Molecular Mobility and Phase Structure of Biodegradable Poly

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Biomacromolecules 2002, 3, 1095-1100

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Molecular Mobility and Phase Structure of Biodegradable Poly(butylene succinate) and Poly(butylene succinate-co-butylene adipate) Kazuhiro Kuwabara,† Zhihua Gan,‡ Takashi Nakamura,§ Hideki Abe,† and Yoshiharu Doi*,†,‡ Polymer Chemistry Laboratory and Characterization Center, RIKEN Institute, Hirosawa 2-1, Wako-shi, Saitama 351-0198, Japan, and Department of Innovative and Engineering Materials, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8502, Japan Received May 23, 2002

Molecular mobility and phase structure of biodegradable poly(butylene succinate) (PBS) and poly(butylene succinate-co-20 mol % butylene adipate) [P(BS-co-20 mol % BA)] have been investigated by high-resolution solid-state 13C NMR. For both samples, two components with different 13C spin-lattice relaxation time (T1C) values have been observed in the crystalline region. The crystalline component with shorter T1C value is assignable to the interface near amorphous phase. The crystalline component with longer T1C value is ascribed to the inside of the crystalline region. On the basis of T1C, it has been concluded that the BA units are not included in the crystalline region of P(BS-co-20 mol % BA). Molecular mobility and higher-ordered structure of amorphous phase have been also compared between the melt and solid state. Variable-temperature high-resolution 13C NMR measurements for the amorphous phase have revealed the remarkable difference in dynamics and structure between the melt and solid state. 1. Introduction With the rise of concern to global environment, active research is in progress for the syntheses, structures, physical properties, and degradation of biodegradable aliphatic polyesters.1-3 Poly(butylene succinate) (PBS) is one of such polyesters, and its good biodegradability in the natural environment has been reported since 1970s.4 However, because it was difficult to synthesize high-molecular-weight polymers,5 PBS had not been an attractive material in a commercial sense for a long time. In 1990, Takiyama et al. succeeded in synthesizing high-molecular-weight PBS by using a new catalyst and coupling reaction.6,7 Since then, many studies have been performed on the solid-state structure,8-15 crystallization behavior,13,14,16 and degradation17-19 of PBS. Random copolymers such as poly(butylene succinate-co-butylene adipate) [P(BS-co-BA)],6,7,17-19 poly(butylene succinate-co-ethylene succinate) [P(BS-co-ES)],13,14,20 and poly(butylene succinate-co-hexamethylene succinate) [P(BS-co-HS)]13,14 have been also developed to control the physical properties and the speed of biodegradation. By X-ray analyses, the crystal structures of PBS and PBS-type copolymers have been determined.8,9,11-14,18 As with the case of other biodegradable aliphatic polyesters,10,12,21-24 information on the solid structure and * To whom correspondence should be addressed. Mailing address: Polymer Chemistry Laboratory, RIKEN Institute, Hirosawa 2-1, Saitama 351-0198, Japan. Phone: +81-48-467-9402. Fax: +81-48-467-4667. E-mail: [email protected]. † Polymer Chemistry Laboratory, RIKEN Institute. ‡ Tokyo Institute of Technology. § Characterization Center, RIKEN Institute.

dynamics is very important for a better understanding of physical properties and designing new biodegradable materials. However, there are few reports for PBS and PBS-type copolymers. Moreover, although a benefit of PBS is its similar processibility and basic physical properties with those of polyolefins,6,7 there are also molecular-level properties which have been studied in polyolefins25-40 but little information in PBS-type polymers. In this work, we have carried out the solid-state 13C NMR characterization for PBS and poly(butylene succinate-co-20 mol % butylene adipate) [P(BS-co-20 mol % BA)]. First, we have extracted the mobile crystalline component assigned to the interface with the amorphous region. In the crystalline region of polyolefins, two or more components with different 13C spin-lattice relaxation time (T1C) values exist.25,28,30,31,33,34,39 The crystalline component with the shorter T1C value is ascribed to the interface with the amorphous region, while the component with a longer T1C value is assigned to the core of the crystalline region.25,30,31,33,34 Such an existence of plural T1C components is reported for polyethylenes,25,31,33,34 isotactic polypropylenes,28,39 and the planar zigzag form of syndiotactic polypropylenes.30,32,39 In some polymers, only one T1C value exist in the crystalline region. Such cases are reported for poly(3-hydroxybutyrate),23 poly(3-hydroxybutyrate-co-3-hydroxyvarelate)s,23 and the helix form of syndiotactic polypropylenes.30,38,39 However, as far as we know, similar T1C analysis has not been performed for PBS and P(BS-co-BA). In this study, we have investigated the 13C spin-lattice relaxation behavior of PBS and P(BS-co-20 mol % BA).

10.1021/bm025575y CCC: $22.00 © 2002 American Chemical Society Published on Web 07/19/2002

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Table 1. Characterization of Polyester Samples Used in This Work

Mwc PBSa 154 000 P(BS-co-20 mol % BA)b 150 000

X-ray Mw/Mnc Tg/°Cd Tm/°Ce crystallinity 2.0 1.8

-33 -50

113 97

0.39 0.34

a Poly(butylene succinate) sample was supplied from Tonen Chemical Corp. Before solid-state 13C NMR measurement, the sample was crystallized at 90 °C for 3 days from the melt. b Poly(butylene succinate-co-20 mol % butylene adipate) sample was supplied from Showa High Polymer Corp. Before solid-state 13C NMR measurement, the sample was crystallized at 90 °C for 3 days from the melt then stored at room temperature for 3 days. c Mw and Mn are relative weight-average and number-average molecular weights, respectively. These molecular weights were determined by GPC method with monodisperse polystyrenes as standard. d Glass transition temperature. e Melting point.

Second, we have examined whether the BA units of P(BS-co-20 mol % BA) are excluded out from the crystalline region or somewhat included in the crystalline area. Generally in polymer science, studies on the exclusion/ inclusion of a second monomer unit in the crystalline region are of great interest. In fact, many studies have been reported for poly(3-hydroxybutyrate)-type21-24 and polyethylenetype26,27,29,31,35,37,40 random copolymers. On the other hand, as far as we know, only two DSC (differential scanning calorimetry) works have been reported for PBS-type random copolymers.14,20 In these works, it has been reported that the ethylene succinate (ES) unit of P(BS-co-ES) is included in the crystalline region but the hexamethylene succinate (HS) unit of P(BS-co-HS) is excluded from the crystalline region.14 Third, we have compared the molecular mobility and higher-ordered structures (conformation and/or chain packing) of the amorphous components between the solid state and the melt. Since the crystallinities of PBS and P(BS-co20 mol % BA) are less than 50%,14 the molecular-level information on the amorphous phase is important. 2. Experimental Section 2.1. Poly(butylene succinate) Homopolymer Sample. A poly(butylene succinate) (PBS) homopolymer sample was supplied by Tonen Chemical Corp. Before solid-state 13C NMR analyses, the sample was isothermally crystallized at 90 °C for 3 days from the melt. 2.2. Poly(butylene succinate-co-butylene adipate) Sample. A poly(butylene succinate-co-20 mol % butylene adipate) [P(BS-co-20 mol % BA)] random copolymer sample was provided from Showa High Polymer Co. Ltd. The content of the BA unit and the degree of randomness were determined by solution-state NMR. For solid-state 13C NMR measurements, the sample was isothermally crystallized at 90 °C for 3 days from the melt and then stored at room temperature for 3 days. Table 1 lists the characteristics of polyester samples used in the present study. Mw and Mn are relative weight-average and number-average molecular weights, respectively. With monodisperse polystyrenes as standard and using a Shimadzu 10A GPC system and 10A refractive index detector with two joint columns of Shodex K-806 and K-802, the relative molecular weights and their distributions of polyester samples were measured by gel permeation chromatography (GPC)

Figure 1. CP/MAS % BA).

13C

NMR spectra of PBS and P(BS-co-20 mol

at 40 °C with chloroform as an eluant at a flow rate of 0.8 mL/min. Glass transition temperature (Tg) and melting point (Tm) were determined by DSC. DSC experiments were carried out on a Perkin-Elmer Pyris differential scanning calorimeter under a nitrogen flow of 30 mL/min. Meltcrystallized polyester samples (ca. 5 mg) encapsulated in the aluminum pans were heated from -50 to 200 °C at a rate of 20 °C/min. Degrees of crystallinity of the samples were calculated from the X-ray diffraction intensities according to Vonk’s method.14,41 For X-ray analyses, PBS and P(BSco-20 mol % BA) samples were melt crystallized at 90 °C for 3 days then stored at room temperature for 3 days. The wide-angle X-ray diffraction (WAXD) patterns of the polyester films were recorded on a Rigaku RINT-2500 system using nickel-filtered Cu KR radiation (λ ) 0.154 nm, 40 kV, 200 mA) in the 2θ range 6-60° at a scan speed of 2 deg/min. 2.3. Solid-State 13C NMR Measurements. High-resolution solid-state 13C NMR measurements were performed on a Chemagnetics Infinity-400 spectrometer under a static magnetic field of 9.4 T. The 1H and 13C radio field strengths γB1/2π were 62.5 kHz. The contact time for the crosspolarization (CP) process was 2.0 ms throughout this work. The magic angle spinning (MAS) rate was set to 8.0 kHz to avoid the overlapping of spinning sidebands on other resonance lines. 13C chemical shifts were expressed as values relative to tetramethylsilane (Me4Si) by using the CH3 line at 17.36 ppm of hexamethylbenzene crystals as an external reference. 13C spin-lattice relaxation times (T1C) were measured by Torchia’s CPT1 pulse sequence.42 3. Results and Discussion 3.1. Detection of Interfacial Region by 13C Spin-Lattice Relaxation Time (T1C) Analyses. Figure 1 shows the CP/ MAS 13C NMR spectra of PBS and P(BS-co-20 mol % BA) measured at room temperature. The assignment of each resonance line is also depicted in this figure. In each spectrum, two resonance lines are observed at about 66 ppm. One is the crystalline resonance line which has the peak maximum at 66 ppm, and the other is the amorphous resonance line which appears as a shoulder at 66-63 ppm. The assignments of the crystalline and amorphous phases are based on the 13C spin-lattice relaxation analyses and variable-temperature experiments shown below.

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Figure 2. 13C spin-lattice relaxation behavior for the 66, 29, and 26 ppm peaks of PBS obtained by the CPT1 pulse sequence. Parts b, d, and f are the enlarged plots of parts a, c, and e, respectively. Table 2. T1C Values for PBS Obtained by the CPT1 Measurement -CH2O-

T1C/s

(fraction)a

-CH2-

66 ppm

29 ppm

26 ppm

96 (0.52) 16 (0.35) 0.66 (0.13)

106 (0.59) 8.6 (0.30) 0.15 (0.11)

77 (0.45) 7.7 (0.25) 0.25 (0.30)

a The fraction was determined by the peak height in the CPT1 measurement. Since the CP efficiency and line width for respective components may be somewhat different, this fraction does not reflect real mole (or weight) fraction for each component.

Figure 2 shows the 13C spin-lattice relaxation behavior for the 66, 29, and 26 ppm peaks of PBS obtained by the CPT1 pulse sequence. Panels b, d, and f of Figure 2 are the enlarged plots of panels a, c, and e of Figure 2, respectively. The obtained T1C values are summarized in Table 2. It is of interest to note that the 66, 29, and 26 ppm peaks include respectively three components with different T1C values. Since the longer T1C value means the less molecular mobility, the components with T1C values of 77-106 s and 0.150.66 s are assigned to the crystalline and amorphous components, respectively. To obtain the information on the medium component with T1C values of 7.7-16 s, we have analyzed the CP/MAS 13C NMR spectra with different T1C relaxation times (τ). Since the peak at about 66 ppm shows the remarkable difference in 13C chemical shifts between the crystalline and amorphous components, we have used this peak for the analyses. The observed spectra are shown in Figure 3. When τ ) 0.1 s (spectrum a), all the three components with different T1C values are reflected. In this spectrum, two resonance lines assigned to the crystalline and amorphous components are observed. When τ ) 70 s (spectrum c), the component with

Figure 3. CP/MAS 13C NMR spectra of PBS, obtained by the CPT1 pulse sequence with different 13C spin-lattice relaxation times (τ).

T1C values of 96 s is selectively reflected. As expected, only the resonance line assigned to the crystalline component is observed. When τ ) 4 s (spectrum b), two components with T1C values of 96 and 16 s are reflected. As it is clearly seen, the line shape of spectrum b is almost the same as that of spectrum c. Therefore, it has been concluded that the medium component is assignable to the crystalline region. Figure 4 and Table 3 show the results of T1C analyses for P(BS-co-20 mol % BA). As is shown, three components with different T1C values of 89-123 s, 7.8-18 s, and 0.37-1.2 s exist. Figure 5 shows the selectively obtained spectra of P(BS-co-20 mol % BA) by using different T1C relaxation times (τ). Similarly to the case of PBS, the medium component with T1C values of 7.8-18 s is also assignable to the crystalline component. The intensity of magnetization at a given τ can be expressed as exp(-τ/T1C). By using long τ, we can reduce the relative intensity of the mobile crystal (shorter T1C crystal) against that of the rigid crystal (longer

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Figure 4. 13C spin-lattice relaxation behavior for the 66, 29, and 26 ppm peaks of P(BS-co-20 mol % BA) obtained by the CPT1 pulse sequence. Parts b, d, and f are the enlarged plots of parts a, c, and e, respectively. Table 3. T1C Values for P(BS-co-20 mol % BA) Obtained by the CPT1 Method -CH2O66 ppm BS and BAb

-CH234 ppm BAc

29 ppm BSd

T1C/s (fraction)a 108 (0.56)

123 (0.64)

18 (0.31)

11 (0.28)

1.2 (0.13) 0.59 (1.00)

26 ppm BS and BAb 89 (0.51) 7.8 (0.22)

0.52 (0.18) 0.37 (0.27)

a The fraction was determined by the peak height in the CPT1 measurement. Since the CP efficiency and line width for respective components may be somewhat different, this fraction does not reflect real mole (or weight) fraction for each component. b The 66 and 26 ppm peaks reflect the contribution from both BS and BA units. c The 34 ppm peak reflects BA unit. The contribution from BS unit is negligible. d The 29 ppm peak reflects BS unit.

T1C crystal). On the other hand, because of T1C relaxation, the NMR measurement with too long τ does not give good signal/noise ratio for detailed analyses. Considering above factors, we selected the appropriate τ values of 70 and 50 s for Figures 3c and 5c, respectively. This is the first report that the existence of two components with different T1C values has been detected for the crystalline region of PBS-type polymers. In polyolefins, the existence of plural components in the crystalline region have been already reported.25,28,30,31,33,34,39 The component with relatively longer T1C value is assigned to the core of crystals, and the crystalline component with shorter T1C value is assigned to the crystalline area near the amorphous.25,30,31,33,34 For PBStype polymers in the solid state, the rigid crystalline component with T1C values of 77-123 s is ascribed to the inner crystalline region, while the mobile crystalline (dis-

Figure 5. CP/MAS 13C NMR spectra of P(BS-co-20 mol % BA), obtained by the CPT1 pulse sequence with different 13C spin-lattice relaxation times (τ).

ordered crystalline) component with T1C values of 7.7-18 s is assignable to the interfacial crystalline region. Judging from the 13C NMR line shapes in Figures 3 and 5, the timeaveraged position of atoms is almost the same between the rigid and mobile crystals. However, molecular mobility is remarkably different between the two crystals. Future works may discuss the processibility, mechanical property, and biodegradability of PBS-type polymers in viewpoint of the existence of two crystalline components with different mobility. 3.2. Partitioning of Butylene Adipate (BA) Units in the Amorphous/Crystalline Regions. In the CP/MAS 13C NMR spectrum of P(BS-co-20 mol % BA), the 34 ppm peak (indicated as h in Figure 1) reflects only the contribution from BA units. We have used this peak to investigate whether the BA units exist only in the amorphous region or both in the amorphous and in crystalline regions. Figure 6 shows

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Figure 6. 13C spin-lattice relaxation behavior for the 34 ppm peak of P(BS-co-20 mol % BA), obtained by the CPT1 pulse sequence.

Figure 8. MAS 13C NMR spectra of PBS and P(BS-co-20 mol % BA) measured at different temperatures, obtained by the 45° single pulse sequence with a recycle delay of 5 s. Dashed lines indicate the resonance positions for the amorphous CH2 carbons.

the enhanced molecular motion is often the case in CP/MAS NMR. From room temperature to 100 °C in Figure 7A, the resonance lines ascribed to the amorphous component are clearly seen. In contrast, at 120 °C (this temperature is higher than the melting point of 113 °C), no remarkable resonance line is observed. This result means that the molecular mobility of amorphous component is quite different between the solid and the melt. In Figure 7B, CP/ MAS 13C NMR spectra of P(BS-co-20 mol % BA) measured at room temperature, 90 °C, and 110 °C are shown. The amorphous resonance lines, which are indicated by dashed lines, are observed at room temperature and 90 °C. At 110 °C (this temperature is higher than the melting point of 97 °C), in contrast, the amorphous line almost disappears. This result also reveals the remarkable difference in molecular mobility between the solid state and melt. Under melt, the whole sample is amorphous. On the other hand, both crystalline and amorphous components exist in the solid state. Therefore, the difference in molecular mobility between the solid state and the melt reflects the restriction by the crystalline chains which connect to the amorphous chains only in the solid state. In polyolefins, it has been also reported that the higherordered structures (conformation and/or chain packing) of amorphous components are temperature-dependent even above glass transition temperature.33,34,36,38 We have investigated the temperature dependence of the higher-ordered structure of PBS and P(BS-co-20 mol % BA) by performing the MAS 13C NMR experiments without CP. Parts A and B of Figure 8 depict the MAS 13C NMR spectra of PBS and P(BS-co-20 mol % BA), respectively, obtained by the 45° single pulse sequence with recycle delays of 5 s. These spectra reflect almost the full component of the amorphous region, and the contribution from the crystalline region is negligible. In both figures, a remarkable difference in chemical shift values is observed at about 29 ppm between 13C

Figure 7. CP/MAS 13C NMR spectra of PBS and P(BS-co-20 mol % BA) measured at different temperatures. Dashed lines indicate the resonance positions for the amorphous CH2 carbons.

the 13C spin-lattice relaxation behavior for the 34 ppm peak of P(BS-co-20 mol % BA) obtained by the CPT1 pulse sequence. Only one component with a T1C value of 0.59 s was observed. This result is in contrast to those of the peaks from BS units (Figure 4 and Table 3), in which longer T1C components ascribed to the crystalline phase exist. Therefore, it has been concluded that the BA units exist only in the amorphous region. 3.3. The Difference in Molecular Mobility between the Solid Amorphous and the Melt. Figure 7A shows the CP/ MAS 13C NMR spectra of PBS measured at different temperatures. Dashed lines at 64.7, 29.9, and 26.1 ppm denote the chemical shift values for the amorphous region in the solid-state. Under CP (cross-polarization), higher molecular mobility induces the less CP efficiency. In other words, higher molecular motion reduces the signal/noise ratio in the CP/MAS 13C NMR spectrum. During CP (crosspolarization), the spin-locked 1H magnetization transfers to the spin-locked 13C magnetization.43 When the sample is melt, since the higher molecular mobility induces the shorter T1FH values (here, T1FH means the 1H spin-lattice relaxation time under spin-locking), the 1H magnetization almost disappears by T1FH relaxation before transferred to the 13C magnetization. The decrease in CP efficiency as a result of

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the solid amorphous and melt. This experimental result means that the chain packing (here, the “chain packing” correlates with the number of carbon and oxygen atoms per volume) and/or conformation in the amorphous phase is different between the solid and melt. As far as we know, this is the first time that the difference in higher-ordered structures between the solid amorphous and melt is detected in PBS and P(BS-co-20 mol % BA). 4. Conclusions Different crystalline and amorphous states in biodegradable poly(butylene succinate) (PBS) homopolymer and poly(butylene succinate-co-20 mol % butylene adipate) [P(BSco-20 mol % BA)] have been characterized by highresolution solid-state 13C NMR spectroscopy. The results obtained in this work are summarized as follows: (1) 13C spin-lattice relaxation time (T1C) analyses for the crystalline region of PBS and P(BS-co-20 mol % BA) have showed the existence of two components with different T1C values. The component with longer T1C value is assigned to the inner crystalline region, while the crystalline component with shorter T1C value is assignable to the interface near amorphous phase. This is the first time that the interface in the crystalline area has been detected. (2) The partitioning of second monomer unit in the crystalline and amorphous regions in partially crystalline polymer is an important subject. In this work, the location of the butylene adipate (BA) unit in P(BS-co-20 mol % BA) has been investigated by T1C. As the result, it has been confirmed that the BA unit is excluded out from the crystalline region. (3) In variable-temperature high-resolution 13C NMR experiments for the amorphous phase of PBS and P(BS-co20 mol % BA), remarkable differences in CP (crosspolarization) efficiency and 13C NMR chemical shift values have been observed between the melt and the solid state. This result means that the molecular mobility and higherordered structure (chain packing and/or conformation) are quite different between the melt and the solid. This is because of the restriction by the coexisting crystalline chains in the solid state. Acknowledgment. The authors are grateful to Tonen Chemical Co. Ltd. for supplying the poly(butylene succinate) homopolymer sample. We also thank Showa High Polymer Co. Ltd. for kindly providing the poly(butylene succinateco-butylene adipate) random copolymer sample Bionolle #3000. We also gratefully acknowledge Dr. Yuka Kobori of Polymer Chemistry Laboratory, RIKEN Institute, for performing GPC measurements. This work was supported by a grant for Ecomolecular Science Research provided to RIKEN Institute and by a SORST (Solution Oriented Research for Science and Technology) grant from Japan Science and Technology Corporation (JST).

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