Article pubs.acs.org/Macromolecules
Di- and Triblock Copolymers Based on Polyethylene and Polyisobutene Blocks. Toward New Thermoplastic Elastomers Edgar Espinosa,† Bernadette Charleux,† Franck D’Agosto,*,† Christophe Boisson,*,† Ranjan Tripathy,‡ Rudolf Faust,‡ and Corinne Soulié-Ziakovic*,§ †
Université de Lyon, Univ. Lyon 1, CPE Lyon, CNRS UMR 5265, Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2), Equipe LCPP Bat 308F, 43 Bd du 11 Novembre 1918, F-69616 Villeurbanne, France ‡ Polymer Science Program, Department of Chemistry, University of Massachusetts Lowell, One, University Avenue, Lowell, Massachusetts 01854, United States § Matière Molle et Chimie, UMR 7167 CNRS-ESPCI, Ecole Supérieure de Physique et Chimie Industrielles de la Ville de Paris, 10 rue Vauquelin 75005 Paris, France
ABSTRACT: Azido end-functionalized polyethylenes (PE-N3, 2250 or 2500 g mol−1, Đ = 1.2, functionality higher than 88%) were reacted with alkyne end-functionalized or telechelic polyisobutenes (PIB-CCH: Mn = 2400 g mol−1, Đ = 1.1; HCCPIB-CCH: Mn = 6900 g mol−1, Đ = 1.2, respectively) using the 1,3-dipolar cycloadddition reaction in a mixture of toluene and dimethylformamide at 110 °C. The reaction led to the quantitative formation of di- and triblock copolymers (PE-b-PIB and PEb-PIB-b-PE) that were characterized by size exclusion chromatography and 1H NMR analyses. The original copolymer PE-b-PIBb-PE that incorporates hard semicrystalline PE blocks and a soft PIB block was further studied by differential scanning calorimetry, optical microscopy, X-ray scattering, and dynamic mechanical analysis. It was shown that the crystallinity of PE segments was not perturbed by the PIB phase and that the Tg of PIB was slightly raised because of chain motion restrictions due to PE crystallites. In addition, mechanical analyses showed that PE-b-PIB-b-PE behaved as a thermoplastic elastomer over a broad temperature range (0−110 °C) with a tensile strength of 4 MPa and an elongation at break of 16%. The use of blocks of higher molar mass should improve the copolymer nanostructuration.
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elimination or chain transfer to metal.2 In these latter cases, a subsequent chemistry step is required to get a highly reactive chain end. Alternatively, the living polymerization of ylide3 and the preparation of end-functionalized polybutadiene or polycycloalkene followed by a hydrogenation step can be employed to prepare model macromolecules.4 Our group implemented a synthetic methodology of choice based on coordinative chain transfer polymerization (CCTP)5 to synthesize linear low molar mass polyethylene chains carrying end groups including, iodine, azide, amine, and thiol.6 Polyisobutylene (PIB) is also a chemically stable fully saturated polymer with low glass transition temperature (Tg).
INTRODUCTION Polyethylene (PE) is a chemically stable fully saturated and highly crystalline polymer. Well-defined polymer architectures incorporating PE segments such as block copolymers may benefit from these properties. The development of these architectures is not an easy task and requires the use of particular synthetic strategies based for instance on living olefin polymerization via coordination chemistry.1 However, these strategies do not offer a wide range of block copolymer structures as the number of monomers that can be polymerized by coordination insertion is rather limited. To expand this range of architectures, coupling reactions can be envisioned. They require the introduction of reactive end groups that could be involved in efficient conjugation chemistries. Functional groups are generally introduced using chain transfer agents or by taking advantage of classical chain transfer reaction, i.e., β-H © XXXX American Chemical Society
Received: March 7, 2013 Revised: April 12, 2013
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was used as solvent. Chemical shift values (δ) are given in ppm with reference internally to tetramethylsilane (TMS). Fourier transform infrared (FTIR) analyses were carried out in a Nicolet 460 FT-IR spectrometer at room temperature over 32 scans (range 650−4000 cm−1). The reaction of cycloaddition was monitored by IR spectroscopy. A drop of reaction medium was withdrawn and deposited on a KBr IR plate. The solvent was evaporated off to form a film which was analyzed by FT-IR. Molar masses and dispersities of polyisobutenes were obtained from size exclusion chromatography analyses using a Waters 717 Plus autosampler, a 515 HPLC pump, a 2410 differential refractometer, a 2487 UV−vis detector, a MiniDawn multiangle laser light scattering (MALLS) detector (measurement angles are 44.7°, 90.0°, and 135.4°) from Wyatt Technology Inc., a ViscoStar viscosity detector from Wyatt, and five Styragel HR columns connected in the following order: 500, 103, 104, 105, and 100 Å. The RI was the concentration detector. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 mL min−1 at room temperature. The results were processed using the Astra 5.4 software from Wyatt Technology Inc. High-temperature size exclusion chromatography (HT-SEC) measurements were performed on PE based materials using a Viscotek High Temperature Triple Detection GPC (HT-GPC) system that incorporates a differential refractive index, a viscometer, and a light scattering detector. 1,2,4-Trichlorobenzene (TCB) was used as the mobile phase at a flow rate of 1 mL min−1. TCB was stabilized with 2,6-di(tert-butyl)-4-methylphenol. All polymers were injected at a concentration of 5 mg mL−1. The separation was carried out on three mixed bed columns (300 × 7.8 mm from Malvern Instrument) and a guard column (75 × 7.5 mm). Columns and detectors were maintained at 150 °C. The Omnisec software was used for data acquisition and data analysis. The molar masses were measured using a calibration curve based on narrow polyethylene standards (170, 395, 750, 1110, 2155, 25 000, 77 500, and 126 000 g mol−1 from Agilent Technologies) using only the refractometer detector. Polarized optical microscopy was performed using a Leica Leitz DM RD light microscope containing a calibrated ocular lens (10×/0.30 PH1). Differential scanning calorimetry (DSC) experiments were performed under helium on a TA Q1000 instrument. Two or three heating cycles were recorded at 10 °C min−1. The glass transition temperature (Tg) is taken as the inflection point of the glass transition step on the last heating. Dynamic mechanical analysis (DMA) experiments were conducted on a TA Q800 apparatus in the film tension geometry. Heating ramps were applied at 3 °C min−1. Rectangular samples of 5.1 mm × 1.4 mm cross section and about 16.1 mm length were tested at 1 Hz and 0.5 μm amplitude. Tensile tests were performed at room temperature on rectangular samples (15 mm × 5 mm × 1.5 mm) using an Instron 5564 tensile machine, with a strain rate of 2 mm min−1. For X-ray experiments, two different setups were employed with the same rotating anode generator (RU-200, Rigaku Co. Ltd.) using Cu Kα radiation. Small-angle X-ray diffraction patterns (SAXS) were recorded on a linear position sensitive detector (LPS 50 INEL) and wide-angle X-ray diffraction patterns (WAXS) on a curve position sensitive detector (CPS 120 INEL). The samples were prepared by increasing the temperature well above any transition temperature. After 10 min at high temperature, the samples were cooled at room temperature at 10 °C min−1. SAXS is capable of delivering structural information on macromolecules between 5 and 25 nm of repeat distances in partially ordered systems of up to 150 nm while WAXS concentrates on structural information between 0.2 and 5 nm. Synthesis Methods. Synthesis of HCC-PIB-CCH. Alkyne telechelic polyisobutene (HCC-PIB-CCH, Mn(SEC THF) = 6900 g mol−1, Đ = 1.2) (Scheme 1) was synthesized by the reaction of Br-Allyl-PIB-Allyl-Br with propargyl alcohol in the presence of KOH, using a procedure analogous to that described for the preparation of alkyne monofunctional PIB.11 The precursor Br-Allyl-PIB-Allyl-Br was obtained by the living polymerization of isobutylene utilizing a difunctional initiator followed by capping with 1,3-butadiene as
This elastomer exhibits excellent thermal, UV, and oxidative stability and good damping and barrier properties. The only way to prepare PIB is by cationic polymerization of isobutene (IB). If living cationic polymerization of IB is now an established process,7 block copolymers that can be produced are only very few.8 Besides, living cationic polymerization provides the most efficient method for the introduction of reactive end groups. However, very few functional PIBs have been reported by employing either functional initiators or functional terminators.9 Recently, Faust and co-workers reported the preparation of functional PIBs by nucleophilic substitution of haloallyl functional PIBs obtained by capping of living PIB chain ends with 1,3-butadiene.10 This general method offers a simple and efficient route to a variety of hydroxy, amino, carboxy, azide, and alkyne functional PIBs.11 As far as we know, block copolymers based on PE and PIB blocks were not reported despite the great interest in combining hard and soft blocks in the same structure.12 Considering the efforts concentrated in the two aforementioned strategies to functionalize chain ends of crystalline linear PE chains and amorphous elastomeric PIB chains, our groups intended to identify an efficient coupling reaction for the synthesis of such block copolymers that would exhibit the unique properties of these two segments. 1,3-Dipolar cylcoaddition between azide and alkyne13 end-functionalized polymers has been undoubtedly shown as an extremely powerful tool to couple polymer chains.14 While many examples of block copolymers have been depicted using this chemistry, its transposition to PE-based block copolymers is not trivial since the difficulty in manipulating PE that is soluble only in a small number of solvents and at high temperature (>90 °C) narrows the reaction conditions that can be employed. However, our experience on performing 1,3-dipolar cylcoaddition on PE carrying azide end groups (PE-N3) shows that this reaction can be very efficient to synthesize macromonomers15 or dye based on PE16 even under these harsh conditions. This drove us to investigate this chemistry to combine PE-N3 and PIB carrying alkyne end groups. The main objective was to implement efficient and simple coupling reaction conditions using relatively low molar mass PE and PIB reactive blocks in order to facilitate the spectroscopic characterizations of the block copolymers. Final copolymers were further analyzed by thermal, optical, X-ray scattering, and mechanical analyses to show their potential as possible new thermoplastic elastomers.
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EXPERIMENTAL SECTION
Materials. Azide end functionalized polyethylenes (PE-N3(A), 88% of functionality, Mn(HT-SEC) = 2500 g mol−1, Đ = 1.2 and PE-N3(B), 94% of functionality, Mn(HT-SEC) = 2250 g mol−1, Đ = 1.2) and alkyne end functionalized polyisobutylene (PIB−CCH, Mn(SEC THF) = 2400 g mol−1, Đ = 1.1) were obtained according to our respective previously published results.11,15 Copper(I) bromide (98%, Aldrich), pentamethyldiethylenetriamine (PMDETA, 99%, Sigma), copper(II) sulfate hydrate (98%, Aldrich), sodium L-ascorbate (99%, Sigma), and anhydrous N,N-dimethylformamide (DMF, SigmaAldrich, 99.8%) were used as received. Toluene (99.9%, Acros) was dried in argon over 3 Å molecular sieves. Analytical Techniques. High-resolution liquid NMR spectroscopy was carried out with a Bruker DRX 400 spectrometer. Spectra were recorded at 363 K using a 5 mm QNP probe for 1H NMR. Polymer samples were examined as 5−15% (w/v) solutions. A mixture of tetrachloroethylene (TCE) and perdeuteriobenzene (C6D6) (2/1 v/v) B
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Scheme 1. Chemical Structure of HCC-PIB-CCH
Scheme 2. Synthetic Strategy To Obtain a PE-b-PIB Block Copolymer by 1,3-Dipolar Cycloaddition between PE-N3(A) and PIB-CCH
described in the literature.11 1H NMR (400 MHz, 90 °C, C6D6/TCE 1/2 v:v, 256 scans, ppm, δ): 7.21−7.22 (br, Ar, m), 5.69 (m, −CH CH−CH2O−, c), 5.45 (m, −C(CH3)2−CH2−CHCH−, d), 3.97 (d, J = 2.5 Hz, −OCH2−CCH, a), 3.93 (d, J = 6 Hz, −CHCH− CH2−O−, b), 2.09 (t, J = 2.5 Hz, −CCH, f), 1.97 (d, J = 7 Hz, −C(CH3)2CH2−CHCH−, e), 1.87 (s, Ar−C(CH3)2−CH2−, i) 1.47 (br, −CH2−, h), 1.14 (br, −CH3, g). Synthesis of PE-b-PIB Diblock Copolymer. 350 mg of PE-N3(A) (0.12 mmol of −N3 group, considering the functionality of PE-N3(A)) was dissolved in dry toluene (70 mL) and DMF (20 mL) at 110 °C under an argon atmosphere. PIB-CCH (468 mg, 0.19 mmol) was dissolved in DMF (9 mL). A solution of CuSO4 (7 mg, 0.046 mmol) and sodium ascorbate (19 mg, 0.093 mmol) in DMF (1 mL) was added to the PIB-CCH solution. The resulting mixture was heated to 110 °C for 10 min and then added to the PE-N3(A) solution. The final mixture was refluxed under a dry argon atmosphere for 45 min. The reaction medium was then cooled to room temperature, and the polymer was precipitated with methanol and recovered after decantation and elimination of the supernatant. Finally the product obtained was dried under vacuum at room temperature for 12 h. Synthesis of PE-b-PIB-b-PE Triblock Copolymer. 574 mg of PEN3(B) (0.24 mmol of −N3 group, considering the functionality of PEN3(B)) was dissolved in dry toluene (100 mL) and DMF (50 mL) at 110 °C under an argon atmosphere. HCC-PIB-CCH (830 mg, 0.12 mmol) was dissolved in DMF (9 mL). A solution of CuBr (10 mg, 0.066 mmol) and PMDETA (12 mg, 0.066 mmol) in DMF (1 mL) was added to the HCC-PIB-CCH solution. The resulting mixture was heated to 110 °C for 10 min and then added to the PEN3(B) solution. The final mixture was refluxed under a dry argon atmosphere for 45 min. The reaction medium was then cooled to room temperature, and the polymer was precipitated with methanol and recovered after decantation and elimination of the supernatant. The product obtained was dried under vacuum at room temperature for 12 h.
surfactant in this toluene solution also containing unreacted PIB-CCH (soluble) and nonfunctional PE (insoluble). Methanol, which is a nonsolvent of PIB, was thus added to the suspension to recover the final polymers. 1H NMR analysis of the precipitated products (Figure 1) showed indeed the complete disappearance of the protons of the methylene adjacent to the azide at 2.9 ppm in PE-N3(A), consistent with FT-IR analyses. The characteristic resonances of the PE and PIB protons of the main chains were observed. New resonances at 3.91 (t, α), 4.56 (s, a) and 7.04 (s, f) ppm were assigned respectively to the methylenes adjacent to the formed triazole between PE and PIB segments and to the proton of the triazole ring. The residual PIB-CCH was also observed (alkyne proton at 2.1 ppm). Considering the apparent high efficiency of the reaction (no remaining PE-N3), a 1 to 1 stoichiometry between the two polymers was thus considered to be ideal to synthesize a clean block copolymer. An experiment in which small quantities of PIB-CCH (0.2 equiv) were successively added to a PE-N3 solution (1 equiv taking into account the functionality of PE-N3 (A)) was thus set up. In addition, the consumption of the azide groups was followed by FTIR analysis performed 10 min after each addition of PIB-CCH (Figure 2). The complete consumption of the azide groups (band at 2100 cm−1) agreed with a stoichiometric amount of PIB-C CH added. The same samples were also analyzed by HT-SEC (Figure 3). As expected when a substoichiometry of PIB-C CH is used, the resulting SEC trace showed a bimodal distribution (0.9 equiv in Figure 3). A population of higher molar mass than the starting homopolymers together with unreacted PE-N3 was observed. The high molar mass fraction was assigned to the formation of the targeted diblock copolymer. When a stoichiometric amount of the two homopolymers was used (1.0 equiv in Figure 3), the HTSEC analysis of the recovered product showed a monomodal distribution corresponding to the desired block copolymer. This result showed the success and the efficiency of the coupling reaction employed. Eventually, when an excess of PIBCCH is used (1.3 equiv in Figure 3), the block copolymer can still be observed together with a population that corresponds to the starting PIB-CCH employed. Additional thermal characterizations (DSC) were performed on the starting PE-N3 and the obtained PE-b-PIB block copolymer. The PE-N3 thermogram revealed a single endotherm of melting at 127 °C characteristic of fusion for semicrystalline polyethylene with an enthalpy of 219 J g−1. Crystallinity of PE-N3 was evaluated to 73% using the heat of fusion ΔH(x) of a 100% crystalline PE chain that can be calculated using eq 1:17
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RESULTS AND DISCUSSION The well-defined polymer building blocks PE-N3 and PIB-C CH were coupled via the 1,3-dipolar cycloaddition, a method that was shown to be particularly efficient for this purpose.14 Generally, polymer coupling reactions are performed in solution at room temperature. The semicrystalline nature of the very linear PEs we are using and their insolubility at low temperature led us to consider nonconventional coupling reaction conditions adapted from our previous results.16 PE-b-PIB. The synthesis of PE-b-PIB block copolymer was first performed by reacting 1 equiv of PE-N3(A) with an excess of PIB-CCH (1.6 equiv) (Scheme 2). The reaction was performed at 110 °C in a mixture of toluene and DMF. After 45 min, FTIR analyses of a sample withdrawn from the reaction medium showed the complete disappearance of the azide band at 2100 cm−1 and thus that the reaction was complete. To recover the product, a first trial was to decrease the temperature to room temperature to precipitate the targeted PE-b-PIB while keeping the unreacted PIB-CCH soluble in toluene. However, in this case, a stable suspension of very fine particles formed, and any attempts to recover the precipitated product failed. This indeed was a good indication of the formation of the targeted block copolymer that can act as a C
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Figure 1. 1H NMR spectra (400 MHz, 90 °C, C6D6/TCE 1/2 v:v, 256 scans) of PE-b-PIB recovered after reaction between PE-N3(A) and PIB-C CH after precipitation. Starred peaks are solvent impurities.
Figure 2. FTIR spectra of the reaction mixture recorded after 10 min of reaction: 1 equiv of −N3 group and 0.2 equiv (1), 0.35 equiv (2) 0.5 equiv (3), 0.7 equiv (4), 0.9 equiv (5), and 1.0 equiv (6) of PIB-C CH.
Figure 3. HT-SEC chromatogram of the PE-N3(A), PIB-CCH, and reaction mixture containing 1 equiv of −N3 group and 0.9, 1.0, and 1.3 equiv of PIB-CCH.
ΔH(x) = 93.22 + 4.249 × 10−3Tm 2(x) − 7.413 × 10−6Tm 3(x)
confirmed that the PE phase consists of extended-chain crystals, as found for n-paraffins, small PE oligomers,20 and PE copolymers.21 The introduction of the amorphous PIB segment induced a decrease in the melting temperature of PE crystalline block at 120 °C (ΔHm = 111.9 J g−1). Using the same maximum value of heat of fusion, diblock crystallinity was estimated to be 37.5%. After normalization to the PIB and PE weight fractions (f PE = 0.51), crystallinity of the PE block was calculated to be 73%. Thus, amorphous PIB phase did not perturb PE crystallinity, which suggested a microphase separation. PE-b-PIB-b-PE. The success of the reaction between PE-N3 and a monoalkyne PIB-CCH drove us to extend this coupling reaction to the synthesis of PE-b-PIB-b-PE triblock
(1)
where Tm(x) is the equilibrium melting temperature of PE with x the number of methylene units per chain, which can be calculated according to eq 2:17,18 ⎛ x − 1.5 ⎞ ⎟ Tm(x) = 414.3⎜ ⎝ x + 5.0 ⎠
(2)
The experimental melting temperature was close to the calculated equilibrium melting temperature (Mn(PE) = 2500 g mol−1, x = 175, Tm(175) = 126.3 °C), and chain length was calculated19 to be 22.5 nm, which is lower than the upper critical limit of 37 nm for PE chain folding. Both of these results D
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Figure 4. 1H NMR spectra (400 MHz, 90 °C, C6D6/TCE 1/2 v:v, 256 scans) of PE-b-PIB-b-PE recovered after reaction between PE-N3(B) and CHC-PIB-CCH after precipitation.
copolymer. In this case, a well-defined α,ω-alkyne functionalized PIB (CHC-PIB-CCH, Mn = 6900 g mol−1) was reacted with 2 equiv of PE-N3 (PE-N3(B), Mn = 2250 g mol−1)). The final product was recovered by precipitation after cooling the reactor and was analyzed by 1H NMR (Figure 4). The absence of residual PE-N3 was confirmed by the absence of any methylene signal at 3.0 ppm. The resonances corresponding to the starting alkyne groups in CHC-PIB-CCH observed at 2.09 ppm also disappeared. In agreement with the 1 H NMR characterization of PE-b-PIB, the formation of the triazole rings was confirmed by the presence of a new resonance at 7.04 ppm corresponding to the protons f of the ring and to the methylene adjacent to the triazole and the oxygen atom at 4.56 ppm (protons a). The new triplets formed at 3.92 ppm corresponded to the methylene α carried by the polyethylene segments and adjacent to the triazole ring. A careful integration of resonances a and α + b confirmed the expected ratio of 2 to 4, in agreement with a complete reaction of the alkyne and azide reactive groups. Additional characterization of the final products was performed by HT-SEC, including a comparison of the chromatograms of the starting PE-N3 and CHC-PIB-C CH (Figure 5). The shift of the molar mass distribution toward higher molar mass and the apparent absence of residual PE or PIB were in agreement with the successful formation of the targeted PE-b-PIB-b-PE triblock copolymer. The bulk behavior of the PE-b-PIB-b-PE triblock copolymer was investigated by thermal (DSC), structural (X-ray scattering, polarized optical microscopy), and mechanical (DMA, tensile test) characterizations. All measurements were performed on samples melted at 130 °C and pressed at 123 °C under 10 tons for 10 min.
Figure 5. HT-SEC analyses of the starting PE-N3 and CHC-PIBCCH and of the final PE-b-PIB-b-PE triblock copolymer recovered after reaction.
The DSC of the PE-b-PIB-b-PE triblock was typical of an immiscible block copolymer (Figure 6A), which was expected since PE and PIB are known to be highly incompatible.22 It displayed a glass transition step at −64 °C corresponding to the Tg of the PIB phase and an endotherm reminiscent of melting at 123 °C (ΔHm = 107 J g−1) characteristic of fusion of semicrystalline polyethylene. As for the diblock copolymer PEb-PIB, the melting temperature was close to the calculated equilibrium melting temperature Tm(x) (Mn(PE) = 2250 g mol−1, x = 158, Tm(158) = 124.7 °C),18 and the calculated chain length was 20.3 nm. Thus, in triblock copolymer as well, the PE phase consists of extended-chain crystals. The glass E
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Figure 6. Thermal analyses of PE-b-PIB-b-PE copolymer: (A) DSC (exo down); (B) polarized optical microscopy at 30 and 127 °C.
transition step corresponded to the Tg of the PIB phase and was slightly higher (−64 °C) than that of PIB homopolymer (−70 °C) because of the constraints exerted by the PE crystalline blocks.23 Indeed, PE crystallites act as a solid wall on which the PIB amorphous chains are firmly anchored, which restrains their chain motion.24 The crystallinity of PE in the copolymer was calculated to 36% (heat of fusion of a 100% crystalline PE chain ΔH(158) = 299 J g−1),17 which agreed with a triblock copolymer where weight fraction of PE segments represented 40% (MnPE = 2250 g mol−1, MnPIB = 6900 g mol−1). After normalization to the PIB and PE weight fractions, crystallinity of each PE block was calculated to be 90%, a value that can be found in the literature for nonfunctionalized PE oligomers of the same molar mass.19 Thus, larger amorphous PIB phase did not perturb PE crystallinity ( f PE = 0.40), as it has already been observed for 80 wt % PIB fraction in PE/PIB blends.25 Polarized optical microscopy images of PE-b-PIB-b-PE (Figure 6B) showed a birefringent texture characteristic of semicrystalline linear polyethylene chains which became isotropic above 126 °C, when chains melted. At room temperature, stressed sample was softly distorted but showed no fracture or crack, characteristic of the PIB elastomeric phase. The WAXS diffraction pattern (Figure 7) exhibited a broad halo at q = 1.06 Å−1 (corresponding to d = 6.63 Å), resulting from nearest neighbors correlation of polyisobutylene units and
the two intense Bragg peaks of crystalline polyethylene, (110) at q = 1.51 Å−1 (d = 4.09 Å) and (200) at q = 1.68 Å−1 (d = 3.73 Å), according to the orthorhombic lattice of polyethylene with a = 7.46 Å, b = 4.9 Å, and c = 2.55 Å. The analysis of X-ray line broadening indicated that the PE crystalline domains were rather large (D110 = 319 Å and D200 = 243 Å). Nevertheless, no peak was observed in the studied SAXS range, even for annealed samples (one night at 125 °C). Indeed, long period of the lamellar structure of a PE2000 was reported to be 179 Å, with no higher observable orders.26 Yet, long-range ordering of PE-b-PIB-b-PE could be expected (lamellar morphology since volume fraction of PE is 0.54) since it has already been encountered for PS-b-PIB-b-PS triblock copolymers,27 PS and PIB being immiscible.28 In our case, not only PE and PIB are known to be highly incompatible29 but also PE crystallizes. We showed that with strongly microphase-separated block copolymers crystallization of hard segments could be constrained into microdomains preexisting in the melt30 or more recently that crystallization can drive nanostructuration.31 Too short blocks could explain this absence of long-range ordering. Indeed, it is well-known that morphology of block copolymers mostly depends on the relative volume fractions of each block, the difference in their solubility parameters, block molar masses, sample preparation, and thermal history.32 Thus, crystalline PE microdomains are separated from amorphous PIB phase however without mesoscopic organization. The dynamic mechanical properties of the PE-b-PIB-b-PE were typical of those of an immiscible block copolymer (Figure 8). The storage modulus G′ began to descend at −52 °C, which corresponds to the Tg of the PIB phase (onset point at −51.2 °C). A well-developed rubbery plateau was observed in the range −20 to 110 °C where PE crystalline blocks melted, with a rubbery plateau modulus of 200 MPa (at 20 °C). Similar thermoplastic elastomeric behaviors have already been encountered for the microphase-separated triblock copolymers PS-b-PIB-b-PS mentioned above33 and for diblock copolymers PIB-b-poly(butylene terephthalate) (PBT), PBT being a crystallizable block.34 In the latter case, the rubbery plateau modulus increased with increasing crystallizable segment content. Modulus value of the PIB-b-PBT with a 70 wt % PBT (Mn(PIB) = 1500 g mol−1; Mn(PBT) = 3500 g mol−1) is close to the one observed for our triblock PE-b-PIB-b-PE (40 wt % PE).
Figure 7. Wide-angle X-ray scattering (WAXS) of PE-b-PIB-b-PE copolymer at 25 °C. F
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PIB with an exact stoichiometry and under simple reaction conditions. As such, this conjugation reaction consists in an excellent coupling tool between PE and PIB that fulfills the criteria of the click chemistry concept originally coined by Sharpless.13 Thermal (DSC) and mechanical (DMA) characterizations of the triblock PE-b-PIB-b-PE were characteristic of highly immiscible block copolymers revealing the glass transition of PIB at −65 °C/−55 °C and the melting temperature of PE at 123 °C/110 °C. The crystallintity of the PE segments was not perturbed by the PIB phase, and the Tg of PIB was slightly raised because of chain motion restrictions due to PE crystallites. Nevertheless, SAXS analyses did not evidence any nanostructuration of the block copolymer, probably because the molar mass of two blocks was not high enough. Mechanical analyses showed PE-b-PIB-b-PE behaved as a thermoplastic elastomer over a broad temperature range (0−110 °C) with a tensile strength of 4 MPa and an elongation at break of 16%. To sum up, these new block copolymers are promising thermoplastic elastomers. TPEs derive their unique properties from thermally reversible cross-links, which are most often in the form of microphase-separated domains. Thus, higher molar mass blocks should help the copolymer nanostructuration. Furthermore, hard (crystallizable PE) and soft (amorphous PIB) segment composition and the molecular architecture (linear, branched, star) would exert a major impact on the morphology of the copolymers and thus on their rubbery plateau modulus and tensile strength.37
Figure 8. DMA traces of PE-b-PIB-b-PE copolymer: storage modulus G′ (plain); loss modulus (dotted line); loss factor tan δ (points).
The loss factor was small over the whole temperature range which is characteristic of an elastic material. The tan δ maximum of PIB is known to be far from the point where G′ begins to drop, which is interpreted in terms of effective chain packing of PIB.35 It is also known to exhibit an asymmetric double-peak structure with a maximum at about −30 °C corresponding to the motion of long segments (slow process) and a shoulder at about −55 °C which corresponds to the motion of local segments that determines the Tg of PIB (α process).36 Here, the peak observed at −34 °C was attributed to the slow process of PIB, but Tg peak was not readily measurable. Figure 9 shows the stress−strain trace for the PE-b-PIB-b-PE block copolymer with a tensile strength of 4 MPa for a 16%
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (C.B.);
[email protected] (F.D.);
[email protected] (C.S-Z.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Edgar Espinosa acknowledges the financial supports from CONACYT programs. Olivier Boyron (C2P2, LCPP Team) is acknowledged for high temperature SEC analyses. Bernadette Charleux thanks the Institut Universitaire de France for her nomination as a senior member.
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Figure 9. Stress/strain profile of PE-b-PIB-b-PE copolymer.
REFERENCES
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elongation. The curve is characteristic of a thermoplastic elastomeric material, with a Young modulus of a semicrystalline polymer above Tg (100 MPa) and a uniform deformation. Even though tensile strength was of interest, elongation at break could be improved. Indeed, the molar mass of the PIB block used in this study was near the entanglement molecular weight of PIB (Me = 7000 g mol−1). Thus, a longer central PIB block having at least the critical molecular weight, i.e., 2Me, should improve the plastic deformation of the material and lead to a better thermoplastic elastomer.
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CONCLUSION New copolymers based on one amorphous PIB block and one or two outer crystalline PE blocks were successfully synthesized. This was achieved by using 1,3-dipolar cycloaddition between PE-N3 and alkyne-terminated or telechelic G
dx.doi.org/10.1021/ma400488d | Macromolecules XXXX, XXX, XXX−XXX
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