Amorphous Phase Structure and Molecular Mobility of

Jan 29, 2002 - Differential scanning calorimetry (DSC), atomic force microscopy (AFM), wide-angle X-ray scattering (WAXD), and solid-state 13C NMR hav...
8 downloads 11 Views 89KB Size
Download PDF
Biomacromolecules 2002, 3, 390-396

390

Crystalline/Amorphous Phase Structure and Molecular Mobility of Biodegradable Poly(butylene adipate-co-butylene terephthalate) and Related Polyesters 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 November 21, 2001; Revised Manuscript Received January 10, 2002

Differential scanning calorimetry (DSC), atomic force microscopy (AFM), wide-angle X-ray scattering (WAXD), and solid-state 13C NMR have been used to investigate the crystalline/amorphous structure and molecular mobility of biodegradable poly(butylene adipate-co-44 mol % butylene terephthalate) [P(BA-co44 mol % BT)] copolyester sample crystallized from the melt. The DSC endothermic peak, which is ascribed to the melting of the crystalline region, was broad relative to those reported for conventional partially crystalline polyesters. In AFM observation, spherulitic morphology was not observed while small particles with a size of about 100 nm were detected. The WAXD pattern of the sample was very broad. These results have indicated that a melt-crystallized P(BA-co-44 mol % BT) sample contains small crystals with a wide distribution in size. A solid-state 13C NMR technique was also used to perform molecular-level and selective analyses for both butylene terephthalate and butylene adipate units. For the butylene terephthalate units, the existence of two components with different microstructure and molecular mobility was detected: one component was assigned to the R-form crystal of poly(butylene terephthalate) homopolymer (PBT) and the other was in amorphous regions. In contrast, all of butylene adipate units were located in amorphous regions. Solid-state NMR data have suggested that sizes of crystalline regions are less than 3 nm. 1. Introduction With an increase in concern about the global environment, many studies on biodegradable aliphatic polyesters are in progress.1-7 To control the melting temperature and processibility, which are generally the weak point of linear aliphatic polyesters, aliphatic-aromatic copolyesters have been developed.2,7-16 It has been reported that some poly(alkylene dicarboxylate-co-alkylene terephthalate)s have good biodegradability and are recognized as ecological materials.7,10-12 As with the case of other polymeric materials, the information on structure and physical properties of poly(alkylene dicarboxylate-co-alkylene terephthalate) is also necessary for designing materials. Although not so many years have passed since the studies on poly(alkylene dicarboxylate-co-alkylene terephthalate)s had started for the purpose of designing ecological materials, there are some reports on the chemical structure, biodegradability, and higher order structures.2,7-16 However, considering the situation for poly(alkylene dicarboxylate)s2,3,17-27 and other aromaticalkylene polymers,28-40 more detailed information is required * 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. ‡ Characterization Center, RIKEN Institute. § Tokyo Institute of Technology.

for the crystalline/noncrystalline microstructure and chain dynamics of poly(alkylene dicarboxylate-co-alkylene terephthalate) copolymers. In this study, therefore, the crystalline/amorphous structure and molecular dynamics of poly(butylene adipate-co- 44 mol % butylene terephthalate) random copolymer [P(BA-co-44 mol % BT)] have been analyzed by differential scanning calorimetry (DSC), atomic force microscopy (AFM), wideangle X-ray scattering (WAXD), and solid-state 13C NMR. The P(BA-co-BT) is, at present, one of the most commercially hopeful materials among biodegradable aliphaticaromatic copolyesters.12,14 Furthermore, as far as we know, no detailed AFM and solid-state 13C NMR analyses have been reported for P(BA-co-BT)s. In this work, the obtained results have been also compared with those for PBA17,21 and PBT28-38 homopolymers. Since the biodegradation processes are significantly affected by the material morphology, the results are of great importance to explain certain effects concerning biodegradability of polyesters. The chemical structures of polymer samples used in this study are depicted in Figure 1. 2. Experimental Section 2.1. Poly(butylene adipate-co-butylene terephthalate) Sample. A P(BA-co-BT) copolymer sample was supplied by BASF Japan. It has been reported that the sample used

10.1021/bm0156476 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/29/2002

Biodegradable Polyesters

Figure 1. Chemical structures of polymer samples used in this work.

in this work is not a simple copolyester but modified with some modular components, e.g., chain extenders (diisocyanates) to increase the molar mass.12 Since the amounts of these modular componentsare very small (in fact, not reflected in NMR spectra), it is likely that these components do not greatly affect our characterization for the solid-state structure and properties. Before characerization and investigation, the sample was purified by dissolving in chroloform and then precipitating from ethanol and finally dried under vaccum conditions. According to the solution-state 13C NMR method,8 the content of the butylene terephthalate unit was determined to be 44 mol %. The degree of randomness was also checked by solution-state 13C NMR,8 with an ideal randomness of 1.022. All the samples used in this study for the DSC, AFM, WAXD, and solid-state NMR were melt-crystallized without drawing. Therefore, only the crystallization temperature and the crystallization time determine the molecular-level structure and dynamics for the crystallized sample. According to the DSC analyses, crystallization time for 3 min was enough to complete the crystallization at a given temperature. Furthermore, the difference in the crystallization temperatures did not greatly influence the shape of DSC curves in the heating process. 2.2. Poly(butylene adipate) Homopolymer Sample. For X-ray and solid-state NMR analyses, poly(butylene adipate) homopolymer (PBA) was used. The PBA sample was purchased from Aldrich (18150-1). 2.3. Poly(butylene terephthalate) Homopolymer Sample. Poly(butylene terephthalate) homopolymer (PBT) was obtained from Aldrich (19094-02). It has been reported that PBT crystal in the R-form can be obtained by conventional melt-crystallization without drawing.28-33,35,37 It is known that the drawing process produces PBT in the β-form.28-31,33,35-37 2.4. DSC Measurements. DSC experiments for the P(BAco-44 mol % BT) copolyester were carried out on a PerkinElmer Pyris differential scanning calorimeter equipped with a CryoFill liquid nitrogen cooling system under a nitrogen flow at a rate of 20 mL/min. The samples of about 3-5 mg were encapsulated in the DSC aluminum pans and then thermally treated under different conditions. For the melting behavior after melt-crystallization, the samples were melted at 180 °C for 2 min and then crystallized at a given temperature. After no exothermic heat flow could be detected on DSC curves, which means that the crystallization was completed, the P(BA-co-44 mol % BT) samples were heated directly from the crystallization temperature to the melt at a

Biomacromolecules, Vol. 3, No. 2, 2002 391

rate of 20 °C/min. For determination of glass-transition temperature Tg, the samples were quenched to -50 °C from the melt and then heated at a rate of 20 °C/min to the melt. The heat flow during the heating process was recorded. 2.5. AFM Observation. The P(BA-co-44 mol % BT) thin films with a thickness of ca. 100 nm were prepared on glass cover slips by solution casting. These thin films were meltcrystallized at a given temperature for a certain period. The surface morphology of the melt-crystallized thin films was examined by AFM (SPI3700/SPA300, Seiko Instrument Inc.) under tapping mode and ambient conditions.23,26 A rectangle cantilever with a spring constant of 13 N/m and a resonance frequency of 131 kHz was utilized in AFM measurement. 2.6. X-ray Measurements. The crystal structures of P(BAco-44 mol % BT), PBA, and PBT were analyzed by the WAXD method. The WAXD patterns were recorded on a Rigaku RINT-2500 system using nickel-filtered Cu KR radiation (λ ) 0.154 nm, 40 kV, 110 mA)25,26 at room temperature. Melt-crystallized films of polymers with a thickness of 0.05 mm were used for WAXD measurements. The PBA and P(BA-co-44 mol % BT) films were obtained by crystallization at 40 and 80 °C, respectively, for 1 day from the melt. The PBT was melt-crystallized at 190 °C for 2 days. 2.7. Solid-State 13C NMR Measurements. High-resolution solid-state 13C NMR measurements were carried out at room temperature on a Chemagnetics Infinity-400 spectrometer under a static magnetic field of 9.4 T. The 1H and 13C field strength, γB1/2π, was 62.5 kHz. The contact time for the cross-polarization (CP) process was 2.0 ms throughout this work. The magic angle spinning (MAS) rate for each measurement 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) and 13C spin-spin relaxation times (T2C) were measured by Torchia’s CPT1 pulse sequence41 and the 13C spin-echo pulse sequence, respectively. Before P(BA-co-44 mol % BT) was measured, the sample was isothermally crystallized at 40 °C for 30 min from the melt and then slowly air-cooled to room temperature. For the analyses of PBA, the sample was kept at room temperature for more than 1 month. Considering the previous reports on the crystal structure and the effect of annealing on PBA,17,21 our sample used for solid-state 13C NMR measurements includes the R-form PBA crystal as well as amorphous component. 3. Results and Discussion 3.1. DSC Analyses of Thermal Properties. Figure 2 shows DSC melting behavior for the P(BA-co-44 mol % BT) samples, crystallized at 0, 10, 20, 30, 40, and 50 °C. Similar endothermic peaks, which have peak maxima at 110 °C, were observed for all the samples crystallized at different temperatures. This melting temperature is about 100 °C lower

392

Biomacromolecules, Vol. 3, No. 2, 2002

Figure 2. DSC melting curves for P(BA-co-44 mol % BT) samples after melt-crystallized at a given temperature and then heating to the melt at a rate of 20 °C/min.

Figure 3. DSC melting curve at a rate of 20 °C/min for P(BA-co-44 mol % BT) after quenching to -50 °C from the melt.

Figure 4. AFM images of P(BA-co-44 mol % BT) film surface after melt-crystallized at 40 °C: (left) topographic image; (right) deflection image.

than that of poly(butylene terephthalate) homopolymer,15,16,38 suggesting that P(BA-co-44 mol % BT) has a poor crystal structure. Moreover, these melting peaks of copolyester are significantly broad in comparison with other aliphatic polyesters1-3,15,17,23,25 and poly(butylene terephthalate).15,38 This result indicates that the crystalline region observed as the endothermic peaks in DSC curves is disordered and not rigid compared to conventional polymer crystals. This indication will be supported by X-ray, AFM, and solid-state 13C NMR results shown below. Figure 3 shows the DSC curve of P(BA-co-44 mol % BT) sample which was quenched to -50 °C from the melt and then heated with a rate of 20 °C/min, indicating that the glasstransition temperature Tg is -28 °C. 3.2. Crystalline/Amorphous Morphology As Revealed by AFM. In Figure 4, AFM images of the melt-crystallized P(BA-co-44 mol % BT) thin film are shown. Spherulitic morphology cannot be detected, while many small particles with a size of about 100 nm were observed. This result also

Kuwabara et al.

Figure 5. WAXD profiles for melt-crystallized films of PBA, P(BAco-44 mol % BT), and PBT.

suggests that the crystalline region of P(BA-co-44 mol % BT) is not well-ordered in comparison with liner aliphatic polyesters6,23,26 and poly(butylene terephthalate). 3.3. Analyses of the Crystalline Region by WAXD. Figure 5 shows the WAXD patterns of the PBA homopolymer, P(BA-co-44 mol % BT) copolymer, and PBT homopolymer. These polymer samples were melt-crystallized, and no drawing process was carried out during crystallization. The diffraction pattern of P(BA-co-44 mol % BT) sample is very broad. This result, in accord with the DSC, AFM, and solid-state 13C NMR data, also reveals the poor crystal structure of this sample. From the peak positions of the weak peaks, it is concluded that the crystal structure of P(BA-co44 mol % BT) is almost the same as that of PBT.28-33 This result suggests that the BA units exist in an amorphous region and are excluded from the crystalline region which is composed of aromatic BT units. 3.4. Solid-State 13C NMR Analyses for the Structure in the Aromatic Region. Figure 6 shows the CP/MAS 13C NMR spectrum of P(BA-co-44 mol % BT) sample and peak assignment for each resonance line. To analyze the microstructure in the aromatic region, an enlarged spectrum is shown in Figure 7. Under CP (cross-polarization), the less mobile crystalline component is predicted to be enhanced compared to the mobile amorphous component. Spectrum a in Figure 7 was obtained by the CP pulse sequence and corresponds to the T1C decay time of 0 s. Spectra b and c were obtained by the CPT1 pulse sequence41 with the T1C decay times of 2 and 4 s, respectively. In general, for solid crystalline polymers at room temperature, the enhanced molecular mobility induces the fast T1C decay in the CPT1 measurements. Accordingly, the component which has the peak maximum at 130 ppm is mobile compared to the other component with the peak maximum of 131 ppm. This result agrees well with the 13C chemical shift analysis shown below. The 13C chemical shift values reported for the amorphous, R-form crystalline, and β-form crystalline components of poly(butylene terephthalate) (PBT)37 are also indicated by arrows in Figure 7. The 13C chemical shift of the mobile component of P(BA-co-44 mol % BT) sample, which is observed as the resonance line at 130 ppm, is similar to that

Biodegradable Polyesters

Figure 6. CP/MAS

13C

Biomacromolecules, Vol. 3, No. 2, 2002 393

NMR spectrum of P(BA-co-44 mol % BT) and peak assignment for each resonance line.

Figure 7. Enlarged CP/MAS 13C NMR spectrum of P(BA-co-44 mol % BT) in the aromatic region. Spectrum a was obtained by the conventional CP pulse sequence. Spectra b and c were measured with the CPT1 pulse sequence by setting the T1C decay time to 2 and 4 s, respectively. For the purpose of comparison, 13C chemical shift values for PBT reported by Gomez et al.37 are also shown.

of PBT in the amorphous form. Moreover, the 13C chemical shift of the less mobile component, which has the peak maximum at 131 ppm, is almost equal to that of PBT in the R-form crystal. Therefore, it has been concluded that P(BAco-44 mol % BT) sample includes the R-form crystalline component as well as the amorphous component reported for PBT homopolymer.28-33,35,37 3.5. Solid-State 13C NMR Analyses for the Structure in the Butylene Adipate Region. In Figure 8, the enlarged CP/MAS 13C NMR spectrum of the butylene adipate units for P(BA-co-44 mol % BT) sample is shown as a solid line. For the purpose of comparison, the crystalline-rich and amorphous-rich spectra of the poly(butylene adipate) (PBA) sample are also shown as dashed lines, which were obtained by the conventional CP pulse sequence and the 13C 45° single pulse sequence with a recycle of 5 s, respectively. The 13C chemical shift value of the butylene adipate units for P(BAco-44 mol % BT) sample is almost identical with that of PBA in the amorphous form. On the other hand, at the position for the 13C chemical shift of PBA in the R-crystal

Figure 8. Enlarged MAS 13C NMR spectra of P(BA-co-44 mol % BT) and PBA for the butylene adipate units: solid line, P(BA-co-44 mol % BT); dashed lines, PBA. The crystalline-rich spectrum for the PBA was obtained by the conventional CP pulse sequence. The amorphous-rich spectrum for the PBA was obtained by the 13C 45° pulse sequence with a recycle delay of 5 s.

form,17,21 even a shoulder is hardly observed although the CP enhances the less mobile component. These results suggest that the solid-state molecular-level structure of the adipate units for P(BA-co-44 mol % BT) sample is similar to that of PBA in the amorphous form. 3.6. Molecular Mobility in the Solid State As Revealed by T1C. As described in DSC analyses, the glass-transition temperature of P(BA-co-44 mol % BT) is remarkably lower than room temperature. For P(BA-co-44 mol % BT) and PBA samples, the T1C data were measured at room temperature between their glass-transition and melting temperatures. Table 1 summarizes the T1C values of each peak for P(BAco-44 mol % BT) sample measured at room temperature. The characters “NPAR” and “PAR” mean the nonprotonated aromatic carbons and aromatic carbons, respectively. As for PAR carbons, two components with different T1C values exist: the T1C value of one component was 0.53 s, while the value of the other component was 22 s. This result reveals that the butylene terephthalate unit plays a major role for the formation of crystalline region in P(BA-co-44 mol % BT) sample. All T1C values of the carbonyl and NPAR carbons are remarkably larger than 1 s. As expected, this result means that the carbonyl and NPAR carbons are less mobile compared to methylene and PAR carbons.

394

Biomacromolecules, Vol. 3, No. 2, 2002

Kuwabara et al.

Table 1. T1C Values for P(BA-co-44 mol % BT) Measured at Room Temperature NPARa

CdO 172 ppm

T1C/s (fraction)

166 ppm

134 ppm 41 (1.00)

11 (1.00)

-CH2O-

PARb 130 ppm

-CH2-

65 ppm

64 ppm

34 ppm

26 ppm

25 ppm

0.83 (1.00)

0.44 (1.00)

0.49 (1.00)

0.43 (1.00)

0.43 (1.00)

22 (0.74)

68 (1.00) 0.53 (0.26)

a

Nonprotonated aromatic carbons. b Protonated aromatic carbons.

Table 2. T1C Values for PBA Sample Measured at Room Temperature CdO

T1C/s (fraction)

-CH2O-

-CH2-

174 ppm

64 ppm

34 ppm

24 ppm

218, 0.44 (0.90) (0.10)

38, 6.5, 0.53 (0.58) (0.31) (0.11)

84, 1.8 (0.80) (0.20)

42, 3.4, 0.43 (0.62) (0.26) (0.12)

Table 3. T2C Values for P(BA-co-44 mol % BT) Sample Measured at Room Temperature -CH2-

T2C/s (fraction)

34 ppm

26 ppm

25 ppm

5.0 (1.00)

5.0 (1.00)

4.7 (1.00)

Figure 9. 13C spin-spin relaxation behavior of the 25 ppm peak for P(BA-co-44 mol % BT) sample.

For methylene chain, the T1C values of all carbons are in the range of 0.43-0.83 s. Even if some carbons exist in the crystalline region, their molecular mobility is almost equal to that in the amorphous region. This result is in contrast to the case of the conventional methylene carbons in the crystalline region (see Table 2, for example). Table 2 shows the T1C values for the PBA homopolymer sample measured at room temperature. In this case, T1C relaxation behavior is obviously different from that for the P(BA-co-44 mol % BT) sample. The PBA sample includes the rigid crystalline methylene carbons with T1C values of 30-90 s. In contrast, no such rigid methylene carbon exists in P(BA-co-44 mol % BT) as shown in Table 1. Therefore, it has been clarified here that the molecular mobility of the butylene adipate unit for P(BA-co-44 mol % BT) is almost the same with that of the amorphous region of PBA homopolymer. 3.7. Molecular Mobility in the Solid State As Revealed by T2C. To obtain more detailed information on the molecular mobility and phase structure for the methylene chain, T2C measurements have been carried out for P(BA-co-44 mol % BT) sample. In the noncrystalline region of melt-crystallized polyethylenes, two components with different T2C values of 0.3-10 ms and 10-100 µs are detected.42-45 The components with the longer and shorter T2C values are assigned to the rubbery-amorphous and crystalline-noncrystalline interfacial regions, respectively. Such assignments for the noncrystalline region of polyethylenes are in good accordance with the results obtained by Raman spectroscopy,46 theoretical calculation,47 and recent 1H pulse NMR analyses.48-52

Figure 10. 1H spin-lattice relaxation behavior in the rotating frame for the 25 ppm peak of P(BA-co-44 mol % BT).

Figure 9 depicts the 13C spin-spin relaxation behavior of the 25 ppm peak assignable to the methylene carbons in P(BA-co-44 mol % BT). As is shown, only one component with a T2C value of 4.7 ms can be detected. Similar T2C relaxation behaviors were also observed for the 26 and 34 ppm peaks of this sample. These results, which are summarized in Table 3, mean that the whole molecular mobility of the methylene chain in P(BA-co-44 mol % BT) is the same with that of the rubbery-amorphous component and different from that of the crystalline-amorphous interfacial component in melt-crystallized polyethylenes.42-45 3.8. Crystalline Size Evaluation by 1H Spin-Lattice Relaxation Time in the Rotating Frame (T1FH). To evaluate the domain sizes in phase-separated partially crystalline polymers and the miscibility in polymer blends, the analysis of 1H spin diffusion is one of the most effective methods.53-56 When a polymer system consists of a single phase in the length scale of about 3 nm, 1H spin-lattice relaxation time in the rotating frame (T1FH) measurement shows only one component.53,54 In contrast, because 1H spin diffusion is ineffective, phase-separated system in the length scale of 3 nm do not give a single component in the T1FH measurement.53-55 Figure 10 shows the 1H spin-lattice relaxation behavior in the rotating frame for the 25 ppm of P(BT-co-44 mol %

Biodegradable Polyesters

BT). The result is only a single component with a T1FH value of 3.2 ms. This result suggests that the sizes of crystalline regions are less than 3 nm. Such a small crystalline size evaluation by solid-state NMR is in good agreement with the data of DSC and WAXD analyses. 4. Conclusions Crystalline/amorphous phase structure and molecular mobility of P(BA-co-44 mol % BT) copolymer and related polyesters have been characterized by differential scanning calorimetry (DSC), atomic force microscopy (AFM), wideangle X-ray scattering (WAXD) and solid-state 13C NMR spectroscopy. The results obtained in this work are summarized as follows: (1) In the heating process of DSC curves, the observed DSC endothermic peak was broad relative to those reported for crystalline aliphatic polyesters and PBT. Moreover, in AFM observation, spherulitic morphology could not be observed, while small particles with a size of about 100 nm were detected. These results suggest that P(BA-co-44 mol % BT) sample has small crystals with a wide distribution in size. (2) The WAXD pattern of P(BA-co-44 mol % BT) also reflected the presence of very small crystals. Considering from the diffraction peaks in the WAXD pattern, it has been indicated that the crystalline region of P(BA-co-44 mol % BT) is composed of butylene terephthalate units and that almost all butylene adipate units exist in a noncrystalline region. This indication agrees well with the solid-state 13C NMR results. (3) For the benzene unit of P(BA-co-44 mol % BT) copolymer, the existence of two components with different molecular-level structure and molecular mobility was detected by CP/MAS 13C NMR. One component was similar to the R-form crystal of PBT homopolymer and the other was similar to the amorphous component of PBT homopolymer. (4) In 13C spin-lattice relaxation time (T1C) measurement, no remarkable difference in molecular mobility was observed between the crystalline and amorphous regions for the methylene carbons of the P(BA-co-44 mol % BT) copolymer. This result means that the methylene chain does not play an important role in the formation of P(BA-co-44 mol % BT) crystal. (5) Similar CP/MAS 13C NMR and T1C analyses were also performed for the butylene adipate units of the P(BA-co-44 mol % BT) copolymer. In this case, the molecular-level structure and molecular mobility were almost the same with those for the amorphous region of PBA homopolymer. Acknowledgment. The authors wish to thank BASF Japan for kindly supplying the poly(butylene adipate-cobutylene terephthalate) random copolymer sample Ecoflex. We are also grateful to Dr. Motonori Yamamoto of BASFAG for his helpful discussion. 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 the Japan Science and Technology Corporation (JST).

Biomacromolecules, Vol. 3, No. 2, 2002 395

References and Notes (1) Doi, Y. Microbial Polyesters; VCH Publishers: New York, 1990. (2) Doi, Y., Fukuda, K., Eds. Biodegradable Plastics and Polymers; Elsevier: Amsterdam, 1994, and related references therein. (3) Ikada, Y.; Tsuji, H. Macromol. Rapid Commun. 2000, 21, 117. (4) Sudesh, K.; Abe, H.; Doi, Y. Prog. Polym. Sci. 2000, 25, 1503. (5) Yamashita, K.; Aoyagi, Y.; Abe, H.; Doi, Y. Biomacromolecules 2001, 2, 25. (6) Kikkawa, Y.; Abe, H.; Iwata, T.; Inoue, Y.; Doi, Y. Biomacromolecules 2001, 2, 940. (7) Mu¨ller, R.-J.; Kleenberg, I.; Deckwer, W.-D. J. Biotechnol. 2001, 86, 87 and related references therein. (8) Witt, U.; Mu¨ller, R.-J.; Deckwer, W.-D. Macromol. Chem. Phys. 1996, 197, 1525. (9) Yoo, E. S.; Im, S. S.; Yoo, Y. Macromol. Symp. 1997, 118, 739. (10) Kleenberg, I.; Hetz, C.; Kroppenstedt, R. M.; Mu¨ller, R.-J.; Deckwer, W.-D. Appl. EnViron. Microbiol. 1998, 64, 1731. (11) Mu¨ller, R.-J.; Witt, U.; Rantze, E.; Deckwer, W.-D. Polym. Degrad. Stab. 1998, 59, 203. (12) Witt, U.; Yamamoto, M.; Seelinger, U.; Mu¨ller, R.-J.; Warzelhan, V. Angew. Chem., Int. Ed. 1999, 38, 1438. (13) Atfani, M.; Brisse, F. Macromolecules 1999, 32, 7741. (14) Witt, U.; Einig, T.; Yamamoto, M.; Kleeberg, I.; Deckwer, W.-D.; Mu¨ller, R.-J. Chemosphere 2001, 44, 289. (15) Li, Z.; Ma, D.; Cheng, X.; Luo, X.; Zhang, G. J. Polym. Sci., Polym. Phys. Ed. 2001, 39, 634. (16) Nagata, M.; Goto, H.; Sakai, W.; Tsutsumi, N.; Yamane, H. Sen-i Gakkaishi 2001, 57, 178. (17) Minke, R.; Blackwell, J. J. Macromol. Sci. Phys. 1979, B16, 407. (18) Ichikawa, Y.; Suzuki, J.; Washiyama, J.; Moteki, Y.; Noguchi, K.; Okuyama, K. Polymer 1994, 35, 3338. (19) Mochizuki, M.; Mukai, K.; Yamada, K.; Ichise, N.; Murase, S.; Iwaya, Y. Macromolecules 1997, 30, 7403. (20) Fujimaki, T. Polym. Degrad. Stab. 1998, 59, 209. (21) Iwata, T.; Doi, Y. Macromol. Chem. Phys. 1999, 200, 2429. (22) Tsuji, M.; Tsujimoto, J.; Murakami, S.; Kohjiya, S. Sen-i Gakkaishi 1999, 55, 511. (23) Gan, Z.; Abe, H.; Doi, Y. Biomacromolecules 2000, 1, 704. (24) Gan, Z.; Abe, H.; Doi, Y. Biomacromolecules 2000, 1, 713. (25) Gan, Z.; Abe, H.; Doi, Y. Biomacromolecules 2001, 2, 313. (26) Gan, Z.; Abe, H.; Kurokawa, H.; Doi, Y. Biomacromolecules 2001, 2, 605. (27) Iwata, T.; Doi, Y.; Isono, K.; Yoshida, Y. Macromolecules 2001, 34, 7343. (28) Yokouchi, M.; Sakakibara, Y.; Chatani, Y.; Tadokoro, H.; Tanaka, T.; Yoda, K. Macromolecules 1976, 9, 266. (29) Hall, I. H.; Pass, M. G. Polymer 1976, 17, 807. (30) Stambaugh, B.; Koenig, J. L.; Lando, J. B. J. Polym. Sci., Polym. Lett. Ed. 1977, 15, 299. (31) Desborough, I. J.; Hall, I. H.Polymer 1977, 18, 825. (32) Bereton, M. G.; Davies, G. R.; Jakeways, R.; Smith, T.; Ward, I. M. Polymer 1978, 19, 17. (33) Stambaugh, B.; Koenig, J. L.; Lando, J. B. J. Polym. Sci., Polym. Phys. Ed. 1979, 17, 1053. (34) Jelinski, L. W.; Dumais, J. J.; Engel, A. K. Macromolecules 1983, 16, 492. (35) Davidson, I. S.; Manuel, A. J.; Ward, I. M. Polymer 1983, 24, 30. (36) Perry, B. C.; Keenig, J. L.; Lando, J. B. Macromolecules 1987, 20, 422. (37) Gomez, M. A.; Cozine, M. H.; Tonelli, A. E. Macromolecules 1988, 21, 388. (38) Guo, M.; Zachman, H. G. Macromolecules 1997, 30, 2746. (39) Ge, J. J.; Guo, M.; Zhang, Z.; Honigfort, P. S.; Mann, I. K.; Wang, S.-Y.; Harris, F. W.; Cheng, S. Z. D. Macromolecules 2000, 33, 3983. (40) Ishida, H.; Horii, F. Macromolecules 2001, 34, 7751. (41) Torchia, D. A. J. Magn. Reson. 1978, 30, 613. (42) Kitamaru, R.; Horii, F.; Murayama, K. Macromolecules 1986, 19, 636. (43) Nakagawa, M.; Horii, F.; Kitamaru, R. Polymer 1990, 31, 323. (44) Kitamaru, R.; Horii, F.; Zhu, Q.; Bassett, D. C.; Olley, R. H. Polymer 1994, 35, 1171. (45) Kuwabara, K.; Kaji, H.; Horii, F.; Bassett, D. C.; Olley, R. H. Macromolecules 1997, 30, 7516. (46) Strobl, G. R.; Hagedorn, W. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 1181. (47) Flory, P. J.; Yoon, D. Y.; Dill, K. A. Macromolecules 1984, 17, 862.

396

Biomacromolecules, Vol. 3, No. 2, 2002

(48) Klu¨ver, W.; Ruland, W. Prog. Colloid Polym. Sci. 1978, 64, 255. (49) Hansen, E. W.; Kristiansen, P. E.; Pedersen, B. J. Phys. Chem. B 1998, 102, 5444. (50) Kristiansen, P. E.; Hansen, E. W.; Pedersen, B. J. Phys. Chem. B 1999, 103, 3552. (51) Uehara, H.; Yamanobe, T.; Komoto, T. Macromolecules 2000, 33, 4861. (52) Kristiansen, P. E.; Hansen, E. W.; Pedersen, B. Polymer 2000, 41, 311.

Kuwabara et al. (53) McBrierty, V. J.; Douglass, D. C.; Kwei, T. K. Macromolecules 1978, 11, 1265. (54) Albert, B.; Je´roˆme, B.; Teyssie´, P.; Smyth, G.; Boyle, N. G.; McBrierty, V. J. Macromolecules 1985, 18, 388. (55) VanderHart, D. L.; Pe´rez, E. Macromolecules 1986, 19, 1902. (56) Cai, W. Z.; Schmidt-Rohr, K.; Egger, N.; Gerharz, B.; Spiess, H. W. Polymer 1993, 34, 267.

BM0156476