Biodegradable Poly(ethylene succinate) (PES). 1. Crystal Growth

Biomacromolecules 2000, 1, 704-712

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Biodegradable Poly(ethylene succinate) (PES). 1. Crystal Growth Kinetics and Morphology Zhihua Gan, Hideki Abe, and Yoshiharu Doi* Polymer Chemistry Laboratory, RIKEN Institute, Hirosawa 2-1, Wako-shi, Saitama 351-0198, Japan Received June 13, 2000; Revised Manuscript Received August 11, 2000

Crystal growth rates of melt-crystallized poly(ethylene succinate) (PES) with a number-average molecular weight Mn of 21 100 and polydispersity of 1.86, respectively, were studied in an extensive temperature range. On the basis of secondary nucleation theory, only one transition between regime III and regime II was found at around 71 °C, but no transition of regime II f I was detected by kinetic analysis. The ratio of nucleation constants KgIII in regime III to KgII in regime II was influenced by the value of activation energy U*, and tended to 2 as predicted by the theory when U* was set as 3688 cal/mol. The morphological changes at spherulitic level and lamellar level in regime II and regime III were examined by using an optical microscope (OM) and atomic force microscope (AFM). Although spherulitic morphologies were found in both regime II and regime III by optical microscopy, further investigations made by following the crystallization process using OM and by checking the lamellar morphology using AFM revealed morphological changes during the crystal growth process and different crystal morphologies in regime II and regime III. Meanwhile by using ultrathin film (∼100 nm in thickness), the morphology of lamellar crystals of lozengeshape outline and composed of single crystals has been studied by AFM technique. The results indicate that there is indeed a transition from regime II to regime III at around 71 °C for the isothermal crystallization of PES with an increasing degree of undercooling. Introduction Biodegradable polymers, regardless of whether they are synthesized by chemical or microbial methods, have been attracting considerable attention in the last 2 decades due to their potential applications in fields related to human life such as environmental protection and the maintenance of physical health. Basic research on the relationship between structure, morphology, and properties as well as efforts to understand the biodegradation mechanism will enable us to design and synthesize a great variety of biodegradable polymers to fulfill the demands in practical applications. Biodegradation rate is undoubtedly one of the most important properties of biodegradable polymers. Many factors affect the biodegradation rates of polymers, such as chemical composition,1,2 stereoregularity,3,4 crystalline structure, and morphology.5-10 As for crystalline structure and morphology, it is known that crystallinity, spherulite size, and lamellar structure can greatly influence the rate of biodegradation. Therefore, studies on crystallization and morphology have to precede research on degradation behavior, and these results will help us to elucidate the degradation mechanism. The crystalline structure and morphology of polymers are influenced greatly by the thermal history and are determined mainly by the nucleation process. On the basis of the nucleation theory established by Hoffman et al (LH model), the crystal growth rate G at a crystallization temperature Tc * Corresponding author and mailing address: Yoshiharu Doi, Polymer Chemistry Laboratory, RIKEN Institute, Hirosawa 2-1, Wako-shi, Saitama 351-0198, Japan. Telephone: +81-48-467-9402. Fax: +81-48-4624667. E-mail: [email protected].

can be expressed by the following equation:11

[

G ) G0 exp -

] [

Kg U* exp R(Tc - T∞) Tc(∆T) f

]

(1)

U* is the activation energy for the transport of segments to the site of crystallization, R is the gas constant, T∞ is the temperature below which the polymer chain motion ceases, ∆T is the degree of supercooling given as Tm0 - Tc where Tm0 is the equilibrium melting point, f is a factor which accounts for the variation in the enthalpy of fusion ∆hf, given as 2Tc/(Tm0 + Tc), and Kg is a nucleation constant, as shown in the following equation: Kg )

nb0σσeTm0 ∆hfk

(2)

σ and σe are the lateral and end-surface free energies, respectively, b0 is molecular thickness, k is the Boltzmann constant, and the n value is dependent on the regime of crystallization. The n values are 4, 2, and 4 for regimes I, II, and III, respectively. Theoretically both the ratios of KgIII to KgII and of KgI to KgII should be 2. Therefore, the regime transition can be calculated from the kinetic treatment according to eq 1 by plotting ln G + U*/R(Tc - T∞) as a function of 1/Tc(∆T)f. The existence of different regimes could then be identified by the changes in the slopes of the lines, which are the values of the nucleation constants Kg at different regimes. To understand how the nucleation process affects the crystalline morphology, two important processes and their

10.1021/bm0000541 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/04/2000

Biodegradable Poly(ethylene succinate)

rates should be considered. First is the rate g at which a nucleus once formed spreads across the front of growing crystal, and second is the rate i at which new nuclei are formed. If g is sufficiently high compared to i, only one nucleus is active on the surface and will complete the layer before the next nucleus is formed. This case is called regime I. Regime II is the case where numerous nuclei form but spread slowly, while regime III is the case where the nucleation rate is much higher than the spreading rate; i.e., i . g. For the polymers in which regime transitions exist, characteristic morphologies may be seen due to the differences in crystal packing in the different regimes. Usually spherulitic and axialitic textures are the representative morphologies for regime II and regime I, respectively. However, even if the kinetic analysis shows two or three regimes for one polymer, sometimes the morphology near the regime transition does not exhibit the corresponding change. From the literature, both the regime transitions of I f II and II f III in the same polymers were reported only for the melt-crystallized polyethylene,12 low molecular mass fraction of isotactic polypropylene,13 cis-polyisoprene,14 and poly(3,3-dimethylthietane).15 The regime transition of I f II has been observed in poly(L-lactide)16 and poly(1,3dioxolane).17 The II f III transition has been found in other polymers such as poly(pivalolactone),18 poly(3-hydroxybutyrate)(PHB),19 poly(p-phenylene sulfide),20 high molecular mass fraction of isotactic polypropylene,13 and syndiotactic polypropylene.21 In this paper, poly(ethylene succinate) (PES), whose chemical structure is shown below,

was chosen to study the crystal growth and crystalline morphology. This work is part of a series of studies on the enzymatic degradation of PES. For the enzymatic degradation of polymer, both the enzyme structure and polymer morphology have great influences on the degradation rate. PES is a good candidate to study these influences because it can be degraded by PHB depolymerases which have binding and catalytic domains, as well as by the action of lipases which have only catalytic domains but are without binding domains.22-24 The crystalline structure and morphology will have great influence on the adsorption of enzymes onto polymers and the subsequent degradation. Several reports are available on the crystal structure and crystal modification of PES,25-27 while relatively less work has been done on the crystallization and morphology.28 Here we report our studies on the crystallization and morphology of PES, and we have obtained results that will be useful in the further elucidation of its enzymatic degradation and mechanism. Experimental Methods Materials. Poly(ethylene succinate) (PES) was used in this study. The molecular weight data was obtained by gel permeation chromatography (GPC) at 40 °C, using a Shimadzu 10A GPC system and 10A refractive index detector

Biomacromolecules, Vol. 1, No. 4, 2000 705

with two joint columns of Shodex K-806 and K-802. Chloroform was used as an eluent at a flow rate of 0.8 mL/ min, and a sample concentration of 2.0 mg/mL was used. Polystyrene standards with a low polydispersity were used to make a molecular weight calibration curve. The weightaverage (Mw) and number-average (Mn) molecular weights for PES sample were determined to be 39 400 and 21 100, respectively, and the molecular weight distribution (Mw/Mn) was 1.86. Chloroform was bought from Kanto Chemical Corp. and used as received. DSC Measurements. The melt-crystallized PES films with thickness of ∼0.05 mm were prepared by a meltcompression method and used for determining PES equilibrium melting point Tm0 by means of differential scanning calorimetry (DSC). The PES small particles were first placed between two Teflon sheets (0.1 mm in thickness) with a Teflon sheet (0.05 mm in thickness) as spacer, and they were compression-molded on a Mini Test Press (Toyoseiki) at 180 °C under a pressure of 150 kg/cm2 for 2 min. Then the samples were transferred into an oven and kept at a given temperature Tc to crystallize isothermally for 3 days. The samples were stored at about -10 °C before characterization. The DSC data of PES samples after isothermal crystallization at different temperatures were recorded on a Shimadzu DSC-50 equipped with a liquid nitrogen cooling accessory under a nitrogen flow of 30 mL/min. Meltcrystallized PES samples of ca. 2 mg were encapsulated in the aluminum pans and heated from -50 °C to 200 °C at a rate of 20 °C/min. The melting temperature Tm was determined from the position of the melting peak in the DSC curves. For Tg measurement, the PES sample was heated at 200 °C for 1 min and then quenched to -150 °C. After that the PES sample was heated to 200 °C at a rate of 20 °C/ min, and the heat flow was recorded. Crystal Growth and Spherulitic Morphology. The measurement of PES crystal growth rate and the observation of the spherulitic morphology were conducted using an optical microscope (Nikkon OPTIPHOTO-2) equipped with cross polarizers and a Linkam hot stage. For the measurement of spherulite growth rate, small pieces of PES films were sandwiched between two microscopic glasses and placed on the hot stage, followed by heating from room temperature to 180 °C at a rate of 80 °C/min and held there for 2 min before rapidly cooling to the crystallization temperature Tc. The sizes of PES spherulites during the growth process were recorded along with the spherulitic morphologies during and after crystallization using the equipped camera. The linear radial growth rate was calculated from the slopes by plotting spherulite radius against crystallization time. Crystal Morphology Observed by AFM. The PES ultrathin films with a thickness of about 100 nm, which were used to study the crystalline morphology by atomic force microscopy (AFM), were prepared by the solution cast method, as described by Abe et al.29 Basically, a droplet of PES solution in chloroform (1% w/v in concentration) was added on a cover glass (18 × 18 mm in dimension) and then covered with a second cover glass to spread the solution between the two glasses. The surfaces of the cover glasses had been cleaned with methanol and dried before the

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Figure 1. DSC melting curves of PES melted-crystallized at different temperatures. Heating rate ) 20 °C/min.

preparation of the ultrathin film. Two glasses were then slid quickly against each other in the opposite direction. The PES ultrathin films formed on the two glass surfaces following the evaporation of chloroform. The PES ultrathin films on the cover glasses were heated at 180 °C for 1 min and then crystallized isothermally at a given temperature for a given period. After crystallization, the crystal morphology in the ultrathin film was observed by an atomic force microscope (AFM) at room temperature. AFM observation was performed using a SPI37000/SPA300 (Seiko Instrument Inc.) in the contact mode. A triangle cantilever made of Si3N4 was used, whose length, spring constant and resonance frequency are 200 µm, 0.021 N/m, and 13 kHz, respectively. Results and Discussion Melting Behavior and Equilibrium Melting Point. For the analysis of crystallization kinetics, in particular by using the Hoffman model to study the regime transition, the equilibrium melting point Tm0 is one of the important parameters and can be obtained from the relationship between the observed melting point Tm and the isothermal crystallization temperature Tc, according to the following equation given by Hoffman and Weeks30

(

Tm ) Tm0 1 -

Tc 1 + γ γ

)

(3)

where γ is the ratio of final to initial lamellar thickness. Plotting the Tm as a function of Tc and extrapolating the straight line to intersect with the line of Tm ) Tc can give the value of Tm0. Figure 1 shows the melting curves of PES after crystallizing at different temperatures. The PES samples showed multiple melting peaks after crystallizing at 60, 70, 80, and 85 °C for 3 days. On the lower temperature range below 80 °C, there is a low and broad melting peak. On the hightemperature range between 80 and 110 °C, the melting curves of PES samples had two main peaks, i.e., peak 1 and peak 2, which represent the melting peaks at relatively lower and higher temperatures, respectively. The shapes and positions of the two peaks were influenced by the crystallization condition. Peak 1 shifted from low to high-temperature region

Figure 2. DSC melting curves at different heating rates for PES meltcrystallized at 80 °C.

Figure 3. Relationship between melting temperature and isothermal crystallization temperature. The extrapolation is based on the Hoffman and Weeks equation, indicating an equilibrium melting point at 115.6 °C.

with increasing Tc, while the position of peak 2 was unchanged, and finally, peak 2 decreased into a shoulder combining with peak 1 for the PES sample crystallized at 85 °C (see the arrow in Figure 1). The multiple melting peaks may come from the formation of different crystalline polymorphs which show different melting points, or from reorganization during the heating process in DSC measurement. To ascertain this, the endothermal behavior of PES after crystallizing at 80 °C was recorded by DSC at different heating rates, as shown in Figure 2. With decreasing heating rate, peak 2 increased, and the broad melting peak at low temperatures below 80 °C disappeared gradually. These results indicate that meltingreorganization occurred in melt-crystallized PES during the heating process. The appearance of the broad peak below 80 °C is related to the laying of PES samples at room temperature before thermal analysis. Peak 1 is the endothermal peak resulting from the melting of original lamella that formed during the primary crystallization process at a given Tc, and peak 2 is the reflection of PES crystals after reorganization during the heating process in DSC measurement. Figure 3 shows the temperatures of two peaks of PES samples crystallizing at different temperatures. Because peak 1 originated from the lamella formed at a given crystallization temperature, the equilibrium melting point Tm0 of 115.6 °C



Figure 4. Variation of crystal growth rates for PES at different crystallization temperatures.

was obtained from the relation of peak 1 and crystallization temperatures, according to eq 3. The glass transition temperature Tg was measured by DSC. PES sample was quenched from the melt to -150 °C and then heated at a rate of 20 °C/min. The Tg was determined to be -10 °C. Crystal Growth Rates and Regime Analysis. The crystal growth rates of melt-crystallized PES at a given temperature were measured by an optical microscope. By plotting the crystal size with time, the crystal growth rates were calculated from the slopes of these straight lines. Figure 4 shows the variations of linear growth rates for PES at different crystallization temperatures. A normal shaped curve was obtained and the maximum growth rate was at about 60 °C. The secondary nucleation theory (LH model) was applied to analyze the crystal growth rate of PES at different crystallization temperatures. Before kinetic analysis, it was important to estimate the values of U* and T∞ in eq 1. Two sets of parameters are usually used for the analysis of crystallization kinetics. One is the empirical “universal” values of U* ) 1500 cal/mol and T∞ ) Tg - 30 K,11 and the other is the Williams-Landel-Ferry (WLF) values of U* ) 4200 cal/mol and T∞ ) Tg - 51.6 K.31 In most cases the first set of parameters for U* and T∞ can fit well the crystallization kinetic data of polymers. However for some polymers such as poly(3-hydroxybutyrate), the U* value is 2750 cal/mol,19 and by using this value the nucleation theory gave the best-fitting curves. In the present study, the universal values of U* ) 1500cal/mol and T∞ ) Tg - 30 K were first applied for the regime analyses of PES crystallization, meanwhile different values of U* such as 3000 (two times U*, 2U*), 4500 (3U*), and 6000 (4U*) cal/mol were also used to study the influences of the chosen parameter on the determination of regime transition. Figure 5 represents the relationship between ln G + U*/ R(Tc - T∞) and 1/Tc(∆T)f by choosing U* as 1500 and 3000 cal/mol, respectively. It shows that it is impossible to fit all the data from 55 to 89 °C on one straight line. The optimum fit was by two straight lines among different range of crystallization temperatures. On the basis of the LH theory, this discontinuity showed a transition from regime III at large supercooling (low crystallization temperature) to regime II at small supercooling (high crystallization temperature). Although the choice of the U* value will lead to the corresponding changes in the line slopes, i.e., the values of

Figure 5. Plots of ln G + U*/R(Tc - T∞) vs 1/Tc(∆T)f for PES by setting the values of U* as 1500 and 3000 cal/mol, respectively. Table 1. Values of Nucleation Constants for PES in Regimes II and III at Various Assumed Values of U* correlation coefficient III,

U*, cal/mol Kg 1500 3000 4500 6000

deg2

104 300 195 100 285 900 376 600

II,

Kg

deg2

83 300 107 200 131 100 155 000

KgIII/KgII

regime III

regime II

1.25 1.82 2.18 2.43

0.9997 0.9971 0.9942 0.9923

0.9981 0.9969 0.9956 0.9944

nucleation constants KgIII and KgII, the regime III f regime II transition is always located at about 71 °C. Furthermore, no transition of regime II to regime I was detected even when the crystallization temperature was increased to 89 °C (supercooling ∆T ) 26 °C). Figure 5 also shows that the values of U* have influence on the slopes of the fitting lines. The results of PES crystallization kinetics analyzed by the LH theory at different values of U* are listed in Table 1. It is found that the linear regression of the kinetic data for the two regimes are well at various U* values. However when the value of U* increased four times, the ratio of KgIII to KgII increased from 1.25 to 2.43, which is in the vicinity of the theoretical value 2. From the experimental data in Table 1, when U* is equal to 3688 cal/mol, the ratio of KgIII to KgII will be 2.0, which is in agreement with the prediction of the LH theory. The reasons for the deviation between experimental and theoretical values in the ratio of KgIII to KgII lie in two aspects; one is the availability of the mobility parameters such as U*, and the other may be the relatively broad distribution of PES molecular weight. The broad distribution of molecular weight mainly results in a nonsharp regime transition. For regime III, due to the large supercooling (low crystallization temperature), the crystallization kinetics are controlled by the transportation rate of polymer segments into the growth front, therefore the kinetic treatments are sensitive to the mobility parameters,32,33 such as activation energy U*. As is shown in Table 1, more change occurred in the values of KgIII (∼3.5-fold) than KgII (∼2-fold) when the U* varies 4 times. On the basis of the results reported in the literature, the value of U* ) 1500 cal/mol is generally suitable for the polymer with regime transition of II to I, which may be due to the relatively small degree of supercooling. However for

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Figure 6. Optical micrographs of PES melt-crystallized at different temperatures: (a) 50 °C; (b) 60 °C; (c) 70 °C; (d) 80 °C.

poly(3-hydroxybutyrate) which shows a regime transition from III to II, the suitable value of U* was 2750 cal/mol and not 1500 cal/mol. Although there is an exceptional example such as poly(pivalolactone), which also has the transition from regime III to regime II but is not sensitive to the various values of U*, in our study, the regime analysis of PES is affected greatly by the values of U*, mainly on the ratio of the nucleation constants in two regimes, and the transition temperature is independent of the U*. Crystalline Morphology in Regime II and Regime III. The above kinetic analysis clearly shows that there is a transition of regime III to regime II at ∼71 °C for meltcrystallized PES; however the ratio of nucleation constants KgIII to KgII is affected by the choice of the U* value. Therefore, it is important to look for an independent evidence to support that there is indeed a regime transition for PES. Because the crystalline morphology of polymer is mainly determined by secondary nucleation process, the number of the nuclei in the growth front of crystals as well as the difference in nucleation rate and lateral spreading rate will lead to corresponding morphological changes under different crystallization conditions. For example in regime II, the nucleation rate and the lateral spreading rate are comparable, and several nuclei are active at the same time. However for regime III, the rate of formation of new nuclei is significantly higher than the spreading rate, and there is little chance for a nucleus to spread out sideways before another nucleus is formed next to it. Therefore, checking the morphological change in a wide range of crystallization temperatures under

a certain experimental conditions will enable us to verify the regime transition. Figure 6 shows the optical micrographs of PES spherulites from the melt at different crystallization temperatures for a sufficiently long time. The spherulites are fully impinged on each other and display the characteristic “Maltese Cross” extinction pattern. There was no abrupt change in morphology when the crystallization temperature was increased from 50 to 80 °C; however, the radiating lines from the center of the PES spherulites became coarse with the increase in temperature. Do these changes in the optical features arise from the secondary nucleation at different regimes? Due to the limited resolution of optical microscope, here the AFM technique, which provides the possibility of examining the crystalline structure at the level of lamellar organization, was used to further study the crystal morphology of PES at different temperatures. Figure 7 shows the AFM deflection images of PES ultrathin films after isothermal crystallization at different temperatures. Two distinct types of morphologies for PES at different temperatures were found. One is the fibrillar like structure radiating from the spherulitic center when the crystallization temperatures were 50 and 60 °C, while the other is the wide flat-on lamellae parallel to the substrates at 70 and 80 °C. It is known that polymer spherulites consist of radiating lamellar crystals which fill in the spherical space through continuous splaying apart and occasional branching. The competition between nucleation and spreading rates in the crystal front at different crystallization conditions will



Figure 7. AFM deflection images of PES ultrathin films melt-crystallized at different temperatures: (a) 50 °C; (b) 60 °C; (c) 70 °C; (d) 80 °C.

result in the corresponding morphological change in the lamellar crystal and subsequently give rise to change in spherulitic morphology. In regime III for PES, since the nucleation rate is much larger than the lateral spreading rate, the lamellae that originated from nuclei grow very fast along the radiating direction as compared to the tangential direction of the spherulitic radii. This results in the formation of fibrillar structures which are the stacks of elongated lamellar crystals with radiating dimension much longer than the other two dimensions, as shown in Figure 7, parts a and b. On the other hand, in regime II, due to the similarity of the two rates, as lamellae grow along the radiating direction, they are also able to grow laterally. In this case, the lamellae will have certain width, such as about 500 nm at 80 °C, as shown in Figure 7d. It is clear from Figure 7 that the sizes of lamellar crystal in the two dimensions (radiating direction and tangential

direction of radii) are different in the two regimes, such difference may come from the difference in nucleation rate and lateral spreading rate, which is expressed kinetically by the different nucleation regimes. Therefore, the morphologies of PES lamellar crystals at different crystallization temperatures in Figure 7 are the reflection of two regimes. From the literature it is known that for most polymers which exhibit nucleation regimes, axialites are only the representatives of regime I and spherulitic structures are observed in regime II. The only exception is poly(pivalolactone) which showed a change from spherulite to axialite at the transition of regime III f regime II.18 However, the reported results are based generally on the crystal aggregates such as spherulites and axialites observed by optical microscopy. Ours is the first study that has been carried out at the level of lamellar crystal by using AFM, and these results

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Figure 8. Optical micrographs of the growth process of PES spherulites at crystallization temperatures of 80 (a-d) and 86 °C (e-h), respectively. (a) 11.5 min; (b) 16.0 min; (c) 20.0 min; (d) 35.0 min; (e) 31.8 min; (f) 43.3 min; (g) 50.9 min; (h) 61.3 min.

give us more direct evidence to understand the nature of different nucleation regimes for polymers. On the other hand, although there are many investigations on the morphological changes corresponding to the regime transition for some polymers, no attention has been paid to

the morphological change during the growth process in a specific regime such as in the crystallization temperature range of regime II. When matured spherulites are formed and impinged on each other, sufficient splaying and branching of lamellae in three dimensions may obscure some


important characteristics so that it cannot be detected by OM, such as in Figure 6 from which we can get no information except for the fact that the radiating lines are becoming coarse with increasing temperature. Following the crystallization process, especially the early stage of crystal growth, may enable us to obtain more useful information about nucleation and crystal growth. Figure 8 shows the growth progress of PES spherulites at 80 (parts a-d) and 86 °C (parts e-h). Obviously in the early stage of crystallization, the crystal morphologies show irregular spherulites, or axialites, and finally develop into spherulites. In regime III when crystallization temperatures were 50 and 60 °C, only spherulites with perfect circular shapes were formed and grow into a large size during the crystallization process (data not shown here). The results in Figure 8 suggest that the final form of the spherulitic structure of PES in regime II has originated from axialite-like structure. It is generally accepted that spherulites evolve from a monolayer lamellar crystal or a single crystal, and the spherical shape is formed gradually from sufficient splaying apart and branching out of the lamellar crystals. During the growth process, a crystal will lose its initial orientation of single crystal. Besides factors such as inhomogeneity of polymer chain length and “impurity” inside the polymer system, we believe that the duration that a crystal growth can maintain its initial single crystal growth habit will depend on the secondary nucleation mechanism in the front of the growing crystal. In regime III, due to the very fast nucleation rate and slow diffusion rate, it is impossible for polymer chains to organize properly according to the single crystal structure. As soon as a nucleus is formed, it will initiate the growth of crystal along the radiating direction very quickly. In this case spherulites are composed of fibrillar structures as shown in Figure 7, parts a and b. For regime I, theoretically the crystal growth can maintain its single crystal growth habit. However in the case of regime II, the situation is more complicated due to the relatively small difference in nucleation and spreading rates. To understand the crystal growth in regime II, the ultrathin film of which the thickness (∼100 nm) was thin enough in comparison to the thickness of several lamellar crystals was prepared, and its lamellar crystal morphology after meltcrystallization was measured by OM and AFM. In this case, the growth of lamellar crystal was limited within two dimensions, and therefore, it was possible to provide a clearer picture about the crystal morphology. Figure 9 shows the optical micrograph of PES ultrathin film melt-crystallized at 85 °C. It is found that the crystal aggregate has lozengeshaped outlines with four “sectors” which are the same with that of PES single crystal.34 The lines inside “sectors” are parallel to the (110) crystal plane of single crystal. The enlarged image observed by AFM in Figure 10 shows that several lozenge-shaped single crystals were formed through screw dislocation along the (110) crystal plane. According to the description of LH theory, in regime I, a nucleus in the crystal front causes the completion of one substrate layer, and the thickness of this layer corresponds to that of a folded polymer chain. In regime II, numerous nuclei occur in the crystal front; however, until now it is yet unclear how the


Figure 9. Optical micrographs (parallel polarizers) of PES ultrathin film melt-crystallized at 85 °C.

Figure 10. AFM deflection image of PES ultrathin film meltcrystallized at 85 °C.

nuclei initiate the growth of new crystals. Our results in Figure 10 indicate that in regime II the new nuclei on (110) plane initiate the crystal growth through screw dislocation and the growing crystals have single-crystal structure. The above results on morphology show that although the spherulites are observed finally in regime II and regime III, different growth mechanisms are involved. The evolution of spherulitic crystals during isothermal crystallization at different temperatures as well as the crystal morphology of melt-crystallized ultrathin film provide an independent evidence besides kinetic analysis to show the existence of a transition from regime II to regime III for poly(ethylene succinate). Conclusions Both regime II and regime III were observed for meltcrystallized poly(ethylene succinate) (PES) at different temperatures by kinetic analysis on the basis of nucleation theory, and the regime III f regime II transition occurred

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at a crystallization temperature of ∼71 °C. The ratio of nucleation constant KgIII to KgII was influenced by the values of U* and had a tendency toward a theoretical value of 2 when U* was 3688 cal/mol. Only spherulitic morphology was seen in both regime II and regime III after complete crystallization; however, by following the crystal growth process, the spherulites in regime II were found to originate from single crystals. AFM results on the ultrathin film showed that, in regime II several nuclei are formed in the crystal front and that they initiate the growth of new crystal according to a single-crystal growth habit through screw dislocation. Acknowledgment. This research was supported by the grant for Ecomolecular Science Research, RIKEN Institute provided by the Science and Technology Agency (STA) of Japan. References and Notes (1) Doi, Y.; Abe, H. Macromol. Symp. 1997, 118, 725. (2) Doi, Y.; Kumagai, Y.; Tanahashi, N.; Mukai, K. In Biodegradable Polymers and Plastics; Vert, M., et al., Eds.; Royal Society of Chemistry: London, 1992; p 139. (3) Abe, H.; Doi, Y. Macromolecules 1996, 29, 8683. (4) Abe, H.; Matsubara, I.; Doi, Y. Macromolecules 1995, 28, 844. (5) Kumagai, Y.; Kanesawa, Y.; Doi, Y. Makromol. Chem. 1992, 193, 53. (6) Koyama, N.; Doi, Y. Macromolecules 1997, 30, 826. (7) Li, S.; McCarthy, S. Macromolecules 1999, 32, 4454. (8) Tsuji, H.; Ikada, Y. J. Polym. Sci. A: Polym. Chem. 1998, 36, 59. (9) Focarete, M. L.; Ceccorulli, G.; Scandola, M.; Kowalczuk, M. Macromolecules 1998, 31, 8485. (10) Tomasi, G.; Scandola, M. J. Macromol. Sci., Pure Appl. Chem. 1995, A32, 671.

Gan et al. (11) Hoffman, J. D.; Davis, G. T.; Lauritzen, J. I.; Jr. In Treatise in Solid State Chemistry, Hannay, N. B., Ed.; Plenum Press: New York, 1976; Vol. 3, Chapter 7, p 497. (12) Hoffman, J. D. Polymer 1983, 24, 3. (13) Cheng, S. Z. D.; Janimak, J. J.; Zhang, A.; Cheng, H. N. Macromolecules 1990, 23, 298. (14) Phillips, P. J.; Vantansever, N. Macromolecules 1987, 20, 2138. (15) Lazcano, S.; Fatou, J. G.; Macro, C.; Bello, A. Polymer 1988, 29, 2076. (16) Vasanthakumari, R.; Pennings, A. J. Polymer 1983, 24, 175. (17) Alamo, R.; Fatou, J. G.; Guzman, J. Polymer 1985, 26, 1595. (18) Roitman, D. B.; Marand, H.; Miller, R. L.; Hoffman, J. D. J. Phys. Chem. 1989, 93, 6919. (19) Barham, P. J.; Keller, A.; Otun, E. L. J. Mater. Sci. 1984, 19, 2781. (20) Lovinger, A. J.; Davis, D. D.; Padden, F. J. Polymer 1985, 26, 1595. (21) Rodriguez-Arnold, J.; Bu, Z.; Cheng, S. Z. D.; Hsieh, E. T.; Johnson, T. W.; Geerts, R. G.; Palackal, S. J.; Hawley, G. R.; Welch, M. B. Polymer 1994, 35, 5194. (22) Scherer, T. M.; Fuller, R. C.; Lenz, R. W.; Goodwin, S. Polym. Degrad. Stab. 1999, 64, 267. (23) Mochizuki, M.; Mukai, K.; Yamada, K.; Ichise, N.; Murase, S.; Iwaya, Y. Macromolecules 1997, 30, 7403. (24) Doi, Y.; Kasuya, K.; Abe, H.; Koyama, N.; Ichiwatari, S.; Takagi, K.; Yoshida, Y. Polym. Degrad. Stab. 1996, 51, 281. (25) Ueda, A. S.; Chatani, Y.; Tadokoro, H. Polym. J. 1971, 2, 387. (26) Noguchi, K.; Ichikawa, Y.; Kondo, S.; Okuyama, K.; Washiyama, J. Polym. Prepr. 1997, 38, 50. (27) Ichikawa, Y.; Washiyama, J.; Moteki, Y.; Noguchi, K.; Okuyama, K. Polym. J. 1995, 27, 1264. (28) Al-Raheil, I. A.; Qudah, A. M. A. Polym. Int. 1995, 37, 249. (29) Abe, H.; Kikkawa, Y.; Iwata, T.; Aoki, H.; Akehata, T.; Doi, Y. Polymer 2000, 3, 867. (30) Hoffman, J. D.; Weeks, J. J. J. Res. Natl. Bur. Std. 1962, 66A, 13. (31) Williams, M. L.; Landel, R. F.; Ferry, J. D. J. Am. Chem. Soc. 1955, 77, 3701. (32) Dalal, E. N.; Phillips, P. J. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 7. (33) Lovinger, A. J.; Davis, D. D.; Padden, F. J. Polymer 1985, 26, 1595. (34) Iwata, T.; Doi, Y. Macromol. Chem. Phys. 1999, 200, 2429.

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Biodegradable Poly(ethylene succinate) (PES). 1. Crystal Growth

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