Article pubs.acs.org/IECR
Novel Poly(butylene fumarate) and Poly(butylene succinate) Multiblock Copolymers Bearing Reactive Carbon−Carbon Double Bonds: Synthesis, Characterization, Cocrystallization, and Properties Liuchun Zheng,† Zhaodong Wang,†,‡ Shaohua Wu,†,‡ Chuncheng Li,*,† Dong Zhang,† and Yaonan Xiao† †
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), Beijing 100190, P. R. China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100049, P. R. China ABSTRACT: Widespread application of biodegradable polyesters is restrained by inferior mechanical properties, higher cost, and lack of reactive functional sites for further modification. A new class of multiblock copolymers bearing reactive sites on the main chains, composed of poly(butylene fumarate) (PBF) and poly(butylene succinate) (PBS), has been designed and synthesized to enhance the biodegradability of PBF and expand its application, while retaining the mechanical properties. The chemical structures of the copolymers were confirmed by 1H NMR spectra and GPC. The two segments are compatible in both the amorphous and crystalline region, and form cocrystals because of their similar nature. The multiblock copolymers possess satisfactory thermal and mechanical properties. The degradation rate of copolymers is substantially accelerated by the copolymerization with PBS. The novel polymer may be directly used as biodegradable materials or be postfunctionalized to modify the physicochemical properties or to endow the polymers with special properties.
1. INTRODUCTION Aliphatic polyesters, as one of the most intensively studied and commercially successfully biodegradable materials,1 have been used in various fields including packing and biomedicine. However, the large-scale commercialization of biodegradable polyesters still suffers from many drawbacks such as inferior mechanical properties, higher cost, high hydrophobicity, and lack of desirable hydrophilicity and reactive functional sites on the main chains for further modification or functionalization.2 PBF is a new type of unsaturated aliphatic polyester developed recently,2−5 bearing favorable carbon−carbon double bonds on the polymer main chains to regulate the physicochemical properties or endow the polyester with specific properties by postmodification or functionalization. Highmolecular-weight and low cost PBF has been successfully synthesized by us via a chain-extension of dihydroxylterminated PBF prepolymers with diisocyanate.3 The resulting PBF possesses various merits such as relative high melting point (Tm) (130−140 °C), good processability, and mechanical properties. Nonetheless, the application of PBF is restricted since its biodegradation rate is too slow to satisfy the requirement of practical application.3 During our previous work, amorphous poly(1,2-propylene succinate) has been introduced to enhance the biodegradability of PBF.5 Though the biodegradtion rate of PBF has been successfully improved, the tensile strength and the flexural strength deteriorate due to the reduced crystalline degree (Xc). On the other hand, PBS is a semicrystalline polymer, and it is regarded as one of the most important members in the family of aliphatic polyesters due to its good thermal properties, excellent processing properties, and favorable biodegradability.6−8 It can also be mentioned that the chemical structure and conformational structure of PBF is similar to that of PBS. Thus, the PBF segment and PBS segment are expected have some © 2013 American Chemical Society
compatibility with each other if the copolymer is synthesized, and the resulting copolymers are promising to possess combined thermal properties, mechanical properties, and biodegradation. Therefore, in this article, biodegradable copolymers composed of PBS and PBF are designed and synthesized. Nikolic et al. synthesized random copolymers of poly(butylene succinate-co-butylene fumarate) (PBSF) with BF units less than 20 mol % by melt copolycondensation and studied the thermal properties and biodegradability.9,10 Recently, Guo et al. also synthesized random PBSF with a much wider range of composition and revealed the strict isomorphism between BS and BF.4 Furthermore, they found that PBF and PBSF are highly efficient polymeric nucleating agents for PBS and its copolymers. However, the crystallization behavior, and other important physical properties such as mechanical properties and biodegradation properties for such copolymers, especially the block copolymers, with a wide range of composition have not been reported so far. The previous work of Wang’s group, our team, and others has proved that chain-extension with diisocyanate, which can take place under mild conditions, is a convenient and efficacious method for synthesizing high-molecular-weight aliphatic polyesters and multiblock copolyesters with good thermal and mechanical properties.11−19 In addition, our previous work has testified that free-radical cross-linking reactions and isomerization of CC on PBF can be prevented by polycondensation and chain-extension under mild conditions, and high-molecular-weight PBF homopolymer and Received: Revised: Accepted: Published: 6147
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multiblock copolymers based on PBS and poly(1,2-propylene succinate) have been successfully synthesized by this technique.3,5 Therefore, multiblock copolymers composed of PBS and PBF have been synthesized with the same strategy in this contribution to improve the biodegradation rate of PBF and expand the utility without sacrificing thermal properties and mechanical properties. Chemical structures, compatibility between the two segments, crystallization behaviors, and biodegradability of polymers in un-cross-linked form have been systematically investigated by 1H NMR, GPC, DSC, WAXD, mechanical testing and enzyme degradation in this article in detail. To the best of our knowledge, it is the first time such novel biodegradable multiblock copolyesters have been reported. The CC along with the main chains of copolymers provides various reactive sites for further modification to modify the physiochemical properties or to endow the polymers with special properties or for the further synthesis of comblike and brushlike polymers with advanced structures. For example, CC has been successfully be derived to reactive and hydrophilic hydroxyls.2
Scheme 1. Synthesis Routes of PBF-Diol (a), PBS-Diol (b) and Multiblock Copolymers (c)
2. EXPRIMENTAL SECTION 2.1. Materials. Fumaric acid (FA), 1,4-butanediol (1,4-BD), and hexamethylene diisocyanate (HDI) were purchased from Alfa Aesar (USA), BASF (Germany), and Bayer (Germany), respectively. Hydroquinone was obtained from Xilong Chemical Corp. (China). The lipase from Pseudomonas cepacia (activity: 37.8 unit/mg) used for enzymatic degradation was purchased from Sigma Aldrich. All the other reagents and solvents, analytical grade, were purchased from Beijing Chemical Reagents Corp. (China) and used without purification. 2.2. Synthesis of Dihydroxyl-Terminated PBF Prepolymer (PBF-Diol). PBF-diol was synthesized from FA and 1,4BD in bulk by a two-step process, i.e., esterification and polycondensation, according to Scheme 1a. Briefly, FA, 1,4-BD, zinc chloride, and hydroquinone were fed to a four-neck roundbottom flask in a 1:3:0.01:0.006 molar ratio. Zinc chloride was added as a catalyst while hydroquinone was introduced as a radical cross-linking inhibitor. The esterification was carried out at 150 °C under nitrogen atmosphere until a theoretical amount of water was separated. Subsequently, the pressure of the reaction system was gradually reduced to 5−15 Pa and maintained for 10 h to synthesize PBF-diol. The structure and molecular weight of PBF-diol were characterized by 1H NMR spectrum. 2.3. Synthesis of Dihydroxyl-Terminated PBS Prepolymer (PBS-Diol). PBS-diol was synthesized by a two-step process, i.e., esterification and polycondensation. The esterification of succinic acid (2 mol) and 1,4-BD (3.3 mol) was carried out at 180 °C under nitrogen atmosphere with Ti(OBt)4 (0.11 mL) as the catalyst until theoretical amount of water was separated. Then the temperature was raised to 230 °C for polycondensation. The pressure of the reaction system was gradually reduced to 5−15 Pa, and maintained for 2 h to synthesize PBS-diol. PBS-diol was characterized by 1H NMR spectrum. 2.4. Synthesis of Multiblock Copolymers and Homopolymers. The chain-extension reaction was accomplished in bulk under nitrogen atmosphere. Typically, a chain-extension reaction of PBF-diol (50 g) and PBS-diol (50 g) was carried out in a silicone oil bath at 150 °C under nitrogen atmosphere to
synthesize PBF-PBS50−50. After the polyester-diols were completely molten, HDI (3.51 g) was added to the reactor under mechanical stirring, and the chain-extension reaction was maintained for 1 h. All the polymers were purified by reprecipitation from their 1,1,2,2-tetrachloroethane (CHCl2CHCl2) solutions by methanol repeatedly, and dried for 12 h at 80 °C before the measurements of 1H NMR spectra and GPC. The synthesis procedures of prepolymers and multiblock copolymers are schematically illustrated in Scheme 1. The feed compositions of homopolymers and copolymers are shown in Table 1. 2.5. Nuclear Magnetic Resonance (NMR) Spectroscopy. The chemical structures of prepolymers and polymers were characterized by NMR spectrometer (Bruker DMX-400) at ambient temperature, using CDCl3 as solvent for PBS-diol as well as PBF-PBS0−100, and CDCl2CDCl2 for the remainder of the samples. 2.6. Gel Permeation Chromatography (GPC). The molecular weight and molecular weight distribution (PDI) were determined by GPC (Waters 1515) equipped with three Waters Styragel columns (HT5, HT4, and HT3) and a differential-refractometer detector. The measurements were taken at 45 °C. CHCl2CHCl2 was used as the eluent at a flow rate of 1.0 mL min−1. The number and weight average molecular weight was calculated by using a calibration curve with monodisperse polystyrene as standards. 2.7. Differential Scanning Calorimetry (DSC). DSC analysis was measured on a DSC Q2000 (Perkin-Elmer instrument) equipped with a CryoFill liquid nitrogen cooling system under N2 atmosphere. Samples were heated to 170 °C and maintained there for 5 min to erase any thermal histories, cooled to −65 °C at a rate of 200 °C min−1 using liquid nitrogen as a cooling agency, and held there for 5 min. After that, the samples were reheated to 170 °C at 20 °C min−1 and held there for 5 min before they were cooled to −65 °C at the 6148
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Table 1. Molecular Weight and Composition of Homopolymers and Multiblock Copolymers PBF (%)
a
PBS (%)
HDI (%)
sample PBF−PBS
Mn (104)
Mw (104)
PDI
feed
found
feed
found
feed
found
100−0 90−10 70−30 50−50 30−70 10−90 0−100
5.61 6.39 7.28 6.82 6.69 6.86 7.69
9.82 16.44 16.26 15.37 16.23 16.63 9.23
1.75 2.57 2.23 2.25 2.42 2.42 1.20
96.45 86.83 67.58 48.30 29.00 9.67 0
96.36 87.06 65.60 47.41 30.31/34.09a 9.38 0
0 9.65 28.96 48.30 67.67 87.07 96.78
0 9.50 30.53 48.82 66.56/62.38a 87.31 96.83
3.55 3.52 3.46 3.40 3.33 3.26 3.22
3.64 3.44 3.87 3.77 3.13/3.53a 3.31 3.17
Composition after degradation for 18 days.
same rate. Both cooling and heating scans were recorded for analysis. 2.8. Wide-Angle X-ray Diffraction (WAXD). WAXD was determined at room temperature with a Rigaku model D/max2B diffractometer using Cu Kα radiation (40 kV, 200 mA). The X-ray wavelength was 0.154 nm. The experimental data were collected from 5° to 40° at a scanning rate of 4° min−1. 2.9. Mechanical Properties. Tensile and flexural testing was conducted with a universal tester (Instron 1122, UK). The tensile strength and elongation at the break of the specimens were measured according to ISO 527 at a crosshead speed of 50 mm/min for dumbbell-shaped specimens with size of 75 mm × 4 mm × 2 mm (length × width × thickness). The flexural properties were examined according to ISO 178 at a crosshead speed of 2 mm/min for samples with size of 80 mm × 10 mm × 4 mm (length × width × thickness). At least five measurements were conducted for each sample, and the average value was reported. The specimens for these tests were all prepared on an injection molding machine (Haake Minijet, German) with an injection pressure of 850 bar. The hold time of pressure is 30 s. The mounding temperature is 45 °C. 2.10. Enzymatic Degradation. Film samples of polymers (10 mg, 10 mm × 10 mm × 0.1 mm) were immersed in small conical flasks containing phosphate buffer solution (pH 6.86) that were kept in a controlled incubator at 45 °C. The media was exchanged every 24 h. The concentration of enzyme in the solution was 20 unit/mL and the enzymatic concentration for the polymer was 5 unit/mg. The media was refreshed every 24 h. Each specimen was taken out at predetermined degradation time intervals, washed with double distilled water, dried at 45 °C in vacuum for 3 h and weighed. The weight loss (%) was calculated from [100(W0 − Wt)]/W0, where W0 was the initial sample weight and Wt was the weight of dried residual specimen after degradation. Three paralleled experiments were performed for each degradation test, and then the average value was reported.
ppm (δH7) and 1.79 ppm (δH8) for methylene protons of 1,4BD residue in PBF repeating units; 6.85 ppm (δH6) for the protons of unsaturated −CHCH− with E-configuration on the PBF repeating units; 3.6−3.7 ppm (δH2) and 1.66 ppm (δH5) for protons on terminal methylene groups. Other characteristic peaks for −CHCH− protons with Zconfiguration20 at around 6.31 ppm and protons from the Ordelt saturation reaction21 of −CHCH− at around 2.9−3.1 ppm cannot be detected in the 1H NMR spectra of PBF-diol and PBF-PBS50−50. It thus confirms that no appreciable side reactions such as isomerization of CC configuration or Ordelt saturation reaction happened during the whole melt polymerization process. Thus, Mn of PBF-diol can be calculated from 1H NMR spectrum by the following equation:
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of PBF-Diol. Dihydroxyl-terminated PBF-diol with number-average molecular weight (Mn) of 5552 was synthesized by the esterification and polycondensation of fumaric acid and 1,4-butanediol with a molar ratio of 1/3. Zinc chloride and hydroquinone were used as catalyst and radical cross-linking inhibitor, respectively (Scheme 1a). The chemical structure and M n of PBF-diol were characterized by 1H NMR spectrum. Since PBF-diol is insoluble in CDCl3, CDCl2CDCl2 was used as the solvent for 1H NMR measurement, and the result is shown in Figure 1a. 1H NMR spectrum of PBF-diol exhibits all the characteristic peaks: 4.24
(2)
Mn(PBF‐diol) = 90 +
A4.24 170 A3.6 − 3.7
(1)
where A4.24 and A3.5−3.7 designate the integral areas of internal and terminal methylene groups and 170 and 90 represent the molecular weights of PBF repeating units and PBF-diol end chains. 3.2. Synthesis and Characterization of PBS-Diol. Dihydroxyl-terminated PBS-diol (Mn 5023) was synthesized by esterification and successive polycondensation of succinic acid and excess 1,4-butanediol (Scheme 1b) as described in the Experimental Section. The chemical structure and Mn of PBSdiol was characterized by 1H NMR spectrum (Figure 1b). The strong peaks located at 4.11 ppm (δH1), 2.60 ppm (δH3), and 1.70 ppm (δH4) are attributed to the protons on PBS repeating units. The weak peaks occurring at 3.5−3.7 ppm (δH2) are assigned to the protons on terminal methylene groups. Integral areas and proton numbers could be employed to calculate Mn of PBS-diol according to the following equation: Mn(PBS‐diol) = 90 +
A4.11 172 A3.5 − 3.7
where A4.11 and A3.5−3.7 are the integral areas of protons on internal and terminal methylene groups and 172 and 90 are the molecular weights of PBS repeating units and PBS-diol end chains. 3.3. Synthesis and Characterization of Copolymers. Copolymer PBF-PBS50−50 with equal composition of PBF and PBS was used as the representative for the characterization of the copolymer. As expected, the 1H NMR spectrum of PBFPBS50−50 (Figure 1c) demonstrates all the characteristics peaks for PBS and PBF repeating units. On the other hand, the peaks located at 3.5−3.7 ppm (δH2) in Figure 1a and Figure 1b, arising form terminal methylene protons of PBF-diol and PBS6149
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(δH11), attributed to the methylene protons of HDI residue in the structure of copolymer molecule, can be observed in Figure 1c. Therefore, the structure of copolymers can be confirmed. Weight compositions of polymers could be deduced from the relationship between peak area and proton number by the following equations: PBF (%) = PBS (%) =
170A4.24
170A4.24 + 172A4.11 + 170A3.14
(3)
170A4.24
172A4.11 + 172A4.11 + 170A3.14
(4)
HDI = 1 − PBF (%) − PBS (%)
(5)
where A4.24 and A4.11 represent peak areas of the corresponding protons, and the numerical values of 170, 172, and 170 are the molecular weights of PBF repeating units, PBS repeating units, and HDI residue in the polymer structure, respectively. The compositions of polymers are listed in Table 1. It can be found that the feed composition and found composition are very close to each other. Therefore, the composition of copolymers can be conveniently controlled and adjusted by changing the feed ratio. The molecular weights and PDI of chain-extended polymers were determined by GPC. The results are listed in Table 1. Evidently, the molecular weight of any chain-extended polymer is much higher than that of the PBF-diol or PBS-diol, suggesting that the chain-extension is effective to achieve polymers with high molecular weight. The high molecular weight is favorable to the mechanical properties of the polymers, which is one of the most important properties for the material application. Therefore, it can be reasonably concluded that multiblock copolymers with high molecular weight have been successfully synthesized via chain-extension of PBF-diol and PBS-diol on the basis of 1H NMR and GPC results. The reactive CC with trans-conformation on PBF has been reserved during the whole melt polymerization. It offers many advantages such as good thermal properties, thermoplasticity and mechanical properties. Furthermore, reactive CC on PBF segment offers the possibility for further modification to modulate the physiochemical properties or to endow specific properties for the copolymers. 3.4. Thermal Transition and Cocrystallization Behaviors. Thermal transition and crystallization behaviors of copolymers were examined by DSC and the corresponding DSC thermograms are given in Figure 2. The fundamental thermal parameters derived from DSC curves are summarized in Table 2. It can be observed from Figure 2a that the glass transition temperature (Tg) of PBF homopolymer (PBFPBS100−0) and PBS homopolymer (PBF-PBS0−100) is unclear, arising from their high crystallinity which will be discussed later on. But Tg of either polymer can be distinguished from the enlargement of the heating curves (see Figure 2b). As shown in Figure 2b and Table 2, PBF shows a Tg around −4 °C, and PBS shows a lower Tg around −33 °C. It is well-known that the higher the rigidity of a polymer chains is, the higher the Tg tends to be. The rigidity of PBF polymer chains is higher than that of PBS due to the presence of rigid CC. As far as multiblock copolymers are regarded, they all show a single Tg, which lies between Tg of PBF and PBS homopolymer and tends to shift to lower temperature region with increasing PBS content. It can be interpreted as that the
Figure 1. 1H NMR of PBF-diol (a), PBS-diol, (b) and PBF-PBS50−50 (c).
diol, completely disappear in the spectrum of PBF-PBS50−50 (see Figure 1c), implying the complete consumption of hydroxyl groups from the prepolymers. Furthermore, new peaks at 3.14 ppm (δH9), 1.50 ppm (δH10), and 1.32 ppm 6150
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Figure 2. DSC heating curves (a), enlargement of heating curves for the glass transition region (b), and cooling curves (c) of polymers. The dash line in Figure 2b indicates the trend of the Tg.
Table 2. Thermal Properties of Homopolymers and Multiblock Copolymers
a
sample PBF−PBS
Tg (°C)
Tc (°C)
Tm (°C)
-ΔHc (J/g)
ΔHm (J/g)
Xca (%)
100−0 90−10 70−30 50−50 30−70 10−90 0−100
−4.5 −14.0 −26.3 −26.2 −28.6/−27.4b −29.9 −32.9
77.0 72.5 71.9 69.7 67.3/74.8b 67.0 63.4
133.2 129.9 130.2 126.9 120.7/128.1b 108.4 107.0
53.3 50.7 51.2 52.3 53.9/67.0b 49.0 55.1
50.3 54.1 52.9 53.6 49.4/52.1b 50.2 48.8
43.1 42.6 43.5 42.1 42.8 43.3 43.7
Determined by WAXD. bThermal parameters after degradation for 18 days.
and linear PBF with trans-conformation bearing reactive CC is a crystalline polymer. PBS homopolymer (PBF-PBS0−100) is also a crystallizable polymer with a lower Tm around 107 °C and crystallization temperature (Tc) around 63 °C. As compared with PBS, the higher Tm of PBF can be interpreted from a thermodynamic concept. Tm is determined by enthalpy of fusion (ΔHm) and change of entropy. As indicated in Table 2, ΔHm of the two polymers is similar to each other because of the similar chemical structure. Thus, the difference in Tm may lie in the change of entropy, which is reversely proportionate to the rigidity of polymer chains. Because of the presence of carbon−carbon double bonds in PBF, the chain rigidity of PBF is higher than PBS, which might be the major reason for the higher Tm of PBF.
introduction of the PBS segment increases the macromolecularchain flexibility by reducing the content of CC content. Since the difference between the Tg of PBS and PBF is larger than 20 °C, Tg can be adopted to evaluate the compatibility between the two segments in the amorphous region. Therefore, it can be safely concluded that the PBF segment is compatible with the PBS segment in the amorphous region irrespective of the composition, which is correlated with the structural similarity. The good compatibility may be favorable for the cocrystallization of the two segments, which will be discussed later on. It can be found that PBF homopolymer (PBF-PBS100−0) shows a sharp endothermic peak around 133 °C during the heating scan and an exothermic peak around 77 °C during the cooling scan, corresponding to the melting and crystallization of PBF crystals, respectively. It suggests that the un-cross-linked 6151
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The crystallization behaviors of the copolymers have been studied in the present work. Generally, crystallization of double crystalline copolymers can be classified into three types: separate crystallization, cocrystallization, and rich-composition crystallization. Cocrystallization and rich-composition crystallization can take place for both random and block copolymers. If the two crystallizable units are incompatible in each crystal lattice, rich-composition crystallization occurs, and the Tm, Tc, and Xc decrease with increasing content of the minor component. It is the most common phenomenon that happens for most of the random copolymers.22,23 If the two crystallizable units are compatible, cocrystallization can take place. Separate crystallization only occurs for block copolymers, and two Tm and two Tc will be distinguished, corresponding to the separate fusion and crystallization of the two crystalline segments. It only happens when the difference between Tm of the two crystalline segments is large enough and the two crystallizable units are incompatible. The separate crystallization behavior has been observed for the PBS and polycaprolactone based multiblock copolymers during our previous work.24 As far as the copolymers are concerned, all the multiblock copolymers show a single Tm (in Figure 2 and Table 2), which lies between Tm of the two homopolymers. Furthermore, it should be mentioned that the copolymers in all compositions show high-level and almost invariable ΔHm and enthalpy of crystallization (ΔHc), which are indicative of Xc. On the basis of the above discussion, it can be hypothesized that the crystallization of the copolymers should also be isodimorphic cocrystallization. In fact, isodimorphic cocrystallization between butylene succinate and butylene fumarate has been revealed by Guo’s groups and supported by two-dimensional WAXD4. Single Tm, which lies between Tm of the two homopolymers, almost invariable ΔHm, and ΔHc are originated from the cocrystallization between butylene succinate and butylene fumarate. The cocrystallization behavior can be interpreted in terms of the compatibility of the two crystals. It has been testified that the lattice parameters of the two polymers are very similar to each other4,25−27 due to their similar chemical and conformational structure. Therefore, the excess free energy of cocrystallization is very small, and the two crystals are well compatible with each other. As a result, only one crystalline phase is formed and isodimorphic cocrystallization takes place.28 The isomorphic cocrystallization will influence the mechanical properties of the copolymers, which will be discussed subsequently. WAXD was also performed to study the crystal structure and calculate the crystallinity degree (Xc). As shown in Figure 3. PBF homopolymer and PBS homopolymer show similar crystal diffraction patterns with four peaks at around 19°, 21°, 23°, and 29°. Since the lattice parameters of PBS and PBF crystals have been determined,4,27 these four peaks of PBF (or PBS) can be ascribed to (020), (021), (110), and (111) planes of monoclinic PBF crystals (or PBS), respectively. All the multiblock copolymers show similar crystal diffraction patterns to PBF homopolymer and PBS homopolymer (PBF-PBS0− 100), except that the peak around 21° becomes less pronounced with increasing PBS content. The above results confirm the isomorphic cocrystallization between PBF and PBS in the crystalline region. It is derived from the fact that the two polymers have similar chemical structure and conformation. It is well-known that Xc is a key parameter for a crystalline polymer that significantly affects mechanical properties. In the
Figure 3. WAXD patterns of homopolymers and multiblock copolymers.
present work, the Xc of the multiblock copolymers was determined from the WAXD pattern according to the ref 29 and the results are given in Table 2. Evidently, Xc of the chainextended polymers varies little with the composition, which is in good agreement with the almost invariable ΔHm (Table 2). 3.5. Mechanical Properties. Since our working objective is to synthesize novel unsaturated aliphatic and block copolymers with good comprehensive properties, which can be used directly as material or be postfunctionalized to modulate physiochemical properties or to endow the polymers with novel properties, processability and mechanical properties are ones in which we are most interested. The mechanical properties of chain-extended PBF have been evaluated in terms of tensile properties and flexural properties in this contribution. As expected, all the five copolymers show satisfactory processability during injection molding of the specimens for mechanical properties testing. And the resulting specimens are white with smooth surface. It is originated from the uncross-linked and linear nature of multiblock copolymers. The stress−strain curves have been given in Figure 4, and the results are summarized in Table 3. To our best knowledge, it is the first time the mechanical properties of PBS and PBF based copolymers have been reported. As indicated in Figure 4 and Table 3, PBS has a tensile strength up to 54.3 MPa, which is much higher than the usual value between 30 and 40 MPa. It is resulted from the strain hardening and orientation of polymer chains in the microcrystals before a break as indicated in Figure 4, which is related with the sample preparation method of injection mounding. The tensile strength of PBF homopolymer (PBF-PBS100−0) is lower than that of the PBS homopolymer (PBF-PBS0−100). Flexural strength and flexural modulus of the two polymers are very close to each other. It is derived from similar chemical, conformational and crystal structure. And it is amusing to find that the copolymers have good mechanical properties close to PBS and PBF. Tensile strength of the copolymers tends to increase with increasing PBS content. The elongation at break, ranging from 120% and 156%, does not vary significantly with the composition. As far as the flexural properties are concerned, including flexural strength and flexural modulus, they hardly change with composition. Therefore, it can be concluded that the copolymers possess good and stable mechanical properties. 6152
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Figure 4. Tensile stress−strain curves of chain-extended polymers with different composition.
Figure 5. Plots of weight loss versus degradation time during enzymatic degradation at 50 °C and pH 6.86.
Table 3. Mechanical Properties of Homopolymers and Multiblock Copolymers sample PBF-PBS 100−0 90−10 70−30 50−50 30−70 10−90 0−100
tensile strength (MPa) 39.6 41.1 42.0 45.2 49.9 51.6 54.3
± ± ± ± ± ± ±
3.0 4.2 1.9 0.6 2.3 5.6 3.9
elongation at break (%) 140 129 138 156 125 120 105
± ± ± ± ± ± ±
17 9 4 28 18 21 15
flexural strength (MPa) 26.7 26.7 26.3 25.9 25.7 26.7 26.8
± ± ± ± ± ± ±
0.2 0.9 1.0 0.9 0.7 1.9 1.7
substantially affected by the composition and increases remarkably with the increase of PBS content, especially when the PBS content is around or higher than 50 wt %. Therefore, it can be concluded that the introduction of the PBS segment indeed enhances the biodegradation rate as expected. Similar results have been observed for the random PBSF with BF less than 20 mol % by Nikolic et al.9 Since the crystallinity degree of the copolymers varies little with composition (see Table 2), the difference in the biodegradation should be originated from molecular mobility, Tm and Tg. It is well regarded that the molecular mobility, which determines the rigidity of the polymer chain, plays a significant role in biodegradation. Aromatic polyesters with rigid benzene rings such as poly(ethylene terephthalate) and poly(butylene terephthalate) are very resistant to fungi or bacteria, and hard to degrade in a natural environment. Previous work of our group also revealed that the introduction of an aromatic poly(1,2-propylene terephthalate) segment substantially decreased the biodegradation rate of PBS.17 As far as the multiblock copolymers are concerned, the incorporation of the PBS segment reduces the content of rigid carbon−carbon double bonds, and thus decreases the rigidity of macromolecular chains.9 Subsequently, Tm and Tg decrease with increasing PBS relative amount. Furthermore, the regular sequential structure of the block copolymers and regular conformational structure of the PBF segment further depress the degradation rate for PBF-rich samples. It has been reported that the random copolymer degraded 100 times faster than the corresponding block one.34 Though the PBF-rich samples biodegrade slowly, the degradation rate for block copolymers with PBS higher than 30% is not so slow, weight loss of which is above 6 wt % upon 18 days of degradation. To study the degradation mechanism, the compositions of PBF-PBS30−70 after enzymatic degradation for 18 days was determined from the 1H NMR spectrum according to eqs 3−5, and the results are given in Table 1. It can be found that PBS content decreases and PBF content increases upon degradation. Therefore, it can be concluded that PBS segment preferentially degrades. However, the value of the reduction of PBS content is less than that of weight loss, suggesting that both PBF and PBS segments take part in the enzymatic degradation. The weight loss of PBF-PBS10−90 upon 18 days degradation exceeds 97
flexural modulus (MPa) 531 542 515 529 508 549 552
± ± ± ± ± ± ±
2 7 24 32 19 21 16
As stated above, the favorable mechanical properties are due to the high molecular weight of the copolymers and regular conformational structure of PBF segment; while the stable mechanical properties are probably because of the very little in change of unit cell parameters originated from isomorphic cocrystallization. 3.6. Enzymatic Degradation of Multiblock Copolymers. To evaluate the biodegradability of the copolymers in a short time scale, enzymatic degradation was performed. It is generally regarded that the biodegradation rates of aliphatic polyesters are significantly affected by many factors, such as chemical structure of repeating unit, hydrophilic−hydrophobic balance, molecular weight, molecular mobility, thermal characteristics, solid-state morphology, and so on.30,31 Especially, the biodegradation rate of the amorphous region is much faster than that of crystal zone, and it has been confirmed that amorphous regions preferentially degrades versus the crystalline regions for copolymers based on PBF and PBS.5,32 DSC for PBF-PBS30−70 after degradation for 18 days was conducted to investigate the degradation mechanism. The ΔHm and ΔHc, indicative of crystallinity degree, in Table 2, increase after degradation. Therefore, it is regarded that the amorphous region preferentially degrades. Moreover, the higher Tm and Tg of a polymer are, the lower its biodegradability tends to be.33 For a copolymer, the final biodegradability also depends on the kind, relative amount, and distribution of the comonomeric units along the polymer chain. Indeed, as shown in Figure 5, PBS degrades much faster than PBF, and the degradation rate of the multiblock copolymer is 6153
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acknowledged for his helpful discussion on the thermal transition, cocrystallization, and mechanical properties.
wt %, which is evidently higher than the PBS content. Therefore, it further confirms the fact that though PBS segment preferentially degrades, both PBS and PBF segment can be degraded by the enzymes. Otherwise, the weight loss of PBF-PBS10−90 should be less than 90 wt %. Furthermore, the weight loss curve of PBF-PBS 30−70 sample shows two slopes. It indicates different degradation rates of two stages, first the faster degradation rate, and later the slower degradation rate. As discussed above, PBS segment preferentially degrades and PBS content decreases with the progress of degradation. Furthermore, the amorphous region preferentially degrades and crystallinity degree increases upon degradation. The first-step degradation rate is faster as compared with the second-step degradation rate, originating from its higher PBS content and lower crystallinity degree. Concisely put, as compared with crystalline regions, amorphous regions preferentially degrade; the PBS segment preferential degrades versus the PBF segment; PBS enhances the degradation rate of the copolymers.
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4. CONCLUSIONS In summary, a series of biodegradable unsaturated multiblock copolymers based on PBF and PBS have been successfully synthesized by chain extension. NMR results confirm that no isomerization of CC configuration or Ordelt reaction of C C took place during the whole melt polymerization process. DSC and WAXD results show that the two segments are compatible in both the amorphous and crystalline region, and isomorphic cocrystallization between PBF and PBS occurs in the crystalline region. The copolymers show good thermal properties, processability, and mechanical properties. The degradation rate of PBF is effectively improved by the incorporation of the PBS segment, especially when the PBS content is around or higher than 50 wt %. These merits may render the novel copolymer to find applications in the area of biodegradable materials. It is the first time there is a report of the mechanical properties and biodegradability of PBF and PBS based copolymers with a wide range of composition, and the first time isomorphic cocrystallization in block copolymers was found. Our work offers a deep understanding of the structure− property relationship of these unsaturated copolymers, which is valuable for the design and synthesis of multiblock copolymers with adjustable biodegradability and desirable thermal and mechanical properties. Furthermore, the reactive CC on the main chain of the copolymers offers various possibilities to adjust the physicochemical properties or to endow the copolymers with specific properties by further functionalization.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-10-6256-2292. Fax: +86-10-6256-2292. E-mail address:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from National Science Fund of China (Grant No. 21104087) and Cultivation Project of Institute of Chemistry Chinese Academy of Science (ICCAS) (Grant No. CMS-PY-201239) is gratefully acknowledged. Dr. Guoming Liu (Institute of Chemistry Chinese Academy of Science) is also 6154
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