Sulfonated Styrene-(ethylene-co-butylene)-styrene/Montmorillonite Clay Nanocomposites: Synthesis, Morphology, and Properties
© to the authors 2007
Received: 20 August 2007
Accepted: 28 November 2007
Published: 18 December 2007
Sulfonated styrene-(ethylene-butylene)-styrene triblock copolymer (SSEBS) was synthesized by reaction of acetyl sulfate with SEBS. SSESB-clay nanocomposites were then prepared from hydrophilic Na-montmorillonite (MT) and organically (quaternary amine) modified hydrophobic nanoclay (OMT) at very low loading. SEBS did not show improvement in properties with MT-based nanocomposites. On sulfonation (3 and 6 weight%) of SEBS, hydrophilic MT clay-based nanocomposites exhibited better mechanical, dynamic mechanical, and thermal properties, and also controlled water–methanol mixture uptake and permeation and AC resistance. Microstructure determined by X-ray diffraction, atomic force microscopy, and transmission electron microscopy due to better dispersion of MT nanoclay particles and interaction of MT with SSEBS matrix was responsible for this effect. The resulting nanocomposites have potential as proton transfer membranes for Fuel Cell applications.
In recent years, there has been considerable interest in special composite materials that consist of a matrix, usually a polymer, filled with plate-like or flake-like inorganic fillers having at least one dimension in nanometer length scale and high aspect ratio. Such fillers can be extremely effective in modifying the properties of polymers. Several orders of change in mechanical, transport, rheological, electrical, or thermal properties have been demonstrated in these composites containing only a few volume percent of nano-filler [1–9].
SEBS, (styrene-ethylene-butylene-styrene) triblock copolymer, chosen as the base material in this work, is extensively used as a thermoplastic elastomer . This is a nonpolar polymer and not compatible with a polar substance. Hence, polar modification of SEBS has gained recent attention. However, till now most researchers have concentrated on the maleation of SEBS with maleic anhydride in organic solution or in the melt or graft copolymerization of SEBS with methacrylic acid in organic solution. Though sulfonation of SEBS has been reported in literature [11–17], no report is available on the montmorillonite clay (MT)-based nanocomposites of sulfonated SEBS. The authors have reported earlier preparation and properties of SEBS–MT-based nanocomposites [18, 19] and numerous other rubber–clay nanocomposites from this laboratory [20–25]. A few reports on polymer composites acting as proton conducting membranes [26, 27] and a few on block copolymer–clay nanocomposites [28–33] are available. Sulfonated SEBS-montmorillonite clay-based nanocomposite as a strong member for controlling the proton transfer is a novel approach in this new field of renewable energy source in a world of crisis of energy.
This is a new approach as all the earlier studies on SEBS-MT clay nanocomposites have concentrated on intercalating the clay after organically modifying it by long-chain amines. Here, in this present work, unmodified clay (MT) has been successfully intercalated and exfoliated by sulfonated SEBS systems. The work reported here is concerned with the synthesis of sulfonated SEBS, and preparation and characteristics of the unmodified montmorillonite clay (MT)-based nanocomposites. Low-cost montmorillonite clay can be used in place of organically modified nanoclays in making nanocomposites with sulfonated SEBS.
(styrene-ethylene-butylene-styrene) triblock copolymer (SEBS) with molecular weight M n = 50,000 and styrene/ethylene-butylene (w/w) = 30/70 was supplied by Shell Chemical Co, USA. Acetic anhydride (Analytical grade) was procured from Aldrich, Milwaukee, WI. 1,2-dichloro ethane (DCE), sulfuric acid (assay content, >99%), methanol, and tetra-hydrofuran (THF, analytical grade) were obtained from Merck Ltd., Mumbai, India. Unmodified sodium montmorillonite clay (MT, having cation exchange capacity = 92.6 meq./100 gm with 2:1 tetrahedral:octahedral layer structure) and long-chain quaternary ammonium ion-modified nanoclay (OMT,Cloisite®20A) were generously supplied by Southern Clay Products, Gonzales, TX, USA. Double deionized water was prepared in this laboratory.
Sulfonation was carried out onto SEBS backbone in an analogous method to that described by Weiss et al.  Acetyl sulfate was synthesized at temperature near to −20 °C as per Scheme 1 in dry oxygen free N2 atmosphere.
A solution of SEBS (10% w/v in DCE) was prepared in a three necked round bottom flask equipped with condenser and the solution was heated to 60 °C and stirred for 4 h for full solubilization of SEBS. O2free dry N2gas was passed through the polymer solution in order to drive out the dissolved oxygen present in the solvent and also in the reaction flask. The required amount of freshly prepared acetyl sulfate was then added drop-wise to the reaction mixture. The reaction mixture (Scheme 2) was maintained at 60 °C under stirring in nitrogen atmosphere. After 2 h of optimized reaction time at this condition, the reaction was stopped by gradually adding an excess of isopropanol for 10 min and cooling to room temperature. Finally, the sulfonated SEBS was isolated, steam stripped in excess of double de-ionized (dd) boiling water, followed by washing several times with boiling and cold dd water (to eliminate the solvent, free acids, and hydrolyze the acetyl sulfate). The product was filtered and dried under vacuum at 70 °C up to a constant weight and was stored in a desiccator to avoid moisture.
Sulfonated SEBS (SSEBS) was dissolved in a THF/methanol mixture (9/1 v/v) and the homogeneous solution was left under stirring for 2 h after which the solvents were evaporated under reduced pressure (about 1 mmHg) at 50 °C for 7 days.
Measurement of Percentage Sulfonation
where, W0 = weight of neat SEBS and Wg = weight of the sulfonic acid-grafted SEBS. Infrared Spectroscopy (Perkin Elmer FTIR–spectrophotometer) and elemental analysis (CHNSO Analyzer, Perkin Elmer) were also performed to quantify the graft percentage. Both the results revealed ∼3 and ∼6 wt.% of sulfonation onto SEBS backbone.
SSEBS Clay Hybrid Nanocomposite Film Preparation
S3SEBS (with 3 wt.% sulfonation to SEBS) and S6SEBS (with 6 wt.% sulfonation to SEBS)/ MT4 and OMT4 clay nanocomposites (with 4 wt.% of clay) were prepared using a THF solvent-casting method. Initially, SSEBS was dissolved in THF overnight and MT or OMT clay at optimized 4 wt.% were suspended in THF for 6 h and stirred for 2 h using a magnetic stirrer. The polymer solution and clay particle suspension were then mixed together at 25 °C and stirred for 1 day in order to complete the mixing. Next, the samples were dried in a hood by evaporating the solvent to get a film thick in the range of 50–60 μm.
Fourier Transform Infrared (FT-IR) Spectroscopic Studies
FT-IR studies were carried out in dispersive mode on thin film samples using Perkin Elmer FTIR–spectrophotometer (model Spectrum RXI,UK), within a range of 400–4,400 cm−1 using a resolution of 4 cm−1. An average of 32 scans have been reported for each sample.
Microstructure by Wide Angle X-ray Diffraction (WAXD)
Wide angle X-ray diffraction analysis of the nanocomposites was carried out in aPANalytical XPert Pro (3040/60 the Netherlands) X-ray diffractometer (operated at 30 kV and 40 mA) at room temperature, equipped with Cu–K a radiation.
where λ = wavelength of the X-ray with Cu–K a target = 0.154 nm,d = interplanar distance of the clay platelets, θ = angle of the incident radiation.
Transmission Electron Microscopy (TEM)
The samples for transmission electron microscopy analysis were prepared by ultra cryo-microtomy using aLeica Ultracut UCT (Wien, Austria). Freshly sharpened glass knives with cutting edge of 45° were used to get the cryosections of 50–70 nm thickness at a sub-ambient temperature of −80 °C using aJEOL 2010, Japan TEM, operating at an accelerating voltage of 200 kV. Selective staining of aromatic moieties in the samples was done with vapor of OsO4.
Phase Imaging by Atomic Force Microscopy (AFM)
The effects of sulfonation on SEBS and of inclusion of inorganic silicate clay layers on the morphology of SEBS and its nanocomposite were investigated by using atomic force microscopy (MultiMode AFM TM from Digital Instruments, Santa Barbara, CA, USA) in air at ambient conditions (25 °C, 60% RH) in the tapping mode using etched silicon probe tips (TESP), with a spring constant in the range of 40 N/m. For each sample, minimum three images were analyzed.
Dynamic Mechanical Thermal Analysis (DMTA)
The dynamic mechanical spectra of the samples were obtained by usingRheometric Scientific DMTA IV, NJ, USA analyzed in tension-compression mode at a constant frequency of 1 Hz, a strain of 0.01%, and a temperature range from −100 to 130 °C at a heating rate of 2 °C/min. The temperature corresponding to the peak in tanδ versus temperature plot was taken as the glass–rubber transition temperature (Tg).
Studies of Mechanical Properties
Tensile properties were measured on dumb-bell specimens at room temperature using a ZWICK Z010 tensile test machine (Zwick Inc., Ulm, Germany). The gauge length and cross-head speed were 25 mm and 500 mm/min, respectively. At least five samples were tested and the average was used.
Thermogravimetric Analysis (TGA)
The thermal degradation analysis of SEBS, grafted SEBS, and their nanocomposites was performed withTGA Q50 of TA Instruments- Waters LLC, USA operated at a heating rate of 20 °C/min in N2atmosphere at a flow rate of 60 mL/min in the temperature range of 25–700 °C.
Water–Methanol Uptake and Permeability
where wwet = weight of wet samples after blotting the surface water–MeOH and wdry = weight of dry sample before wetting. Permeability of water–MeOH (80–20) mix into free air through the films of SSEBS and MT4-based nanocomposites was measured by diffusion process with an airtight glass diffusion cell.
AC Resistance and Proton Conductivity
AC electrical resistance was measured at room temperature for film samples in the transverse direction with a two probe INSTEK LCR meter (LCR 819, Taiwan) operating in AC frequency range from 0.4 to 10 kHz. The proton conductivity was measured in an indirect process for the water–methanol-immersed samples after wiping out the surface water and measuring the resistance employing the same set up.
Results and Discussion
Sulfonation of SEBS
Morphological Shift on Sulfonation
Microstructure of SSEBS–Clay Nanocomposites by WAXD
Microstructure of Nanocomposite by TEM, AFM, and DMTA
Stress–strain Properties of Resulting SSEBS and their MT4-Based Nanocomposites
Tensile stress–strain of sulfonated SEBS-nanocomposites
Tensile strength (MPa)
Modulus at 50% strain (MPa)
Breaking elongation (%)
23.6 ± 0.5
2.6 ± 0.2
520 ± 10
24.2 ± 0.5
2.2 ± 0.1
530 ± 15
31.6 ± 0.7
3.5 ± 0.3
580 ± 20
20.5 ± 0.3
2.4 ± 0.2
500 ± 10
26.1 ± 0.5
2.9 ± 0.2
500 ± 10
20.0 ± 0.3
2.3 ± 0.1
460 ± 10
28.3 ± 0.6
3.2 ± 0.2
480 ± 8
Proper dispersion of fine reinforcing MT clay platelets is responsible for this enhanced physical property as compared to sulfonated SEBSs.
Thermal Degradation Properties
Water–Methanol Uptake Properties
AC Resistance in Dry and Wet Modes
SEBS has been sulfonated at two different levels of sulfonation (3 and 6 wt.%) by in situ-prepared acetyl sulfate. FTIR spectra and DMTA analysis confirm that grafting has taken place at the end PS blocks of SEBS.
On sulfonation, micro-phase-separated morphology has been shifted from purely cylindrical for neat SEBS to distorted-spherical mixed one for S3SEBS and distorted one for S6SEBS.
Unmodified montmorillonite clay (MT)-based nanocomposites have been synthesized based on these sulfonated SEBS following solution intercalation process.
The dispersion of MT clays in neat SEBS matrix was a real problem as evidenced from morphology and reflected in its properties. Hydrophilic MT clays have been better dispersed and intercalated in these SSEBS matrices and MT clay-based nanocomposites exhibit enhanced mechanical and thermal properties as compared SEBS-MT and SEBS-OMT. XRD and TEM studies reveal better interaction and dispersion of MT with SSEBS matrix. Remarkable improvement in thermal degradation resistance for S6SEBS-MT4 is observed.
Water–MeOH uptake and permeation is much improved for corresponding nanocomposites making them potential candidate for DMFC.
AC resistance is shown to decrease on water–MeOH wet samples with nanocomposites posing restrictions for electricity flow owing to the torturous path in the matrix.
From this study, it is proved that organic modification of clay is not mandatory in making polymer–clay nanocomposites. Polar modification of the SEBS matrix by sulfonation enables cheaper MT clays to be used to synthesize excellent nanocomposites with enhanced physico-mechanical, thermal, water swelling, and electrical properties.
Anirban Ganguly acknowledges the scholarship grant in NDF category by AICTE, New Delhi, India.
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