Polymorphic Behavior and Enzymatic Degradation of Poly(butylene

Jan 26, 2015 - ... isotactic poly(1-butene). Bao Wang , Alfred Menyhard , Giovanni C Alfonso , Alejandro J Müller , Dario Cavallo. Polymer Internatio...
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Polymorphic Behavior and Enzymatic Degradation of Poly(butylene adipate) in the Presence of Hexagonal Boron Nitride Nanosheets Yi-Ren Tang, Jun Xu,* and Bao-Hua Guo* †

Advanced Materials Laboratory of Ministry of Education, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Hexagonal boron nitride nanosheets (BNNSs) were prepared by a facile chemical exfoliation method. Then cosolution film casting method was used to obtain poly(butylene adipate) (PBA)/BNNSs nanocomposites. The incorporated BNNSs changed the formation condition of the polymorphic crystals of PBA. The results of differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) showed that BNNSs significantly facilitated the formation of α-form crystals for isothermal and nonisothermal crystallization. Compared to pure PBA at 28 °C, the α-form crystals would not disappear until the temperature decreased to 13 °C with the addition of 0.5 wt % BNNSs. In addition, the excellent heterogeneous nucleation ability of BNNSs was identified by the nonisothermal and isothermal crystallization process. The enzymatic degradation experiments exhibited that PBA/BNNSs nanocomposites possessed a considerably lower degradation rate than neat PBA. This work demonstrates that BNNSs is an efficient material to regulate the formation of polymorphic crystals and the degradation behavior of PBA.

1. INTRODUCTION Due to the characteristics of mechanical strength, lightweight structure, resistance to chemical influences, and low costs, polymeric materials play a significant role in everyday life. On the other hand, the problem of plastic disposal arises. With the development of the polyester industry in the last two decades, biodegradable polyesters with comparable properties to the commodity plastics offer an attractive solution to the problem of white pollution caused by improper disposal of nondegradable plastics.1,2 In addition, biodegradable polyesters have gained significant importance in the fields of tissue engineering, endosseous stabilizer implants, artificial skin, and controlled drug delivery.3,4 Among the various physical− chemical properties, controllable biodegradability is the most important property for the extensive use of polyester materials. Besides the chemical structure, the microstructure of the condensed state also plays a crucial role in the biodegradability of polyesters. Poly(butylene adipate) (PBA) is a typical crystallizable biodegradable polyester with the distinct feature of being polymorphic, which has attracted wide attention from scientific research and industry.5−9 Thus, the investigation of polymorphic crystals would be of great help in clarifying the degradation kinetics and further regulating the degradation rate of PBA. According to prior research, polymorphism of PBA crystals is mainly dependent on the crystallization temperature. PBA shows pure α-form crystals, a mixture of both α-form and βform crystals, and pure β-form crystals with crystallization temperature decreases.10,11 On the other hand, during the melting recrystallization process, the remaining content of αnuclei decides the final crystal form of PBA regardless of crystallization temperature.12 The recent research in our lab found that the molecular weight and its distribution also had a significant influence on the polymorphic crystals of PBA.8 α© XXXX American Chemical Society

Form crystal possesses 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°, while for the β-form crystal, an orthorhombic unit cell with dimensions of a = 5.05 Å, b = 7.36 Å, and c (fiber axis) = 14.67 Å has been identified.5,6 The solid−solid phase transition of βto-α-form crystals occurs at elevated temperatures, and the equilibrium melting point (Tm0) of the α-form crystal is higher than that of the β-form crystal.11 The transformation and Tm0 indicate that the α-form crystals are thermodynamically stable phase, whereas β-form crystals are kinetically favorable phase. Due to the different packing manners, α-form crystals show the fastest degradation rate, while the mixture of the two crystals forms show the lowest degradation rate.13 Besides the crystallization temperature, blending, 14,15 nucleating agents,16−18 copolymerization,19,20 and epitaxial growth21,22 are also used to control the formation of polymorphic crystals in PBA. Among various regulation methods, nucleating agents have been considered greatly significant for their low content and high efficiency. Various research studies on the effect of nucleating agents, on the polymorphic behavior during the nonisothermal crystallization, such as small organic molecule uracil16 and multimethyl-benzilidene sorbitol (TM6),23 inclusion complex α-cyclodextrin,17 one-dimensional material MWCNT,18 and inorganic clays,24 have been reported. Although these nucleating agents can promote the formation of α-form crystals at high cooling rate, there are few reports of the effect of nucleating agents on the polymorphic behavior during the isothermal crystallization process. In addition, the effect of two-dimensional materials on the polymorphic Received: November 24, 2014 Revised: January 18, 2015 Accepted: January 26, 2015

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suspension and then mixed with PBA/chloroform solution together followed by gentle ultrasonic treatment (60 w) for 3 h. At last, the PBA/BNNSs solution was cast on a glass disk for the evaporation of the solvent. The samples were dried in vacuum for 24 h before use. Characterization. High-resolution transmission electron microscopy (HRTEM; JEOL, JEM-2010) was operated at an accelerating voltage of 120 kV. The HRTEM samples were prepared by drying a droplet of the 0.05 mg/mL BNNSs chloroform solution on a lacy carbon grid. Atomic force microscopy (AFM; Shimadzu SPM-9700) was employed in tapping mode. The AFM samples were prepared by solution casting 50 μL of 0.05 mg/mL BNNSs chloroform solution on a newly cleaved mica and dried in a vacuum oven for 8 h before the AFM measurements. UV−vis spectra were recorded for the 0.1 mg/mL of BNNSs chloroform solution on a Pgeneral TU1810 twin-beam spectrophotometer with a spectral range from 200 to 900 nm. Thermal analysis of the specimens was performed by differential scanning calorimetry (DSC) using a Shimadzu DSC-60 instrument. The measurements were carried out under a nitrogen atmosphere, and indium and zinc standards were used for calibration. The specimens of about 2−5 mg were placed in a seal aluminum pan during each process. During the nonisothermal crystallization process, specimens were first melted at 80 °C for 3 min to eliminate thermal history, cooled to 0 °C at different cooling rates ranging from 25 to 0.5 °C/ min, and then reheated at a rate of 10 °C/min to 80 °C. The peak temperature of endothermic and exothermal peak was identified as the melting point (Tm) and crystallization temperature (Tc), respectively. For the isothermal crystallization process, a heating stage was used to erase the thermal history of the sample which is sealed in the aluminum pan, and then the pan was transferred quickly to a Linkam THMS600 hot stage (with liquid nitrogen subcooler) at fixed temperature for isothermal crystallization. Usually, this procedure can decrease the cooling time from minutes to less than 5 s compared to the traditional DSC cooling process. After the complete isothermal crystallization, the aluminum pans were transformed to DSC and further heated to 80 °C at a rate of 10 °C/min to get the melting peak. Wide-angle X-ray diffraction (WAXD) analysis was carried out at room temperature using a Rigaku D/max2500+/PC Xray diffractometer with Cu Kα radiation. Scanning was performed with 2θ from 5° to 30° at a rate of 4°/min with a step of 0.02°. The nonisothermal samples were first annealed at 80 °C for 3 min to erase thermal history and then cooled to 0 °C at various cooling rates with a temperature controller (Linkam THMS600). The isothermal samples were prepared as well as the DSC isothermal samples. A Leica polarized optical microscope (DM2500P) equipped with a Linkam THMS600 hot stage was used to observe the crystal morphology of PBA/BNNSs nanocomposites. For enzymatic degradation experiment, the following procedure was applied: (a) 10 mm × 10 mm × 0.3 mm films of PBA and its nanocomposites were obtained by hot-press and then quenched to the specified temperature; (b) 50 μL of lipase solution (1 mg/mL in 0.1 M phosphate buffer solution) was added to 1 mL of phosphate buffer solution with a piece of the sample; (c) the reaction mixture was incubated in a vial with water bath at 37 °C, and the degradation rate of sample was calculated by the weight loss in 3 h interval. Before the measurements, the samples were washed with deionized water

behavior of PBA has not been reported to our knowledge. Additionally, the different enzymatic degradation rates of αand β-form crystals provide us a new method to modulate the degradation rate by regulating the polymorphic behavior of PBA via nucleating agent. Hexagonal boron nitride (h-BN) is a common nucleating agent for polyester.24−26 Driven by the numerous intriguing results on graphene, hexagonal boron nitride nanosheets (BNNSs) prepared by exfoliating h-BN powder have attracted wide attention as one of the most important graphene-like twodimensional materials. The high elastic modulus, superthermal conductivity, insulation, and optical properties are the unique performance of BNNSs.27−32 Furthermore, a distinct nucleation ability of hydroxyl-functional hexagonal boron nitride nanosheets (OH-BNNSs) on the poly(butylene succinate) (PBS) has been reported in our latest research in detail.33 Because of their tremendous surface area, BNNSs may also act as a nucleating agent and polymorphic and degradation controller of PBA. In this work, BNNSs instead of OH-BNNSs were successfully prepared by chemical exfoliation34 for the purpose of preparing finely dispersed BNNSs in chloroform solution. Then the PBA/BNNSs nanocomposites were obtained at low BNNSS loading via the solution casting method. The effects of BNNSs on the crystallization kinetics and the formation of polymorphic crystals under isothermal and nonisothermal conditions were studied by DSC and WAXD. Degradation experiments were further carried out to directly evaluate the effect of BNNSs on the degradation behavior of PBA. The results presented here not only clarify the relationship between biodegradation and crystallization mechanisms but also supply a new and highly efficient material to regulate the polymorphic crystals structure and degradation rate of PBA.

2. EXPERIMENTAL SECTION Materials and Sample Preparation. PBA (Mw = 1.2 × 104 g/mol) and lipase (Pseudomonas species type XIII) were purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd. Hexagonal boron nitride (h-BN) powder was purchased from Aladdin (Shanghai) Industrial Co., Ltd. H2SO4 (98%, w/w) and KMnO4 were received from Beijing Chemical Works. A chemical exfoliation method34 was used to obtain hexagonal boron nitride nanosheets (BNNSs): 25 mL of concentrated H2SO4 (98%, w/w) and 1 g of h-BN powder were mixed and stirred in a 250 mL one-neck round-bottomed flask. Afterward, 0.5 g of KMnO4 was added slowly. After its addition, the suspension temperature was controlled at 0 °C for 12 h under vigorous stirring. Later, 10 mL of H2O2 (30%, w/w) was added into the suspensions drop by drop. Finally, the asobtained “milky” solution was centrifuged at 5000 rpm for 10 min in order to gain few-layer BNNSs. The filtrate was washed with deionized water until the pH value of the supernatant reached 7. The final products were dried in a vacuum oven at 40 °C for 24 h. About 50 mg of BNNSs could be obtained from 1 g of h-BN powder. In our experiment, a series of PBA/BNNSs nanocomposites were prepared by a cosolution film casting process. The nanocomposites were abbreviated as P-0, P-0.1, P-0.3, P-0.5, and P-1 with the content of BNNSs being 0, 0.1, 0.3, 0.5, and 1 wt %. In the case of P-0.3, 1.994 g of PBA was fully dissolved in 20 mL of chloroform. Six milligrams (6 mg) of BNNSs was dispersed in 20 mL of chloroform with mild ultrasonic treatment (60 W) for 2 h in order to obtain a uniform B

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Figure 1. (a) AFM height image of BNNSs and the corresponding height profile (inset), the length of inserted bar is 500 nm; (b) HRTEM image of BNNSs, the length of inserted bar is 5 nm, and the corresponding electron diffraction (ED) pattern of the region, which shows the hexagonal structure of BNNSs; (c) UV−vis spectrum of 0.1 mg/mL BNNSs chloroform solution and its Tyndall effect (inset).

Figure 2. (a) DSC curves of isothermally crystallized P-0 at a heating rate of 10 °C/min; (b) WAXD patterns of P-0 isothermally crystallized at various temperatures.

until the pH value of filtrate was 7 and then dried in vacuum for 24 h.

in the HRTEM image (Figure 1b), it is obvious that the powder of h-BN has been exfoliated to the individual few-layer BNNSs. The corresponding electron diffraction pattern reveals a typical sixfold symmetry nature of h-BN, which means that the synthesis process does not destroy the hexagonal crystals of BNNSs. The well-crystallized BNNSs are significantly important for its nucleation capability for PBA. Figure 1c shows a symmetrically shaped band gap peak around 200 nm in the UV−vis absorption spectrum for 0.1 mg/mL of BNNSs chloroform solution that corresponds with previous observations.35,36 The fine dispersion of the suspended materials does not exhibit any visible precipitates within a few days. Effect of BNNSs on the Polymorphic Crystals Control of Isothermally Crystallized PBA. In the above section, the finely dispersed few-layer BNNSs have been studied. In this part, we combine DSC and WAXD results to trace the

3. RESULTS AND DISCUSSION Characterization of h-BN Nanosheets. The thickness of BNNSs is very important for the “size effect”. When the thickness of lamellar materials decreases to atomic level, they will exhibit completely different and excellent properties such as optical, mechanical, thermal conductivity, and nucleation property, compared to the macroscopic lamellar materials.27−33 The AFM height image (Figure 1a) indicates that few-layer BNNSs with a lateral size between 500 nm and 2 um and a thickness close to 1 nm were successfully prepared by chemical exfoliation. Figure 1a also proves the presence of substantial fraction of 2−3 layer BNNSs and a uniform thickness distribution of BNNSs layers. From the marked yellow circle C

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Figure 3. (a) DSC curves of isothermally crystallized P-1 at a heating rate of 10 °C/min; (b) WAXD patterns of P-1 isothermally crystallized at various temperatures.

method to distinguish the polymorphic crystals. It is interesting to find that the peak area ratio of β(110) and α(110) in Figure 3b increases gradually until α(110) disappears at 11 °C. This means that the neat β-form, α-form, and α/β mixed type crystals are formed at Tc < 12 °C, Tc > 31 °C, and Tc = 12−31 °C. Supporting Information Figures S1−S3 show the WAXD and DSC patterns of P-0.1, P-0.3, and P-0.5. The temperature at which the α-form crystals vanish, identified by the α(110) peak, changes from 28 to 24, 20, and 13 °C for P-0.1, P-0.3, and P-0.5, respectively. The above results are summarized in Figure 4.

polymorphic crystals of isothermally crystallized PBA/BNNSs nanocomposites. Figure 2 and Figure 3 show the melting behavior and WAXD patterns of P-0 and P-1 at different crystallization temperatures so as to distinguish polymorphic crystals. In order to ensure consistency, the melting samples were prepared by isothermal crystallization in the sealed aluminum pan (with sample inside) on the Linkam THMS600 hot stage. As shown in Figure 2a, multiple melting peaks are found for all crystallization temperatures, corresponding to the fusion of polymorphic crystals and melting−recrystallization. Two melting peaks Tm1 and Tm2 are detected at the high crystallization temperatures (above 30 °C). The lower Tm1 represents the melting of initial α-form crystals, and the higher Tm2 originates from the melting−recrystallization of α-form crystals during the heating process.10,11 Nevertheless, the double melting peaks Tm1 and Tm2 are substituted by the triple melting peaks: Tm3 (lower), Tm4 (middle), and Tm5 (higher temperature) when the crystallization temperature of P-0 decreases. Tm3 corresponds to Tm1, which represents the melting of the original α-form crystals, Tm4 arises from the melting of the recrystallized α-form crystals and the initial β-form crystals, and Tm5 can be attributed to the α-form crystals transformed from β-form crystal during the heating process.11,37 The triple melting peaks (Tm3, Tm4, and Tm5) at crystallization temperatures ranging from 31 to 29 °C indicate mixed crystals of α- and β-form. P-0 only forms pure β-form crystals with the crystallization temperature further decreasing. The melting peaks of Tm6 and Tm7 represent the melting of original β-form crystals and α-form crystals that are transformed from the β-form crystals, respectively. WAXD is a more direct method to identify the crystal structure of polymers and gives a further proof of polymorphic transformation, as shown in Figure 2b. When crystallized above 31 °C, neat PBA shows three main characteristic diffraction peaks that correspond to the crystal planes (110), (020), and (021) of α-form crystal.15 However, as the crystallization temperature decreases, β-form crystal planes (110) and (020) arise and the α-form crystal plane (110) degenerates to a shoulder peak of the β-form crystal plane (110). The WAXD results confirm the findings made by DSC. Therefore, it can be concluded that PBA forms neat β-form, α-form, and α/β mixed type of crystals at Tc < 29 °C, Tc > 31 °C, and Tc = 29−31 °C, respectively. Because of the low content of α-form crystals at some temperatures for PBA/BNNSs nanocomposites, Tm3 is not distinguishable with DSC. Therefore, WAXD is a crucial

Figure 4. Temperature dependence of the polymorphic crystals of PBA and PBA/BNNSs nanocomposites.

Two important conclusions could be deduced. First, β-form crystals appear almost consistently at 31 °C for all samples regardless of the BNNSs content. Additionally, the temperature at which the α-form crystals vanish decreases with the increasing amount of BNNSs and finally levels off when the content of BNNSs is larger than 0.5 wt %. We speculate that there may exist two mechanisms responsible for the observed selective nucleation of α-form PBA. First, considering the specific selectivity of BNNSs on α-form PBA and the lattice parameter of both forms of PBA with BNNSs cannot be matched, we conjecture that preferential crystallization of αform crystals is due to the bulk “memory effect” of PBA molecular chains which are anchored at the surface of BNNSs. The similar chain adsorption is a very common interaction between polymer and inorganic filler,38 especially in the twodimension material39 for its tremendous surface area. From ref 11 and Figure 2a, even if PBA forms pure β-form crystals at low crystallization temperature, PBA will melt as α-form crystals D

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Figure 5. (a) Melting behavior of neat nonisothermally crystallized PBA at the heating rate of 10 °C/min; (b) WAXD patterns of neat nonisothermal crystallized PBA.

Figure 6. (a) Melting behavior of neat nonisothermally crystallized PBA/BNNSs nanocomposites at the heating rate of 10 °C/min; (b) WAXD patterns of neat nonisothermal crystallized PBA/BNNSs nanocomposites.

only exhibit the ability of selective nucleation of α-form nucleus. Moreover, the inorganic nucleating agents of uracil,16 α-cyclodextrin,17 and MWCNT18 have been proved effective only in the α-form crystals. These two reasons may explain the extended formation temperature range of α-form crystals. Effect of BNNSs on the Polymorphic Crystals Control of Nonisothermally Crystallized PBA. In the manufacturing process, the products always experience a nonisothermal cooling process. Therefore, it is important for practical application to investigate the effect of BNNSs on the polymorphic crystals of nonisothermally crystallized PBA. The variation of polymorphic crystals in nonisothermally crystallized PBA/nanocomposites was investigated by the same method as the isothermally crystallized samples. The main characteristics of the α-form crystal are the melting peaks of Tm1 and Tm2 and the diffraction peak (110), (020), and (021) of α-form. The melting peaks of Tm6 and Tm7 and the diffraction peaks (110), (020) of β form are used to distinguish β-form crystal. Three melting peaks of Tm3, Tm4, and Tm5 and mixed diffraction peaks of α-form crystals and β-form crystals represent the mixed crystals. From the DSC and WAXD results in Figure 5, we can conclude that PBA forms only α-form crystals at cooling rates not higher than 5 °C/min, and a mixture of α-form crystals and β-form crystals at cooling rates above 10 °C/min. All the PBA/BNNSs nanocomposites show only two crystal characteristic melting peaks Tm1 and Tm2 of α-form at cooling rates below 25 °C/min (Figure 6a). However, P-0.1 shows a weak shoulder peak β (110) which indicates a mixture of αform and β-form crystals at a cooling rate of 25 °C/min (Figure 6b). The absence of the mixed crystals melting peak Tm5 is due

due to the solid−solid phase-transition at the elevated temperature. The molecular chains of PBA that are on the surface of BNNSs at the melting temperature cannot form random coil completely due to the strong interaction. Thus, the residual molecular chains with α-form ordered structure can induce the formation of α-form crystals. The similar phenomenon of memory effect on PBA α-form crystals has been reported by Wu and Woo.12 Second, we can separate the crystallization temperature into two regions. In the crystallization temperature range of forming mixed crystals, α-nuclei and β-nuclei grow competitively. α-Nuclei have faster primary nucleation rate than β-nuclei; thus, the spherulite center consists of α-form crystals. β-form crystals develop on the periphery of α-form spherulite, namely, the cross-nucleation.8 So even if BNNSs decrease the nucleation barrier, the primary nucleation rate of α-nuclei is still higher than that of β-nuclei at high-temperature range. Besides, the nuclei density increases tremendously due to the heterogeneous nucleation property of BNNSs (the results are demonstrated in the morphology part). After α-nuclei form, the spherulites impinge almost immediately. β-Form crystals have no time to (secondary) nucleate on the periphery of α-form spherulite. Consequently, the content of β-form crystals decreases. When the temperature is sufficiently low, the diffusion energy instead of nucleation energy contributes primarily to the crystallization. The nucleation barrier for PBA is low enough, and the contribution of BNNSs is negligible. The similar spherulite size of PBA and PBA/BNNSs nanocomposites crystallized at 10 °C gives the evidence of the negligible nucleation ability of BNNSs at low temperature (Supporting Information Figure S4). In these temperature ranges, only the β-form crystals form. So BNNSs E

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Figure 7. DSC curves of PBA and PBA/BNNSs nanocomposites during (a) cooling from the melt at a rate of 10 °C/min and (b) subsequent heating at the same rate.

Figure 8. Relative crystallinity as a function of time for (a) P-0 and (b) P-1, during isothermal melting crystallization.

to the negligible fusion enthalpy of the rare β-form crystals. Except P-0.1 at 25 °C/min, all samples show the same characteristic diffraction peaks of α-form crystal. This means that after adding 0.1 wt % BNNSs, β-form crystals will not form at cooling rates below 20 °C/min. Moreover, with increasing addition of BNNSs, only α-form crystals develop for all the investigated cooling rates. Obviously, the formation of polymorphic crystals of PBA depends on the cooling rate (Figure 5). The higher the cooling rate, the shorter the crystallization time at each temperature, and therefore from a kinetics viewpoint PBA preferably crystallizes in the metastable β-form. The formation of the thermodynamically more stable α-form crystals occur at lower cooling rates. With the addition of BNNSs, nanocomposites are inclined to form α-form crystals at high cooling rates. The reason is that the BNNSs accelerate the heterogeneous nucleation of PBA, so that the nanocomposites can crystallize faster in the high-temperature range and form pure α-form crystals. Furthermore, the heterogeneous nucleation property of BNNSs was investigated. Figure 7 shows the crystallization of PBA and PBA/BNNSs nanocomposites during cooling and the subsequent melting curves at the same cooling and heating rate of 10 °C/min. It is interesting that PBA shows a two-stage crystallization behavior. The first and second stages of the crystallization peak temperatures are 31.7 and 30.2 °C, which correspond to the formation of α-form and β-form crystals, respectively.17 This observation gives further evidence that neat PBA forms mixed crystals at cooling rates between 25 and 10 °C/min (for cooling rates not higher than 10 °C/min, the crystallization curves during cooling with only one stage of αform crystals is not shown here). The crystallization temperatures of the PBA/BNNSs nanocomposites increase to 33.3, 33.7, 34.0, and 35.5 °C with an increase of BNNSs content

from 0.1, 0.3, 0.5, to 1 wt %. Obviously, BNNSs enhance the heterogeneous nucleation ability of PBA, but the increasing tendency almost levels off with more BNNSs. Figure 7b shows the melting curves of PBA/nanocomposites. All the curves show two endothermic peaks of α-form crystals except P-0 with an extra endothermic peak of Tm5. Though both derived from original α-form crystals, the lamellar hinder effect of BNNSs causes the decrease of Tm1 compared to Tm3. The working mechanism of the lamellar hinder effect is that the lamellar structure will restrict the molecular motion of polymer chains and thus decrease the crystal lamellar thickness. Fornes et al. reported that the Tm of low molecular weight nylon-6/ organically modified montmorillonite (OMMT) composites decreased 7 °C if the content of OMMT was 6.6 wt %.40 Fabbri et al. obtained similar results for the poly(butylene terephthalate)/graphene composites.41 Effect of BNNSs on the Isothermal Crystallization of PBA. To further investigate the nucleation capacity of BNNSs, isothermal crystallizations of PBA and its nanocomposites were executed in the temperature ranging from 39 to 45 °C, at intervals of 2 °C. Figure 8 shows the typical sigmoid shape of the relative degree of crystallinity versus crystallization time of P-0 (a) and P-1 (b) at different Tc (the original bell-shaped DSC curves of P-0 and P-1 are shown in the Supporting Information Figure S5. As the Tc increases, the crystallization time is prolonged as well as the induction time for the nucleation in the early stage. This indicates that nucleation rather than diffusion is the limiting factor for crystallization in this temperature range. In contrast to P-0, the crystallization time decreases considerably at the same temperature. In the case of 45 °C, P-0 needs 40 min to accomplish the crystallization, while P-1 only needs 9 min, demonstrating F

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Figure 9. Avrami plots of (a) P-0 and (b) P-1 during isothermal melting crystallization.

three-dimensional growth of PBA and PBA/BNNSs nanocomposites. As deduced from the n in Table 1, BNNSs do not change the crystallization mechanism of PBA in the nanocomposites. Generally, BNNSs have a prominent heterogeneous nucleation property regarding PBA. Spherulite Morphology. Figure 10 shows the spherulite morphology of P-0, P-0.1, P-0.3, and P-1 at 45 °C. When the crystallization temperature is 45 °C, only α-form crystals form, and the spherulite size of P-0 is relatively large in Figure 10a. However, the spherulite size continuously decreases with the increasing content of BNNSs until they are invisible. This is due to the large surface area of BNNSs that provides sufficient heterogeneous nucleation sites for PBA and reduces the free energy barrier of the primary crystallization. The results of the spherulite morphology are consistent with the DSC results in the previous section. Enzymatic Degradation. Enzymatic degradation experiments were carried out to evaluate the effect of BNNSs on the degradation behavior of PBA. In the above section, we found that the addition of BNNSs could change the polymorphic crystals form, morphology, and size of PBA spherulites. Therefore, the biodegradation property of PBA should be affected by BNNSs. Figure 11 reveals the effect of the BNNSs content on the weight loss of PBA at different crystallization temperatures. Obviously, the enzymatic degradation rate decreases gradually with the increase of BNNSs content independent of the crystallization temperature. As seen in Figure 11a, in P-0 only β-form crystals are formed, while P-0.3 and P-1 are composed of mixed crystals of α-form and β-form. From ref 13, we know that the mixed PBA crystals of α-form and β-form possess slower degradation rate than the individual α-form crystals. Our results in Supporting Information Figure S5 also exhibit a similar phenomena. Thus, the degradation rate decreases for the α-form crystals that emerge from adding BNNSs at 25 °C. More interestingly, BNNSs can also decrease the degradation rate at 35 °C when all samples only form α-form crystals. This means that BNNSs can also affect the degradation rate of PBA by heterogeneous nucleation and its lamellar structure can block the invasion of the small molecules. Gan et al. found a similar phenomena in the composites of PBA/TM6 and proved that the heterogeneous nucleation played a dominant role in determining the biodegradation rate when the content of TM6 was high.23 Looking back at the WAXD measurements at 32 °C in Figure 2b and 3b, the relative intensity of the α(110) and α(020) diffraction peaks changes considerably after adding BNNSs. The intensity variance suggests that BNNSs can change the optimal growth direction of PBA crystals by heterogeneous nucleation, which affects the degradation rate of

that the isothermal crystallization of PBA could be greatly increased by adding BNNSs. To study the effect of BNNSs on the crystallization mechanism of PBA, the well-known Avrami equation42−44 was employed to analyze the crystallization kinetics of PBA and PBA/BNNSs nanocomposites 1 − X t = exp( −kt n)

(1)

where Xt is the relative degree of crystallinity, k the crystallization rate constant (min−1), and n the Avrami exponent related to the nature of the nucleation mechanism and the growth dimension of the crystals. Figure 9 shows the Avrami plots of P-0 and P-1; all the linear correlation factors R2 are greater than 0.99. The Avrami parameters n and k which were calculated from the slope and intercept are summarized in Table 1 for reasons of comparison, as well as the crystallization Table 1. Avrami Exponents of Neat PBA and PBA/BNNSs Nanocomposites Isothermally Crystallized at Specified Temperatures sample

crystallization temp (°C)

n

k (min−n)

t0.5 (min)

P-0

39 41 43 45 41 43 45 47 41 43 45 47 41 43 45 47

3.10 3.31 3.65 3.23 3.34 3.35 3.66 4.06 3.00 2.96 3.21 3.37 2.90 2.96 2.92 3.38

8.73 × 10−2 9.40 × 10−3 3.37 × 10−4 3.86 × 10−5 1.56 × 10−1 1.17 × 10−2 3.89 × 10−4 2.16 × 10−6 2.23 × 10−1 2.30 × 10−2 1.19 × 10−3 3.76 × 10−5 1.27 × 10° 1.42 × 10−1 1.10 × 10−2 4.21 × 10−4

2.0 3.8 8.5 20.1 1.6 3.2 7.6 18.0 1.4 3.0 6.7 16.2 0.8 1.7 4.0 8.8

P-0.3

P-0.5

P-1

half-time (t0.5). t0.5 describes the time until the sample achieves half of its final crystallinity and can be calculated by the following equation:

t0.5 =

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

(2)

As shown in Table 1, the Avrami parameter n ranges from 3 to 4 at the chosen temperatures regardless of the content of BNNSs. Other literature also reports the approximate value of n for pure PBA at the same temperature.45 This demonstrates the G

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Figure 10. Spherulitic morphologies of (a) P-0, (b) P-0.1, (c) P-0.3, and (d) P-1 at 45 °C; the length of inserted bar is 50 μm.

Figure 11. Weight change of PBA films and its nanocomposite films at the crystallization temperature of (a) 25 °C and (b) 35 °C during the enzymatic degradation.

DSC and WAXD results, we found that BNNSs could induce α-form crystals during nonisothermal and isothermal crystallization. Only 0.1 wt % BNNSs could induce neat α-form crystals in PBA at the cooling rate of 20 °C/min. In isothermally crystallized samples, the temperature at which the α-form crystals disappeared changed from 28 to 13 °C after adding 0.5 wt % BNNSs. The enzymatic degradation experiments provided direct evidence that BNNSs could regulate the degradation rate of PBA via regulating the formation of the polymorphic crystals, facilitating heterogeneous nucleation, and the characteristic of lamellar hinder effect. Thus, this work provides an effective material to regulate the formation of the polymorphic crystals and to further control the degradation rate of PBA.

the PBA definitely. Moreover, the difference of weight loss between different temperature decreases as the BNNSs content increases, as indicated by Supporting Information Figure S6. This means that the heterogeneous nucleation and lamellar hinder effect occupies the dominant role in deciding the degradation rate regardless of the polymorphic crystals. To summarize, BNNSs can control the degradation rate of PBA via regulation of the polymorphic crystals, its heterogeneous nucleation property, and lamellar hinder effect.

4. CONCLUSIONS Few-layer BNNSs with uniform thickness were prepared successfully via a facile chemical exfoliation method. The heterogeneous nucleation capacity of BNNSs was studied by isothermal and nonisothermal crystallization. Combining the H

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ASSOCIATED CONTENT

S Supporting Information *

DSC, WAXD, and POM graphs of P-0, P-0.1, P-0.3, P-0.5, and P-1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86-10-62784740. Fax: +86-10-62784550. E-mail: [email protected]. *E-mail: [email protected]. Tel.: +86-10-62784550. Fax: +86-10-62784550. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National High-Tech R&D Program of China (863 Program) (Grant No. 2011AA02A203), the National Natural Science Foundation of China (Grant No. 21274077, 21374054), and the Sino-German Center for Research Promotion.

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