Article pubs.acs.org/IECR
Novel Unsaturated Aliphatic Polyesters: Synthesis, Characterization, and Properties of Multiblock Copolymers Composing of Poly(Butylene Fumarate) and Poly(1,2-Propylene Succinate) Liuchun Zheng,† Zhaodong Wang,†,‡ 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 various disadvantages such as inadequate thermomechanical properties, high cost, and lack of reactive sites for further modification. Poly(butylene fumarate) (PBF), as a novel and low-cost aliphatic polyester, possesses good physical properties and reactive double bonds along with the polymer backbones which can be potentially modified to endow the polyester with specific properties. However, the biodegradation rate of PBF is too slow to meet the requirement of practical application. In this contribution, amorphous poly(1,2-propylene succinate) (PPS) was copolymerized with PBF to decrease the crystallinity (Xc) and accelerate the biodegradability of PBF via chain-extension with diisocyanate. The chemical structures of the copolymers were confirmed by 1H NMR spectra and gel permeation chromatography (GPC). The crystal structure and physicochemical properties were investigated by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), mechanical testing, and enzymatic degradation in detail. Both the impact strength and degradation rate of the copolymers are effectively improved by the introduction of amorphous PPS; while the melting point (Tm) and crystallization temperature (Tc) are hardly reduced by the incorporation of PPS. The novel polymer may be directed used as low-cost biodegradable materials or be further functionalized with special properties for particular applications. previous work.14,15 In the present work, the amorphous PPS segment was chosen as the soft segment to decrease the Xc and accelerate the biodegradation of PBF. The previous work of Wang’s group, our team, and others has testified that polyester based multiblock copolymers, which can be synthesized by chain-extension with diisocyanate at low cost, have versatile merits, including regular sequential structure, high molecular weight, good thermomechanical properties, and convenient control over chemical structures and fundamental properties.16−20 Another fastinating advantage of this synthetic techinique is that no cross-linking reaction of CC derived from fumaric acid happened during chain extension, and high-molecular-weight PBF with linear structure has been successfully synthesized by this technique. Therefore, multiblock copolymers composed of PBF and PPS have been designed and synthesized by the same strategy in present work to enhance the biodegradation rate of PBF. Chemical structures, crystal structures, thermal properties, mechanical properties, and biodegradability of un-cross-linked multiblock copolymers have been systematically studied by 1H NMR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), mechanical testing, and enzyme degradation in detail. Up to now, this kind of biodegradable copolyester has not been reported to the best of our knowledge. The reactive
1. INTRODUCTION In recent decades, given the urgent need to solve serious waste environmental pollution associated with the traditional nondegradable polymers, biodegradable aliphatic polyesters have attracted considerable interest. They are regarded as the most economically competitive and commercially successfully biodegradable materials. Unfortunately, as compared with the traditional plastic, the widespread use of these biodegradable polyesters still suffers from the drawbacks including relatively poor thermomechanical properties and higher cost.1−3 In particular, aliphatic polyesters are always highly hydrophobic, and lack of desirable hydrophilicity and reactive sites for further modification or functionalization, which considerably hamper their use for specific applications.4−7 Therefore, development of functional polyesters with satisfactory thermomechanical properties is one of the most challenging and valuable topics. PBF, as a novel unsaturated aliphatic polyester developed by Guo’s group8 and us9 recently, has the charming carbon− carbon double bonds along with polymer main chains. Double bonds on PBF polymer chains offer various possibilities. For example, double bonds have the advantages of being promisingly derived and postfunctionalized to endow the polyester with specific properties or directly for cross-linking. They also have the potential for the synthesis of comblike and brushlike polymers with advanced structures.10−13 Regretfully, the application of PBF is greatly restricted by its slow biodegradation rate. PPS is an amorphous polyester, which can be synthesized at low cost and has already been successfully used to enhance the impact strength of poly(butylene succinate) (PBS) during our © 2012 American Chemical Society
Received: Revised: Accepted: Published: 14107
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2.3. Synthesis of Dihydroxyl-Terminated PPS Prepolymer (PPS-diol). The synthesis procedure of PPS-diol, as described in Scheme1b, was reported in our previous paper.14 2.4. Synthesis of Multiblock Copolymers and Homopolymers. Chain-extension reaction was conducted in bulk under nitrogen atmosphere and the molar ratio of diisocyanate/ polyester-diols was set to be 1.1/1. Typically, a chain-extension reaction of PBF-diol (70 g) and PPS-diol (30 g) was carried out in a silicone oil bath at 150 °C under nitrogen atmosphere to synthesize PBF-PPS70-30. After the polyester-diols were completely molten, HDI (3.49 g) was added to the reactor under mechanical stirring. And it was maintained for 1 h to complete the chain-extension reaction. 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 procedure of multiblock copolymers is schematically illustrated in Scheme 1c. 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 investigated by NMR spectrometer (Bruker DMX-400) at room temperature, using CDCl3 as the solvent for PPS-diol, and CDCl2CDCl2 for the rest 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. And 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, and then cooled to −70 °C at a rate of 200 °C min−1 using liquid nitrogen as 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 −70 °C at the 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 Ragaku Model D/max2B diffractometer using Cu Kα radiation (40 kV, 200 mA), and the experimental data were collected from 5° to 40° at a scanning rate of 4° min−1. 2.9. Mechanical Properties. The notched izod impact strength of the polymer samples was measured with an impact testing machine (CSI-137C, USA) according to ISO 180.
carbon−carbon double bonds along with the backbones of copolymers provide a wide range of possibilities for further modification or cross-linking to meet a variety of requirements.
2. EXPERIMENTAL SECTION 2.1. Materials. Fumaric acid (FA), 1,4-butanediol (1,4-BD), and hexamethylene diisocyanate (HDI) were purchased from Alfa Aesar (USA), BASF (German), and Bayer (German), 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 (USA). 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 esterification and polycondensation, according to Scheme 1a. Briefly, FA, 1,4-BD, zinc chloride, and hydroScheme 1. Synthesis Routes of Polyester-Diols and Multiblock Copolymers
quinone were fed to a four-neck round-bottom 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 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.
Table 1. Molecular Weight and Composition of Homopolymers and Multiblock Copolymers PBF (%) 4
4
PPS (%)
HDI (%)
sample
Mn (10 )
Mw (10 )
PDI
feed
found
feed
found
feed
found
PBF-PPS100-0 PBF-PPS95-5 PBF-PPS90-10 PBF-PPS80-20 PBF-PPS70-30
5.02 5.34 5.72 6.63 5.89
10.9 10.4 12.3 13.7 13.4
2.16 1.95 2.14 2.07 1.72
96.76 91.89 87.04 77.34 67.65
96.69 91.92 87.14 76.84 68.25
0 4.84 9.67 19.33 28.99
0 4.76 9.51 19.88 28.33
3.24 3.27 3.29 3.33 3.36
3.31 3.32 3.35 3.28 3.42
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Tensile and flexural testing was conducted with a universal tester (Instron 1122, UK). The tensile strength and elongation at break of the specimens were measured according to ISO 527 at a crosshead speed of 50 mm/min for dumbbell-shaped specimens (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. The dimensions of specimens for the impact and flexural testing were 80 mm × 10 mm × 4 mm (length × width × thickness). The reported result was the average value from measurements of at least five specimens. The specimens for these tests were all prepared on an injection molding machine (Haake Minijet, German). 2.10. Enzymatic Degradation. Film samples of polymers (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 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.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of PBF-diol. Dihydroxyl-terminated PBF-diol was synthesized from fumaric acid and 1,4-butanediol at a molar ratio of 1/3 to increase the content of terminal hydroxyl by a two-step reaction of esterification and polycondensation as described in the Experimental Section, with zinc chloride and hydroquinone as the catalyst and radical cross-linking inhibitor, respectively (Scheme 1a). The chemical structure and molecular weight of PBF-diol were characterized by the 1H NMR spectrum. CDCl2CDCl2 was used as the solvent for 1H NMR measurement of PBF-diol because it cannot be completely soluble in CDCl3, and the resulting 1H NMR spectrum is shown in Figure 1a. It can be observed that 1H NMR spectrum only displays the characteristic peaks of PBF-diol: 6.85 ppm (δH1) for the protons on CHCH with E-configuration (trans CHCH) for FA residue in PBF repeating units; 4.24 ppm (δH2) and 1.79 ppm (δH4) for methylene protons of 1,4-BD residue in PBF repeating units; 3.5−3.7 ppm (δH3) and 1.66 ppm (δH5) for terminal methylene groups. Other characteristic signals for CHCH protons with Z-configuration21 (cis CH CH) at around 6.31 ppm and protons from the Ordelt saturation reaction22 of CHCH at around 2.9−3.1 ppm cannot be detected in the 1H NMR spectra of PBF-diol and PBF-PPS70-30. Therefore, it can be unequivocally concluded that that no appreciable isomerization of CC configuration or Ordelt saturation reaction took place during the whole melt polymerization process. Thus, number-average molecular weight (Mn) of PBF-diol can be calculated from 1H NMR spectrum by the following equation: M n(PBF‐diol) = 90 +
A4.24 × 170 A3.6 − 3.7
Figure 1. 1H NMR of (a) PBF-diol, (b) PPS-diol, and (c) PBF-PPS7030.
where A4.24 and A3.5−3.7 designate the integral areas of internal and terminal methylene protons, and 170 and 90 represent the molecular weights of PBF repeating units and PBF-diol end chains. The Mn of PBF-diol is found to be 5518.
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Figure 2. DSC curves of homopolymer and multiblock copolymers: (a) heating scan, (b) cooling scan.
3.2. Synthesis and Characterization of PPS-diol. Dihydroxyl-terminated PPS-diol was synthesized by esterification and successive polycondensation of succinic acid and excess 1,2-propanediol (Scheme 1b). The chemical structure of PPS-diol was confirmed by 1H NMR spectrum (Figure 1b). 1H NMR spectrum of PPS-diol reveals all the characteristic signals of protons on PPS repeating units (5.13 (δH6), 4.11 (δH7), 2.62 (δH8), and 1.23 ppm (δH9)) and protons on terminal methylene groups (3.5−3.7 ppm (δH3)). Integral areas and proton numbers are employed to deduce Mn of PPS-diol according to the following equation: M n(PPS‐diol) = 76 +
A4.11 × 158 A3.5 − 3.7
PBF(%) =
170 × A4.24 170 × A4.24 + 158 × A4.11 + 170 × A3.14 (3)
PPS(%) =
158 × A4.11 170 × A4.24 + 158 × A4.11 + 170 × A3.14 (4)
HDI = 1 − PBF(%) − PPS(%)
(5)
where A4.24 and A4.11 mean peak areas of the corresponding protons, and the numerical values of 170, 158, and 170 designate the molecular weights of PBF repeating units, PPS repeating units, and HDI residue in the urethane groups of polymer structure, respectively. The compositions of polymers are given in Table 1. Evidently, the found compositions of each sample are very close to the feed compositions. Therefore, the multiblock copolymers have the favorable merit of convenient controllability over the structure, compositions, and subsequent properties of copolymers by simply varying the feed ratio. The molecular weights and PDI of PBF homopolymer and multiblock copolymers were measured by means of GPC, and the corresponding data are summarized in Table 1. It can be observed that the weight-average molecular weight (Mw) of any copolymer is higher than 10 × 104 and high-molecular-weight copolymers have been synthesized. Combined with the 1H NMR analyses, we can reach a conclusion that the copolymers have been successfully synthesized and chain-extension is very effective for the synthesis of high-molecular-weight copolyesters. It is well established that the molecular weight, which is a very significant parameter for polymers, determines the mechanical properties and the potential application range of material. Thus, the copolymers with high molecular weight will be promising to have better mechanical properties and prone to find some applications as a biodegradable material. 3.4. Melting and Crystallization Behaviors of Multiblock Copolymers and Homopolymer. DSC was employed to study the melting and cystallization behaviors of copolymers. The relevant DSC results are shown in Figure 2 and Table 2. As indicated in Figure 2a, all the PBF homopolymer and the copolymers show very strong melting peaks during the second heating scans, suggesting that the polymers are semicrystalline polymers. As indicated in Figure 2a and Table 2, the position of
(2)
where A4.11 and A3.5−3.7 are the integral areas of protons on internal and terminal methylene groups, and 158 and 76 are the molecular weights of PPS repeating units and PPS-diol end chains. The Mn of synthesized PPS-diol is calculated to be 4868. 3.3. Synthesis and Characterization of Copolymers. Multiblock copolymers have been synthesized by chainextension of PBF-diol and PPS-diol at 150 °C in bulk with HDI, as described in Scheme 1c. Copolymer PBF-PPS70-30 with highest content of PPS was given as the representative for the characterization of the copolymer. Evidently, the 1H NMR spectrum of PBF-PPS70-30 demonstrates all the characteristic signals belonging to PBF and PPS repeating units. At the mean time, the characteristic signals at 3.5−3.7 ppm (δH3), arising from the terminal methylene protons of PBF-diol and PPS-diol, disappear thoroughly. It is due to the fact that all the hydroxyl groups from the prepolymers have been reacted completely with isocyanate groups and converted to urethane groups. Consequently, new signals of the methylene protons on HDI residue in urethane groups of copolymer molecule appear after chain-extension, which can be distinguishable in the enlargement of 1H NMR spectrum of PBF-PPS70-30 (3.14 (δH10), 1.50 (δH11) and 1.32 ppm (δH12)). Thus, it can be undoubtedly concluded that the multiblock copolymers composed of PBF and PPS have been successfully synthesized. The weight compositions of polymers, including the weight fraction of PBF, PPS, and HDI can be calculated from the peak areas and proton numbers according to the following equations: 14110
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Table 2. Thermal Properties of Homopolymers and Multiblock Copolymers
a
sample
Tg (°C)
Tc (°C)
Tm (°C)
−ΔHc (J/g)
ΔHm (J/g)
Xca (%)
PBF-PPS100-0 PBF-PPS95-5 PBF-PPS90-10 PBF-PPS80-20 PBF-PPS70-30
−10.4 −10.1 −9.0 −8.3 −5.7
78.6 79.3 80.0 76.7 77.2
129.8 128.6 132.2 128.0 130.8
52.6 47.8 48.4 39.3 33.8
51.3 46.7 45.3 37.3 32.2
45.6 43.8 41.3 36.9 31.2
Determined by WAXD.
the melting peak and the value of Tm vary little with increasing PPS content. Generally, Tm is correlated with crystal size or perfectness. The slight change in the value of Tm with composition indicates that the crystal size or perfectness of PBF is not greatly affected by the introduction of amorphous PPS segment. Similar results have been obtained on the multiblock copolymers during our previous work.14,15 It is ascribed to the fact the melting point of crystallizable segment in a block copolymer is mainly determined by the average sequential length of the crystallizable segment, thus almost independent of the composition. Since Tm is one the most significant thermal parameters, which determines the upperlimit temperature for material application, the multiblock copolymers with relatively higher Tm tend to find more applications. At the mean time, all the multiblock copolymers show single crystallization temperature (Tc). Similar to the trend of the Tm, the value of Tc varies little with the composition in the investigated range. On the other hand, both the values for enthalpy of fusion (ΔHm) and crystallization (ΔHc) reduce gradually with increasing PPS content. Since the values of ΔHm and ΔHc are closely associated with Xc, the reduced values of ΔHm and ΔH c is attributed to the decrease in composition of crystallizable fraction with the increasing amorphous PPS content. Generally, Xc has considerable influence on material mechanical properties, which will be discussed later on in this contribution. In short, it can be deduced from DSC results that the introduction of PPS hardly decreases the Tm and Tc but reduces the Xc. 3.5. Wide-Angle X-ray Diffraction (WAXD). WAXD was carried out to investigate the effect of PPS segment on the crystal structure of PBF, and the resulting WAXD profiles have been shown in Figure 3. It has been reported that both the crystal structure and cell parameters of PBF are almost the same to that of PBS since the chemical structure, volume, and trans configuration of butylene fumarate are very close to that of butylene succinate.8 Thus, the three sharp diffraction peaks of PBF homopolymer (PBF-PPS100-0) at around 19.0°, 21.2°, and 23.2° can be ascribed to the (020), (021), and (110) planes of PBF, respectively. As indicated in Figure 3, copolymers show similar WAXD patterns to that of PBF-PPS100-0, and no evident shift in the peak positions can be observed, suggesting that the copolymerization with PPS does not modify the crystal structure and lattice parameters of PBF. Therefore, the PPS segment may be rejected from the crystals. It is well-regarded that Xc is an significant parameter for the mechanical properties of a crystalline polymer. In the present work, Xc of multiblock copolymers was determined from WAXD pattern according to ref 23, and the results are summarized in Table 2. It can be found that Xc of the chain-
Figure 3. WAXD patterns of chain-extended polymers.
extended polymers decreases gradually from 45.6% for the PBF homopolymer (PBF-PPS100-0) to 31.2% for PBF-PPS70-30 with increasing PPS content in the composition. It consists well with the DSC analysis. Since Xc is a key parameter affecting mechanical properties, it will be employed to analyze the mechanical properties later on. 3.6. Mechanical Properties. It is generally regarded that the mechanical properties are very significant for potential utility of material. Therefore, the mechanical properties of PBF homopolymer and multiblock copolymers have been investigated in the work. It is worth noting that all the four copolymers show good processability during the inject molding of the specimens for mechanical properties testing. Notched izod impact strengths, tensile properties, and flexural properties are tabulated in Table 3. It can be observed that the impact strength of the polymers increases gradually with increasing PPS contents, and the copolymers become unbreakable and possess excellent toughness when PPS content is around or higher than 20%. Thus, the toughness of PBF has been effectively enhanced by the introduction of PPS segment. The super toughness of multiblock copolymer is attributed to the reduced Xc, which has been confirmed by the DSC and WAXD, the high molecular weight and regular sequential structure as well as the allophanate branching.15,24,25 However, the elongation at break shows an opposite trend and decreases with increasing PPS content. These seemingly contradictory results may be explained in terms of Xc and allophanate branching. On one hand, HDI needed for chain-extension increases with increasing PPS content (see Table 1) because Mn of PPS-diol is lower than that of PBF-diol. Consequently, the number of allophanate branching points, resulting from the reaction of excess HDI with urethane groups, increases with increase of PPS content. On the other hand, Xc decreases with increment in PPS content (see Table 2). Since both the reduced Xc and increased number of allophanate branching points contribute to enhancement of impact strength, impact strength increases steadily with increasing PPS content. As far as the elongation at break is concerned, though the reduced Xc favors to the increase of the elongation at break, the increased number of allophanate branching points decreases the elongation at break. The ultimate result of decrease in elongation at break is due to the dominate role of the latter. 14111
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Table 3. Mechanical Properties of Homopolymers and Multiblock Copolymers sample PBF-PPS100-0 PBF-PPS95-5 PBF-PPS90-10 PBF-PPS80-20 PBF-PPS70-30 a
tensile strength (MPa)
elongation at break (%)
± ± ± ± ±
140 ± 65 81 ± 22 76 ± 10 63 ± 11 37 ± 9
41.0 37.2 33.8 27.5 19.2
5.1 3.0 3.9 2.0 1.4
flexural strength (MPa)
flexural modulus (MPa)
impact strength (J/m)
± ± ± ± ±
531 ± 2 426 ± 5 348 ± 10 264 ± 7 149 ± 5
160 ± 3 216 ± 5 235 ± 6 a a
26.7 21.1 17.8 13.5 7.2
0.2 0.2 0.1 0.2 0.3
Unbreakable.
On the other hand, the tensile strength, flexural strength, and flexural modulus of copolymers decrease with increasing PPS content. It is mainly attributed to the reduced Xc. Therefore, it can be concluded that the mechanical properties of the copolymers are controllable and adjustable, and the copolymers can be ranged from rigid plastics to soft elastomers with increasing PPS content. 3.7. Enzymatic Degradation of Multiblock Copolymers. As mentioned before, the main aim of this work is to improve the biodegradability of PBF through copolymerization with amorphous PPS. Therefore, the degradation rate is the focus of our attention and has been evaluated by determining the weight loss of PBF homopolymer and multiblock copolymer during enzymatic degradation. The dependence of degradation rate upon PPS content is given in Figure 4. As
reduced Xc by the introduction amorphous PPS segment, which has been evidenced by the data of DSC and WAXD. Furthermore, it can be observed that the total weight loss for PBF-PPS70-30 is 38%, which is obvious higher than the content of PPS segment. Thus, it can be deduced that PBF segment can be degraded by the enzyme. In order to further confirm this conclusion, we have also measured the 1H NMR of the residual polymers after enzymatic degradation, and the results are shown in Figure 5. It can be observed that there is
Figure 5. 1H NMR spectra of PBF-PPS70-30 during the degradation process: (a) at day 0 and (b) at day 18.
no obvious difference between 1H NMR spectra of PBFPPS70-30 before and after degradation, except the relative peak intensities. The compositions have been calculated from 1H NMR spectrum according to eqs 3−5, and the results are summarized in Table 4. The values of the reduction of PPS content upon degradation are also listed in Table 4 and compared with that of weight loss in order to confirm whether PBF can be degraded by the enzyme or not. If biodegraded parts in the copolymers were only PPS region, the value of weight loss would be close or equal to the reduction of PPS content. Or else, both PBF and PPS segment degraded during enzymatic degradation. It can be found from Table 4 that the value of the weight loss for any copolymer is evidently higher than the reduction in PPS content. Therefore, it can be undoubtedly concluded that both PBF and PPS segment took part in the enzymatic degradation. Furthermore, it can be observed that content of PPS decreases, while that of PBF increases upon degradation. It testifies the fact that PPS segment degrades faster than the PBF segment and accelerates the degradation because of the preferential degradation of amorphous fraction.
Figure 4. Weight loss for polymers as a function of time during the degradation process with a lipase from Pseudomonas cepacia at 50 °C and pH 6.86.
shown in Figure 4, PBF homopolymer (PBF-PPS100-0) degrades rather slowly, and the total weight loss is less than 2% after 10 days of degradation. In contrast, all the multiblock copolymers degrade faster and the degradation rate increases rapidly with increasing PPS content. The total weight loss approaches nearly to 20% upon 10 days of degradation for PBF-PPS70-30, which is 10 times that of the unmodified PBF. Therefore, it can be safely concluded that the incorporation of PPS segment indeed enhances the biodegradation rate as expected and the biodegradation rate of copolymers is regulatable by varying the feed ratio. Generally, biodegradability of aliphatic polyesters is greatly affected by crystallinity and degradation usually initiates from amorphous domains. The faster biodegradation of the copolymers is ascribed to the 14112
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Table 4. Composition Variation of Homopolymers and Multiblock Copolymers upon Degradation PBF (%)
PPS (%)
weight loss
0 day
18 days
0 day
18 days
reduction
0 day
18 days
PBF-PPS90-10 PBF-PPS80-20 PBF-PPS70-30
6 16 38
87.14 76.84 68.25
89.45 83.53 77.76
9.51 19.88 28.33
7.13 13.31 18.56
−2.38 −6.57 −9.77
3.35 3.28 3.42
3.42 3.16 3.68
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4. CONCLUSIONS A series of biodegradable PBF-PPS multiblock copolymers bearing reactive carbon−carbon double bonds have been successfully synthesized by chain extension with HDI. The chemical structures, crystal structures, thermal properties, mechanical properties, and biodegradation behaviors of the copolymers were systematically investigated by 1H NMR, GPC, DSC, WAXD, mechanical testing, and enzymatic degradation. The following detailed conclusions were achieved: (1) 1H NMR results confirm that no isomerization of CC configuration or Ordelt reaction of CC took place during the whole melt polymerization process. (2) The DSC and WAXD results show that the crystal structures, Tm, and Tc of PBF are not greatly affected by copolymerization, but the crystallinity reduces after copolymerization. (3) The impact strength of PBF is effectively improved by copolymerization with PPS and the copolymers are unbreakable when PPS content is about or higher than 20%, while the tensile strength, elongation at break, flexural strength, and flexural modulus show a decreased trend with the introduction of PPS. The mechanical properties of the copolymers are controllable and adjustable, and the copolymers can be ranged from rigid plastics to soft elastomers with increasing PPS content. (4) The biodegradability of PBF has been substantially enhanced by copolymerization with PPS, especially when PPS content is above 20%. Both PBF and PPS segment can be degraded by the enzyme. These merits may render the novel copolymer directly find applications in the area of biodegradable materials. The carbon−carbon double bonds on the main chain of the copolymers further provide the reactive sites to potentially endow the copolymers with advanced structures or desirable properties by post modification, functionalization or crosslinking.
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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 the National Science Fund of China (Grant No. 21104087) and the Cultivation Project of Institute of Chemistry Chinese Academy of Science (ICCAS) (Grant No. CMS-PY-201239) is gratefully acknowledged.
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HDI (%)
sample
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