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

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<ul><li><p>The gas transport properties of amine-containing polyurethaneand poly(urethane-urea) membranes</p><p>Liang-Siong Teo, Chuh-Yung Chen*, Jen-Feng Kuo</p><p>Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 701, ROC</p><p>Received 2 September 1997; received in revised form 6 November 1997; accepted 7 November 1997</p><p>Abstract</p><p>A series of amine-containing polyurethanes and poly(urethane-urea)s based on 4,40-diphenylmethane diisocyanate andeither poly(ethylene glycol) of molecular weights 400 or 600 were prepared as gas separation membranes. The amine</p><p>functional groups of N-methyldiethanolamine (MDEA) and/or tetraethylenepentamine (TEPA) were introduced into the hard</p><p>segment as a chain extender. The gas transport data of He, H2, O2, N2, CH4 and CO2 in these polymer membranes were</p><p>determined by using the Barrer's high-vacuum technique and the time-lag method. The restriction of chain mobility has been</p><p>shown by the formation of hydrogen bonding in the soft segment and hard-segment domains, resulting in the increase in the</p><p>density, glass transition temperature of soft segments (Tgs). The separation mechanism of various gas pairs used in industrial</p><p>processes is also discussed. Effect of pressure on permeability of the gases above and below Tgs was studied. It was found that</p><p>the gas permeability increased or decreased with upstream pressure above Tgs, and should be described by a modied free-</p><p>volume model. On the other hand, the condensable CO2 exhibits a minimum permeability at a certain upstream pressure below</p><p>Tgs. The permeability of He and H2 were pressure independent above and below the Tgs. # 1998 Elsevier Science B.V.</p><p>Keywords: Gas separations; Gas and vapor permeation; Membrane preparation and structure; Polyurethane membrane;</p><p>Permselectivity</p><p>1. Introduction</p><p>Membrane-based separation has been utilized in</p><p>many chemical industries. It provides an energy-ef-</p><p>cient, compact, modular, and green-house process.</p><p>Applications of polymeric membranes as gas separa-</p><p>tion membranes are used in a wide variety of areas [1],</p><p>such as carbon dioxide recovery in petroleum applica-</p><p>tions, removal of helium gas from natural gas, recov-</p><p>ery of hydrogen from ammonia plant purge streams,</p><p>oxygen/nitrogen separation, etc.</p><p>Polyurethane is a segmented copolymer composed</p><p>of alternating soft and hard segments. The study of</p><p>transport properties of penetrant gases through poly-</p><p>urethane membranes is particularly interesting [213].</p><p>It is possible to introduce controlled changes in polyol</p><p>length and chemical nature and the proportions of</p><p>the constituents which make up the soft and hard</p><p>segments of polyurethanes. These changes result</p><p>in different physicochemical properties. Various</p><p>researchers [4,7,10,11] have shown the gas perme-</p><p>ability of polyurethane membranes increases with the</p><p>decrease of hard segment content and increase of soft</p><p>segment molecular weight. In addition, correlation has</p><p>Journal of Membrane Science 141 (1998) 9199</p><p>*Corresponding address. Fax: +886 6 2344496</p><p>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</p></li><li><p>been established between the gas permeability and the</p><p>chemical nature of polyols and chain extenders. It</p><p>affects the transport properties of polyurethane mem-</p><p>branes with the result of changing the phase-separated</p><p>domain morphology, polyfunctional cross-linking,</p><p>crystallinity, density and glass transition of the mem-</p><p>branes.</p><p>The transport of gases through polymer membranes</p><p>depends strongly on whether the polymer is in its</p><p>rubbery or glassy state. Usually, the gas sorption</p><p>process in glassy polymers (TTg) because of their</p><p>nonequilibrium state [1416]. In our previous paper</p><p>[17] we chose the CO2 afnity reagents N-methyl-</p><p>diethanolamine (MDEA) and tetraethylenepentamine</p><p>(TEPA) as chain extenders to prepare amine-contain-</p><p>ing polyurethanes (PU) and poly(urethane-urea)s</p><p>(PUU) and studied the permeation and sorption iso-</p><p>therm of CO2. This study showed that the sorption</p><p>isotherms obey Henry's law above Tgs, while below</p><p>Tgs the sorption isotherms follow a dual-mode sorption</p><p>model. The aim of the present paper is to study the</p><p>transport properties of various gases in these amine-</p><p>containing PU and PUU membranes. Relationships</p><p>between transport properties and chemical structure</p><p>are discussed.</p><p>2. Experimental</p><p>2.1. Polymers</p><p>The PU and PUU membranes chosen for the present</p><p>study were prepared by a two-step process in N,N-</p><p>dimethylformamide (DMF). Poly(ethylene glycol)</p><p>(PEG) of molecular weight 400 or 600 was used as</p><p>the soft segment component. The hard segment was</p><p>formed from 4,40-diphenylmethane diisocyanate(MDI) and chain extended with N-methyldiethanol-</p><p>amine (MDEA) and/or tetraethylenepentamine</p><p>(TEPA). Details of the polymerization procedure have</p><p>been described previously [17]. The chemical struc-</p><p>tures are shown in Scheme 1.</p><p>2.2. Film casting</p><p>The polymers were used in the form of non-porous</p><p>planar membranes which were prepared from DMF</p><p>solution containing 15 wt% polymer. These solutions</p><p>were centrifuged at 2000 rpm to remove any gel</p><p>particles or undissolved impurities. Then a certain</p><p>amount of the solution was placed in a glass cylinder</p><p>with a clean polyethylene (PE) lm attached to the</p><p>bottom. To prevent dust pollution, the top of the glass</p><p>cylinder was covered with a lter paper. The system</p><p>was placed in a nitrogen-purged oven at 808C anddried for about 24 h. Finally, the polymer membranes</p><p>were removed from the PE substrate and put in a</p><p>vacuum at 508C for about 3 days to remove theremaining solvent. The thickness of the dry mem-</p><p>branes varied from 120 to 180 mm.</p><p>2.3. Characterization of polymer membranes</p><p>The composition of each polymer membrane stu-</p><p>died was determined by a Heraeus CHN-O-RAPID</p><p>Elemental Analyzer (EA). Density of each polymer</p><p>membrane was measured by the immersion method</p><p>[18], which included weighing of samples in air and in</p><p>distilled water. Thermal transition temperatures of</p><p>each polymer membrane were obtained by a DuPont</p><p>DSC 2910 differential scanning calorimeter linked to</p><p>a Thermal Analysis 2000 system for data acquisition.</p><p>The samples were scanned from70 to 2508C at a rateof 108C/min under nitrogen purging.</p><p>2.4. Permeation test</p><p>The permeation of pure gases through the polymer</p><p>membranes was measured using Barrer's high-</p><p>vacuum method [19]. The purity of gases (He, H2,</p><p>N2, O2, CH4, and CO2) used was 99.5% or higher. The</p><p>cumulative amount of gas passing through the poly-</p><p>mer membrane in a constant volume receiving cham-</p><p>ber (V) was measured by the pressure increase on the</p><p>downstream side (pd) with a pressure transducer (MKS</p><p>Baratron 222B with a full-scale range of 10 Torr</p><p>connected to an MKS Type PDR-C-1C readout).</p><p>The upstream pressure (pu) was monitored with a</p><p>pressure transducer (Honeywell). At steady gas ow,</p><p>the permeability coefcient (P) was determined fromthe slope (dp/dt) of the linear portion of pd vs. time plot</p><p>using the following equation:</p><p>P 273 V Ldpdt</p><p>760 T A pu pd (1)</p><p>92 L.-S. Teo et al. / Journal of Membrane Science 141 (1998) 9199</p></li><li><p>where L is the thickness of the membrane, T is the</p><p>absolute temperature, and A is the area of the mem-</p><p>brane. The downstream pressure, pd, can be regarded</p><p>as negligible since pdpu.The diffusion coefcient (D) was determined by the</p><p>time-lag method [20], and is represented as</p><p>D L2</p><p>6(2)</p><p>where is the time lag, i.e. the intercept obtained byextrapolating the linear region of pd vs. time plot to the</p><p>time axis.</p><p>The solubility coefcient (S) was then calculatedfrom the equation of</p><p>P D S (3)</p><p>3. Results and discussion</p><p>Table 1 summarizes the characteristic properties of</p><p>the amine-containing PU and PUU membranes. It</p><p>shows that the hard segment content of the polymer</p><p>membranes is in the range of 5164%. The density of</p><p>the polymer membranes is in the range of 1.01.2 g/</p><p>Scheme 1.</p><p>L.-S. Teo et al. / Journal of Membrane Science 141 (1998) 9199 93</p></li><li><p>cm3, increasing with the decrease of molecular weight</p><p>of the PEG soft segment but with increase of the TEPA</p><p>content. This indicates that the packing density of the</p><p>PEG-400 polymer membranes is larger than that of</p><p>PEG-600 ones, while TPUU membranes comprise the</p><p>larger packing density. In addition, PEG-400 polymer</p><p>membranes show a glass transition temperature of</p><p>PEG soft segment (Tgs) at 15278C, while thePEG600 ones exhibit a lower Tgs of 188C to68C. Our previous paper [21] showed that a sig-nicant amount of hydrogen bonding between the soft</p><p>and hard segments in the soft segment domains.</p><p>Therefore, it seems that for PEG-400 more hard seg-</p><p>ments are dissolved in soft segment domains causing</p><p>more stiffness and a higher Tgs. Thus, PEG-400 poly-</p><p>mer membranes exhibit less free volume than those of</p><p>PEG-600. On the other hand, the microcrystalline</p><p>ordering of hard segment domains above 2008C wasnot found [22,23]. This indicates that the polymer</p><p>membranes are amorphous polymers. Our previous</p><p>paper [21] also showed that the MPU membranes</p><p>exhibited only loosely packed urethane hydrogen</p><p>bonding in the hard segment domains. However, the</p><p>TPUU membranes present more hydrogen bonded</p><p>urea carbonyl with a mixed state of three-dimensional</p><p>hydrogen bonding as well as conventional interurea</p><p>bonds and that of interurethane bonds. It seems that</p><p>the hydrogen packing density increases with increas-</p><p>ing TEPA content. It is consistent with the increase in</p><p>the density of the polymer membranes (Table 1).</p><p>3.1. Pressure dependency of permeability above Tgs</p><p>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 -600polymer membranes, respectively. The measured tem-</p><p>perature is higher than that of the Tgs of the polymer</p><p>membranes studied. In the gures, all of the lines were</p><p>obtained from least-square ts of the experimental</p><p>data. It shows that the semilog plot of P vs. pu is linear.The permeability of He and H2 are pressure indepen-</p><p>Table 1</p><p>Characteristics of the amine-containing polyurethane and poly-</p><p>(urea-urethane) membranes</p><p>Polymer Hard segment</p><p>content (wt%)</p><p>Density</p><p>(g/cm3)</p><p>Tgs(8C)</p><p>MPU4 61.4 (60.8) a 1.16 27</p><p>MTPUU4 62.7 (62.8) 1.18 21</p><p>TPUU4 63.9 (63.3) 1.21 15</p><p>MPU6 51.5 (50.8) 1.04 6MTPUU6 52.8 (52.2) 1.09 13TPUU6 54.1 (53.5) 1.12 18a Calculated values.</p><p>Fig. 1. Effect of upstream pressure on permeability for various gases in the PEG-400 polymer membranes above Tgs: (a) MPU4; (b)</p><p>MTPUU4; (c) TPUU4.</p><p>94 L.-S. Teo et al. / Journal of Membrane Science 141 (1998) 9199</p></li><li><p>dent while those of O2, N2 and CH4 decrease with</p><p>increasing pressure. However, the permeability of CO2increases with increasing pressure. According to Stern</p><p>[24] and Sada [25,26] and co-workers, such a linear</p><p>relationship above Tgs indicates that the permeation of</p><p>the polymer membranes for all the gases might be</p><p>described by the modied free-volume model. In the</p><p>case of pupd, the pressure dependency of perme-ability is given by the relation [24]:</p><p>ln P ln P0 12mpu (4)</p><p>where P(0) is the value of P at zero penetrant pressureand m is a measure of pressure dependence of P.Hence, a linear decrease encountered in the cases</p><p>of O2, N2 and CH4 system implies that the increase</p><p>in the hydrostatic pressure on the high pressure</p><p>side (pu) causes a decrease in the free volume. While</p><p>a linear increase for the CO2 system indicates that</p><p>the increase in concentration of the dissolved</p><p>penetrant of condensable CO2 overwhelms the</p><p>hydrostatic pressure effect and causes an increase</p><p>in the free volume.</p><p>Fig. 2. Effect of upstream pressure on permeability for various gases in the PEG-600 polymer membranes above Tgs: (a) MPU6; (b)</p><p>MTPUU6; (c) TPUU6.</p><p>Fig. 3. Effect of upstream pressure on permeability for various gases in the PEG-600 polymer membranes below Tgs: (a) MPU6; (b)</p><p>MTPUU6; (c) TPUU6.</p><p>L.-S. Teo et al. / Journal of Membrane Science 141 (1998) 9199 95</p></li><li><p>3.2. Pressure dependency of permeability below Tgs</p><p>Fig. 3 shows the semilogarithm plot of P vs. pu at158C for PEG-600 polymer membranes. The mea-sured temperature is lower than that of the Tgs of the</p><p>polymer membranes studied. The permeability of He</p><p>and H2 are relatively pressure independent, while the</p><p>permeability of O2, N2 and CH4 decrease with increas-</p><p>ing pu. These results are consistent with the results of</p><p>other literature [2730]. Therefore, it is in qualitative</p><p>agreement with the predictions of the dual-mobility</p><p>model in glassy polymers [14,27,28]:</p><p>P kDDD DHC0Hb</p><p>1 bp (5)</p><p>where the nomenclature used above is the same as</p><p>reported elsewhere [14,27,28]. It is noted that the</p><p>pressure dependence on permeation of CO2 exhibits</p><p>a minimum permeability at a certain upstream pres-</p><p>sure (pc). The permeability decreases for pupc,</p><p>such that the plasticization of the polymers increases</p><p>chain mobility substantially. The diffusivity of CO2increases with sorbed concentration much more</p><p>rapidly than the solubility decreases causing the per-</p><p>meability to increase with the pressure.</p><p>3.3. Structurepermeation relationship</p><p>Table 2 shows the permeability of various gases and</p><p>permselectivity A=B of gas pairs in the polymermembranes studied at 358C and 10 atm. Obviously,the values of P increase as the molecular weight ofPEG soft segment increases from 400 to 600. In</p><p>contrast, P decreases with increasing TEPA content.This shows that the free volume of the polymer</p><p>membranes studied increases with increasing mole-</p><p>cular weight of PEG but decreases with TEPA content.</p><p>However, the affect on permeability of the increase in</p><p>TEPA content is not as signicant as the change of</p><p>PEG molecular weight. This indicates that the gases</p><p>mainly pass through the PEG soft segment domains,</p><p>and the loosely packed hard segment domains are</p><p>minor passages. The values of permselectivity,</p><p>A=B, for the gas pairs in the polymer membraneswere calculated by the ratio of the pure gas perme-</p><p>ability values. These fall in the range of 1232 for</p><p>CO2/CH4, 1519 for H2/N2, 2335 for He/N2, 2028</p><p>for He/CH4, and 3.74.7 for O2/N2. The selectivity</p><p>values for these polymer membranes have attracted</p><p>interest in gas separation of industrial process com-</p><p>pared with other literature [31].</p><p>Fig. 4. CO2 permeability at temperatures below Tgs as function of upstream pressure for the PEG-600 polymer membranes: (a) MPU6; (b)</p><p>MTPUU6; (c) TPUU6.</p><p>96 L.-S. Teo et al. / Journal of Membrane Science 141 (1998) 9199</p></li><li><p>Basically, the permeation of gas through a dense,</p><p>non-porous membrane is generally analyzed using the</p><p>solutiondiffusion model. Therefore, the permselec-</p><p>tivity of a membrane can be dened by the equation:</p><p>A=B PA=PB DA=DBSA=SB (6)where D is diffusivity coefcient, S is the solubility</p><p>coefcient, DA/DB is the mobility selectivity, andSA/SB is the solubility selectivity [32]. The valuesof D and DA/DB as well as S and SA/SB for variousgases in the polymer membranes studied are listed in</p><p>Tables 3 and 4, respectively. These data show that in a</p><p>given polymer the diffusivity coefcients decrease in</p><p>the penetrant gas order: DHe &gt; DH2 &gt; DO2 &gt; DN2 &gt;</p><p>Table 2</p><p>Permeability and permselectivity for various gases in the...</p></li></ul>