Article pubs.acs.org/IECR
Biobased Thermoplastic Poly(ester urethane) Elastomers Consisting of Poly(butylene succinate) and Poly(propylene succinate) Shao-Long Li,† Fang Wu,† Yu-Zhong Wang,*,† and Jian-Bing Zeng*,‡ †
Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China ‡ College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China ABSTRACT: In this study, a series of biobased and biodegradable thermoplastic poly(ester urethane)s (PEUs) with different compositions were synthesized via chain extension reaction of dihydroxyl terminated poly(propylene succinate) (HO-PPS-OH) and poly(butylene succinate) (HO-PBS-OH) with 4,4′-methylenediphenyl diisocyanate (MDI) as a chain extender. The thermal behaviors of PEUs were characterized by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). All the PEUs showed a single glass transition temperature shifting with compositions; only one melting peak was observed when the feeding weight ratio of PPS to PBS was less than 8:2, and the crystallization ability of the PEUs decreased gradually with the increase in PPS content. The tensile and tensile hysteresis tests suggest that the PEUs showed the tensile behaviors of elastomers when the weight ratios of PPS/PBS were 7/3, 6/4, and 5/5, and the tensile hysteresis value and Young’s modulus increased with increase in PBS content. The tensile strength and elongation at break of the three PEU elasomers exceeded 37 MPa and 1600%. Enzymatic (Candida rugosa lipase) hydrolysis study showed that the degradation rate increased with PPS fraction, according to the weight loss measurement and scanning electron microscopy observations.
1. INTRODUCTION
of the physical cross-links, the hard segments determine mechanical strength.18,19 PBS is usually synthesized through esterification and polycondensation of 1,4-butanediol (BD) and succinic acid (SA), both of which can be obtained from the biomass resources.20 It is a so-called “even”-numbered biobased and biodegradable polyester plastic. PBS is regarded as one of the most promising materials to replace some traditional plastics such as polyethylene (PE) and polypropylene (PP) for the similar processability, thermal stability, and mechanical properties. Besides, it has a high degree of crystallinity and one of the highest melting temperatures in polyesters (Tm ≈ 113 °C).21−24 Therefore, the crystals of PBS, which will form strong physical cross-links, can be used to replace the “diisocyanate hard phase”. In recent years, 1,3-propanediol (PD) is successfully produced from biological fermentation with sufficient purity which garnered poly(propylene succinate) (PPS) increasing attention.25−27 PPS is a typical biobased “odd”-numbered polyester and showed special physicochemical properties. Its biodegradation rate is much higher than poly(ethylene succinate) (PES), poly(butylene succinate) (PBS), and even poly(propylene adipate) (PPA) and poly(propylene sebacate) (PPSe) because of the low crystallinity.28,29 Moreover, its melting point of about 43−45 °C is the lowest of the above polyesters (43−45 °C).30 Thus, PPS is a good candidate for the soft phase, since its crystallization ability
In recent decades, biobased and biodegradable polymers have attracted considerable attention and were considered the most promising “green” materials to address the “white waste” problem and petroleum crisis.1,2 Among them, aliphatic polyesters have been playing an increasingly important role in replacing common polymeric materials, since they can be chemically synthesized and their molecular structure and physical properties are readily controllable and tunable.3 However, most of the aliphatic polyesters are semicrystalline plastics, like poly(butylene succinate) (PBS),4,5 poly(lactic acid) (PLA),6−8 and microbial polyesters,9 not elastomers. Biodegradable polymeric elastomers with well-tuned properties are usually designed from polyurethane structure, yielding biodegradable polyurethane elastomers (BPUEs). Most of the research on BPUEs to date focuses on the medical material and tissue engineering applications because of their biodegradability and biocompatibility.10−12 However, less attention has been paid touting BPUEs as a replacement material for traditional nondegradable elastomer materials such as TPO and SBS and to improve the tensile properties of BPUEs, since the strengths and elongations of the elastomers are usually less than 10 MPa and 500%, respectively.13−15 Biodegradable PEUs, which are usually synthesized by a polydiol, a diisocyanate, and a small molecular chain extender, consist of two phases, i.e., hard and soft phases.16,17 The polydiol is an aliphatic polyester precursor with low-Tg, which is the soft segment and provides the elastomeric behavior. The residues of diisocyanate and chain extender after the reaction with high-Tg act as the hard segment because of the hydrogen bond interaction and other van der Waals’ force. On the basis © XXXX American Chemical Society
Received: February 16, 2015 Revised: May 28, 2015 Accepted: June 4, 2015
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DOI: 10.1021/acs.iecr.5b00637 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
After all the reactants were molten, the bulk was vacuumed and purged with nitrogen three times. Then, a predetermined amount of MDI was charged into the reactor by stirring quickly for about 2 h until the Weissenberg effect appeared. The obtained block copolyesters were dissolved in chloroform and precipitated with a 5-fold amount of methanol. The precipitates were dried at 60 °C in a vacuum oven to constant weights before 1H NMR and GPC measurements. 2.4. Nuclear Magnetic Resonance (NMR) Spectroscopy. The chemical structures of prepolymers and PEUs were recorded by NMR spectrometer (Bruker AC-P 400) at ambient temperature. The samples were dissolved in deuterated chloroform (CDCl3) at a concentration of 0.005 mg/mL. Tetramethylsilane was used as the internal reference standard. 2.5. Gel Permeation Chromatography (GPC). GPC was used to determine the average molecular weight and molecular weight distribution (PDI) of the PEUs. The instrument is equipped with a model 1515 pump, a Waters model 717 autosampler, and a model 2414 refractive index detector. All samples were dissolved in CHCl3 at a concentration of 2.5 mg/ mL. CHCl3 was used as the elution solvent, and the flow rate of eluent was 1.0 mL/min. The experiments were operated at 35 °C. 2.6. Differential Scanning Calorimeter (DSC). A TA Instruments DSC-Q200 was used to study the thermal behavior of the PEUs. Samples with 5 mg were quenched from room temperature to 0 °C, then heated to 140 °C at a scanning rate of 10 °C/min and held for 4 min, then quenched to −50 °C and maintained for 3 min, finally reheated to 140 °C at a heating rate of 10 °C/min. All the procedures were carried out under N2 atmosphere. Both the first and second heating scans of the PEUs were recorded for thermal behavior analysis. 2.7. Wide-Angle X-ray Diffraction (WAXD). X-ray diffraction measurements of the samples were recorded with an X-ray diffractometer (Philips X’Pert X-ray diffractometer) using Cu Kα radiation. The experimental data were measured at a scan rate of 2°/min scanning from 10° to 40°. 2.8. Thermogravimetric Analysis (TGA). The thermal stabilities of PPSU, PBSU, and PPBUs were determined through thermogravimetric analysis. The thermograms were recorded on a TG 209 F1 (NETZSCH, Germany) thermogravimetric analyzer from room temperature to 600 °C at a heating rate of 10 °C/min under N2 atmosphere. 2.9. Mechanical Property Measurement. The stress− strain and hysteresis tests of the PEUs were performed on an Instron Universal testing machine (model 4302, Instron Engineering Corporation, Canton, MA) at a crosshead speed of 200 mm/min and room temperature. The specimens were prepared by hot pressing and cutting with a dumbbell-shaped cutter. The thickness and width of the specimens were 0.4 and 4 mm, respectively, and the length of the sample between the two pneumatic grips of the testing machine was 24 mm. Five measurements were carried out for each sample, and the average results were reported. For hysteresis tests, the samples were stretched to 300% elongation at a crosshead speed of 200 mm/min and then immediately reversing the crosshead at the same speed. Shore A hardness of the flat surface of PPBUs with 5 mm thick specimen were measured by a LAC-J Shore durometer (Zhejiang, China). All the tests were carried out at room temperature. 2.10. Enzymatic Degradation. The phosphate buffer solution (pH 7.4) containing 0.1 mg/mL Candida rugosa lipase was prepared first. Then PEU films with dimensions of 10 mm
will be further compressed and provide elastic behavior after being copolymerized with another high melting point segment. Papageorgiou et al.31 have synthesized a complete series of fully biobased and biodegradable poly(butylene-co-propylene succinate) (PBPSu) random copolymers. The copolyesters showed faster enzymatic degradation rate with increasing PSu content. Nevertheless, the melting points and mechanical properties decreased sharply. Xu et al.32 and Lu et al.33 have been reported the isothermal and nonisothermal crystallization kinetics of the copolyesters, respectively. Chain-extension reaction between aliphatic polyester precursors and diisocyanate is an efficient way to synthesize multiblock poly(ester urethane) (PEU) with high Tm and good mechanical performance, as reported in our previous paper and Li’s group’s paper.21,23 In the present article, a series of biobased and biodegradable thermoplastic PEU were synthesized by chain extension of HOPPS-OH and HO-PBS-OH precursors with MDI as a chain extender. We expect that high tensile strength elastomers could be obtained when the PBS and PPS segments perform their respective functions under suitable composition. The structures, molecular weights, crystallization behavior, mechanical properties, elastic behaviors, and enzymatic degradation were measured by 1H NMR, GPC, DSC, WAXD, mechanical testing, enzymatic degradation testing, and SEM. To our best knowledge, no similar research has been done before.
2. EXPERIMENTAL SECTION 2.1. Materials. Succinic acid (SA), 1,3-propanediol (PD), 1,4-butanediol (BD), Na2HPO4, and NaH2PO4 of AR grade were purchased from the Kelong Chemical Co. (Chengdu, China). 4,4′-Methylenediphenyl diisocyanate (MDI, AR grade) was supplied by Alfa Aesar Co. The lipase from Candida rugosa (CRL, activity: 700 unit/mg) was purchased from the SigmaAldrich Chemical Co. Tetraisopropyl titanate (AR grade) was also bought from Sigma-Aldrich and was dissolved in anhydrous toluene to prepare a 0.3 g/mL solution. All the other materials and solvents with AR grades were used without any treatment. 2.2. Synthesis of HO-PPS-OH and HO-PBS-OH Precursors. To obtain precursors with hydroxyl-terminated groups, the raw material diol should be added more than diacid. Previous investigations suggested that when the feed mole ratio of diol to diacid was 1.2:1, the obtained prepolymers were nearly terminated with dihydroxyl groups.34,35 Typically, HOPPS-OH was synthesized by esterification and subsequent polycondensation reactions. The esterification of 1,3-PD (1.2 mol) and succinic acid (1 mol) was carried out in a roundbottom flask. The mixture was purged by nitrogen and mechanical stirrer to distill off the water at 180 °C for 4 h. Then 0.1 wt % catalyst tetraisopropyl titanate was added to the flask and heated to 220 °C under reduced pressure (
