Download - The gas transport properties of amine-containing polyurethane and poly(urethane-urea) membranes

The gas transport properties of amine-containing polyurethaneand poly(urethane-urea) membranes

Liang-Siong Teo, Chuh-Yung Chen*, Jen-Feng Kuo

Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 701, ROC

Received 2 September 1997; received in revised form 6 November 1997; accepted 7 November 1997

Abstract

A series of amine-containing polyurethanes and poly(urethane-urea)s based on 4,40-diphenylmethane diisocyanate and

either poly(ethylene glycol) of molecular weights 400 or 600 were prepared as gas separation membranes. The amine

functional groups of N-methyldiethanolamine (MDEA) and/or tetraethylenepentamine (TEPA) were introduced into the hard

segment as a chain extender. The gas transport data of He, H2, O2, N2, CH4 and CO2 in these polymer membranes were

determined by using the Barrer's high-vacuum technique and the time-lag method. The restriction of chain mobility has been

shown by the formation of hydrogen bonding in the soft segment and hard-segment domains, resulting in the increase in the

density, glass transition temperature of soft segments (Tgs). The separation mechanism of various gas pairs used in industrial

processes is also discussed. Effect of pressure on permeability of the gases above and below Tgs was studied. It was found that

the gas permeability increased or decreased with upstream pressure above Tgs, and should be described by a modi®ed free-

volume model. On the other hand, the condensable CO2 exhibits a minimum permeability at a certain upstream pressure below

Tgs. The permeability of He and H2 were pressure independent above and below the Tgs. # 1998 Elsevier Science B.V.

Keywords: Gas separations; Gas and vapor permeation; Membrane preparation and structure; Polyurethane membrane;

Permselectivity

1. Introduction

Membrane-based separation has been utilized in

many chemical industries. It provides an energy-ef®-

cient, compact, modular, and green-house process.

Applications of polymeric membranes as gas separa-

tion membranes are used in a wide variety of areas [1],

such as carbon dioxide recovery in petroleum applica-

tions, removal of helium gas from natural gas, recov-

ery of hydrogen from ammonia plant purge streams,

oxygen/nitrogen separation, etc.

Polyurethane is a segmented copolymer composed

of alternating soft and hard segments. The study of

transport properties of penetrant gases through poly-

urethane membranes is particularly interesting [2±13].

It is possible to introduce controlled changes in polyol

length and chemical nature and the proportions of

the constituents which make up the soft and hard

segments of polyurethanes. These changes result

in different physicochemical properties. Various

researchers [4,7,10,11] have shown the gas perme-

ability of polyurethane membranes increases with the

decrease of hard segment content and increase of soft

segment molecular weight. In addition, correlation has

Journal of Membrane Science 141 (1998) 91±99

*Corresponding address. Fax: +886 6 2344496

0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 3 7 6 - 7 3 8 8 ( 9 7 ) 0 0 2 9 3 - 7

been established between the gas permeability and the

chemical nature of polyols and chain extenders. It

affects the transport properties of polyurethane mem-

branes with the result of changing the phase-separated

domain morphology, polyfunctional cross-linking,

crystallinity, density and glass transition of the mem-

branes.

The transport of gases through polymer membranes

depends strongly on whether the polymer is in its

rubbery or glassy state. Usually, the gas sorption

process in glassy polymers (T<Tg) is more complex

than in rubbery polymers (T>Tg) because of their

nonequilibrium state [14±16]. In our previous paper

[17] we chose the CO2 af®nity reagents N-methyl-

diethanolamine (MDEA) and tetraethylenepentamine

(TEPA) as chain extenders to prepare amine-contain-

ing polyurethanes (PU) and poly(urethane-urea)s

(PUU) and studied the permeation and sorption iso-

therm of CO2. This study showed that the sorption

isotherms obey Henry's law above Tgs, while below

Tgs the sorption isotherms follow a dual-mode sorption

model. The aim of the present paper is to study the

transport properties of various gases in these amine-

containing PU and PUU membranes. Relationships

between transport properties and chemical structure

are discussed.

2. Experimental

2.1. Polymers

The PU and PUU membranes chosen for the present

study were prepared by a two-step process in N,N-

dimethylformamide (DMF). Poly(ethylene glycol)

(PEG) of molecular weight 400 or 600 was used as

the soft segment component. The hard segment was

formed from 4,40-diphenylmethane diisocyanate

(MDI) and chain extended with N-methyldiethanol-

amine (MDEA) and/or tetraethylenepentamine

(TEPA). Details of the polymerization procedure have

been described previously [17]. The chemical struc-

tures are shown in Scheme 1.

2.2. Film casting

The polymers were used in the form of non-porous

planar membranes which were prepared from DMF

solution containing 15 wt% polymer. These solutions

were centrifuged at 2000 rpm to remove any gel

particles or undissolved impurities. Then a certain

amount of the solution was placed in a glass cylinder

with a clean polyethylene (PE) ®lm attached to the

bottom. To prevent dust pollution, the top of the glass

cylinder was covered with a ®lter paper. The system

was placed in a nitrogen-purged oven at 808C and

dried for about 24 h. Finally, the polymer membranes

were removed from the PE substrate and put in a

vacuum at 508C for about 3 days to remove the

remaining solvent. The thickness of the dry mem-

branes varied from 120 to 180 mm.

2.3. Characterization of polymer membranes

The composition of each polymer membrane stu-

died was determined by a Heraeus CHN-O-RAPID

Elemental Analyzer (EA). Density of each polymer

membrane was measured by the immersion method

[18], which included weighing of samples in air and in

distilled water. Thermal transition temperatures of

each polymer membrane were obtained by a DuPont

DSC 2910 differential scanning calorimeter linked to

a Thermal Analysis 2000 system for data acquisition.

The samples were scanned fromÿ70 to 2508C at a rate

of 108C/min under nitrogen purging.

2.4. Permeation test

The permeation of pure gases through the polymer

membranes was measured using Barrer's high-

vacuum method [19]. The purity of gases (He, H2,

N2, O2, CH4, and CO2) used was 99.5% or higher. The

cumulative amount of gas passing through the poly-

mer membrane in a constant volume receiving cham-

ber (V) was measured by the pressure increase on the

downstream side (pd) with a pressure transducer (MKS

Baratron 222B with a full-scale range of 10 Torr

connected to an MKS Type PDR-C-1C readout).

The upstream pressure (pu) was monitored with a

pressure transducer (Honeywell). At steady gas ¯ow,

the permeability coef®cient (P) was determined from

the slope (dp/dt) of the linear portion of pd vs. time plot

using the following equation:

P � 273� V � L� dpdt

760� T � A� �pu ÿ pd� (1)

92 L.-S. Teo et al. / Journal of Membrane Science 141 (1998) 91±99

where L is the thickness of the membrane, T is the

absolute temperature, and A is the area of the mem-

brane. The downstream pressure, pd, can be regarded

as negligible since pd�pu.

The diffusion coef®cient (D) was determined by the

time-lag method [20], and is represented as

D � L2

6�(2)

where � is the time lag, i.e. the intercept obtained by

extrapolating the linear region of pd vs. time plot to the

time axis.

The solubility coef®cient (S) was then calculated

from the equation of

P � D� S (3)

3. Results and discussion

Table 1 summarizes the characteristic properties of

the amine-containing PU and PUU membranes. It

shows that the hard segment content of the polymer

membranes is in the range of 51±64%. The density of

the polymer membranes is in the range of 1.0±1.2 g/

Scheme 1.

L.-S. Teo et al. / Journal of Membrane Science 141 (1998) 91±99 93

cm3, increasing with the decrease of molecular weight

of the PEG soft segment but with increase of the TEPA

content. This indicates that the packing density of the

PEG-400 polymer membranes is larger than that of

PEG-600 ones, while TPUU membranes comprise the

larger packing density. In addition, PEG-400 polymer

membranes show a glass transition temperature of

PEG soft segment (Tgs) at 15±278C, while the

PEG600 ones exhibit a lower Tgs of ÿ188C to

ÿ68C. Our previous paper [21] showed that a sig-

ni®cant amount of hydrogen bonding between the soft

and hard segments in the soft segment domains.

Therefore, it seems that for PEG-400 more hard seg-

ments are dissolved in soft segment domains causing

more stiffness and a higher Tgs. Thus, PEG-400 poly-

mer membranes exhibit less free volume than those of

PEG-600. On the other hand, the microcrystalline

ordering of hard segment domains above 2008C was

not found [22,23]. This indicates that the polymer

membranes are amorphous polymers. Our previous

paper [21] also showed that the MPU membranes

exhibited only loosely packed urethane hydrogen

bonding in the hard segment domains. However, the

TPUU membranes present more hydrogen bonded

urea carbonyl with a mixed state of three-dimensional

hydrogen bonding as well as conventional interurea

bonds and that of interurethane bonds. It seems that

the hydrogen packing density increases with increas-

ing TEPA content. It is consistent with the increase in

the density of the polymer membranes (Table 1).

3.1. Pressure dependency of permeability above Tgs

Figs. 1 and 2 show the results of permeability (P)

measurements in the form of semilogarithm of P vs.

upstream pressure (pu) at 358C for PEG-400 and -600

polymer membranes, respectively. The measured tem-

perature is higher than that of the Tgs of the polymer

membranes studied. In the ®gures, all of the lines were

obtained from least-square ®ts of the experimental

data. It shows that the semilog plot of P vs. pu is linear.

The permeability of He and H2 are pressure indepen-

Table 1

Characteristics of the amine-containing polyurethane and poly-

(urea-urethane) membranes

Polymer Hard segment

content (wt%)

Density

(g/cm3)

Tgs

(8C)

MPU4 61.4 (60.8) a 1.16 27

MTPUU4 62.7 (62.8) 1.18 21

TPUU4 63.9 (63.3) 1.21 15

MPU6 51.5 (50.8) 1.04 ÿ6

MTPUU6 52.8 (52.2) 1.09 ÿ13

TPUU6 54.1 (53.5) 1.12 ÿ18

a Calculated values.

Fig. 1. Effect of upstream pressure on permeability for various gases in the PEG-400 polymer membranes above Tgs: (a) MPU4; (b)

MTPUU4; (c) TPUU4.


dent while those of O2, N2 and CH4 decrease with

increasing pressure. However, the permeability of CO2

increases with increasing pressure. According to Stern

[24] and Sada [25,26] and co-workers, such a linear

relationship above Tgs indicates that the permeation of

the polymer membranes for all the gases might be

described by the modi®ed free-volume model. In the

case of pu�pd, the pressure dependency of perme-

ability is given by the relation [24]:

ln P � ln P�0� � 12mpu (4)

where P(0) is the value of P at zero penetrant pressure

and m is a measure of pressure dependence of P.

Hence, a linear decrease encountered in the cases

of O2, N2 and CH4 system implies that the increase

in the hydrostatic pressure on the high pressure

side (pu) causes a decrease in the free volume. While

a linear increase for the CO2 system indicates that

the increase in concentration of the dissolved

penetrant of condensable CO2 overwhelms the

hydrostatic pressure effect and causes an increase

in the free volume.

Fig. 2. Effect of upstream pressure on permeability for various gases in the PEG-600 polymer membranes above Tgs: (a) MPU6; (b)

MTPUU6; (c) TPUU6.

Fig. 3. Effect of upstream pressure on permeability for various gases in the PEG-600 polymer membranes below Tgs: (a) MPU6; (b)

MTPUU6; (c) TPUU6.


3.2. Pressure dependency of permeability below Tgs

Fig. 3 shows the semilogarithm plot of P vs. pu at

158C for PEG-600 polymer membranes. The mea-

sured temperature is lower than that of the Tgs of the

polymer membranes studied. The permeability of He

and H2 are relatively pressure independent, while the

permeability of O2, N2 and CH4 decrease with increas-

ing pu. These results are consistent with the results of

other literature [27±30]. Therefore, it is in qualitative

agreement with the predictions of the dual-mobility

model in glassy polymers [14,27,28]:

P � kDDD � DHC0Hb

1� bp(5)

where the nomenclature used above is the same as

reported elsewhere [14,27,28]. It is noted that the

pressure dependence on permeation of CO2 exhibits

a minimum permeability at a certain upstream pres-

sure (pc). The permeability decreases for pu<pc. How-

ever, when applied pressure is larger than pc, the

permeability increases with the pressure. A similar

pressure dependence of the permeability of CO2 at

different temperatures below Tgs is shown in Fig. 4.

Thus, a plasticization effect of CO2 occurs for pu>pc,

such that the plasticization of the polymers increases

chain mobility substantially. The diffusivity of CO2

increases with sorbed concentration much more

rapidly than the solubility decreases causing the per-

meability to increase with the pressure.

3.3. Structure±permeation relationship

Table 2 shows the permeability of various gases and

permselectivity ��A=B� of gas pairs in the polymer

membranes studied at 358C and 10 atm. Obviously,

the values of P increase as the molecular weight of

PEG soft segment increases from 400 to 600. In

contrast, P decreases with increasing TEPA content.

This shows that the free volume of the polymer

membranes studied increases with increasing mole-

cular weight of PEG but decreases with TEPA content.

However, the affect on permeability of the increase in

TEPA content is not as signi®cant as the change of

PEG molecular weight. This indicates that the gases

mainly pass through the PEG soft segment domains,

and the loosely packed hard segment domains are

minor passages. The values of permselectivity,

�A=B, for the gas pairs in the polymer membranes

were calculated by the ratio of the pure gas perme-

ability values. These fall in the range of 12±32 for

CO2/CH4, 15±19 for H2/N2, 23±35 for He/N2, 20±28

for He/CH4, and 3.7±4.7 for O2/N2. The selectivity

values for these polymer membranes have attracted

interest in gas separation of industrial process com-

pared with other literature [31].

Fig. 4. CO2 permeability at temperatures below Tgs as function of upstream pressure for the PEG-600 polymer membranes: (a) MPU6; (b)

MTPUU6; (c) TPUU6.


Basically, the permeation of gas through a dense,

non-porous membrane is generally analyzed using the

solution±diffusion model. Therefore, the permselec-

tivity of a membrane can be de®ned by the equation:

�A=B � PA=PB � �DA=DB��SA=SB� (6)

where D is diffusivity coef®cient, S is the solubility

coef®cient, DA/DB is the mobility selectivity, and

SA/SB is the solubility selectivity [32]. The values

of D and DA/DB as well as S and SA/SB for various

gases in the polymer membranes studied are listed in

Tables 3 and 4, respectively. These data show that in a

given polymer the diffusivity coef®cients decrease in

the penetrant gas order: DHe > DH2> DO2

> DN2>

Table 2

Permeability and permselectivity for various gases in the amine-containing polyurethane and poly(urea-urethane) membranes at 358C and

10 atm

Polymer Permeability a Permselectivity b

PHe PH2PO2

PN2PCH4

PCO2�CO2=CH4

�O2=N2�H2=N2

�He=CH4�He=N2

MPU4 1.95 1.29 0.317 0.0847 0.0969 1.12 12 3.7 15 20 23

MTPUU4 1.79 1.11 0.274 0.0629 0.0828 1.07 13 4.0 16 22 26

TPUU4 1.73 1.05 0.255 0.0558 0.0744 1.01 14 4.6 19 23 31

MPU6 11.3 6.08 1.75 0.374 0.419 12.9 31 4.7 16 27 30

MTPUU6 10.5 5.55 1.51 0.334 0.390 12.3 32 4.5 17 27 31

TPUU6 10.3 5.13 1.40 0.298 0.369 11.9 32 4.5 17 28 35

a Barrer (�10ÿ10 cm3(STP) cm2 cmÿ3 of polym. sÿ1 cmHgÿ1).b Ratio of pure gas permeability.

Table 3

Diffusivity coefficients a and diffusivity selectivity for various gases in the amine-containing polyurethane and poly(urea-urethane)

membranes at 358C and 10 atm

Polymer DHe DH2DO2

DN2DCH4

DCO2

DCO2

DCH4

DO2

DN2

DHe

DN2

DHe

DCH4

DH2

DN2

MPU4 698 283 22.5 8.33 3.98 1.61 0.40 2.7 84 175 34

MTPUU4 619 234 16.8 5.76 3.10 1.29 0.42 2.9 107 200 41

TPUU4 558 197 12.9 3.96 2.29 1.04 0.45 3.3 141 244 50

MPU6 3180 1100 73.9 21.4 10.4 4.04 0.39 3.5 149 306 51

MTPUU6 2860 899 58.0 17.0 8.3 3.20 0.39 3.4 168 345 53

TPUU6 2610 733 46.5 13.1 6.30 2.59 0.41 3.5 199 414 56

a �10ÿ8 cm2 sÿ1.

Table 4

Solubility coefficients a and solubility selectivity for various gases in the amine-containing polyurethane and poly(urea-urethane) membranes

at 358C and 10 atm

Polymer SHe SH2SO2

SN2SCH4

SCO2

SCO2

SCH4

SO2

SN2

SHe

SN2

SHe

SCH4

SH2

SN2

MPU4 0.211 0.346 1.07 0.773 1.85 52.9 29 1.4 0.27 0.11 0.45

MTPUU4 0.220 0.360 1.24 0.914 2.03 63.0 31 1.4 0.24 0.11 0.39

TPUU4 0.235 0.405 1.50 1.07 2.47 73.8 30 1.4 0.22 0.10 0.38

MPU6 0.270 0.421 1.80 1.33 3.06 243 79 1.4 0.20 0.09 0.32

MTPUU6 0.279 0.469 1.98 1.49 3.57 292 82 1.3 0.19 0.08 0.31

TPUU6 0.301 0.532 2.29 1.73 4.45 343 77 1.3 0.17 0.07 0.31

a �10ÿ2 cm3(STP) cmÿ3 of polym. atmÿ1.


DCH4> DCO2

. This shows a general trend that the Dvalues in a given polymer decrease with increasing

Lennard-Jones diameter [33]. On the other hand, in a

given polymer the solubility coef®cients increase in

the penetrant gas order: SCO2> SCH4

> SO2> SN2

>SH2

> SHe. This shows that the solubility coef®cient in

the polymers increases with the increase in the con-

densibility of the penetrant gas. This is consistent with

the results reported for many other elastomers [34].

As shown in Table 3, an increase in the value of Dfor a given penetrant gas follows an increase in the

values of P, i.e., the greater the free volume of the

polymer membrane is, the easier the penetrant gas can

diffuse through the polymer membrane. It is note-

worthy that for condensable CO2, the S values of the

PEG6 polymer membranes increase 4.6-fold over the

values of PEG4, while for the non-condensable gases,

the S values increase by 1.2±1.8 fold (Table 4). The Svalues of CO2 also increase more than those of non-

condensable gases with increasing TEPA content. This

might be due to a speci®c interaction between the free

amine groups of TEPA and CO2 molecules as shown in

FTIR spectra previously [17]. Furthermore, informa-

tion on the permselectivity of the polymers studied in

term of SA/SB and DA/DB were discussed. Because

this two ratios represent the contributions of the

differences in the diffusivity and solubility, respec-

tively, of different gases to the permselectivity. There-

fore, it is an index of the assessment of the gas

separation mechanism. In this study, the CO2/CH4

gas pair shows the value of SA/SB is far larger than

the DA/DB, Consequently, indicating that the perm-

selectivity of the gas pair is controlled by solubility

selectivity. On the other hand, for He/N2, He/CH4, and

H2/N2, the ratio DA/DB is much larger than the SA/SB

ratio. Therefore, the separation mechanism of these

gas pairs is controlled by diffusivity selectivity. How-

ever, both solubility and diffusivity selectivity are

important in the separation of O2 and N2.

4. Conclusion

The amine-containing polyurethane and poly(ur-

ethane-urea) membranes studied are amorphous poly-

mers. The free volume of these polymer membranes

are related to hydrogen bonding in the soft segment

and hard segment domains, which is supported by the

density of the polymer membranes. Those with higher

packing density (less free volume) were less perme-

able. The dependence of gases permeability on pene-

trant pressure in the polymer membranes studied is

satisfactorily represented by a modi®ed free-volume

model above Tgs. However, below Tgs, the pressure

dependence of CO2 permeability exhibited a mini-

mum caused by the plasticization effect of conden-

sable CO2.

Acknowledgements

The authors are grateful to the National Science

Council of the Republic of China for its ®nancial

support (NSC87-2216-E-006-003) and Prof. E.M.

Woo and Prof. J.-C Lin of the Department of Chemical

Engineering for their editorial comments on this

manuscript.

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