Polymer Membranes for Gas and Vapor Separation - American

Chapter 7

Effects of Physical Aging on Gas Permeability and Molecular Motion in Poly(1-trimethylsilyl-1-propyne) 1

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Kazukiyo Nagai , B. D. Freeman , Tetsuya Watanabe , and Tsutomu Nakagawa Downloaded by NORTH CAROLINA STATE UNIV on December 24, 2012 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch007

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Department of Chemical Engineering, North Carolina State University, Box 7905, Raleigh, NC 27695-7905 Department of Industrial Chemistry, Meiji University, Higashi-mita, Tama-ku, Kawasaki 214, Japan 2

The effect of physical aging on gas permeability and molecular motion of poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes synthesized using various catalysts was studied. During aging, the gas permeability of PTMSP membranes synthesized using TaCl and TaCl -Ph Bi was dramatically reduced, but the gas permeability of PTMSP synthesized using N b C l was stable. The decrease in gas permeability of PTMSP membranes synthesized using various catalysts was correlated with changes in the non-equilibrium excess free volume in the polymer matrix. In the case of PTMSP synthesized using N b C l , no change in C T and Si T (NMR spin-lattice relaxation times) was observed during aging. However, C T of the backbone chain carbons of PTMSP membranes synthesized using TaCl and TaCl -Ph Bi increased upon aging. This change probably accompanied the relaxation of non-equilibrium excess free volume of PTMSP membranes synthesized these catalysts. 5

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Polymeric membranes must have good permeabilities, permselectivities, and long-term stability. Moreover, they must be compatible with the process environment in which they will be used. However, polymers with high permeability tend to have low selectivity and vice versa (1). Poly(l-trimethylsilyl-l-propyne) (PTMSP) has the highest mtrinsic gas permeability of all known synthetic polymers. It was first synthesized by Masuda and Higashimura in the 1980's (2). However, PTMSP has the lowest selectivities of all known polymers and exhibits unstable gas permeabihty (3). PTMSP has a glass transition temperature of more than 250°C (2). In all glassy polymers, small-scale polymer segmental motions lead to relaxation of nonequihbrium excess free volume and, as a result, the physical properties of glassy polymers drift over time. Because PTMSP has more free volume than other glassy polymers, a dramatic decline in gas permeabihty occurs when the non-equihbrium excess volume in PTMSP relaxes (3,4). Membrane contamination via absorption of organics (such as pump oil vapor) also decreases gas permeabihty of PTMSP membranes (4). In the absence of such contaminants, the decrease in gas permeabiUty

© 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.

95

96 in PTMSP membranes is due to a relaxation of non-equilibrium excess free volume, a process known as physical aging (4). As the amount of excess free volume is reduced during physical aging, the constraints on molecular motion increase, and polymer segmental mobility decreases. The catalyst used for polymerization influences several properties such as the solubility of PTMSP in various solvents (7). Even though the cis-trans content of PTMSP has not been quantitatively determined, Costa et al. (5) and Izumikawa et al. (6) reported that the polymerization catalyst N b C l produced a more regular chain configuration than TaCl . In addition, PTMSP synthesized using N b C l had a cis-rich structure relative to that prepared with TaCl . In this study, the effects of physical aging on gas permeabihty and molecular motion of PTMSP membranes synthesized using various catalysts are reported. 5

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Downloaded by NORTH CAROLINA STATE UNIV on December 24, 2012 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch007

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Experimental Polymers. The PTMSP samples were previously synthesized (4,8), utilizing Masuda et α/.'s methods (2,7).

CH

CH

3

I

Catalyst

C EC H C—Si—CH 3

CH

3

• H C — Si— C H

3

3

CH

3

3

3

Poly( 1 -trimethylsilyl-1 -propyne) (PTMSP) Polymerization catalysts included tantalum (V) chloride (TaCl ), niobium (V) chloride (NbCl ), and TaCl -triphenyl bismuthine (Ph Bi) as a cocatalyst (TaCl :Ph Bi=l:l). The weight-average molecular weights ( M , g/mol) of the PTMSP samples were 8 6 x l 0 (TaCl ), 260xl0 (TaCl -Ph Bi), and 3 5 x l 0 (NbCl ). The mtrinsic 5

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w

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viscosities ([η], dLg" ) were 6.0 (TaCl ), 9.3 (TaCl -Ph Bi), and 0.7 (NbCl ), and were measured in toluene at 30°C. No thermal events were recorded in differential scanning calorimetry scans at temperatures up to 250° for all PTMSP samples. These samples were, therefore, glassy under the conditions used in this study. 5

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Membranes. PTMSP membranes with uniform thicknesses of 60 to 200 μπι were prepared by casting on a horizontal glass plate from a solution of the polymer dissolved in toluene. To prevent physical aging and contamination, all membranes were immersed in methanol until just before beginning gas permeation, gas sorption, and N M R experiments. The methanol treatment was performed at least 48 hours at room temperature in order to attain the equilibrium sorption of methanol in the PTMSP membranes (i.e., to prepare samples with the same initial condition). PTMSP membranes swell when immersed in methanol. The experiments were started after the macroscopic contraction of the swollen membrane was finished. The detailed


97 procedures before rjeginning the experiments were reported previously (8). Aged membranes were prepared by storing them in a vacuum vessel at 30°C. Gas Permeation and Sorption Measurements. The gas permeabihty was determined by the vacuum-pressure method using an M K S Baratron pressure transducer. The upstream pressure was up to 1 atmosphere. The gases used were hydrogen (H ), nitrogen (N ), oxygen (0 ), carbon dioxide (C0 ), and methane (CH4). Sorption isotherms for propane (C H ) were determined at 35°C using a gravimetric quartz spring balance operating at pressures up to 1 atmosphere. ?

2

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NMR Measurements. High resolution, solid-state N M R measurements were performed at 30°C using a JEOL J N M 400 spectrometer (9). The spin-lattice relaxation times (Tj) were obtained by the CP T, pulse sequence. The same T, measurements were performed on at least three separate membranes prepared at the same time from the same casting solution. Results and Discussion Gas Permeability. Figure 1 presents N permeabihty coefficients for as-cast and aged PTMSP membranes synthesized using various catalysts. The initial value of an as-cast membrane was measured between 24 and 48 hours after the membrane was removed from methanol. Gas permeabilities of PTMSP membranes synthesized using TaCl and TaCl Ph Bi were dramatically lower than those of PTMSP membranes synthesized using NbCl . For other gases (e.g., H , 0 , C 0 , and CH ), the same trend was observed. Figure 2 presents the relationship between kinetic diameter (10) and aging ratio of PTMSP membranes synthesized using various catalysts. The aging ratio is the permeabiHty coefficient of a membrane stored for 12 days relative to its initial permeability [P(12 days)ZP(as-cast)]. The PTMSP synthesized using N b C l had stable gas permeability coefficients, but the gas permeability of PTMSP synthesized using TaCl and TaCl -Ph Bi was reduced upon aging. With increasing kinetic diameter, the aging ratio slightly decreased. The ideal separation factors changed with age because the aging ratio of each gas was different. Table I summarizes the ideal separation factors of as-cast and aged PTMSP membranes synthesized using various catalysts. PTMSP synthesized using TaCl -Ph Bi, which exhibited the most distinct decline in gas permeabihty upon aging, showed a clear increase in separation factor after aging. However, in the case of P(0 )/P(N ), where there is a very small difference in kinetic diameter, the change in separation factor with aging was small. PTMSP synthesized using N b C l , which had more stable gas permeabihty values than PTMSP synthesized using TaCl or TaCl Ph Bi, showed the smallest change in separation factor. Based on these gas permeation results, aging of gas permeabihty appears to depend on the chain configuration of PTMSP since the polymerization catalyst, which is believed to be important in determining chain configuration, influenced aging behavior. With the aging conditions used, the gas permeability of PTMSP synthesized using NbCl was more stable than that of PTMSP membranes synthesized using TaCl or TaCl -Ph Bi. This result suggests that the non-equihbrium state of FTMSP synthesized using NbCl is more stable than that of polymers prepared with TaCl or TaCl -Ph Bi. In general, the permeability can be represented as the product of gas solubility and gas diffusivity. In previous studies of light gases (4,11), the reduction in gas permeabihty of PTMSP upon aging depended on gas diffusivity more than gas 2

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16 14

•

Gas: Ν


12

As-cast 12 days

1Ο 8 6 4 2

TaCI

TaCI -Ph B i 5

Figure 1.

NbCI

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Permeability coefficients of nitrogen in as-cast and aged PTMSP membranes at 30°C.

Figure 2. Relationship between aging ratio and kinetic diameter of the PTMSP membranes synthesized using various catalysts. Aging ratio: Permeability coefficient (12 days)/ Permeability coefficient (as-cast).


99 solubility. Gas diffusivity, in turn, depends on free volume and molecular motion of the polymer chains. Table I.

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Ideal Separation Factors of As-cast and Aged PTMSP Membranes Synthesized Using Various Catalysts

Catalyst

H /N 2

C(VN

2


Initial Aged Initial Aged

4

Initial Aged Initial Aged

2.7

2.9

1.5

1.6

4.9

5.6

1.9

2.2

TaCl -Ph Bi

2.5

3.0

1.4

1.6

4.7

6.5

1.9

2.5

NbCl

2.6

2.8

1.4

1.5

4.9

5.3

1.9

2.0

TaCl

5

5

8

c(ycH

2

3

5

Aging protocol: stored in a vacuum vessel for 12 days at 30°C.

Free Volume. The Langmuir sorption capacity, C ' , of the dual-mode sorption model (12,13) characterizes gas sorption in the non-equilibrium excess free volume of a glassy polymer. The total sorbed gas concentration in a glassy polymer, which is a summation of Henry's law dissolution (C ) and Langmuir type hole filling (C ), is given by: H

D

C=C

D

H

+ C =k p H

+^ l + bp

D

(1)

where k is the Henry's law coefficient, and b is the hole-affinity parameter. Based on parameters estimated from sorption isotherms for C H in as-cast and aged PTMSP membranes, only C ' showed a distinct change as a result of the aging protocol. Figure 3 presents C values for C H in as-cast and aged PTMSP membranes. The C ' values for PTMSP synthesized using TaCl -Ph Bi was dramatically reduced upon aging, indicating a decrease in the non-equihbrium excess free volume during the aging process. On the other hand, the C value for PTMSP synthesized using NbCl was practically independent of aging, suggesting that the polymerization catalyst strongly influences aging behavior in PTMSP. When the aging protocol includes thermal annealing at various temperatures, the same trend in C ' is observed (4). That is, both gas permeabihty and C of PTMSP are reduced by thermal treatment (4). Furthermore, aging-induced decreases in gas permeabihty of PTMSP synthesized using various catalysts are correlated systematically with changes in C ' . Therefore, aging appears to reduce the nonequihbrium excess free volume of PTMSP which, in turn, reduces the Langmuir sorption capacity and the gas permeabihty. D

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H

H

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g

H

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H

H

H

Molecular Motion. Spin-lattice relaxation times, T,, determined using solid-state N M R provide a practical method to characterize molecular motion of polymers in the solid state. In mis regard, Figure 4 presents S i T, values for as-cast and aged PTMSP membranes. Based on the results in this table, the spin lattice relaxation time is not sensitive to aging for PTMSP synthesized with the three catalysts considered in this study. Table Π summarizes the C Tj of each distinct carbon in as-cast and aged PTMSP membranes. As previously described (9), for PTMSP synthesized using NbCl , no changes in Tj values were observed as a result of aging. In contrast, the T, values of the backbone chain carbons (C and C ) of PTMSP synthesized using TaCl 29

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d


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100

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s

0

Ο

Gas:C H


Ε

•5

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•

e

60

As-cast 14 days

ο H

Φ υ

20

TaCI

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Figure 3.

NbCI

TaCI -Ph Bi 3

C values for propane in as-cast and aged PTMSP membranes. H

30 2 9

•

Si

20

As-cast 14 days

es

10

TaCI

TaCI -Ph BI 5

Figure 4.

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S

NbCI

3

S i T, of as-cast and aged PTMSP membranes.


101 and TaCl -Ph Bi decreased upon aging. The T, values of side chain carbons (C and C ) were almost constant. The reduction in backbone chain carbon spin lattice relaxation time is probably closely coupled to the relaxation of non-equilibrium excess free volume of PTMSP synthesized with TaCl and TaCl -Ph Bi. 5

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a

b

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Table IL

1 3

C T, of C - C Membranes a


Catalyst

a

5

3

b

in the As-cast and Aged PTMSP

d

C [s]

C [s]

a

C [s]

b

C [s]

c

d

Initial Aeed Initial Aeed Initial Aeed Initial Aeed TaCl

2.4

2.4

5.9

6.4

18.9

26.1

17.8

27.9

TaCl -Ph Bi

2.3

2.8

5.8

5.6

22.2

27.4

21.9

25.2

NbCl

2.6

2.6

5.2

5.0

25.6

25.9

27.7

26.2

5

5

3

5

a

Repeat Unit Structure: C H - C = C - S i ( C H )

b

Aging protocol: stored in a vacuum vessel for 14 days at 30°C.

b

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d

c

a

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Conclusions The effect of physical aging on gas permeability of pxMSP synthesized using various catalysts was studied. With the aging protocol used in this study, the gas permeabihty of PTMSP synthesized using TaCl and TaCl -Ph Bi was dramatically reduced after aging. However, the gas permeability of PTMSP synthesized using NbCl was stable. The C values of PTMSP prepared with different catalysts also exhibited the same behavior. Based on Tj measurements, only backbone chain carbons of PTMSP synthesized using TaCl and TaCl -Ph Bi decreased as a result of aging. This decrease in molecular motion is probably closely related to the relaxation of non-equilibrium excess free volume of PTMSP synthesized with TaCl and TaCl -Ph Bi. Based on these composite data, the non-equilibrium state of PTMSP prepared using N b C l appears to be more stable than that prepared using TaCl and TaCl -Ph Bi. 5

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Literature Cited 1. Robeson, L . M. J. Membrane Sci. 1991, 62, 165-185. 2. Masuda, T.; Isobe, E.; Higashimura, T.; Takada, K. J. Am. Chem. Sci. 1983, 105, 7473-7474. 3. For example, Shimomura, H.; Nakanishi, K.; Odani, H . ; Kurata, M.; Masuda, T.; Higashimura, T. Kobunshi Ronbunshu 1986, 43, 747-753. 4. For example, Nagai, K.; Nakagawa, T, J. Membrane Sci. 1995, 105, 261-272. 5. Costa, G.; Grosso, Α.; Sacchi, M. C.; Stein, P. C., Zetta, L . Macromolecules 1991,24, 2858-2861. 6. Izumikawa, H.; Masuda, T.; Higashimura, T. Polym. Bull. 1991, 27, 193-199. 7. Masuda, T.; Isobe, E.; Higashimura, T. Macromolecules 1985, 18, 841-845. 8. Nagai, K.; Higuchi, Α.; Nakagawa, T. J. Polym. Sci.: Part B: Polym. Phys. 1995, 33, 289-298. 9. Nagai, K.; Watanabe, T.; Nakagawa, T. Polym. J. 1996, 10, 933-935. 10. Breck, D. W. Zeolite Molecular Sieves; Wiley; N Y , 1974; p. 636. 11. Nakagawa, T.; Saito, T.; Asakawa, S., Saito, Y . Gas Sep. Purif. 1988, 2, 3-8. 12. Barrer, R. M.; Barrie, J. Α.; Slater, J. J. Polym. Sci. 1958, 27, 177-197. 13. Michaels, A . S.; Vieth. W. R.; Barrie, J. A . J. Appl. Phys. 1963, 34, 1-13.


Polymer Membranes for Gas and Vapor Separation - American

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