Changes in Gas-Transport Properties with the Phase Structure of

Jun 28, 2010 - K. W. Song, K. R. Ka and C. K. Kim* ... blending resulted in phase structure changes: As the PPO content in the blend increased, the ha...
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Changes in Gas-Transport Properties with the Phase Structure of Blends Containing Styrene-Butadiene-Styrene Triblock Copolymer and Poly(2,6-dimethyl-1,4-phenylene oxide) K. W. Song, K. R. Ka, and C. K. Kim* School of Chemical Engineering & Materials Science, Chung-Ang UniVersity, 221 Huksuk-Dong, Dongjak-Gu, Seoul 156-756, Korea

Blend films prepared from styrene-butadiene-styrene triblock copolymer (SBS) and poly(2,6-dimethyl-1,4phenylene oxide) (PPO) in various compositions were examined to explore their application as gas separation membranes. PPO formed a single-phase mixture with hard styrene segments in SBS regardless of the blend composition. Therefore, an increase in the hard-phase volume with PPO blending resulted in phase structure changes: As the PPO content in the blend increased, the hard phase that was dispersed in the matrix formed by butadiene soft segments was converted into a continuous lamellar structure and finally formed a matrix of the blend by phase inversion. A decrease in the gas permeability and an increase in the gas selectivity were observed with increasing PPO content. The continuous soft phase was changed to the discrete phase in blends containing between 40 and 50 wt % PPO. As a result, an abrupt decrease in the gas permeability was observed at around these blend compositions. Permeability coefficients showed a small decrease as the upstream gas pressure increased when the hard phase formed a matrix, whereas they were not changed regardless of the upstream gas pressure when the soft segments formed the continuous phase. Plasticization caused by a polar penetrant was not observed in the SBS/PPO blend membranes. Our results show that blend films exhibiting desired gas permselectivity can be fabricated by controlling the phase-separated structure of SBS/PPO blends. Introduction Polymeric materials and their blends have been used in gastransport applications such as gas separation and as barriers for packing.1-17 Many efforts to produce new polymers and modify conventional polymers through blending have been made to obtain high permselective membranes.12-16 Gas separation membranes prepared by blending are of special interest because they allow the advantages of each material to be combined into a single product. A membrane needs to provide a reasonable gas flux, separation ability, and good thermal and mechanical properties to conduct effectively. The above criteria might be met by a block copolymer composed of a hard block and a soft block.17 The balance between the hard and soft blocks provides good separation without a loss of permeability. The hard segments (glassy segments) in a block copolymer provide mechanical and thermal resistance and excellent gas selectivity, whereas the soft segments (rubbery segments) provide good permeability. Appropriately designed triblock copolymers of styrene-butadienestyrene (SBS) exhibit thermoplastic elastomeric behavior, combining the good processability of thermoplastics with rubber elasticity.18 SBS elastomer is a highly permeable polymer for most gases, but its gas selectivity and mechanical strength are low. Therefore, the modification of SBS is necessary for its use as material for gas separation membranes. In previous works, SBS was modified by graft copolymerization with functional monomers to enhance its gas selectivity and mechanical strength.19-24 Although graft polymerization of SBS was somewhat effective in increasing the gas selectivity, it is a timeconsuming and expensive process in industrial applications. The blending of SBS with other polymers having high gas permselectivity, thermal resitance, and mechanical strength offers * To whom correspondence should be addressed. E-mail: ckkim@ cau.ac.kr.

an attractive opportunity for developing novel membranes that overcome the drawbacks of SBS membranes. Poly(2,6-dimethyl1,4-phenylene oxide) (PPO) might be an attractive material for blending with SBS because of its miscibility with polystyrene and its high gas permselectivity, thermal resistance, high strength, chemical resistance, and stiffness.1,17,25-27 The gas-transport properties of blends composed of glassy polymers generally depend on the volume fraction of each component.1-10 Furthermore, the transport rates of various gases through blend membranes are especially sensitive to the microstructure of the polymer mixture when each component of the blend exists in different states (i.e., rubbery and glassy states) at the application temperature.10,11,16-19,28-30 To develop blend membranes having a high permselectivity, changes in the gas transport with the microstructure of blends should be understood. It is known that polystyrene (PS) homopolymer is completely miscible with PPO at all molecular weights and composition ranges.25-27 When the styrene segments in SBS block copolymer are mixed with PPO by blending, there is an increase in the hard-phase volume. When an SBS thermoplastic elastomer contains about 30 wt % styrene in the form of end blocks, the soft phase is continuous, whereas the hard phase is dispersed in the form of cylinders.24 Blends might appear to have a cocontinuous morphology at intermediate compositions, and finally, phase inversion (i.e., a dispersed rubbery phase in a glassy matrix) might be observed with increased PPO content in the blend. As a result, the addition of PPO with a glass transition temperature (Tg) higher than that of PS could raise the softening point of the hard phase and change the blend from a thermoplastic elastomer to a leathery material and, finally, to a hard glassy thermoplastic. If the styrene segments mix with the PPO to form the hard phase, then by changing the mixing ratio of SBS to PPO, one can produce membranes for gas separation exhibiting high permselectivity, excellent thermal resistance, and improved mechanical properties. To confirm this

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hypothesis, gas separation membranes were fabricated from SBS/PPO blends with various compositions. Subsequently, the mixing behavior between the styrene segments in SBS and PPO, the changes in morphology with blend composition, and the gas permselectivity were explored. Materials and Procedure Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased j W ) 224000 g/mol, M jn ) from Aldrich (Milwaukee, WI; M 32000 g/mol). Styrene-dutadiene-styrene (SBS) triblock copolymer was also purchased from Aldrich (30 wt % styrene; j W ) 140000 g/mol). Polystyrene (PS) was of commercial M j W ) 187000 g/mol, M j n ) 96000 g/mol) grade (GPPS-15NF; M and was supplied by LG Chemicals (Seoul, South Korea). The blend was prepared as a film by casting solutions containing 5 wt % total polymers in chloroform onto a glass plate at room temperature. The cast solutions were dried at 30 °C for 1 day until most of the solvent had evaporated. After solvent evaporation, the blend films were floated off the plate with deionized water. The resulting films were dried further in a vacuum oven by slowly increasing the temperature to 100 °C. They were kept at this final temperature for 1 week to ensure that all residual solvent had been eliminated, and then they were quenched to room temperature. The glass transition temperatures (Tg) of SBS, PPO, and their blends were measured at a heating rate of 20 °C/min using a differential scanning calorimeter (TA Instruments, New Castle, DE; model DSC-2010). The first scan was run to 240 °C to erase the previous thermal history due to sample preparation, and then the sample was quenched to room temperature to start the second scan. Tg was defined as the onset of the change in the heat capacity during the second scan. Although SBS characteristically has glass transitions associated with both rubbery and glassy phases, only the hard-phase (or glassy) transition range was examined by differential scanning calorimetry (DSC) because the point of interest was the phase behavior of the PPO and the styrene segments. The morphologies of the blends were investigated by highresolution transmission electron microscopy (HR-TEM; JEOL, Tokyo, Japan; model JEM-3010). The films were stained for 1 h in a 2% solution of osmium tetroxide (to stain the butadiene segments). Specimens embedded in epoxy resin were sectioned with an ultramicrotome (RMC, Tucson, AZ; model MT7000) at room temperature. Gas permeability coefficients for various pure gases were determined using a constant-volume method with equipment and procedures described in previous studies.9,10,31 The measurements were performed at various upstream pressures and 35 °C for all gases. The gas permeability coefficients of blends were also examined for mixtures of CH4 and CO2 (50/50 vol %) at a partial pressure of 0.9 MPa. The permeated gases were analyzed by gas chromatography (GC; Varian, Palo Alto, CA; model 3900 GC) for detection. A purge stream was supplied to build up the desired pressure on the feed side of the film. At the other cell side, the permeated gases entered the suction of the pump and were detected by GC. Five specimens of each membrane were tested, and the results were averaged. Results and Discussion Thermal Behavior and Morphology Changes of Blend Films. Figure 1 shows selected DSC thermograms for PPO, the hard-phase transition region of SBS, and SBS/PPO blends observed during the second scan. It is known that the phase transition temperature (Tg) of the butadiene segments occurs at

Figure 1. DSC thermograms of SBS/PPO blends: (a) SBS, (b) SBS/PPO ) 80/20, (c) SBS/PPO ) 40/60, (d) PPO.

Figure 2. Changes in Tg of SBS/PPO blends as a function of blend composition. Note that the solid line indicates the Tg values of the SBS/ PPO blends predicted with the Fox equation.

around -80 °C.18 However, it could not be detected by DSC because of instrument limitations. The Tg value of the styrene segments in the pure block copolymer (Figure 1a) was below what would be expected for pure polystyrene (about 100 °C). This difference was attributed to the relatively low molecular weight of PS in the copolymer, the diffuse interphase between the component domains, and surface energy effects.18 The blends exhibited a single Tg at an intermediate temperature that fell between the Tg values of the styrene segments and PPO. As summarized in Figure 2, blends prepared with other compositions also showed a single Tg, and the Tg values of the blends were always similar to those approximated using the Fox equation. It is known that PS/PPO blends are completely miscible at all composition ranges and do not undergo phase separation until reaching the thermal degradation temperature (about 400 °C).32 The results observed here indicate that the styrene segments of the copolymer and PPO also formed one phase for all blend compositions.

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Figure 4. Carbon dioxide permeability coefficients as a function of PPO content in SBS/PPO blends.

Figure 3. Microphotographs of the SBS/PPO blends observed by HR-TEM: (a) SBS, (b) SBS/PPO ) 80/20, (c) SBS/PPO ) 60/40, (d) SBS/PPO ) 50/50, (e) SBS/PPO ) 40/60. Note that specimens were stained with osmium tetraoxide to yield a dark rubber phase.

The phase morphologies of the SBS/PPO blends were examined by HR-TEM with osmium teraoxide used to stain the phase formed by the butadiene midblock. In the photomicrographs shown here, this phase appears dark. It is known that, when the SBS thermoplastic elastomer contains about 30% styrene by weight in the form of end blocks, the soft phase is continuous, whereas the hard phase is dispersed in the form of cylinders (see Figure 3a).18 Figure 3b is a photomicrograph of the SBS/PPO ) 80/20 blend corresponding to 44 wt % hard phase (styrene block and PPO) and 56 wt % soft phase (butadiene block). In this two-dimensional view, both phases appear to have continuous characteristics. There was an increase in the hard-phase volume, in which both phases appeared to form a lamellar structure. As shown in Figure 3c for the SBS/ PPO ) 60/40 blend corresponding to 58 wt % hard phase and 42 wt % soft phase, even though both phases remained continuous, the hard phase better resembled a true matrix, which generally features a thick lamellar structure with greater connectivity. When the blend contained 50 wt % PPO (65 wt % hard phase and 35 wt % soft phase), a morphological feature not seen at lower PPO contents was observed (Figure 3d). The ordered structure observed at low PPO content became disordered when the lamellar structure of the soft phase was converted into an irregularly shaped network structure that could be construed as more typical of a dispersed phase. Figure 3e shows a microphotograph for the SBS/PPO ) 40/60 blend (72 wt % hard phase and 28 wt % soft phase). The hard phase is clearly continuous, whereas the soft phase is dispersed and has a spherical shape. The results observed here indicate that styrene end blocks in SBS and PPO formed a single, mixed hard phase regardless of blend composition. As a result, the hard phase

that had a dispersed characteristic in SBS was converted into a continuous skeletal phase (lamellar structure) with increased PPO content in the blend, and finally, a phase inversion (formation of the dispersed phase with soft segments) occurred when the blend contained more than 50 wt % PPO. Gas Permeation Properties. The membrane process is a lowcost means of separating gas mixtures such as flue gases (CO2 separation from a CO2/N2 mixture), natural gases (CH4 separation from a CH4/CO2 mixture), and air (O2 separation from an O2/N2 mixture) when high-purity gas streams are not vital. For gas separation, the polymeric membrane has to meet certain criteria such as a reasonable permselectivity and good thermal and mechanical properties. The poor thermal and mechanical properties of SBS block copolymers might be enhanced by blending with PPO. The permselectivities of the SBS/PPO blend films for these gases were examined to explore application of the films as gas separation membranes. The permeability coefficients for carbon dioxide, methane, oxygen, and nitrogen as a function of blend composition are shown in Figures 4 and 5. The gas permeability of the SBS triblock copolymer is mainly determined by the butadiene rubber phase, which forms a matrix in SBS. As shown in Figures 4 and 5, the membrane fabricated from SBS triblock copolymer exhibited a high permeability coefficient for all gases. Note that the permeability coefficients of the PS homopolymer for carbon dioxide, methane, oxygen, and nitrogen were measured to be 12.2, 0.71, 1.2, and 0.22 barrer [1 barrer ) 1010 cm3(STP) cm/ (cm2 s cmHg)], respectively. Gas permeability decreased with increasing PPO content. When blends contained PPO between 40 wt % and 50 wt %, the gas permeability abruptly decreased. The rate of decrease of gas permeability with PPO content when the SBS/PPO blends contained more than 50 wt % PPO was lower than that when the SBS/PPO blends contained less than 40 wt % PPO. The linear additive rule, which is generally used to explain the gas permeability behavior of blend membranes, has some shortages in explaining the dramatic decrease in the gas permeability through SBS/PPO blend membranes.1,2,12-16,31 The concept of percolation theory has been employed to explain the dramatic changes in the gas permeabilities of block-copolymer and blend membranes composed of a rubbery polymer and a glassy polymer.28-30 A dramatic increase in the gas permeability through hetrophase blend membranes composed of polyisoprene

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Figure 5. Permeability coefficients of various gases as a function of PPO content in SBS/PPO blends.

(PI) and PPO has been observed at a certain blend composition where the minor component (the more permeable component, PI) starts to form continuous channels across the membrane, termed percolation threshold.28 As a result, the gas permeability examined as a function of block copolymer composition or blend composition increases sigmoidally with the content of the more permeable component. The gas permeability behavior of SBS/ PPO blend membranes observed here can also be explained with this theory. As shown in Figure 3, the soft phase of SBS composed of butadiene segments was continuous, whereas the hard phase composed of styrene segments was dispersed in the form of cylinders and spheres. In this two-dimensional view, when SBS was blended with PPO, both phases appeared to have continuous characteristics. Both the soft phase and the hard phase composed of styrene segments and PPO formed a cocontinuous phase (lamellar structure) when the blend contained between 20 and 40 wt % PPO. In this case, gas permeability continuously decreased with increasing PPO content in the blend. Conversion of the continuous soft phase to the discrete phase occurred in blends containing between 40 and 50 wt % PPO. The number of continuous channels formed by soft segments decreased with increasing PPO content as described by percolation theory. As a result, the gas permeability decreased abruptly near PPO contents of 40-50 wt %. When blends contained 60 wt % PPO or more, the soft segments formed a spherically dispersed phase. Because gas permeability mainly depends on the continuous hard phase in this case, a smaller decrease in the gas permeability with PPO content was observed. Figure 6 shows the permeability coefficients of various blends for carbon dioxide, methane, and oxygen as a function of upstream driving pressure. As shown in Figure 6a for the SBS/ PPO ) 80/20 blend, the permeability coefficients remained almost constant irrespective of the upstream pressure. When soft segments formed the continuous phase, the relatively negligible upstream pressure dependence of permeability indicated that the permeation occurred primarily in the Henry region as described in a dual-sorption model.1 However, when the soft segments formed the discrete phase (Figure 6b for SBS/PPO ) 80/20 blends), the permeability coefficients decreased with increasing upstream pressure, demonstrating dual-mode transport behavior in common glassy polymers.1 The permeability coef-

Figure 6. Permeability coefficients of various gases as a function of upstream driving pressure: (a) SBS/PPO ) 80/20 blend membrane, (b) SBS/ PPO ) 20/80 blend membrane.

ficient of CO2 showed a small but significant decrease as the upstream gas pressure increased, and methane and oxygen showed smaller decreases. This means that the gas permeation through the blend membrane was determined mostly by the glassy hard phase when hard segments formed the continuous phase and soft segments formed the discrete phase. Figure 7 shows the permeability coefficients for CO2 and CH4 of SBS/PPO membranes measured with the pure and mixed gas (CO2/CH4 ) 50/50) at a partial pressure of 0.9 MPa. In all cases gas permeability coefficients measured with the mixed gas were identical with those measured with the pure gases with in experimental accuracy of the measurements. Thus, it can be considered that until a partial pressure of 0.9 MPa for CO2 and for CH4 the effects of plasticization caused by polar penetrant (CO2) seem to be negligible for these blends.1-3,33 These results indicate that the mixed CO2/CH4 gas permeability behavior in SBS/PPO blend membrane can be predicted by pure-gas measurement. The ideal separation factor, or selectivity, is given by the ratio of permeability coefficients. Figure 8 shows the gas selectivities of the blends for CO2/N2, CO2/CH4, and O2/N2. The selectivities of the SBS membrane for the CO2/N2, CO2/CH4, and O2/ N2 gas pairs were 17.9, 8.1, and 2.9, respectively. The gas selectivities of this membrane were always lower than those of

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continuous lamellar structure when the blend contained about 20 wt % PPO. The ordered lamellar structure observed until 40 wt % PPO changed into a disordered network structure that could be construed as more typical of a dispersed phase. Then, phase inversion, in which soft segments formed the dispersed phase, was observed when the blend contained more than 50 wt % PPO. A decrease in the gas permeability and an increase in the gas selectivity were observed with increasing PPO content. The observed abrupt decrease in the gas permeability of blends containing between 40 and 50 wt % PPO can be interpreted in terms of percolation theory. When the hard phase formed a matrix, the permeability coefficients decreased with increasing upstream pressure, demonstrating dual-mode transport behavior. Plasticization of the SBS/PPO blend membranes caused by CO2 polar penetrant was not observed at any blend composition. Blend membranes having the desired gas permselectivity could be fabricated by controlling the microstructure of the SBS/PPO blends. Acknowledgment Figure 7. Mixed- and pure-gas permeability coefficients for CO2 and CH4 as a function of SBS/PPO blend composition at a partial pressure of 9 atm.

This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (200900899). Literature Cited

Figure 8. Changes in gas selectivities for CO2/N2, CO2/CH4, and O2/N2 pairs as a function of PPO content in SBS/PPO blends.

the membranes of PS (CO2/N2 selectivity ) 20.5, CO2/CH4 selectivity ) 17.2, O2/N2 selectivity ) 5.5) and PPO (CO2/N2 selectivity ) 23.4, CO2/CH4 selectivity ) 12.8, O2/N2 selectivity ) 3.9), which are glassy polymers. The selectivity of the blend membrane increased with increasing PPO content. An abrupt increase in the gas selectivity was also observed when the blend contained between 40 and 50 wt % PPO. These results indicate that blend films exhibiting desired gas permselectivity could be fabricated by controlling the phase-separated structure of the SBS/PPO blends. Summary The phase behaviors of SBS/PPO blends, their morphology changes with blend composition, and their permeabilities for various gases were examined. Hard styrene segments in an SBS triblock copolymer formed a one-phase mixture with PPO. The hard phase with a dispersed characteristic in SBS became a

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ReceiVed for reView March 9, 2010 ReVised manuscript receiVed May 3, 2010 Accepted June 16, 2010 IE100565Q