Effects of CO2 exposure on gas transport properties of glassy polymers

Journal of Membrane Science, 32 (1987) 195-205 Elsevier Science Publishers B.V., Amsterdam - Printed

195 in The Netherlands

EFFECTS OF CO2 EXPOSURE ON GAS TRANSPORT PROPERTIES OF GLASSY POLYMERS

J.S.

CHIOU

and D.R. PAUL

Department

of Chemical Engineering

Texas at Austin, (Received

Austin,

October

and Center for Polymer

Research,

The L’niLlerszty of

TX 78712 (U.S.A.)

151986;

accepted

in revised form December

18, 1986)

and CO, exposure

on transport

Summary The effects

of CO, driving pressure

polycarbonates

and two acrylic polymers

bility decreases

with increased

urements

a decrease

are described.

driving pressure

in permeability

history

For the polycarbonates.

while exposure

and an increase

each response is exactly the opposite. The effects processes involving CO, at high pressures.

properties

to CO, causes in subsequent

in the diffusion

may be important

for two

the CO, permeameas-

time lag. For the acrylics. for membrane

separation

Introduction The sorption and transport behavior or CO, in glassy polymers has been intensively investigated in recent years because of potential applications in several areas such as packaging of carbonated beverages and separation of CO, and CH, mixtures. High levels of CO* sorption can plasticize the polymer and cause significant changes in its characteristics. For example, CO, can induce crystallization like liquids or vapors do [ 11. The mechanical properties of the polymer may be altered when CO* is sorbed [ 2,3]. Gas sorption and transport properties of glassy polymers are also changed after exposure to CO, gas. For instance, it is well known that gas sorption is enhanced after a glassy polymer is exposed to CO, at a certain pressure for a period of time, say 20 atm for one day. This is the so-called “conditioning” effect, in which the glassy polymer is swollen by the dissolved COa, and after degassing, it cannot fully return to its original state within the time scale over which subsequent transport experiments are usually performed. In addition, exposure of bisphenol-A polycarbonate to high pressures of CO, causes a slight decrease in the CO, permeability and a rather large increase in the diffusion time lag [ 41 in subsequent experiments compared with the original. Here, we demonstrate a similar response for a polycarbonate based on chloral bisphenol, but two acrylic-based polymers show entirely opposite responses to CO, exposure. Interestingly, the two polycarbonates exhibit permeabilities which decrease with CO, driving pressure, similar to that usually observed for glassy polymers, whereas, the two acrylic0376-7388/87/$03.50

0 1987 Elsevier

Science

Publishers

B.V.

196 TABLE 1 Polymers used in this study Designation

Chemical description

PC

bisphenol-A polycarbonate

BCPC

bisphenol chloral polycarbonate

PMMA

Korad ACV

Structure

Source

T,. ‘C

Densny. g/cm 1

_O*&:*O-E-

Genera1 E1ectrlc c ()

7

148

1.20

_O_,,FJ?&

General Electrrc Co

164

1.392

106

1 188

90 ( matrrx 1

1.133

poly (methyl methacrylate) acrylic multipolymer made by multistage emulsion polymerization

cI/ck F”‘ -%

~7 ;=” P

Rohm and Haas Co Plexiglas V ( 811

1

0.3

glassy matrix (predominately formed from MMA) with a complex rubbery disperse phase

Polymer Extruded Products, Inc

based glassy polymers show CO, permeabilities which increase with increased driving pressure. Our purpose here is to call attention to these two different patterns of behavior as they may have significance for current and future membrane separations involving CO, or other gases (or vapors 1 which may significantly interact with the polymer from which the membrane is fabricated. An important question to be resolved by future research is the reason why some glassy polymers show one response while other polymers show an opposite response. Experimental

The physical characteristics and related information for the polymers employed in this study are listed in Table 1. The Korad ACV was a commercial film product with a nominal thickness of 3 mil. It is a complex material made by a multistage emulsion polymerization process which has a glassy matrix that is essentially poly( methyl methacrylate) with crosslinked acrylic rubber inclusions [ 51. The poly( methyl methacrylate) (PMMA 1 film was prepared by extrusion in our laboratories. The bisphenol chloral polycarbonate (BCPC 1 [ 61 film was made by solution casting using methylene chloride as the solvent. The film, which was cast on a glass plate, was dried in an air oven by raising the temperature gradually from 25°C to 190°C over a period of one week and kept at 190°C for two days. After drying, the film was quenched to room temperature. All gas permeabilities were measured at 35’ C using a high-pressure permeation cell whose design and operation have been described elsewhere

197

p2 (atm) Fig. 1. Effect of CO, exposure history on the CO, permeability for BCPC

at 35’C.

coefficient

versus drlcmg pressure

The numbers indicate the sequence of measurement.

[ 71. The upstream pressure ranged from 1 to 25 atm while the downstream pressure was effectively zero. To examine the effect of COP gas exposure on gas transport properties, gas permeability coefficients were measured for polymer films using three CO, exposure histories, identified here as virgin, vectored and conditioned. The experiments for the “virgin” case refer to a first cycle of permeability measurements (from 1 to 20 or 25 atm) made on the untreated film by sequentially increasing the driving pressure from low to high values. “Yectored” films refer to those after the first cycle of CO, permeability measurements For the “conditioned” experiments, the films were first exposed to CO, gas uniformly in a high pressure chamber at 20 or 25 atm for 24 hours and then degassed for 24 hours before they were installed in the permeation cell. Whenever the time lag was measured, the permeation cell was thoroughly evacuated for at least ten diffusion time lags before starting a transient permeation measurement. Results and discussion Bisphenol chloral polycarbonate Bisphenol chloral polycarbonate has been reported to have mechanical properties similar to bisphenol-A polycarbonate [ 61. Our recent studies with BCPC show that its gas sorption and permeation properties are also rather similar to PC, and both exhibit comparable CO, exposure effects. Figure 1 shows CO2 permeability coefficients for a BCPC film as a function of CO, driving pressure. The numbers indicate the sequence of measurements. For a given CO, driving pressure, it is clear that prior exposure to CO, ( either by vectoring or conditioning in the previously defined terminology 1 results in a

198

I

I

I

1

Till,

BCPC/CO, ,l

= IS

.

/

/

I

2

I

condithoned

@ vectored

a

L

1

/ 35°C mll

/llllll

5

I

IO

1

20

pz (afm)

Fig. 2. Effect of CO, exposure history on the CO, diffusional

time lag for BCPC at 3.S

(

decreased permeability relative to that observed during a first exposure to CO, (i.e. virgin case). Figure 2 shows an analogous pattern of experiments for the diffusion time lag, which is increased by prior CO, exposure. Similar responses for b&phenol-A polycarbonate [ 41 have been rationalized by changes which the CO, exposure evidently causes in the various sorption and transport coefficients of the dual sorption/dual mobility model [ 81 which has been successfully used to describe the behavior of gases in most glassy polymers. Measurements of permeability coefficients and time lags for other gases m BCPC films following various exposures to CO, reveal similar trends as shown in Figs. 1 and 2 for which CO, itself was used as the probe. This is illustrated for CH, in Figs. 3 and 4, where it is seen that exposure to CO, reduces the CH, permeability coefficient and increases its time lag.

Km-ad ACV An extensive study of gas sorption and transport in Korad ADV film was recently reported [ 51. This is one of a very few glassy polymers known presently whose permeability coefficient for CO, increases with driving pressure (see Fig. 5). Similar behavior has been noted for PMMA [ 9 ] : and since the matrix of Korad ACV is closely related to PMMA this analogous response IS not surprising. Figure 5 also shows that prior CO, exposure increases the permeability coefficient for CO, in Korad ACV which is the opposite to the effect shown in Fig. 1 for BCPC. As seen in Fig. 6, time lags for CO, transport are somewhat lower in the conditioned Korad ACV film relative to the virgin case, which is again the opposite of the response for BCPC noted in Fig. 2. Permeability coefficients for the lower solubility gases I e.g. He, I%*, Ar, and CH, in Korad ACV have been found to be virtually independent of driving

199

0.26 0.24 n

-0x

a

0

0.20 0

t .

0.18C

virg,n

(not

vectored

exposed

to co,1

by 250tm

condltloned

CO2

by 25atm

I

I

I

2

I

I

CO,

Illill

5 p2

I

IO

,

20

(atm)

Fig. 3. Effect of CO, exposure history on the CH, permeability

coeffictrnt

for WPC

at 35

(

pressure as observed for poly (methyl methacrylate 1 (9 j and poly f vinyl chloride) [lo]. Such behavior is consistent with the total immobilization limit [ 111 of the dual mobility model mentioned earlier. Figure 7 shows permeabll ity coefficients and time lags for argon in Korad ADV before and after (20, vectoring. The permeability coefficient is increased by about, 20% by vectormg while the time lag is decreased by up to 10% (in the low-pressure range 1,

.

condItIoned

@ vectored

35 L-

I

5CPC/CH,/35T ! = 1.8ml1

40 .'_

I

2

by25atm by 25 otm

0 vlrgln (not

exposed

5 pr Cotm)

IO

COP COP

i

:

to Con)

20

Fig. 4. Effect of CO, exposure history on the CH, diffusional

time lag tar H(‘PC

at 35 (

200

x

a

I

1

0

5

Fig. 5. Effect

I

1

IO I5 pr (otm)

of CO, exposure

20

history

on the CO, permeability

ccwffklent

for Korad A( ‘L at 35

‘C

Poly(methy1 methacrylate) Sorption and transport properties of extruded PM51A films for several gases including He, NS, Ar, CH4, and CO, have been reported previously 1121. In these studies, CO2 permeability coefficients were measured using condltloned films while permeability coefficients for other gases were measured usmg virgin films; reasons for these choices have been given [ 12 ] To illustrate the CO, exposure effect, CO, permeability coefficients for vlrgln and conditioned films, plotted versus the driving pressure, are compared m Fig 8 For six other gases, He, Hz, Ar, N2, O2 and CH,, whose permeability coefficients are independent

Korad l

. q

ACV/CO,

/35

I st cycle (virgin) 2 nd cycle } (vectored 3 rd cycle

“C

i

p;, (atm) Fig. 6. Effect

of CO, exposure

history

on the CO, diffusional

time lag t(~r Korad AC\ at 3:) -C

-

-1

=o /

Korad

-

0 A

ACV

I

/Ar/35’C

I

“lrgl”

vectored for

24

by 20 o:m hours

CO,

at

35°C

Fig. 7. Permeability coefficient and time lag for Ar in Korad ACV before i atm vectoring with CO,.

l

I and after ( A a It!

of the pressure (like that shown in Fig. 8)) a comparison 1~ given m Table 2. As observed for Korad ACV film, the permeability coefficients are increased (see Fig. 8 and Table 2) but the time lags are decreased 1nc)t shown 1 by (‘( Ii conditioning of PMMA films. It is noteworthy that the relative change in permeability coefficients caused by CO, exposure increases with the molecular size of the penetrant gas, as shown in the last column of Table 2 Figure 9 shows a plot of the ratio of permeability coefficients for the conditioned film to the as-extruded film, PC/P,, versus the Lennard-Jones collision diameter of the gas (for COZ, a kinetic molecular diameter is used as explamed previously [ 13 1 I The results shown in Fig. 9 indicate that the virgin film I’; dilated by (‘0,

0.2

I 0

I IO

1

I 20

1

I 30

p2 (atml

Fig. 8. Comparison of CO, permeability coefficients at 35’ C in PMMA her+)x-t‘1l I and after I A 1 25 atm conditioning with CO,. For ( 0 ) , see Figs. 10 and 11,

202 TABLE 2 Comparison of CO, permeabilities at 35’ C for extruded PMMA film before and after 25 atm CO, conditioning Gas

As-extruded P.dIa1

CO,-conditioned P,.‘“N

PJPa

He H, 0, co, Ar Ne

7.62 x 10 I” (‘I 3.73 x 10 “I 8.75X106” 3.10x lo-” ‘c1 2.13 x 10 ‘” ‘d’ 8.25xlOV”“” 3 50~10 1.3 tdl

9.39 x 4.70 x 1.26x 4.91 x 4.09x 1.54 x 9.33 x

1.23 1.26 1.44 1.58 1.92 2.10 2.67

CK,

10 10 10 10 10 10 10

II I” I1 I’ ‘, ’ :i

Ii ‘.>

‘“‘In units of [ cm”( STP) -cm/( set-cm’-cmHg)] . ‘“‘This value is 19% higher than that reported before [ 121, presumably due to different orientation in the extruded film. ‘“‘Data taken at pe = 1 atm. ‘d’Data from Ref. (12).

conditioning such that the amount and/or distribution of free volume in the polymer is altered to favor a greater increase in transport rates for larger molecules than for small molecules. It should be noted that the magnitude of the CO, exposure effect depends on the length of time the polymer was vectored or conditioned. This is because glassy polymers are not in an equilibrium state, and the relatively immobile chains require a period of time to achieve a new organizational state. The CO, exposure time, 24 hours, adopted for this study may not be enough for such a

4

z,t;I;g; ,i :2

‘2.6

3.0

IJ (A,

3.4

3.8

Fig. 9. Ratio of permeability coefficients in CO,-conditioned PMMA, P, , to those as-extruded film, P,, plotted versus molecular diameter of the gas probe. All data at 35 C.

203

1.0 I

0

50

I IO0

I

150 t

I 200 (hr)

I

250

300

I I 350

Fig. 10. Change in the permeability coefficient for CO, in PMMA at 25 atm driving pressure with vectoring time. All data at 35°C. dilation process to reach completion, especially for polymers with relatively rigid chains like PMMA. Figure 10 gives an example where the CO, permeability coefficient is plotted versus the time the polymer had been vectored. In this experiment, the upstream pressure was kept constant at 25 atm and the permeability was periodically measured over a period of 330 hours. As seen, the permeability initially increases rapidly but levels off after about 200 hours. Note that the diffusion time lag is only about 1.5 hours, which is much shorter than the time span of these permeability measurements. The increase of permeability with time is obviously not because of insufficient time to reach a “steady” state concentration profile across the film but is because the polymer continues to relax towards a new organizational state in the presence cf dissolved gas. After completion of the vectoring experiment, the upstream pressure was decreased to 13.5 atm and gas permeation was again recorded against time as shown in Fig. 11. As seen, the permeability decreases with time, indicating that a slow collapse of free volume is occurring. Note that even at 300 hours, which is the time required to essentially complete the previous vectoring experiment (see Fig. lo), the collapse process is still in progress. This is because chain mobility at 13.5 atm of CO, is less than that at 25 atm so that longer time is needed to effect the relaxation process. Also notice that the permeability coefficient after recovery at 13.5 atm is still higher than in the conditioned state at the same driving pressure (see Fig. 8). Similar results have been observed for polycarbonate [ 141 and poly (vinyl chloride) [ lo]. These latter studies showed that gas permeability coefficients measured subsequent to a previous CO, exposure are strongly affected by the extent the polymer is permitted to return to the original undisturbed state. Related changes occur in glassy polymers as a result of exposure to vapors at much lower pressures. For example, hysteresis

204

PMMA /cop/

1

PI.2 ;; ii

N’

; f

I .I

9

0 _ x a

. I\

e

= 2.8

ml1

!

55 “5

35”

pr = 13.5 atm

i

I

1.ot I

0

I

100

I

I

200

300

/

400

I

i

I

500

t (hr)

Fig. 11. Change in CO, permeability coefficient with time at 13.5 atm driving pressure for PMMA film vectored at 25 atm for 330 hours.

behavior has been observed during sorption and desorption of vinyl chloride vapor in poly (vinyl chloride) [ 151. Conclusions

Table 3 summarizes the observations reported here for two different polycarbonates and two acrylic-based polymers. For the former, the CO, permeability coefficient decreases with increasing driving pressure which is similar to the response for many other glassy polymers; while for the latter, the CO, permeability coefficient increases with increasing driving pressure which may be therefore regarded as atypical. The response for the acrylic polymers is believed to be the result of significant plasticization by CO, causing increased rates of segmental motions and hence increased permeability coefficients for CO,. It is interesting to compare the behavior of the two important polymers used as membranes in current commercial applications for gas separations, viz. TABLE 3 Summary of gas transport responses Polymer type

Polycarbonates Acrylics

Direction of change in CO, permeability coefficient with increased driving pressure

Permeability coefficient caused by CO, exposure

Diffusional time lag caused by CO? exposure

decrease increase

decrease increase

increase decrease

polysulfone and cellulose acetate. Polysulfone [ 161 responds in the manner of the polycarbonates, while cellulose acetate [ 171 responds like the acrylics. The polycarbonates and the acrylics also respond oppositely in terms of the effects of CO2 exposure on subsequent measurement of gas transport characteristics. Exposure to CO, reduces the permeability coefficient and increases the diffusional time lag for the polycarbonates, while for the acrylics the permeability coefficient is increased and the time lag is decreased. Sorption of significant quantities of CO2 in glassy polymers perturbs their local segmental organization. Upon removal of the C02, there is a slow relaxation of the perturbed state, but it is not yet known whether full restoration to the original state (before CO, exposure) can occur or not. From the observations here, which use gas transport as a probe for the change in glassy state organization, it is clear that the nature of the changes caused by CO, exposure is not qualitatively the same for all polymers, e.g. the polycarbonates versus the acrylics. Further work is needed to better understand these differences in conditioning effects and whether they are indeed related to the extent of interaction of CO, with the polymer, viz. plasticization, or not. Acknowledgment This research was supported University of Texas.

by the Separations

Research

Program

at The

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

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