Poly(ester urethane) - American Chemical Society

Biomacromolecules 2005, 6, 2740-2747

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Poly(ester urethane)s Consisting of Poly[(R)-3-hydroxybutyrate] and Poly(ethylene glycol) as Candidate Biomaterials: Characterization and Mechanical Property Study Xu Li,† Xian Jun Loh,† Ke Wang,† Chaobin He,† and Jun Li*,†,‡ Institute of Materials Research and Engineering (IMRE), National University of Singapore, 3 Research Link, Singapore 117602, and Division of Bioengineering, Faculty of Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore Received April 1, 2005; Revised Manuscript Received May 24, 2005

Poly(ester urethane)s with poly[(R)-3-hydroxybutyrate] (PHB) as the hard and hydrophobic segment and poly(ethylene glycol) (PEG) as the soft and hydrophilic segment were synthesized from telechelic hydroxylated PHB (PHB-diol) and PEG using 1,6-hexamethylene diisocyanate as a nontoxic coupling reagent. Their chemical structures and molecular characteristics were studied by gel permeation chromatography, 1H NMR, and Fourier transform infrared spectroscopy. Results of differential scanning calorimetry and X-ray diffraction indicated that the PHB segment and PEG segment in the poly(ester urethane)s formed separate crystalline phases with lower crystallinity and a lower melting point than those of their corresponding precursors, except no PHB crystalline phase was observed in those with a relatively low PHB fraction. Thermogravimetric analysis showed that the poly(ester urethane)s had better thermal stability than their precursors. The segment compositions were calculated from the two-step thermal decomposition profiles, which were in good agreement with those obtained from 1H NMR. Water contact angle measurement and water swelling analysis revealed that both surface hydrophilicity and bulk hydrophilicity of the poly(ester urethane)s were enhanced by incorporating the PEG segment into PHB polymer chains. The mechanical properties of the poly(ester urethane)s were also assessed by tensile strength measurement. It was found that the poly(ester urethane)s were ductile, while natural source PHB is brittle. Young’s modulus and the stress at break increased with increasing PHB segment length or PEG segment length, whereas the strain at break increased with increasing PEG segment length or decreasing PHB segment length. Introduction Poly[(R)-3-hydroxybutyrate] (PHB) is a stereoregular biodegradable and biocompatible polyester produced by many strains of bacteria as an intracellular carbon and energy storage material.1 Due to its natural origin, PHB can be obtained in exceptionally pure form without retaining any catalyst. The low toxicity of PHB is partly due to its degradation product D-3-hydroxybutyrate, a normal constituent of human blood.2 Moreover, PHB is a thermoplastic polyester with mechanical properties close to those of isotactic polypropylene, which can be extruded, moulded, and spun using conventional processing equipment. Therefore, PHB may be suitable for a variety of biomedical applications, such as uses as drug carriers and tissue engineering scaffolds. However, its applications suffer from its brittleness and substantial hydrophobicity. The brittleness of PHB is largely due to the crack growth within the large spherulites, which could be circumvented by controlling its crystallization.3 Thereby, many attempts have been employed to adjust PHB crystallization kinetics * To whom correspondence should be addressed. E-mail: [email protected], [email protected]. Phone: +65-6874-8376 or 7273. Fax: +65-68747273. † IMRE. ‡ Faculty of Engineering.

including annealing,4,5 blending,6-8 and copolymerization.9-15 Among these, copolymerization could be an efficient approach for such a purpose. The introduction of other units into PHB chains can disturb the regular alignment of PHB chains, its morphologic structure is changed, which leads to retardation of the crystallinization of PHB, and its mechanical properties are subsequently improved. Bacterially synthesized copolymers of 3-hydroxybutyric acid (3HB) with other hydroxyalkanoate units have been developed with lower crystallinity and higher flexibility.9-12 However, due to the limitation of the fermentation medium and high cost, it is difficult to use these copolymers for diverse applications. Chemically synthesized copolymers of PHB with other polyesters have been successfully developed to improve the mechanical properties of PHB.13,14 A copolyester containing a PHB block and a poly[(R)-3-hydroxyoctanoate] (PHO) block has been synthesized through polycondensation of telechelic hydroxylated PHB (PHB-diol) and telechelic hydroxylated PHO (PHO-diol) with terephthaloyl chloride.14 Through incorporation of the soft PHO block together with the hard PHB block in the copolymer, the copolymer showed improved mechanical properties compared with pure PHB. Poly(ethylene glycol) (PEG), or poly(ethylene oxide) (PEO), as a hydrophilic and biocompatible polyether, has been widely used in biomedical research and application.15

10.1021/bm050234g CCC: $30.25 © 2005 American Chemical Society Published on Web 07/06/2005

Poly(ester urethane)s as Candidate Biomaterials

Considerable effort has been focused on the preparation of PEG/polyester block copolymers to improve the hydrophilicity and physicomechanical and biological properties of polyesters.16-23 Huh et al. prepared poly(ethylene glycol)/ poly(L-lactic acid) (PEG/PLLA) alternating block copolymer by polycondensation between PEG and dicarboxylated PLLA in the presence of dicyclohexyl carbodiimide and 4-(dimethylamino)pyridine.17 They reported some unique properties of the multiblock copolymers, such as temperaturedependent swelling, optical transmittance, and improved mechanical properties. Poly(urethane)s consisting of a PEO block and a poly(-caprolactone) block have been synthesized through one-step condensation copolymerization in the presence of a coupling reagent, 1,6-hexamethylene diisocyanate (HDI).20 Recently, we reported the synthesis and characterization of a new amphiphilic PEO/PHB triblock copolymer.21 The new triblock copolymer is expected to form micelles in aqueous medium and may be used for drug delivery applications. As new biodegradable and biocompatible polymeric biomaterials for use in tissue engineering as cell scaffolding materials, higher molecular weight copolymers with better mechanical strength and tunable hydrophilicity are expected. Hirt et al. reported the synthesis and characterization of poly(ester urethane)s with poly{[(R)-3-hydroxybutyric acid]-co-[(R)-3-hydroxyvaleric acid]} as the hard segment and poly(-caprolactone) (PCL) or poly[(adipic acid)-alt-(1,4-butanediol; diethylene glycol; ethylene glycol)] (Diorez) as the soft segment.13 The mechanical properties of the materials could be tailored by changing their composition. Their studies also indicated that such poly(ester urethane)s were suitable for fabrication of nerve guidance channels.24 It is hoped that poly(ester urethane)s with PEG as the soft segment instead of PCL or Diorez should be tunable in hydrophilicity, which can facilitate the incorporation and the transportation of nutrients within the materials. At the same time, the aggregation of the PEG segment on the material surface may retard the unwanted protein absorption, and thus facilitate cell affinity and cell proliferation on the material surface. Above all, a poly(ester urethane) containing multiple blocks of PHB and PEG with various segment lengths and tunable hydrophilicity may be a promising biomaterial for different applications. During our work on the synthesis and characterization of the poly(PHB/PEG urethane)s with different segment lengths of PHB-diol and PEG using HDI as a nontoxic coupling reagent, Zhao et al. reported the preparation of similar copolymers in a short paper.25 However, there were only Fourier transform infrared (FTIR) and 13C NMR results for the characterization of the copolymers, while information was still lacking on the molecular weights and segment lengths, as well as detailed and convincing characterization data and proof to support the structures of the multiblock polyurethane copolymers. More importantly, the physicochemical and biological properties of polymeric biomaterials such as mechanical strength, biodegradability, and biocompatibility are dependent not only on the chemical structures of the monomeric units but also on their morphological structures, such as crystallinity and phase behaviors.

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Herein, we successfully synthesized a series of poly(PHB/ PEG urethane)s with various PHB and PEG segment lengths. Their chemical structures and molecular characteristics were studied using 1H NMR, gel permeation chromatography (GPC), and FTIR. Their thermal properties, phase separation, crystallization behaviors, and hydrophilicity, which are essential factors determining the properties of the bulky materials, were studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (XRD), and water contact angle testing. Their water-swelling property and mechanical property have also been studied, as the copolymers are to be used as candidate biomaterials. Experimental Section Materials. Natural source PHB was supplied by Aldrich, and purified by being dissolved in chloroform followed by filtration and subsequent precipitation in hexane before use. The Mn and Mw of the purified PHB were 8.7 × 104 and 2.3 × 105, respectively. PEGs with Mn values of ca. 2000, 3400, 4600, and 8000 were purchased from Aldrich. PEGs were purified by being dissolved in dichloromethane followed by precipitation in diethyl ether and vacuum-drying before use. Their Mn and Mw were found to be 1960 and 2060, 3250 and 3380, 4150 and 4400, and 7950 and 8200, respectively. Bis(2-methoxyethyl) ether (diglyme; 99%), ethylene glycol (99%), dibutyltin dilaurate (95%) HDI (98%), methanol, diethyl ether, and 1,2-dichloroethane (99.8%) were purchased from Aldrich. Diglyme was dried with molecular sieves, and 1,2-dichloroethane was distilled over CaH2 before use. Synthesis of Poly(PHB/PEG urethane)s. Telechelic PHB-diol prepolymers with various molecular weights were prepared by transesterification between the natural source PHB and ethylene glycol using dibutyltin dilaurate in diglyme as reported previously.21,26 The yields were about 80%. Poly(PHB/PEG urethane)s were synthesized from PHBdiol and PEG with molar ratios of PHB to PEG fixed at 1:1 using HDI as a coupling reagent. The amount of HDI added was equivalent to that of the reactive hydroxyl groups in the solution. Typically, 0.52 g of PHB-diol (Mn ) 1740, 3.0 × 10-4 mol) and 0.98 g of PEG (Mn ) 3250, 3.0 × 10-4 mol) were dried in a 250 mL two-neck flask at 50 °C under high vacuum overnight. Then, 20 mL of anhydrous 1,2dichloroethane was added to the flask, and any trace water in the system was removed through azeotropic distillation, with only 3 mL of 1,2-dichloroethane being left in the flask. When the flask was cooled to 75 °C, 0.10 g of HDI (6.0 × 10-4 mol) and two drops of dibutyltin dilaurate (∼8 × 10-3 g) were added sequentially. The reaction mixture was stirred at 75 °C under dried nitrogen for 48 h. The resultant poly(PHB/PEG urethane) was precipitated from diethyl ether, and further purified by being redissolved in 1,2-dichloroethane followed by precipitation in a mixture of methanol and diethyl ether to remove the remaining dibutyltin dilaurate. A series of poly(PHB/PEG urethane)s with various PHB and PEG segment lengths were prepared, and their numberaverage molecular weight and polydispersity are given in

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Table 1. Molecular Characteristics of Poly(PHB/PEG urethane)s b

poly(PHB/PEG Mn Mw/ urethane)a (×103) Mnb ∑Mnc PHE(17-20) PHE(17-33) PHE(17-42) PHE(17-80) PHE(11-33) PHE(32-33)

30.4 34.4 37.5 37.1 25.9 33.8

1.57 1.53 1.42 1.20 1.51 1.47

3700 5000 5890 9690 4360 6490

PHB content (wt %) degree of polymerizationd NMRe TGAf 8.20 6.89 6.36 3.83 5.93 5.21

38.4 32.5 25.0 12.7 20.4 45.7

40.7 30.9 26.0 16.0 18.3 48.8

a Poly(PHB/PEG urethane)s are denoted PHE, and the numbers in parentheses show the indicative molecular weight of the respective precursor in hundreds. b Determined by GPC. c Calculated from ∑Mn ) Mn(PHB) + Mn(PEG). d Calculated from Mn/∑Mn. e Calculated from 1H NMR results. f Calculated from TGA results.

Table 1. The yield was 90% or above after isolation and purification. Measurements. GPC analysis was carried out with a Shimadzu SCL-10A and LC-8A system equipped with two Phenogel 5 µm, 50 and 1000 Å columns (size 300 × 4.6 mm) in series and a Shimadzu RID-10A refractive index detector. THF was used as the eluent at a flow rate of 0.30 mL/min at 40 °C. Monodispersed poly(ethylene glycol) standards were used to obtain a calibration curve. The 1H NMR spectra were recorded on a Bruker AV-400 NMR spectrometer at 400 MHz at room temperature. The 1 H NMR measurements were carried out with an acquisition time of 3.2 s, a pulse repetition time of 2.0 s, a 30° pulse width, a 5208 Hz spectral width, and 32K data points. Chemical shifts were referred to the solvent peaks (δ ) 7.3 ppm for CHCl3). FTIR spectra of the polymer films coated on a KBr pellet were recorded on a Bio-Rad 165 FT-IR spectrophotometer; 64 scans were signal-averaged with a resolution of 2 cm-1 at room temperature. DSC measurements were performed on a TA Instruments 2920 differential scanning calorimeter equipped with an autocool accessory and calibrated using indium. The following protocol was used for each sample: heating from room temperature to +170 °C at 20 °C min-1, holding at +170 °C for 2 min, cooling from +170 to -30 °C at 5 °C min-1, and finally reheating from -30 to +170 °C at 5 °C min-1. Data were collected during the second heating run. Transition temperatures were taken as peak maxima. TGA was carried out on a TA Instruments SDT 2960. Samples were heated at 20 °C min-1 from room temperature to 800 °C in a dynamic nitrogen atmosphere (flow rate 70 mL min-1). Wide-angle XRD measurements were carried out on a Bruker GADDS diffractometer with an area detector operating under Cu KR (1.5418 Å) radiation (40 kV, 40 mA) at room temperature. Film samples were mounted onto the sample holder with double-sided adhesive tape. Poly(PHB/PEG urethane) thin films with a thickness of 100 nm were prepared from 10 mg/mL 1,2-dichloroethane solution through spin-coating onto clean glass slides, and dried in high vacuum overnight. Static water contact angles of the films were measured by the sessile method at 25 °C in an air atmosphere using an NRL-100-00-(230) contact angle goniometer (Rame`-Hart Inc., New Jersey). The telescope with a magnification power of 23× was equipped with a protractor of 1° graduation. For each contact angle reported,

at least five readings from different parts of the film surface were averaged. Each angle reported was reliable to 2°. Poly(PHB/PEG urethane) films were prepared by casting a 10 wt % chloroform solution of the polymers onto a poly(tetrafluoroethylene) dish. After slow evaporation of chloroform, the remaining solvent in the films was removed by drying the films in high vacuum at 50 °C for 2 days. Then, the dried films were cut to a small size and immersed in deionized water at 37 °C. At predetermined time intervals, hydrated samples were picked up and weighed after the surface water was blotted up with Kimwipes. The water contents were then calculated on the basis of the weight difference of the film before and after swelling. The percentage of water uptake was calculated using the following equation: water uptake (%) ) 100(Ww - Wd)/Wd where Wd and Ww are the weights of the sample film before and after being immersed in water, respectively. Tensile strength measurements were performed using an Instron 5543 microforce tester at 25 °C with a deformation rate of 45 mm‚min-1. The polymer films were prepared according to the method described above. The sample dimensions were 20 mm × 6 mm × 80-110 µm with a free length of 10 mm. The average of four measured values was taken for each sample. Results and Discussion Synthesis and Characterization of Poly(PHB/PEG urethane)s. Telechelic PHB-diols with lower molecular weight were obtained through transesterification between high-molecular-weight natural source PHB and ethylene glycol using dibutyltin dilaurate as catalyst.21,26 The transesterification was allowed to proceed for a few hours to overnight to produce three PHB-diols with Mn values of 1100, 1740, and 3240 as determined by GPC. The reaction of a hydroxy group of PHB-diol and PEG with an isocyanate of HDI in the presence of dibutyltin dilaurate led to the formation of poly(PHB/PEG urethane)s. The procedures for the synthesis of PHB-diol and poly(PHB/PEG urethane)s are presented in Scheme 1. Owing to the moisture-sensitive nature, any trace water in the system was removed through azeotropic distillation, and the reaction was carried out in dried 1,2-dichloroethane under a nitrogen atmosphere. The target poly(PHB/PEG urethane)s were isolated and purified from the reaction mixture by repeated precipitation from a mixture of methanol and diethyl ether. A series of poly(PHB/PEG urethane)s with various PHB and PEG segment lengths were synthesized, and their molecular weights and molecular weight distributions were determined by GPC (Table 1). A typical GPC chromatograph for one of the poly(PHB/PEG urethane)s together with its corresponding precursors is shown in Figure 1. The observation of a unimodal peak in the GPC chromatograph of the purified poly(PHB/PEG urethane) with a nonoverlapping nature with those of the corresponding precursors indicates that a complete reaction took place with no precursor remaining unreacted. All the poly(PHB/PEG urethane)s



Figure 1. GPC curves of PHE(17-33) and its PHB and PEG precursors: (a) PHB-diol (Mn ) 1740); (b) PEG (Mn ) 3250); (c) PHE(17-33) (Mw ) 52.6 × 103, Mn ) 34.4 × 103, Mw/Mn ) 1.53). Figure 3. FTIR spectra of PHE(17-33) and its PHB and PEG precursors: (a) PHB-diol (Mn ) 1740); (b) PHE(17-33); (c) PEG (Mn ) 3250).

Figure 2.

1H

NMR (400 MHz) spectrum of PHE(17-33) in CDCl3.

synthesized had a relatively narrow molecular weight distribution and a high molecular weight, with the polydispersity ranging from 1.20 to 1.60 and Mn from 2.00 × 104 to 3.75 × 104. The chemical structure of poly(PHB/PEG urethane)s was verified by 1H NMR spectroscopy. Figure 2 shows the 1H NMR spectrum of PHE(17-33) in CDCl3, in which all Scheme 1. Synthesis of PHB-diol and Poly(PHB/PEG urethane)s

proton signals belonging to both the PHB and PEG segments are confirmed. However, due to the low fraction of HDI (2-5 wt %) used in polycondensation, the proton signals originating from methylene of HDI could not be observed apparently. While the proton signals corresponding to methylene in repeated units of PEG segments are observed at 3.64 ppm, the signals at 5.25 ppm are assigned to methine in the repeated unit of PHB segments. As the content of HDI among the starting materials is below 5 wt %, the compositions of poly(PHB/PEG urethane)s could be determined from the integration ratio of resonances at 3.64 and 5.25 ppm within the limits of 1H NMR precision, and the results are shown in Table 1. FTIR has been a useful tool to obtain information on polymer copolymerization. As a typical example, Figure 3 shows FTIR spectra of PHE(17-33) and its PEG and PHB precursors. For PEG (Figure 3c), the characteristic C-O-C stretching vibration of the repeated -OCH2CH2- units is

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Table 2. Transition Temperatures, Corresponding Enthalpies, Crystallinity, and Decomposition Temperatures for Poly(PHB/PEG urethane)s

Tmb (°C) poly(PHB/PEG

urethane)a

PHB-diol (Mn ) 1740) PEG (Mn ) 7950) PHE(17-20) PHE(17-33) PHE(17-42) PHE(17-80) PHE(11-33) PHE(32-33)

PEG

∆Hmc (J/g) PHB

PEG

135.0 61.6 31.4 42.7 45.8 55.7 40.9 40.6

126.0 125.6 123.8 82.2 148.5

PHB

Xcd PEG

96.6 163.5 39.4 60.0 72.2 97.4 77.3 64.0

31.2 18.6 13.6 14.4 65.2

Tde (°C) PHB

PEG

65.9 79.8 19.2 29.3 35.2 47.5 37.7 31.2

21.3 12.7 9.3 9.8 44.5

PHB 253.6

386.0 391.3 389.8 393.8 394.6 393.3 399.6

279.2 267.7 270.2 269.2 271.3 260.4

a The poly(PHB/PEG urethane)s are denoted PHE, and the numbers in parentheses show the indicative molecular weight of the respective precursor in hundreds. b Melting point determined in the DSC second heating run. For the PHB segment having a multipeak due to melting-recrystallization, the Tm value for the second peak is given. c Enthalpy change during melting determined in the DSC second heating run. ∆Hm ) ∆Hi/Wi, where ∆Hi is the area of the endothermic peak for the PEG or PHB segment read from the DSC curves and Wi is the weight fraction of the corresponding segment. d Crystallinity calculated from melting enthalpies. Reference values of 205.0 and 146.6 J/g for completely crystallized PEG21and PHB21 were used, respectively. e Temperature at which 10% mass loss has occurred from TGA curves.

Figure 4. TGA curves of PHE(17-33) and its PHB and PEG precursors: (a) PEG (Mn ) 3250); (b) PHE(17-33); (c) PHB-diol (Mn ) 1740).

observed at 1102 cm-1. The bands at 963 and 843 cm-1 are characteristic of the crystalline phase of PEG.27 An intensive carbonyl stretching band at 1723 cm-1 characterizes the FTIR spectrum of pure PHB-diol as shown in Figure 3a. It is clearly seen that, in Figure 3b, all the characteristic absorptions for PHB-diol and PEG appear in the spectrum of PHE(17-33), which confirms the formation of poly(PHB/PEG urethane)s. Thermal Properties and Crystallization. The thermal stability of poly(PHB/PEG urethane)s was evaluated using TGA. Figure 4 shows the TGA scan results for PHE(1733) compared with its PHB and PEG precursors. The degradation of pure PHB-diol starts at 218 °C and is completed at 295 °C (Figure 4c), while that of pure PEG starts at 333 °C (Figure 4a), at which pure PHB-diol has completed the degradation. PHE(17-33) undergoes a twostep thermal degradation, with the first step occurring between 227 and 303 °C and the second step between 350 and 433 °C (Figure 4b). In comparison with the TGA curves of pure PHB-diol and pure PEG, the first weight loss is attributed to the decomposition of the PHB segment and the second weight loss to the decomposition of the PEG segment. Therefore, the composition of PHE(17-33) could be calculated from the two-step degradation profile. Similar weight loss curves were also observed for other poly(PHB/PEG

Figure 5. DSC curves of PHB-diol (Mn ) 1740), PEG (Mn ) 8000), and poly(PHB/PEG urethane)s with identical PHB segment lengths of Mn ) 1740 in the second heating run: (a) PHB-diol (Mn ) 1740); (b) PHE(17-20); (c) PHE(17-33); (d) PHE(17-42); (e) PHE(1780); (f) PEG (Mn ) 8000).

urethane)s. The PHB contents estimated from TGA results are listed in Table 1, which are in good agreement with those calculated from 1H NMR. The temperature at which 10% mass loss has occurred for each polymer segment is usually used as the decomposition temperature (Td)28 to quantitatively evaluate the thermal stability. The results for the poly(PHB/ PEG urethane)s are listed in Table 2, which indicate that the poly(PHB/PEG urethane)s have better thermal stability than their respective precursors. Crystallization behavior of PHB and PEG segments in the poly(PHB/PEG urethane)s was studied using DSC. The DSC thermograms of a series of the poly(PHB/PEG urethane)s with identical PHB segment lengths and their precursors are shown in Figure 5. In addition, numerical values corresponding to the thermal transition and crystallinity of each segment are presented in Table 2. Both PEG and PHB are crystalline polymers.1,27 While PEG (Mn ) 8000) shows a sharp and strong melting peak at 61.6 °C (Figure 5f), a broad melting peak at 135.0 °C is observed with PHB-diol (Mn ) 1740) (Figure 5a). With the poly(PHB/PEG urethane)s except PHE(17-80), both PHB and PEG melting transitions are observed, indicating the coexistence of PHB and PEG crystalline phases in the poly(PHB/PEG urethane)s (Figure 5b-


Figure 6. XRD results of PHB-diol (Mn ) 1740), PEG (Mn ) 8000), and poly(PHB/PEG urethane)s with identical PHB segment lengths of Mn ) 1740: (a) PHB-diol (Mn ) 1740); (b) PHE(17-20); (c) PHE(17-33); (d) PHE(17-42); (e) PHE(17-80); (f) PEG (Mn ) 8000).

e). However, the melting transition corresponding to PHB or PEG segments shifted to lower temperature with lower crystallinity in comparison with that of their PHB or PEG precursors. The decreases in Tm and crystallinity are determined by the relative fractions of PHB and PEG segments in the poly(PHB/PEG urethane)s. For example, the Tm and crystallinity of the PEG segment decrease from 55.7 °C and 47.5% for PHE(17-80) to 31.4 °C and 19.2% for PHE(1720), respectively. With a relatively low fraction, the PHB segment is unable to crystallize, and no melting transition is observed as shown in Figure 5e for PHE(17-80). Similar tendencies in melting temperature and crystallinity were also observed for the series of poly(PHB/PEG urethane)s with identical PEG segment lengths, as shown in Table 2. When poly(PHB/PEG urethane)s are cooled from the molten state, the PHB segment solidifies and crystallizes first at higher temperature, and the PEG segment is then excluded and confined between PHB laminae. Thus, the mobility of the PEG segment is hindered, and its crystallization is restricted. At the same time, due to the interference of the PEG segment, the crystallization of the PHB segment is also retarded. Figure 6 shows the XRD patterns of typical poly(PHB/ PEG urethane) samples and their PEG and PHB precursors. With pure PHB-diol (Mn ) 1740), as shown in Figure 6a, the peaks at 2θ ) 13.6° and 17.0° are characteristic of crystalline PHB. Due to the interference from the PEG segment, the relative intensity of these two peaks decreases with increasing PEG segment fraction in the poly(PHB/PEG urethane)s (Figure 6b-d). For PHE(17-80) (Figure 6e), no obvious characteristic reflection peaks for crystalline PHB appear, which indicates that the PHB segment is unable to crystallize when the PEG segment fraction is too high. The observation of characteristic peaks of crystalline PEG at 2θ ) 19.3° and 23.5° (Figure 6c,d) in the poly(PHB/PEG urethane)s indicates the formation of a PEG crystalline phase as in pure crystalline PEG (Figure 6f). As XRD measurements were carried out at room temperature, which is close to the Tm of the PEG segment in PHE(17-20), no obvious characteristic reflection peaks for the crystalline PEG segment are observed for PHE(17-20) as shown in Figure 6b. The broadening of the peaks in the patterns for PHE(17-


Figure 7. Expansions of the FTIR spectra of PHB-diol (Mn ) 1740), PHE(17-33), and PHE(17-80): (a) PHB-diol (Mn ) 1740); (b) PHE(17-33); (c) PHE(17-80).

Figure 8. Water contact angles of natural source PHB and poly(PHB/PEG urethane)s with identical PHB segment lengths of Mn ) 1740.

20) and PHE(17-33) also indicates that the two poly(PHB/ PEG urethane)s have lower crystallinity than others with longer PEG block lengths at room temperature. The crystallization behavior of the poly(PHB/PEG urethane)s was also studied using FTIR. The characteristic carbonyl stretching band of PHB at 1723 cm-1 can be resolved into an intensive band at 1723 cm-1 and a weak shoulder at 1736 cm-1, corresponding to the carbonyl stretching band of the crystalline PHB phase and that of the amorphous PHB region, respectively.29 Figure 7 shows the expansion of the carbonyl stretching region of FTIR spectra for PHB-diol (Mn ) 1740), PHE(17-33), and PHE(17-80). With increasing PEG segment fraction, the peak at 1723 cm-1 decreases sharply whereas the relative intensity of the shoulder at 1736 cm-1 increases sharply, indicating that the increasing PEG segment fraction interferes with the crystallization of the PHB segment. These results are in good agreement with the DSC and XRD results. Hydrophilicity. The surface hydrophilicity of some typical poly(PHB/PEG urethane)s, as characterized by the static water contact angle, is shown in Figure 8. The water contact angle of the poly(PHB/PEG urethane)s with identical PHB segments (Mn ) 1740) decreases with increasing PEG segment content, indicating that the surface hydrophilicity

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Figure 9. Dependence of the water uptake of natural source PHB and poly(PHB/PEG urethane)s with identical PHB segment lengths of Mn ) 1740 on the immersion time: (a) PHE(17-80); (b) PHE(17-42); (c) PHE(17-33); (d) PHE(17-20); (e) natural source PHB.

Li et al.

Figure 11. Stress-strain diagrams of natural source PHB and poly(PHB/PEG urethane)s with identical PHB segment lengths of Mn ) 1740: (a) PHE(17-20); (b) PHE(17-33); (c) PHE(17-42); (d) PHE(17-80); (e) natural PHB. Table 3. Mechanical Properties of Natural Source PHB and Poly(PHB/PEG urethane)s poly(PHB/PEG urethane)a Eb (MPa) PHB PHE(17-20) PHE(17-33) PHE(17-42) PHE(17-80) PHE(11-33) PHE(32-33)

Figure 10. Water uptake of poly(PHB/PEG urethane)s against the weight fraction of the PEG segment.

is enhanced through incorporation of the PEG segment. However, the changes are not so significant compared with that of bulk hydrophilicity, which will be discussed in the next paragraph. A similar tendency of surface hydrophilicity is also observed with poly(PHB/PEG urethane)s of other compositions. The bulk hydrophilicity and swelling property of poly(PHB/PEG urethane)s were measured by water uptake. The typical results are shown in Figure 9. In general, the test films reach equilibrium weight after 15 min of immersion in deionized water at 37 °C. The equilibrium water uptake of natural source PHB is only 5% due to its hydrophobic nature. In contrast, for the poly(PHB/PEG urethane)s with identical PHB segments (Mn ) 1740), the equilibrium water uptake increases from 55% for PHE(17-20) to 575% for PHE(17-80), with increasing PEG segment length from 2000 to 8000. In Figure 10, the equilibrium water uptake is plotted against the weight fraction of the PEG segment in the poly(PHB/PEG urethane)s. It clearly shows that the incorporation of the hydrophilic PEG segment leads to significant improvement in the hydrophilicity of the poly(PHB/PEG urethane)s. Mechanical Properties. Figure 11 shows the stress-strain diagrams of natural source PHB and the poly(PHB/PEG urethane)s with identical PHB segments (Mn ) 1740) but

σyb (MPa) yb (%) σbb(MPa)

bb (%)

1143 ( 72 25.7 ( 3.6 2.3 ( 0.6 21 ( 1 5.1 ( 0.2 227 ( 46 37 ( 5 8.9 ( 0.3 943 ( 126 65 ( 4 7.3 ( 0.4 20 ( 3 11.0 ( 0.3 1408 ( 39 120 ( 7 10.2 ( 1.2 14 ( 2 10.7 ( 0.4 1912 ( 120 41 ( 3 5.9 ( 0.4 15 ( 1 6.5 ( 0.1 1112 ( 61 78 ( 1 8.9 ( 0.1 11 ( 1

a The poly(PHB/PEG urethane)s are denoted PHE, and the numbers in parentheses show the indicative molecular weight of the respective precursor in hundreds. b The mechanical properties were assessed using an Instron 5543 at a deformation rate of 45 mm‚min-1 at 25 °C. E ) Young’s modulus, σy ) stress at yield, y ) strain at yield, σb ) stress at break, and b ) strain at break.

varying PEG segment lengths. In Table 3 are summarized their tensile properties. The natural source PHB fails in a brittle manner with Young’s modulus of 1143 MPa, stress at break of 25.7 MPa, and strain at break of only 2%. Compared with those of natural source PHB, Young’s modulus and the stress at break of poly(PHB/PEG urethane)s decrease to 21-120 and 5-11 MPa, respectively, whereas their strains at break increase tremendously to 227-1912% for different PEG segment lengths. The observed mechanical properties of the poly(PHB/PEG urethane)s can be correlated with their thermal properties. It is known that the brittleness of natural source PHB is caused by its high crystallinity and heterogeneous secondary crystallization.3 Our DSC and XRD studies have shown that the crystallinity of the PHB segment in the poly(PHB/PEG urethane)s decreases. Thus, the incorporation of the soft PEG segment into PHB makes the poly(PHB/PEG urethane)s ductile. The increase of Young’s modulus from 21 MPa for PHE(17-20) to 120 MPa for PHE(17-80) is due to the increase of crystallinity of the longer PEG segment as shown in Table 2. The yielding process with PHE(17-42) and PHE(1780) is assumed to be a result of deformation of the PEG crystalline phase driven by external stress. The influence of the PHB hard segment on the mechanical properties of the poly(PHB/ PEG urethane)s is also assessed


by fixing the PEG segment length and varying the PHB segment length. As shown in Table 3, with increasing PHB segment length, Young’s modulus and the stress at break increase whereas the strain at break decreases. Conclusions Potentially biodegradable and biocompatible poly(PHB/ PEG urethane)s with various PHB and PEG segment lengths have been successfully synthesized in the presence of HDI as a coupling agent. Their chemical structure and molecular characteristics were studied with GPC, 1H NMR, and FTIR, which confirmed the architecture of the poly(PHB/PEG urethane)s. The GPC results indicated that the synthesized poly(PHB/PEG urethane)s had high molecular weights with relatively narrow molecular weight distributions. The contents of the PHB segment in the poly(PHB/PEG urethane)s were calculated from the 1H NMR results and ranged from 12 to 46 wt %. The thermal stability of the poly(PHB/PEG urethane)s was studied by TGA, and two separate thermal degradation steps corresponding to PHB and PEG segments were observed, from which the segment contents were calculated and the results were in good agreement with those from the 1H NMR measurements. Both PHB and PEG segments in the poly(PHB/PEG urethane)s presented better thermal stability than their respective precursors. The XRD and DSC results indicated that PHB or PEG segments in the poly(PHB/PEG urethane)s formed separate crystalline phases, except for PHE(17-80) with a relatively low PHB fraction. Due to the mutual interference between PHB and PEG segments, crystallizations of PHB and PEG segments were restricted, and their melting point and crystallinity decreased compared with those of their respective precursors. The decrease in crystallinity of the PHB segment was also proved by FTIR results. Compared with the limited decrease in melting point, a significant decrease in PHB crystallinity was observed. Through incorporation of PEG segments, the hydrophilicity of the poly(PHB/PEG urethane)s was improved and determined by the PEG segment content, as revealed by water contact angle measurement and water swelling analysis. The mechanical properties of the poly(PHB/PEG urethane)s were assessed by stress-strain measurements. The results of the tensile strength test indicated that the poly(PHB/PEG urethane)s were ductile and had very wide mechanical properties that could be modulated by changing the copolymer composition. The poly(PHB/PEG urethane)s may be suitable candidates as biomaterials since they are potentially biodegradable and biocompatible.


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BM050234G

Poly(ester urethane) - American Chemical Society

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