Relation between Gas Permeabilities and Structure of Polyimides

Chapter 14

Relation between Gas Permeabilities and Structure of Polyimides Yusei Hirayama, Toshimune Yoshinaga, Shunsuke Nakanishi, and Yoshihiro Kusuki

Downloaded by UNIV OF GUELPH LIBRARY on June 15, 2012 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch014

Polymer Laboratory (Chiba), U B E Industries, Ltd., 8-1 Goi-Minamikaigan, Ichihara, Chiba 290-0045, Japan

In this study, several polyimides were prepared and the relation between their gas permeation properties and chemical structure was determined and compared with data from previous studies. Permeability coefficients and diffusion coefficients, D, of the polyimides were obtained by a time-lag method for He, CO , O , N , and CH . The correlation of log(D) vs. reciprocal fractional free volume was poor due to the polyimides having substituents, such as halogens, in the diamine unit. Better linear relations were observed for plots of log(D) vs. storage modulus and cohesive energy density for these polyimides. These results suggest that gas diffusivity is influenced by mobility of polymer chains due to intermolecular interaction. Gas molecules in the glassy polymers diffuse more easily as segmental mobility increases, and estimation of gas diffusivities may be possible by the use of factors such as cohesive energy density. 2

2

2

4

Many papers have been published regarding the gas permeation properties of polyimides(1-7). Polyimides are very useful polymers for the analysis of relations between gas permeation properties and polymer structure, because many different structures, based on diacids and diamines, can be prepared from existing compounds. We have previously reported relations between free volume or cohesive energy density and gas diffusivity(8,9). In this study, we prepared five kinds of polyimides and explored correlations between gas diffusivity and parameters such as storage modulus, cohesive energy density, and free volume. These results were compared with data from previous studies. We also discuss the merits of the use of each of these parameters to correlate gas permeability properties of different polymers. Gas diffusivity and permeabihty are generally understood to depend sensitively on free volume, which corresponds to the amount of free space, in a polymer matrix(10,11). Generally, linear correlations between iog(D) or log(P) and reciprocal free volume are observed(l,3,11-21). Gas diffusion coefficients, D , are often correlated with fractional free volume of polymers, V , as follows, f

D=A exp(-B /V ) 0

194

0

(1)

f

© 1999 American Chemical Society

In Polymer Membranes for Gas and Vapor Separation; Freeman, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

195 where Arj and BQ are adjustable parameters(20). The fractional free volume was calculated as follows: Vf=(V-V )/V

(2)

0

where V is the specific molar volume at the temperature of the diffusivity measurements, and VQ is the volume occupied by the repeat unit of the polymer. VQ was estimated as 1 . 3 V , where V is the van der Waals volume of the repeat unit estimated by Bondi's method(22). The cohesive energy density, C E D , of each polymer was calculated by the group contribution method of Fedors(23). The C E D is expressed by Equation 3:


W

CED=AE/V

w

(3)

C

where Δ Ε is the hypothetical evaporative energy and V is the molar volume of the polymer(23). c

Experimental Materials. The polyimides were prepared from tetracarboxylic dianhydrides (such as B P D A and 6FDA) and various diamines as shown in Figure 1. Polymerization and film preparation protocol have been previously described(S). The polyimides were prepared from B P D A or 6 F D A and the diamine by polycondensation in p-chlorophenol. This 10-20 w t % polyimide solution was cast onto a clean glass plate and the p-chlorophenol was evaporated at 100 °C in a clean oven. The film on the glass plate was annealed at 300 °C, and after being peeled off, dried at 80-100 °C for 5 more hours. Measurements. The details of all measurements were described in previous papers(8,9). The permeation properties of the polyimide films were measured by a time-lag method(27,24) for H e , CO2, O2, N2, and CH4 at a upstream pressure of about 2.5 kg/cm2 and a downstream pressure of 10"^ torr in the temperature range from 35 to 100 °C. A n apparent permeability coefficient, P , was determined from the slope of downstream pressure vs. time curves at steady state conditions. A n apparent diffusion coefficient, D , was calculated from the lag-time, θ , using the following equation, D= β 2/(6 0)

(4)

where β is the film thickness. A n apparent solubility coefficient, S, was calculated from Equation 5: S=P/D

(5)

Factoring the permeability into diffusivity and solubility terms, the permselectivity of components A and Β may be expressed using Equation 5 as: P /P =(D /DB)(S /SB) A

B

A

A


(6)


196

Ο General Structure: — Ν

Ar II

Ο Ar(Diacid)-part

BPDA:

Ο Ν—R— II

ο

3©T@C

6FDA: ) § Γ ^ @ [

R(Diamine)-part TFDM: - ^ ^ - α « - ^ -

MBHA:

802

MASN:"(θΠ "(§Γ

^ C H 2 ^ -

Figure 1. Chemical structures of polyimides


197 where D / D g is referred to as the mobility selectivity of components A a n d B , and S / S B is the solubility selectivity. Dynamic thermal mechanical properties were analyzed using an autovibron dynamic mechanical visco-elastometer. The elastic modulus, E , is given by the following equation, A

A

IB2 | '|2 |E»|2


=

E

(7)

+

where E ' and E " are the storage modulus and loss modulus, respectively. Density data were obtained at 35°C using a conventional density gradient column method. Wide angle X-ray diffraction ( W A X D ) patterns were measured using C u K a radiation having a wavelength of 1 . 5 4 Â . d-Spacing values were obtained from the W A X D patterns. R e s u l t s and D i s c u s s i o n Physical properties of the polyimide films are listed i n Tables I, II and III. Based on W A X D spectra and D S C results, the five polyimides were determined to be amorphous. Chemical structures and characteristics of the amorphous B P D A and 6 F D A polyimide films from the previous study are shown in Figure 2, and Tables I V and V , respectively(#,9). In Table V , we present new permeability and diffusivity data at 80 °C of the polyimides reported in our previous study. Table I. Characteristics of polyimide films T

No. 101 102 103 104 105

Polyimide BPDA-TFDM BPDA-TCDM BPDA-MBHA BPDA-MASN 6FDA-DABA

g

307 304

-

283 309

d-Spacing

d g/cm 1.42 1.45 1.38 1.42 1.51

3

A 5.8 5.8 5.1 5.5 5.3

Vf

E'

CED

0.135 0.135 0.099 0.111 0.153

GPa 2.1 2.5 4.4 3.7 2.3

J/cm 1090 1100 1390 1050 850

3

Except for B P D A - M A S N , the polyimides in this study had side chains containing polar substituents. The order of glass transition temperatures, T g , was: 6FDA-DABA=BPDA-TFDM>BPDA-TCDM>BPDA-MASN. No peaks which could be ascribed to a glass transition were observed in the DSC thermogram of B P D A - M B H A . From W A X D , the order of d-spacing was: B PDA-TCDM> Β PDA-TFDM= Β PDA- M A S N>6FDA- D A B A> Β PDA- M B H A . Based on the permeation studies, the order of the permeability and diffusion coefficients was:


198 Table II. Permeability coefficients and permselectivities of polyimide films

No.

Polyimide

101 102 103 104 105

BPDA-TFDM BPDA-TCDM BPDA-MBHA BPDA-MASN 6FDA-DABA 10

P a t 3 5 ° C , Barrer He CO2 19.9 10.3 4.6 4.4 31

3

5.7 1.25 0.22 0.173 3.4

Permselectivity at 35 °C θ2 1.53 0.37 10.059 0.51 1.01

CO2/CH4

O2/N2

25 40

5.4 8.0 9.0 11 8.0

60 63

2


1 Barrer is 1 0 " cm (STP) · cm/(cm ·s-cmHg)

No.

Polyimide

101 102 103 104 105

BPDA-TFDM BPDA-TCDM BPDA-MBHA BPDA-MASN 6FDA-DABA

P a t 8 0 °C, Barrer He CO2 35 18.2 9.2 8.2 51

9.8 1.99 0.56 0.37 6.3

Permselectivity at 80 °C 02 3.0 0.74 '0.187 10.139 1.99

CO2/CH4

O2/N2

16 22 29 33 32

4.2 6.0 6.5 7.6 5.8

Table III. Diffusion coefficients and mobility ιselectivities of polyimide films 1 0

No.

Polyimide

101 102 103 104 105


No.

Polyimide

101 102 103 104 105


2

Dat35BPDA-MBHA>BPDA-MASN. A similar order was observed in the storage modulus and C E D . Gas solubility coefficients of the five polyimide films were essentially independent of polymer structure and similar to those of the polyimide films in our previous study(8). B P D A - M B H A had smaller values of V f and E ' and larger diffusion coefficients than B P D A - H A B . This result suggests that introducing -CH2-


199 Table IV. Characteristics of polyimides in previous studies(8,9)


No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 31 32 33 34 35 36 37 38

d-Spacing, Â BPDA- D A D M DADS PASN HHP MFA MBA MCA BAPE BAPS BAPP HFBAPP MDT CDM MDX HAB TSN 6FDA- DADE DADM MDT MDX CDM TSN BAPE HAB

5.8 5.2 5.5 6.2 5.8 3.8 6.1 4.9 5.2 5.4 6.0 5.5 5.9 6.2 5.3 5.8 5.8 5.8 5.9 6.2 5.8 5.8 5.5 5.9

Vf 0.110 0.117 0.117 0.163 0.163 0.161 0.164 0.151 0.135 0.142 0.162 0.173 0.176 0.175 0.145 0.118 0.173 0.155 0.168 0.164 0.182 0.146 0.170 0.182

E \ GPa

CED,J/cm

2.3 2.8 3.5 2.9 3.1 4.2 3.0 3.2 2.7 2.2 3.5 2.6 2.6 5.7 4.6 2.5 2.1 2.7 2.2 3.4 3.1 2.2 4.0

3

990 1050 1050 740 1040 1030 1050 880 900 840 850 910 1050 850 1480 1060 740 720 680 650 770 770 700 980

E' isreportedatSS'C.

between the two phenylene rings in the diamine component increased chain flexibility and, in turn, increased diffusion coefficients. Gas diffusivities in B P D A - T C D M , which contains four bulky chloride substituents on two phenylene rings, were larger than those in the dichloro substituted analogs, Β P D A - C D M and B P D A - M C A . Similarly, gas diffusivities in a tetrafluoro substituted polyimide, B P D A - T F D M , were larger than those of its disubstituted analog, B P D A - M F A . These tetrasubstituted polyimides had smaller values of V f and E ' at 35 °C than their disubstituted analogs. The calculated C H ) values of the polyimides containing four bulky and polar substituents (i.e. B P D A - T F D M and B P D A - T C D M ) increase little compared with those of the disubstituted analogs, B P D A - M F A and B P D A M C A , respectively. In tetramethyl and dimethyl substituted polymers, B P D A - M D X and B P D A - M D T , respectively, gas diffusion coefficients in the tetrasubstituted



1.00 0.77 1.75 16.8 0.54 0.32 0.34 1.25 1.85 2.8 7.3 1.41 0.98 22 0.031 2.7 13.8 15.1 8.8 44 6.8 56 7.9 4.9

4.6 3.7 7.0 34 4.3 3.6 3.7 4.1 6.0 7.5 18.3 8.9 6.9 32 1.09 12 34 33 35 74 30 104 18.3 30

1 BPDA-DADM 2 DADS 3 PASN 4 HHP 5 MFA 6 MBA 7 MCA 8 BAPE 9 BAPS 10 BAPP HFBAPP 11 12 MDT 13 CDM 14 MDX 15 HAB 16 TSN 31 6FDA-DADE 32 DADM 33 MDT 34 MDX 35 CDM 36 TSN 37 BAPE HAB 38

2

C0

He

No.

35 -o-^)-so2^)^o^)CH3 0

BAPP: -^>- "^>~f " - © - ° - ^ > CH3 CF3

0

HFBAPP: - ^ > " ^ > " Ç - © ^ ° ^ @ ^ CF3

Figure 2. Diamines used in previous studies(#,9)


203


polymer, B P D A - M D X , were much larger than those of its disubstituted analog, B P D A - M D T . This result is interesting since these polymers have very similar fractional free volume values. The calculated C E D and storage modulus values of B P D A - M D X were smaller than those of B P D A - M D T . These composite results suggest that, for gas diffusion in these substituted B P D A - D A D M polyimides, effects related to increasing segmental mobility override those associated with decreasing free volume. C E D values, which are sensitive to inter molecular interaction and values of storage modulus decrease with increasing segmental mobility despite the introduction of bulky and polar substituents such as halogens and fractional free volume values for substituted B P D A - D A D M polyimides are almost the same. Relation between diffusivity and cohesive energy density. Gas diffusion through polymers has been described by various models(76). For example, Meares(25) relates the energy required to open a gap i n the polymer matrix large enough to permit a diffusion step to the energy of activation for diffusion, E ^ , as follows: 2

E = (7t/4)- σ · λ

CED

d

(8)

where σ is the mean penetrant size, and À is the length of the diffusional jump. This model presumes that as a gas molecule passes through a polymer matrix, it is hindered by polymer chains(75,25,26). These barriers to gas transport are the rate-limiting factors in penetrant diffusion. For the penetrant molecule to jump from one equilibrium position to another, a gap in the polymer matrix of at least the size of the gas molecule must be created. The bending of polymer chains to create the gap may be envisioned to scale roughly with the length of the rigid elements of the polymer backbone, (i.e. the distance between flexible bonds). The diffusion jump length, λ , w i l l be determined by the size of these elements. Here, we assumed that E and the apparent energy of activation for diffusion are equivalent. The Arrhenius behavior of D has been described as follows(20,27,24,27), d

D=D exp(-E /RT) 0

d

(9)

where R is the universal gas constant, and Τ is absolute temperature. It is known that D Q and E are related as follows, d

ln(Dtj)=aE -c d

(10)

where a is approximately 0.001/R and c is a constant which is different for glassy and rubbery polymers(27,27,28). Based on Equations 8, 9 and 10, the C E D , which is a measure of the cohesive forces inside the polymer matrix, should correlate with diffusivity. Figures 3 and 4 present correlations between diffusion coefficients, log(D), and calculated C E D . For all polyimides examined, including the polyimides in this study, good correlations between log(D) and C E D were observed for each gas in spite of differences in measurement temperature. Polyimides such as B P D A - C D M , - M F A , - M B A , - M C A , - H A B , - M B H A , 6 F D A - C D M , - H A B , etc., exhibit relatively low diffusivities even though they have bulky substituents. The mobility of segments in these polyimides, which


204

10-7


ft ο ο ο

10-8

JO CM

ΙΟ"

C02, 35 °C

6

10-7 m CM

ι*

ε υ CM

ε «10-8

10-9

10-11

CM

ê

Ο Ο ο 10-10

©

©

Ο Û 10-9

500

750 1000 1250 1500 CED, J/cm3

10-10

10-8

10-7

10-9

10-8

500

750

1000 1250 1500

CED, J/cm3

500

750 1000 1250 1500 CED, J/cm3

500

750

1000 1250 1500

CED, J/cm3

®: This stud idy o: Polyimides i n previous studiesfS,^) • : Polyimides containing polar substituents(S, 9) Figure 3. Correlation between D and CED at 3 5 ^


205


ΙΟ"

6

On,

80 °C

ο

Relation between Gas Permeabilities and Structure of Polyimides

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