Research Note pubs.acs.org/IECR
Effect of Uracil on the Isothermal Melt Crystallization Kinetics and Polymorphic Crystals Control of Biodegradable Poly(butylene adipate) Mengting Weng, Yingran He, and Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *
ABSTRACT: Biodegradable poly(butylene adipate) (PBA) and uracil composites were prepared via a solution and casting method at low uracil loadings in this work. The isothermal melt crystallization rate has been increased apparently in the composites by the addition of uracil, which acts as a nucleating agent for the crystallization of PBA. PBA may crystallize in the αor β-form under different conditions. During the nonisothermal melt crystallization, the α-form crystals of neat PBA are formed at only relatively slow cooling rate; however, the formation of α-form crystals can be induced by uracil in the composites at relatively faster cooling rates. The formation of polymorphic crystals of PBA may be regulated by changing cooling rate and the uracil loading, which provides an efficient way of controlling the polymorphic crystals formation and biodegradation behaviors.
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INTRODUCTION Biodegradable poly(butylene adipate) (PBA) has recently received considerable attention, which may crystallize in the α- or β-form under different conditions.1−6 For the α-form, it has a monoclinic unit cell with dimensions of a = 6.70 Å, b = 8.00 Å, c (fiber axis) = 14.20 Å, and β = 45.5°, and for the βform, it has an orthorhombic unit cell with dimensions of a = 5.05 Å, b = 7.36 Å, and c (fiber axis) = 14.67 Å.7,8 Gan et al. found that crystallization temperature was a key factor of determining the crystalline structure of PBA.9,10 According to their study, the pure α-form crystals of PBA were developed at crystallization temperatures above 31 °C, a mixture of both αand β-form crystals was formed between 29 and 31 °C, and the pure β-form crystals were produced at crystallization temperatures below 29 °C. The equilibrium melting point for the αform crystals is greater than that for the β-form ones, which indicates the α-form is thermodynamically more stable.10 Upon annealing at elevated temperatures, the metastable β-form crystals of PBA may transform to the α-form ones.9−16 It is well-known that the biodegradation behaviors of semicrystalline polymers must be influenced by the polymorphic crystals.17 In the case of PBA, Gan et al. found that the degradation rate for the α-form crystals was faster than that for the β-form ones.18−21 Controlling the formation of polymorphic crystals is believed to be an effective way of regulating the biodegradation behavior of PBA, because the polymorphic crystals show different biodegradation kinetics. In addition to crystallization temperature, the incorporation of nucleating agent is found to be another effective way of regulating the formation of polymorphic crystals of PBA, which can not only enhance the crystallization behaviors but also affect the formation of polymorphic crystals.21−24 Pan et al. have recently studied the effect of uracil as a novel nucleating agent for the crystallization behaviors of some biodegradable polymers, including poly(3-hydroxybutyrate), poly[(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)] copoly© 2012 American Chemical Society
mers with HHx content of 5, 10, and 18 mol %, and poly(Llactide).25,26 Both the nonisothermal melt crystallization behaviors and isothermal melt crystallization rates of these biodegradable polymers have been enhanced apparently after the addition of a small amount of uracil, which is indicative of its nucleating agent effect.25,26 In comparison to other nucleating agents, uracil does not deteriorate the biocompatibility of biodegradable polyesters, because it is one of the naturally occurring pyrimidines found in DNA.25 In this note, PBA/uracil composites have been successfully prepared via a solution and casting method at low uracil loadings; moreover, the effect of uracil on the isothermal melt crystallization kinetics and regulation of the formation of polymorphic crystals of PBA was investigated with various techniques. The objectives of this research are to study not only the effect of uracil on the crystallization kinetics but also especially its effect on the polymorphic crystals control of PBA. It is of great interest to investigate whether the addition of uracil may affect the polymorphic crystals formation and subsequently influence the biodegradation behavior of PBA. So far, the polymorphic crystals control of PBA has been achieved in most cases, under isothermal melt crystallization; however, it should be very interesting and important to control the polymorphic crystals formation of PBA under nonisothermal melt crystallization from both academic and practical application viewpoints. It is expected that the research reported herein will be of interest and help with the crystallization study of PBA from both academic and practical application viewpoints. Received: Revised: Accepted: Published: 13862
September 9, 2012 October 5, 2012 October 12, 2012 October 12, 2012 dx.doi.org/10.1021/ie302423n | Ind. Eng. Chem. Res. 2012, 51, 13862−13868
Industrial & Engineering Chemistry Research
Research Note
Figure 1. Development of relative crystallinity with crystallization time for (a) neat PBA and (b) PBA 0.1 at different crystallization temperature (Tc) values.
Figure 2. Avrami plots of (a) neat PBA and (b) PBA 0.1.
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EXPERIMENTAL SECTION PBA (Mw = 1.2 × 104 g/mol) and uracil (Mw = 112.09) were purchased from Sigma−Aldrich (Shanghai) Trading Co., Ltd. The PBA/uracil composites were prepared with the addition of 0.1, 0.5, and 1 wt % uracil contents. In the case of PBA/uracil composite with 0.1 wt % uracil, 1.998 g PBA was placed into 20 mL chloroform and stirred for 30 min. Two milligrams (2 mg) of uracil was added into 10 mL of chloroform and then sonicated with a KQ 700 DE ultrasonic generator for 30 min to make a uniform suspension. The uracil suspension then was added to the PBA solution and stirred for 3 h. The PBA/uracil solution was poured into a dish to evaporate the solvent at room temperature for 12 h. The sample was dried at 30 °C under vacuum for 72 h to remove the solvent completely. For brevity, the composites containing 0.1, 0.5, and 1 wt % of uracil are abbreviated hereafter as PBA 0.1, PBA 0.5, and PBA 1. Thermal analysis was performed using a TA Instruments differential scanning calorimeter (DSC) Q100 with Universal Analysis 2000 software. The weight of the samples varied between 4 mg and 6 mg, and all the operations were performed under a nitrogen purge. In the case of isothermal melt crystallization, the samples were heated to 70 °C at 20 °C/min, held for 3 min to erase any thermal history, cooled to the crystallization temperature (Tc) at 60 °C/min, and held for a period of time until the crystallization was complete. The exothermal traces were recorded for the later data analysis. In the case of nonisothermal melt crystallization, the samples were heated at 20 °C/min to 70 °C (first heating), held for 3 min to erase any previous thermal history, and then cooled to 0 °C at various cooling rate (first cooling), such as 0.5, 1, 2, 3, 5, 10, 15, 20, and 25 °C/min. The samples were further heated to 70 °C again at a rate of 10 °C/min (second heating) to study the subsequent melting behaviors. The nonisothermal melt crystallization behaviors were investigated from the first cooling
traces, and the subsequent melting behaviors were studied from the second heating traces. The spherulitic morphology of neat PBA and the PBA/uracil composites was observed with an optical microscope (POM) (Olympus BX51) equipped with a temperature controller (Linkam THMS 600). The samples were first annealed at 70 °C for 3 min to erase any thermal history and then cooled to 41 °C at 60 °C/min. Wide-angle X-ray diffraction (WAXD) patterns were recorded using a Rigaku D/Max 2500 VB2t/PC X-ray diffractometer from 10° to 30° at 2°/min at 40 kV and 200 mA. The samples were first annealed at 70 °C for 3 min to erase any thermal history and then cooled to 0 °C at various cooling rates with a temperature controller (Linkam THMS 600/CI94).
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RESULTS AND DISCUSSION Effect of Uracil on the Isothermal Melt Crystallization Kinetics of PBA. The overall isothermal melt crystallization kinetics of neat PBA and its three composites was studied via differential scanning calorimetry (DSC) in a wide crystallization temperature range of 33−45 °C. Because the samples were crystallized above 31 °C in this work, neat PBA and its three composites should crystallize in α-form. Figure 1 shows the development of relative crystallinity with crystallization time for neat PBA and PBA 0.1 in a crystallization temperature range of 33−41 °C. It is obvious from Figure 1 that crystallization time is prolonged as the crystallization temperature (Tc) increases for both neat PBA and PBA 0.1, suggesting that the crystallization rate is reduced at higher Tc. It is also interesting to note that the crystallization time is shorter in PBA 0.1 than in neat PBA at the same Tc. For example, ∼24 min were needed for neat PBA, while only 14 min were required for PBA 0.1 to finish crystallization at 41 °C, indicating that the incorporation 13863
dx.doi.org/10.1021/ie302423n | Ind. Eng. Chem. Res. 2012, 51, 13862−13868
Industrial & Engineering Chemistry Research
Research Note
of uracil has apparently accelerated the isothermal melt crystallization process of PBA in the composite. The well-known Avrami equation is further used to analyze the isothermal melt crystallization kinetics of neat PBA and the PBA/uracil composites. According to the Avrami equation, the relative crystallinity (Xt) develops with crystallization time (t) as 1 − X t = exp( −kt n)
(1)
where n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals, and k is a composite rate constant involving both nucleation and growth rate parameters.27,28 Figure 2 shows the Avrami plots for both neat PBA and PBA 0.1 crystallized at different Tc values, from which the Avrami parameters n and k were obtained from the slopes and the intercepts, respectively. For comparison, the obtained Avrami parameters are summarized in Table S1 in the Supporting Information for neat PBA and its three composites. As shown in Table S1 in the Supporting Information, the n values vary between 2 and 3 for neat PBA and its three composites with different uracil loading within the investigated Tc values. The average values of n are ∼2.4, despite Tc and the uracil loading, indicating that the addition of uracil does not change the crystallization mechanism of PBA in the composites.29 Because the unit of k is min−n and n is not constant in this work for all the samples at each Tc, it is not better to compare the overall crystallization rate from the k values directly. Therefore, the crystallization half-life time (t0.5), the time required to half completion of the final crystallinity of the samples, is employed to discuss crystallization kinetics of neat PBA and the PBA/uracil composites. The t0.5 values were calculated through the following equation:
t0.5 =
⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠
Figure 3. Optical micrographs of spherulitic morphology of (a) neat PBA, (b) PBA 0.1, (c) PBA 0.5, and (d) PBA 1 after complete crystallization at 41 °C.
Effect of Uracil on the Polymorphic Crystals Control of PBA. In the above section, the isothermal melt crystallization kinetics and spherulitic morphology of neat PBA and the PBA/uracil composites were studied in a crystallization temperature range where only α-form crystals can be formed. In this section, the nonisothermal melt crystallization behaviors of neat PBA and its composites were further studied with DSC at different cooling rates to investigate the influence of uracil on the polymorphic control of PBA crystals. Figures 4a and 4b show the nonisothermal melt crystallization behaviors at 1 °C/min and subsequent melting behaviors at 10 °C/min for neat PBA and its three composites, respectively. It is clear from Figure 4a that a two-stage crystallization behavior is found for neat PBA. The first crystallization peak temperature (Tp) is at 33.2 °C, corresponding to the α-form crystallization in the higher-temperature region, while the second Tp is at 32.0 °C, corresponding to the β-form crystallization in the lower-temperature region. As shown in Figure 4a, there is an intersection at 32.3 °C between the two stages, suggesting that the crystalline phase consists of both α- and β-form crystals for neat PBA during the nonisothermal melt crystallization at 1 °C/min. For the composites, the first Tp values are 37.7 °C for PBA 0.1, 39.2 °C for PBA 0.5, and 38.7 °C for PBA 1, respectively, indicating that the addition of uracil shifts the crystallization of PBA upward to the high-temperature range. It is obvious that the nonisothermal melt crystallization of the PBA matrix has been enhanced by uracil; however, increasing the uracil loading from 0.1 to 1 wt % affects the Tp values slightly. Because the crystallization end temperatures are greater than 32.3 °C for PBA 0.1, PBA 0.5, and PBA 1, only the α-form crystals are formed in the composites. 10 Figure 4b illustrates the subsequent melting behaviors for neat PBA and its three composites at 10 °C/min. As shown in Figure 4b, double melting endotherms are found for all the samples, corresponding to the fusion of the α-form crystals. Melting temperature Tm1 is attributed to the melting of the original α-form crystals formed during nonisothermal melt crystallization, and melting temperature Tm2 is attributed to the melting of the α-form crystals formed through melting and recrystallization of the αform crystals during the heating process.10 Relative to neat
(2)
The overall isothermal melt crystallization rate can thus be easily described by the reciprocal of t0.5 (i.e., 1/t0.5). Table S1 in the Supporting Information also lists all the 1/t0.5 values for neat PBA and its composites at different Tc values for comparison. The 1/t0.5 values are found to decrease with increasing Tc for all the samples, suggesting that the overall isothermal crystallization rates are reduced at higher Tc, because of small supercooling. The 1/t0.5 values are greater in the PBA/ uracil composites than in neat PBA at a given Tc; moreover, the 1/t0.5 values increase slightly with increasing the uracil loading in the composites, indicating again that the overall isothermal melt crystallization rates of PBA have been enhanced by the presence of uracil as a nucleating agent. It is interesting to investigate the effect of uracil on the spherulitic morphology of PBA. Figure 3 illustrates a series of POM images for neat PBA and its three composites crystallized at 41 °C. As shown in Figure 3, the size of PBA spherulites becomes smaller, and the number of PBA spherulites becomes greater as the uracil loading in the composites increases, compared to neat PBA. It is clear that uracil provides more nucleating sites for PBA to crystallize, resulting in the restricted growth of spherulites. In brief, the spherulitic morphology study indicates that the nucleation density of PBA spherulites in the composites has been improved, apparently because of the nucleating agent effect of uracil, which is consistent with the DSC results in the previous section. 13864
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Figure 4. (a) Nonisothermal melt crystallization at 1 °C/min and (b) subsequent melting behavior at 10 °C/min of neat PBA and its composites.
Figure 5. Nonisothermal melt crystallization behavior of (a) neat PBA, (b) PBA 0.1, (c) PBA 0.5, and (d) PBA 1 at various cooling rates.
Figure 6. Subsequent melting behaviors at 10 °C/min of (a) neat PBA, (b) PBA 0.1, (c) PBA 0.5, and (d) PBA 1. 13865
dx.doi.org/10.1021/ie302423n | Ind. Eng. Chem. Res. 2012, 51, 13862−13868
Industrial & Engineering Chemistry Research
Research Note
Figure 7. WAXD patterns of neat PBA and its composites crystallized at different cooling rates.
PBA, the Tm1 values shift slightly upward to high temperature in the composites and level off when the uracil loading is 0.5 wt % and above. In addition, the Tm2 values shift downward to low temperature very slightly in the composites relative to neat PBA, and the variation of Tm2 is
