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Effects of neutralization with Et3Al on structure and properties in sulfonated styrenic pentablock copolymers
Journal of Membrane Science 428 (2013) 516–522
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Effects of neutralization with Et3Al on structure and properties in sulfonated styrenic pentablock copolymers Jae-Hong Choi a, Carl L. Willis b, Karen I. Winey a,n a b
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA Kraton Performance Polymers, Inc, Houston, TX 77084, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2012 Received in revised form 25 October 2012 Accepted 27 October 2012 Available online 5 November 2012
The effect of neutralization with Et3Al on morphologies in a sulfonated pentablock copolymer solution and cast membrane was investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The sulfonated pentablock copolymer in a non-polar solvent mixture (cyclohexane/ heptane) persists as spherical micelles after neutralization where the micelle core of sulfonated polystyrene (SS) incorporates Al (3þ) ions and the corona is solvated hydrogenated isoprene (HI) and t-butylstyrene (tBS). Membranes cast from this micellar solution exhibit a bicontinuous microphase separated morphology with interconnected SS microdomains. The aluminum-neutralized membrane with a continuous transport channel shows excellent water vapor transport rate (WVTR) (22 kg/(m2 day)), which is comparable to the WVTR of membranes in the sulfonic acid form. The strong interaction between Al(3þ) ions and the sulfonic acid group in the aluminum-neutralized membrane results in significantly lower water uptake ( 30 wt%) that provides much-improved mechanical stability in the wet state. & 2012 Elsevier B.V. All rights reserved.
Keywords: Pentablock copolymer Neutralization Morphology X-ray scattering Water vapor transport rate
1. Introduction Polymers with sulfonated groups have been widely used as functional materials for applications including polymer electrolyte membrane (PEM) fuel cells [1–10] and reverse osmosis (RO) membranes [11–14] due to their unique combination of mechanical strength and ion transport. Sulfonated block copolymers have attracted considerable attention for these diverse applications because one can design and control their nanoscale, selfassembled morphologies to improve desired properties including mechanical stability, water uptake and ionic conductivity. For many applications, there are a number of demanding requirements [15–17]: high ion or water transport properties, good chemical and mechanical stability, and low reactant permeability. Recently, researchers have tried to synthesize new sulfonated block copolymer membranes and to understand structure– property relationships of these materials to control their morphology and optimize performance [2,5–10]. Previously, in efforts to create new materials that achieve targeted membrane properties, we presented a novel sulfonated pentablock copolymer, poly(t-butylstyrene-b-hydrogenated isoprene-b-sulfonated styrene-b-hydrogenated isoprene-b-t-butylstyrene) (tBS-HISS-HI-tBS). In this sulfonated pentablock copolymer, the tBS end block provides good mechanical strength in both dry and wet states
n
Corresponding author. Tel.: þ1 215 898 0593; fax: þ1 215 573 2128. E-mail address: [email protected] (K.I. Winey).
0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.10.051
and the HI block gives additional toughness to avoid severe brittleness in the dry state, which is a critical attribute in membranes. The SS middle block, which is selectively sulfonated to a desired ion exchange capacity (IEC), provides high hydrophilicity and enables high ion and water transport. In two previous studies we have reported the effect of sulfonation level on the spherical micellar morphology of these sulfonated pentablock copolymers in cyclohexane/heptane solutions [18] and the implications of the microphase separation of these solutions on membrane morphology and properties [19]. Sulfonated pentablock copolymer membranes with a high sulfonation levels (1.5 and 2.0 mequiv./g IECs) exhibit bicontinuous microphase separated morphologies with interconnected SS microdomains that enable good water vapor transport rate [19]. Also, when exposed to liquid water, the extent of water uptake and the increase in primary spacing of the membranes are greater at higher sulfonation levels, suggesting that absorbed water plasticizes the hydrophilic SS microdomains. The plasticized hydrophilic microdomains lead to deterioration of membrane performance for reverse osmosis, desalination, and humidification/dehumidification applications due to reduced mechanical strength. Others have reported that one method to minimize the amount of water uptake in membranes is to neutralize the sulfonic acid groups with monovalent and multivalent metal counterions to cross-link the membranes [16,20–22]. In the present study, we investigate the effects of neutralization with Et3Al on morphologies of both a sulfonated pentablock copolymer solution in a non-polar solvent mixture and a membrane
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cast from this micellar solution. The solution and membrane morphologies are determined by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Previously, Geise et al. reported the effects of aluminum-neutralization on membrane properties including permeability, selectivity, and mechanical and transport properties of these sulfonated pentablock copolymers [23]. In this study, we mainly focus on a fundamental understanding of the effect of aluminum-neutralization on selfassembled morphologies of both solutions and membranes. These determined morphologies are correlated to both transport and mechanical properties of membranes. Additionally, it is difficult to obtain the equilibrium morphologies in these sulfonated block copolymers due to the strong interaction parameter and the difficulty in preparing equilibrium morphologies due to the high sensitivity to processing conditions. Thus, thorough studies of the self-assembled morphologies in these pentablock copolymer solutions and membranes are essential for constructing structure– property relationships.
2. Experimental 2.1. Materials The sulfonated pentablock copolymers in this study were prepared and provided by Kraton Polymers LLC. The pentablock copolymer of tBS–HI–S–HI–tBS was synthesized via anionic polymerization. After polymerization the polyisoprene blocks were hydrogenated (HI). The molecular weight of the unsulfonated pentablock copolymer is approximately 15–10–28–10–15 kg/mol [14,19]. The middle styrene block of the pentablock copolymer was selectively sulfonated to an ion exchange capacity (IEC) of 2.0 milliequivalents of sulfonic acid per gram of dry polymer (mequiv./g). The structure of the studied pentablock copolymer and detailed synthetic procedures have been described elsewhere [19,24,25]. The concentration of sulfonated pentablock copolymer solutions is 10 wt% in a mixed solvent of cyclohexane and heptane. (approximately 28:72 by weight). The neutralization was conducted by adding triethylaluminum (Et3Al) to the stirred sulfonated pentablock copolymer in a cyclohexane/heptane solution in an inert atmosphere. An exotherm ( 20 1C) was observed upon addition of 1 mol of neutralizing agent (Et3Al) per equivalent of sulfonic acid (1 mol/ equivalent of –SO3H) to the sulfonated pentablock copolymer solution [23,26]. Because three sulfonated groups are needed to neutralize a single Al3 þ ion [27], this neutralization in solution has been referred to as ionic crosslinking as depicted in Scheme 1. There was no significant increase in viscosity and no change in visual appearance upon neutralization of the micellar solution. The aluminum leaching experiment with aqueous acid also showed that there was no significant amount of excess aluminum in the membrane [23]. It should be noted that because this metal
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compound may be a hazardous substance and reacts vigorously with oxygen, the metal compound must be handled in dispersed form, or as a solution, in an inert solvent or diluents in the absence of oxygen [26]. The acid-form and aluminum-neutralized pentablock copolymer membranes were prepared by hand casting from their solutions in cyclohexane/heptane. Both solutions were cast onto a silicanized glass plate and then solvents were evaporated at room temperature and 50% relative humidity. The measured film thickness of both the acid-form and aluminum-neutralized membranes is 25 mm. 2.2. Small-angle X-ray scattering About 1 ml of each micellar solution was loaded into a capillary tube ( 1 mm diameter) and the capillary tube was flame sealed. Cyclohexane was also loaded into a capillary and studied using SAXS, so that the incoherent scattering from the solvent could be subtracted from the block copolymer solutions. Small-angle X-ray scattering was performed on the membranes and solutions. The Cu X-rays were generated from a Nonius FR 591 rotating-anode generator operated at 40 kV and 85 mA. The bright, highly collimated beam was obtained via Osmic Max-Flux optics and pinhole collimation in an integral vacuum system. The scattering data were collected with a Bruker Hi-Star two-dimensional detector with a sample to detector distance of 150 cm. Using the Datasqueeze software [28], 2-D scattering patterns were converted to 1-D plots with azimuthal angle integration. The scattering intensity was corrected for the primary beam intensity. The corrected scatterings from the cyclohexane capillary and an empty cell were subtracted from the solution and membrane data, respectively. Note that there is no difference in scattering patterns between using cyclohexane and using mixture of cyclohexane/heptane as a background. To characterize the microphase separated morphology of membranes exposed to liquid water, a small piece of membrane and deionized (DI) water were loaded into a capillary tube (1 mm diameter) and flame-sealed. The swelling of the membranes was characterized as a function of time. The scattering from a capillary filled with DI water was substracted from the pentablock copolymer membrane scattering. The scattering data of solutions were modeled as modified hard spheres where micelles are treated as monodisperse hard spheres distributed with liquidlike order in a uniform matrix [29]. The model parameters include the radius of the micelle core (R), the radius of closest approach (RCA) that limits the spatial correlation between micelles, and the number density of micelles (n). The parameter RCA is also size of the micelle with a core of SS and a corona of tBS and HI swollen by the non-polar solvent mixture. This model uses the Percus–Yevick [30] total correlation function to account for correlations between all micelles in the system. 2.3. Transmission electron microscopy (TEM)
Scheme 1. Neutralization with Et3Al of the sulfonated polystyrene monomeric units in the midblock of the tBS–HI–SS–HI–tBS pentablock copolymer. Dashed line (---) indicates a partially ionic bond.
The aluminum-neutralized pentablock copolymer solution was diluted to 0.5 wt % by adding cyclohexane to the original solutions (10 wt %). TEM samples were prepared by placing a small droplet of solution on a carbon-coated copper grid and solvent was rapidly evaporated at 80 1C in a vacuum oven to prevent changes in micelle shape and size. The TEM specimens were dried under these conditions for 2 days before imaging. The aluminum-neutralized membrane was sectioned at 60 1C using a Reichert-Jung ultramicrotome with a diamond knife to a nominal thickness of 40–70 nm. The dried TEM specimens from a dilute solution and ultrathin sections of the membranes were
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examined in a JEOL 2010 F field emission transmission electron microscope. Images were recorded at an accelerating voltage of 200 kV. 2.4. Water vapor transport rate (WVTR) measurement The water vapor transport rate was measured by the inverted cup method (ASTM E96) at 23 1C and 50% RH in an environmental chamber controlling humidity and temperature. The cup was filled with DI water and the membrane was placed over the cup. The cup was then inverted so that the water directly contacted the membrane. The cup was weighed after various periods of time to determine the weight of evaporated water. The weight change provides a water vapor transport rate (kg/(m2 day)). Because water accumulation at the membrane/air interface during the test can lead to inaccurate measurements, a low speed fan was installed below the inverted cup to promote the evaporation of water at the membrane/air interface. 2.5. Water uptake measurement The membranes were dried under vacuum at room temperature and the dry mass of the membrane measured. The membrane was then soaked in de-ionized water for 24 h and the wet mass weighed again. The water uptake of each sample was calculated as follows: Water uptake ð%Þ ¼
W wet W dry 100 W dry
Fig. 1. Transmission electron micrograph of aluminum-neutralized IEC ¼ 2.0 pentablock copolymer solution (0.5 wt % dilute solution) deposited on a carbon support grid. Dark domains indicate the micelle cores.
ð1Þ
where Wwet and Wdry are the mass of the wet and dry membrane, respectively. 2.6. Tensile property measurement Tensile property measurements were performed on a tensile tester (MTS EM System 6430 with load frame 2208) equipped with a custom-built chamber for wet-state measurement. Mechanical property data were measured for both dry and wet membranes. In this study, dry membranes are as-cast membranes that were equilibrated at room conditions after casting and wet membranes are membranes that were equilibrated in liquid DI water for 24 h prior to the measurement. Detailed conditions for the experiments were described elsewhere [23].
3. Results and discussion 3.1. Morphological characterization of micellar solutions Fig. 1 shows a TEM micrograph of aluminum-neutralized pentablock micelles after rapidly drying from a dilute solution. Because the contrast is mainly provided by the difference in electron densities, the micelle core containing the charged group (–[SO3]3Al) appears dark in TEM. The neutralized solution shows spherical micelles with a core diameter of 20 nm. The acid-form precursor solution also contains spherical micelles in a non-polar solvent mixture as reported previously [18]. Fig. 2 shows small angle X-ray scattering profiles of acid-form and aluminum-neutralized IEC¼2.0 sulfonated pentablock copolymers in a non-polar solvent mixture. Both systems exhibit scattering profiles consistent with spherical micellar structures. The primary scattering peak in the aluminum-neutralized solution shifts to slightly lower q relative to the acid-form pentablock copolymer solution without any other significant changes in SAXS profile. This indicates that the neutralized solution retains its spherical micellar structure and the distance between cores
increases slightly. This scattering result is consistent with the absence of a significant increase in viscosity of the micellar solution upon neutralization. Because ionic crosslinking has occurred selectively in the SS cores of the micelles upon neutralization, there is no significant change in the micelle size or micellar volume fraction to change the viscosity. Previously, we reported that increasing the micellar volume fraction increased the solution viscosity in the acid-form pentablock copolymers [18]. Quantitative information about the size and separation of spherical micelles in these pentablock copolymer solutions can be obtained by fitting the data to a modified hard sphere model [29]. The experimental data show good agreement with this model. The discrepancy at q is attributed primarily to size polydispersity of the micelles or broad interfaces between core and solvated corona in the solutions, whereas the model assumes monodisperse spheres and sharp interfaces. Table 1 summarizes the fitting parameters for acid-form and aluminum-neutralized solutions. The size of micelle core found by TEM is consistent with the X-ray scattering result. The diameter of micelle core (2R) increases slightly ( 5%) upon neutralization, while the closest approach distance between cores (2RCA) increases 20%. Neutralization of the SS core increases the incompatibility between core (ionic) and corona (non-ionic) and this appears to cause the HI and tBS chains in the corona to become significantly more stretched and thereby increases RCA. Fig. 3 shows morphological changes that occur when sulfonated pentablock copolymer solution is neutralized with Et3Al, based on X-ray scattering and TEM results. The pentablock copolymer (IEC¼2.0) in a non-polar solvent (a mixed solvent of cyclohexane and heptane) forms spherical micelles both before and after solution neutralization. The neutralized micelles have cores with diameters of 21 nm containing SS and Al (3þ) ions. Upon neutralization the HI–tBS blocks expand to increase the corona thickness. 3.2. Morphological characterization of membranes Fig. 4 shows the through-plane X-ray scattering profiles for as-received IEC¼2.0 sulfonated pentablock copolymer precursor
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Fig. 2. X-ray scattering intensity as a function of scattering vector q for (a) acid-form IEC¼2.0 sulfonated pentablock copolymer solution and (b) aluminum-neutralized solution. Experimental data (square) and the model fit (solid line) for monodisperse spherical micelles.
Table 1 Fitting parameters for Kinning–Thomas model for acid-form and aluminumneutralized IEC ¼2.0 sulfonated pentablock copolymer solutions. 2R, 2RCA, and n represent the diameter of the micelle core, the closest approach distance between cores, and the number of micelles per unit volume, respectively. Sample
2R (nm)
2RCA (nm)
n (10 6/ nm3)
Acid-form Aluminum-neutralized
20.4 21.4
42.2 51.0
9.71 4.58
Fig. 4. Through-plane X-ray scattering intensity as a function of scattering vector q for as received acid-form and aluminum-neutralized membranes.
Fig. 3. Schematic of the spherical micelles and the size changes in sulfonated pentablock copolymer solution upon neutralization. Spherical micelles in neutralized solution contain a dense core of SS and Al(3þ ) ions and a corona of HI–tBS swollen by solvent. The tBS, HI, SS blocks and Al ions are shown in green, red, blue and yellow, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(acid-form) and neutralized membranes cast from 10 wt% cyclohexane/heptane solutions. Both as-received pentablock copolymer membranes show broad scattering features and their primary scattering spacings (d¼2p/q1, q1 is the position of the primary peak) are 24.7 and 23.5 nm for the acid-form and aluminumneutralized materials, respectively. These sizes are somewhat larger (10–20%) than the micelle core diameters (2R in Table 1). These membranes show anisotropic 2-D scattering patterns for inplane X-ray scattering and the primary scattering spacings are smaller perpendicular the membrane, even smaller than the micellar cores in the acid-form and aluminum-neutralized pentablock solutions. From our previous study of these acid-form
pentablock copolymer membranes, this suggests that both as-received membranes exhibit bicontinuous morphology with interconnected SS microdomains [19]. The TEM micrograph of the aluminum-neutralized pentablock copolymer membrane, Fig. 5, reveals disordered interconnected microdomains. The dark microdomains in the image correspond to the SS microdomains with Al. The measured interdomain spacing of the SS microdomains from the TEM image is 19– 22 nm, which is consistent with X-ray scattering result. A similar bicontinuous morphology in IEC¼2.0 acid-form pentablock copolymer membrane has previously been reported [19]. Thus, both the acid-form and aluminum-neutralized membranes exhibit bicontinuous morphology with interconnected SS microdomains based on X-ray scattering and TEM results. X-ray scattering was performed on the membranes after soaking in water for 1 and 80 days as shown in Fig. 6. The membranes remain microphase separated which is consistent with water selectively entering the SS microdomains. The extent of swelling as indicated by the shift of the primary peak to lower scattering angle is much more pronounced for the acid-form pentablock copolymer membrane. The ratio of the primary spacing in the swollen state (d) to the primary spacing in the dry state (as-received) (d0) was calculated. The ratio of d/d0 indicates the relative increment of spacing in a wet state, which also corresponds to how much water was absorbed into the hydrophilic SS microdomains. There is a 53% increase in microdomain
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of the hydrophilic SS microdomains, such that the hydrogen bonds between sulfonic acid centers in the dry state are destroyed when exposed to water [19]. This results in a greater increase in SS microdomain spacing in the acid-form membrane when exposed to liquid water. On the other hand, water only slightly plasticizes the ion-containing microdomains in aluminumneutralized membranes. The ionic interactions are retained even in the wet state, because Al(3þ) ions have a stronger interaction with the oxygen center in the sulfonic acid than with the oxygen in water. The absorption of water reaches a maximum within one day, as indicated by no change in the primary spacing of the aluminum-neutralized membrane when equilibrated with liquid water up to 80 days. In contrast, the acid-form pentablock copolymer membrane continues to evolve after 1 day in water. These X-ray scattering results of membranes suggest that the aluminum-neutralized membrane exhibits different morphological behavior when exposed to water compared to the membrane in the acid form. This will be correlated to water transport and mechanical properties of these membranes in the next section. 3.3. Structure–property relationships in membranes
Fig. 5. Transmission electron micrograph of aluminum-neutralized IEC ¼2.0 pentablock copolymer membrane. This is an unstained specimen and dark microdomains indicate the SS microdomains with Al.
Fig. 6. X-ray scattering results for as-received and swollen IEC ¼2.0 pentablock copolymers in acid-form and aluminum-neutralized membranes. The swollen membranes were immersed in water for 1 and 80 days.
spacing for the acid-form membrane when exposed to water for 1 day, while there is only an 8% increase for the neutralized membrane, Table 2. The acid-form membranes absorb more water into the SS microdomains and this leads to plasticization
Table 2 summarizes transport and mechanical properties for acid-form and aluminum-neutralized IEC¼2.0 sulfonated pentablock copolymer membranes. The swelling data from X-ray scattering presented above is consistent with the water uptake property of these membranes. The neutralized membrane takes up just 29% by weight when immersed in water, while the acidform membrane absorbs 140% by weight. A decrease in water uptake in aluminum-neutralized membranes can be attributed to the chemical cross-linking of the SS blocks. Similar behavior has been reported in other sulfonated membranes [20,22]. The lower water uptake in the aluminum-neutralized membrane provides better dimensional stability which is a critical factor for membranes in reverse osmosis, desalination, and humidification/ dehumidification device applications, because sagging membranes in the presence of water vapor and liquid water impede performance. The water vapor transport rates (WVTR) are comparable for the acid-form and neutralized membranes (Table 2), indicating that both membranes have continuous SS microdomains for water transport. This is consistent with a bicontinuous morphology with interconnected SS microdomains inferred from the X-ray scattering and TEM data. The small decrease in WVTR from 24 to 22 kg/(m2 day) after aluminum-neutralization can be attributed to lower water uptake in the neutralized membrane when exposed to liquid water [14,23]. Note that the wet neutralized membrane has smaller SS microdomains than the wet acid-form membrane, but still exhibits similar water transport rates. This indicates that the effect of the size of the SS microdomains on water transport property is much less significant than the effect of connectivity of the SS microdomains for transport in this sulfonated pentablock copolymer system. By comparison, we previously found a WVTR of 2.5 kg/(m2 day) for acid-form pentablock copolymer membranes (IEC ¼0.4–1.0 mequiv./g) with discrete SS microdomains [19]. Aluminum neutralization has a significant effect on mechanical properties of the membranes, Table 2. In the wet state, the acid-form membrane shows a significantly lower modulus than the dry acid-form membrane and has no yield point. This is attributed to the softening of the membrane resulting from the hydrolysis of hydrogen bonds between sulfonic acid groups in SS microdomains when exposed to water. Importantly, the aluminum-neutralized membrane retains mechanical strength in the wet state. This indicates that water only slightly plasticizes
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Table 2 Summary of properties for IEC ¼2.0 sulfonated pentablock copolymer in acid-form and aluminum-neutralized membranes. The d-spacings for the dry and wet states correspond to the calculated primary spacings from X-ray scattering results for as-received membranes and membranes swollen in water for 1 day, respectively. The error of mechanical properties is roughly7 15% of the reported values [23]. Sample
Structure d-Spacing (dry) (nm)
Acid-form 24.7 Aluminum23.5 neutralized
Properties d-Spacing (wet) (nm)
Water uptake (wt %)
WVTR (kg/ (m2 day))
Modulus (dry) (MPa)
Modulus Yield stress (wet) (MPa) (dry) (MPa)
Yield stress (wet) (MPa)
Tensile stress at break (dry) (MPa)
Tensile stress at break (wet) (MPa)
37.4 25.6
140 29
24 22
455 365
21.4 648
no yield 8.27
16.5 9.65
8.27 8.27
the SS microphases, which is consistent with reduced water uptake in aluminum-neutralized membranes relative to the acid-form membrane. Also, note that the wet neutralized membrane is stiffer (higher modulus) than the dry neutralized membrane, which can be attributed to the topological constraints of highly cross-linked neutralized membranes in the wet state. The polymer chains of SS block are already extended (expanded chain conformation) in the swollen state, which results in a higher modulus under stress than the dry neutralized membrane.
12.4 8.27
[4]
[5]
[6]
[7]
4. Conclusion [8]
We examined the effect of neutralization of IEC¼2.0 sulfonated pentablock copolymer with Et3Al on the solution morphology in non-polar solvents (cyclohexane/heptane) and the membrane morphology and properties. The aluminum-neutralized solution retains spherical micelles where the micelle contains a core of SS and Al (3þ) ions and a corona of solvated HI-tBS. The aluminumneutralized membrane with continuous SS microdomains shows excellent water transport properties that are comparable to the acid-form membrane. The strong interaction between Al(3þ ) ions and the oxygen center in the sulfonic acid (cross-linking) in the aluminum-neutralized membrane restricts water uptake and provides much improved mechanical properties and better dimensional stability. The aluminum-neutralized membrane with these improved properties could have an important role in various membrane-based applications. This study demonstrates that the understanding of the structure–property relationships is a key part of the development of new membranes with improved properties.
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[28] P.A. Heiney, Datasqueeze: a software tool for powder and small angle X-ray diffraction analysis, Commun. Powder Diffr. Newsletter 32 (2005) 9. [29] D.J. Kinning, E.L. Thomas, Hard-sphere interactions between spherical domains in diblock copolymers, Macromolecules 17 (1984) 1712. [30] J.K. Percus, G.J. Yevick, Analysis of classical statistical mechanics by means of collective coordinates, Phys. Rev. 110 (1958) 1.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Effects of neutralization with Et3Al on structure and properties in sulfonated styrenic pentablock copolymers Jae-Hong Choi a, Carl L. Willis b, Karen I. Winey a,n a b
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA Kraton Performance Polymers, Inc, Houston, TX 77084, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2012 Received in revised form 25 October 2012 Accepted 27 October 2012 Available online 5 November 2012
The effect of neutralization with Et3Al on morphologies in a sulfonated pentablock copolymer solution and cast membrane was investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The sulfonated pentablock copolymer in a non-polar solvent mixture (cyclohexane/ heptane) persists as spherical micelles after neutralization where the micelle core of sulfonated polystyrene (SS) incorporates Al (3þ) ions and the corona is solvated hydrogenated isoprene (HI) and t-butylstyrene (tBS). Membranes cast from this micellar solution exhibit a bicontinuous microphase separated morphology with interconnected SS microdomains. The aluminum-neutralized membrane with a continuous transport channel shows excellent water vapor transport rate (WVTR) (22 kg/(m2 day)), which is comparable to the WVTR of membranes in the sulfonic acid form. The strong interaction between Al(3þ) ions and the sulfonic acid group in the aluminum-neutralized membrane results in significantly lower water uptake ( 30 wt%) that provides much-improved mechanical stability in the wet state. & 2012 Elsevier B.V. All rights reserved.
Keywords: Pentablock copolymer Neutralization Morphology X-ray scattering Water vapor transport rate
1. Introduction Polymers with sulfonated groups have been widely used as functional materials for applications including polymer electrolyte membrane (PEM) fuel cells [1–10] and reverse osmosis (RO) membranes [11–14] due to their unique combination of mechanical strength and ion transport. Sulfonated block copolymers have attracted considerable attention for these diverse applications because one can design and control their nanoscale, selfassembled morphologies to improve desired properties including mechanical stability, water uptake and ionic conductivity. For many applications, there are a number of demanding requirements [15–17]: high ion or water transport properties, good chemical and mechanical stability, and low reactant permeability. Recently, researchers have tried to synthesize new sulfonated block copolymer membranes and to understand structure– property relationships of these materials to control their morphology and optimize performance [2,5–10]. Previously, in efforts to create new materials that achieve targeted membrane properties, we presented a novel sulfonated pentablock copolymer, poly(t-butylstyrene-b-hydrogenated isoprene-b-sulfonated styrene-b-hydrogenated isoprene-b-t-butylstyrene) (tBS-HISS-HI-tBS). In this sulfonated pentablock copolymer, the tBS end block provides good mechanical strength in both dry and wet states
n
Corresponding author. Tel.: þ1 215 898 0593; fax: þ1 215 573 2128. E-mail address: [email protected] (K.I. Winey).
0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.10.051
and the HI block gives additional toughness to avoid severe brittleness in the dry state, which is a critical attribute in membranes. The SS middle block, which is selectively sulfonated to a desired ion exchange capacity (IEC), provides high hydrophilicity and enables high ion and water transport. In two previous studies we have reported the effect of sulfonation level on the spherical micellar morphology of these sulfonated pentablock copolymers in cyclohexane/heptane solutions [18] and the implications of the microphase separation of these solutions on membrane morphology and properties [19]. Sulfonated pentablock copolymer membranes with a high sulfonation levels (1.5 and 2.0 mequiv./g IECs) exhibit bicontinuous microphase separated morphologies with interconnected SS microdomains that enable good water vapor transport rate [19]. Also, when exposed to liquid water, the extent of water uptake and the increase in primary spacing of the membranes are greater at higher sulfonation levels, suggesting that absorbed water plasticizes the hydrophilic SS microdomains. The plasticized hydrophilic microdomains lead to deterioration of membrane performance for reverse osmosis, desalination, and humidification/dehumidification applications due to reduced mechanical strength. Others have reported that one method to minimize the amount of water uptake in membranes is to neutralize the sulfonic acid groups with monovalent and multivalent metal counterions to cross-link the membranes [16,20–22]. In the present study, we investigate the effects of neutralization with Et3Al on morphologies of both a sulfonated pentablock copolymer solution in a non-polar solvent mixture and a membrane
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cast from this micellar solution. The solution and membrane morphologies are determined by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Previously, Geise et al. reported the effects of aluminum-neutralization on membrane properties including permeability, selectivity, and mechanical and transport properties of these sulfonated pentablock copolymers [23]. In this study, we mainly focus on a fundamental understanding of the effect of aluminum-neutralization on selfassembled morphologies of both solutions and membranes. These determined morphologies are correlated to both transport and mechanical properties of membranes. Additionally, it is difficult to obtain the equilibrium morphologies in these sulfonated block copolymers due to the strong interaction parameter and the difficulty in preparing equilibrium morphologies due to the high sensitivity to processing conditions. Thus, thorough studies of the self-assembled morphologies in these pentablock copolymer solutions and membranes are essential for constructing structure– property relationships.
2. Experimental 2.1. Materials The sulfonated pentablock copolymers in this study were prepared and provided by Kraton Polymers LLC. The pentablock copolymer of tBS–HI–S–HI–tBS was synthesized via anionic polymerization. After polymerization the polyisoprene blocks were hydrogenated (HI). The molecular weight of the unsulfonated pentablock copolymer is approximately 15–10–28–10–15 kg/mol [14,19]. The middle styrene block of the pentablock copolymer was selectively sulfonated to an ion exchange capacity (IEC) of 2.0 milliequivalents of sulfonic acid per gram of dry polymer (mequiv./g). The structure of the studied pentablock copolymer and detailed synthetic procedures have been described elsewhere [19,24,25]. The concentration of sulfonated pentablock copolymer solutions is 10 wt% in a mixed solvent of cyclohexane and heptane. (approximately 28:72 by weight). The neutralization was conducted by adding triethylaluminum (Et3Al) to the stirred sulfonated pentablock copolymer in a cyclohexane/heptane solution in an inert atmosphere. An exotherm ( 20 1C) was observed upon addition of 1 mol of neutralizing agent (Et3Al) per equivalent of sulfonic acid (1 mol/ equivalent of –SO3H) to the sulfonated pentablock copolymer solution [23,26]. Because three sulfonated groups are needed to neutralize a single Al3 þ ion [27], this neutralization in solution has been referred to as ionic crosslinking as depicted in Scheme 1. There was no significant increase in viscosity and no change in visual appearance upon neutralization of the micellar solution. The aluminum leaching experiment with aqueous acid also showed that there was no significant amount of excess aluminum in the membrane [23]. It should be noted that because this metal
517
compound may be a hazardous substance and reacts vigorously with oxygen, the metal compound must be handled in dispersed form, or as a solution, in an inert solvent or diluents in the absence of oxygen [26]. The acid-form and aluminum-neutralized pentablock copolymer membranes were prepared by hand casting from their solutions in cyclohexane/heptane. Both solutions were cast onto a silicanized glass plate and then solvents were evaporated at room temperature and 50% relative humidity. The measured film thickness of both the acid-form and aluminum-neutralized membranes is 25 mm. 2.2. Small-angle X-ray scattering About 1 ml of each micellar solution was loaded into a capillary tube ( 1 mm diameter) and the capillary tube was flame sealed. Cyclohexane was also loaded into a capillary and studied using SAXS, so that the incoherent scattering from the solvent could be subtracted from the block copolymer solutions. Small-angle X-ray scattering was performed on the membranes and solutions. The Cu X-rays were generated from a Nonius FR 591 rotating-anode generator operated at 40 kV and 85 mA. The bright, highly collimated beam was obtained via Osmic Max-Flux optics and pinhole collimation in an integral vacuum system. The scattering data were collected with a Bruker Hi-Star two-dimensional detector with a sample to detector distance of 150 cm. Using the Datasqueeze software [28], 2-D scattering patterns were converted to 1-D plots with azimuthal angle integration. The scattering intensity was corrected for the primary beam intensity. The corrected scatterings from the cyclohexane capillary and an empty cell were subtracted from the solution and membrane data, respectively. Note that there is no difference in scattering patterns between using cyclohexane and using mixture of cyclohexane/heptane as a background. To characterize the microphase separated morphology of membranes exposed to liquid water, a small piece of membrane and deionized (DI) water were loaded into a capillary tube (1 mm diameter) and flame-sealed. The swelling of the membranes was characterized as a function of time. The scattering from a capillary filled with DI water was substracted from the pentablock copolymer membrane scattering. The scattering data of solutions were modeled as modified hard spheres where micelles are treated as monodisperse hard spheres distributed with liquidlike order in a uniform matrix [29]. The model parameters include the radius of the micelle core (R), the radius of closest approach (RCA) that limits the spatial correlation between micelles, and the number density of micelles (n). The parameter RCA is also size of the micelle with a core of SS and a corona of tBS and HI swollen by the non-polar solvent mixture. This model uses the Percus–Yevick [30] total correlation function to account for correlations between all micelles in the system. 2.3. Transmission electron microscopy (TEM)
Scheme 1. Neutralization with Et3Al of the sulfonated polystyrene monomeric units in the midblock of the tBS–HI–SS–HI–tBS pentablock copolymer. Dashed line (---) indicates a partially ionic bond.
The aluminum-neutralized pentablock copolymer solution was diluted to 0.5 wt % by adding cyclohexane to the original solutions (10 wt %). TEM samples were prepared by placing a small droplet of solution on a carbon-coated copper grid and solvent was rapidly evaporated at 80 1C in a vacuum oven to prevent changes in micelle shape and size. The TEM specimens were dried under these conditions for 2 days before imaging. The aluminum-neutralized membrane was sectioned at 60 1C using a Reichert-Jung ultramicrotome with a diamond knife to a nominal thickness of 40–70 nm. The dried TEM specimens from a dilute solution and ultrathin sections of the membranes were
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examined in a JEOL 2010 F field emission transmission electron microscope. Images were recorded at an accelerating voltage of 200 kV. 2.4. Water vapor transport rate (WVTR) measurement The water vapor transport rate was measured by the inverted cup method (ASTM E96) at 23 1C and 50% RH in an environmental chamber controlling humidity and temperature. The cup was filled with DI water and the membrane was placed over the cup. The cup was then inverted so that the water directly contacted the membrane. The cup was weighed after various periods of time to determine the weight of evaporated water. The weight change provides a water vapor transport rate (kg/(m2 day)). Because water accumulation at the membrane/air interface during the test can lead to inaccurate measurements, a low speed fan was installed below the inverted cup to promote the evaporation of water at the membrane/air interface. 2.5. Water uptake measurement The membranes were dried under vacuum at room temperature and the dry mass of the membrane measured. The membrane was then soaked in de-ionized water for 24 h and the wet mass weighed again. The water uptake of each sample was calculated as follows: Water uptake ð%Þ ¼
W wet W dry 100 W dry
Fig. 1. Transmission electron micrograph of aluminum-neutralized IEC ¼ 2.0 pentablock copolymer solution (0.5 wt % dilute solution) deposited on a carbon support grid. Dark domains indicate the micelle cores.
ð1Þ
where Wwet and Wdry are the mass of the wet and dry membrane, respectively. 2.6. Tensile property measurement Tensile property measurements were performed on a tensile tester (MTS EM System 6430 with load frame 2208) equipped with a custom-built chamber for wet-state measurement. Mechanical property data were measured for both dry and wet membranes. In this study, dry membranes are as-cast membranes that were equilibrated at room conditions after casting and wet membranes are membranes that were equilibrated in liquid DI water for 24 h prior to the measurement. Detailed conditions for the experiments were described elsewhere [23].
3. Results and discussion 3.1. Morphological characterization of micellar solutions Fig. 1 shows a TEM micrograph of aluminum-neutralized pentablock micelles after rapidly drying from a dilute solution. Because the contrast is mainly provided by the difference in electron densities, the micelle core containing the charged group (–[SO3]3Al) appears dark in TEM. The neutralized solution shows spherical micelles with a core diameter of 20 nm. The acid-form precursor solution also contains spherical micelles in a non-polar solvent mixture as reported previously [18]. Fig. 2 shows small angle X-ray scattering profiles of acid-form and aluminum-neutralized IEC¼2.0 sulfonated pentablock copolymers in a non-polar solvent mixture. Both systems exhibit scattering profiles consistent with spherical micellar structures. The primary scattering peak in the aluminum-neutralized solution shifts to slightly lower q relative to the acid-form pentablock copolymer solution without any other significant changes in SAXS profile. This indicates that the neutralized solution retains its spherical micellar structure and the distance between cores
increases slightly. This scattering result is consistent with the absence of a significant increase in viscosity of the micellar solution upon neutralization. Because ionic crosslinking has occurred selectively in the SS cores of the micelles upon neutralization, there is no significant change in the micelle size or micellar volume fraction to change the viscosity. Previously, we reported that increasing the micellar volume fraction increased the solution viscosity in the acid-form pentablock copolymers [18]. Quantitative information about the size and separation of spherical micelles in these pentablock copolymer solutions can be obtained by fitting the data to a modified hard sphere model [29]. The experimental data show good agreement with this model. The discrepancy at q is attributed primarily to size polydispersity of the micelles or broad interfaces between core and solvated corona in the solutions, whereas the model assumes monodisperse spheres and sharp interfaces. Table 1 summarizes the fitting parameters for acid-form and aluminum-neutralized solutions. The size of micelle core found by TEM is consistent with the X-ray scattering result. The diameter of micelle core (2R) increases slightly ( 5%) upon neutralization, while the closest approach distance between cores (2RCA) increases 20%. Neutralization of the SS core increases the incompatibility between core (ionic) and corona (non-ionic) and this appears to cause the HI and tBS chains in the corona to become significantly more stretched and thereby increases RCA. Fig. 3 shows morphological changes that occur when sulfonated pentablock copolymer solution is neutralized with Et3Al, based on X-ray scattering and TEM results. The pentablock copolymer (IEC¼2.0) in a non-polar solvent (a mixed solvent of cyclohexane and heptane) forms spherical micelles both before and after solution neutralization. The neutralized micelles have cores with diameters of 21 nm containing SS and Al (3þ) ions. Upon neutralization the HI–tBS blocks expand to increase the corona thickness. 3.2. Morphological characterization of membranes Fig. 4 shows the through-plane X-ray scattering profiles for as-received IEC¼2.0 sulfonated pentablock copolymer precursor
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519
Fig. 2. X-ray scattering intensity as a function of scattering vector q for (a) acid-form IEC¼2.0 sulfonated pentablock copolymer solution and (b) aluminum-neutralized solution. Experimental data (square) and the model fit (solid line) for monodisperse spherical micelles.
Table 1 Fitting parameters for Kinning–Thomas model for acid-form and aluminumneutralized IEC ¼2.0 sulfonated pentablock copolymer solutions. 2R, 2RCA, and n represent the diameter of the micelle core, the closest approach distance between cores, and the number of micelles per unit volume, respectively. Sample
2R (nm)
2RCA (nm)
n (10 6/ nm3)
Acid-form Aluminum-neutralized
20.4 21.4
42.2 51.0
9.71 4.58
Fig. 4. Through-plane X-ray scattering intensity as a function of scattering vector q for as received acid-form and aluminum-neutralized membranes.
Fig. 3. Schematic of the spherical micelles and the size changes in sulfonated pentablock copolymer solution upon neutralization. Spherical micelles in neutralized solution contain a dense core of SS and Al(3þ ) ions and a corona of HI–tBS swollen by solvent. The tBS, HI, SS blocks and Al ions are shown in green, red, blue and yellow, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(acid-form) and neutralized membranes cast from 10 wt% cyclohexane/heptane solutions. Both as-received pentablock copolymer membranes show broad scattering features and their primary scattering spacings (d¼2p/q1, q1 is the position of the primary peak) are 24.7 and 23.5 nm for the acid-form and aluminumneutralized materials, respectively. These sizes are somewhat larger (10–20%) than the micelle core diameters (2R in Table 1). These membranes show anisotropic 2-D scattering patterns for inplane X-ray scattering and the primary scattering spacings are smaller perpendicular the membrane, even smaller than the micellar cores in the acid-form and aluminum-neutralized pentablock solutions. From our previous study of these acid-form
pentablock copolymer membranes, this suggests that both as-received membranes exhibit bicontinuous morphology with interconnected SS microdomains [19]. The TEM micrograph of the aluminum-neutralized pentablock copolymer membrane, Fig. 5, reveals disordered interconnected microdomains. The dark microdomains in the image correspond to the SS microdomains with Al. The measured interdomain spacing of the SS microdomains from the TEM image is 19– 22 nm, which is consistent with X-ray scattering result. A similar bicontinuous morphology in IEC¼2.0 acid-form pentablock copolymer membrane has previously been reported [19]. Thus, both the acid-form and aluminum-neutralized membranes exhibit bicontinuous morphology with interconnected SS microdomains based on X-ray scattering and TEM results. X-ray scattering was performed on the membranes after soaking in water for 1 and 80 days as shown in Fig. 6. The membranes remain microphase separated which is consistent with water selectively entering the SS microdomains. The extent of swelling as indicated by the shift of the primary peak to lower scattering angle is much more pronounced for the acid-form pentablock copolymer membrane. The ratio of the primary spacing in the swollen state (d) to the primary spacing in the dry state (as-received) (d0) was calculated. The ratio of d/d0 indicates the relative increment of spacing in a wet state, which also corresponds to how much water was absorbed into the hydrophilic SS microdomains. There is a 53% increase in microdomain
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of the hydrophilic SS microdomains, such that the hydrogen bonds between sulfonic acid centers in the dry state are destroyed when exposed to water [19]. This results in a greater increase in SS microdomain spacing in the acid-form membrane when exposed to liquid water. On the other hand, water only slightly plasticizes the ion-containing microdomains in aluminumneutralized membranes. The ionic interactions are retained even in the wet state, because Al(3þ) ions have a stronger interaction with the oxygen center in the sulfonic acid than with the oxygen in water. The absorption of water reaches a maximum within one day, as indicated by no change in the primary spacing of the aluminum-neutralized membrane when equilibrated with liquid water up to 80 days. In contrast, the acid-form pentablock copolymer membrane continues to evolve after 1 day in water. These X-ray scattering results of membranes suggest that the aluminum-neutralized membrane exhibits different morphological behavior when exposed to water compared to the membrane in the acid form. This will be correlated to water transport and mechanical properties of these membranes in the next section. 3.3. Structure–property relationships in membranes
Fig. 5. Transmission electron micrograph of aluminum-neutralized IEC ¼2.0 pentablock copolymer membrane. This is an unstained specimen and dark microdomains indicate the SS microdomains with Al.
Fig. 6. X-ray scattering results for as-received and swollen IEC ¼2.0 pentablock copolymers in acid-form and aluminum-neutralized membranes. The swollen membranes were immersed in water for 1 and 80 days.
spacing for the acid-form membrane when exposed to water for 1 day, while there is only an 8% increase for the neutralized membrane, Table 2. The acid-form membranes absorb more water into the SS microdomains and this leads to plasticization
Table 2 summarizes transport and mechanical properties for acid-form and aluminum-neutralized IEC¼2.0 sulfonated pentablock copolymer membranes. The swelling data from X-ray scattering presented above is consistent with the water uptake property of these membranes. The neutralized membrane takes up just 29% by weight when immersed in water, while the acidform membrane absorbs 140% by weight. A decrease in water uptake in aluminum-neutralized membranes can be attributed to the chemical cross-linking of the SS blocks. Similar behavior has been reported in other sulfonated membranes [20,22]. The lower water uptake in the aluminum-neutralized membrane provides better dimensional stability which is a critical factor for membranes in reverse osmosis, desalination, and humidification/ dehumidification device applications, because sagging membranes in the presence of water vapor and liquid water impede performance. The water vapor transport rates (WVTR) are comparable for the acid-form and neutralized membranes (Table 2), indicating that both membranes have continuous SS microdomains for water transport. This is consistent with a bicontinuous morphology with interconnected SS microdomains inferred from the X-ray scattering and TEM data. The small decrease in WVTR from 24 to 22 kg/(m2 day) after aluminum-neutralization can be attributed to lower water uptake in the neutralized membrane when exposed to liquid water [14,23]. Note that the wet neutralized membrane has smaller SS microdomains than the wet acid-form membrane, but still exhibits similar water transport rates. This indicates that the effect of the size of the SS microdomains on water transport property is much less significant than the effect of connectivity of the SS microdomains for transport in this sulfonated pentablock copolymer system. By comparison, we previously found a WVTR of 2.5 kg/(m2 day) for acid-form pentablock copolymer membranes (IEC ¼0.4–1.0 mequiv./g) with discrete SS microdomains [19]. Aluminum neutralization has a significant effect on mechanical properties of the membranes, Table 2. In the wet state, the acid-form membrane shows a significantly lower modulus than the dry acid-form membrane and has no yield point. This is attributed to the softening of the membrane resulting from the hydrolysis of hydrogen bonds between sulfonic acid groups in SS microdomains when exposed to water. Importantly, the aluminum-neutralized membrane retains mechanical strength in the wet state. This indicates that water only slightly plasticizes
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Table 2 Summary of properties for IEC ¼2.0 sulfonated pentablock copolymer in acid-form and aluminum-neutralized membranes. The d-spacings for the dry and wet states correspond to the calculated primary spacings from X-ray scattering results for as-received membranes and membranes swollen in water for 1 day, respectively. The error of mechanical properties is roughly7 15% of the reported values [23]. Sample
Structure d-Spacing (dry) (nm)
Acid-form 24.7 Aluminum23.5 neutralized
Properties d-Spacing (wet) (nm)
Water uptake (wt %)
WVTR (kg/ (m2 day))
Modulus (dry) (MPa)
Modulus Yield stress (wet) (MPa) (dry) (MPa)
Yield stress (wet) (MPa)
Tensile stress at break (dry) (MPa)
Tensile stress at break (wet) (MPa)
37.4 25.6
140 29
24 22
455 365
21.4 648
no yield 8.27
16.5 9.65
8.27 8.27
the SS microphases, which is consistent with reduced water uptake in aluminum-neutralized membranes relative to the acid-form membrane. Also, note that the wet neutralized membrane is stiffer (higher modulus) than the dry neutralized membrane, which can be attributed to the topological constraints of highly cross-linked neutralized membranes in the wet state. The polymer chains of SS block are already extended (expanded chain conformation) in the swollen state, which results in a higher modulus under stress than the dry neutralized membrane.
12.4 8.27
[4]
[5]
[6]
[7]
4. Conclusion [8]
We examined the effect of neutralization of IEC¼2.0 sulfonated pentablock copolymer with Et3Al on the solution morphology in non-polar solvents (cyclohexane/heptane) and the membrane morphology and properties. The aluminum-neutralized solution retains spherical micelles where the micelle contains a core of SS and Al (3þ) ions and a corona of solvated HI-tBS. The aluminumneutralized membrane with continuous SS microdomains shows excellent water transport properties that are comparable to the acid-form membrane. The strong interaction between Al(3þ ) ions and the oxygen center in the sulfonic acid (cross-linking) in the aluminum-neutralized membrane restricts water uptake and provides much improved mechanical properties and better dimensional stability. The aluminum-neutralized membrane with these improved properties could have an important role in various membrane-based applications. This study demonstrates that the understanding of the structure–property relationships is a key part of the development of new membranes with improved properties.
Acknowledgements This work was supported by Kraton Performance Polymers, Inc. The authors thank Kraton Polymer scientists for useful discussions. We acknowledge the financial support of the U.S. Army Research Office under grant no. W911NF-07-1-0452 Ionic Liquids in Electro-Active Devices (ILEAD) MURI. We acknowledge the use of facilities at the University of Pennsylvania funded in part by the MRSEC Program of the National Science Foundation (#DMR11-20901). References [1] A. Taeger, C. Vogel, D. Lehmann, D. Jehnichen, H. Komber, J. Meier-Haack, N.A. Ochoa, S.P. Nunes, K.V. Peinemann, Ion exchange membranes derived from sulfonaed polyaramides, React. Funct. Polym. 57 (2003) 77. [2] Y.A. Elabd, C.W. Walker, F.L. Beyer, Triblock copolymer ionomer membranes Part II. Structure characterization and its effects on transport properties and direct methanol fuel cell performance, J. Membr. Sci. 231 (2004) 181. [3] Y.S. Kim, M.A. Hickner, L. Dong, B.S. Pivovar, J.E. McGrath, Sulfonated poly(arylene ether sulfone) copolymer proton exchange membranes:
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