Physical properties of solution cast poly(styrene–butadiene–styrene)/heteroaromatic poly(azo-thiourea) blends

Abstract

The heteroaromatic azo-polymer poly(azo-thiourea) has been prepared using 4-(4-aminobenzyl)benzenamine and diazonium salt solution of 2,6-diaminopyridine. The polymer was processable using polar organic solvents and had high molar mass of 62 × 10³ gmol⁻¹. Various concentrations of azo-filler were blended in solution phase using tetrahydrofuran or dimethylformamide with poly(styrene–butadiene–styrene). Morphology, thermal, and mechanical properties of styrene–butadiene–styrene/poly(azo-thiourea)blends have been studied using field-emission scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and tensile tests. Field-emission scanning electron microscopy micrographs of styrene–butadiene–styrene/poly(azo-thiourea) blends revealed fine granular filler dispersion in the matrix and development of conducting pathways. Accordingly, filler content from 10 to 60 wt% increased the conductivity from 0.99 × 10⁻¹ to 1.61 S cm⁻¹. A relationship between poly(azo-thiourea) loading and thermal stability of the materials was evident. Temperature for 10 wt% thermogravimetric weight loss increased from 467℃ to 483℃, while glass transition was enhanced from 151℃ to 155℃. Thermal and conductivity properties showed better results relative to pristine elastomer but less significant than the conducting filler used. Tensile properties were also dependent on the solvent used. In the dimethylformamide cast system, elongation at break was 345–387% versus the tetrahydrofuran system at 33–38%. The dimethylformamide cast blends tensile modulus was 599–769 MPa versus 876–1032 MPa for the tetrahydrofuran system. Thermal and conductivity behaviors were the same for both hybrid systems. Stress-induced birefringence in these blends has also been studied to find out the suitability of materials for waveguide applications.

Keywords

Poly(azo-thiourea)styrene–butadiene–styrene conducting pathways tensile strength gravimetric loss structure–property relationship

Introduction

With the discovery of intrinsically conducting polymers, a fascinating subject of research has been initiated due to the attractive characteristics and several application potentials of these materials.¹ Polymers with intrinsic electrical conductivity have a wide range of commercial relevance such as rechargeable batteries, electro-rheological materials, and sensors.^2,3 However, these intrinsically conductive polymers may have poor thermal stability along with deprived mechanical properties.^4,5 A successful approach to surmount these tribulations has been the fabrication of such composite or blend materials that enclose conductive polymer dispersed in thermoplastic elastomer. In this regard, various techniques have been exploited to introduce a conductive polymer into an insulating matrix such as thermomechanical mixing, solution mixing and electrochemical methods.^6,7 Among the conducting polymers, polyaniline (PANI) is quite promising because of its ease of synthesis, low cost, and tunable properties.⁸ The conductivity of PANI can be improved through the selection of a suitable dopant and level of doping and also by controlling its structure during synthesis.⁹ Elastomer blends have been constantly exploited in various industries to attain finest conciliation of physical properties along with processability, price, etc. Incidentally, styrene–butadiene block copolymers constitute an essential group of thermoplastic elastomers. Consequently, poly(styrene–butadiene–styrene) (SBS) triblock copolymer has been subjugated as matrix for conductive materials. Among heterogeneous systems, one constituted by SBS as insulating matrix is rather beneficial because of its thermoplastic-elastomeric nature and exceptional mishmash of physical properties and solvent miscibility.¹⁰ SBS may present a morphology segregated into different phases attributable to thermodynamic incompatibility of polystyrene and polybutadiene blocks.¹¹ Some studies regarding SBS block copolymer melt-blended with PANI have been found in the literature. Furthermore, research has also been conducted related to conducting SBS/PANI blends by solution process.^12–14 Conducting blends of SBS/PANI or polypyrrole have also been developed.^15–18

In this article, we reported on an electrically conductive, soluble, and thermally stable heteroaromatic poly(azo-thiourea) (PAT) as filler. The aim of this study was to investigate electrically conductive blends produced by combining an elastomeric material SBS with different amounts of PAT using a solution mixing method. Solution blending of SBS with PAT, instead of some traditional conducting polymeric filler, produced high-performance blend materials. Mechanical strength, electrical conductivity, heat stability, and morphology of SBS/PAT blends have been studied as a function of the increasing heteroaromatic azo-filler content. Comparison of the physical properties with respect to pure SBS and PAT and structure–property relationship in SBS/PAT blends has also been made. Owing to the outstanding properties, novel materials can be utilized in electronic, automotive and other engineering film applications.

Experimental

Materials

Poly(styrene-co-butadiene) (styrene 45 wt%), color: off-white; form: rubbery chunk; density: 0.965 g mL⁻¹ at 25℃, 2,6-daminopyridine (97%), and 4-(4-aminobenzyl)benzenamine (ABA) (97%) were provided by Aldrich. Ammonium thiocyanate (98%) and tetrahydrofuran (THF) (99.5%) were procured from Merck.

Measurements

NMR spectra were scanned at room temperature using BRUKER Spectrometer (300.13 MHz for ¹H NMR) in deuterated dimethyl sulfoxide (DMSO-d₆). IR spectra were recorded using Fourier transform infrared (FTIR) Spectrometer, Model No. FTSW 300 MX, manufactured by BIO-RAD (4 cm⁻¹ resolution). The number and weight-average molecular weight (M_n and M_w) were calculated through gel permeation chromatography using THF as an eluent and refractive index detector. Field-emission scanning electron microscopy (FESEM) of freeze fractured samples was performed using JSM5910, JEOL Japan. Samples were first frozen by placing in liquid nitrogen, and then fractured for morphology study. Thermal stability was verified by METTLER TOLEDO thermogravimetric analysis (TGA)/SDTA 851^e thermo gravimetric analyzer using 1–5 mg of the sample in Al₂O₃ crucible at a heating rate of 10 C min⁻¹. Differential scanning calorimetry (DSC) was performed by METTLER TOLEDO DSC 822^e differential scanning calorimeter taking 5–10 mg of samples in aluminum pans and heated at a rate 10℃ min⁻¹. Electrical conductivity of thin films was measured using a Keithley 614 electrometer and the four-probe method. Stress–strain response of the samples was obtained on Universal Testing Machine INSTRON 4206 according to the ASTM 638 method. A crosshead speed of 100 mm min⁻¹ was used during the test. Bright field microscopy was performed using an Olympus CKX41 inverted microscope equipped with a Basler area scan camera.

Synthesis of monomer

Synthesis of 1-(4-(4-thiocarbamoylaminobenzyl)-phenyl)thiourea

On a steam bath, ABA (0.2 mol), concentrated HCl (16 mL), ammonium thiocyanate (0.2 mol), and deaerated water (120 mL) were mixed and heated in a porcelain dish (2 h). The mixture was allowed to cool down to room temperature. The mixture was then evaporated to dryness (6–7 h). The product was boiled with small quantity of charcoal (ethanol), filtered, and cooled. Finally, 1 -(4-(4-thiocarbamoylaminobenzyl)-phenyl)thiourea (TCABPT) was re-crystallized from methanol and dried under vacuum at 90℃ (36 h).¹⁹ Elemental Anal. Calcd for C₁₅H₁₆N₄S₂ (%): C, 56.93; H, 5.10; N, 17.71. Found (%): C, 56.39; H, 4.92; N, 17.21. FTIR (KBr): 3409 cm⁻¹, 3336 cm⁻¹ (N–H stretch), 3016 cm⁻¹ (aromatic C–H stretch), 2917 cm⁻¹ (aliphatic C–H stretch), 1595 cm⁻¹ (N–H bend), 1411 cm⁻¹ (C–N stretch), 1155 cm⁻¹ (C = S stretch). ¹H NMR (300.13 MHz, DMSO-d₆, δ ppm): 4.21 (s, 4H, H_a), 9.45 (s, 2H, H_b), 6.31 (d, 4H, H_c), 6.77 (d, 4H, H_d), 3.72 (s, 2H, H_e). ¹³C NMR (75.47 MHz, DMSO-d₆, δ ppm): thiocarbonyl C=S 173.2 (C₁), 131.4 (C₂), 127.2 (C₃), 129.9 (C₄), 137.2 (C₅), 47.9 (C₅).

Diazotization

2,6-Diaminopyridine (0.04 mol) was first added to concentrated hydrochloric acid (10 mL) and then dissolved in distilled water (50 mL). The mixture was placed in a freezing-bath to cool to −10℃, ensuing a brown-colored precipitate. To the above suspension, 0.04 mol sodium nitrite solution (20 mL) was added with constant stirring. After complete addition, the mixture was stirred for 1 h at −10℃, to avoid the decomposition of diazonium salt. At the end, excess nitrite was removed by the addition of urea (2 g) with stirring of 20 min, resulting in a brown-colored diazonium salt solution.

Synthesis of PAT

TCABPT (0.02 mol) was dissolved in sodium hydroxide (10%) and the diazonium salt solution obtained (see section ‘Diazotization’) was added dropwise keeping the temperature below −10℃. After 3 h of stirring, the mixture was acidified with dilute HCl (10%). The resulting precipitate was filtered, washed with hot water, and then with hydrochloric acid (3%). The precipitate was again dissolved in sodium hydroxide (3%) followed by re-precipitation in HCl.²⁰ The polymer was dried under vacuum at 80℃. FTIR (KBr) (Figure 1): 3244 and 1595 cm⁻¹ (sec. amine N–H stretch and bend), 3088 cm⁻¹ (aromatic C–H stretch), 2827 and 2893 cm⁻¹ (aliphatic C–H stretch), 1414 cm⁻¹ (−N = N− stretch), 1124 cm⁻¹ (C = S stretch), and 827 cm⁻¹ (C–S stretching). ¹H NMR (300.13 MHz, DMSO-d₆, δ ppm) (Figure 2): 8.83 (s), 9.95 (s) (N–Hs chemical shifts), 7.19 (d), 7.34 (t) (pyridine unit), 6.33 (d), 6.71 (d) (diamine unit), 3.31 (CH₂).

Figure 1.

Fourier transform infrared spectrum of poly(azo-thiourea).

Figure 2.

¹H NMR spectrum of poly(azo-thiourea).

Fabrication of PAT/SBS blends

Solution blending technique was employed for the preparation of novel materials. Solutions of pure SBS and pure PAT were prepared in THF and dimethylformamide (DMF), individually. PAT solution was added to the solution of SBS at desired proportions (10, 30, and 60 wt%). SBS/PAT 10 was, thus, prepared by mixing a solution of SBS dissolved in THF with the solution of 10 wt% PAT. Similarly, elastomer dissolved in THF was blended with 30 and 60 wt% PAT (THF). The mixed solutions were ultrasonicated (30 W and 50 Hz) for 5 h. Thin blend films were prepared by pouring the blend solution on glass Petri dishes, and the solvent was evaporated at 60℃ for 24 h. Similarly, thin films for pure SBS and PAT were casted. The films obtained were subjected to various characterization techniques.

Results and discussion

Preparation of PAT and SBS/PAT blends

Mechanical testing

With the addition, a continued increase in tensile modulus was obvious, as it showed an increase up to 1032 MPa for 60 wt% PAT, and from 876 MPa for 10 wt% PAT in the matrix. Toughness, estimated by integrating the area under stress–strain curve, of blend with 10 wt% conducting heteroaromatic filler showed a value of 17.0 J m⁻³ that increased with further addition of reinforcement, as 18.4 J m⁻³ for 30 wt%, and a maximum of 20.0 J m⁻³ for 60 wt% content.

To study the effect of the solvent used on the tensile properties of SBS/PAT blends, another series of same hybrids was cast using DMF as solvent. It is generally known that the mechanical properties of solution cast styrenic block copolymer elastomer systems strongly depend on the type of solvent. Depending on the composition of thermoplastic elastomer, casting from some solvents may result in extremely stiff and brittle materials. On the other hand, casting the same polymer from some other solvents would result in conventional elastomeric properties; however, the chemical composition of the elastomer is the same.¹⁰ In our case, THF cast neat SBS and blends were stiffer in nature, whereas casting of the same SBS/PAT blends system using DMF resulted in less rigid materials having larger strain values. From the data presented in Table 2, it can be seen that DMF solvent created a less stiff material with enhanced elongation at break around 345–387%. However, the tensile modulus suffered a decrease of 599–769 MPa. Evidently, use of a more polar aprotic solvent DMF renders the hybrids with the conventional elastomeric properties.

The mechanical properties of the blends were, thus, improved with the addition of conducting polymer and tensile measurement indicated that azo-filler imparted strength to the elastomer. Currently, the research work on embrittling, fracture, and toughening mechanisms of blends is still in progress in our laboratory.

Morphological aspect

Tensile properties of SBS/PAT blends, cast from THF solvent, mentioned in Table 1 and Figure 3, clearly indicated that the addition of azo-polymer significantly led to substantial improvement in their values. Ultimate stress at break for SBS/PAT blend increased with the increase in conducting filler content. Filler of 10 wt% containing blend had tensile strength 28.1 MPa. The maximum stress value was observed for SBS/PAT 60 (60 wt%) as 30.1 MPa, while 30 wt% filler containing blend had the value of 29.4 intermediate between SBS/PAT 10 and 60. The enhancing effect of filler on the stiffness and strength was probably due to the incorporation of rigid-rod filler in the elastomer. A similar increasing trend with respect to the level of PAT content was also evident in the tensile modulus calculated from the initial slope of the stress–strain curve.

Table 1.

Mechanical properties of SBS/PAT blends (tetrahydrofuran).

Sample	Maximum stress (MPa) ± 0.1	Maximum strain ± 5 (%)	Tensile modulus (MPa) ± 0.1	Toughness (Jm⁻³) ± 0.3
PAT	24.2	30	566	15.3
SBS/PAT 10 (90:10)	28.1	33	876	17.0
SBS/PAT 30 (70:30)	29.4	35	997	18.4
SBS/PAT 60 (40:60)	30.1	38	1032	20.0
Neat SBS	20.3	27	123	3.1

SBS: styrene–butadiene–styrene; PAT: poly(azo-thiourea).

Table 2.

Mechanical properties of SBS/poly(azo-thiourea) (PAT) blends (dimethylformamide).

Sample	Maximum stress (MPa) ± 0.1	Maximum strain ± 5 (%)	Tensile modulus (MPa) ± 0.1	Toughness (Jm⁻³) ± 0.3
PAT	24.2	30	566	15.3
SBS/PAT 10 (90:10)	27.8	345	599	15.7
SBS/PAT 30 (70:30)	28.3	356	688	16.7
SBS/PAT 60 (40:60)	29.9	387	769	17.6
Neat SBS	19.5	334	99	3.0

SBS: styrene–butadiene–styrene; PAT: poly(azo-thiourea).

Figure 3.

Stress–strain curves of SBS and SBS/PAT blends.

FESEM images of new SBS/PAT blends are shown in Figure 4(a) to (d). We can see that the conducting filler was distributed in grainy form along with the matrix chains, forming a granular morphology. Because of fine solvent miscibility of heteroaromatic azo-polymer, these blends contain finely dispersed SBS and PAT phases forming a continuous structure that bridges the dispersed conducting phase, thus, promoting the conductivity of the material. Additionally, some holes/gaps formed during blending can be observed in the fractured specimen. Owing to the chemical affinity of hydrophobic chains of PAT with SBS, the matrix and azo-polymer were fairly compatible. The size of dispersed heteroaromatic azo-phase was increased from 10 to 60 wt% composition; the effect was more pronounced at 60 wt% filler content (Figure 4(d)). A better adhesion of dispersed phase in the matrix can be recognized in the micrograph along with a decrease in the number of holes. No conducting polymer aggregates were perceived in these micrographs. Higher level of molecular mixing constrained the chain conformation mobility and thus improved the conductivity performance of new blends. For better understanding of the results, micrographs for pure SBS (Figure 4(e)) and PAT (Figure 4(f)) are also shown. This morphology composed of dispersed nano-pores was accountable for fine conductivity in new blends as compared with previous SBS/PANI.²¹

Figure 4.

FESEM images of: (a) SBS/PAT 10 (1 µm), (b) SBS/PAT 10 (300 nm), (c) SBS/PAT 30 (500 nm), (d) SBS/PAT 60 (500 nm), (e) SBS (500 nm), and (f) PAT (500 nm).

To gain better insight into the blend morphology, a thin film of SBS/PAT 10 was prepared for optical microscopic study. Figure 5 shows that the blend exhibited a typical two-phase morphology composed of round drops of diameter of several micrometers. The matrix which was the majority phase was expected to change to the continuous phase while filler was dispersed as spheres. This observation also supported the high-resolution studies by scanning electron microscopy.

Figure 5.

Microscopic image of bright field of SBS/PAT 10 blend.

Thermal stability

Thermal stability of pure SBS, SBS/PAT blends, and heteroaromatic azo-filler (THF or DMF cast) was investigated through TGA and DSC. Figure 6 depicts TGA thermograms of pure PAT, elastomer, and SBS/PAT blends and data are presented in Table 3. Azo-polymer showed somewhat two-stage decomposition starting at 473℃ and main weight loss occurred around 500℃ and 600℃. Thermogravimetric curve exhibited initial decomposition T₀ at 433℃, 10% gravimetric loss at 467℃ and the decomposition continued up to 541℃, for 10 wt% filler blend. In contrast, increasing azo-filler from 30 to 60 wt%, the degradation occurred at further higher temperature. SBS/PAT 30 exhibited T₀ of 444℃, T₁₀ of 472℃, and T_max up to 601℃. Whereas, SBS/PAT 60 showed higher heat stability amid new blends i.e. T₀ of 462℃, T₁₀ of 483℃, and T_max was up to 611℃. Inclusion of reinforcement having rigid-rod azo design increased the heat stability of novel elastomeric blends significantly. Char yield of these blends was also high, 55–58% at 650℃, indicating good heat stability relative to SBS. However, the thermal stability values of pure PAT were higher, relative to blends, owing to rigid heteroaromatic structure. Figure 7 represents DSC thermograms of SBS/PAT blends containing 10–60 wt% filler and filler without SBS. Glass transition of neat PAT appeared at higher temperature 205℃ relative to those blended with the elastomer. Thermal transition for SBS/PAT 10–60 was recorded in the range 151–155℃. As obvious from the thermograms, a shift in T_g occurred to the right with the increasing filler content. The 90:10 blend showed relatively lower T_g value of 151℃ due to the flexibility of PAT backbone caused by the substitution of rigid bonds by freely rotating elastomer chains. However, SBS/PAT 30 and 60 were higher, 153℃ and 155℃, respectively. So, T_g values of new blends were, intermediate between that of the pure matrix (205℃) and the filler (−58℃). Thermal data, hence, exposed that the adding up of azo-polymer to SBS has affirmative effect on the thermal properties. Compared with the previously prepared SBS/PANI blends, the new materials were thermally more stable.^22,23 Here, the change of solvent THF or DMF did not seem to affect the thermal properties or glass transition of new blends. The reason behind was that the thermal properties of polymeric systems depend on the chemical nature of backbone. The hybrids encompassed same chemical structure regardless of the solvent used in the processing, therefore depicted similar heat resistance.

Figure 6.

Thermogravimetric analysis curves of poly(azo-thiourea) and blends at a heating rate of 10℃min⁻¹ in N₂.

Figure 7.

Differential scanning calorimetry thermograms of poly(azo-thiourea) and blends at heating rate of 10℃min⁻¹ in N₂.

Scheme 1 illustrates the preparation of PAT, having the exclusive combination of azo and thiourea linkages, via simple route. High molar mass PAT (62 × 10³ gmol⁻¹) showed good solvent miscibility especially in THF. Structural architecture of PAT renders fine electrical conductivity and heat stability of new materials. Afterward, the conducting polymer was exploited as reinforcement in SBS elastomeric matrix to enhance the conducting, mechanical, and thermal properties of new materials. The increase in PAT content was, therefore, found to enhance the conductivity and other physical characteristics of SBS/PAT system.

Scheme 1.

Scheme for the synthesis of poly(azo-thiourea).

Table 3.

Thermal analyses data of poly(azo-thiourea) (PAT) and blends (tetrahydrofuran or dimethylformamide).

Sample	T_g (℃)	T₀ (℃)	T₁₀ (℃)	T_max (℃)	Y_c at 650℃ (%)
Neat SBS	−58	414	434	476	21
SBS/PAT 10 (90:10)	151	433	467	541	55
SBS/PAT 30 (70:30)	153	444	472	601	57
SBS/PAT 60 (40:60)	155	462	483	611	58
PAT	205	473	489	625	61

T_g: glass transition temperature; T₀: initial decomposition temperature; T₁₀: temperature for 10% weight loss; T_max: maximum decomposition temperature; Y_c: char yield; weight of polymer remained; SBS: styrene–butadiene–styrene; PAT: poly(azo-thiourea).

Electrical conductivity

In SBS/PAT blends cast from THF or DMF, the reliance of conductivity on PAT content was analogous to that of reported insulating polymer matrices loaded with conducting fillers. Table 4 estimates the electrical conductivity of new heteroaromatic azo-polymer and blends with different compositions. Increase in the conductivity of new SBS-based materials was attributable to azo and heteroaromatic pyridine moieties introduced in the filler. The conductivity of SBS was increased with PAT loading; SBS/PAT 10 had 0.99 × 10⁻¹ S cm⁻¹, and SBS/PAT 30 owned 1.54 and 1.61 S cm⁻¹ for SBS/PAT 60. These values were found superior than the inherent conductivity of the elastomeric blends of PANI.^23–25 Nevertheless, the conductivity of blends was lower than that of pure PAT i.e. 1.99 S cm⁻¹ having wholly azo-backbone structure.

Table 4.

Conductivity measurement of SBS/PAT blends (tetrahydrofuran or dimethylformamide).

Sample	Conductivity (Scm⁻¹)
SBS	0.1 × 10⁻⁷
SBS/PAT 10	0.99 × 10⁻¹
SBS/PAT 30	1.54
SBS/PAT 60	1.61
PAT	1.99

SBS: styrene–butadiene–styrene; PAT: poly(azo-thiourea).

The good conductivity was also obvious from the morphological investigations as azo-polymer along with SBS, formed certain conducting pathways. In the conductivity measurements, results obtained for THF or DMF solvent cast hybrids were essentially the same as shown in Table 4. The conductivity of SBS-based materials was actually due to the azo and heteroaromatic pyridine structure introduced in the matrix, therefore remained unaffected by the change of solvent.

Stress-induced birefringence in blends

Birefringence in polymer films measures polymer orientation, which is related to physical properties of the films. The polymer science studies have spotlighted the relationships between orientation of amorphous and semicrystalline polymers and their shrinkage, tensile properties, and tear resistance. For low orientations polymers, the stress and strain were related to birefringence.²⁶ The development and use of a birefringence technique for on-line or off-line quantitative measurement of orientation in transparent films, sheets and bottles have been significant in the materials science.²⁷ Similarly, the applications of a birefringence technique (on-line or off-line quantitative measurement of biaxial orientation) have been explored in transparent films and sheets of different thickness in which birefringence values from 0.0005 to 0.25 can be measured.²⁸ Moreover, polymers have been explored extensively in fabricating planar waveguides which plays an important role in the telecommunications industry.²⁹

Except for pristine rubbery matrix, all reported waveguide polymers occur in a glassy state and are associated with a high birefringence. In order to reduce birefringence to a level that is suitable for waveguide fabrication, researchers have modified existing polymer systems chemically to control or compensate for molecular orientation.³⁰ This approach has significantly reduced orientation-related birefringence, although the requirement for quality waveguides has not yet been met, the reason being that, in addition to orientation-related birefringence, there is much stress-related birefringence in polymers. Unless this type of birefringence is reduced or eliminated, polymers will still not be suitable for waveguide applications. It is well known that birefringence in polymers is related to both molecular orientation and stress. Stress-related birefringence arises from the stress sensitivity of polymeric optical properties as follows:

Δ n = C σ

where Δn represents birefringence, C is the photoelastic constant of polymers, and σ is the stress generated in polymers. Stress in polymers originates from two sources, namely shrinkage and mismatched thermal expansion. Shrinkage is caused by chemical reactions occurring during fabrication. Mismatched thermal expansion occurs when processing is carried out at high temperature and the coefficient of thermal expansion (CTE) of polymer is much higher than that of waveguide substrate, which is often silicon. By calculating shrinkage-related and mismatched CTE-related stress using equation σ = ɛE, it is possible to estimate the birefringence caused by chemical shrinkage (Δncs) and mismatched CTE (Δncte), as shown in Table 5 (Hinds Instruments Birefringence measurement technology). As can be seen from the data, reaction shrinkage and mismatched CTE can cause a total birefringence in the order of 10⁴. This value is unacceptable for applications in which a birefringence of 5 × 10⁻⁵ or less is required for waveguide materials. In fact, even if orientation-related birefringence is completely eliminated using chemical modification, the remaining stress-related birefringence is still too high for polymers to be deemed usable.

Table 5.

Estimation of stress-caused birefringence in SBS/PAT 10.

Material	Linear curing shrinkage (%)	CTE at <T_g (10⁻⁶ per ℃)	Δncs (10⁻⁴)	Δncte (10⁻⁴)
SBS/PAT 10	2	69	2.2	1.0

SBS: styrene–butadiene–styrene; PAT: poly(azo-thiourea); CTE: coefficient of thermal expansion; cs: chemical shrinkage.

Conclusion

The conducting heteroaromatic PAT have been prepared in this attempt using the solution blending technique. The article also demonstrated that the solution blending of SBS elastomer with PAT in THF or DMF resulted in new high-performance materials with improved properties compared with the pristine materials. Novel blends presented good electrical conductivity ranging from 0.99 × 10⁻¹ to 1.61 S cm⁻¹; however, the conductivity value of pure azo-polymer was rather high 1.99 S cm⁻¹. Owing to the affinity of hydrophobic chains of PAT with SBS, a fine granular morphology was observed promoting the conducting path ways. High-mechanical performance of the conductive blends was also observed. Tensile strength, modulus as well as toughness increased with the addition of the conductive component. Additionally, tensile properties were dependent on the solvent used. In DMF cast system, the elongation at break increased by 345–387% (33–38% in THF), whereas the tensile modulus of DMF cast blends suffered a decrease of 599–769 MPa. So the conventional elastomeric properties were observed in more polar aprotic solvent DMF, although, thermal and conductivity behaviors of the hybrids were independent of the solvent used. Thermal stability investigation suggested that blending SBS with 10–60 wt% PAT could produce heat stable materials with 10% gravimetric loss of 467–483℃ and glass transition of 151–155℃. Moreover, new heteroaromatic filler exhibited higher glass transition, thermal, and electrically conducting properties relative to the elastomer used and blends prepared owing to rigid backbone aromatic azo structure. Exclusive thermal, mechanical, and conducting properties supported by morphological profile also supported the fine miscibility in SBS/PAT blends.

Footnotes

Acknowledgments

A Kausar thanks the Higher Education Commission (HEC) of Pakistan for the financial support.

Conflict of interest

None declared.

Biographies

Ayesha Kausar is Assistant Professor at National Centre for Physics, Quaid-i-Azam University, Islamabad, Pakistan. She obtained her PhD from the Quaid-i-Azam University, Islamabad/KAIST (Korea Advanced Institute of Science & Technology), Graduate School of EEWS, Daejeon, Republic of Korea. Her Research Interests includes (a) Synthesis, characterization and structure-property relationship of new types of polymeric materials. (b) Synthesis of nano-materials, including organic inorganic nanocomposites/ hybrid materials. (c) Polymeric blends via incorporation of nano-particles into thin polymer films. (d) Exploration of practical and potential prospects of the novel synthesized materials (new polymers and nanomaterials) counting morphological, mechanical, thermal, electrical, conducting, etc. (e) Flame retardant materials. (f) Proton conducting fuel cell membranes (g) Polymer Li-ion battery, etc.

Syed Tajammul Hussain is Director of National Centre for Physics, Quaid-i-Azam University, Islamabad, Pakistan. He received PhD from Heterogeneous Nano Catalyst development, characterization and its industrial applications, UMIST, Manchester, UK. He also has Post Doc in Nano catalyst for fuel cell industries from Centre for Surface and Material Analysis (UMIST), Manchester, UK. His Research Interests are Nano catalysts development for environment/energy/biomedical/agriculture applications, photocatalysts for solar cell applications, carbon nano tubes manufacturing and its application in industries, catalytic conversion of natural gas to higher hydrocarbons, etc.

References

Bhadraa

Khastgir

Singhaa

. Progress in preparation, processing and applications of polyaniline. Prog Polym Sci 2009; 34: 783–810.

Schoch

Jr . Update on electrically conductive polymers and their applications, IEEE. Electric Insulat Mag 1994; 10: 29–32.

Angelopoulos

. Conducting polymers in microelectronics. IBM J Res Dev 2001; 45: 57–75.

Gospodinova

Terlemezyan

. Conducting polymers prepared by oxidative polymerization: Polyaniline. Prog Polym Sci 1998; 23: 1443–1484.

Foot

PJS

Kaiser

. Conducting polymers, Kirk-Othmer encyclopedia of chemical technology Vol. 7, New York, NY: Wiley, pp. 513–513.

Koning

Van duin

Pagnoulle

. Strategies for compatibilization of polymer blends. Prog Polym Sci 1998; 23: 707–757.

Kroeze

Brinke

Hadziioannou

. Compatibilization of low-density polyethylene/polystyrene blends by segmented EB(PS-block-EB)n block copolymers. Polym Bull 1997; 38: 203–210.

Bhadra

Chattopadhyay

Singha

. Improvement of conductivity of electrochemically synthesized polyaniline. J Appl Polym Sci 2008; 108: 57–64.

Bhadra

Singha

Khastgir

. Polyaniline by new miniemulsion polymerization and the effect of reducing agent on conductivity. Synth Met 2006; 156: 1148–1154.

10.

Holden

Legge

Styrenic thermoplastic elastomers. In: Holden

Legge

Quirk

(eds). Thermoplastic elastomers, 2nd ed. New York, NY: Hanser, 1996, pp. 47–47.

11.

Leyva

Soares

Khastgir

. Dynamic-mechanical and dielectric relaxations of SBS block copolymer: Polyaniline blends prepared by mechanical mixing. Polymer 2002; 43: 7505–7513.

12.

Davies

Ryan

Wilde

. Processable forms of conductive polyaniline. Synth Met 1995; 69: 209–210.

13.

Leyva

Barra

GMO

Gorelova

. Conducting SBS block copolymer–polyaniline blends prepared by mechanical mixing. J Appl Polym Sci 2001; 80: 626–633.

14.

Leyva

Barra

GMO

Soares

. Conductive polyaniline–SBS blends prepared in solution. Synth Met 2001; 123: 443–449.

15.

Souza

Soares

Mantovani

. Blends of styrene butadiene styrene TRI block copolymer/polyaniline – characterization by WAXS. Polymer 2006; 47: 2163–2171.

16.

Xie

H-Q

Y-M

. Change of conductivity of polyaniline/(styrene–butadiene–styrene) triblock copolymer composites during mechanical deformation. J Appl Polym Sci 2000; 77: 2156–2164.

17.

Ruckenstein

Sun

. Polyaniline-containing electrical conductive composite prepared by two inverted emulsion pathways. Synth Met 1995; 74: 107–113.

18.

Ruckenstein

Hong

. Inverted emulsion pathway to polypyrrole and polypyrrole elastomer composites. Synth Met 1994; 66: 249–256.

19.

Kausar

Zulfiqar

Ali

. Novel Poly(thiourea-ether-imide)s derived from 4,4′-oxydiphenyl bis(thiourea): Probing the possibility for high temperature applications. Polym Int 2011; 60: 564–570.

20.

Sivadhayanithy

Ravikumar

Rengaswamy

. Design and synthesis of new thermally stable polyesters containing azo and phenylthiourea groups. High Perform Polym 2007; 19: 62–77.

21.

Kim

Lee

Park

. End functionalization of styrene–butadiene rubber with poly(ethylene glycol)–poly(dimethylsiloxane) terminator. Polym J 2001; 34: 674–681.

22.

Leyva

Barra

Soares

. PAni.DBSA/SBS blends prepared from “in situ” polymerization: Electric, dielectric and dynamic-mechanical properties. Polímeros 2001; 12: 197–205.

23.

Cruz-Estrada

Folkes

. In-situ production of electrically conductive fibres in polyaniline-SBS blends. J Mater Sci 2000; 35: 5065–5069.

24.

Leyva

Barra

GMO

Soares

. Conductive polyaniline-SBS blends prepared in solution. Synth Met 2001; 123: 443–449.

25.

Souza

Jr Pinto

de Oliveira

. Evaluation of electrical properties of SBS/Pani blends plasticized with DOP and CNSL using an empirical statistical model. Polym Test 2007; 26: 720–728.

26.

Ajji

Zhang

. Correlations between orientation and some properties of polymer films and sheets. J Plast Film Sheet 2002; 18: 105–116.

27.

Ajji

Guevremont

Matthews

. Measurements of absolute birefringence of biaxially oriented films and sheets on-line or off-line. J Plast Film Sheet 1999; 15: 256–268.

28.

Ajji

. A technique for online biaxial birefringence measurements and its application. J Plast Film Sheet 2004; 20: 31–41.

29.

King

J-S

Whang

W-T

Lee

W-C

. The generation of biaxial optical anisotropies in polyimide films by an uniaxial stretch method. Jpn J Appl Phys 2006; 45: L501–L504.

30.

Zhang

Liu

Xiao

. Novel approach to reducing stress-caused birefringence in polymers. J Mater Sci 2004; 39: 1415–1417.