CO2 Plasticization in Polyimide Membranes: Coupling of Permeation,

Sorption, and Polymer Swelling

John D. Wind, Stephen M. Sirard, Donald R. Paul, Peter F. Green, Keith P. Johnston, and William J. Koros*

The University of Texas at Austin and *The Georgia Institute of Technology

C2

C1

!

AIChE National MeetingSan Francisco 2003

Overview• Motivation

u Membrane limitations• Mixed gas performance • Materials and crosslinking scheme• Decoupling of physical and chemical changes with heat

treatmentu Free volume vs. physical agingu Permeationu Sorptionu Swelling by in-situ ellipsometryu Long-term relaxations: polymer swelling and CO2 diffusion

• Conclusions and Future Work

C2

C1

!

• Remove CO2 from natural gas mixtures ⇒ maximize gas heating value and minimize corrosion

• $1.3 Trillion gas off-spec in U.S.‡

Motivation for Project

‡ Meyer, H.S., Volume and Distribution of Subquality Natural Gas in the United States, 2000, Gas Research Institute.

75% CH43% C2H63% C3-C51% C6+5-20% CO2Water Sat’d750-4000 psia

< 2% CO210°F Dew Point

Permeate to FlareOr Sequestration50-1500 psia Product

To Pipeline

Heat Exchanger

• Goal: inhibit plasticization: permselectivity losses from excessive polymer chain segmental motion CH4 loss ($$)

• Polyimides Superior transport properties relative to cellulose acetate Need to be more robust

Highly Permeable and Stable Materials in Mixed Gas

Adapted from L. Robeson,J. Membr. Sci, 1991

1

10

100

1000

0.1 1 10 100 1000

CO2 Permeability (Barrers)

Sele

ctiv

ity C

O2/C

H4

Robeson's 1991 Upper Limit

Commercially Attractive Region

50/50 CO2/CH435°C, 25 atm

Lit. Datapure gas, < 10 atm

Cellulose Acetate Matrimid®

(50/50 mixed gas25 atm)

J. Membr. Sci, 2003, in press.

Polymer Structure

Crosslinking Agents

Butylene Glycol(BG)

OHOH

CF3 CF3O

O O

O

N

COOH

CF3 CF3

O

O

O

O

CH3 CH3

CH3

O N N N

2 1

CH2OHHOCH2

1,4-cyclohexanedimethanol(CHDM)

Covalent Crosslinking Mechanism

Solid State(vacuum)

(2) TransesterificationO

HO--RO

+O

O------R-------OO

HO--R--OH2 Δ

Tg ~ 360 °C

In Solution

(1) MonoesterificationOHO HO--R

HO--R--OH+ + H2O

O

H+

Macromolecules, 2003, 36, 1882-1886.

Generalized mechanism1. Simpler transition to hollow fibers2. Allows for spectroscopic characterization along

reaction pathway

Heat Treatment:Physical Aging and Crosslinking

Annealing and Crosslinking: significant changes to polymer structure below Tg (~ 360°C)

Polymer6FDA-DAM:DABA 2:1

0

40

80

120

160

100 150 200 250 300

CO

2 Per

mea

bilit

y (B

arre

rs)

Annealing Temperature (oC)

Free Acid Groups

CHDM

BG

Macromolecules, 2003, 36, 1882-1886.

0

50

100

150

200

250

0 10 20 30 40 50 60

CO

2 Per

mea

bilit

y (B

arre

rs)

CO2 Feed Pressure (atm)

CO2 Permeation Isotherms6FDA-DAM:DABA 2:1

Free Acid Groups130°C

295°C

Annealing temps = better plasticization resistance

Macromolecules, 2003, 36, 6433-6441.

220°C

0

20

40

60

80

100

0 10 20 30 40 50 60 70

CO

2 Per

mea

bilit

y (B

arre

rs)

CO2 Feed Pressure (atm)

CHDM monoester

100°C

295°C

220°C

Crosslinking increases permeability

CHDM Monoester

0

20

40

60

80

100

120

0 10 20 30 40 50C

O2 C

once

ntra

tion

(cm

3 (STP

)/cm

3 pol

ymer

)CO

2 Pressure (atm)

100°C

295°C

Free Volume Created

CO2 Sorption Isotherms

Free Acid Groups

6FDA-DAM:DABA 2:1 at 35°C

0

50

100

150

200

0 10 20 30 40 50

CO

2 Con

cent

ratio

n

(cm

3(ST

P)/c

m3 p

olym

er)

Pressure (atm)

100°C

295°C

Physical Aging

Macromolecules, 2003, 36, 6433-6441.

Sorption modes affected in different ways

Pg

PMMA,35°C

Basic Principle:Reflection of light causes a changein polarization state of light

Extract thickness and refractive index, independently

Input-Optical model-Known parameters

Output:- Dilation, sorption-Tg, Pg (Sirard et al, Macromolecules, 2002)

70o

Sample

P

CO2cylinder

Spectroscopic Ellipsometry

0

5

10

15

20

25

0 20 40 60 80 100

Swel

ling

(%)

Pressure (atm)

Swelling and Plasticization

Film Thickness ~ 120 nm hollow fiber skin thickness

plasticization pressures

Increased ThermalTreatment (130°C, 220°C, 295°C

Sorption

Desorption

Similar results for free acid polymer

Polymer6FDA-DAM:DABA 2:1CHDM Monoester

Macromolecules, 2003, 36, 6433-6441.

Long-Term Stability: CHDM Monoester

0

3

6

9

12

15

0 5 10 15 20 25 30

Swel

ling

(%)

Time (h)

100°C220°C

295°C

Swelling40 atm CO2 @ 35°C

Permeability

1.0

1.5

2.0

2.5

3.0

3.5

0 5 10 15 20

P t/P0

Time (days)

100°C

220°C295°C

Correlation between swelling and permeability KineticsAmplitude of instability

Macromolecules, 2003, 36, 6442-6448.

100

110

120

130

0 20 40 60 80

CO

2 Con

cent

ratio

n

cm3 (S

TP)/c

m3 p

olym

er

Time (h)

CO2 Sorption Kinetics: CHDM Crosslinking

• Sorption does not describe permeation stability well at all!• Long-term swelling without much sorption (Bohning and Springer

Polymer, 1998)

100°C

295°C

Permeability Increases at 48 hr.230 %

7 %

40 atm CO2 @ 35°C

Summary: Crosslinking and Plasticization

Temperature (°C)

kD (cm3 (STP)/ (cm3 atm)

CH’

(cm3(STP)/ cm3)

100 1.8 31 295 0.81 60

Sorption

AAA

AHAADA p

pbbCkC ⎟⎟

⎠

⎞⎜⎜⎝

⎛

++=1

'

Henry’s law

Langmuir+

Dual Mode Sorption:

Decreased Chain Mobility and Swelling

Increased Free Volume and Permeability

Swelling and PermeationThermal

Treatment (°C) Plasticization

Pressure (atm)

Concentration cm3(STP)/cm3

polymer

CO2 Partial Molar Volume

(cm3/mole) 130 Free Acid 12 38 27

220 Free Acid 30 73 28

100 CHDM 12 33 28

220 CHDM 36 64 31

pT

CO Chh

,

0400,222

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

∂

⎟⎠⎞

⎜⎝⎛∂

=υ

Plasticization pressure in permeation correlates with CO2 sorbed partial molar volumeSupported by other data in literature (Bohning and Springer, Polymer, 1998)

Conclusions and Recommendations• Covalent crosslinking can stabilize membranes against

plasticization• Thermal treatment is significant in determining transport

properties and long-term stability• Controlling swelling is key to controlling plasticization• Need to assess the long-term stability in presence of

multicomponent high pressure natural gas streams• Need to understand effect of crosslinking on diffusion

coefficients of various penetrants

n-butanepropane toluene

Fundingu U.S. Department of Energyu National Science Foundation

Collaborationsu Dr. Ryan Burnsu Joseph Pham

Acknowledgements

Relaxation Times

⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

−

−

∞ R0

0

τexp1 t

YYYY

Rt φ

Relaxation-controlled Permeation, Swelling , and Sorption all follow same functional form:

Annealing Temperature

(°C)

Permeation (h)

Swelling (h)

Sorption (h)

100 16 0.57 6.8 295 47 5.3 5.8

Crosslinking slows relaxations

Thin film swelling fast relaxationsimportant for practical membranes!

CHDM Monoester (τ values)

Proposed Fundamental Cause of Plasticization

pT

CO Chh

,

0400,22~2

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

∂

⎟⎠⎞

⎜⎝⎛∂

=υ

Partial Molar Volume

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70

CO

2 Par

tial M

olar

Vol

ume

(cm

3 /mol

e)Pressure (atm)

Rubbery Asymptote

Plasticization Threshold

Swelling normalized by concentration

100 °C220 °C

Hypothesis: ↑ Polymer segmental mobility ↑ CO2 partial molar volume ↑ polymer “free volume”

supported by other data in literature (Bohning and Springer, Polymer, 1998)