Synthesis and properties of a new high-temperature liquid crystalline thermoplastic elastomer based on mesogen-jacketed liquid crystalline polymer

Synthesis and properties of a new high-temperature liquid crystalline thermoplastic elastomer based on mesogen-jacketed liquid crystalline polymer

Polymer 108 (2017) 50e57 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Synthesis and properti...

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Polymer 108 (2017) 50e57

Contents lists available at ScienceDirect

Polymer journal homepage: www.elsevier.com/locate/polymer

Synthesis and properties of a new high-temperature liquid crystalline thermoplastic elastomer based on mesogen-jacketed liquid crystalline polymer Zhen-Yu Zhang a, Qi-Kai Zhang a, Jian-Peng Yu b, Yi-Xian Wu b, **, Zhihao Shen a, **, Xing-He Fan a, * a

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers (Ministry of Education), Beijing University of Chemical Technology, Beijing 100029, China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 August 2016 Received in revised form 20 October 2016 Accepted 18 November 2016 Available online 19 November 2016

In order to obtain high-temperature resistant thermoplastic elastomers (TPEs), we designed and synthesized a new kind of ABA triblock copolymer containing a mesogen-jacketed liquid crystalline polymer, poly(40 -(methoxy)-2-vinylbiphenyl-4-methyl ether) (PMVBP), as the hard blocks and polybutadiene (PB) as the soft block. PB was synthesized by ring-opening metathesis polymerization in the presence of a chain transfer agent, and PMVBP was synthesized by nitroxide-mediated radical polymerization. The glass transition temperature of the PMVBP block is 186e211  C, which is much higher than that of polystyrene. More importantly, PMVBP can maintain its liquid crystallinity even when the temperature is as high as 270  C, which can enhance the mechanical properties of TPEs based on PMVBP, especially at high temperatures. All the resulting PMVBP-PB-PMVBP triblock copolymer samples form lamellar microphase-separated structures after combined treatment of solvent annealing and thermal annealing. They are good candidates for TPEs with a high operating temperature of more than 200  C. © 2016 Published by Elsevier Ltd.

Keywords: Liquid crystalline polymer Thermoplastic elastomer High-temperature range

1. Introduction Thermoplastic elastomers (TPEs) are usually composed of two resin segments (hard segments) and a rubber segment (soft segment) of triblock copolymers [1e3]. The hard segments in the triblock copolymers form physical crosslinking points, while the soft segment is highly elastic and contributes flexibility. The physical crosslinking points can reversibly break or form with the change of temperature, leading to the plastic processing properties of TPEs. Because TPEs have many unique physical and mechanical properties, they are known as the “third-generation synthetic rubbers” [4]. In the past decades, TPEs have experienced extremely rapid development and have been widely used in almost all areas of rubber products, such as building materials, medical equipment, and communications materials [5].

* Corresponding author. ** Corresponding authors. E-mail addresses: [email protected] (Z. Shen), [email protected] (X.-H. Fan). http://dx.doi.org/10.1016/j.polymer.2016.11.043 0032-3861/© 2016 Published by Elsevier Ltd.

There are many kinds of TPEs, among which the most common products are polystyrene-b-polybutadiene-b-polystyrene (SBS) and its corresponding hydrogenated derivatives [6e9]. SBS has many advantages, such as good tensile strength, elasticity, abrasion resistance, fatigue resistance, easiness for dyeing, and relatively low cost. However, the hard segment, polystyrene (PS), of SBS is easy to soften at high temperatures, which limits the high-temperature applications of SBS [10,11]. With the development of polymerization technology, how to obtain high-temperature resistant TPEs is a research focus in the field in recent years. The application of metal-catalyzed ringopening metathesis polymerization (ROMP) brings new development of polyolefins [12e14]. Bifunctional polyolefins such as polybutadiene (PB) can be facilely synthesized by ROMP in the presence of symmetric acyclic olefin chain transfer agents (CTAs). This gives significant opportunities for the development of PB, polyisoprene (PI), and their corresponding TPEs [15e17]. Many studies have focused on the relationship between the chain structure and the phase structure in TPEs [10,18e22]. With the variation of the chain structure, the phase structure generally

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also changes, which is an effective way to improve the performance of TPEs. Introducing liquid crystalline (LC) polymers to TPEs to obtain liquid crystalline thermoplastic elastomers (LCTPEs) has become another strategy in obtaining high-temperature resistant TPEs [23e26]. The relationship among the liquid crystallinity, the microphase-separated structures, and the mechanical properties of these LCTPEs has been explored. The presence of liquid crystallinity in the hard segment renders high glass transition temperatures (Tg's) and extra physical networks, which can increase the high service temperature and improve the mechanical properties of TPEs. Mesogen-jacketed LC polymers (MJLCPs) are semi-rigid LC polymers, which have relatively large side groups laterally attached to the main chain through a very short spacer or just a C-C bond [27e29]. From the chemical viewpoint, MJLCPs are side-chain polymers, which can be readily synthesized by chain polymerization. However, from the physical viewpoint, the properties of MJLCPs are similar to those of main-chain LC polymers. MJLCPs usually are rigid or semi-rigid, with high Tg values and high LC-toisotropic transition temperatures. Generally, the liquid crystallinity of MJLCPs is dependent on their molecular weights (MWs) [30e32]. Compared with PS, most MJLCPs with a PS main chain have higher mechanical strengths and higher Tg's. Yi et al. synthesized LCTPEs containing a typical MJLCP, poly{2,5bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS), as the hard segments and poly-n-butyl (PnBA) as the soft block [33]. Because the chemical properties of these two blocks are different, microphase separation occurs in bulk. The introduction of the MJLCP as the hard segments leads to stable physical networks, which are still stable above the Tg of PMPCS. The sample has a high melt viscosity, a high elongation at break, and a low tensile modulus. However, only when the MW of PMPCS is above 10  103 g/mol, can columnar LC phases form, which limits the synthesis and application of PMPCS-based LCTPEs [30,31]. In addition, because the elasticity of the soft segment PnBA is poor, the mechanical properties of this kind of LCTPEs are unsatisfactory. Liu et al. introduced an MJLCP as the soft segment and poly(4-vinylpyridine) as the hard segment to obtain an MJLCP-based TPE [34]. However, the Tg of the soft segment was too high to afford elasticity, and the position and the quantity of the metal coordination in the hard segment were not controllable. In addition, the mechanical properties of the TPE were not superior. In our recent work [35], we prepared new LCTPEs with PMPCS as hard blocks and PB as the soft block, and the materials showed relatively good mechanical properties. However, the service temperatures of these TPEs were not very high. In our previous work [32], we synthesized a new MJLCP, poly[40 -(methoxy)-2vinylbiphenyl-4-methyl ether] (PMVBP), which has a relatively high Tg of 173e208  C. And a hexagonal columnar (Colh) LC phase forms when the number-averaged MW (Mn) of PMVBP is only above 5.3  103 g/mol, which is a low threshold MW for the LC formation for MJLCPs. In this work, we combined ROMP chain transfer (ROMP-CT) with nitroxide-mediated radical polymerization (NMRP) to synthesize an ABA triblock copolymer containing the MJLCP, PMVBP, as the hard segments and PB as the soft segment. We investigated the relationship between the mechanical properties and the compositions of the resultant TPEs. The chemical structure of the target triblock copolymer, PMVBP-b-PB-b-PMVBP (V-B-V), is shown in Chart 1. 2. Materials and methods 2.1. Materials cis-2-Butene-1,4-diol (J&K, 97%), 2-bromo-2-methylpropionyl

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Chart 1. Chemical structure of V-B-V.

bromide (J&K, 98%), 2,2,6,6-tetramethylpiperidinyloxy (TEMPO), borane-tetrahydrofuran complex (J&K, 1.0 M solution in tetrahydrofuran, THF), Grubbs' catalyst (second generation, Sigma-Aldrich, 98%), and N,N,N0 ,N00 ,N00 -pentamethyl diethylenetriamine (PMDETA, 98%, TCI) were used without further purification. The cyclic olefin monomer 1,5-cyclooctadiene (COD, J&K, 98.5%) was pretreated by borane-THF complex (1.0 M solution in THF), and then distilled under a reduced pressure. Triethylamine (TEA) was distilled from potassium hydroxide (KOH). Dichloromethane (DCM), THF, and chlorobenzene (PhCl) were purified by the M. Braun solvent purification system. All other reagents were commercially available and used as received. 2.2. Measurements The chemical structures of the intermediates, the chain transfer agent, the monomer, and the polymers were characterized by 1 H/13C NMR, high-resolution mass spectrometry (HR-MS), and elemental analysis (EA). 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were obtained with a Bruker ARX400 spectrometer using deuterated chloroform as the solvent and tetramethylsilane as the internal standard at ambient temperature. HRMS spectra were recorded on a Bruker Apex IV Fourier-transform ion cyclotron resonance mass spectrometer by electrospray ionization (ESI). EA was carried out with an Elementar Vario EL instrument. Other characterization methods, such as gel permeation chromatographic (GPC) measurements, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), polarized light microscopy (PLM), one-dimensional wide-angle X-ray scattering (1D WAXS), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), dynamic mechanical analysis (DMA), and tensile measurements were performed according to the procedures previously described [33,34]. 2.3. Synthetic procedures The synthetic route of V-B-V is shown in Scheme 1. The synthetic details are described below. 2.3.1. Synthesis of chain transfer agent 1,2-Bis(2,2,6,6Tetramethylpiperidinooxyisobutyryloxy)-2-butene, CTA-NMRP The synthetic route of the chain transfer agent is shown in Scheme 2. The method of synthesizing 1,2-bis(bromoisobutyryloxy)2-butene has been reported in our previous work [35]. A mixture of 1,2-bis(bromoisobutyryloxy)-2-butene (2.00 g, 5.30 mmol), TEMPO (1.85 g, 11.7 mmol), PMDETA (56.0 mL, 0.260 mmol), and DMF (30.0 mL) were added into a polymerization tube. After the air in the tube was removed by three freeze-pump-thaw cycles, Cu (1.00 g, 15.6 mmol) and CuBr (37.4 mg, 0.260 mmol) were added into the tube under freezing conditions. After another three freeze-pumpthaw cycles, the tube was sealed under vacuum. Then the mixture was thawed, and the polymerization was conducted at ambient

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polymerization procedure, a mixture of the monomer COD (8.00 g, 73.9 mmol), chain transfer agent CTA-NMRP (0.270 g, 0.490 mmol), and DCM (45.0 mL) were added into a polymerization tube with a magnetic stir bar. After stirring and degassing by three freezepump-thaw cycles, air in the solvent was thoroughly removed. The second-generation Grubbs' catalyst (6.28 mg, 7.40 mmol) was quickly added into the polymerization tube under freezing conditions. After another three freeze-pump-thaw cycles, the tube was sealed under vacuum. The mixture was thawed, and the polymerization was carried out at ambient temperature for 24 h. Then the polymerization tube was broken. The resultant mixture was diluted with DCM (20.0 mL) and passed through a neutral alumina column to remove the metal catalyst. The product was diluted with THF and finally precipitated into a large volume of methanol to obtain a white product. In order to remove the unreacted monomer, the product was redissolved in DCM and reprecipitated in methanol for 3 times. After purification, the telechelic PB macroinitiator was dried under vacuum for 24 h. Yield: 91%. The Mn of the macroinitiator is 24.9  103 g/mol, and the polydispersity index (PDI) is 1.65. The sample is named PB-NMRP. 2.3.4. Synthesis of the monomer of the hard segment The monomer of PMVBP, 40 -(methoxy)-2-vinylbiphenyl-4methyl ether (MVBP), was synthesized following our previously published method [32].

Scheme 1. Synthetic Route of V-B-V.

temperature for 24 h. The polymerization tube was then broken. The resulting mixture was diluted with DCM (20.0 mL) and passed through a neutral alumina column to remove the metal catalyst. Solvent was removed by rotary evaporation. The crude product was purified by column chromatography on silica gel with DCM/petroleum ether (v/v ¼ 1/1) as the eluent to afford the product as a white powder. Yield: 61%. 1H NMR (400 MHz, d, ppm, CDCl3): 1.00 (s, 12H, -CH3), 1.14 (s, 12H, -CH2-), 1.48 (s, 24H, -CH3), 4.74e4.75 (d, 4H, -CH2), 5.78e5.80 (m, 2H, ¼CH-). MS (ESI): found (M þ H)þ/z, 539.40; calcd. (M þ H)þ/z, 539.40.

2.3.5. Synthesis of ABA triblock copolymers V-B-V The triblock copolymers V-B-V were synthesized by NMRP in chlorobenzene with the telechelic PB as the macroinitiator. In a typical polymerization procedure, MVBP (63.4 mg, 0.260 mmol), macroinitiator (0.400 g, 0.880 mmol), and PhCl (4.00 mL) were placed into a 10 mL polymerization tube containing a magnetic stir bar. After three freeze-pump-thaw cycles, the tube was sealed under vacuum. Then the tube was placed into an oil bath at 130  C. After a certain period of time, the polymerization was quenched with liquid nitrogen. The raw product was diluted with DCM and added into a large volume of methanol. All triblock copolymers were extracted by reflux in n-hexane in a Soxhlet extractor for 48 h to remove the unreacted PB-NMRP macroinitiator. By filtration and drying in vacuum, the triblock copolymer V-B-V was obtained as a white product at a yield of 35%. 3. Results and discussion

2.3.2. Pretreatment of the monomer COD Because of the presence of the isomeric 4-vinylcyclohexene, the monomer COD was pretreated by borane-THF complex (1.0 M solution in THF) before polymerization. The method of pretreatment was described in the literature [36]. 2.3.3. Synthesis of the telechelic PB macroinitiator The macroinitiator, the telechelic PB with TEMPO as functional groups in both ends, was synthesized by ROMP-CT. In a typical

Scheme 2. Synthetic Route of the Chain Transfer Agent.

3.1. Synthesis and characterization of V-B-V There are many methods to obtain NMRP initiators from stable nitroxyl radicals. Hawker et al. synthesized an NMRP initiator by TEMPO and styrene or a styrene derivative by using the Jecorb catalyst [37,38]. Matyjaszewski et al. employed atom transfer radical addition (ATRA) to obtain NMRP initiators [39]. We chose ATRA to obtain CTA-NMRP because of its simple operation and higher yields, as shown in Scheme 2. With the presence of the chain transfer agent and the secondgeneration Grubbs' catalyst, the PB-NMRP macroinitiator was obtained. The disappearance of the allylic proton resonances of CTANMRP (4.74e4.75 ppm) and the appearance of resonances assigned to the polymer end groups (4.56e4.58 ppm) demonstrate the complete consumption of CTA-NMRP, as shown in Fig. 1. As shown in Table 1, a series of V-B-V triblock copolymers with the same MW of the soft segment (Mn ¼ 24.9  103 g/mol) and different MWs of the hard segments were synthesized through NMRP initiated by the same PB-NMRP macroinitiator. GPC results give the Mn and PDI values of all V-B-V triblock copolymers (Fig. 2). All triblock copolymers were characterized by 1H NMR, with Fig. 1

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as an example. The Mn values of all triblock copolymers range from 38.3  103 to 87.3  103 g/mol, and the volume fractions of PMVBP are in the range of 33e71% by using densities of 1.01 g/cm3 for PB [40] and 1.08 g/cm3 for PMVBP [41]. 3.2. Thermal properties of V-B-V triblock copolymers TGA results indicate that all triblock copolymers exhibit excellent thermal stabilities, with 5% weight loss temperatures above 360  C in a nitrogen atmosphere as shown in Fig. S1 in Supplementary Content. For DSC experiments, all triblock copolymers were first heated to 270  C at a rate of 20  C/min to eliminate thermal history, followed by a manual cooling process using liquid nitrogen to 130  C at a rate of 10  C/min. The DSC experiments were then performed from 130 to 250  C at a heating rate of 20  C/ min. Because of the rigidity of PMVBP, the Tg's of all the block copolymers are difficult to be observed (Fig. S2 in Supplementary Content). Compared with other MJLCP-based LCTPEs [31,35], the Tg of the PMVBP block in V-B-V is much higher, which greatly increases the upper temperature limit of TPEs. For all triblock copolymers, the endothermic process between 0 and 50  C can be attributed to the melting of the PB blocks. 3.3. Liquid crystalline properties of V-B-V The mesomorphic behaviors of all triblock copolymers were first

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investigated by PLM. Because the Tg of PMVBP is quite high, thin films were prepared on glass substrates by solution-casting. Polymers with different MWs show very different behaviors during heating processes. The sample V-B-V-33 with an Mn of 13.4  103 g/ mol of PMVBP shows no birefringence (Fig. 3a). However, V-B-V-55 with an Mn of 33.2  103 g/mol of PMVBP exhibits a large area of birefringence at 250  C, and no typical texture is observed during further heating (Fig. 3b). With further increasing MW of PMVBP, birefringence was similar to that of V-B-V-55 during heating, as shown in Fig. 3c and d. Because the information from PLM experiments was not sufficient to determine the LC phase structures of PMVBP in the triblock copolymers, variable-temperature 1D WAXS experiments were carried out. For V-B-V-33 (Fig. 4a), there is only a weak scattering halo at a q value of 4.98 nm1 (with a d-spacing of 1.26 nm) in the low-angle region at ambient temperature. During the heating process, the width of the broad low-angle halo remains almost the same even when the sample is heated to 260  C. The scattering halo originates from the disordered lateral packing of PMVBP chains. With the combination of the PLM result, it can be concluded that PMVBP in V-B-V-33 is not liquid crystalline. The phase behavior of V-B-V-55 was also examined by 1D WAXS experiments. As shown in Fig. 4b, the WAXS profile only shows a weak low-angle halo at low temperatures. During the first heating, a sharp diffraction peak at a q value of 5.08 nm1 (with a d-spacing of 1.24 nm) develops at 230  C which is higher than the Tg of

Fig. 1. 1H NMR spectra of CTA-NMRP (bottom), PB-NMRP (middle), and V-B-V-64 (top) in CDCl3.

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Table 1 MWs, PDI values, compositions, Tg values, and spacing of the triblock copolymers. Sample

Mn,.NMR Mn, PMVBP PDIa fbPMVBP(%) Tg, Mn, GPC (  103 g/mol)a (  103 g/mol)b (  103 g/mol) (and corresponding polymerization degree of PMVBP)b

PB-NMRP V-B-V-33 V-B-V-55 V-B-V-64 V-B-V-71

24.9 29.1 32.7 30.2 47.6

a b c d e f

24.1 38.3 58.1 72.9 87.3

f 13.4 33.2 48.0 62.4

(55) (138) (199) (259)

1.65 1.76 1.67 1.78 1.57

f 33 55 64 71

c  PB( C)

f 105 106 107 105

Tg,

c  PMVBP( C)

f 186 201 204 211

d-spacing(nm)d lamellar spacing (nm)e

d 26.8 36.1 40.3 42.1

d 29 39 44 47

Determined by GPC in THF using PS standards. Determined from.1H NMR results. Determined from DMA results. Determined from SAXS results. Determined from TEM results. Not applicable.

Fig. 2. GPC curves of the V-B-V triblock copolymers.

Fig. 3. PLM micrographs of V-B-V-33 (a), V-B-V-55 (b), V-B-V-64 (c), and V-B-V-71 (d) at 250  C.

PMVBP. This diffraction peak remains during the cooling process. The appearance of the diffraction indicates the development of an ordered structure. Along with the PLM result, such an ordered structure is likely a columnar nematic (Coln) LC phase owing to the lack of higher-order diffractions in the low-angle region, unlike the Colh phase for the PMVBP homopolymer in our previous work [32]. V-B-V-64 and V-B-V-71 exhibit similar 1D WAXS results (Fig. S3 in Supplementary Content) to those of V-B-V-55, indicating the formation of a columnar LC phase. Therefore, when the Mn of the PMVBP block is above 33.2  103 g/mol, the PMVBP block in the corresponding triblock copolymer forms a Coln phase. 3.4. Self-assembled structures of V-B-V samples in bulk Because the Tg of PMVBP is about 205  C, the segmental motion

Fig. 4. 1D WAXS profiles of V-B-V-33 (a) during the first heating and V-B-V-55 (b) during the first heating and the subsequent cooling processes.

of PMVBP is frozen at ambient temperature, which limits the formation of ordered microphase-separated structures. Thus, it is necessary to pretreat all triblock copolymers by combining solvent annealing and thermal annealing before SAXS and TEM experiments. All triblock copolymers were annealed in a toluene atmosphere at 90  C for 5 days and then thermally annealed at 240  C for 12 h. After that, all triblock copolymers were investigated by SAXS at ambient temperature. Fig. 5 shows the SAXS profiles of all the VB-V samples. With V-B-V-33 as an example, two scattering peaks with a scattering vector ratio of 1:2 are observed, indicating a lamellar (LAM) structure. The layer spacing is 26.8 nm. V-B-V-55, VB-V-64, and V-B-V-71 exhibit similar SAXS profiles, and the corresponding layer spacing values are 31.2, 40.3, and 42.1 nm, respectively. The layer spacing increases with increasing MW of the triblock copolymers, as expected. TEM experiments were also carried out to confirm the self-

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assembled nanostructures of the triblock copolymers. In order to enhance the contrast between the constitutive blocks, OsO4 was used to selectively stain the PB block. As shown in Fig. 6, LAM structures are clearly observed for all triblock copolymers. The lamellar spacing values from TEM results are in good agreement with those from SAXS measurements, as shown in Table 1. PMVBP homopolymers with Mn values above 5.3  103 g/mol can form a Colh LC phase [32]. However, for V-B-V-33 with a total Mn values of 13.4  103 g/mol for the two hard PMVBP blocks, no LC structure can be observed. For the other three V-B-V samples, only Coln phases are observed for the PMVBP block, while the PMVBP homopolymers with similar MWs exhibit more ordered Colh phases. The different behaviors between the PMVBP block in the triblock copolymer and the PMVBP homopolymer can be attributed to the confinement effect of the LAM structure of the triblock copolymer. For V-B-V-33, the volume fraction of PMVBP is relatively low, and the Mn of the PMVBP block is relatively small. The LAM structure of the triblock copolymer limits the motion of the PMVBP chains, which hinders the ordered packing of PMVBP. With the increase in the Mn and the volume fraction of PMVBP, the influence of the LAM structure still exists but weakens, and PMVBP in the other triblock copolymers (V-B-V-55, V-B-V-64, and V-B-V-71) forms the Coln LC phase, as shown in Fig. 7. 3.5. Mechanical properties of V-B-V triblock copolymers Mechanical properties of these materials were studied by DMA. DMA curves in Fig. 8 illustrate the temperature dependence of the storage modulus (G0 ), loss modulus (G00 ), and loss factor (tand). For V-B-V-33 (Fig. 8a), two transitions and a well-defined rubbery plateau in between the two transitions during the heating process are observed in the G0 vs temperature curve. The transition temperatures around 50  C and 210  C are consistent with the Tg values of PB and PMVBP, respectively, in the triblock copolymer. When the temperature is above the Tg of PMVBP, V-B-V-33 enters into the liquid flow region quickly. The above results indicate TPE properties of V-B-V-33. In the curve of tand vs temperature, again the two peaks around 50  C and 210  C are attributed to the glass transitions of the two immiscible blocks. For V-B-V-55, when the temperature is below the Tg of PMVBP, the DMA curves are similar to those of V-B-V-33, as shown in Fig. 8b. However, when the temperature is above the Tg of the PMVBP block, even at about 270  C (upper limit of the DMA instrument), the sample does not enter into the liquid flow region. Instead, a new rubbery plateau appears, which can be attributed to

Fig. 5. SAXS profiles of all V-B-V triblock copolymers after solvent annealing at 90  C for 5 days and thermal annealing at 240  C for 12 h.

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Fig. 6. TEM micrographs of V-B-V-33 (a), V-B-V-55 (b), V-B-V-64 (c), and V-B-V-71 (d).

the formation of the Coln LC phase of the PMVBP block at high temperatures. The LC phase provides a new physical network which can maintain the high modulus. For V-B-V-64 and V-B-V-71, because the contents of the PMVBP rod block are quite high, these two block copolymers are not elastic, but only plastic. The stress-stain relationships of V-B-V-33, V-B-V-55, and V-B-V64 are shown in Fig. 9. For V-B-V-33, with the strain value increasing from 0% to the breaking point at 400%, the tensile stress slightly increases, and the maximum elongation at break is 409%. For V-B-V-55, the stress-strain curve exhibits a linear relationship until a well-defined yield point. After the yield point, the curve is similar to that of V-B-V-33, and the maximum elongation at break is 692%. The stretching strength at 300% of V-B-V-33 and V-B-V-55 are 1.1 and 3.2 MPa, respectively. These materials are good candidates for TPEs. However, for V-B-V-64, because the content of PMVBP is too high, the sample is not elastic, but only plastic, with the rupture at 4% observed in the stress-strain curve. Compared with our previously reported MJLCP-based LCTPE MB-M-55 which has a similar mass fraction (45 wt%) of PB to that of V-B-V-55 having 43 wt% of PB. The maximum elongation at break and the tensile strength of M-B-M-55 are 736% and 10.6 MPa, respectively, and the stretching strength at 300% is 6.9 MPa. For VB-V-55, the maximum elongation at break and the tensile strength of V-B-V-55 are 692% and 4.5 MPa, respectively, and the stretching strength at 300% is 3.2 MPa. Therefore, although these data of V-BV-55 are not as good as those of M-B-M-55, V-B-V-55 has a higher storage modulus, and the temperatures of the two rubbery plateaus

Fig. 7. Apparent phase diagram of V-B-V.

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Fig. 10. Comparison of DMA results of V-B-V-55 and M-B-M-55.

formation of new physical crosslinking points owing to the LC phase of PMVBP can maintain the modulus at a relatively high level even when the temperature is above the Tg of PMVBP. These materials are good candidates for TPEs with a wide operating temperature range and a high upper temperature limit. Acknowledgments

Fig. 8. DMA results of V-B-V-33 (a) and V-B-V-55 (b).

This work was supported by the National Natural Science Foundation of China (Grants 21134001) and the National Basic Research Program of China (973 Program, Project 2011CB606004). Appendix A. Supplementary data

are higher (Fig. 10), owing to the higher Tg of PMVBP compared to that of PMPCS in M-B-M. Therefore, V-B-V-55 can be used at higher temperatures compared with M-B-M-55. Compared with PMPCS, PMVBP has a higher Tg, and the rubbery plateau temperature (210  C) of V-B-V is much higher than that (150  C) of M-B-M. Therefore, V-B-V is a better TPE candidate than M-B-M and SBS for high-temperature applications. However, the high Tg also brings the difficulties in the adjustment of the polymer chains during the stretching of V-B-V, especially at ambient temperature. Consequently, the mechanical properties of V-B-V are not as good as those of M-B-M. 4. Conclusions In this work, we demonstrate that introducing a high-Tg MJLCP is an effective approach to improve the service temperature of TPEs. All the new ABA triblock copolymer V-B-V samples form lamellar microphase-separated nanostructures after proper treatment. The samples with fPMVBP values of 33% and 55% exhibit the typical properties of TPEs, and the sample V-B-V-55 is an LCTPE. The

Fig. 9. Stress-strain curves of V-B-V.

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