Transport properties of crosslinkable polyimide blends

journal of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 136 (1997) 249-259

Transport properties of crosslinkable polyimide blends a *

M a r y E. R e z a c ' , E. T o d d S o r e n s e n a, H a s k e l l W . B e c k h a m b a School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100, USA b School of Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100, USA Received 17 May 1997; received in revised form 24 June 1997; accepted 30 June 1997

Abstract The use of polymeric membranes for separation of chemicaUy aggressive media, or at elevated temperatures, has been limited by membrane availability. While a number of polymers are both resistant to chemical dissolution and thermally stable to over 300°C, none has been shown to be simultaneously capable of the selective transport of gases at high rates. The research reported here analyzes the influence of solid-state crosslinking of polyimides to achieve this unique combination of properties. Polyimide blends consisting of an inert polymer base with a diacetylene-functionalized additive were prepared and the properties evaluated before and after crosslinking by thermal annealing. Crosslinking rendered the resultant polymers insoluble in what were previously solvents, but had no measurable influence on gas permeabilities or selectivities. The permeability/permselectivity of these new crosslinked polyimide blends are competitive with the best known polymers. Moreover, the combined ease of processing and resultant stability are unmatched.

Keywords: Polyimide blends; Crosslinking; Gas transport; Diacetylene-containing polyimides

1. Introduction Membranes for the separation of nonreactive gases at temperatures from about 0 to 100°C are readily available [1,2]. The number of commercial installations of such devices has grown rapidly in the past few decades [3]. New applications of membranes will be in the separation of chemically aggressive materials and/or at temperatures outside the current operating window [4]. To be applied successfully in these situations, a polymer must meet the following criteria: 1. resistant to chemical attack, 2. thermally stable to several hundred degrees Celsius, *Corresponding author. Fax: +1 404 894 2866; e-mail: mary.rezac @che.gatech.edu. 0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PI1 S 0 3 7 6 - 7 3 8 8 ( 9 7 ) 0 0 170- 1

3. mechanically strong in each of these environments, 4. easily processed into the asymmetric structures of high performance membranes, 5. able to selectively separate gases while providing for a high throughput of one component. Specialty polymers with outstanding stability are available (for example, Teflon, Kevlar, certain polyimides). However, the inherent stability of these materials makes them very difficult to process either from solution or from the melt [5-10]. Thus, the cost of the resultant products are high and the types of structures which can be produced are limited. Useful asymmetric gas separation membrane structures, for example, have not yet been produced from such 'stable' polymers [11].

250

M.E. Rezac et al./Journal of Membrane Science 136 (1997) 249-259

This research examines the possibility of forming polyimide blend membranes from a soluble polyimide and a crosslinkable imide oligomer. The polyimide of choice has attractive gas transport properties, is easily processed into asymmetric membranes, and has moderate thermal stability. However, it is also susceptible to chemical attack and its modulus is insufficient to allow the resultant membranes to be used over about 100°C. Solid-state crosslinking should improve stability and strength. This paper reports on the bulk properties of these polymer blends.

2. Background Polyimides have proven useful for the formation of gas-separation membranes because of their attractive combination of properties [12-14]. Polyimides have unusually high permeabilities and permselectivities [5,15,16]. Some are readily soluble in typical membrane casting solvents and high performance membranes have been prepared therefrom [ 17,18]. Certain members of the polyimide family are resistant to chemical attack and stable to temperatures above 400°C. However, no single material possesses all of these attributes. Those that are chemically resistant are difficult to process, and vice versa. Further, softening of the polymer has limited their use as membranes to moderate temperatures (less than about 100°C) [19]. Crosslinking of the polymer matrix is a convenient way to improve resistance to chemical dissolution while simultaneously enhancing the high temperature modulus of the material [20,21]. However, crosslinking has been shown to result in a significant decrease in the gas permeability [22-25]. Borgoyne and coworkers, for example, have reported on the photoinitiated crosslinking of polyimide copolymers containing 5 mol% alkene groups [22]. The degree of crosslinking is controlled by the concentration of the reactive vinyl groups. Nitrogen permeabilities following crosslinking were reduced to only 10-20% of their initial values. These dramatic reductions in permeability were associated with modest increases in oxygen/nitrogen selectivities by a factor of about 1.2-2. In the 1970s, a series of acetylene-terminated imide oligomers was introduced for aerospace and electronic applications [26]. The materials were first prepared by

Landis, Bilow, and co-workers under sponsorship of the U.S. Airforce Materials Laboratory [27-29]. These oligomers are readily soluble in a wide variety of common solvents, and undergo crosslinking upon heating without release of volatiles [30]. The crosslinked resins are thermally stable to over 400°C. Yet, because of the presence of a high concentration of reactive end groups, the crosslinked materials are brittle [26]. These limitations have been overcome by the formation of polymer blends with semi-interpenetrating networks [31 ]. St. Clair and co-workers have reported the mechanical and thermal properties of these blends [32,33]. Only very limited data relating to gas transport is available. Bos has recently reported on the ability of similar blends to transport carbon dioxide [34]. Blends of Thermid FA-700 with Matrimid ® polyimide exhibited a reduced tendency to plasticize when exposed to high pressures of carbon dioxide. Bos also suggests that the materials exhibited improved chemical resistance, although this was not the focus of the study. Crosslinking without embrittlement may also be achieved by the use of diacetylene-functionalized polymers [35]. By exposure to controlled radiation dosages, or annealing conditions, the diacetylene groups can be activated to cross-polymerize adjacent polymer chains which are correctly aligned [36]. Crosslinking can be initiated by thermal treatment, ultraviolet radiation, or with other energy sources [37]. The cross-polymerization reaction dominates for low energy doses, and occurs with no by-product formation. Thus, components produced from diacetylenecontaining materials and subsequently crosslinked do so with minimal change in bulk density. Application of higher energy doses results in more random reactions of the unsaturated constituents in addition to the wellcontrolled cross-polymerization reaction [38]. By limiting the energy doses used, and thus restricting the crosslinking to cross-polymerization, the impact on transport properties is expected to be limited. Some diacetylene-containing polyimides have been reported [39-41]. In the research reported here, we evaluate the gas transport, chemical resistance, and bulk properties of a series of polyimide blends. The blends are prepared from a standard polyimide which is known to have good transport properties but limited chemical

M.E. Rezac et al./Journal of Membrane Science 136 (1997) 249-259

resistance, and a diacetylene-functionalized imide oligomer which provides a mechanism for solid-state crosslinking. Fig. 2 represents the DSC trace of the pure 1,16FDA-DIA pentamer. DSC traces of all blend samples were similar with proportionally smaller exotherms. The magnitude of the exotherm activated by thermal annealing at 240°C was directly proportional to the content of the 1,1-6FDA-DIA in the blend.

251

linkable component in an inert base were evaluated. Descriptions of each component follow. 3.1.1.1. Nonreactive material. The polyimide from

4J-hexafluoroisopropylidenebis(phthalic anhydride) (6FDA) and 4,#-isopropylidene dianiline (IPDA) was employed as the nonreactive component in the polyimide blends. This polyimide, 6FDA-IPDA (Fig. 1), was evaluated because of its outstanding physical properties and processability [42]. Previous research has shown this material to possess a combination of high permeability and high permselectivity as demonstrated in Table 1. Further, 6FDA-IPDA is soluble in methylene chloride, can be prepared in high molecular weight, and is thermally stable to over 300°C [42]. The 6FDA-IPDA sample employed here was synthesized following the procedure of Husk et al. [45].

3. Experimental 3.1. Materials 3.1.1. Polymers

This study involved the investigation of a series of polyimide blends. Blends of 0-25 wt% of a cross-

3.1.1.2. Reactive material. The reactive component

O

was designed to be structurally similar to the nonreactive 6FDA-IPDA polyimide. Such structural similarity has been shown to increase the probability of forming miscible polymer blends. As shown in Fig. 1, the reactive material was synthesized from 6FDA, and contains aliphatic diacetylene groups and terminal acetylene groups. It was prepared by the oxidative coupling of the N,N'-dipropargyl dimide monomer synthesized from two equivalents of propargyl amine and one equivalent 6FDA. Details of the synthesis have been provided [46]. By controlling the reaction conditions, the molecular weight of this component was kept low to enhance miscibility with the high molecular weight 6FDAIPDA. End-group analysis by tH NMR spectroscopy revealed a number-average-degree of polymerization

O 6FDA - IPDA

O

H'~C..~CCH2-N ~ O

I / [ ~ N - CH2C-~C'~nH O

1,1-6FDA-DIA Fig. 1. Repeat structure of 6FDA-IPDA polyimide (top) and 1,16FDA-DIA pentamer (bottom).

Table 1 Transport properties of gases in 6FDA-IPDA homopolymer at 10 atm and 35°C Permeability (barrer)

This study Kim [42] Jaervelin [43] Pfromm [44]

Selectivity

He

CO2

N2

02 (2 atm)

CH4

O2/N2 (2 atm)

CO2/N2

He/N2

61.2 71.2 -68.0

25.0 30.0 28 .0 --

1.19 1.34 1.3 1.47

6.33 7.53 6.63 7.50

0.67 0.70 ---

5.3 5.6 5.1 5.1

21.0 22.4 21.2 --

51.4 53.1 -46.2

1 barrer = 10 - l ° cm3(STP) c m / c m 2 s cmHg.

252

M.E. Rezac et al./Journal of Membrane Science 136 (1997) 249-259

of 5. The pentamer was dried to a constant weight and stored in a dessicator prior to use. We refer to these diacetylene-containing materials as 1,1-6FDA-DIA to signify the anhydride from which they were produced (6FDA), and the number of methylene spacers (1 and 1) separating the diacetylene groups (DIA) from the imide nitrogens.

3.1.2. Chemicals 3.1.2.1. Solvents. All polymer samples were cast from methylene chloride solutions (Aldrich, 99%). The solvent was used as received. 3.1.2.2. Pressurized gases. Pressurized gases were obtained from Air Products and used as received. Gas purity is as follows: helium, 99.997%; hydrogen, 99.995%; carbon dioxide, 99.0%; oxygen, 99.8%; nitrogen 99.999%; and methane, 99.0%. 3.2. Methods 3.2.1. Sample preparation Blend samples were prepared by dissolving the weighed quantities of each component in methylene chloride. The solutions were filtered through a 0.45 I,tm filter (Whatman) into a casting ring on a leveled glass plate. Films were dried for a minimum of 24 h before removal from the casting plate. Films were further dried under atmospheric conditions to facilitate the removal of residual solvent. Since the 1,16FDA-DIA component can be thermally reacted, care was taken to dry the films at room temperature. Following 10 days drying, differential scanning calorimetry (DSC) showed an endotherm at approximately 50°C (near the boiling point of methylene chloride) and an exotherm at approximately 250°C, corresponding to the crosslinking reaction. The magnitude of the endotherm decreased with drying time until no trace of solvent was detected following 42 days of drying. Samples containing 1, 2, 5, and 10 wt% 1,1-6FDADIA were dried for a minimum of 50 days prior to analysis. The magnitude of the exotherm at 250°C did not change with drying time. A 25 wt% 1,1-6FDA-DIA sample was added late in the study. No 'original' (uncrosslinked) properties were measured for this sample. Rather, following removal from the casting plate, the sample was dried

under atmospheric conditions for 3 days and then crosslinked as the other samples. This thermal treatment also served to remove residual solvent in the case of the 25 wt% blend. DSC performed on the fully crosslinked 25 wt% sample showed no trace of solvent. The pure 6FDA-IPDA sample (0 wt% 1,1-6FDADIA) contained no thermally reactive moieties. Therefore, this sample was dried under vacuum and heat in a series of steps to a final temperature of 280°C where it was held for 2 days. The sample was tested immediately following this drying process. Sample thickness (typically 20-30 ~tm) was calculated by measuring the weight of the film, its crosssectional area, and density. Thickness was calculated as the weight of the sample divided by the product of area times density.

3.2.2. Crosslinking DSC was employed to measure the reaction exotherm of the polymer blends and the time required for full reaction. When heating at 15°C/min under nitrogen, an exotherm was observed in the 200--450°C range, as can be seen in Fig. 2. This exotherm appears to consist of at least two overlapping peaks. The first is rather sharp and has a maximum intensity at approximately 230°C. The second is much broader and flatter with a maximum intensity at approximately 340°C. At least three different types of thermally-initiated reac-

• ''

I . . . .

A

I . . . .

I

. . . .

I

. . . .

I

. . . .

I . . . .

I

' ' '

Originar

I

I

@ X v

Annealed

~Z .

.

.

.

.

.

.

.

I

. . . .

I

. . . .

I

. . . .

I . . . .

I . i

i i i

ii

I

100 150 200 250 300 350 400 450 500

Temperature (°C) Fig. 2. Differential scanning calorimetry thermogram of the pure 1,1-6FDA-DIA pentamer. Original and after annealing at 240°C for 4 h, under nitrogen, 15°C/min.

253

M.E. Rezac et al./Journal of Membrane Science 136 (1997) 249-259

tions can occur in these systems: (1) cross-polymerization of the diacetylene units, (2) a second more random reaction of the unsaturated diacetylene moieties, and (3) reaction of the ethynyl end groups [26]. The exact nature of these reactions and the resultant products are unknown, but are currently under investigation, Blend samples used in permeation and density measurements were thermally annealed under vacuum at 240°C for 4 h. In doing so, the first exothermic peak disappears and solvent resistance is imparted (see Results). The second, broad peak remains (Fig. 2) even upon further annealing. The secondary reaction associated with the exotherm in the 300--400°C range is not activated by annealing at 240°C (as employed here). Thus, use of the 'crosslinked' materials prepared here at temperatures to about 300°C should be possible without changes in degree of crosslinking. At temperatures above 300°C, further reaction is possible which may influence the properties of the polymers. 3.2.3. Gas transport measurement

Permeation measurements were made using a constant-volume/variable-pressure apparatus [47] at 35°C. Each gas was evaluated at 1.5, 2,5, 6 and 10 atm upstream pressure with a permeate pressure of less than 10 torr. The error of the permeability coefficient associated with the system was determined to be ±6% for the slowest gas. The largest source of error was the measurement of the sample thickness (described above). While thickness directly impacts the measured permeability, it does not influence the selectivities reported here. 3.2.4. Density measurement

The bulk density of the polymer films were measured by flotation in a density gradient column at 23 +0.1°C. Aqueous calcium nitrate solutions of varying density were employed. The column was calibrated using glass beads of known density (Amer-

ican Density Materials, Inc.). Film samples were allowed to equilibrate in the column for a minimum of 48 h prior to a reading.

4. Results 4.1. Sample appearance

Following drying, all samples were optically clear and remained so throughout the experimentation (including crosslinking). Sample color ranged from pale lemon to burnt orange (0-25 wt% 1,1-6FDA-DIA) with darker hues at the higher 1,1-6FDA-DIA concentrations. Following crosslinking, the films darkened. Qualitatively, the magnitude of the color change was directly related to the 1,1-6FDA-DIA content. 4.2. Bulk density

Bulk density, as measured by flotation, is reported in Table 2 for each sample both before and after crosslinking. The density of the bulk polymer samples increases with increasing 1,1-6FDA-DIA content in the blend. This slight increase will later be correlated with the occupied volume of the 1,1-6FDA-DIA component. Following thermal annealing to induce crosslinking, each sample exhibited an increase in density of less than 1% of the total value. The measured density correlates well with blend composition. This density increase suggests that slight volume contraction occurs upon crosslinking. The density of the pure 1,1-6FDA-DIA pentamer was not evaluated due to the inability to prepare a film sample. However, the extrapolated densities for the pure 1,1-6FDA-DIA are 1.471 and 1.493 g / c m 3 for the original and crosslinked samples, respectively. The correlation coefficients (R2's) for these data are 0.974 and 0.998 for the original and crosslinked samples.

Table 2 Density of 1,1-6FDA-DIA/6FDA-IPDApolymerblends before and after crosslinking a 1,1-6FDA-DIAin blend

0 wt%

1 wt%

2 wt%

5 wt%

10 wt%

25 wt%

Initial density (g/cm3) After crosslinking (g/cm3)

1.355 1.356

-1.357

1.355 1~360

1.359 1.362

1.360 1.369

1.382 1.388

a Error of each value -4-0.001g/cm3

M.E. Rezac et al./Journal of Membrane Science 136 (1997) 249-259

254 100

. . , ,..,...He(closed)

_- _-

,

H2(open)

_

100

..........

i .........

i,

100 ~

CO2

10

10 ~

g

02

a--a

a

a

=

•

=N2

e

a CH 4

=

,,I

0

, , , I , ,

2

, I , , , I

, , , I , ,

-

~

I 1

)riginal 5 wt% 1, I-6FDA-D1A 0. l

(open)'

~

CO2

~

°2

N

2 CH4

0 1 I'O'r'ig'in"al(Blends ,I

4 6 8 10 Pressure (atm)

0

12

0

0

0.1

0.2

0.3

Volume fraction 1, I-6FDA-DIA in Blend

Fig. 3. Gas permeability as a function of feed pressure for a 5 wt% 1,1-6FDA-DIA/95 wt% 6FDA-IPDA blend prior to crosslinking, T = 35°C (1 batter = 10-1° cm3(STP)cm/crn2 s cmHg).

Fig. 4. Gas permeability as a function of volume fraction 1,16FDA-DIA in blend prior to crosslinking (left) and following crosslinking (right), Feed pressure = 10 atm, T = 35°C (1 batter = l0 -10 cm 3(STP) cm/cm 2 s cmHg).

4.3. Initial polymer permeation properties The permeability of each polymer sample was measured as a function of upstream pressure. Fig. 3 presents representative results for the 5 wt% 1,16FDA-DIA/95 wt% 6FDA-IPDA sample. The permeability of all gases is either independent of applied pressure (helium, hydrogen, oxygen, nitrogen) or decreased with increasing feed pressure (carbon dioxide, methane). This behavior provides a clear indication that the properties measured are those of a nonporous solid [48]. The behavior of all other films was similar. Blend theory suggests that the logarithm of permeability should correlate with the volume fraction of component one in the blend [49]. The permeability results for both the original and crosslinked films are presented graphically in this form. To achieve this, the measured weight fractions were converted to volume fractions using: ~6FDA-IPDA in blend z ~Z~6FDA_IPDAin blend )<

R6FDA-IPDA Pblend

(1) where P6FDA-IPDAis the density of the pure 6FDAIPDA homopolymer,/gblend is the density of the polyimide blend, and 06FDA-IPOAin blend and ~ 6 F D A - I P D A in blend are the calculated volume fraction and measured weight fraction of 6FDA-IPDA in the blend, respectively. The volume fraction of 1,1-6FDA-DIA in the blend was calculated from: 41,1-6FDA-DIA in blend •

1-

~6FDA-IPDA in blend

(2)

Fig. 4 shows permeabilities at 10 atm upstream pressure and 35°C as a function of the calculated

volume fraction of 1,1-6FDA-DIA in the uncrosslinked polymer blend. Gas permeabilities decreased as the content of 1,1-6FDA-DIA in the blend increased. Nitrogen permeability, for example, decreased by 50% as the 1,1-6FDA-DIA content increased from 0-10 wt%. The transport properties measured for the pure 6FDA-IPDA homopolymer are in acceptable agreement with those reported previously, as is evident in Table 1. Both the polymer synthesis and the film preparation and drying technique employed in this study were identical to those previously employed for this polyimide [42].

4.4. Influence of crosslinking The measured permeabilities before and after crosslinking are nearly identical as is evident in Fig. 4. This result is in stark contrast to the influence of crosslinking on transport previously reported for other polyimides [22-25]. Permeabilities of those crosslinked polyimides decreased by as much as a factor of twenty. Here, essentially no change in the transport properties were observed. This fact is of great importance since now a gas-separation membrane with simultaneously outstanding transport properties and chemical resistance can be obtained, while maintaining facile processing.

4.5. Chemical resistance The resistance of the crosslinked polyimide blends to a number of chemicals was evaluated. The ability of

M.E. Rezac et al./Journal of Membrane Science 136 (1997) 249-259 Table 3 Resistance of polyimide blend samples to attack by methylene chloride at 25°C. Evaluation after 9 months of exposure unless otherwise noted wt% 1,1-6FDA-DIA in blend

Response to CH2C12 exposure

0 1 2 5 10, 20, 25, 35, 50, 75, 100

Rapid dissolution Dissolves within 2 min Dissolves within 10 min Sample swells Insoluble, no visible swelling

methylene chloride, acetone, and N-methyl pyrrolidone to solvate the crosslinked polymers was measured. Prior to crosslinking, placement of a bulk sample with any blend composition in methylene chloride resulted in complete and nearly instantaneous dissolution. Following crosslinking, dissolution was either not possible, or required extended time to be achieved. The results of this analysis are summarized in Table 3. Several additional samples that were not analyzed for gas transport have been included here. The stability of the 10 wt% 1,1-6FDADIA/90 wt% 6FDA-IPDA polymer blend in liquid acetone was also evaluated. Acetone was selected because of its ability to swell or dissolve many polymers and its importance in many chemical processes. The crosslinked 10 wt% 1,1-6FDA-DIA blend swelled slightly, but did not dissolve even after 6 months exposure in excess acetone. Finally, the stability of a few of these crosslinked blends was measured in hot N-methyl pyrrolidone (NMP). The soluble mass fraction in the presence of NMP was measured as follows: 1. approximately 0.15 g of crosslinked polymer film was placed in 20 ml of NMP at room temperature and stirred for 2 h, 2. the samples were heated to approximately 80°C and stirred for 1 h, 3. the samples were cooled and filtered through a 0.45 ~tm Teflon filter, 4. the residue on the filter was washed with NMP and then acetone, 5. the filter and any residue were vacuum dried at 100°C for 24 h (to a constant weight), 6. the residue was weighed and defined as the 'insoluble mass.'

255

Table 4 Resistance of polyimide blend samples to attack by N-methyl pyrrolidone. Evaluation as detailed in text wt% 1,1-6FDA-DIA in blend

wt% mass retained as insoluble

5 10 25

5.4 74.0 97.1

Results from this evaluation are presented in Table 4. Comparison of Tables 3 and 4 indicates that the results are directionally consistent: The crosslinked blends are not soluble in either hot NMP nor methylene chloride. Exposure to hot NMP resulted in nearly 95% mass loss for the blend containing 5 wt% 1,16FDA-DIA. In contrast, the 25 wt% crosslinked sample had essentially no mass loss following exposure to hot NMP. Yet, the material retains flexibility and outstanding transport character.

5. Discussion

5.1. Comparison to other crosslinked polymer systems Crosslinking of polymer systems has been shown to decrease gas permeability with simultaneous increases in permselectivity [22-25]. This is believed to result from a reduction in the mobility of the polymer caused by formation of interchain covalent bonds. It is difficult to directly compare the influence of crosslinking on permeability for the different crosslinkable systems because the extent of crosslinking is unknown. However, an estimate of the maximum degree of crosslinking can be made by assuming complete conversion of all crosslinkable units. For the diacetylene-functionalized polyimide blends reported here, the crosslinkable 1,1-6FDA-DIA components contain two unsaturated bonds in each repeat unit. Thus, for a polymer blend which undergoes complete conversion of every unsaturated linkage, the maximum crosslink density is two times the mol% of crosslinkable repeat units. For the blend containing 1 wt% 1,1-6FDA-DIA, the maximum number of crosslinks per average repeat unit is 0.02. These values are tabulated in Table 5 for all blend compositions. Also presented in Table 5 are the maximum crosslink densities for other crosslink-

M.E. Rezac et al./Journal of Membrane Science 136 (1997) 249-259

256

e~

o

0

o

¢)

o e~

o o

¢.

M.E. Rezac

et al./Journal

of Membrane

able systems reported in the literature, assuming complete conversion of the crosslinkable component. The maximum degree of crosslinking for the diacetylene-containing blends evaluated here is equal to or greater than that of other crosslinkable systems reported in the literature as is seen in Table 5. These values may greatly overstate the number of crosslinks actually present, and do not distinguish crosslinks which effectively restrict molecular mobility from those that have no influence. Indeed, the efficiency of crosslinking using a benzophenone moiety is very low. Lin and co-workers [21] have shown that in solid polymers only 45% of the benzophenone units are located at reactive sites and that the efficiency of crosslink formation is of the order of 0.03. Thus, it is perhaps even more interesting that these systems exhibit such drops in permeabilities upon crosslinking (Table 5), even though the crosslinking is inefficient. The diacetylene-containing polyimide blends contain comparatively higher concentrations of crosslinkable moieties, and yet the permeabilities are unaffected by crosslinking. 5.2. Comparison to other polymer systems The polyimide blends evaluated here exhibit an attractive combination of gas permeabilities and permselectivities. To place these materials in context to others available, the permeabilities (as exemplified by the 5 wt% blend) were compared to the 'upper bound' materials documented by Robeson [50]. We anticipate these polyimide blends may find application in the recovery of hydrogen from hydrocarbon streams at high temperatures. Therefore, Fig. 5 presents the ideal hydrogen/methane selectivity versus hydrogen permeability for a variety of polymers. The small crosses represent materials with the best combination of transport properties from Robeson's 1991 review. The properties of the 5 wt% blend analyzed here are competitive with those reported by Robeson. Further, the polyimides evaluated here can be easily processed from solution and then made chemically resistant through the solid-state crosslinking reaction. The other 'upper bound' materials include poly(methyl methacrylate) (PMMA), poly[1-(trimethyl silyl) propyne] (PTMSP), and several polyimides [50]. PTMSP has been plagued with degradation of transport properties caused by physical aging and by

Science

257

136 (1997) 249-259

1000

100

10 • CrosslinkedBlend l~ Original Blend ....... , ........ , ........ i

1

......

10 100 1000 10000 Hydrogen Permeability (barrer) Fig. 5. Correlation of hydrogen/methane selectivity with hydrogen permeability. The small cross represents the polymers with the best combination of properties as reported by Robeson [50]. The line is the 'upper bound' proposed by Robeson. The blend data reported is for the 5 w t % 1 , 1 - 6 F D A - D I A / 9 5 w t % 6FDA-IPDA. Feed pressure = 10atm, T=35°C (1 barter= 10 -1° cm3(STP) cm/ cm 2 s cmHg).

its susceptibility to chemical attack by oxygen at elevated temperatures. PMMA has found numerous commercial applications outside of the membrane arena, but has a glass transition temperature of only about 100°C and therefore is of limited utility for hightemperature applications. The polyimides included in Robeson's 'upper bound' materials were all prepared as polyamic acids and thermally imidized in the solid state [ 15]. Imidization occurs with the release of water and an increase in the polymer density. Therefore, preparation of asymmetric membranes with thin, defect-free separating layers may be complicated by significant shrinkage that commonly occurs during the imidization step. 5.3. Blend theory A number of theories have been utilized in the characterization of the influence of blend composition on gas permeability. The simplest of these is an empirical model used to describe transport in both homogenous and heterogeneous binary blends and copolymers [49]: lnPblend =

(~1

lnP1 + ~b2 l n P 2

(3)

where PBlend is the gas permeability of the blend, q51 and ~b2are the volume fractions of polymers 1 and 2 in the blend, and Pt and P2 are the permeabilities of homopolymers 1 and 2. Maeda and Paul [51] have

258

M.E. Rezac et al./Journal of Membrane Science 136 (1997) 249-259

demonstrated the ability to extend this theory to blends of widely varying molecular weight. They demonstrated the excellent fit of this model for blends of polymer with low molecular weight antiplasticizers. The molecular weights of the antiplasticizers studied by Maeda and Paul are of the same magnitude as the 1,1-6FDA-DIA pentamer utilized here. This model fits our data well, as is demonstrated by the solid lines in Fig. 4. Thermodynamic considerations indicate that two compounds will form miscible blends if the free energy of mixing is negative [52]. While it is often difficult to accurately calculate the free energy, it is known that the tendency of additives to form miscible blends with polymers is increased as the molecular weight of the additive decreases [52]. This, along with the other circumstantial evidence reported here (optical clarity of films, linear relation between polymer content and the logarithm of permeation) suggests that these blends are probably homogeneous. Thermal analysis to check for the presence of a single glass transition is precluded by the exotherm occuring from 170 to 400°C (Fig. 2).

6. Conclusions Crosslinking a polyimide blend has resulted in a material which simultaneously has outstanding permselectivity properties and chemical resistance. No measurable decrease in permeability was observed upon crosslinking by annealing at 240°C. Yet, the chemical resistance of the crosslinked films with over 10 wt% 1,1-6FDA-DIA pentamer was dramatically improved. The transport properties are equivalent to the best materials previously reported. Polymer processing was facilitated by the ease of dissolution of both the polymer and pentamer in simple solvents, followed by solid-state thermally initiated crosslinking. Blends of a crosslinkable diacetylene-containing 6FDA-based imide pentamer with 6FDA-IPDA polyimide appear to be miscible and follow simple mixing rules both in the original and crosslinked forms.

Acknowledgements Although the research described in this article has been funded in part by the United States Environ-

mental Protection Agency under assistance agreement number R824727 to MER and HWB, it has not been subjected to the Agency's peer and administrative review and therefore may not necessarily reflect the views of the Agency and no official endorsement should be inferred. Special thanks to Dr. Tim Wang for assistance with the preparation of the polyimide and Njeri Karangu for preparation of the diacetylene-containing pentamer.

References [1] J. Haggin, New generation of membranes developed for industrial separations, Chem. Eng., June (1988) 7. [2] R.W. Spillman, Economics of gas separation membranes, Chem. Eng. Prog., 85 (1989) 41. [3] W.J. Koros and G.K. Fleming, Membrane-based gas separation, J. Membr. Sci., 83 (1993) 1. [4] J.M.S. Henis, Commercial and practical aspects of gas separation membranes, in D.R. Paul and Y.P. Yampol'skii (Eds.), Polymeric Gas Separation Membranes, CRC Press, Boca Raton, 1994. [5] T.M. Hurtz, Textile Res. J., 80 (1950) 786. [6] W.E Gorham, J. Polym. Sci., Part A-l, 4 (1966) 3027. [7] RE. Cassidy, Thermally Stable Polymers, Marcel Dekker, New York, 1980. [8] M. Haubs, F. Herold, C. Krieg, U. Meyer-Blumenroth, J. Schneider, R. Wagener and J. Wildhardt, Membranes of highperformance polymers, Makromol. Chem., Makromol. Symp., 50 (1991) 67. [9] H. Hoehn, and J.W. Richter, Aromatic polyimide, polyester and polyamide separation membranes, US Patent 3,899,309 (1975). [10] D.R.B. Walker and W.J. Koros, Transport characteristics of a polypyrrolone for gas separations, J. Membr. Sci., 55 (1991) 99. [11] B.C. Anderson, L.R. Bartron and J.W. Collette, Trends in polymer development, Adv. Tech., (1980). [12] C.E. Sroog, Polyimides, Prog. Polym. Sci., 16 (1991) 561. [13] K.L. Mittal (Ed.), Polyimides, Plenum Press, New York, 1984. [14] M.I. Bessonov, Polyimides: Thermally Stable Polymers, Plenum Press, New York, 1987. [15] S.A. Stern, Y. Mi, H. Yamamoto and A.K. St. Clair, Structure/permeability relationships of polyimide membranes. Applications to the separation of gas mixtures, J. Polym. Sci., Polym. Phys., 27 (1989) 1887. [16] M. Langsam and W.F. Burgoyne, Effects of diamine monomer structure on the gas permeability of polyimides. I. Bridged diamines, J. Polym. Sci., Polym. Chem., 31 (1993) 909. [17] I. Pinnau, and W.J. Koros, Defect-free ultrahigh flux asymmetric membranes, US Patent 4,902,422 (1990) and I.

M.E. Rezac et al./Journal of Membrane Science 136 (1997) 249-259

[18]

[19]

[20] [21]

[22]

[23]

[24]

[25] [26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

Pinnau, Skin Formation of Integral-Asymmetric Gas Separation Membranes made by Dry/Wet Phase Inversion, Ph.D. Dissertation, The University of Texas at Austin, 1991. K. Haray, K. Obata, N. Itoh, Y. Shindo, T. Hakuta and H. Yoshitome, Gas permeation and separation by an asymmetric polyimide hollow fiber membrane, J. Membr. Sci., 41 (1989) 23. M.R. Pixton and D.R. Paul, Relationships between structure and transport properties for polymers with aromatic backbones, in D,R. Paul and Y.P. Yampol'skii (Eds.), Polymeric Gas Separation Membranes, CRC Press, Boca Raton, 1994. R.J. Young and P.A. Lovell, Introduction to Polymers, Chapman and Hall Publishers, 1991. A.A. Lin, V.R. Sastri, G. Tesoro, A. Reiser and R. Eachus, On the cross-linking mechanism of benzophenone-containing polyimides, Macromolecules, 21 (1988) 1165. W.E Burgoyne, Jr., M. Langsam, M.E. Ford, and J.P. Casey, Membranes formed from unsaturated polyimides, US Patent 4,931,182 (1990). I.K. Meier, M. Langsam and H.C. Klotz, Selectivity enhancement via photooxidative surface modification on polyimide air separation membranes, J. Membr. Sci., 94 (1994) 195. I.K. Meier and M. Langsam, Photochemically Induced Oxidative Surface Modifications of Polyimide Films, J. Polym. Sci., Polym. Chem., 31 (1993) 83. R.A. Hayes, Polyimide gas separation membranes, US Patent 4,717,393 (1988). S. Alam, L.D. Kandpal and I.K. Varma, Ethynyl-terminated imide oligomers, J. M. S.-Rev. Macromol. Chem. Phys., C33(3) (1993) 291. R. H. Boschan and A. L. Landis, Development of HighTemperature Addition Cured Adhesives, Air Force Contract Report AFML-TR-74-89, July 1974. N. Bilow, R.H. Boschan and A.L. Landis, Acetylenesubstituted aromatic primary amines and the process of making them, US Patent 3,928,450 (1975). N. Bilow, A.L. Landis and L.J. Miller, Copolymer of polyimide oligomers and terephthalonitrile N,N-dioxide and their methods of preparation, US Patent 3,864,309 (1975). D.J. Capo and J.E. Schoenberg, An acetylenic-terminated fluorinated polyimide, properties and applications, SAMPE J., March/April (1987) 35. A.O. Hanky and T.L. St. Clair, Semi-2-interpenetrating polymer networks of high-temperature systems, SAMPE J., 30 (1985) 912. T.L. St. Clair, ACS Interdisciplinary symposium on recent advances in polyimides and other high-temperature polymers, Reno, NV, July 13-16, 1987. H. Zeng and K. Mai, Makromol. Chem., 187 (1986) 1787.

259

[34] A. Bos, High Pressure CO2/CH4 Separation with Glassy Polymer Membranes, Ph.D. Dissertation, University of Twente, 1996. [35] H.W. Beckham and M.E Rubner, Polymer, 32 (1991) 1821. [36] G. Wegner, Makromol. Chem., 134 (1970) 219. [37] V. Enkelmann, Adv. Polym. Sci., 63 (1984) 91. [38] H.W. Beckham and H.W. Spiess, Macromol. Chem. Phys., 195 (1994) 1471. [39] R. Giesa, M. Klapper and R.C. Schulz, Makromol. Chem., Makromol. Symp., 44 (1991) 1. [40] H.S.-I. Chao and M.A. Vallance, J. Polym. Sci.: Part A: Polym. Chem., 28 (1990) 1209. [41] G.E D'Alelio and P.A. Waitkus, US Patents 4,331,601 (1982) and 4,405,521 (1983). [42] T.H. Kim, W.J. Koros, G.R. Husk and K.C. O'Brien, Relationship between gas separation properties and chemical structure in a series of aromatic polyimides, J. Membr. Sci., 37 (1988) 45; and T.H. Kim, Ph.D. Dissertation, The University of Texas at Austin, 1987. [43] H. Jaervelin, unpublished results, The University of Texas at Austin, 1990. [44] P.H. Pfromm, Gas Transport Properties and Aging of Thin and Thick Films made from Amorphous Glassy Polymers, Ph.D. Dissertation, The University of Texas at Austin, 1994. [45] G.R. Husk, P.E. Cassidy and K.L. Gebert, Synthesis and characterization of a series of polyimides, Macromolecules, 21 (1988) 1234. [46] N.T. Karangu, M.E. Rezac and H.W. Beckham, Polym. Prepr., Am. Chem. Soc. Div. Polym. Mater., 76 (1997) 316. [47] K.C. O'Brien, W.J. Koros, T.A. Barbari and E.S. Sanders, New technique for the measurement of multicomponent gas transport through polymeric films, J. Membr. Sci., 29 (1986) 229. [48] R.M. Barrer, Diffusion in and through Solids, Cambridge University Press, 1941. [49] J.H. Petropoulos, Mechanisms and theories for sorption and diffusion of gases in polymers, in D.R. Paul and Y.P. Yampol'skii (Eds.), Polymeric Gas Separation Membranes, CRC Press, Boca Raton, 1994. [50] L. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci., 62 (1991) 165. [51 ] Y. Maeda and D.R. Paul, Effect of antiplasticization on gas sorption and transport. III. Free volume interpretation, J. Polym. Sci., Polym. Phys., 25 (1987) 1005. [52] D.R. Paul, J.W. Barlow and H. Keskkula, Mark-BikalesOverberger-Menges: Encyclopedia of Polymer Science and Engineering, Vol. 12, John Wiley and Sons, Inc., New York, 1988.