Article pubs.acs.org/Macromolecules
Tailoring the Morphologies and Mechanical Properties of Styrene− Butadiene−Styrene Triblock Copolymers by the Incorporation of Thiol Functionalized Benzoxazine Jing Bai,† Zixing Shi,†,* Jie Yin,† and Ming Tian‡,* †
School of Chemistry & Chemical Engineering, State Key Laboratory of Metal Matrix Composite Materials, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China ‡ State Key Lab of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Benzoxazine-functionalized poly(styrene-b-butadiene-b-styrene) (SBS) has been successfully synthesized via the thiol−ene click reaction. Unlike the typical method for fabrication of blends of SBS and thermosetting resins, the benzoxazine could be directly attached on to the chains of PB domains of this triblock copolymer without any prechemical modification for SBS via the incorporation of thiol functionalized benzoxazine (PTMP-BZ). AFM characterization shows that both thiol−ene and subsequent benzoxazine ring-opening reactions have a profound influence on the final morphologies of SBS, which undergoes great change from the cylinders for the pure SBS to different types of lamella structure for the SBS with different contents of the benzoxazines and the results obtained from AFM indicate that the interaction between PB and PS domains is strengthened after two reaction steps and this is responsible for the substantial improvement on the mechanical properties of material including tensile strength and storage modulus. In the meantime, the resilence of SBS is also improved significantly by the incorporation of benzoxazine and the modified SBS blends could recover its original shape without residual elongation after the tests of cyclic tensile stress−strain.
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to increase the compatibility between two polymers.10 However, it is not particularly desirable to use this kind of epoxidized SBS since the modification method usually involves the use of toxic reagent which threaten the environment, involve long times for the purification of the modified products as well as the degradation of SBS. In this article, we used a simple in situ modification method to increase the compatibility between the thermosetting resin and SBS based on thiol−ene click reaction. Instead of the epoxy resins, benzoxazine thermosetting resin was used in this article as the cross-linking center for the SBS matrix. In fact, benzoxazine has been seldom incorporated in the SBS matrix. Only Wonchalerm Rungswang reported the change of morphology for nanofibers produced from the blends of polystyrene-b-poly(ethylene-co-1-butene)-b-polystyrene triblock copolymer (SEBS) and benzoxazine based on π−π interaction.18 Benzoxazines are synthesized with a primary amine, a phenolic derivative, and formaldehyde.19−21 Owing to the immense molecular design flexibility, the structures of benzoxazines could be easily tailored to fit various applications. By the thermally activated ring-opening polymerization, benzoxazines could form the cross-linked center in the SBS system without the release of byproducts.19,22 Here, a kind of
INTRODUCTION Poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymers form one of the main families of thermoplastic elastomers (TPE), which can be subjected to thermoplastic molding at high temperatures and used as rubber at room temperature with the presence of physical cross-links introduced by vitrification of PS domains.1 They are widely used in various applications, such as footwear, impact modifiers in engineering plastics, and adhesives due to their good balance of mechanical properties along with favorable processability and recyclability.2−7 In recent years, the mechanical properties of SBS could be controlled by blending with several kinds of polymer.8−10 Depending on the procedure used for the fabrication, different kinds of morphology on the nanometer scale were obtained since the chain of the polymer could only be confined into the different domains of SBS based on their compatibility.10 Therefore, the relationship between the morphology and mechanical properties would be established, which is useful in designing the SBS blends to improve their properties.9,11−13 Thermosetting polymers are usually added into the SBS blends for two reasons.14−17 One is to increase the toughness for the thermosetting polymers provided by SBS as thermosetting materials are used as matrix. The other is to enhance the mechanical properties of the triblock copolymers by the formation of cross-linking center from thermosetting resins. In general, these two kinds of polymers are immiscible and SBS is usually modified by the attachment of epoxy group © 2014 American Chemical Society
Received: February 24, 2014 Revised: April 3, 2014 Published: April 16, 2014 2964
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4 mm and length of 30 mm were cut from the cast films. Data analyses were based on five measurements on each sample performed at the same conditions. The fracture surfaces of all specimens were examined with Nova Nano SEM NPE219 scanning electron microscope (SEM) at an activation voltage of 5 kV. Synthesis of Benzoxazine with Allyl Groups (BZ). To a 100 mL dry three-neck flask equipped with a mechanical stirring device were added 2-allylphenol (1.34 g), paraformaldehyde (0.60 g), and dodecylamine (1.84 g) under nitrogen flow (Scheme 1). The mixture
benzoxazine was designed to incorporate into the chain of SBS via the UV-induced thiol−ene click reaction in the presence of thiol functionalized benzoxazine. The thiol−ene reaction, known for over 100 years, is the hydrothiolation of a CC bond which has been most widely employed for preparing nearperfect networks and films.23,24 The UV-induced thiol−ene click reaction effectively combines the classical benefits of click reactions with the advantages of a photoinitiated polymerization which can be activated at a specific time and location. Therefore, this method is a powerful and efficient method to improve the compatibility between different polymers by in situ chemical modification.25,26 In this article, allyl groups in benzoxazine monomers could be easily linked with the 1,2-butadienes for the polybutadiene domains of SBS in the presence of PTMP-BZ via the thiol−ene reaction.27 Therefore, the formation of chemical bonds promoted the miscibility between the SBS copolymer with the uncured benzoxazine system and then the film underwent thermal treatment to initiate ring-open polymerization of the benzoxazine precursor. Therefore, the chain of SBS was further cross-linked together by the formation of polybenzoxazine network. The aim of the present contribution is the design of nanostructured materials stiffened by the incorporation of thermosetting benzoxazine as modifier for SBS at low contents and evaluation the effect of this modifier on the phase behavior and mechanical properties of SBS. It is found that the strengthened interaction between PB and PS phase by the incorporation of benzoxazine could lead to the great change in its morphology and substantial reinforcement in its mechanical properties and this method is quite convenient and effective.
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Scheme 1. Synthesis Process of Benzoxazine with Allyl Groups (BZ)
was heated and refluxed at 100 °C for 6 h with stirring. After cooling to room temperature, the transparent yellow compound was washed with dilute hydrochloric acid, sodium hydroxide solution, and water in sequence for 3 times and dried in a vacuum at 50 °C for 10 h. Synthesis of the Reactive Thiol-Functionalized Benzoxazine (PTMP-BZ). PTMP-BZ was synthesized by grafting the benzoxazine with allyl group to pentaerythritol tetrakis(3-mercaptopropionate) (PTMP) via thiol−ene photoclick reaction (Scheme 2) according to our previous reports. Benzoxazine was introduced into PTMP to get PTMP-BZ. PTMP (1 mmol), benzoxazine with allyl group (2 mmol) and trace amount of photoinitiator I907 were dissolved in chloroform. The solution was exposed under an ultraviolet lamp at 365 nm, and then the mixture was evaporated by rotary to remove solvent. The obtained PTMP-BZ was characterized by FT-IR and 1H NMR. About two benzoxazine monomers were successfully grafted to one PTMP molecule, which was almost the same to the feed ratio. Preparation of the SBS-BZ Films. The mixtures of SBS modified with 0.5, 1, 1.5, 2, and 2.5 wt % PTMP-BZ system, named throughout the paper as SBS-BZ, were prepared in the following way: First, triblock copolymer and PTMP-BZ were dissolved in toluene and then cast onto glass plate and dried at room temperature for 12 h upon ultraviolet (UV) light exposure. The films were named based on the content of PTMP-BZ as UV-0.5, UV-1, UV-1.5, UV-2, and UV-2.5 respectively. Second, these films underwent thermal treatment to initiate ring-open polymerization of the benzoxazine from 80 to 220 °C at 10 K/h in oven. After this process, the films were called as OPB0.5, OPB-1, OPB- 1.5, OPB-2, and OPB-2.5 orderly.
EXPERIMENTAL SECTION
Materials. Toluene, chloroform, dodecylamine(>97%), and paraformaldehyde(>94%) were purchased from Sinopharm Chemical Reagent Co., Ltd., 2-allylphenol(>97%) and pentaerythritol tetra(3mercaptopropionate) (>85%)were purchased from TCI Chemical CO., Ltd. Poly(styrene-b-butadiene-b-styrene) (Mw ∼ 153000− 185000) was purchased from Sigma-Aldrich. All the reagents were used as received. Measurements. 1H NMR spectra were recorded on a Varian Mercury Plus 400 MHz instrument with CDCl3 as the solvent and tetramethylsilane (TMS) as an internal standard at room temperature. Fourier transform infrared (FTIR) spectra measurements were carried out on a Spectrum 100 Fourier transformation infrared absorption spectrometer (PerkinElmer, Inc., Waltham, MA) from 3200 to 800 cm−1 at a resolution of 4 cm−1. In all cases, 64 scans at a resolution of 2 cm−1 were used to record the spectra. The samples were prepared by dropping the solution onto KBr films. The morphology of the samples was examined by tapping-mode atomic force microscopy (TM-AFM). TM-AFM measurements were carried out in a SII Nanonavi E-sweep under ambient conditions. The measurements were performed using commercial Si cantilevers with a nominal spring constant and resonance frequency at about 40 N/m and 300 kHz, respectively (AFM Probes, NSC11). Differential scanning calorimetry (DSC) analysis was carried out with DSC 6200 (Seiko Instrument Inc.) at a heating rate of 5 K/min from 323 to 573 K and nitrogen flow rate of 50 mL/min; about 5 mg samples were sealed in aluminum hermetic pans and lids for the tests. The dynamic mechanical tests were carried out on a (DMTA) (TA Q800, TA Instruments. USA) under the temperature range from 153 to 393 K. The frequency used is 1.0 Hz and heating rate is 10 K/min. The film samples with thicknesses of 0.2−0.4 mm were obtained by a solution-casting method. The specimens were cut from the films with width of 4 mm and length of 30 mm. The tensile property of films was measured with an Instron 4465 instrument at room temperature with a humidity of about 30% at a crosshead speed of 100 mm/min. Dumbbell specimens with width of
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RESULTS AND DISCUSSION The Analysis of Structure by NMR, FTIR, and DSC. The benzoxazine monomer with allyl was successfully prepared by the condensation of 2-allylphenol, dodecylamine, and paraformaldehyde. The structure of the (BZ) was confirmed by spectral analyses including 1H NMR and FTIR spectra. As shown in Figure 1A, the 1H NMR spectrum of the BZ monomers exhibits the typical proton signals attributed to the oxazine ring at 4.87 and 3.97 ppm for N−CH2−O and Ar− CH2−N, respectively. The resonances of equal integrated intensity allylphenol to the methylene groups in the oxazine cyclic are the characteristic chemical shifts of benzoxazine structure, which could prove the successful synthesis of benzoxazine monomer with allyl group. 2965
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Scheme 2. Synthesis of PTMP-BZ
Figure 1. 1H NMR spectrum of BZ (A) and PTMP-BZ (B).
Figure 2. (A) FTIR of BZ and PTMP-BZ (B) DSC curves of BZ and PTMP-BZ.
the oxazine structure at 921 cm−1 also indicates the formation of benzoxazine ring. Strong band at 1644 cm−1 is related with the CC of allyl group on 2-allylphenol. All these signals confirm the successful synthesis of benzoxazine monomers with allyl group.
Figure 2A shows the FTIR spectra of the synthesized BZ. As illustrated in Figure 2A, the peaks at 1466 and 1220 cm−1 are assigned to the trisubstituted benzene ring vibration and the antisymmetric C−O−C stretching mode in the oxazine ring, respectively. The appearance of another characteristic band of 2966
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Scheme 3. Process of the Formation of the Films
Figure 3. Picture of the SBS, UV-1.5, and OPB-1.5, from left to right.
According to the published literature24−26,28 and our previous work,29,30 PTMP-BZ could be synthesized with PTMP and BZ via photo induced thiol−ene click reaction. However, Beyazkilic and his co-workers have found that radicalmediated thiol−ene photopolymerization was simultaneously accompanied by the occurrence of thiol−benzoxazine ringopening reaction.28 In the meantime, there is literature focused on the methods to prevent the thiol−benzoxazine ring-opening reaction,31,32 which shows that the thiol−benzoxazine reaction was catalyzed with the presence of acids and inhibited by the incorporation of basic chemicals since the protonation of the amine plays an important role in the thiol−benzoxazine ringopening reaction. On the basis of this mechanism, it is possible to suppress the thiol−benzoxazine ring-opening reaction by the addition of base which is stronger than benzoxazine to impede the protonation of the benzoxazine by the thiol. In this work, the thiol-benzoxazine ring-opening reaction was quenched by the addition of triethyl amine and the PTMP-BZ was successfully synthesized via the thiol−ene reaction. The structure analysis of the PTMP-BZ by the 1H NMR spectrum is demonstrated in Figure 1B. As compared in Figure 1A, the disappearance of the double bond signal in Figure 1B proves the success for the thiol−ene click reaction and the existence of typical proton signals attributed to the oxazine ring at 4.87 and 3.97 ppm for N−CH2−O and Ar−CH2−N suggests the suppression of the thiol-benzoxazine reaction. The structure of PTMP-BZ is also supported by FTIR spectra, and the attribution of each band is also signed in Figure 2B. In comparison with BZ, the existence of benzoxazine groups in PTMP-BZ is illustrated by the characteristic absorption peaks for the benzoxazine ring at 1466, 1220, and 921 cm−1. The new band located at 2480 cm−1 is assigned to S−H, indicating the
existence of residual thiol groups. The other new band appears at 1745 cm−1 belonging to the CO on the PTMP monomers. The disappearance of the band at 1644 cm−1 in FTIR spectrum of PTMP-BZ proves that the vinyl groups have taken part in the thiol−ene click reaction. To further prove the existence of oxazine structure on the BZ and PTMP-BZ molecules, DSC experiments were conducted to investigate the thermal ring-opening behavior for the obtained products. As shown in Figure 2B obtained from DSC measurements, a single exothermic peak is found, which is related with the ring-opening polymerization of the benzoxazine. Both exothermic peaks are fairly broad, with total enthalpies of about 18 and 43 kJ/mol for BZ and PTMP-BZ, respectively. The Fabrication of the SBS Film with Polybenzoxazine. All SBS films with different percentages of PTMP-BZ were prepared by a solution-casting method. The mechanism for the preparation of film was illustrated in Scheme 3. The mixture solution was cast onto glass plates and subjected to radiation under 365 nm UV-light for 12 h on the process of drying. After upon ultraviolet (UV) light exposure, the thiol− ene click reaction happened between the PTMP-BZ and 1, 2butadienes in the polybutadiene block of SBS. In this procedure, the benzoxazine rings were grafted to the chains of SBS through the thiol−ene reaction during the formation of the films and the materials are named as UV series, including UV-0.5, UV-1, UV-1.5, UV-2, and UV-2.5 based on the content of PTMP-BZ. After the ultraviolet (UV) light exposure, the films underwent thermal treatment to induce the attached benzoxazine ring-opening polymerization in the SBS network and this series of materials is named as OPB (ring-opening polymerization of benzoxazine) series. The pictures of the 2967
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Figure 4. Gel fraction (A) and the swelling ratio (B) of the series composites with the different PTMP-BZ content.
composite films with 1.5 wt % PTMP-BZ (UV-1.5 and OPB1.5) and the pure SBS are shown in Figure 3. All the films are homogeneous and transparent. The Change of Solubility after Two Steps of Modification Gel Fraction. Figure 4A shows some typical plots of the insoluble fraction as a function of contents of PTMP-BZ before and after two reaction steps. It is found that solvent resistance is improved by the incorporation of the PTMP-BZ via thiol−ene click reaction and the behavior of insolubility is further enhanced by the subsequent ring-opening polymerization of the benzoxazine attached on the chains of SBS due to the increased cross-linking density. For the UV series, at low PTMP-BZ content, the SBS polymer remains partly soluble and the gel fraction is low and the gel fraction is increased by the increase of the PTMP-BZ content. For example, the gel fraction increased from 7 to 58.5% when the content of PTMP-BZ increased from 0.5 to 2.5 wt %. For the OPB series, the gel fraction is further increased from 43.1 to 77% as the concentration of PTMP-BZ increases from 0.5 to 2.5 wt %. Compared with the UV series, the OPB series have higher gel fraction at the same PTMP-BZ content. For example, the gel fraction is enhanced from 7% for UV-0.5 to 43.1% for OPB-0.5 (Table 1).
The Swelling Ratio. Figure 4B shows how the swelling ratio (swollen solvent weight/dry polymer weight) is changed with the content of the PTMP-BZ after two reaction steps. Just like the change in the gel fraction, the swelling ratio also undergoes great change after two step reactions. With the presence of PTMP-BZ after thiol−ene reaction, the swelling ratio decreased with increasing the content of PTMP and after the thermal ring-opening reaction, the swelling ratio is further reduced. For example, after the thiol−ene reaction, the swelling ratio is reduced from 83.47 to 30.92 as the content of PTMP is increased from 0.5 to 2.5 wt % and after the thermal ringopening reaction; the swelling ratio is reduced from 67.62 to 24.94 for the sample with 1.5 wt % PTMP-BZ. Such great change in the solvent resistance (the gel fraction and swelling ratio) for SBS is attributed to the cross-linking reaction taken place after two steps of reaction. During the first step of thiol−ene reaction, two thiol groups from PTMP-BZ would have chance to link two different chains of SBS to form the ladder structure. Therefore, the solubility of SBS is reduced which reflects the increased gel fraction and decreased swelling ratio. After the second step of ring-opening reaction, the attached benzoxazine could further connect the different chains of SBS together and the increased cross-linking degree further improve the solvent resistance and the gel fraction is further increased accompanied by more reduced swelling ratio. SEM Characterization for the Fracture Surface of the Modified SBS. To comprehend the inner structure, to evaluate compatibility of PTMP-BZ with SBS matrix and to analyze internal reasons for the change of mechanical properties for the SBS after the reaction of modification, the SEM was first applied to characterize the fracture surface of SBS composites. As shown in Figure 5, the fracture surfaces for the series of
Table 1. Date of Gel Fraction and Swelling Ratio content of PTMP-BZ (wt %) gel fraction (%) swelling ratio
UV OPB UV OPB
0.5
1
1.5
2
2.5
7.0 43.1 83.5 76.3
23.8 54.7 90.2 47.7
47.5 71.9 67.6 24.9
55.1 72.2 40.8 23.8
58.5 77.0 30.9 18.1
Figure 5. SEM images: (A) SBS; (B) SBS with 1.5 wt % PTMP-BZ before thermal treatment (UV-1.5); (C) SBS with 1.5 wt % PTMP-BZ after thermal treatment (OPB-1.5). 2968
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specimens show different degrees of roughness. The surfaces of the film with 1.5 wt % PTMP-BZ (Figure 5B) even show a little smoother than the pure SBS matrix (Figure 5A), which shows that the phase compatibility between PB and PS chains is improved with the presence of PTMP-BZ via thiol−ene reaction. After the second step of thermal ring-opening reaction, the roughness of fracture surface for SBS composites (Figure 5C) is increased as compared with that of UV-1.5 and this phenomenon may be related with the new phase balance between the PB and PS domains caused by the increased crosslinking density. In addition, all the SEM images do not present any observable dispersed phase in the cryogenically fractured surfaces for the SBS by the incorporation of PTMP-BZ, which could explain the good transparency for the film with 1.5 wt % PTMP-BZ as shown in Figure 3. On the basis of the investigation of transparency of composite materials from Khanarian33 and Maruhashi and Iida,34 the size of dispersed domain in our system is very small, at least below 0.1 μm without the occurrence of the macro-phase separation. The Analysis of Morphology for SBS after Two Step Reactions and AFM Analysis of PB and PS Phase in the SBS. AFM is a powerful tool for the investigation of phase morphology at nanoscale. For the SBS copolymer, AFM can easily detect the PB’s or PS’s phase based on their different hardness. Commonly, cantilever oscillation acting on hard surface (PS domains) loses energy and generates smaller phase contrast. Hence, PS phase is the brighter region reflecting in phase image. On the contrary, PB’s domain is reflected as the dark region. Morphologies’ transformations after two reactions are vividly recorded by AFM in tapping mode. Figure 6A exhibits the morphology of SBS in their original state and Figure 6B is the schematic illustration for the SBS
Figure 6. (A) Morphology for SBS. (B) Schematic illustration for the SBS microstructure.
Figure 7. AFM phase images of different samples including OPB and UV series.
microstructure.35 It is found that PS domains occur as a cylinders dispersed in the PB matrix. Therefore, it seems like that PS domains are dispersed phase and PB domains are the continuous phase since the PS block only accounts for 30% in SBS we have selected. Figure 7 illustrates the change of morphology for SBS after two sequential reactions by the incorporation of PTMP-BZ from 0.5 wt % to 2.5 wt %. As shown in Figure 7, different morphologies are found with the presence of PTMP-BZ after the thiol−ene reaction and the morphologies of SBS change from the cylinders to the lamella with increasing the content of PTMP-BZ from 0.5 wt % to 1.0 wt %. As the concentration of PTMP-BZ increases from 1.5 to 2 wt %, the disappearance of the lamella structure is accompanied by the emergence of the homogeneous phase structure. With further increasing the
amount of PTMP-BZ, the lamella structure appears again. It is also observed that the thickness of each lamella is decreased with the increasing loading of PTMP-BZ. Furthermore, PS domain has gradually changed from the initial dispersed phase to the continuous phase with increasing PTMP-BZ contents. After the second ring-opening reaction, the morphologies also underwent obvious changes in comparison with those after the thiol−ene reaction. As shown in Figure 7, the width of lamella is further reduced followed by the appearance of much more branch points in the lamella structure after the ringopening reaction, which indicates that the interactions between PB and PS domains become stronger after the ring-opening reaction. For example, the width of PS and PB domains are decreased from 60 and 50 nm to 50 and 45 nm respectively after ring-opening reaction with the presence of 0.5 wt % 2969
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Figure 8. (A) Stress−strain curves of composites with different content of PTMP-BZ for photo curing process. (B) Stress−strain curves of composites with different content of PTMP-BZ for thermal curing process. (C) Breaking stress. (D) Comparison of ultimate strain of different composites.
Table 2. Mechanical Properties of the SBS Composites content of PTMP-BZ (wt %) ultimate strain (%) breaking stress (MPa)
UV OPB UV OPB
0 (SBS) 932.35 1.86
0.5 944.68 1069.64 10.81 17.10
1 1084.75 1019.18 16.09 21.31
1.5 1012.38 919.57 21.27 22.28
2 981.86 900.40 19.21 21.11
2.5 869.65 755.90 13.72 16.08
PB domains than that for PS domains and the ability of chain movement for PB domains is greatly reduced by the formation of chemical bonding among different PB chains after two reactions and the reduced ability of chain movement for PB domains has made the original cylinders structure with PS as dispersed domain and PB as a continuous domain changed into the lamella with both PS and PB are acted as two continuous domains. Therefore, the interaction between PS domains and PB domains is strengthened, which plays an important role on the formation of the final morphologies as well as their final mechanical properties which would be discussed later. The Mechanical Properties of SBS after Two Reaction Steps. On the basis of the investigation of morphologies for SBS after thiol−ene and ring-opening reactions, it is found that morphologies take various appearances as the result of the improved interaction between PB and PS phase on the progress of the reaction. As it is known that the mechanical properties are closely related with the morphologies for the SBS, it is very interesting to make investigation on the change of mechanical properties under different morphologies after two sequent reactions. Parts A and B of Figure 8 are typical stress−strain curves for the SBS with the presence of different contents of PTMP-BZ after two steps of reactions and its data are summarized on
PTMP-BZ. Quite different morphologies are observed for samples by further increasing the content of PTMP-BZ from 1.5 wt % to 2.5 wt %. For the sample with 1.5 wt % PTMP-BZ after thiol−ene reaction, the PB phase is covered on the whole surface and the PS phase was interwoven with PB phase. After the ring-opening reaction, the PS domains are changed as worm like micelles in the PB domain. For the sample with 2 wt % PTMP-BZ after thiol−ene reaction, the PB phase is acted as dots dispersed in PS domains. After ring-opening reaction, the lamella structure appears again. For the sample with 2.5 wt % after thiol−ene reaction, more PS domains are found in the lamella structure than PB domains. After the ring-opening reaction the phase inversion has taken place in the sample with more PB phase occupied on the surface. All these changes for morphologies were closely related with the movement of polymer chains. It is well-known that PS possesses plastic behavior with low chain movement and PB shows rubber behavior with high chain movement at room temperature. On the basis of their abilities of chain movement and content, PS domains usually form aggregates and constitute the physical cross-linking point in the PB matrix due to the incompatibility between PS and PB. Since the thiol−ene and ring-opening reaction usually happened at the PB phase, the effect of reaction on the ability of chain movement is greater for 2970
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Figure 9. Tensile stress at elongation of 300% (A), 500% (B), and 700% (C) of the samples.
Figure 10. Storage modulus of composites with different contents of PTMP-BZ before and after thermal curing process.
Table 2 and Figure 8C. It is surprising to find that the incorporation of PTMP-BZ into the SBS matrix shows a significant reinforcement effect on the tensile strength after two reactions. In the meantime, the elongation of SBS is not sacrificed or even improved, which is not always observed in the traditional reinforced polymer blends. For example, as the content of PTMP-BZ is about 0.5 wt %, the ultimate strain increases from 932.35% to 944.68% after thiol−ene reaction, further increases to 1069.64% after ring-opening reaction. The final tensile strength at break shows an almost 9-fold increase on that of pure SBS after two reactions. At the same time, the elongation at break is still kept at about 1000%. As the content of PTMP-BZ is increased, the tensile strength continues to increase with the maximum data of 22.28 MPa observed for the sample at the 1.5 wt % PTMP-BZ, almost 12 times higher than that for the pure SBS. Further increasing the concentration of PTMP-BZ leads to a reduction on the tensile strength at break, which is still much higher than that for the pure SBS. Therefore, it is concluded that mechanical properties of SBS undergoes great change by the incorporation of PTMP-BZ after two reactions. The reason may be closely related with the increased phase interaction that also accounts for the great varieties in morphologies of SBS. As the content of PTMP-BZ is below 1.5 wt %, the improved phase interaction makes morphologies of SBS changed into lamellar structure and mechanical properties are also simultaneously increased by the improved phase interaction. For the sample with 1.5 wt % PTMP-BZ, the continuous improved phase interaction resulted in the occurrence of the most homogeneous morphologies coupled with the appearance of maximum tensile strength at break.
Further increasing concentration of PTMP-BZ (2−2.5 wt %) leads to the reappearance of lamellar structure and the increased strong phase interaction at higher loading of PTMP-BZ results in the great reduction on the ability of chain movement for SBS, which is responsible for the reduction on the tensile strength at break since the sample was broken at lower elongation. In addition, the tensile strength at definite elongation and the storage modulus also reflect the interaction between PB and PS phase for SBS. Figure 9 shows three tensile strengths at the elongation of 300, 500, and 700% for five contents of PTMP-BZ after two reactions. As shown in Figure 9, it is found that the tensile stress at definite elongation increases with increasing content PTMP-BZ after two reactions. For example, the tensile stress at 700% elongation increases from 1.40 to 8.75 MPa as the content of TMP-BZ increases from 0.5 to 2.5 wt %. Figure 10 is also the plots of storage modulus as functions of temperature for SBS containing different loading of PTMP-BZ after two reactions. As illustrated in Figure 10, the storage modulus of SBS also increases gradually with the increase of concentration of PTMP-BZ as the temperature was scanned from −110 to +100 °C, consistent with the trends obtained through the tensile strength at definite elongation. Therefore, the results from the tensile strength at definite elongation and the storage modulus also indicate that the phase interaction between PB and PS phase in SBS is enhanced with increasing concentration of PTMP-BZ. In addition, the elastic resilience is very important property for elastomer. However, it is usually found that elastomer is inclined to lose the elastic resilience on the course of the reinforcement provided by the physical and chemical methods. In other words, the elastomer is easily transformed into the 2971
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Figure 11. Cyclic tensile stress−strain curves for the samples with different content of PTMP-BZ before (A) and after thermal treatment (B).
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CONCLUSION Benzoxazine group has been successfully attached to the chain of PB chains of the elastomer of SBS via the thiol−ene reaction with the presence of PTMP-BZ. The incorporation of benzoxazine into the SBS matrix increases the interaction between PB and PS domains, which makes the morphologies of SBS change from the cylinders for the pure SBS to different types of lamella structure and PS has become a continuous domains instead of original dispersed domains. The mechanical properties and solvent resistance of SBS are substantially increased by the strong phase interaction from the presence of benzoxazine. The breaking strength of SBS is some nine times greater than that of the pure sample as the content of PTMPBZ is only 0.5 wt % and the elongation is not even reduced but improved, quite different from the traditional reinforced polymer system. The results show that this method in our paper is quite effective method for improving the mechanical properties of elastomers in a facile and convenient way.
plastics to lose the resilience since the chain of elastomer becomes too stiff as a result of the reinforced effect. Therefore, three parameters including tensile strength, elongation and elastic resilience should be considered for the reinforcement of elastomer materials. Figure 11 demonstrates cyclic tensile stress−strain curves after the 200% elongation for samples to evaluate the elastic resilience. It is found that all the samples modified by PTMP-BZ show good elastic resilience and the tensile stresses for the loading and unloading are both improved by increasing the concentration of PTMP-BZ. After the samples were stretched up to 200%, they could recover their original size except that the pure SBS sample left the residual elongation. This comparison indicates that the elastic resilience is improved by the incorporation of PTMP-BZ. In addition, it is also observed that loading and unloading paths of the stress−strain curves differ pronouncedly to form hysteresis loops for all the samples and the area of hysteresis loops could be an important parameter to evaluate a material’s elastic resilience. Table 3 summarizes the area of hysteresis loops for
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Table 3. Area of Hysteresis Loops for All the Samples content of PTMPBZ (wt %) UV OPB
AUTHOR INFORMATION
Corresponding Authors
SBS
0.5
1
1.5
2
2.5
76.04
70.08 45.50
70.70 46.26
52.18 40.45
58.02 43.22
62.05 43.15
*(Z.S.) E-mail:
[email protected]. Fax: + 86-21-54747445. Telephone: + 86-21-54743268. *(M.T.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China (No. 50973062) and National Basic Research Program of China (Grant No. 2011CB606003) for the support. Additionally, we also acknowledge the staff of Instrumental Analysis Center of Shanghai Jiao Tong University for the measurements.
all the samples. As shown in Table 3, it is found that the area of hysteresis is gradually reduced from 76.04 to 52.18 with increasing contents of PTMP-BZ from 0 to 1.5 wt %, and then the area of hysteresis increased to 62.05 when the contents of PTMP-BZ continued to increase to 2.5 wt % and this indicates that the change of elastic resilience is in the same trend as that of breaking strength and storage modulus. After the thermal treatment, the area of hysteresis is decreased. Therefore, the ability of elastic recovery is improved through thermal treatment. On the basis of the investigation of mechanical properties for the SBS modified by PTMP-BZ, both the elastic resilience and tensile strength are improved greatly without the sacrifice of the elongation. This improvement is quite different from the traditional reinforced elastomer and the improved phase interaction between PB and PS domains in SBS can explain the inner reasons for the improvement.
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