1. Introduction
World-wide demands for a clean energy system have been urged to develop an effective and economic fuel cells, which can produce electric power to generate energy with low/zero production of poisonous gases through the redox chemical reaction of fuels and oxygen. Among these fuel cell systems, polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) using proton conductive membrane such as NafionⓇ have been extensively explored for the applications to electric vehicles as well as portable devices[1,2]. Thus, ionomers or proton conducting membranes have garnered attention from widespread fields of interests due to its interesting properties. Application in various fields such as sensors, ion exchange, water purification, and most recently, fuel cells has motivated a lot of researchers to study on the said types of materials[3]. Ionomers are polymers which contains electrolyte groups in their hydrocarbon backbone. It can be easily synthesized by addition of ionic groups such as phosphonic acid, carbonic acid and sulfonic acid to non-ionic polymers. Incorporation of such ionic functional groups adds enhanced properties such as increased strength, hydrophilicity and proton conductivity[4].
Several approaches can be performed to synthesize ionomers. This includes a free radical copolymerization of a nonionic monomer with another type of monomer that can be chemically modified to an ionic functional group. An example of this method is the free radical polymerization of poly(ethylene-co-methacrylic acid) or poly(styrene-co-methacrylic acid) where methacrylic acid contains a polar carboxylic acid[5]. Another method is by post reaction of a preformed non polar hydrocarbon polymer. Hydrocarbon polymers containing polar groups, as sulfonic groups (-SO3-), which can hold high amounts of water are particularly fascinated for higher proton transport and relatively cheaper to synthesize than perfluorinated polymers. Sulfonic groups can be added to aromatic hydrocarbon polymers in the backbone by post-sulfonation using aromatic electrophilic substitution. It has been known that the introductions of sulfonic groups enhance physico-chemical properties such as mechanical strength, hydrophilicity, and proton conductivity. As for the block copolymers sulfonated in an elastomeric block, academic interest has been made, because the blocks show a highly ordered morphology of both ionic and non-ionic blocks as long as only one of the blocks is sulfonated[6-8].
Several researches have been reported to improve the physical properties of membrane using composite membranes of Nafion with inorganic fillers[9], others attempted blending 2 kinds of polymers[10] while others used crosslinking[11], or grafting[12]. As reported by our group, Kim et al. tried controlling the morphology of the sulfonated membrane by studied the effect of the types of casting solvent used[13] and selective swelling of the middle block of sulfonated styrene(ethylene-butylene) styrene (SEBS) triblock copolymer[14]. Hickner et al. identified critical features essential for fuel cell membranes and influencing factors that can be tuned such as tunable parameters and structural parameters[15].
As reported early[13,14], the phase separation of block copolymers between non-ionic domains and ionic domain forms well-interconnected ion clusters for a high proton transport. As the ionic domains increase, owing to a proper amounts of sulfonic groups, these clusters can be interconnected to form ionic cluster channels that facilitate the transport of protons. However, the other non-ionic block domains play role of an effective barriers for methanol. Thus, here in this article, we aim to synthesize and characterize a polyelectrolyte by middle block sulfonation of the star block copolymer with different sulfonation levels and to prepare proton exchange membranes with enhanced proton conductivity and water transport from the sulfonated polymer for mainly fuel cell application and other applications.
2. Experimental
2.1. Materials
The poly(styrene-butadiene-styrene) (SBS) was purchased from LG chemicals. The star block copolymer used has 50 wt% styrene and molecular weight of 217,000 g/mol. A linear SBS with 60 wt% styrene and 84,000 g/mol molecular weight was also purchased from the same company. 1,4-dioxane, chlorosulfonic acid and dimethyl sulfoxide, ethanol was purchased from various companies and was used as received.
2.1.1. Polymer purification
The SBS was soaked in n-hexane with a ratio of 3 wt% to remove the oil extenders. The step was repeated until the washings became clear and colorless. The polymer was dried to remove residual hexane, after which it was dissolved in chloroform and precipitated in methanol to remove the stabilizers. The polymer was separated by filtration and then dried under vacuum at 25°C.
2.1.2. Sulfonation
Sulfonation of the double bond in the butadiene block of the polymer was achieved by reaction with chlorosulfonic acid. SBS was dissolved in 100 mL 1,4-dioxane in a flame dried reactor coupled with a dropping column and then calculated amount of chlorosulfonic acid in 1,4-dioxane (3 vol%) was added under nitrogen environment using a syringe. The solution was let to react for 4 hours and then precipitated in hexane. The sulfonated polymer was then washed with water and ethanol until the washings have a neutral pH. The same procedure was performed for a linear SBS polymer.
2.2. Proton exchange membrane preparation and characterization
Membranes were prepared by dissolving 3 wt% of the sulfonated SBS (SSBS) in DMSO and casting in Teflon dishes in a drying oven at 60°C for 7 days. Residual solvent was evaporated in vacuum at the same temperature for 1 day. The resulting membranes were annealed in vacuum at 110°C for 6 hours.
2.2.1. Fourier transform infrared spectroscopy
Sulfonation was verified using FT-IR (Varian Resolutions). Thin films of the membrane were analyzed with 32 scans and resolution of 4 cm-1.
2.2.2. Atomic force microscopy
Phase images with a phase scale of 40° and 1 μm × 1 μm scan size was recorded using True non-contact atomic force microscopy (TNC-AFM), Park system XE-100. Samples with 35 wt% water content were analyzed at ambient conditions.
2.2.3. Thermal stability
TGA was employed to determine the thermal stability (weight loss) of SBS and SSBS using TA instruments, TGA 2950. All samples were vacuum dried prior to TGA analysis. Typical heating rate of 10 °C/min was used from 30°C to 800°C.
2.2.4. Ion exchange capacity (IEC)
Ion exchange capacity was taken using methods reported elsewhere[14,20]. Samples were soaked into 0.1 M NaCl to substitute the protons in the sulfonic acid groups with Na+ and after 24 hours, titrated with 0.01 M NaOH to determine the amount of released protons. IEC was computed using the formula:
2.2.5. Water and vapor uptake
Water and vapor uptake was performed by soaking the membranes in liquid water or exposing the membrane in a controlled humidity chamber for 5 days. Table 1 shows the salts used to control the humidity of the chambers. The membranes are then blot dried using Kimwipes and weighed in an analytical balance. After weighing, the membranes are dried under vacuum at 100°C for 24 hours. The dried membranes are then weighed and then water and vapor uptake was calculated using the following equation (2):
2.3. Proton conductivity and methanol permeability
2.3.1. Proton Conductivity
Proton conductivity of hydrated membranes was measured using electrochemical impedance spectroscopy (EIS), Zahner IM6ex, in AC mode with a frequency range of 100 mHz~1 MHz and 50 mV amplitude. A four probe proton conductivity cell similar to the one reported elsewhere was used[20]. Impedance spectroscopy is an indirect but reliable method of analyzing proton conductivity. Proton conductivity was calculated using the following equation:
where σ is the proton conductivity (S/cm), L is the length of membrane (cm), R is the impedance (Ω), W is the membrane width (cm), and d is the membrane thickness (cm), respectively.
Proton conductivity under different temperatures was performed by placing the four-point probe cell into a chamber to control humidity. The chamber which is shown in Fig. 2, reference[20] was then placed in a water bath to control the temperature. All temperature effect experiments were done under 99% RH. Fig. 1
2.3.2. Methanol permeability
Permeability analysis was based on procedures reported previously[14]. Using a home-made diffusion cell, one compartment of the cell, compartment B (VB = 48.2 mL), was filled with deionized water while other compartment, compartment B (VA = 55.4 mL), was filled with 15 wt.% methanol (DC Chemical, Extra pure grade) solution in deionized water. The membrane that with a diffusion area of 12.76 cm2 was sandwiched by Teflon O-rings to effectively seal the membrane as previously illustrated in the reference[14,20]. The concentration of compartment A was kept constant by feeding a solution with 15 wt% concentration. The methanol concentration changes with respect to time of compartment B from the methanol which diffused through the membrane was detected using a RI detector connected to compartment B . The detected refractive values with respect to time were converted to concentration using a calibration curve of known methanol concentrations. To compute for the methanol permeability, the following equation was used.
where: CB is concentration of compartment B , CA is the concentration of compartment A, A is the effective area of the membrane, L is the membrane thickness, t is the time, tO is the lag time, D is the methanol diffusivity, K is the methanol solubility, respectively.
Methanol permeability is defined as the product of diffusivity to solubility, DK, and can be calculated from the slope of the concentration change of CB with time.
3. Results and Discussion
3.1. Sulfonation
The sulfonation of a four arm star tri-block copolymer yields an ionomer with ionic block located at the inner part of the star as illustrated in Fig. 1(a). The double bond in the butadiene blocks are most likely to react with the sulfonating agent than the stable aromatic ring of polystyrene due to the stronger reactivity from the former as shown in Fig. 1(b). Chlorosulfonic acid was chosen as the sulfonating agent because of its well-known reactivity. This strong acid has also been used as a common sulfonating agent for several ionomer syntheses[16,17]. The effective reaction of sulfonation on the butadiene blocks was qualitatively verified using FT-IR spectrophotometry. Fig. 2 shows the FT-IR spectra of pristine SBS and SSBS with different degrees of sulfonation. The asymmetric stretch of the S=O group can be observed at 1,155 cm-1 and symmetric stretch of S-O at 696 cm-1. It is apparent that as the degree of sulfonation increases, the intensity of the peaks also increases. The characteristic peaks representing the substituted phenyl rings of polystyrene were not observed showing that sulfonation proceeded in the middle block rather than the terminal ends.
The obtained values of IEC, degree of sulfonation (DS) and equivalent weight (EW) are listed in Table 2. Increasing IEC, DS, and EW is attributed to increasing amount of sulfonyl groups in the polymer and can be controlled by regulating the amount of chlorosulfonic acid charged to the reaction vessel.
As listed in Table 2, the effective reactivity of sulfonation as a function of degree of sulfonation achieved. The degree of sulfonation was calculated from the ratio of the moles of sulfonic acid added to the moles of double bonds in the butadiene block. The resulting degree of sulfonation was lower than the expected degree of sulfonation with respect to the amount of sulfonating agent added, which are denoted as SSBS50, SSBS100, and SSBS200, respectively. Nevertheless, sulfonation increased as more amount of sulfonating agent was added. The effective reactivity of sulfonations are 8.6, 17.2 and 25.9% for SSBS8, SSBS17 and SSBS25, respectively.
3.2. Thermal properties
The thermal properties of the SBS and the SSBS with different degrees of sulfonation are shown in Fig. 3. Minimum and major degradations are represented by the slope of the weight loss for each sample. Degradation temperature can be taken from the change in slope of the thermogravimetric graph. It can be observed that the degradation temperature of SBS is around 477°C which is similar to the reported values[18]. For the sulfonated samples, three degradation temperatures can be observed at 177, 272 and 442°C. The first weight decrease from 0~175°C can be attributed to the removal of the water bounded to the polymer due to the hygroscopic nature of the sulfonated polymer. The high temperature for the removal of water can be explained by the fact that water bounded to the sulfonic acids is harder to remove than loosely bound water. The second slope which ends around 272°C can be attributed to the removal of sulfonic acids. The desulfonation temperature observed is similar to those reported for aromatic ring sulfonated block copolymers by several references[19,20]. The increase in slope with increasing degree of sulfonation is an indication of the amount of sulfonic acid present in the ionomer. The third degradation at 442°C temperature is attributed to the degradation of polystyrene and the polybutadiene blocks. The 37°C decrease in degradation temperature is contrary to those reported for aromatic ring sulfonated block copolymers. This is due to when the double bond in the polybutadiene block was sulfonated, the polymer backbone became less thermally stable, hence, a lower degradation temperature.
3.3. Water uptake
As expected, an increase in IEC has a proportional increase in water uptake. A proportional increase in water uptake was observed with the IEC. This can be explained by the amount of water absorbed by the sulfonic acid groups in the polymer. Furthermore, the specific hydration, λ, which is the number of moles of water absorbed per sulfonic acid, also increases with IEC. This is due to the amount sulfonic acid present in the membrane. Sulfonic acid groups are the ones responsible for the enhanced hydrophilicity, thus, increasing water uptake. In addition, the location of the -SO3H in the flexible butadiene block provides enhanced water uptake as opposed to polymers sulfonated at rigid styrene or sulfone blocks.
The vapor absorption capacity of the SSBS at different percent relative humidity is shown in Fig. 4. It can be seen that from 33 %RH to 75 %RH, there is a slow absorption of water vapor for all sulfonation degrees. From 75 %RH to 97 %RH, the vapor uptake for SSBS8 was almost doubled and for SSBS17 and SSBS25 was almost tripled.
3.4. Proton conductivity and methanol permeability
For numerical comparison, the transport properties of SSBS such as proton conductivity and methanol permeability are listed in Table 4. It can be seen that the proton conductivity increases with increasing degree of sulfonation. This can be attributed to the amount of sulfonic acid present in the SSBS. For the sample with lower IEC with nation 117, SSBS8, the proton conductivity is lower with a value of 0.0231 S/cm. This can be explained by the more acidic nature of the sulfonic acid end groups of nation due to the longer chain functional group of Nafion. Moreover, even with an increase of IEC of 1.58 mmol/g for SSBS17 the proton conductivity, 0.074 S/cm, is still lower than Nafion. However, a remarkable value of proton conductivity was observed for SSBS25 with a value of 0.114 S/cm. Table 3
The methanol crossover was determined by letting 15 wt% methanol diffuse through the membrane for a certain amount of time and using the aforementioned equation for calculation of permeability value. Remarkably, the obtained values of methanol permeability for all samples were observed to be lower than Nafion. It can also be observed that there was a large jump in the value of methanol permeability of SSBS8 to SSBS17. From this, it is safe to assume that the percolation threshold of SSBS can be seen within this range of degree of sulfonation. The results of proton conductivity also agree with this trend since there is also a large jump from the proton conductivity values of SSBS8 to SSBS17. The percolation threshold is the formation of the channels where H+ can pass through and also have been reported for other sulfonated polymers with different degrees of sulfonation.
As a measure of efficiency, the ratios of the desired to undesired solute permeability were taken. This quantity is referred to as the selectivity and is the ratio of the proton conductivity to methanol permeability. It was calculated using the formula α = σ/P, where α is the selectivity of proton conductivity to methanol permeability (S-cm/s), σ is proton conductivity (S/cm), P is methanol permeability (cm2/s).
Fig. 5 shows that the selectivity for all membranes is higher than the value for Nafion. This basically means that the sulfonated styrene butadiene styrene favors proton more than methanol. Interestingly, the selectivity for SSBS25 is higher than that of SSBS17 and SSBS8 in spite of having the highest methanol permeability value.
The role of water in the transport properties of SSBS can be seen in Fig. 6. The addition of sulfonic acids in the block copolymer corresponds to enhancements in both water uptake and proton conductivity. Conductivity was also observed to be in direct relationship with the specific hydration number, λ, as seen in Fig. 7. Several reports mentioned the importance of water in the transport of protons in ionomers. Several reasons are the swelling of the polymer by water forms ionic channels which provides pathways for ion transport and the hydrated sulfonic acid groups provides transport sites for protons by means of two known mechanisms, Grotthus and vehicular mechanism depending on the amount of water. The amount of water is significant in water transport because the interionic distance is determined by the amount of water content within the ionomer. Also, tortuosity, which is the twistedness of the ionic channels is affected by water uptake and is responsible for the architecture of the membrane structure[15]. With higher water content, interionic distance is closer and there are more sites for proton transport. In addition, a swelled membrane decreases tortuosity because of the more connected ionic channels are brought about by swelling with water. For the sulfonated samples it can be observed that together with water uptake, proton conductivity increased for membranes with increasing degree of sulfonation. It should be noted that proton conductivity measurements were taken at fully hydrated state, meaning all ionic groups are saturated with water and ionic channels are formed within the membranes. The variation in proton conductivities can be explained by the differences interionic distances for each membrane. As expected, the membrane with the highest degree of sulfonation has closer interionic distances since it has more sulfonic acid to absorb water and connect the ionic pathways within the membrane matrix.
The effect of raising the temperature to the proton conductivity of SSBS is shown in Fig. 8. The proton conductivity was taken from 30°C to 80°C at 100% RH for the sulfonated block copolymer and Nafion in a closed chamber. All membranes showed conductivity increase when temperature was rose which is a similar trend with other sulfonated membranes[21,22]. Remark-ably, SSBS25 exhibited a dramatic increase of conductivity for SSBS8 contrary to slow increase for SSBS8 when the temperature was raised, while SSBS17 showed a similar rate with Nafion. Moreover, SSBS25 showed a superior conductivity of 0.1723 S/cm compared to 0.1337 S/cm for Nafion at 80°C. It is well established that at higher temperatures, water sorption also increases as reported by others[23,24]. This can explain the increase of proton conductivity at higher temperatures for all membranes. For the sulfonated block copolymers with increasing IEC, the enhancement of conductivity values was observed to be directly proportional to the IEC.
3.5. Morphology
Morphological analysis of the sulfonated block copolymers was performed using True non-contact AFM. Phase images of membranes with 20 wt% water content were taken at ambient condition with a size scale of 1 μm × 1 μm. Micro scale features such as ion clusters and ionic channels were investigated and showed in Fig. 9.
Hydrophilic phases represented by the darker areas and hydrophobic phases represented by the light areas was identified for all the membrane samples. It is apparent that interconnection and clustering of the dark areas varies with different samples. Interestingly, the size of hydrophilic areas becomes smaller when degree of sulfonation increases, contrary to the trend observed for other sulfonated block copolymers where the size of ionic channels increases with sulfonation degree. However, the trends agree when it comes to interconnection of the hydrophilic areas with increasing sulfonation. For SSBS8, the hydrophilic phases are clustered together and showing few continuities. This shows that ionic channels are not completely formed due to the limited amount of sulfonic acid groups that constitute such channels. This structure explains the lower value of proton conductivity for SSBS8. The percolation threshold can be observed for SSBS17 where the darker areas begun to connect to each other. This observation can be supported by the rise in water uptake with the increase in sulfonation degree as mentioned earlier. This means that at degrees of sulfonation higher than 17 mol%, a continuous ionic matrix can be expected for sulfonated membranes. This supports the dramatic increase in proton conductivity of SSBS17 as well as the activation energy of proton conductivity is the lowest value, 7.4 kJ/mol listed in Table 4.
Compared with the lower sulfonation degree membranes, the darker areas of the SSBS25 can be observed to be more connected. This explains the higher proton transport properties of SSBS25. The continuous and close interionic sites provide easier transport for protons within the membrane matrix. However, it is intriguing as to why the channels are smaller, as opposed to what was expected for higher IEC ionomers. But this phenomenon can be a reasonable explanation as to why the methanol permeability is lower than the value of Nafion. Narrower channels can be an obstacle for methanol transport which involves a bigger particle compared to a proton, hence the larger selectivity of protons to methanol.
4. Conclusions
Successful sulfonation of the poly(styrene-butadiene- styrene) star tri block copolymer using chlorosulfonic acid was verified by FTIR spectroscopy. Ion exchange capacity determination using titration showed that 8, 17 and 25 mol% sulfonation was achieved. Increasing degree of sulfonation resulted to enhanced water uptake properties. Outstanding transport properties were observed for the sulfonated samples. Remarkable proton conductivities with 0.0231, 0.0724 and 0.114 S/cm was observed for SSBS8, SSBS17 and SSBS25, respectively. The conductivity was also observed to increase remarkably when the temperature was raised. Moreover, methanol permeability was lower compared to Nafion. To determine the efficiency of the desired property, selectivity was calculated and it was found out that selectivites for all samples surpasses that of Nafion. Morphological studies showed closer interionic distance and increasing connectivity of ionic channels with increasing degree of sulfonation.
