Metastability and Transformation of Polymorphic Crystals in

The of the PBA α crystal was found to be higher than that of the β crystal, indicating that the PBA α crystal form is a structurally stable phase a...
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Biomacromolecules 2004, 5, 371-378

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Metastability and Transformation of Polymorphic Crystals in Biodegradable Poly(butylene adipate) Zhihua Gan,*,† Kazuhiro Kuwabara,† Hideki Abe,†,‡ Tadahisa Iwata,† and Yoshiharu Doi†,‡ SORST Group of Japan Science and Technology Corporation (JST), Polymer Chemistry Laboratory, RIKEN Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan, and Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received September 30, 2003; Revised Manuscript Received November 28, 2003

Polymorphism phenomenon of melt-crystallized poly(butylene adipate) (PBA) has been studied by wideangle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). It has been found that the isothermal crystallization leads to the formation of PBA polymorphic crystals, simply by changing the crystallization temperature. The PBA R crystal, β crystal, and the mixture of two crystal forms grow at the crystallization temperatures above 32 °C, below 27 °C, and between these two temperatures, respectively. The relationship between PBA polymorphism and melting behaviors has been analyzed by the assignments of multiple melting peaks. Accordingly, the equilibrium melting temperatures T°m of both R and β crystals were determined by Hoffman-Weeks and Gibbs-Thomson equations for the purpose of understanding the structural metastability. The T°m of the PBA R crystal was found to be higher than that of the β crystal, indicating that the PBA R crystal form is a structurally stable phase and that the β crystal form is a metastable phase. The analysis of growth kinetics of PBA polymorphic crystals indicates that the metastable PBA β crystal is indeed the kinetically preferential result. Based on the thermal and kinetic results, the phenomenon of stability inversion with crystal size in melt-crystallized PBA was recognized, in terms of the growth mechanisms of PBA R and β crystals and the transformation of β to R crystals. The PBA β f R crystal transformation takes place at a sufficiently high annealing temperature, and the transformation has been evident to be a solid-solid-phase transition process accompanied by the thickening of lamellar crystals. The molecular motion of polymer chains in both crystalline and amorphous phases has been discussed to understand the thickening and phase transformation behaviors. 1. Introduction The important role of metastability in polymer phase transitions has been reviewed by Keller et al.1,2 Two classes of metastability have been proposed and thoroughly discussed. One is classical metastability, or structural metastability, which is defined as a state between two stable phases when a phase transformation proceeds through, according to the Ostwald’s rule of stages.3 Many polymers possess more than one crystal form of which the polymer chains are packed in crystal lattices with different symmetries. Among the crystal forms, all but one are metastable states at a specific temperature and pressure. The metastable phases are considered to fall into one of the multiple local free energy minima in Gibbs free energy profiles and will ultimately transform into the thermodynamically stable state of global free energy minimum. The reason the metastable states exist is because of the kinetics which provides a favorable pathway for the polymer to fall into the local free energy minimum. Another class of metastability is circumstantial metastability, or morphological metastability, which represents a state * To whom correspondence should be addressed. Tel: +81-48-467-9403. Fax: +81-48-462-4667. E-mail: [email protected]. † RIKEN Institute. ‡ Tokyo Institute of Technology.

that failed to reach the ultimate stability due to the phase size. The morphological metastability is an essentially important phenomenon in polymer polymorphism. In reality, an equilibrium stable state of crystalline polymer with infinite crystal size, i.e., extended chain crystals, is never obtained due to the nature of long chain of polymer. Instead of that, lamellar crystals with folded chains are generally formed in crystalline polymers, and the lamellar thickness is determined by crystallization kinetics. From the viewpoint of morphological metastability, each lamellar crystal represents a crystal form and does not reach the equilibrium state. It belongs to the morphological metastability. The study of both structural metastability and morphological metastability is necessary for understanding the growth mechanism and transformation of polymer polymorphisms. For this reason, proper research samples are indispensable. The usual stretch and annealing induced polymorphisms are not suitable because of the difficulty in correlating lamellar thickness with crystallization kinetics. On the contrary, the temperature-induced polymorphic crystals are more suitable, because they include both classes of metastability, and the size of each crystal form may have its own dependence on crystallization temperature. One sample is syndiotactic polystyrene (sPS) which has been widely studied on the

10.1021/bm0343850 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/08/2004

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polymorphisms.4-8 Although the kinetically preferential growth of the metastable crystal phase has been found in sPS polymorphisms and the phenomenon of stability inversion with crystal size has been proposed, no experimental results of polymorphic crystal size have been reported.4,5 One possible reason is that the phase transformation took place after keeping it at the crystallization temperature for a long time.5 Compared to sPS, poly(butylene adipate) (PBA) is one of the most suitable substances. In a previous paper, we have reported the formation of PBA R and β crystals at different crystallization temperatures.9 Unlike sPS, the temperature-induced PBA polymorphism is stable at crystallization temperatures, which allows us to study the metastability of each polymorphic crystal. In addition, PBA R and β crystals show the corresponding melting behavior, which allows us to correlate the crystal structure with crystal size, melting point, and crystallization temperature, and subsequently to demonstrate the mechanisms of polymorphic crystal growth and transformation. The biodegradable polymer with polymorphic crystals is of great interest, because it allows us to study the relationship between chain conformation and biodegradability. It can be predicted that the polymer chains, despite the same chemical structure, may have the different biodegradability. This is not only due to the size of crystal but also to the different spatial orientation of polymer chains in the polymorphic crystal lattice. The results on PBA polymorphism and enzymatic degradation will be reported later in another paper. In the present work, attention was concentrated on the metastability and transformation of PBA polymorphic crystals. We first correlated the multiple melting peaks of meltcrystallized PBA with the polymorphic crystals for the purpose of determining their equilibrium melting temperatures T°m. This provides us direct evidence to know the structural metastability of polymorphic crystals. Furthermore, the thermal behavior, lamellar thickness, and phase transformation of PBA polymorphism at an elevated temperature Ta were examined by means of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). Finally, the growth mechanism, metastability, and transformation of PBA polymorphisms were discussed. 2. Experimental Section 2.1. Materials. Poly(butylene adipate) (PBA) was a courtesy sample provided by BASF AG company. It was purified by first dissolving in chloroform and then precipitating from methanol before investigation. The number-average molecular weight Mn and polydispersity Mw/Mn are 40 000 and 1.7, respectively, measured by GPC at 40 °C with narrow-distributed polystyrenes as standards and chloroform as eluant. 2.2. Differential Scanning Calorimetry (DSC). The isothermal crystallization at a given temperature, annealing treatment at an elevated temperature, and the subsequent melting behavior of PBA samples were carried out on a Pyris 1 DSC differential scanning calorimeter (Perkin-Elmer Instruments) equipped with a CryoFill liquid nitrogen cooling

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system and operated under nitrogen at a flow rate of 20 mL/ min. The temperature and heat flow at different heating rates of this apparatus were carefully calibrated by using standard materials before measurement. About 5 mg of the polymer samples were encapsulated in the DSC aluminum pans and then thermally treated on the DSC sample holder. For isothermal crystallization, the PBA samples were first heated to 75 °C for 3 min and then cooled at a rate of 400 °C/min to a given temperature Tc for isothermal crystallization. After a certain time length, the PBA samples were heated directly from Tc to the melting state at a rate of 20 °C/min. Both the heat flows of crystallization at Tc and the heating scan at a given rate were recorded for the analyses of crystallization kinetics and thermal properties. The position tp of the exothermal peak during the isothermal crystallization process was determined, and its reciprocal value, 1/tp, was utilized as the crystallization rate in this work. To study the transformation behavior of PBA polymorphism, the meltcrystallized PBA samples were heated at a rate of 60 °C/ min to an elevated temperature Ta for different time length, then quenched to 25 °C, and finally heated again to the melting state at a rate of 20 °C/min. The heat flow was recorded for the analyses of crystal thickening and transformation after annealing treatment. 2.3. Wide-Angle X-ray Diffraction (WAXD) and SmallAngle X-ray Scattering (SAXS). Melt-crystallized PBA films for WAXD and SAXS experiments were prepared by sandwiching the polymer samples between Teflon sheets with a Teflon film of 0.1 mm in thickness as a spacer and then compression molded on a Mini Test Press (Toyoseiki) by heating at 75 °C for 2 min under a pressure of 15 kg cm-2. After that, the polymer films were transferred rapidly into a water bath at a designed temperature for isothermal crystallization. After completion of crystallization, the polymer films were used for X-ray diffraction characterizations. Annealing treatments of melt-crystallized PBA films for the investigation of polymorphic transformation were carried out in an oven at an elevated temperature Ta. The WAXD patterns of melt-crystallized and annealed PBA films were recorded at room temperature on a Rigaku RAD-IIIB system with a nickel-filtered Cu KR radiation (wavelength λ ) 0.154 nm, 40 kV and 110 mA) in the 2θ range from 6 to 60° at a scanning step of 0.02°. The SAXS analyses were carried out at the same Rigaku RAD-IIIB system in the 2θ range from 0.1 to 3° at a scanning step of 0.05°. The typical plots of relative SAXS scattering intensities vs scattering vector q (defined as q ) 4π sin θ/λ) were shown in the insets of Figure 7. In the present work, the pseudo two-phase model and one-dimensional correlation functions were applied to analyze the SAXS data for calculating the long period Lc, lamellar thickness lc, and amorphous thickness la, according to the methods of Vonk and Hsiao.10,11 3. Results and Discussion 3.1. Identification of Multiple Melting Peaks. In a previous paper, the temperature-induced PBA polymorphism has been reported and the influences of crystallization conditions on polymorphism have been also discussed.9 The

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Figure 1. DSC melting curves of poly(butylene adipate) (PBA) after melt-crystallized at different temperatures Tc. The heating rate is 20 °C/min. The inset is the WAXD patterns of PBA films after meltcrystallized at 25, 35, and 30 °C, corresponding to the β crystal, the R crystal, and the mixture of two crystal forms, respectively.

further detailed investigation in the present work indicates that the R and β crystals are formed at crystallization temperatures above 32 °C and below 27 °C, respectively, and the mixture of two crystal forms is formed between 27 and 32 °C. Figure 1 shows the typical melting behaviors of PBA polymorphic crystals after being melt-crystallized at different temperatures. Both the PBA R and β crystals show double melting behaviors. However, the dependences of melting peaks on the crystallization temperature are obviously different. This suggests a respectively corresponding relationship of two crystal forms between the melting behavior and crystal structure. Generally, the crystal grown at higher temperature has higher thermodynamic stability. Therefore, it is rational to regard the PBA R crystal as a more stable phase compared to the β crystal. This assumption is supported by the reported results on PBA β f a crystal transformation.12,13 However, the thermodynamic evidence is still needed to determine the stability of two crystal forms, especially for the unusual phenomenon in Figure 1 which shows that the PBA β crystal grown at lower crystallization temperatures shows higher melting temperatures compared to the R crystal. To determine the stability, we must first assign the multiple melting peaks, and then only are we able to analyze the metastability of PBA polymorphic crystals from the relationship among the crystallization temperature, melting temperature, and crystal size. The appearance of multiple melting peaks in DSC diagrams during the heating process is a long-standing phenomenon and still a controversial issue in polymer thermal analyses. Great efforts have been made to explain the origins of multiple peaks in terms of partial melting and recrystallization during the heating process,14,15 crystals with different lamellar thicknesses,16-19 and crystals with different structures,4,6 etc. In the case of temperature-induced PBA polymorphic crystals, the multiple melting behaviors are suggested to be closely related to the recrystallization and phase transition during heating scan. The β crystal shows double melting peaks Tm1 and Tm2 on its DSC melting curve, and both peaks shift toward higher temperatures with raising crystallization temperature (Figure

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Figure 2. Melting temperatures of double peaks of the PBA β crystals (Tc ) 25 °C) as a function of heating rates. The inset is the corresponding DSC melting curves.

1). During the heating process, thermal energy may be provided to the β crystal not only to melt the original crystal but also to overcome the energy barrier for phase transformation and/or lamellar thickening. Therefore, the double peaks can be assigned to three possibilities: (1) both Tm1 and Tm2 melting peaks arise from the R crystal formed by transformation from the β crystal during heating process; (2) both melting peaks arise from the β crystal, the lower one and higher one correspond respectively to the original and the recrystallized β crystal; and (3) the lower peak is the melting peak of the original β crystal, and the higher peak arises from the R crystal formed by transformation from the β crystal during heating process. For all of the possibilities, the time scale of the experiment, i.e., the heating rate in the DSC scan, is of great influence on recrystallization and/or on the β f R crystal transformation. For the first possibility, a slow heating rate allows the β crystal to have enough time for its transformation into the R crystal; therefore, the enthalpy of melting peak Tm1 should increase. However, as shown in the inset of Figure 2, the Tm1 peak area decreases in enthalpy and almost disappears at a very slow heating rate. These results indicate that the Tm1 peak can be only from the original β crystal, and that the Tm2 may arise from either recrystallized β crystal or the R crystal formed by transformation during heating process. To prove it, the corresponding crystal structure with melting peak Tm2 was examined by WAXD which will be discussed later in detail in Figure 6A. The results clearly indicated that phase transformation takes place during the DSC heating process. Some original β crystals melt to give the Tm1 peak, and some may transform into R crystals to give the peak Tm2. Therefore, it can be concluded that the melting peak Tm1 arises from the original β crystal and that the Tm2 peak arises from the R crystal which is formed by transformation from the β crystal during the DSC heating process. The R crystal also shows double melting behavior (see Figure 1). At the same heating rate, the lower melting peak Tm3 shifts toward higher temperature with increasing crystallization temperature Tc, whereas the location of the higher melting peak Tm4 is almost independent of Tc. However, the corresponding enthalpies of Tm4 show a significant decrease at a higher Tc. Our WAXD results have confirmed that both melting peaks Tm3 and Tm4 arise from the R crystal (WAXD data not shown). Further examination by changing the heating

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rate in DSC experiments indicated that recrystallization takes place during the heating process. Therefore, it is clear that the lower melting peak Tm3 corresponds to the original R crystal, and the higher one Tm4 arises from recrystallized R crystal during heating process. The assignment of triple melting peaks of PBA mixed polymorphic crystals grown around 30 °C is more complicated. However, it still can be concluded on the basis of DSC and WAXD results that the melting peak Tm5 arises from the original R crystal; Tm6 arises from the recrystallized R crystal and the original β crystal; and Tm7 arises from the R crystal formed from the β crystal by transformation during heating process. 3.2. Thermodynamic Stability of Polymorphic Crystals. According to the classical concept of metastable states, a structurally stable state is a system with sufficiently large phase size so that the size and other kinetic effects have no influences on it. In the case of crystalline polymers, such kinds of systems refer to an infinite stack of extended chain crystals with an equilibrium melting temperature T°m. Therefore, the determination of T°m is of fundamental importance in understanding the structural stability of polymorphism in polymer systems. However, in reality, crystalline polymers are generally composed of chain-folded lamellar crystals which are regarded as morphologically metastable phases. These chain-folded lamellar crystals with a thickness lc show an apparent melting temperature Tm which is dependent on the crystallization temperature Tc. The apparent Tm is always lower than the equilibrium melting point T°m due to the depression of lamellar thickness. Fortunately, several methods have been established to calculate T°m by extrapolation on the basis of experimentally obtained Tm values. One is the Hoffman-Weeks method20 which has been often utilized because of its straightforward experimental implementation and its analytical simplicity based only on DSC data. It is expressed as

(

Tm ) T°m 1 -

Tc 1 + γ γ

)

(1)

where the γ is equal to lc/l* in which lc and l* are the lamellar thickness and the thickness of the critical nucleus at Tc. Another method is the Gibbs-Thomson method20 which is given as

(

Tm ) T°m 1 -

2σe lc∆H

)

(2)

where ∆H is the heat of fusion and σe is the surface free energy of the basal chain-folded lamellae. It is obvious that Tm is a linear function of the reciprocal thickness 1/lc with a negative slope T°m(2σe/∆H) and an intercept T°m. Mostly this equation is closely obeyed by a crystalline polymer with a growth habit of a chain-folded lamellar crystal. In the case of melt-crystallized PBA polymorphic crystals, the R and β crystals have their own dependences on the crystallization temperature Tc, as shown in Figure 1. Therefore, it allows us to first correlate the melting temperature Tm with the crystallization temperature Tc or the lamellar thickness lc for the two crystal forms and then to determine

Figure 3. Melting temperatures of the PBA R and β crystals as a function of inverse lamellar thickness 1/lc. The equilibrium melting temperature T°m of each crystal form is determined by the GibbsThomson equation. The pointing lines (r) toward 1/lc ) 0 denote either the isothermal growth pathway at a temperature or the transformation process at an annealing temperature. The intersection point of two Tm vs 1/lc lines is defined as a triple point Q, at which temperature TQ all three phases (the melt L, the R crystal, and the β crystal) can coexist as stable phases. Two crystallization regions are indicated, one above TQ and one below TQ which are representative of the growth regions A and B, respectively. The pointing line (M f N) describes the thickening and transformation processes at an annealing temperature. The thickness of lR/ A and lβ/B are defined as the critical nucleation size of the R crystal in region A and of the β crystal in region B, respectively. The thickness of l/tr indicates the limiting size for the β to R crystal transformation.1

the respective T°m by applying the Hoffman-Weeks and Gibbs-Thomson equations. Before application of the two methods, the precise determination of melting temperature Tm in DSC is extremely important. Especially for β crystals, lamellar thickening and phase transformation take place during the DSC heating scan (to be discussed later), resulting in the possible shift of the melting peak Tm1 of the original β crystal to a higher temperature. Therefore, the higher heating rate is needed to prevent the thickening and/or transformation, but it may also cause the shift of Tm1 to a higher temperature due to the thermal lag of the DSC sample pan. To find the optimum DSC experimental conditions, the influence of heating rate on melting temperature Tm1 was checked, and the results are presented in Figure 2. It was found in Figure 2 that the original β crystal shows a higher melting point Tm1 at lower heating rates. This is due to the obvious thickening of original crystals during the slower heating process. When the heating rates are raised, the Tm1 tended to a lower value and finally kept unchanged, indicating that the influence of thickening on Tm1 can be ignored at proper higher heating rate. Therefore, based on the results in Figure 2, the Tm1 measured at the heating rate of 20 °C/min is reliable and is able to reflect the real melting temperature of original β crystal. In this work, the heating rate of 20 °C/min was applied to measure the melting temperatures of both original R and β crystals. The Gibbs-Thomson equation was first applied for determining T°m’s of the PBA R and β crystals. As shown in Figure 3, each one of the PBA R and β crystals shows a linear relation of melting temperature Tm vs the inverse lamellar thickness 1/lc. By extrapolating the 1/lc to zero, the T°m’s of the PBA R and β crystals were determined as 73 and 58 °C, respectively. These results have clearly proven that the PBA R crystal with a higher T°m is thermodynami-

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Figure 4. Melting temperatures of PBA R and β crystals as a function of crystallization temperature. The Hoffman-Weeks equation is applied for the determination of equilibrium melting temperature T°m of each crystal form.

Figure 5. Growth rates of PBA polymorphic crystals as a function of crystallization temperature. Three temperature regions are corresponding to the growth rates of the β crystal, the R crystal, and their mixture, respectively.

cally more stable than the β crystal. Thus, according to the concept of metastability, the PBA R crystal is a structurally stable phase, whereas the β crystal is the metastable phase. Figure 4 shows the melting temperatures Tm of the PBA R and β crystals at different crystallization temperatures Tc. Good linear relationships between Tm and Tc are exhibited for two crystal forms. On the basis of Hoffman-Weeks method, two intersection points corresponding to the respective equilibrium melting temperatures T°m of the PBA R and β crystals were obtained by extrapolating the fitting lines of Tm vs Tc to intersect with the equilibrium line Tm ) Tc. It was found from Figure 4 that the PBA R crystal has a T°m of 64 °C, whereas the β crystal shows a lower T°m of 54 °C. Thus, the determination of T°m by using the HoffmanWeeks equation has shown again that the PBA R crystal is a structurally stable phase, whereas the β crystal is a metastable phase. The two methods gave the T°m values with some difference for PBA R or β crystals. In addition to the inevitable experimental errors, the question of applicability of the two methods possibly remained, especially for the HoffmanWeeks equation, because both the linear fitting and nonlinear fitting have been reported to give different T°m for a polymer.21 However, even though the difference in T°m values exists, the final conclusion should be undoubtedly that PBA R crystal has a higher T°m than the β crystal so that the R crystal is the stable phase and the β crystal is the metastable phase. 3.3. Stability Inversion and Crystallization Kinetics. An interesting point should be noted from Figure 3, that there is an intersection point Q between two fitting lines Tm vs 1/lc of the PBA R and β crystals. The PBA β crystal was found to have a lower T°m than the R crystal. However, at the temperatures below TQ (region B), the structurally metastable β crystal surprisingly showed a higher melting temperature than the stable R crystal. This obviously experimental evidence that the structurally metastable β crystal has been inversed into a more stable phase with a higher melting temperature compared to the R crystal, because of the relatively small lamellar thickness. Although the thermodynamic stability of PBA R and β crystal forms have been determined, it has to be known that

each of the PBA R and β crystals is also the morphologically metastable phase due to the nature of the chain-folded lamellar crystal. The growth of the chain-folded lamellar crystal is a kinetics process, and the lamellar thickness determined by crystallization kinetics is therefore recognized as a distinct metastable structure. So the stability of each PBA polymorphic crystal is practically determined by lamellar thickness lc and represented by the melting temperatures Tm. Actually, Figure 3 is also a phase stability diagram for PBA polymorphic crystals to connect their thermodynamics with kinetics emerging from crystal size dependence. The Tm vs 1/lc line represented the minimum lamellar thickness which is stable at each temperature for PBA two crystal forms. The smallest stable lamellar thickness, i.e., the critical nucleus, represented the free energy barrier for crystal growth. Therefore, from the viewpoint of kinetics, the smaller critical nucleus is greatly favorable for the faster start and evolution of polymer crystal growth in the course of isothermal crystallization. Based on this diagram, the growth mechanism of PBA polymorphism can be clearly demonstrated. In Figure 3, the two temperature regions designated as region A above TQ and region B below TQ represented the regions of different growth mechanisms. Region A is the usually considered situation where the PBA melting phase (LA) goes directly into the stable PBA R crystal phase for crystallization. On the contrary, in region B, even though it is the structurally metastable phase in its infinite size, the PBA β crystal evolves first from the melting phase (LB) and becomes the stable phase due to the smaller crystal size, whereas the growth of the structurally stable R crystal is hindered from its relative large size. Therefore, the stability inversion with crystal size for PBA polymorphic crystals occurs in this region. Obviously, the crystal size is the essential factor for the phenomenon of stability inversion. Due to the smaller crystal size, the PBA β crystal shows a faster growth kinetics compared to the R crystal. As shown in Figure 5, at crystallization temperatures below 27°C where the stability inversion with crystal size takes place, the PBA β crystal showed a growth rate significantly faster than the R crystal which data was predicted by the fitting curve based on the experimental data. Therefore, it can be concluded from the results in Figures 3 and 5 that the metastable PBA β

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Figure 6. WAXD patterns of PBA β crystal (Tc ) 25 °C) upon thermal treatment. (A) Before and after annealing at different temperatures Ta for 1 h; (B) Annealing at 49 °C for different time length.

crystal is indeed the result of kinetically preferential growth due to the smaller crystal size. 3.4. Transformation of PBA Polymorphism. It is doubtless that the metastable PBA β crystal will finally transform into the thermodynamically stable phase, i.e., the R crystal. However, no detectable change in the PBA β crystal structure was found at crystallization temperature in our experiments, indicating that the β f R crystal transformation is hard to take place during isothermal crystallization process, unlike syndiotactic polystyrene in which the isothermal crystallization induced transformation.5 This suggested that the crystallization temperature Tc at which the β crystal grows is not high enough to provide sufficient energy to overcome the free energy barrier of the phase transition. In this work, the annealing treatment on the β crystal at an elevated temperature Ta (Ta > Tc) was carried out and the results are shown in Figure 6A. After annealing at 40 °C, the WAXD pattern did not change obviously, indicating that the β f R crystal transformation did not take place at this temperature. After annealing at 45 °C, the transformation was observed. A small but clear shoulder peak R(110) emerged along the WAXD pattern, and this peak evolved into a separated peak as the annealing temperature rose to 49°C (Figure 6A). The time dependence of diffraction peaks at annealing temperature of 49°C in Figure 6B clearly shows the β f R crystal transformation process. The R(110) peak came up as a shoulder peak first, and finally, developed into a separated peak with intensity exceeding β(110) peak. In Figure 6A, three new peaks of R(110), R(020), and R(021) which were not observed in the β crystal appeared after annealing at 56 °C, indicating that PBA β crystal has been completely transformed into the R crystal. Both WAXD and DSC results with annealing time demonstrated that transformation at 56 °C is very fast, almost immediately (data not shown). This strongly proves that the melting peak Tm2 in Figure 1 arises from the R crystal which is transformed from β crystal during DSC heating process. In addition to the change of crystal structure revealed by WAXD, the change of crystal size during annealing process was also followed quantitatively by small-angle X-ray scattering (SAXS). As discussed before, the growth of metastable PBA β crystal is due to the stability inversion

Figure 7. Variation of lamellar thickness lc with annealing temperature (A) and annealing time (B). The insets are the corresponding smallangle X-ray scattering patterns of intensity vs scattering vector q for the PBA β crystals (Tc ) 25 °C) before and after annealing treatments.

with crystal size, i.e., the relative small crystal size of the β crystal compared to the R crystal. Therefore, when the β crystal transforms into the R crystal at an elevated temperature Ta, as denoted by the pointing line M f N in Figure 3, crystal thickening should be involved. It was evident in Figure 7 that the elevated temperature Ta above the original crystallization temperature Tc promotes the thickening of lamellar crystals (Figure 7A) and that the lamellar thickening is kinetically dependent on the annealing time (Figure 7B). Further observation from Figure 7 suggests that transformation may be favorable for lamellar thickening, because lamellar thickness increases greatly during the phase transformation process. The slower thickening in the later part of the phase transition process in Figure 7 may be due to the consumption of limited amorphous chains in the interlamellar phases. The DSC result in Figure 8 also provided the evidence for PBA crystal thickening and transformation after annealing treatment. Combined with WAXD results in Figure 6A, annealing the PBA β crystal at 40°C for 1 h only resulted in the lamellar thickening but did not alter the crystal structure. As a result, the melting peak Tm1 of the original β crystal not only shifted toward higher temperature region but also increased the relative enthalpy, as shown in Figure 8. However, as the annealing temperature rose to 45 °C or higher, both the crystal thickening and phase transformation took place. Further analysis on the crystal size and structure in Figures 6B and 7B clearly indicated that the thickening and phase transformation took place simultaneously with annealing time. In this case, the DSC curves in Figure 8

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Figure 9. Illustration of PBA lamellar thickening and phase transformation upon annealing at elevated temperatures Ta.

Figure 8. DSC melting curves of the PBA β crystal (Tc) 25 °C) after annealing at 40, 45, and 49 °C for 1 h. The heating rate is 20 °C/min.

showed that the Tm1 peak still shifted toward higher temperature due to the thickening, but the relative enthalpy decreased obviously due to the transformation. After annealing at the elevated temperatures Ta, a small endothermic peak denoted by the arrow in Figure 8 appeared with the melting temperature Ts ca. 5 °C above the Ta. Such a small peak induced by annealing has been reported in other polymers.19 In the case of PBA polymorphic crystals, it is suggested that this small peak Ts arises from the crystallization of amorphous chains in the interlamellar regions at the annealing temperature Ta and that these annealing-induced crystals have R crystal structure. 3.5. Phase Transformation Mechanism. The inset of Figure 6A shows that the annealing temperatures 45 and 49 °C at which the β f R crystal transformation took place are lower than the melting temperature Tm1 of original β crystal. Therefore, it can be evidenced that the PBA β f R crystal transformation is a solid-solid-phase transition process. Because of the different conformation of the PBA R and β crystals, the solid-solid-phase transition of PBA polymorphism involves the molecular motion of polymer chains in both crystalline and amorphous phases. WAXD and SAXS results reveal that PBA β f R crystal transformation took place during the lamellar thickening process; however, the thickening did not necessarily cause the transformation (e.g., at annealing temperature of 40 °C). The essential factor governing the likelihood of thickening is the polymer chains in the amorphous phase, whereas the crystal transformation undoubtedly involves the change of chain conformation in the crystalline phase due to the different chain conformations of R and β crystals. It can be predicted that the necessary energy for driving polymer chain motion in amorphous and crystalline phases is different. Based on the results discussed above, it can be concluded

that only enough energy at sufficiently high annealing temperature Ta will activate the motion of polymer chains in the β crystalline phase and subsequently resulted in the transformation of β f R crystals. Therefore, the PBA β f R crystal transformation mechanism can be illustrated by Figure 9. At a lower annealing temperature Ta (e.g., 40 °C), the β crystal only increased in size with no changes in structure, as confirmed by WAXD and SAXS results in Figures 6 and 7. It was found that this annealing temperature is about 10 °C lower than the temperature where the β crystal starts to melt (see the inset in Figure 6A). However, the provided energy at this Ta is enough for the motion of amorphous polymer chains in the interlamellar regions. It has been known that the β crystal grows at a fast crystallization rate (see Figure 5), so the β crystal may have defects on the chainfolded surfaces. Therefore, the amorphous polymer chains on the chain-folded surface of the β crystal are movable for partial crystallization, resulting in the lamellar thickening. At a relative high annealing temperature Ta (e.g., 45 and 49 °C), not only lamellar thickening but also the β f R crystal transformation took place, as revealed by Figures 6 and 7. This Ta is still lower than the temperature where the β crystal starts to melt. In this case, the changes of crystal size and structure are not due to the partial melt and recrystallization but due to the solid-solid-phase transformation, i.e., the polymer chains packed in the β crystal lattice acquire enough energy and are able to change their conformation to that of the R crystal. With a lamella as the model, it is rational to assume that both the chains with β crystal conformation and the chains with R crystal conformation are possible to coexist in a lamella during the transformation process, because the β f R crystal transformation itself is a kinetic process. The amount ratio of polymer chains with R and β crystal conformations depends on the annealing temperature and time. The WAXD results in Figure 6 have proved this assumption. If the annealing time is long enough, all of the polymer chains with a β crystal conformation in

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the lamellae will convert into those with a R crystal conformation. As shown in Figure 9, the PBA R crystal has a monoclinic unit cell (a ) 0.673 nm, b ) 0.794 nm, c ) 1.420 nm, and β ) 45.5°) with an axially compressed planar zigzag conformation, whereas the β crystal is the orthorhombic unit cell (a ) 0.506 nm, b ) 0.735 nm, and c ) 1.467 nm) with a planar zigzag conformation12,13,22 From the viewpoint of a crystal lattice structure, the PBA β crystal has a different chain conformation, packing manner, and setting angle in the crystal lattice from the R crystal. For example, the β crystal has a longer c value (1.467 nm) than teh R crystal (c ) 1.420 nm). Therefore, the β f R crystal transformation involves three main types of molecular motion of polymer chains in the crystal lattice: shifting along the c axis, rotation around the c axis, and slight shrinking along the c axis. Such motions of polymer chains change the chain conformation and packing in crystal lattice from the kinetically preferential packing manner (β crystal) to the thermodynamically stable packing state with the lowest free energy (R crystal). 4. Conclusions Temperature-induced poly(butylene adipate) R and β crystals with the characteristic melting behaviors in their DSC diagrams provide us a suitable substance to study the phase metastability and transformation of polymer polymorphism. By assigning the multiple melting peaks and correlating melting temperature Tm with crystallization temperature Tc and lamellar thickness lc, the equilibrium melting temperatures T°m of polymeric crystals have been determined for understanding the structural stability of polymorphic crystals. The PBA R crystal with a higher T°m is a thermodynamically stable phase, whereas the β crystal with a lower T°m is a metastable phase. At crystallization temperatures below 27 °C, the metastable β crystal is kinetically preferential growth due to the stability inversion with crystal size. The metastable β crystal is stable at its crystallization temperature Tc and transforms into the R crystal after annealing at an elevated temperature Ta (Ta > Tc). The lamellar thickening and β f R crystal transformation at annealing temperatures are both solid-solid-phase transition processes, which involve the molecular motion of polymer chains in both amorphous and crystalline phases. Thickening is attributed

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to the partial crystallization of interlamellar amorphous polymer chains on the chain-folded surface of crystal, whereas the β f R crystal transformation is due to the motion of polymer chains packed in the β crystal lattice which acquire enough energy for changing their conformation to R crystals. Based on the crystal lattice structure, three kinds of polymer chain motion in lattice cell, i.e., shift, rotation, and shrink, have been proposed for demonstrating the β f R crystal transformation process. Acknowledgment. This work was supported by the SORST (Solution Oriented Research for Science and Technology) grant from Japan Science and Technology Corporation (JST). The authors also thank M. Yamamoto of the BASF AG company for his kindly supply of poly(butylene adipate) sample. References and Notes (1) Keller, A.; Cheng, S. Z. D. Polymer 1998, 39, 4461. (2) Cheng, S. Z. D.; Zhu, L.; Li, C. Y.; Honigfort, P. S.; Keller, A. Thermochim. Acta 1999, 332, 105. (3) Ostwald, W. Z. Physik. Chem. 1897, 22, 286. (4) Ho, R. M.; Lin, C. P.; Tsai, H. Y.; Woo, E. M. Macromolecules 2000, 33, 6517. (5) Ho, R. M.; Lin, C. P.; Hseih, P. Y.; Chung, T. M. Macromolecules 2001, 34, 6727. (6) Sun, Y. S.; Woo, E. M. Macromolecules 1999, 32, 7836. (7) Claudio, D. S.; Odda, R. D. B.; Massimo, D. G.; Finizia, A. Polymer 2003, 44, 1861. (8) Li, Y.; He, J.; Wei, Q.; Hu, X. Polymer 2002, 43, 2489. (9) Gan, Z.; Abe, H.; Doi, Y. Macromol. Chem. Phys. 2002, 203, 2369. (10) Vonk, C. G. J. Appl. Crystallogr. 1973, 6, 148. (11) Hsiao, B. S.; Gardner, K. H.; Wu, D. Q,; Chu, B. Polymer 1993, 34, 3986. (12) Minke, R.; Blackwell, J. J. Macromol. Sci., Phys. 1979, B16, 407. (13) Minke, R.; Blackwell, J. J. Macromol. Sci., Phys. 1979, B18, 233. (14) Liu, T.; Yan, S.; Bonnet, M.; Lieberwirth, I.; Rogausch, K. D.; Petermann, J. J. Mater. Sci. 2000, 35, 5047 and references therein. (15) Al-Hussein, M.; Strobl, G. Eur. Phys. J. E 2001, 6, 305. (16) Denchev, Z.; Nogales, A.; Sics, I.; Ezquerra, T. A.; Balta-Calleja, F. J. J. Polym. Sci. Part B: Polym. Phys. 2001, 39, 881. (17) Hsiao, B. S.; Wang, Z.; Yeh, F.; Gao, Y.; Sheth, K. C. Polymer 1999, 40, 3515. (18) Verma, R.; Marand, H.; Hsiao, B. Macromolecules 1996, 29, 7767. (19) Verma, R. K.; Hsiao, B. S. Trends Polym. Sci., 1996, 4, 312. (20) Hoffman, J. D.; Davis, G. T.; Lauritzen, J. I., Jr. In Theatise on Solid State Chemistry, Volume 3, Crystalline and Noncrystalline Solids; Hannay, N. B., Ed.; Plenum Press: New York, 1976, Chapter 7; pp497. (21) Marand, H.; Xu, J.; Srinivas, S. Macromolecules 1998, 32, 8219. (22) Pouget, E.; Almontassir, A.; Casas, M. T.; Puiggali, J. Macromolecules 2003, 36, 698.

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