One-Pot Synthesis of Supertough, Sustainable Polyester Thermoplastic Elastomers Using Block-Like, Gradient Copolymer as Soft Midblock

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022One-Pot Synthesis of Supertough, Sustainable Polyester Thermoplastic Elastomers Using Block-Like, Gradient Copolymer as Soft Midblock Wuchao Zhao, Chengkai Li, Xiao Yang, Jianghua He, Xuan Pang, Yuetao Zhang, Yongfeng Men and Xuesi Chen Wuchao Zhao State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Chengkai Li State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Xiao Yang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Google Scholar More articles by this author , Jianghua He State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Xuan Pang Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Google Scholar More articles by this author , Yuetao Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Yongfeng Men *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Google Scholar More articles by this author and Xuesi Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100897 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail It remains challenging to synthesize supertough thermoplastic elastomers (TPEs) since the stretchability and tensile strength are mutually exclusive. Here, we report a one-pot strategy for the preparation of sustainable, triblock polyester TPEs consisting of poly(l-lactide) (PLLA) hard segments and poly(ε-caprolactone)-co-poly(δ-valerolactone) (PCVL) soft segments. The TPEs were synthesized successfully with high stretchability (up to 2100%) and strong tensile strength (up to 71.5 MPa) without requiring specific functionalized groups by simply adjusting the polymer microstructures, which, in turn, exhibited a world’s record toughness of 445 MJ/m3. Systematic investigation revealed that the block-like, gradient microstructure of PCVL improved the ductility by providing a flexible elastic network and enhancing the tensile strength through strain-induced crystallization. The practicability of this strategy was well demonstrated by lifting a water tank over 30,000 times heavier than itself and easy scale-up experiments. Download figure Download PowerPoint Introduction High-performance thermoplastic elastomers (TPEs), especially supertough TPEs with both high stretchability and strong tensile strength, have attracted much intense attention due to their wide applications in both engineering and biomedical fields.1–6 So far, the development of supertough materials to attain both tensile strength and stretchability has traditionally been a compromise between hardness and ductility since these properties are generally mutually exclusive.7,8 Increasing the tensile strength is often achieved at the expense of elasticity and vice versa. To enhance the mechanical performance, several effective methods have been applied to the TPEs syntheses through smart design and precise synthesis, making remarkable progress of achieving toughness up to 387 MJ/m3, with characteristics such as filler-reinforcement,9,10 double networks of two components blends,7,11 multicomponent cross-linking,12,13 and biomimetic strategy based on multiple hydrogen bonds.14–16 Clearly, introducing specific functionalized groups is essential for the toughening of elastomers. Without these reinforcement strategies, it seems an impossible mission to synthesize TPEs with both high stretchability and strong tensile strength. ABA triblock (A-B-A-type copolymer) TPEs, composed of the hard end block A and soft midblock B, primarily the biorenewable and biodegradable polyester-based TPEs, have been widely used in the industrial field, medical science, and as traditional commodities, due to their ubiquitous biodegradable, renewable, and biocompatible properties.1,2,6,17,18 Several research groups have made significant contributions to this field and have pioneered a series of aliphatic polyesters TPEs with impressive mechanical properties.6,18 Typically, poly(lactide) (PLA), poly(cyclohexene-alt-phthalate), or poly(α-methylene-γ-butyrolactone) served as hard end block A, whereas poly(ε-caprolactone) (PCL), poly(δ-valerolactone) (PVL), poly(menthide), and their homologs, or random copolymers with reduced crystallinity, acted as soft middle block B, respectively.19–31 Furthermore, multiblock polyesters, cross-linking polyesters, and end-functionalized polyesters were also reported to achieve TPEs with improved mechanical performance.32–36 So far, the reported polyester-based TPEs might exhibit either high elongation at break, up to 2100%, or tensile strength of ∼37 MPa (Figure 1a); it remains an enormous challenge to prepare TPEs with both high stretchability and strong mechanical strength of the above-mentioned order of magnitude. Figure 1 | (a) Mechanical performance of aliphatic polyester TPEs in this work compared with other related studies. (b) Synthesis of triblock copolyester TPEs PLLA-PCVL-PLLA by living/controlled Mg(II)/BDM catalyst system. Download figure Download PowerPoint In this context, an organomagnesium complex was combined with 1,4-benzenedimethanol (BDM) initiator to prepare a series of triblock polyester TPEs consisting of poly(l-lactide) (PLLA) hard segments and homopolymer (PCL or PVL) or copolymer poly(ε-caprolactone)-co-poly(δ-valerolactone) (PCVL) soft segment, from the living/controlled ring-opening polymerization (ROP) of renewable lactones. By changing the monomer feed ratio, the contents of soft or hard segments in TPEs are easily adjustable, thus, furnishing high-performance polyester TPEs with high elongation at break (up to 2100%) and strong tensile strength (up to 71.5 MPa) (Figure 1a). Further, systematic characterizations of these fabricated TPEs by differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD), in comparison with control experiments, revealed that the block-like, gradient polyester PCVL played an essential role in toughening of TPEs. To the best of our knowledge, this is the first time a polyester TPE had been generated with a world’s record toughness of 445 MJ/m3, achieved successfully from the ROP of traditional cyclic lactones by simply adjusting the composition and microstructure of polymers. Experimental Section General information All syntheses and manipulations of air- and moisture-sensitive materials were carried out in flamed Schlenk-type glassware on a dual-manifold Schlenk line, a high-vacuum line, or an argon-filled glovebox. Benzyl alcohol (BnOH) and nBu2Mg were purchased from J&K Scientific Ltd., and tetrahydrofuran (THF) [high-performance liquid chromatography (HPLC) grade] was purchased from Sigma (Shanghai, China). N,N,N′-trimethylethylenediamine, hydrobromic acid, BDM, 2,4-di-tert-butylphenol, and l-lactide (LLA) were purchased from Energy Chemical Co., Ltd. (Shanghai, China). All chemicals were used as received unless otherwise specified as follows. ε-Caprolactone (ε-CL; J&K Scientific Ltd., Beijing, China) and δ-valerolactone (δ-VL; Adamas-beta, Shanghai, China) were dried over CaH2, distilled under nitrogen, and stored in a glovebox at −35 °C. Toluene (TOL) and benzene were refluxed over sodium/potassium alloy, followed by distillation under a nitrogen atmosphere; hexane and dichloromethane were refluxed over CaH2, followed by distillation under nitrogen atmosphere. All solvents were stored over molecular sieves of 4 Å. General polymerization procedures Polymerizations were performed in 60 mL glass reactors inside the glovebox for runs carried out at room temperature (RT). In a typical polymerization procedure, a predetermined amount of Mg(II) complex solution and BnOH or BDM initiator were first mixed in TOL for 2 min. A 200 molar equiv of monomer ε-CL (0.50 M in TOL) was added to the mixture inside a glovebox. After the first batch of monomers had reached total consumption, a 25 molar equiv of LLA (0.25 M in dichloromethane (DCM)) was added without quenching. After a measured time interval, a 0.2 mL aliquot was taken from the reaction mixture with a syringe and quickly quenched into a 4-mL vial containing 0.6 mL of undried “wet” CDCl3 stabilized by 250 ppm of Butylated hydroxytoluene (BHT-H); the quenched aliquots were later analyzed by 1H NMR to obtain the percent monomer conversion data. After the polymerization was stirred for the stated reaction time, the polymer was immediately precipitated by 200 mL of hexane, stirred for 1 h, filtered, washed with hexane, and dried in a vacuum oven at 50 °C overnight to constant weight. Polymer characterizations NMR spectra were recorded on Bruker Avance II 500 (Zurich, Switzerland, 500 MHz, 1H; 126 MHz, 13C) instrument at RT in CDCl3. Gel permeation chromatography (GPC) analyses were performed on a Waters 1515 instrument (Milford, MA) equipped with a guard column MIXED 7.5 × 50 mm PL column and two MIXED-C 7.5 × 300 columns and a differential refractive index (DRI) detector using THF (HPLC grade) as the eluent at 35 °C with a flow rate of 1 mL/min. The weight–average molar masses (Mw) and molecular weight distribution (MWD) (Mw/Mn) of the polymer samples were determined by WYATT DAWAN 8+ light scattering (LS) detector at 35 °C and a flow rate of 1 mL/min. The DRI increment (dn/dc) value of 0.084 mL/g was used for PVL and 0.076 mL/g for PCL, and 0.042 mL/g for PLA. Chromatograms were processed with Waters Breeze 2 software. Thermal properties were measured on a TA Instruments (New Castle, DE) Discovery DSC series DSC 25. Polymer samples were first heated to 200 °C at 10 °C/min, equilibrated at this temperature for 2 min, then cooled down to −80 °C at 10 °C/min, held at this temperature for 2 min, and then reheated to 200 °C at 10 °C/min. SAXS experiments were performed on the BL16B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF) at RT. The wavelength of the X-rays was 0.932 Å, and two-dimensional (2D) SAXS patterns were recorded using a MAR-CCD detector (Norderstedt, Germany) at a sample-to-detector distance of 5745 mm. All 2D figures were transformed to one-dimensional (1D) curves via Fit2D software ( https://www.esrf.fr/computing/scientific/FIT2D/). WAXD was performed on a customized microfocus WAXD system. A focused Cu Kα X-ray source (GeniX3D, Xenocs SA, Isere, France), generated at 50 kV and 0.6 mA and employed in this setup. The wavelength of X-ray radiation is 0.154 nm. 2D WAXD patterns were collected by a semiconductor detector with a resolution of 487 pixels × 195 pixels (pixel size = 172 μm) (Pilatus 100 K, DECTRIS AG, Baden, Switzerland) at a sample-to-detector distance of 45.8 mm. Each WAXD pattern was background corrected according to standard procedure utilizing the “Fit2D00” program.37 Atomic force microscopy (AFM) test was performed on Bruker Dimension FastScan (Karlsruhe, Germany), and the sample films were prepared by solvent vapor annealing. TPE films (thickness ∼150–300 μm) were prepared by dissolving triblock copolymers in chloroform and casting them on a glass plate. Then the solvent was allowed to evaporate for 24 h at RT, followed by removing the residual solvent under vacuum for 24 h. Dog-bone-shaped specimens were die-cut from the prepared films with a width of 2.0 mm and a length of 12 mm. Uniaxial tensile tests were conducted on an Instron universal testing machine (Model 5944; Cambridge, MA) equipped with a 2 kN load cell operated at a crosshead speed of 15 mm/min. Tests were performed at RT, and at least three measurements were performed for each sample. Toughness was calculated as the integral area under the stress–strain curve according to eq 1.38 Toughness : W = ∫ strain = 0 strain = strain max ( stress ) d ( strain ) (1)where stress, tensile stress, MPa; strain, the tensile strain was expressed as a percentage (%). The true strain : ϵ = ln l l 0 = ln ( Δ l + l 0 ) l 0 = ln ( 1 + ε ) (2)where l, tensile length; l0, initial length. The true stress : σ = P S = P S 0 ( 1 + ε ) (3)where, P, load, N; S0, initial area, m2. Results and Discussion Synthesis of ABA triblock polyester TPEs To synthesize triblock polyester TPEs, it is highly demanded to develop more efficient polymerization systems. Based on our previous experiences,39–41 a new butylmagnesium 2,4-di-tert-butyl-6-((methyl(2-(methylamino)ethyl)amino)methyl phenolate (Mg(II)) was synthesized and combined with BnOH to promote living/controlled ROP of renewable cyclic lactones (see Supporting Information Table S1 and Figure S1). The living nature of this Mg(II)/BnOH system was verified by successful synthesis of well-defined diblock polymer PCL-b-PLLA, revealed by GPC analysis (see Supporting Information Figure S2b) and NMR spectroscopy (see Supporting Information Figures S5 and S6). PLA is known to be a perfect hard segment candidate for TPEs synthesis due to its biorenewable, biocompatible and biodegradable properties, as well as high stiffness,42–44 whereas random copolymers exhibiting decreased crystallinity and enhanced flexibility could serve as a better choice of soft midblock.45–47 By replacing BnOH with BDM and through sequential monomer addition method, a series of ABA-type triblock polyester TPEs composed of random PCVL soft midblock (mass ratio of ε-CL:δ-VL = 1:1 is fixed for all TPEs synthesis) and PLLA hard end block were synthesized through sequential addition of monomer method at RT in the one-pot process (Figure 1b). GPC traces clearly showed the gradual shift to a higher molecular weight region with an increase in the contents of either the soft segment or hard segment (see Supporting Information Figure S3). 1H NMR spectra confirmed that the integral ratios of soft PCVL segment to PLLA hard segments were consistent with the monomer feed ratio and 13C NMR spectra revealed no observation of transesterification between PLLA and PCVL (see Supporting Information Figures S7–S24 and Table S3). Microphase separation Compared with TPE1 or TPE2 composed of PCL or PVL soft segments (see Supporting Information Figure S29), TPE3 to TPE9 exhibited a lower Tm value in the range of 24.7–29.2 °C attributed to soft segment PCVL and a Tm between 150.9 and 160.6 °C attributed to hard segment PLLA (see Supporting Information Figure S30–S32), thus, confirming the microphase separation of PCVL soft midblock and PLLA hard end block. Furthermore, SAXS was employed to study the morphology and effects of midblock composition on the microphase separation of the bulk triblock TPEs at RT. As a result, all of them exhibited the principal peaks indicative of microphase separation (Figures 2a and 2b), and the correlation lengths of the domains (d = 2π/q*) calculated from SAXS data are in the range of 27.3–52.3 nm (see Supporting Information Table S2). Further, as the soft segment contents increased, the primary scattering peaks gradually shifted toward the low q side, indicating the enlargement of microphase-separated domains. Moreover, AFM results also clearly demonstrated the microphase separation of TPE8 with high PLLA contents, consistent with the results obtained by the SAXS test (see Supporting Information Figures S44 and S45). These results clearly demonstrated the immiscibility of soft midblock PCVL and hard end block PLLA due to their differences in the polarity and crystallinity, indicating that all these TPEs are composed of a well-defined interpenetrated network of hard crystal skeleton and entangled soft “amorphous” network. Figure 2 | SAXS profiles of (a) 1D curves and (b) 2D curves for TPE3–TPE5 obtained at RT. Download figure Download PowerPoint Mechanical properties of TPEs To measure mechanical properties, TPE films were prepared from these triblock copolymers by a solvent-casting method. They were colorless and highly transparent without air bubbles. Uniaxial tensile testing showed that TPE1 or TPE2 with either PCL or PVL as soft segment exhibited comparable ultimate tensile strength (35.5 vs 35.4 MPa) and different elongation at break (1200% vs 953%). However, both showed clear yield points, followed by a cold drawing, indicating plastic deformation at a low elongation of 8–20% (see Supporting Information Figure S34).48 Although TPE3 with PCVL soft midblock showed weaker tensile strength (13.6 MPa), the elongation at break was significantly enhanced to 1552%, and no yield stress and cold stretching were observed (Figure 3), thus, confirming that PCVL is a better candidate for the soft segment in TPEs syntheses than PCL or PVL. By fixing the length of hard segments, increasing the ε-CL/δ-VL molar ratio from 200:228 to 300:342 and 400:456 led to the production of TPEs with high elongation at break from 1552% to 2000% and 2100% and strong tensile strength from 13.6 MPa to 33.8 MPa and 46.3 MPa, hence, exhibiting increased toughness from 109 MJ/m3 to 296 MJ/m3 and 445 MJ/m3, respectively (Figure 3). To the best of our knowledge, TPE5 with the longest soft segment (ε-CL:δ-VL = 400:456) represented supertough aliphatic polyester TPEs with the highest toughness value of 445 MJ/m3 so far. This phenomenon, in particular, attracted our attention since it contradicted a previous study that showed that elongation at break and tensile strength were mutually exclusive.8 Moreover, we investigated the impact of hard block contents on the mechanical properties of TPEs. It turned out that for TPE5 to TPE7 with the same soft block contents (ε-CL:δ-VL = 400:456) but varying hard end block contents from 25 to 50 and 75, the ultimate tensile strength values increased from 46.3 MPa to 56.4 MPa and 71.5 MPa, whereas the elongation at break decreased from 2100% to 1678% and 1365%, respectively (runs 5 vs 6 and 7, Table 1 and Figure 3). A similar trend was observed for TPEs with a fixed 300:342 ε-CL:δ-VL molor ratio, consistent with previous literatures.22,25,26 We observed that the higher the hard domain contents, the harder the crystal skeleton (or physical cross-linking points), thus, enhancing the tensile strength of TPEs, but sacrificed elongation at break.49 These results revealed that PCVL soft midblock played an essentially important role in affecting the toughness of the TPEs (vide infra). Figure 3 | The stress–strain curves of different TPEs. Download figure Download PowerPoint Table 1 | Summary of Ultimate Tensile Properties of the Renewable Aliphatic Polyester TPEsa Run Sample Name Sampleb Mwc (kg/mol) Đc εbd (%) σbd (MPa) Toughness (MJ/m3)d 1 TPE1 25PLLA-400PCL-25PLLA 41.6 1.08 1200 35.4 2 TPE2 3 TPE3 13.6 109 4 1.08 33.8 296 5 TPE5 46.3 445 12 6 56.4 TPE7 71.5 TPE8 TPE9 10 12 samples were from polymerization performed in mixed solvents of (TOL) and at temperature with = = = M in the monomer feed ratio to the molecular weight (Mw) and determined by GPC using a elongation at tensile strength. values and standard were obtained from the tensile test of three samples with an rate of 15 Toughness was calculated as the area under the stress–strain curve according to eq between two exhibited improved mechanical properties and properties compared with their polymers. it could be to the and of the By simply with was exhibited significantly enhanced tensile strength MPa), high stretchability and toughness of MJ/m3, hence, the significant impact of on the of mechanical properties Table Supporting Information Figure Compared with engineering stress–strain the stress–strain curves were also for the mechanical performance of As in Supporting Information Figure TPE7 exhibited the true tensile strength up to MPa, comparable with engineering a of TPE7 sample with mm could a water tank successfully (see Supporting Information is over 30,000 times heavier than Moreover, 50 of TPE5 could be synthesized from a high polymerization in a one-pot process and employed to prepare a × size TPE (see Supporting Information Figure these results demonstrated the practicability of this method. To into the toughening we the of and to the tensile deformation of these TPE using true stress σ as a of = Supporting Information Figure The elastic network could be obtained from the of the curves Supporting Information Table and revealed that the was enhanced with the hard segment contents and the highest was we that these TPEs a flexible elastic due to the lower of crystallinity of PCVL from the of ε-CL and and the of PLLA end This led to a soft segment with Tm to RT and elastic networks with low crystallinity that were entangled “amorphous” networks under such These results well high stretchability could be achieved by our strategy but the of the tensile strength was also enhanced at the same As in Supporting Information Figure we observed two at different strain indicating these TPE samples exhibited tensile deformation of two networks of different TPE7 (see Supporting Information Figure as an the could be determined by the of two from two Based on the of two we the network had a higher than the first It is known that the of the and of network to the of strain-induced To we performed in WAXD to the of TPE3 to TPE5 with different soft block contents stretching at RT. to the initial crystallinity value of TPE3 was (Figure we observed the diffraction patterns of (see Supporting Information Figure analysis showed its crystallinity increasing with a sample was reached for stretching of (Figure indicating the of the As control experiments, the strain of TPE1 and TPE2 composed of either PCL or PVL soft by WAXD (Figures and that both samples exhibited high crystallinity might be attributed to the nature of PCL or PVL. Although we observed the stretching (see Supporting Information Figures and the total crystallinity of both samples indicating that the crystallinity was well the of yield in both TPEs (Figure with an increase in soft segment contents, both and TPE5 exhibited higher crystallinity and stretching in comparison with TPE3 (Figures and also Supporting Information Figures and that molecular weight but also the microstructure of soft segment of TPEs were related to the (vide infra). these results demonstrated that the soft segment PCVL a flexible elastic network to elongation TPEs but also played a significant role in affecting the for the in both tensile strength and elongation at break, achieved by our Mg(II)/BDM system. Then to the in these triblock polyester-based has reported the of PCVL as a soft segment and PLA as a hard segment system for TPEs syntheses with the of and system. However, the generated elongation at break was than with a tensile strength more than Figure 4 | analysis of WAXD for TPEs with different soft segment Download figure Download PowerPoint To the to the we the polymer composition changing with the reaction time by both and Mg(II)/BDM catalyst It turned out that the polymerization rate of is similar to that of ε-CL for NMR spectra revealed that the PCVL was a more random copolymer we observed an ratio of the and and and and and the polymer composition the polymerization (see Supporting Information Figure In the polymerization rate of is times than that of ε-CL for As in Figure the monomer conversion reached and for and ε-CL min, with of and for and indicating that the PVL was generated at the of The polymerization rate of down with its The of and increased indicating that the random PCVL was as the at this After conversion the integral of increased that the PCL was generated in the of the polymerization In with the that a was

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