Melt Crystallization and Crystal Transition of Poly(butylene adipate

Notable band shifts in the 1300-1100 cm-1 region indicate that the twist of polymer chains in the R-form is located in the C-O-C and C-O linkages. Mor...
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J. Phys. Chem. B 2008, 112, 3311-3314

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Melt Crystallization and Crystal Transition of Poly(butylene adipate) Revealed by Infrared Spectroscopy Chao Yan,† Ying Zhang,† Yun Hu,‡ Yukihiro Ozaki,‡ Deyan Shen,§ Zhihua Gan,§ Shouke Yan,*,§ and Isao Takahashi*,† Department of Physics, and Department of Chemistry, School of Science and Technology, Kwansei-Gakuin UniVersity, Gakuen, Sanda 669-1337, Japan, and State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed: September 7, 2007; In Final Form: December 18, 2007

The structure evolution of poly(butylene adipate) (PBA) during isothermal melt crystallization and phase transition processes is investigated by Fourier transform infrared spectroscopy (FTIR). Detailed IR spectra analysis and band assignment are performed to disclose the bands sensitive to the R-form crystalline order of PBA. It is revealed from the in situ IR study that the functionalities within PBA chains alter simultaneously during the melt crystallization process. From the analysis of the spectral changes, it is found that band shifts take place during the phase transition process of PBA from its metastable β-form crystal to the stable R-form. Notable band shifts in the 1300-1100 cm-1 region indicate that the twist of polymer chains in the R-form is located in the C-O-C and C-O linkages. Moreover, the results elucidated that the different segments of molecular chains tune up their conformations synchronously during the β to R crystal transition process of PBA. It is suggested that the βR phase transition process proceeds randomly throughout the solid at a constant rate.

1. Introduction

SCHEME 1: Chemical Structure of Poly(butylene adipate)

Aliphatic polyesters are recently of great interest due to their biodegradability for environmental and ecological advantages.1,2 Therefore, they were extensively studied during the past years.3-17 For biodegradable polymers, their degradation rates were most frequently concerned. It was well documented that the degradation rate of a polymer depends not only on the chemical structure but also on its supermolecular structures. For example, Doi and co-workers found that the degradation rate of polyesters is inversely proportional to the lamellar thickness.18 Moreover, the chain packing manner, that is, crystalline modification, has also a remarkable influence on the degradation rate of the biodegradable polymers. For example, poly(butylene adipate) (PBA), a semicrystalline polyester (see Scheme 1) exhibiting two kinds of crystal forms designated as R- and β-forms,19,20 shows more rapid biodegradation rate in its R-form than in β-form, although the R-form has thicker lamellae.21 It is suggested that the faster biodegradation rate of R-form PBA originates from its higher molecular mobility.22 Taking into account that the polymorphism is an often encountered phenomenon of many types of polyester,18,19,23-29 it is of great importance to understand molecular arrangements in the different crystal forms and their changes during the phase transition process. Polymorphic phenomenon of PBA was first reported by Minke and Blackwell.19,20 The R-PBA is found thermodynamically more stable than its β-counterpart. It has been well documented that temperature is an important factor in governing a specific type of crystal formation;25,26 for example, PBA chains * Corresponding authors. E-mail: [email protected] (I.T.); [email protected] (S.Y.). † Department of Physics, Kwansei-Gakuin University. ‡ Department of Chemistry, Kwansei-Gakuin University. § The Chinese Academy of Sciences.

pack in R-form by melt crystallization at temperature above 32 °C, while the β-form crystal is formed at temperature below 27 °C. Up to date, even though some DSC and X-ray investigations have been performed on the polymorphic phenomenon of PBA,30 little attention has been paid to conformation changes in the different crystalline forms and during their transition process of PBA. This is actually of great importance for understanding the mechanical and thermal properties as well as biodegradability. Therefore, the structural evolution of PBA during isothermal crystallization and crystal transition processes is investigated by infrared spectroscopy. The purpose of this Article is to present the detailed experimental procedure and some results concerning the crystallization as well as the phase transition process of PBA. 2. Experimental Section The PBA used in the present study was produced by BASF AG Ludwigshafen, Germany. The weight-average molecular weight of PBA is about 4 × 104, with a polydispersity of 1.7. Its melting temperature was measured to be 57 °C. The sample was purified by reprecipitation in methanol from a chloroform solution and dried in vacuum for one week. The PBA samples for melt crystallization and crystal transition were cast on a KBr window from its 1% (m/v) chloroform solution. The thusprepared films were placed under vacuum at room temperature for 48 h to completely remove the residue solvent.

10.1021/jp077195i CCC: $40.75 © 2008 American Chemical Society Published on Web 02/28/2008

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Yan et al.

TABLE 1: Band Assignments of PBA in the r and β Modifications position

assignment

position

assignment

1735 1729 1462 1417 1399 1370 1263

CdO stretching in amorphous CdO stretching in β-form crystalline CH2 bending in R-form crystalline CH2 bending in β-form crystalline CH2 wagging in R-form crystalline CH2 wagging in β-form crystalline C-O-C asymmetric stretching in β-form crystalline C-O stretching in β-form crystalline CH2 sketch deformation in β-form crystalline CH2 sketch deformation in β-form crystalline CH2 sketch deformation in R-form crystalline

1731 1464 1419 1401 1392 1369 1260

CdO stretching R-form crystalline CH2 bending in β-form crystalline CH2 bending in R-form crystalline CH2 wagging in β-form crystalline CH2 wagging in amorphous CH2 wagging in R-form crystalline C-O-C asymmetric stretching in R-form crystalline C-O stretching in R-form crystalline CH2 sketch deformation in R-form crystalline CH2 sketch deformation in β-form crystalline CH2 rocking in crystalline

1175 960 930 909

For IR spectroscopic measurements, a Nicolet Magna 870 spectrometer equipped with a MCT detector was used. For the melt crystallization experiment of PBA, the as-prepared sample was set on a homemade variable-temperature cell, which was placed in the compartment of the spectrometer. The sample was first heated to 77 °C at 5 °C/min for 5 min to erase the thermal history and then cooled to the required temperature for the IR measurement of isothermal melt crystallization. For the crystal transition experiments of PBA, the solution cast film at room temperature was directly transferred to the variable-temperature cell at the desired temperature. The IR spectra were recollected at a 4 cm-1 resolution with a 2 min interval during the melt crystallization and 1 min interval in the crystal transition process. During the experimental process, the sample was protected with dry N2 gas. 3. Results and Discussion For IR spectroscopic characterization, correct band assignments are the essential requirement for a reasonable analysis. For this purpose, time-dependent IR spectra of PBA were collected during the isothermal crystallization process (see Figure S1 in the Supporting Information). The crystallization temperature was chosen at 48 °C because (i) the crystallization of PBA at this temperature is suitable to be followed by infrared spectroscopy, and (ii) the PBA grows in its R-crystal with the chains packed in a monoclinic unit cell, as confirmed by the X-ray diffraction shown in Figure S3 of the Supporting Information. To our best knowledge, there is no report on the IR analysis of PBA. Therefore, we have made the assignments of the IR bands referring to those for polyethylene,31 poly(carprolactone),32,33 poly(butylene succinate) (PBS),34 and the related polyesters with tetramethylene glycol units, for example, poly(butylene terephthalate).35-37 As summarized in Table 1, the band at 1735 cm-1 is assigned to the CdO stretching in amorphous phase because it shifts gradually to 1731 cm-1 during the melt crystallization process (see Figure S2 in the Supporting Information), while the band at 1392 cm-1 is assigned to the CH2 wagging vibration in amorphous phase. The bands at 1731, 1462, 1419, 1399, 1369, 1260, 1171, 959, 909, and 734 cm-1 show significant intensity variation with time, reflecting chain conformational changes caused by ordered chain packing during crystallization, and therefore are assigned to the crystalline bands of PBA in its R-form. Through a careful comparison of the IR spectra of PBA in its R- and β-forms (see Figure S4 in the Supporting Information), the bands at 1729, 1464, 1417, 1401, 1370, 1263, 1175, 960, 930, and 910 cm-1 are found to be the characteristic bands of the β-PBA. According to these assignments, the crystallization as well as the phase transition

1170 959 910 734

processes of PBA can be successfully followed by the conformational changes. Figure 1a shows the intensities of the bands corresponding to the R form crystals plotted as a function of crystallization time during the crystallization process. The appearance of the sigmoid shapes of these plots reveals simply that the isothermal crystallization process of PBA at 48 °C proceeds via the nucleation and growth of the spherulites, which is in agreement with the optical microscopic observation. Kinetic analysis of the crystallization process according to the IR data obtains an Avrami exponent of about 3.35 (see Supporting Information), indicating again that PBA crystals grow in spherulites at 48 °C. Figure 1b shows the normalized peak intensities of the bands shown in Figure 1a versus the crystallization time. From Figure 1b, the simultaneous intensity changes of all of these bands demonstrate a cooperative structural change of various functionalities within PBA during the crystallization. This is different from the crystallization of PHB and PLLA. It was documented that the arrangements of the C-O-C and C-C linkages in PHB occur with different rates during melt crystallization,38 while sequential change for the methyl and ester group during the crystallization of PLLA takes place.39 It is well documented that among the crystalline modifications of a polymorphic polymer, there is generally one phase thermodynamically more stable than the others. The metastable phases are able to transit into the stable phase under suitable

Figure 1. (a) IR band intensity as a function of time during the isothermal crystallization of PBA at 48 °C. (b) Corresponding normalized peak intensity versus time.

Melt Crystallization and Crystal Transition of PBA

Figure 2. (a) IR spectra of PBA recorded during the annealing process at 49 °C. The arrows indicate the directions of the band changes with annealing time. (b) The second derivatives before and after the crystal transition process.

Figure 3. Normalized intensity changes of the PBA R-form crystalline sensitive bands at 1419, 1369, 1260, and 1170 cm-1 as a function of annealing time at 49 °C.

thermal conditions. In the present case, it is known that the monoclinic R-form PBA is more stable than the orthorhombic β-form and the β to R transition takes place under proper conditions. The phase transition process is, however, of less concern so far. Therefore, the phase transition process of PBA from the β-form to the R-form was followed by IR spectroscopy. To elucidate the structure evolution in the βR transition process of PBA, the IR spectra of solution cast β-PBA film were recorded during the annealing process at 49 °C (see Figure 2a). Many band shifts have been identified in the IR spectra during annealing. Especially the bands in region of 1300-1100 cm-1 involved in the C-O-C and C-O vibration are very sensitive to the annealing time. This can be more clearly seen from the second derivatives of spectra before and after annealing displayed in Figure 2b. The band shifts, for example, shifts of the C-O-C asymmetric stretching band in β-form at 12631260 cm-1 related to the C-O-C asymmetric stretching in R-form and the C-O stretching band in β-form at 1175-1170 cm-1 (the C-O stretching band in R-form), indicate unambiguously the occurrence of the βR phase transition. Figure 3 shows the plots of the normalized peak intensity changes of the bands at 1419, 1369, 1260, and 1170 cm-1 as a function of annealing time at 49 °C. From Figure 3, it is clear that substantial intensity increases take place during the first half hour of annealing,

J. Phys. Chem. B, Vol. 112, No. 11, 2008 3313 followed by very gradual increases in a long time range. Moreover, all of these bands start to change at the same time and alter synchronously. Because the bands at 1419, 1369, 1260, and 1170 cm-1 are assigned, respectively, to CH2 bending, CH2 wagging, C-O-C asymmetric, and symmetric stretching, the above feature demonstrates that the segments of PBA molecular chains modify their conformations simultaneously during phase transition. According to the above IR results, several aspects concerning the rearrangement of the PBA chains during the βR phase transition can be addressed. First, it is well documented that the PBA β-form is characterized by an orthorhombic unit cell with all trans conformation, while the R-form is characterized by a monoclinic unit cell with distorted molecular chains. Minke and Blackwell19 suggested that the distortions in the R-form of PBA might be located in the diacid moiety. Boyd et al.40 provided similar results in their calculation study of the structure and packing in crystalline aliphatic polyesters. Recently, Noguchi et al.41 investigated the R-form PBA samples by synchrotron X-ray fiber diffraction in detail. They pointed out that the torsional angle of CH2-CH2-O-C(dO) is significantly deviated from a trans value in the PBA R-form crystal. Moreover, on the basis of the results of both X-ray fiber diffraction of uniaxially oriented samples and electron diffraction patterns obtained from solution grown single crystals in R-form, Pouget et al.42 postulated a more compressed chain conformation of the R-form PBA crystal. All of these results suggested that the chain distortion of R-PBA takes place in the ester part. From our IR results, it is found that the shifts and intensity changes of the IR bands in the region from 1300 to 1100 cm-1 are highly sensitive to the crystal transition. Considering that these bands are strongly correlated to the C-O-C and C-O stretching modes resulting from different intermolecular interactions of ester groups, the relatively huge peak shift and intensity change of them indicate that there are indeed intense twists in the segment of C-O-C and C-O linkages during the βR transition process of PBA. This also indicates the occurrence of molecular distortion in the ester part and is in agreement with the results reported in the literature.19,40-42 Second, on the basis of the results of SAXS, WAXD, and DSC, Gan et al.26 have supposed a mechanism of PBA crystal transition process. They argued that the PBA crystal transition includes three kinds of molecular motions: shifting along the c-axis, rotation around the c-axis, and slight shrinking along the c-axis. In the present case, the simultaneous changes of the bands associated with CH2 bending, CH2 wagging, C-O-C asymmetric, and symmetric stretching undoubtedly demonstrate a cooperative response of all PBA chain segments during these suggested molecular chain motions. Third, to get more information about the βR transformation process, we have fitted the degree of transformation X with annealing time t. The X is defined as X ) (Ht - H0)/(H∞ H0), where H is the band height. As shown in Figure 4, a good linear fit is found between the ln(1 - X) and t. The reason for the deviation at the beginning is not clear now. It may originate from the experimental process because the IR spectra were recorded soon after the sample was transferred to the temperature cell at 49 °C from room temperature. Nevertheless, the linear relationship between the ln(1 - X) and t indicates that the degree of transformation X follows the law: X ) 1 - e-at. This implies that the βR transition process of PBA proceeds randomly throughout the solid at a constant rate.

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Figure 4. The relationship between the ln(1 - X) and the annealing time t during the phase transition, where X is the degree of transformation. The line represents the least-squares linear fit.

4. Conclusion IR band assignments of PBA in its R and β phases are carefully made. The formation processes of R-PBA through isothermal crystallization and βR crystal transition have been studied by real time infrared spectroscopy. During isothermal crystallization at 48 °C, the simultaneous evolution of different IR bands demonstrates that the functionalities in PBA chains change cooperatively during the melt crystallization, which is different from the crystallization of PHB and PLLA. During the βR crystal transition at 49 °C, distinct peak shifts of IR bands of the β-PBA to the positions associated with their R-counterparts in the region of 1300-1100 cm-1 suggest that the twist of PBA chains in R-form occurs in the range of C-O-C and C-O linkages, which confirms the findings reported in the literature. The simultaneous intensity changes of the IR bands associated with different groups during the crystal transition indicate a cooperative adjustment of the PBA chain segments. Moreover, a linear relationship between the ln(1 - X) and t indicates that the βR transformation process of PBA proceeds randomly throughout the solid at a constant rate. Acknowledgment. The financial support of the Outstanding Youth Fund and the National Natural Science Foundations of China (No. 50521302, 20574079, 20634050, 20423003, and 20604031) is gratefully acknowledged. Supporting Information Available: Time-dependent IR spectra of PBA during isothermal crystallization, X-ray diffraction profile of PBA crystallized at 48 °C, IR spectra of solution cast β-PBA recorded before and after annealing at 49 °C, Avrami plot of the band at 1260 cm-1 during isothermal crystallization at 48 °C, and IR spectra in the carbonyl stretching region of PBA during the crystal transition. This material is available free of charge via the Internet at http://pubs.acs. org. References and Notes (1) Huang, S. J. Encyclopedia of Polymer Science and Engineering; Wiley- Interscience: New York, 1985; Vol. 2.

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