Novel nanostructured semicrystalline ionomers by chemoselective sulfonation of multiblock copolymers of syndiotactic polystyrene with polybutadiene

Abstract

Novel semi-crystalline sulfonated copolymers (sPS(B-SA)) have been synthesized by sulfonation of multiblock copolymers of syndiotactic polystyrene with cis-1,4-polybutadiene (sPSB), using a two-stage solution process comprising the addition of thiolacetic acid to the butadiene units, followed by the in situ oxidation of the thioacetyl moieties with performic acid. The sulfonation process is quantitative, chemoselective, cheap and more environmentally benign than similar methods previously reported; in addition it allows achieving high ion exchange capacity values (up to 4.48 equiv. per kg). The sPS(B-SA)s are crystalline, preserving the native polymorphic behavior of the crystalline syndiotactic polystyrene segments. Thin films of sPS(B-SA) samples show a phase separated morphology at the nanometer scale evidenced by tapping mode and tunneling current atomic force microscopy; proton conductive regions embedded in a hard-hydrophobic matrix of syndiotactic polystyrene were actually observed.

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Introduction

Much effort in the last few decades has been aimed at synthesizing high performance perfluoroalkyl and aryl sulfonated polymers under eco-friendly conditions and low production cost. Sulfonated polymers are of interest as superacid catalysts,^1a–c sealing agents, adhesives, emulsifiers, surfactants, compatibilizers and in membrane science as ion exchange resins for technical and medical applications² Moreover one of the most relevant use is in polymer electrolyte membrane (PEM) for fuel cells (FC)³ where the PEM is typically an ionomer containing a high number of ionic groups tethered to the main chain. These polar groups allow, after appropriate hydration, an efficient transport of protons. Typical examples include sulfonated perfluoro-polymers, as e.g. Nafion, or arylsulfonated polymers, based on polystyrene or polyarylketones; all of these polymers are characterized by a random distribution of sulfonic groups along the main chain. Few detailed study on the structural properties of Nafion^4a,b and polystyrene^4c based PEMs are available because of the amorphous nature of these polymers. It is generally accepted that the phase segregation creates a nanoscale morphology where the ionic domains assure proton mobility³ by the proton hopping^5a among the water molecules or physical transport^5b by hydrophilic molecules (usually water) through the ionic network across the membrane. Not less relevant is the role of the hydrophobic regions which act as scaffold and contribute to define the mechanical properties of the membrane. The efficiency of the PEM, determining the overall the power output of the cell, depends on all mentioned factors.

Sulfonation of styrenic or aromatic copolymers is typically obtained randomly in para-position of the aromatic rings using: (i) concentrated sulfuric acid; (ii) sulfur trioxide; (iii) chlorosulfonic acid; (iv) acetyl sulfate.⁶ Noteworthy when stereoregular polymers undergo this reaction, the crystalline phases are readily converted into the amorphous ones even at low degree of sulfonation, determining fast degradation of the physical mechanical properties of the polymer thin films. Sulfonation of butadiene units in styrene–butadiene rubber (SBR) is carried out using: (i) sulfur trioxide in vapour phase^7a or in solution, in presence of Lewis bases as triethyl phosphate,^7b tetrahydrofuran, dioxane, or amines;^7c (ii) chlorosulfonic acid in diethyl ether;^7d (iii) concentrated sulfuric acid^7e or its mixtures with alkyl hypochlorite;^7f (iv) bisulfites^7g combined to dioxygen, hydrogen peroxide, metallic catalysts, or peroxo derivates;^7h (v) acetyl sulfate.⁷ These sulfonation routes need harsh conditions and lead to poor results in terms of conversion, selectivity and environmental sustainability. Interestingly a sulfonation degree of about 80% of the butadiene units in SBS copolymers has been recently reported by Spontak et al. using fuming sulfuric acid dropwise added to 1,4-dioxane polymer solutions.⁸

Recent studies have shown that sulfonated block⁹ and multiblock copolymers¹⁰ exhibit proton conductivity higher than the random counterparts. Actually the block copolymers enable the formation of a phase separated morphology that is recognized as the main issue responsible of the enhancement in proton conductivity in the PEMs. Self-assembly of the polymer segments leads to the formation of continuous hydrophilic domains^10a in which the ionic groups guarantee an efficient proton transport and the hard hydrophobic regions act as scaffold for the membrane. Thus a selective and efficient sulfonation route allowing to preserve crystallinity and morphology in polymer films is a topic of high interest.

In the recent years, we extensively investigated the synthesis and properties of syndiotactic polystyrene (sPS)¹¹ and the metal catalyzed stereospecific copolymerization of styrene with conjugated dienes¹² to produce block copolymers containing crystalline segments of stereoregular polystyrene. sPS is a semicrystalline thermoplastics characterized by high T_g value (105 °C), high melting temperature (270 °C), excellent thermal stability and chemical inertness.^11,13 Among the five crystalline forms described for this polymer, the δ and ε forms are noteworthy since they present nanometric voids and channels, respectively, in which a variety of organic molecules can be clathrated leading to co-crystalline structure.¹³ This crystalline behavior results quite singular in polymeric materials,^13c and several applications in molecular separation, capture of volatile organic compound (VOC), drug release, chemical sensor, optics and catalysis have been proposed for this nanoporous polymer.¹³ The synthesis of multiblock copolymers of syndiotactic polystyrene and 1,4-cis-polybutadiene (sPSB) was developed in our group affording high stereo- and regio-regular polymer segments with a syndiotacticity index for the styrene segments >95 mol% and 1,4-cis/1,2-vinyl butadiene insertion of 85/15.¹² sPSB copolymers showed the same complex polymorphism of sPS when the composition is rich in styrene, i.e. styrene block length ≥9 and x_S ≥ 0.40. Notably thin films of sPSB exhibit separated phase morphology in the nanometer scale,^12e and are active compatibilizers of PB/PS blends.^12d Recently, sPSBs were applied as support of gold nanoparticles where the nanoporosity of the polymer matrix allowed to address a remarkable selectivity in aerobic oxidation catalysis of alcohols.^12a,c

We recently reported sulfonation of polybutadiene (PB) and of triblock copolymers polystyrene-b-polybutadiene-b-polystyrene (SBS) using a highly efficient and selective route that results inexpensive, environmental-benign and allows to achieve high IEC values.¹⁴ This synthetic approach has been applied to stereoregular sPSBs for the synthesis of novel semi-crystalline nanostructured sulfonated styrenic polymers.

Experimental

General procedure and materials

The manipulation of air- and moisture-sensitive compounds was performed under nitrogen atmosphere using standard Schlenk techniques and a MBraun glovebox. Toluene (Carlo Erba, 99.5%) was used as received or predried with calcium chloride, refluxed for 48 h over sodium and distilled before use in moisture- and oxygen-sensitive reactions. Multiblock syndiotactic polystyrene-co-1,4-cis-polybutadiene samples were synthesized according to the literature procedure.¹² Thiolacetic acid (96%, Sigma-Aldrich), benzophenone (>99%, Sigma-Aldrich), formic acid (98%, Carlo Erba), hydrogen peroxide (35 wt% in water, Carlo Erba), potassium bromide (Sigma-Aldrich), silver paste (10 wt% dispersion of silver nanoparticles (<100 nm) in ethylene glycol; Sigma-Aldrich), indium tin oxide (ITO)-coated glass slide (8–12 Ω sq⁻¹; Sigma-Aldrich), Nafion® (20 wt% in isopropanol and water; Sigma-Aldrich), dimethyl sulfoxide-d₆ (DMSO-d₆; Euriso-top), and 1,1,2,2-tetrachloethane-d₂ (TCE-d₂; Euriso-top) were used as received.

Measurements and characterizations

NMR spectra were collected on AVANCE Bruker spectrometers (600, 400, 300 and 250 MHz for ¹H): the chemical shifts were referenced to tetramethylsilane as external reference, using the residual protio signal of the deuterated solvents. Elemental analysis was performed on a CHNS Thermo Scientific Flash EA 1112 equipped with a thermal conductivity detector. FT-IR measurements were carried out on a Bruker Vertex 70 spectrometer equipped with a DTGS detector and a Ge/KBr beam splitter. The samples were analysed in the form of KBr disks. Powder wide angle X-ray diffraction patterns were obtained, in reflection mode, with an automatic Bruker D8 powder diffractometer using the nickel-filtered Cu K_α radiation. The DSC measurements were carried out in air on a TA Instrument DSC Q20 calorimeter (heating rate = 10 °C min⁻¹). The TGA and TGA-IR measurements were performed in air on a Netzsch TG 209 F1 (heating rate = 10 °C min⁻¹) coupled with a Bruker Vertex 70 FTIR spectrometer by means of a PTFE transfer line and a gas cell thermostated at 200 °C. The TGA-IR single acquisition run, of the gas evolved from the sample heating, consists of 30 scans within 28 s, with a resolution of 2 cm⁻¹ and a delay of 2 s between the runs, repeated during the whole thermogravimetric analysis. UV irradiations were performed on polymer solutions at wavelength of 365 nm and power of 100 W in a UV incubator Bio-Link BLX from Vilber Lourmat. GPC analysis was carried out at 135 °C using 1,2-dichlorobenzene as carrier solvent with a flow rate of 1.0 mL min⁻¹ on a GPCV 2000 from Waters Instruments equipped with a viscosimeter and a refractive index as detectors, through a four columns set purchased from PSS-USA with a particle size of 10 μm and pore size of 106, 105, 104 and 103 Å. The calibration was performed with polystyrene standards with molecular weight in the range of 10⁶ to 10² Da. TEM analyses were carried out with a JEM 3010 electron microscope from JEOL operating at 300 kV, with a point-to-point resolution of 0.17 nm (at Scherzer defocus). Specimens for TEM analysis were sonicated in 2-propanol, transferred (10 mL) onto a copper grid covered with a lacey carbon film supplied from Assing, and then stained by immersion into a lead(II) acetate water solution. AFM micrographs of thin films were collected in air at room temperature with a Dimension 3100 coupled with a Bruker Nanoscope V controller operating in tapping mode or in tunneling current mode. The specimens to be analysed were obtained by spin-coating using a spin-coater SPIN150 from SPS Europe (2000 rpm per s²; 1500 rpm for 30 s) of chloroform polymer solution (30 μL; 0.2 wt%) onto glass slides for the sPSB sample and by deposition of DMSO polymer solutions (30 μL; 0.2 wt%) at 80 °C onto glass slides for the sPS(B-SA) copolymer. The film thickness was evaluated with AFM by scratching the polymer film and found in the range 0.5–1 μm. Commercial probe tips with nominal spring constants of 20–100 Nm⁻¹, resonance frequencies in the range of 200–400 kHz, and tip radius of 5–10 nm were used. The TUNA-AFM measurements were performed using platinum-coated probes with nominal spring constants of 35 Nm⁻¹ and electrically conductive tip of 20 nm. The samples were deposited from DMSO solutions (0.2 wt%; 10 μL; 80 °C) onto glass slide coated with ITO and electrically connected to the nanoscope with silver paste and aluminium foils. The support was heated at 250 °C for 1 h before the polymer deposition, while the specimen was annealed at 110 °C for 15 min before TUNA-AFM analysis. The micrographs were analysed using the software Nanoscope Analysis v1.40 r2sr1 from Bruker.

Cyclic voltammograms were recorded with a Metrohm 757 VA Computrace combined with a AUTOLAB potentiostat–galvanostat. The working electrode was a glassy carbon microelectrode covered with PEEK as insulator supplied by Metrohm, with 1.0 and 3.5 mm inner and outer radii, respectively. The counter electrode was a platinum grid, and a standard Ag/AgCl/KCl_(sat.) electrode was used as reference. Voltammetric experiments were performed at 25 °C, in a single-compartment three-electrode cell, spanning the 1.5 to −1.5 V interval, at 0.1 V s⁻¹ scan rate. The 0.1 M HClO₄ electrolyte medium was purged with a high-purity nitrogen stream. Stable and reproducible voltammetric curves were obtained within 5–6 sweeps. The electrochemical properties were evaluated on the basis of the data recorded at the tenth sweep. For the preparation of the polymer membranes with a thickness of 100 ± 10 μm onto the working electrode were deposed 38.5 mg of a 20 wt% solution of Nafion® in isopropanol and water or 19.3 mg of a 20 wt% solution of sample 5-SA of Table 1 in chloroform.

Table 1 sPSB, sPS(B-TA) and sPS(B-SA) copolymer

Sample^a	Composition^b								ΦTA^b^c	ΦSA^b^d	S^e		Mn^f	Mn/Mw^f	Cryst.^g
	S		B		B-TA		B-SA				NMR	EA^h
	(mol%)	(wt%)	(mol%)	(wt%)	(mol%)	(wt%)	(mol%)	(wt%)	(mol%)	(mol%)	(eq. kg^−1)	(eq. kg^−1)	(kDa)		(%)
a All samples were reproduced at least three times. Thioacetylated and sulfonated derivatives were labelled respectively with -TA and -SA.b Determined by NMR.c Thioacetylation degree of butadiene units.d Sulfonation degree of butadiene units.e Sulfur content evaluated by NMR and EA: for the sulfonated derivatives this value correspond to the IEC.f Determined by GPC.g Relative crystallinity of the sPS fraction estimated by DSC assuming a ΔHm reference value of 53.2 J g^−1 for highly crystalline sPS.^15h Value corrected subtracting the water content evaluated from TGA (see ESI).i Not resolved.
1	44.3	60.5	55.7	39.5	—	—	—	—	—	—	—	—	47.7	1.61	19
1-TA	44.3	39.5	2.6	1.2	53.1	59.2	—	—	95.2	—	4.55	n.d.	47.1	1.59	32
1-SA	44.3	38.5	0	0	0	0	52.9	60.3	—	95.0	4.43	4.48	n.d.	n.d.	n.r.^i
2	54.6	69.8	45.4	30.2	—	—	—	—	—	—	—	—	56.4	2.02	19
2-TA	54.6	49.7	2.1	1.0	43.3	49.3	—	—	95.3	—	3.78	n.d.	54.9	1.92	23
2-SA	54.6	48.6	0	0	0	0	42.8	50.1	—	94.3	3.68	3.58	n.d.	n.d.	n.r.^i
3	68.7	80.8	31.3	19.2	—	—	—	—	—	—	—	—	39.9	1.97	12
3-TA	68.7	64.2	1.1	0.5	30.2	35.3	—	—	96.3	—	2.71	n.d.	37.5	1.74	24
3-SA	68.7	63.2	0	0	0	0	29.9	36.1	—	95.4	2.65	n.d.	n.d.	n.d.	n.r.^i
4	82.3	89.9	17.7	10.0	—	—	—	—	—	—	—	—	73.2	1.90	30
4-TA	82.3	79.3	0.9	0.5	16.8	20.2	—	—	94.7	—	1.55	n.d.	76.5	1.88	26
4-SA	82.3	78.6	0	0	0	0	16.7	20.9	—	94.3	1.54	1.23	n.d.	n.d.	21
5	88.1	93.4	11.9	6.6	—	—	—	—	—	—	—	—	48.0	3.17	31
5-TA	88.1	86.1	0.9	0.5	11.0	13.5	—	—	92.6	—	1.03	n.d.	47.7	2.96	31
5-SA	88.1	85.6	0	0	0	0	11.0	13.9	—	92.4	1.03	1.08	n.d.	n.d.	19

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Protocol for thioacetylation of sPSB (example given for sample 1-TA, Table 1)

1.20 g of sample 1 (8.76 mmol of butadiene units) were treated with 200 mL of toluene in a two-neck 500 mL round bottom flask under vigorous stirring at room temperature for 72 hours. The resulting slurry was then refluxed for 1 hour to yield a clear polymer gel. The reactor was cooled to 50 °C, and BZP (0.014 g, 73 μmol; BZP/TAA molar ratio = 0.0083) and TAA (0.71 mL, 9.64 mmol; TAA/B molar ratio = 1.1) were added in the order. The reactor was irradiated at 365 nm and power of 100 W in an UV incubator for 4 h. The reaction was monitored by coagulation of 5 mL of the polymer gel in methanol. The polymer was recovered by filtration dried in vacuum and analysed by NMR to check the degree of functionalization.

Protocol for sulfonation of sPSB (example given for sample 1-SA, Table 1)

Formic acid (9.3 mL; 0.25 mol; HCOOH/TAA molar ratio = 25) was added at 50 °C to the polymer gel resulting from the thioacetylation step. Hydrogen peroxide (4.1 mL, 48.2 mmol; 35 wt% in water; HCOOH/H₂O₂ molar ratio = 5) was slowly added to the polymer solution in about 15 min.

CAUTION: the reaction is autocatalytic and strongly exothermic! The flask was then allowed to cool at room temperature and 300 mL of acetonitrile were added followed by in vacuo distillation of the solvent at 35 °C, avoiding the complete drying of the polymer. Additional 200 mL of acetonitrile were added and the solvent was distilled off until the formation of a reddish-brown polymer gel. Further addition of a plenty of acetonitrile affords the precipitation of the polymer permitting its recovery by filtration. The polymer was dried in vacuum on a PTFE sheet at room temperature for 48 hours.

Results and discussion

The addition of thiolacetic acid (TAA) to alkenes is usually promoted by strong acids, hydrogen peroxide combined with radical initiators as 2,2′-azobis(2-methyl-propionitrile) (AIBN),^16a coupling agents as cerium(IV) and iron(III) salts,^16b or indium(III) complex,^16c montmorillonite K10 clay.^16d These reactions are partially chemoselective and side reactions, as e.g. crosslinking of polymer chains, are possible. Moreover the poor solubility of the inorganic catalysts in organic solvents severely limits their use in the thioacetylation of hydrophobic unsaturated polymers. The quantitative and chemoselective addition of TAA to the olefinic C=C double bond in sPSB was successfully obtained optimizing the previously reported procedure.¹⁴ The reaction resulted more challenging than in the case of amorphous PB and SBS, since the syndiotactic polystyrene crystalline domains make the sPSBs sparingly soluble in all organic solvents. Thus the polymers were preliminary swollen in toluene (3.0 wt%) and then refluxed in this solvent for 1 h to yield clear polymer gels. The addition of 1.1 equiv. of TAA at 50 °C under UV radiation (365 nm; 100 W) in the presence of 8 mequiv. of benzophenone (BZP) as radical activator leads to the thioacetylated derivatives (sPS(B-TA)) in high yields and excellent functionalization degree (see Scheme 1 and Table 1). This synthetic route does not require further purification steps of the reaction products and results fast, efficient and selective.

Scheme 1 Functionalization of sPSB multiblock copolymers.

The toluene solution of sPS(B-TA) was treated with performic acid, generated in situ by reaction of formic acid with hydrogen peroxide, for the final oxidation of the thioacetyl moieties to sulfonic groups. The reaction is highly exothermic and auto-catalytic since the acidic groups, produced at the beginning of the reaction, further catalyze the generation of peracid and so the oxidation of the thioacetyl groups. sPS(B-SA)s were obtained in quantitative yields in only 1 h (Scheme 1 and Table 1).

The typical FT-IR spectrum of a sPS(B-TA) copolymer (sample 2-TA, Table 1 and Fig. 1b) shows the absence of the olefinic signals and the presence of new sharp adsorption bands at 1689 cm⁻¹ and 953 cm⁻¹ attributed respectively to the C=O and C–S stretching of the thioacetyl moiety.¹⁷ In the FT-IR spectrum of the corresponding sulfonated polymer (sample 2-SA, Table 1 and Fig. 1c) the characteristic signals for the sulfonic moiety are at 1348 cm⁻¹ (antisymmetric stretching, ν_a, of SO₂ moiety), 1155 cm⁻¹ (symmetric stretching, ν_s, of SO₂), 1031 cm⁻¹ (stretching, ν, of S=O) and 903 cm⁻¹ (stretching, ν, of S–O). The broad peak at 3455 cm⁻¹ is attributed to the OH stretching of sorbed water after exposition of the hygroscopic sample to air.¹⁷

Fig. 1 FTIR spectra of: (a) sPSB (sample 2, Table 1); (b) sPSB(B-TA) (sample 2-TA, Table 1); (c) sPS(B-SA) (entry 2-SA, Table 1).

Quantitative evaluation of the functionalization degree was performed by means of NMR spectroscopy adopting the polystyrene segments as an internal standard (Fig. 2 and S1 and 2†). The ¹H signals at 3.47 and 2.85 ppm were attributed to the methine (B_1,4-TA–H₂) and the methylene (B_1,2-TA–H₄) protons of 1-TA (Fig. 2a), whereas the thioacetyl methyl group (B_1,4-TA–H₆ and B_1,2-TA–H₆) was observed at 2.33 ppm. The corresponding ¹³C signal (Fig. 2d and d′) were found at 44.37 ppm (B_1,4-TA–C₂), 27.71 ppm (B_1,2-TA–C₄) and 196 ppm (B_1,4-TA–C₆ and B_1,2-TA–C₆) (see Fig. S1 and ESI, Table S1† for the complete NMR assignments).

Fig. 2 Chart for the NMR signal assignments for the functionalized butadiene units (a). ¹H NMR spectra of 1-TA (b) and of 1-SA (c). ¹³C NMR spectra of 1-TA (d) and of 1-SA (e) with the corresponding magnifications respectively in d′ and e′. The characteristic signals of polystyrene were not labelled for simplicity. The spectra of the thioacetylated derivative were acquired in TCE-d₂ (*) at 25 °C, while the sulfonated in TCE-d₂/DMSO-d₆ (#) solvents mixture (v/v = 1/1) at 90 °C.

The full NMR characterization of amphiphilic sulfonated polymers is challenging, since these polymers are poor soluble in the most common organic solvents and few NMR reference data are available in the literature. The ¹H NMR spectrum of 1-SA in TCE-d₂/DMSO-d₆ (v/v = 1 : 1) at 90 °C shows the methine and methylene signals of the sulfonated 1,4-cis (B_1,4-SA–H₂) and 1,2-vinyl (B_1,2-SA–H₄) butadiene units at 2.61 and 2.74 ppm, respectively (Fig. 2c); the corresponding ¹³C signals are observed at 58.2 ppm (B_1,4-SA–C₂) and 47.1 ppm (B_1,2-SA–C₄) (Fig. 2e and e′ and S2†). Not functionalized butadiene units or by-products resulting from oxidation of the olefinic carbon–carbon bonds with peracids are under the limit of spectroscopic detection, confirming that a saturated polymer backbone was obtained.

The sulfonation degrees (Φ_SA) and the corresponding ion exchange capacity (IEC) values were determined by elemental analysis (EA) and found in good agreement with the NMR data (see Table 1). It is worth noting that high IEC values were obtained at low sulfonation degree, as a result of the low molecular weight of the sulfonated butadiene unit (136.2 g mol⁻¹ vs. 444.1 g mol⁻¹ of the sulfonated monomer of Nafion).

The number average molecular weights M_n and the polydispersity index (PDI = M_w/M_n), determined by GPC for both the sPS(B-TA)s and pristine sPSBs samples, are very close to each other (see Table 1 and Fig. S3–6†) ruling out polymer crosslink or backbone degradation during the thioacetylation reaction. Unfortunately the sPS(B-SA)s are sparingly soluble in the conventional solvents used for the GPC analysis and this hampered the evaluation of the polymer average molecular weights (see Table S2† for the solubility properties of the sPS(B-SA)s).

Differential scanning calorimetry (DSC) and thermogravimetric analysis coupled with infrared spectroscopy (TGA-IR) allowed assessing the thermal properties and stability of the sPS(B-TA)s and sPS(B-SA)s. The DSC curve of 1-TA exhibits melting of crystalline sPS segments at 239 °C whereas the corresponding sulfonated derivative, 1-SA, shows a more complex thermal behavior (Fig. 3). The DSC trace can be divided in three main regions, corresponding to the weight loss steps observed in the TGA measurements, namely 70–190, 205–280 and 280–480 °C (Fig. 3b).

Fig. 3 DSC (blue curves) and TGA (red dashed curves) profiles of sample 1-TA (a) and 1-SA (b). Plot of FTIR spectra of the gas evolved during the thermal decomposition of the sample 1-SA by TGA-IR analysis (c), where the dotted blue curve highlights the SO₂ evolution (vibration at 1360 cm⁻¹).

In the first region the two endo-peaks at 93.0 and 135.4 °C account for the release of water molecules clustered by the sulfonic acid groups,¹⁸ whereas the broad endo peak centred at 258 °C is attributed to the desulfonation reaction, as confirmed by FT-IR analysis of the gas evolved during the thermal decomposition, corresponding to water and sulfur dioxide in the first region and second region, respectively (Fig. 3c). Finally the thermal decomposition of the polymer backbone starts at 380 °C or higher temperature (see Fig. S7–S14† for the other samples of Table 1). The weight loss of 26.7% observed for the desulfonation stage well compares with the value of 22.3% estimated by NMR considering the concentration of the sulfonic groups. The desulfonation of 1-SA begins at about 200 °C (Fig. 3c), at the same temperature reported for the perfluorosulfonic polymers or even higher to that of aryl sulfonated polymers, confirming the potential of the sPS(B-SA)s in membrane science.^18–20

Random and non-selective sulfonation of semicrystalline polymer typically leads to the loss of crystallinity in the polymer films also at low degree of functionalization. Attempts to obtain selective sulfonation of the amorphous phase of crystalline sPS in solid state resulted in fast degradation the crystallinity.²¹ The procedure herein described successfully allowed to preserve crystallinity in sPS(B-SA), also at high concentration of sulfonic groups, permitting to reach high value of IEC. Actually the DSC trace of 1-TA shows the same melting point of the sPS segments observed in the pristine sPSB (see Table 1 and Fig. 3a) whereas this endo- peak overlaps in 1-SA the desulfonation endotherm (Fig. 3b). The latter effect is particularly prominent in 2-SA and 3-SA where the mole concentration of the sulfonic groups is high (DSC traces in Fig. S7 and 8†). The presence of crystalline sPS was confirmed by the powder wide angle X-ray diffraction (WAXD; Fig. 4 and S15–S18†) analysis of the sPS(B-SA). The sample 5 exhibits crystallinity corresponding to the δ form in which toluene molecules, resulting from the solution process, are hosted¹³ in the nanocavities of the crystalline polymer phase (one toluene molecule per cavity) (Fig. 4a). The WAXD patterns of 5-TA and 5-SA, displayed in Fig. 4b and c, show that the native co-crystalline form is preserved during all the functionalization process. The annealing of 5-SA at 180 or 120 °C affords the conversion of the δ crystalline form into the expected β and γ forms¹³ respectively (Fig. 4d and e). These crystalline phases show high melting temperature that assures high chemical and thermo-mechanical stability to the polymer phase.^21c

Fig. 4 WAXD patterns of: (a) sample 5, δ form; (b) sample 5-TA, δ form; (c) sample 5-SA, δ form; (d) sample 5-SA, β form; (e) sample 5-SA, γ form.

The crystallinity degree of sPS(B-TA) and sPS(B-SA) was evaluated by DSC considering the area of the melting peak in the first heating run and assuming the reference value for the melting enthalpy for highly crystalline sPS of 52.3 J g⁻¹⁵ (Table 1). sPS(B-TA)s and sPS(B-SA)s exhibit crystallinity of sPS domains up to a styrene concentration of 40 mol%, that approximately corresponds to a styrene block length of 9 units;^12b that is the same limit composition observed in sPSB.^12e

The sample 5-SA, showing IEC of 1.03 equiv. per kg close to that of Nafion (0.9 equiv. per kg), was chosen for a deep morphological investigation at the nanometre scale. The TEM micrograph of 5-SA powders is shown in Fig. 5: the crude polymer was suspended in isopropanol by sonication, deposed onto a TEM grid and analysed after staining with lead(II) acetate. The polymer particles are homogenously stained with lead(II) (dark regions in Fig. 5a and b), as confirmed additionally by the energy dispersive X-ray analysis (EDX, Fig. 5c and S21†). This finding could probably result from the treatment of the polymer powders with isopropanol; actually the polar solvent could promote the ejection of the sulfonic groups onto the surface of the polymer particles and the extensive staining of the particle surface as a consequence. The phase segregation appeared clearly in Nafion only after thermal or electro-poling treatments.²² However, crystalline not stained polymer regions were detected in the high resolution (HR) TEM of the same specimen (Fig. 5d): the electron diffraction pattern originated by the same area (Fig. 5e and S22†) is in agreement with the WAXD analysis of this sample and suggests the existence of sPS crystallinity.

Fig. 5 (a) TEM micrograph of sample 5-SA stained with lead(II) acetate; (b) detail of a polymer particle with the corresponding EDX microanalysis in (c); (d) HRTEM with sPS crystalline plane highlighted by red lines (taken approximately from the centre of (b)) and the corresponding SAED pattern in (d).

Tapping mode (TM-AFM)^{11c,12d,23,24} and tunneling current mode (TUNA-AFM)^14,25 atomic force microscopy allowed to shed light on the morphology of the proton conductive regions at the nanometre scale of the sulfonated polymers. The TM-AFM scanning of the sample surface provides information about the topography and phase distribution, whereas TUNA-AFM analysis of the thin film deposited on electrical conductive indium tin oxide (ITO) substrate enables the detection of ultralow currents crossing the film when a bias voltage is applied between the sample holder and the tip. Preliminarily, the TM-AFM analysis of a thin film of sample 5 (sPSB) showed in the height image (Fig. 6a) a flat surface characterized by the presence of smooth circular irregular pores of average diameter of 30 ± 21 nm (size distribution diagram in Fig. S19†). These pores, in dark colour in the phase contrast image (Fig. 6b) consist of a soft material dispersed in a predominant hard matrix (bright relief in Fig. 6b).²⁵ Unfortunately the TM-AFM technique does not allow to discriminate if the hard matrix is crystalline as well as if the observed circular soft domains are isolated spheres or vertically aligned cylindrical array. On the basis of the chemical composition and the tensile properties of the two polymer segments, we tentatively attributed the soft phase to PB and the hard matrix to sPS. A quite similar morphology was observed in the corresponding thin film of 5-SA, where isolated and smaller irregular circular domains of 21 ± 12 nm (size distribution diagram in Fig. S20†) were detected. The TUNA-AFM analysis of 5-SA (Fig. 6e and f) deposited onto ITO surface, showed electrical conductive regions quite similar in shape, dimension and distribution to those evidenced by TM-AFM, pointing out that the embedded pores consist of sulfonated proton conductive regions.

Fig. 6 AFM micrographs of the sample: 5 height (a) and phase (b); 5-SA height (c), phase (d) and TUNA current ((e and f); DC sample bias 0.500 V).

Although all of the samples 1–5-SA produce clear polymer solutions in conventional polar solvents as DMSO, DMAc and DMF or mixture of solvents as TCE/DMSO (see ESI, Table S2†), membranes of these sulfonated polymers, suitable for the determination of the electrical and mechanical properties, could not be obtained by casting of dry polymer solutions or hot-pressing. Cyclic voltammetry (CV) is an alternative useful tool for the assessment of the electrical properties in semiconductors; noteworthy this technique needs very small amount of sample (in the order of milligrams) and allows fast acquisition.²⁶ In electrochemical processes, the Tafel equation (see ESI, eqn (S1)†) anticipates that the plot of the overpotential vs. the logarithm of the current (log i) is linear at high polarization values of the electrode; the same occurs at low polarization values when the overpotential is plotted vs. the current i. Due to the similarity of this equation with the Ohm's law (eqn (S3)†), both the electrical resistance (eqn (4)) and the corresponding conductivity (σ, eqn (S4)†) can be calculated from the exchange current (i₀) extrapolated in the Tafel plot (see Fig. S23†). A polymer membrane of 5-SA was thus deposed onto the working electrode by casting of a chloroform solution to perform the CV measurements. For a head to head comparison, a polymer film of 100 ± 10 μm of Nafion® was deposed on the electrode by casting of a commercial solution of the polymer in isopropanol and water. Thus cyclic voltammograms were acquired at sweep rate of 0.1 V s⁻¹ in the range of 1.5 V and −1.5 V, under nitrogen atmosphere using 0.1 M HClO₄ aqueous solution as electrolyte. The sample 5-SA yielded a proton conductivity of 0.077 ± 0.013 S cm⁻¹ very close to that found for Nafion (0.060 ± 0.010 S cm⁻¹) under the same conditions. These values fall in the range 0.050–0.140 S cm⁻¹ typically observed for Nafion at 25 °C at variance of the technique and the experimental conditions adopted for the measurement.²⁷

Conclusions

This paper deals with the synthesis and characterization of novel semicrystalline sulfonated styrenic block copolymers. The synthetic route involves the addition of thiolacetic acid to butadiene units of sPSB, promoted by UV radiation and benzophenone as radical activator, followed by the oxidation of the thioacetyl groups using in situ generated performic acid. The overall reaction pathways is cost effective, eco-sustainable and highly selective towards the olefinic carbon–carbon double bonds, leaving the aromatic rings of the styrene units totally not reacted. The comparison of the M_n values of the sPS(B-TA)s and pristine polymers rule out the possibility of covalent crosslinking or degradation of the polymer chains during the synthetic process. The chemical structure of the functionalized polymers was determined by NMR and FTIR spectroscopy. The synthesized copolymer are thermally stable, with thermal decomposition of the sPS(B-TA)s occurring at temperature values close to those of the sPSBs (380 °C) whereas that of the sPS(B-SA)s is comparable to that of commercial aryl and perfluoro sulfonated polymers. The native crystallinity of the sPSBs as well as the polymorphic behaviour of the crystalline segments of syndiotactic polystyrene is preserved when using the solution process herein described. In fact, the WAXD analysis of 5-SA powders and the HRTEM micrograph of the same sample after staining with lead(II) acetate showed the presence of crystalline not stained polystyrenic domains. We believe that such crystallinity improves the oxidative stability of the styrenic domains under harsh oxidation conditions as experienced in the synthetic procedure involving the use of performic acid: actually the samples 1–5-SA resulted soluble in polar solvents leading to clear, scantly viscous solutions at high temperature. In agreement with this finding previous studies have shown that sulfonated syndiotactic polystyrene samples are barely reactive in Fenton test.^21c This is a crucial issue for the potential application of the title sulfonated polymers in PEMFC.

The AFM analysis of 5-SA highlighted a phase separated morphology in which the alkylsulfonated segments segregate to form irregular and disordered circular domains embedded in the hydrophobic crystalline sPS matrix. Proton conductivity of 0.077 ± 0.013 S cm⁻¹ was measured at 25 °C for this sample exhibiting IEC value similar to that of Nafion.

Some preliminary studies have also shown that this novel class of sulfonated polymers are robust super acid catalyst in esterification reaction for the biodiesel production.²⁸

Acknowledgements

Financial support is acknowledged from the Ministero dell'Istruzione dell'Università e della Ricerca (MIUR, Roma, Italy for FARB-2013), the Regione Campania (POR FSE, project: “MAteriali e STrutture Intelligenti”, MASTRI, Code 4-17-3, CUPB25B09000010007) and the Centro di Tecnologie Integrate per la Salute (Project PONa3_00138) for the 600 MHz NMR instrumental time. The authors are also grateful to Prof. Ermanno Vasca, Dr Tonino Caruso, Dr Patrizia Oliva, Dr Patrizia Iannece and Dr Ivano Immediata from Università degli Studi di Salerno and Enrica Fontananova from Consiglio Nazionale delle Ricerche for technical assistance.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Further NMR, GPC, DSC, TGA-IR, WAXD, AFM and TEM characterizations and solubility properties of the novel synthesized copolymers. See DOI: 10.1039/c4ra13253j

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