Novel Biodegradable and Double Crystalline Multiblock Copolymers

Article pubs.acs.org/IECR

Novel Biodegradable and Double Crystalline Multiblock Copolymers Comprising of Poly(butylene succinate) and Poly(ε-caprolactone): Synthesis, Characterization, and Properties Liuchun Zheng,† Chuncheng Li,*,† Zhaodong Wang,†,‡ Jin Wang,†,‡ Yaonan Xiao,† Dong Zhang,† and Guohu Guan† †

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 S Supporting Information *

ABSTRACT: A series of double crystalline multiblock copolymers composed of poly(butylene succinate) (PBS) and poly(εcaprolactone) (PCL) have been successfully synthesized with hexamethylene diisocyanate (HDI) as a chain extender. The copolymers were systematically characterized by 1H NMR, GPC, TGA, DSC, WAXD, and mechanical testing. The results indicate that the PBS segment is immiscible with the PCL segment in the amorphous region. The copolymers follow a two-stage degradation behavior, and thermal stability increases with increasing PBS content. PBS and PCL in the copolymers crystallize and melt separately. The mechanical properties of the copolymers can be conveniently adjusted from rigid plastics to flexible elastomers by changing the feed composition. The impact strength is substantially improved by the incorporation of the PCL segment.

1. INTRODUCTION In the past few decades, biodegradable aliphatic polyesters1−13 have attracted considerate attention from both academia and industry, due to their superior biodegradability and good comprehensive properties.14−16 PBS, as one of the most extensively investigated and developed aliphatic polyesters, has a relative high melting temperature (Tm around 110 °C), excellent processing properties, and thermal stability.17−19 Unfortunately, the insufficient impact resistance and slow biodegradation rate of PBS greatly restrict its widespread applications. Therefore, PBS has been copolymerized with other aliphatic polyester units, such as butylene adipate,20,21 propylene succinate,22 L-lactate,23 ω-pentadecalactone,24 butylene 2methyl succinate25 and so forth. Although these biodegradable aliphatic units, especially those that have side groups,25,26 could effectively improve the degradation rate of PBS by reducing crystallinity degree, they simultaneously deteriorate tensile strength and sharply reduce Tm since their sequential structures are entirely random.20−26 On the other hand, previous work of Wang’s group, our team, and other groups has proved that multiblock copolymers with regular sequential structure and high molecular weight can be conveniently prepared by chain extension of two dihydroxytelechelic polyester prepolymers with diisocyanate.27−35 Due to the merits of regular sequential structure and high molecular weight, this type of multiblock copolymer usually possesses evidently better thermal and mechanical properties as compared with their corresponding random copolyesters and homopolyesters.30−35 Previous work of our team32,33 reveals that the rubbery poly(1,2-propylene succinate) segment can enhance the impact strength of PBS by 18 © 2012 American Chemical Society

times without obviously decreasing the tensile strength and Tm;32 the rigid and glassy poly(1,2-propylene terephthalate) segment can simultaneously enhance the impact the strength of PBS by 628% and the tensile strength by 64%.33 Furthermore, structures and mechanical properties of this kind of multiblock copolymer are controllable and adjustable by simply varying the feed ratio.27−30 PCL, another promising aliphatic polyester with well-defined crystalline structure and good flexible properties, is fully biodegradable and biocompatible. It is regretful that PCL inherently has shortcomings of low melting point, thermal stability, and tensile strength, which greatly hinder its extensive application. To combine the advantages, PCL has been copolymerized or blended with PBS. Up to now, very limited work has been reported on this respect. Cao et al. synthesized random copolyesters of poly(butylene succinate-co-ε-caprolactone)s.36 Due to the random sequential structure, the melting point of the copolymers decreases rapidly with increasing ε-caprolactone unit, which will reduce its applicability. Nugroho37 and Qiu38 et al. have prepared the PCL and PBS blends and found that although the thermal stability of PCL is indeed improved after blending, PBS is completely immiscible with PCL.38 Thus, it is very hard to obtain PBS and PCL based material with excellent mechanical and thermal properties by simple copolymerization or blending. Received: Revised: Accepted: Published: 7264

March 3, 2012 April 28, 2012 May 3, 2012 May 3, 2012 dx.doi.org/10.1021/ie300576z | Ind. Eng. Chem. Res. 2012, 51, 7264−7272

Industrial & Engineering Chemistry Research

Article

Scheme 1. Synthesis Routes of PBS-diol (a), PCL-diol (b), and Multiblock Copolymers (c)

Table 1. Composition and Molecular Weight for Multiblock Copolymers and Homopolymer feed composition (wt %)

found composition (wt %)

sample

PBS

PCL

HDI

PBS

PCL

HDI

Mn (× 104)

Mw (× 104)

PDI

PBS-PCL100-0 PBS-PCL90-10 PBS-PCL70-30 PBS-PCL50-50 PBS-PCL30-70 PBS-PCL10-90 PBS-PCL0-100 PBS-PCL50-50 (1) PBS-PCL50-50 (2)

96.38 86.73 67.46 48.18 28.91 9.64 0 45.98 48.88

0 9.64 28.91 48.18 67.45 86.72 96.36 45.98 48.88

3.62 3.63 3.63 3.63 3.64 3.64 3.64 8.04 2.24

95.96 86.85 68.34 49.91 31.68 11.05 0 46.54 49.42

0 9.32 28.13 46.36 64.82 85.41 96.41 45.46 48.34

4.04 3.83 3.53 3.72 3.50 3.54 3.59 8.00 2.24

9.38 12.6 12.6 10.3 4.23 4.47 3.11 1.21 0.96

50.9 40.2 31.0 31.0 9.64 9.36 6.94 3.74 2.20

5.43 3.19 2.46 3.01 2.28 2.09 2.23 1.69 2.29

succinic acid (2 mol) and 1,4-butanediol (3.3 mol) was carried out at 180 °C under nitrogen atmosphere with Ti(OBt)4 (0.11 mL) as the catalyst until the 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 its 1H NMR spectrum. 2.3. Synthesis of Dihydroxytelechelic-PCL (PCL-diol). PCL-diol was synthesized by ring-opening polymerization of εCL with Sn(Oct)2 and 1,4-butanediol as catalyst and initiator, respectively. In a dry four neck flask, which was evacuated and purged with nitrogen three times, ε-CL (3 mol), Sn(Oct)2 (0.009 mol), and 1,4-butanediol (0.0711 mol) were introduced to prepare PCL-diol. After that, the exhausting-refilling process was repeated three times. Then the temperature was raised to 130 °C, and the ring-opening polymerization reaction proceeded for 12 h with a mechanical stirrer. PCL-diol was characterized by 1H NMR spectrum. 2.4. Synthesis of Multiblock Copolymers and Homopolymers. Chain-extension reaction was accomplished in bulk under nitrogen atmosphere, and the molar ratio of

On the basis of previous results and analysis, these issues may be overcome if a multiblock copolymer can be synthesized. Therefore, in the present work, double crystalline multiblock copolymers with PBS as the hard segment and PCL as the soft segment were synthesized and systematically characterized. To the best of our knowledge, it is the first time to report the synthesis, characterization, and properties of such copolymers.

2. EXPRIMENTAL SECTION 2.1. Materials. ε-Caprolactone (ε-CL) and 1,4-butanediol were purchased from Alfa Aesar (USA) and BASF (German) and purified by vacuum distillation over CaH2 before use. Stannous (II) octoate (Sn(Oct)2) and HDI were bought from Sigma-Aldrich (USA) and Bayer (German) and used as received. Titanium(IV) butoxide (Ti(OBt)4) was obtained from Beijing Chemical Reagents Corp. (China) and distilled before use. All the other reagents and solvents, analytical grade, were purchased from Beijing Chemical Reagents Corp. (China) and used without purification. 2.2. Synthesis of Dihydroxytelechelic-PBS (PBS-diol). PBS-diol was synthesized by a two-step process, that is, esterification and polycondensation. The esterification of 7265

dx.doi.org/10.1021/ie300576z | Ind. Eng. Chem. Res. 2012, 51, 7264−7272


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diisocyanate/polyester-diols was fixed at 1.1/1. Typically, chainextension reaction of PBS-diol (70 g) and PCL-diol (30 g) was carried out in a silicone oil bath at 130 °C under nitrogen atmosphere. After the polyester-diols were completely molten, HDI (3.77 g) was added to the reactor by dripping with vigorous mechanical stirring. And it was maintained for 1 h to complete the chain-extension reaction. All the polymers were purified by reprecipitation from their chloroform solutions by methanol repeatedly and dried for 12 h at 80 °C before the measurements of 1H NMR spectra and GPC. The synthesis routes 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. 1H NMR spectra were acquired with Bruker DMX-400 NMR spectrometer at room temperature using tetramethylsilane and CDCl3 as internal standard and solvent, respectively. The sample concentration was 10 mg/mL. 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 35 °C. Chloroform was used as the eluent at a flow rate of 1.0 mL min−1, and a sample concentration of 2.5 mg/mL was employed. The molecular weights were calibrated with polystyrene standards. 2.7. Thermogravimetric Analysis (TGA). TGA of samples (2−3 mg) was carried out under N2 atmosphere at a heating rate of 20 °C min−1 with a Perkin-Elmer TGA-7. 2.8. 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 (7−8 mg) were heated to 150 °C and maintained there for 5 min to eliminate thermal histories and then quenched to −120 °C as fast as possible using liquid nitrogen as the cooling agent and held there for 5 min. After that, the samples were reheated to 150 °C at 20 °C min−1 and held there for 5 min before they were cooled to −120 °C at the same rate. Both cooling and heating scans were recorded for analysis. 2.9. 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. 2.10. 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. Tensile and flexural testing was conducted on a universal tester (Instron 1122, UK). The tensile properties of the specimens were measured according to ISO 527 at a constant 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 determined 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). All the results were taken as an average from measurements of at least five specimens. The specimens for these tests were all prepared on an injection molding machine (Haake Minijet, German).

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of PBS-diol and PCL-diol. PBS-diol was prepared by the esterification and polycondensation of succinic acid with excess 1,4-butanediol, and the molar ratio of succinic acid/1,4-butanediol was set to be 1/1.65 (Scheme 1a). The structure of PBS-diol was characterized by 1H NMR spectrum (Figure 1a). The very

Figure 1. 1H NMR spectra of (a) PBS-diol, (b) PCL-diol, and (c) PBS-PCL50-50.

typical signals occurring at 4.11 ppm (δH1), 2.60 ppm (δH3), and 1.70 ppm (δH4) belong to the repeating units of PBS, whereas the small signal located at 3.5−3.7 ppm (δH2) is assigned to the terminal methylene groups. The number-average molecular weight (Mn) of PBS-diol was calculated from the 1H NMR spectrum according to the following equation: Aδ = 4.11 M n(PBS ‐ diol) = 90 + × 172 Aδ = 3.5 − 3.7 (1) 7266



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Figure 2. TG (a) and DTG (b) curves of chain-extended polymers.

where Aδ=4.11 and Aδ=3.5−3.7 represent the integral areas of internal and terminal methylene groups, 172 is the mass of PBS repeating units (−OCCH2CH2COOCH2CH2CH2CH2O−), and 90 is the gross mass of PBS-diol end chains. PCL-diol was prepared using 1,4-butanediol as initiator for the controlled ring-opening polymerization of ε-CL in the presence of Sn(Oct)2 as catalyst (Scheme 1b). The molecular structure of PCL-diol was characterized by the 1H NMR spectrum (Figure 1b). As indicated in Figure 1b, the very strong signals occurring at 4.06 ppm (δH5), 2.30 ppm (δH6), 1.50−1.80 ppm (δH7), and 1.37 ppm (δH8) are assigned to the methylene protons of PCL repeat units. The triplet at 3.5−3.7 ppm (δH2) belongs to the methylene protons connecting with the terminal hydroxyl groups. Mn of PCL-diol was calculated from the 1H NMR spectrum by the following equation: M n(PCL‐diol) = 90 + 2 ×

Aδ = 4.06 × 114 Aδ = 3.5 − 3.7

weights of the two copolymers (in Table 1) are much lower than that of PBS-PCL50-50 based on prepolymers with Mn around 5000. In order to obtain materials with good mechanical property and processing property, the molar ratio of HDI to the total prepolymers (R value) was fixed at 1.1/1.39−41 This is because our previous works have confirmed that this R value is beneficial for the formation of slightly cross-linked or branched structure, which can prominently enhance the mechanical properties. The chemical structures of the copolymers were characterized by 1H NMR spectra. The 1H NMR spectrum and the corresponding assignments of the representative copolymer PBS-PCL50-50 are demonstrated in Figure 1c. It can be found that all the characteristics signals, belonging to repeating units of PBS and PCL, still exist in the spectrum of PBS-PCL50-50. Meanwhile, the peaks at 3.5−3.7 ppm (δH2) in Figure 1a,b, attributed to terminal methylene protons of PBS-diol and PCLdiol, completely disappear in the spectrum of copolymer PBSPCL50-50. This indicates that all the hydroxyl groups from the prepolymers were thoroughly reacted. Furthermore, the signal at 3.14 ppm (δH9) is reasonably attributed to the methylene protons of HDI residue in the structure of the copolymer, while the signals of the other protons belonging to HDI residue in the structure of copolymer located around 1.53 ppm (δH7) and 1.37 ppm (δH8) overlap with those of PCL. Therefore, the signal at 3.14 ppm is employed for the determination of the composition. Weight compositions of the resulting chain-extended polymers have been calculated based on the integral areas from 1H NMR spectra according to the following equations:

(2)

where Aδ=4.06 and Aδ=3.5−3.7 are the integral areas of internal and terminal methylene groups and the values of 114 and 90 are the molecular weights of PCL repeating units and gross mass of PCL-diol end chains, respectively. The Mn values of PBS-diol and PCL-diol are 4913 and 4889, respectively. 3.2. Synthesis and Characterization of Multiblock Copolymers and Homopolymers. Multiblock copolymers and homopolymers were synthesized via chain-extension of the prepolymers in bulk using HDI as a chain extender at 130 °C for 1 h (Scheme 1c). In this article, the copolymers with various compositions have been synthesized to study the relationship between the chemical structure and properties. For comparison, PBS and PCL homopolymers, that is, chain-extended polymers originated from PBS-diol or PCL-diol, have also been synthesized and named as PBS-PCL100-0 and 0-100, respectively. The prepolymers with Mn around 5000 are chosen for the synthesis of homopolymers and copolymers as it has been testified that the prepolymers with this Mn have the highest reactivity, resulting in the highest Mn of chain-extended polymers and thus good mechanical properties.27,32 During the experiment, we have also synthesized another four PBS-diol and PCL-diol with Mn around 2000 and 8000, that is, PBS2198, PCL2037, PBS7800, and PCL8291 (the number behind the prepolymer indicates its Mn). And we have prepared copolymer PBS-PCL50-50(1) from 50 g of PBS2000 and 50 g of PCL2000, as well as copolymer PBS-PCL50-50(2) from 50 g of PBS8000 and 50 g of PCL8000. It is regretful that molecular

PCL (%) =

172Aδ = 2.60

114(2Aδ = 2.30) + 114(2Aδ = 2.30) + 170Aδ = 3.14 (3)

172Aδ = 2.60 172Aδ = 2.60 + 114(2Aδ = 2.30) + 170Aδ = 3.14

PBS (%) =

(4)

HDI (%) =

172Aδ = 2.60

170Aδ = 3.14 + 114(2Aδ = 2.30) + 170Aδ = 3.14 (5)

where 2Aδ=2.30, Aδ=2.60, and Aδ=3.14 are the integral areas of methylene protons of the PCL unit, PBS unit, and HDI residue in the structure of polymers, respectively. The numbers 114, 172, and 170 are the molecular weights of repeating units of 7267



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Figure 3. DSC curves of chain-extended polymers with different compositions: (a) heating scan, (b) cooling scan.

Table 2. Thermal Transition and Crystallization Data of Chain-Extended Polymers Tg (°C)

Tc (°C)

Tm (°C)

−ΔHc (J/g)

ΔHm (J/g)

Xc b

sample

Tg1

Tg2

Tc1

Tc2

Tm1

Tm2

−ΔHc1

−ΔHc2

ΔHm1

ΔHm2

Xc‑PCL

Xc‑PBS

Xc‑T

PBS-PCL100-0 PBS-PCL90-10 PBS-PCL70-30 PBS-PCL50-50 PBS-PCL30-70 PBS-PCL10-90 PBS-PCL0-100

a a −59.1 −58.8 −59.3 −59.3 −61.7

−37.3 −36.4 −39.1 −38.4 a a a

a 21.0 23.1 23.9 19.9 19.5 21.1

74.0 66.7 62.8 58.3 a a a

a 49.4 49.3 49.6 49.3 49.4 50.7

106.5 106.3 106.2 107.7 107.7 106.9 a

a 0.6 7.7 17.1 31.2 40.1 49.3

48.7 44.1 34.0 25.7 a a a

a 0.7 7.2 16.6 27.3 37.9 51.3

43.9 40.2 33.3 30.1 17.0 5.5 a

a 0.75 7.7 17.8 29.3 40.7 55.1

39.8 36.41 30.2 27.3 15.4 5.0 a

39.8 37.2 37.9 45.1 44.7 45.7 55.1

Undetectable. bCrystallinity degree of PBS segment (Xc‑PBS) was calculated by (ΔHm1/ΔH0m1), ΔH0m1 = 110.4 J/g;47 crystallinity degree of PCL segment (Xc‑PCL) was calculated by (ΔHm2/ΔH0m2), ΔH0m2 = 93.1 J/g;48 and total crystallinity degree (Xc‑T) was calculated by Xc‑PBS + Xc‑PCL. a

not so evident and overlaps slightly with that of the degradation of the PCL segment. Thus, the undetectable of Tmax2 for PBSPCL10-90 can be ascribed to the overlapping with that of degradation of PCL segment. As expected, the copolymerization with PBS improves the thermal stability of PCL evidently. The decomposition temperature of polymers at a weight loss of 5% (T5%) (see Table 1s in Figure 2a) increases monotonously along with increasing PBS segment in the copolymers. It agrees well with the results observed by Cao et al.36 It is worth noting that T5% of any copolymer is above 290 °C under nitrogen atmosphere, which is evidently higher than the chain-extension temperature (130 °C) and the processing temperature (150−180 °C) of the copolymers. Thus, it can be undoubtedly concluded that the copolymers have attractive thermal stability. 3.4. Thermal Properties and Crystallization Behaviors of Multiblock Copolymers and Homopolymers. Thermal transition and crystallization behaviors of multiblock copolymers and homopolymers as well as the compatibility between the two segments in the copolymers were studied by DSC. The corresponding DSC thermograms are shown in Figure 3, and the fundamental data have been listed in Table 2. As reflected in Figure 3 and Table 2, neat PBS (PBSPCL100-0) exhibits a glass transition temperature (Tg2) at around −37 °C; neat PCL (PBS-PCL0-100) displays a glass transition temperature (Tg1) around −62 °C. In the copolymers PBS-PCL90-10, 30-70, and 10-90, a single Tg located at −36 °C or −59 °C, which is very close to that of the rich segment, has been detected. In the case of copolymers PBS-PCL70-30 and 50-50, two composition-independent Tg values located at around −59 °C and −39 °C, corresponding to Tg of the PCL and PBS segments, can be found. Since the difference between the Tg of the two segments is more than 20 °C and the Tg for

PCL, PBS, and the HDI residue in the structure of polymers, respectively. The compositions, Mn, weight-average molecular weight (Mw), and PDI of chain-extended polymers are presented in Table 1. As shown in Table 1, the found composition is very close to the feed composition. Thus the composition of copolymers can be facilely controlled by varying the feed ratio. Furthermore, the molecular weights of the resulting copolymers are very high, confirming the effectiveness of the chainextension reaction. 3.3. Thermal Stability of Multiblock Copolymers and Homopolymers. It is known that thermal stability is an important property for the applicability of aliphatic polyesters. In this paper, thermal stability of multiblock copolymers and homopolymers was studied by determining the weight loss during heating via TGA, and the results are summarized in Figure 2 and Table 1s (located in part a of Figure 2 and in the Supporting Information). Apparently, PBS (PBS-PCL100-0) and PCL (PBS-PCL0-100) degrade in a single step. However, the degradation behavior becomes quite different after copolymerization (see derivative curves). Multiblock copolymers with 10−70 wt % PCL go through a two-stage degradation. Obviously, the first stage thermal degradation, with a degradation peak (Tmax1) close to that of PCL (PBSPCL0-100), is the degradation of PCL segment, and the second stage, with a degradation peak (Tmax2) close to that of PBS (PBS-PCL100-0), is due to the degradation of the PBS segment. It thus further testifies to the block structure of copolymers. Similar results have been reported for the block copolymers based on PLA and PBS.42−44 As indicated in Figure 2 and Table 1s (Supporting Information), the degradation peak of the PBS segment tends to become flat and shifts to the lower temperature region with the decrease in the PBS content. And the degradation peak of the PBS segment for PBS-PCL30-70 is 7268



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multiblock copolymers reduces with reducing its content. While the trend of total crystallinity degree of the two compositions (Xc‑T) is more complicated, it is slightly smaller than that of the PBS homopolymer when PCL is the poor composition and larger than that of the PBS homopolymer when PCL is the rich composition. 3.5. Wide-Angle X-ray Diffraction (WAXD). WAXD experiments were further performed to study the crystal structure of multiblock copolymers, and the corresponding WAXD patterns have been shown in Figure 4. As indicated in

the copolymers is almost composition independent, it can be safely concluded that the PBS segment is incompatible with the PCL segment in the amorphous phase. With regard to crystallization behaviors, all the chainextended polymers are able to crystallize irrespective of composition. Neat PBS (PBS-PCL100-0) shows a melting point (Tm2) around 107 °C and a crystallization temperature (Tc2) around 70 °C; neat PCL (PBS-PCL0-100) exhibits a melting point (Tm1) around 52 °C and a crystallization temperature (Tc1) around 21 °C. All the copolymers show two separate melting peaks during the heating scans and are identified with Tm1 and Tm2 from low temperature to high temperature. Evidently, Tm1 is the melting of PCL segment; Tm2 is due to the fusion of the PBS segment. This phenomenon is arising from the block structure and separate crystallization and melting of the two segments. On the other hand, both Tm1 and Tm2 vary little with the composition. This suggests that Tm of one segment is not greatly affected by the other segment. It agrees well with our previous results on the PBS-b-poly(1,2propylene succinate) and PBS-b-poly(1,2-propylene terephthalate) and can be attributed to the fact that the melting point of a crystalline segment in a block copolymer is predominately determined by its average segment length rather than its content.45,46 In the case of corresponding random copolymer poly(butylene succinate-co-ε-caprolactone)s, only one melting peak is observed for all the compositions, and the melting point decreased sharply from 116 °C to around 76 °C when 23 wt % CL (CL/BS = 0.46) was incorporated, due to the rapid decreased sequential length.36 Since melting point is one of the most important thermal parameters, multiblock copolymers with higher Tm, as compared to the corresponding random copolymers, are promising to find more applications. Though all the copolymers show two separate melting peaks during the heating scans, only PBS-PCL90-10, 70-30, and 50-50 display two well-defined crystallization peaks during the cooling scans, corresponding to the crystallization of the two crystalline segments. In contrast with the crystallization peak of the PCL segment in PBS-PCL70-30, the crystallization peak of the PBS segment in PBS-PCL30-70 is so flat that it cannot be determined accurately. This may be ascribed to the fact that the crystallization of PBS (PBS-PCL100-0) itself is difficult as compared with PCL (PBS-PCL0-100). As indicated in Figure 3b, the crystallization peak of PBS (PBS-PCL100-0) is much smoother than that of PCL (PBS-PCL0-100). The appearance of two separate crystallization peaks of the copolymers confirms the fact that the two segments in the block copolymers crystallize separately. The undetectable crystallization peak of the poor-composition segment for PBS-PCL10-90 may be due to the very slow crystallization rate. The higher crystallization temperature, which belongs to the crystallization of the PBS segment, decreases gradually with the increase of the PCL segment, implying that the crystallization rate of the PBS segment is reduced with increasing PCL segment. However, with respect to the crystallization temperature of the PCL segment, that is, Tc1, it varies little with the composition, indicating that the crystallization of the PCL segment is not significantly affected by the crystallized PBS. Crystallinity degree (Xc), which has considerable influence on material mechanical properties, is related to the amount of crystals. As indicated in Table 2, the crystallinity degree of PBS homopolymer (PBS-PCL100-0) is around 39.8%, while that of PCL homopolymer is as high as 55.5%. The crystallinity degree of the PBS segment (Xc‑PBS) (or PCL segment, Xc‑PCL) in the

Figure 4. WXRD patterns of chain-extended polymers.

Figure 4, chain-extended PBS (PBS-PCL100-0) shows two strong diffraction peaks at around 19.5° and 22.5°, while chainextended PCL, that is, PBS-PCL0-100, demonstrates sharp diffraction peaks at 21.4°, 22.0°, and 23.7°. The WAXD patterns of the multiblock copolymers of PBS-PCL70-30, 5050, and 30-70 evidently show all the diffraction peaks of both segments and no evident shifts can be observed. Furthermore, the intensity of the diffraction peaks of one segment decreases regularly with decreasing its content. Combined with the results of DSC, it can be reasonably concluded that PBS and PCL crystallize separately in the multiblock copolymers and form their own crystals. The copolymerization of crystalline PBS with crystalline PCL does not modify the crystal structure of PBS or PCL but only reduces the crystallinity degree. The simultaneous presence of PCL and PBS crystals is related with the regular sequential structure and separate crystallization of the copolymers. For PBS-PCL90-10 and 10-90, the diffraction peaks of the poor component are not evident due to the very low content. 3.6. Mechanical Properties. It is generally recognized that the mechanical properties are extremely significant for potential application and one of our working objectives is to improve the impact strength of PBS by copolymerization with PCL. Therefore, the mechanical properties are the ones we are most interested in. The mechanical properties of multiblock copolymers as well as homopolymers have been studied and evaluated by notched izod impact strengths, tensile properties, and flexural properties in the work. The corresponding stress− strain curves are presented in Figure 5, and the average parameters of mechanical properties collected from at least five samples are summarized in Table 3. It is noteworthy in Table 3 that PCL homopolymer and all five of the multiblock copolymers are unbreakable during the impact experiment and possess good impact resistance. As shown in Figure 5 and Table 3, the elongation at the break increases substantially with increase of PCL content. Thus, it can be concluded that the toughness of PBS has been enhanced 7269



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Figure 5. Tensile (a) and flexural (b) stress−strain curves of chain-extended polymers with different compositions.

Table 3. Mechanical Properties of Chain-Extended Polymers sample PBS-PCL100-0 PBS-PCL90-10 PBS-PCL70-30 PBS-PCL50-50 PBS-PCL30-70 PBS-PCL10-90 PBS-PCL0-100 a

tensile strength (MPa) 61.3 55.5 42.9 29.0 23.3 28.0 30.3

± ± ± ± ± ± ±

elongation at break (%)

6.3 1.4 1.7 2.4 1.0 3.4 1.2

105 226 275 463 570 712 961

± ± ± ± ± ± ±

flexural strength (MPa)

14 15 13 11 17 80 70

29.5 20.7 15.3 15.0 13.3 12.5 15.7

± ± ± ± ± ± ±

flexural modulus (MPa)

impact strength (J/m)

± ± ± ± ± ± ±

259 ± 6 a a a a a a

0.0 0.5 0.1 1.1 0.3 0.9 0.8

608 412 304 332 314 309 340

6 9 2 12 6 7 17

Unbreakable when doing the impact experiment.

effectively by flexible and crystalline PCL. The tensile strength, flexural strength, and flexural modulus show a decreased trend with increasing PCL content. As discussed before, the introduction of PCL segment evidently does not reduce the Xc‑T when PCL content is less than 30% and raises the Xc‑T when PCL content is higher than 30%. Thus, the improvement of the toughness cannot be aroused from the reduction of Xc‑T but can be ascribed to the increased flexibility of macromolecular chains by PCL segment. The high molecular weight and regular sequential structure by chain extension also contribute to the good toughness. Furthermore, as in the results revealed by our previous work, when the R value is greater than 1.0, allophanate cross-linked and branched structure, resulting from the reaction of excess −NCO groups with urethane groups, also prominently improves the impact strength, tensile strength, flexural strength, and flexural modulus of the multiblock copolymers.34 Since HDI is added to the reactor by dripping with vigorous stirring, the uniform dispersion of HDI can be achieved. On the other hand, the reacting time is fixed to be 1 h. Therefore, stable mechanical properties of copolymers can be obtained. Furthermore, as shown in Figure 5 and Table 3, the mechanical properties of the copolymers are adjustable from rigid plastics to soft elastomers with the increase of PCL content. When the PCL content ranges from 10 to 30 wt %, the multiblock copolymers are unbreakable when doing the impact experiment and possess satisfactory tensile strength and superior impact resistance.

segment is incompatible with the PCL segment in the amorphous region. PBS and PCL segments in the copolymers crystallize and melt separately, and the crystallization of the PBS segment happens first, followed by that of the PCL segment. Tm of one segment is hardly decreased by the introduction of the other segment. The data of mechanical testing indicate that the impact strength of multiblock copolymers is significantly enhanced with the incorporation of the PCL segment, and the copolymers are unbreakable when the content of PCL is around or higher than 10 wt %. Though the tensile strength, flexural strength, and flexural modulus tend to decrease with the increase of PCL segment, the copolymers with 10−30 wt % PCL possess desirable tensile properties and excellent impact resistance. The good mechanical properties are ascribed to the flexibility and crystallinity of PCL chains, high molecular weight, regular sequential structure, and allophanate branching. These novel multiblock copolymers are expected to find applications in the area of biodegradable and environmentally friendly polymer materials.

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ASSOCIATED CONTENT

* Supporting Information S

Enlarged version of Table 1s from Figure 2a. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-106-256-0029. Fax: +86-106-256-0029. E-mail address: [email protected].

4. CONCLUSIONS In conclusion, novel double crystalline multiblock copolymers comprising of PBS and PCL have been successfully synthesized with HDI as a chain extender. The data of TGA suggest that the copolymers follow a two-stage degradation behavior, and the thermal stability of PCL is effectively improved after copolymerization. Results of DSC suggest that the PBS

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 National High Technology Research and 7270



Article

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Development Program of China (Grant No. 2009AA033601) is gratefully acknowledged.

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Novel Biodegradable and Double Crystalline Multiblock Copolymers

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