in- correct for glassy polymers, it provides an esti- mate of the average diffusion and solubility coefficients. In fact, in the experimental condi- tions used in this investigation the effective ox- ygen affinity constant is considerably depressed by the presence of adsorbed water. This result follows from the data of Yang et al. [ 261 which evaluated the affinity constant of water in Kap- ton at three different temperatures. Based on these findings, a value of bH20 at 25°C equal to 225 1 atm --I was obtained. The value of bo2, de- termined from O2 sorption isotherms is of the order of 10-l atm- ‘. As a consequence, the cor- responding value for b& is of the order of 10 p-5 atm- ’ and the value of b&p,, is << 1 forp,, = 1 atm. Referring to the arguments reported in the introduction, it can be hence reasonably as- sumed that De= DA".

The average diffusivities and solubilities are reported in Figs. 7 and 8. As can be noted (Fig. 7) the dependence of diffusivity on sample thickness is qualitatively similar to that reported in the case of the tests performed in the dry state even if the stepwise increase of the diffusivity is

5.0 IO-’

4.0 I o-9 1

3 3.0 10-9

a” 2.0 IO-’

ii

1.0 IO-’ ,1&y ,,,,,.,,,,,,,,,, 1 ,,,,,,

0 20 40 60 80 100 120 140

Thickness [pm]

Fig. 7. Average diffusivity DLh in cm2 s-‘, of oxygen through water-saturated Kapton polyimide versus film thickness at 25°C. Lines do not represent any model hut are drawn only to assist reading.

0.001 ‘. . ’ s ’ . ’ . ’ ‘. ’ ’ 0 20 40 60 80 100 120 140

Thickness [pm]

Fig. 8. Oxygen solubility in water-saturated Kapton polyi- mide versus film thickness at 25°C. Lines do not represent any model but are drawn only to assist reading.

much higher. In fact, due to the reduced value of oxygen-polymer affinity, the increase of excess free volume does no counteract the tortuosity depression as the thickness increases. The reduc- tion of tortuosity is, therefore, the dominating effect that determines the higher diffusivity in- crement with thickness compared to the case of dry oxygen.

In contrast to the data reported in dry condi- tions the oxygen solubility (Fig. 8) shows a marked decrease as a function of the thickness. Since the Langmuir contribution to oxygen sorp- tion is depressed by the presence of water, the only effect to be taken into account is the in- crease of the crystallinity level with thickness that consequently brings about a reduction of sorp- tion capacity.

Based on these observations it can be con- cluded that the increase of permeability with thickness reported in Fig. 3 in the case of water- saturated polymer is determined by the circum- stance that diffusivity increment prevails on sol- ubility depression

5. Conclusions

Both in the case of dry and water-saturated polymer films an increase of oxygen permeabil- ity through Kapton was observed as a function of the thickness. In the dry tests both solubility and average diffusivity were detected to increase with thickness, while in the tests performed with water-saturated Kapton the increase of diffusiv-

140 G. Mensitieri et al. /Journal ofMembrane Science 89 (1994) 131-141

ity was coupled with a decrease in solubility. These behaviors have been explained in terms of: (i ) different morphologies characterizing the different samples, and (ii) of competition of water and oxygen for sorption on the Langmuir sites in the case of water-saturated polymer films. The increase of the amount of ordered regions of crystallinity with thickness reduces the sorption capacity, while the different molecular chains orientation enhances the tortuosity as thickness decreases and leads to a depression of the value of the diffusion coefficient. Finally, the excess free volume, that increases with thickness, leads to an increase of the amount of adsorbed oxygen. All these three factors affect the oxygen permea- bility properties of Kapton polyimide in the dry state. The effect of excess free volume is much less pronounced in the experiments performed in the water-saturated state where only crystallinity level and tortuosity come into play.

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[ 1OlD.R. Paul and W.J. Koros, Effect of partial immobiliz- ing sorption on permeability and the diffusion time lag, J. Polym. Sci., Polym. Phys. Ed., 14 (1976) 675.

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[ 12lC.H. Klute, Diffusion of small molecules in semicrys- talline polymers: water in polyethylene, J. Appl. Polym. Sci., 10 (1959) 340.

[ 13 ] A. Peterlin, Dependence of diffusive transport on mor- phology of crystalline polymers, J. Macromol. Sci. Phys., Bll (1975) 57.

[ 141s. Isoda, H. Shimada, M. Kochi and H. Kambe, Molec- ular aggregation of solid aromatic polymers. I. Small- angle X-ray scattering from aromatic polyimide film, J. Polym. Sci., Polym. Phys. Ed., 19 ( 1981) 1293.

[ 15 ] K.L. Mittal (Ed. ), Polyimides, Synthesis, Characteri- zation and Applications, Vol. 1 and 2, Plenum Press, New York, 1984.

6. Acknowledgements [ 16lR.B. Comizzoli, R.P. Frankenthal, P.C. Milner and J.D.

Sinclair, Corrosion of electronic materials and devices, Science, 234 (1986) 340.

The financial support of CNR (Progetto Fin- alizzato Chimica Fine e Secondaria) is gratefully acknowledged.

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