Role of Poly(butylene fumarate) on Crystallization Behavior of Poly

Jul 3, 2013 - The details of poly(butylene fumarate) (PBF) as highly effective nucleating agent for poly(butylene succinate) (PBS) were systematically...
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Role of Poly(butylene fumarate) on Crystallization Behavior of Poly(butylene succinate) Hai-Mu Ye,#,‡ Yi-Ren Tang,# Jun Xu, and Bao-Hua Guo* Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: The details of poly(butylene fumarate) (PBF) as highly effective nucleating agent for poly(butylene succinate) (PBS) were systematically studied via X-ray diffraction, differential scanning calorimeter, polarized optical microscopy, and atom force microscopy. All results show that PBF can significantly improve the melt-crystallization temperature and the degree of crystallinity of PBS during the nonisothermal crystallization process. Both crystallization time span and spherulitic size of PBS decrease drastically with the addition of a small amount of PBF, which shows that PBF not only enhances the primary nucleation of PBS by epitaxial mechanism, but also greatly accelerates the secondary nucleation during spherulite growth. The secondary nucleation parameters of PBS, Kg and G0, are notably improved just with a small amount of PBF. Furthermore, the appearance of wrinkles on PBF-nucleated PBS ultrathin film visually suggests that PBF indeed affects the subsequent growth behavior, besides the primary nucleation.

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INTRODUCTION In recent decades, development of eco-friendly and renewable polymers to substitute petroleum-based materials in some areas has obtained increasing attention because of the demand of constructing a sustainable world. Owing to the remarkable biodegradability, biocompatibility, thermal stability, and mechanical properties, aliphatic polyesters synthesized by polycondensation from diacids and diols have become the focus of polymer industrial application and academic research.1 Among those semicrystalline polyesters, poly(butylene succinate) (PBS) and its copolyesters are particularly important for their biomass-based resource, appropriate degradation rate, relatively high melting temperatures, and good processability.2,3 However, practical application of PBS has been limited due to its lower crystallization rate, softness, and so on. Therefore, the crystallization behavior of PBS, including nucleation, crystal structure, and morphology, and crystallization mechanism, has received intense study.4−12 Nucleation is an important topic in polymer science since it affects the crystallization rate, crystalline structure, and the final properties of semicrystalline polymers. In some methods to improve nucleation rate, the nucleating agent, or nucleant, can create a large number of primary nuclei, resulting in smaller spherulites and improved optical and mechanical properties. Enhancing the crystallization rate of PBS is greatly beneficial for its industrial processing and application. So various nucleating agents for PBS have been studied and exploited, including functionalized organoclay,13 carbon nanotubes,14−16 and attapulgite17, as well as an inclusion complex,9 etc. Recently, our lab has developed a series of highly effective nucleating agents, poly(butylene fumarate) (PBF), and the random copolyesters of butylene succinate and butylene fumarate (PBSF), for PBS based on the unique characteristic of isomorphism and the epitaxial nucleation mechanism.18 PBF and PBSF provide new cheaper and easily dispersed nucleating agents for PBS, and the polymeric nucleating agent can also avoid some disadvantages of small molecular nucleating agent, that is, easy leaching. In the previous research,18 we mainly © 2013 American Chemical Society

focused on the verification of strict isomorphism formed between butylene succinate and butylene fumarate. The nucleating ability of PBF and PBSF on PBS was just briefly mentioned. In this research, we will present more information on the nucleation mechanism, and demonstrate that PBF can enhance not only the primary nucleating rate, but also the secondary nucleation process. Both aspects are helpful for accelerating the overall crystallization rate of PBS.

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EXPERIMENTAL SECTION

Materials and Sample Preparation. PBS and PBF were synthesized by a two-step reaction of esterification and polycondensation in the molten state, as reported in previous paper.18 Both products were purified by dissolution−precipitation before use. The number-average molecular weight (Mn) and polydisperse index (PDI) of PBS are 4.46 × 104 g/mol and 1.47, respectively; and those of PBF are 2.95 × 104 g/mol and 1.86, respectively. Chemical structures of PBS and PBF are illustrated in Scheme 1. PBS specimens with different amounts of PBF as nucleating agent were prepared by a cosolution film casting process, the additive weight percentages of PBF were selected as 0.1 wt %, 0.5 wt %, 1 wt %, 2 wt %, and 4 wt % in PBS matrix. Polymer dissolved in chloroform was first heated to 50 °C and maintained for 3 h to get a homogeneous solution, and then cast in a glass disk to evaporate the solvent. The resultant film was dried in vacuum at 50 °C for 2 days before use. X-ray Diffraction. Wide angle X-ray diffractograms (WAXD) of PBS and PBF samples were recorded at ambient conditions on a PGENERAL XD-3 instrument using graphitefiltered Cu Kα radiation. The supplied power was 1.4 kW. The Received: Revised: Accepted: Published: 10682

March 29, 2013 June 21, 2013 July 3, 2013 July 3, 2013 dx.doi.org/10.1021/ie4010018 | Ind. Eng. Chem. Res. 2013, 52, 10682−10689

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Scheme 1. Chemical Structures of PBS and PBF

Figure 1. The WAXD diffractograms of PBF isothermally crystallized at (a) 20 °C, (b) 80 °C, and (c) 100 °C, and PBS isothermally crystallized at (d) 20 °C, (e) 80 °C, and (f) 100 °C, respectively.

requirement for epitaxial nucleation is at least one-dimension lattice matching between polymer matrix and nucleating agent, and the usually acceptable range of lattice mismatch is less than 15%.19 Figure 1 shows the WAXD diffractograms of PBF and PBS isothermally crystallized at 20, 80, and 100 °C, respectively. The diffraction peak positions of PBF remain almost independent of crystallization temperature, while those of PBS shift more obviously and are related to the different chain packing densities in the crystalline state. The section area of ab plane of PBS is slightly larger than that of PBF.18 The lattice mismatch values between PBF and PBS from 20 to 100 °C are calculated, and both the mismatch values for a axis and b axis are less than 5%. This provides the basic condition for epitaxial nucleation at a large range of crystallization temperature. To make the nucleating behavior visual, PBF fiber was immerged into molten PBS film under 130 °C, then the sample was reheated to 135 °C and isothermally annealed for 10 min to eliminate, if any, disturbances during the introduction of the PBF fiber. After those procedures, the film was isothermally crystallized at 108 °C. Figure 2 displays that PBS uniformly crystallized from the PBF fiber surface; the ultrahigh nucleating density of PBS on the surface of PBF fiber further supports epitaxial nucleation mechanism. The width of the PBS crystal

scanning was carried out with 2θ from 14° to 30° at a rate of 4°/min. The scanning interval is 0.01°. Differential Scanning Calorimetry (DSC). The nonisothermal and isothermal crystallization behaviors of all specimens were measured using Shimadzu DSC-60 equipment. A specimen of about 3 mg was held in aluminum seal during each process at a heating or cooling rate of 10 °C/min under nitrogen atmosphere. Indium and zinc standards were used for the temperature and enthalpy calibration before measurement. During the nonisothermal crystallization process, the specimen was melted at 160 °C for 3 min to eliminate thermal history and cooled to 30 °C, followed then by a reheating process to 160 °C. During the isothermal crystallization process, the specimen was melted at 160 °C for 3 min, then quenched to a preset isothermal temperature and maintained until the crystallization process was completed. After that, the specimen was reheated to 160 °C again. Polarized Optical Microscopy (POM). The spherulite morphologies and radial growth rates of different specimens isothermally crystallized at various temperatures were observed by a Leica polarized optical microscope (DM2500P) equipped with a Linkam THMS600 hot stage. Atom Force Microscopy (AFM). Atom force microscopy (Shimadzu, 9500-J3) was utilized to characterize the ultrathin film morphology of different specimens under tapping mode. The specimens for observation were first prepared through spin-coating the 0.25% (w/v) chloroform solution on freshly cleaved mica at 4000 rpm for 2 min; then the specimens were kept in vacuum at 50 °C for overnight to remove the residual solvent. After that, the films were melted at 160 °C for 2 min following by isothermal crystallization at 104 °C for 36 h.

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RESULTS AND DISCUSSION Lattice Matching. The high nucleating effect for PBF in the PBS matrix was attributed to the epitaxial nucleation mechanism. The cell parameters of α for PBS are a = 5.232 Å, b = 9.057 Å, c = 10.90 Å, and β = 123.87°, and those of PBF are a = 5.012, b = 9.258, c = 10.93, and β = 120.90°.18 The essential

Figure 2. The POM images of PBS melt-crystallized from PBF fiber surface at 108 °C for (a) 0 min and (b) 12 min. The length of inserted bar in each image is 100 μm. 10683

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Figure 3. (a) DSC curves of PBS with different amounts of PBF during cooling from melt, and (b) the subsequent heating process at a rate of 10 °C/min.

Table 1. Thermal Parameters of Neat PBS and PBF-Nucleated PBS sample neat PBS PBS PBS PBS PBS

PBS + 0.1 wt % PBF + 0.5 wt % PBF + 1 wt % PBF + 2 wt % PBF + 4 wt % PBF

Tc (°C)

ΔHc (J/g)

74.4 88.8 91.4 92.5 92.9 94.4

68.7 70.2 73.3 74.2 75.7 76.9

Tm1 (°C)

Tm2 (°C)

ΔHm (J/g)

Xc‑DSC (%)

103.8 105.3 105.9 106.5 107.7

113.8 113.2 113.2 113.1 113.1 113.3

75.9 77.4 82.0 83.3 86.1 89.1

68.6 70.0 74.2 75.4 77.9 80.6

reached 6.5 μm when the isothermal crystallization time was 12 min. The average growing rate was obtained as 0.54 μm/min, which was the same as the radial growing rate of neat PBS spherulite at 108 °C. This indicates that PBS immediately started to grow from the surface of the PBF crystal once it was quenched to the isothermal temperature. Nonisothermal Crystallization. Evaluation of the PBF additive effect on the crystallization behavior of PBS matrix was carried out by DSC. Figure 3 shows the DSC curves of the melt-crystallization and subsequent heating processes of neat PBS and PBF-nucleated PBS. The crystallization temperature (Tc) increases with the increasing amount of PBF. The Tc value increases from 74.4 °C to 88.8 °C and 91.4 °C, respectively, when PBF contents are 0.l wt % and 0.5 wt %; then the increasing tendency becomes slow and finally almost levels off with more PBF, and the Tc value reaches 94.4 °C as the PBF content is 4 wt %. This reveals that an optimal nucleating effect for the PBS matrix could be achieved with only 0.5 wt % PBF, and confirms that PBF is the highest efficiency nucleating agent for PBS so far. The high efficiency of PBF should be due to not only the matching of the cell unit, but also the good dispersion of PBF in the PBS matrix. The crystallization signal of PBF was concealed due to its low content. The insert in Figure 3a shows that the PBF crystallized before PBS during the nonisothermal process and exhibited a Tc value at around 110 °C, which reveals that PBF crystals serve as the nucleating agents in the samples. In Figure 3b, only neat PBS shows a notable exothermal peak during the heating process at around 97 °C, indicated by an arrow. Similar phenomena are usually observed in polymers with a lower crystallization rate, for example, poly(L-lactide),20 poly(ethylene terephthalate),21 and the copolymers of PBS.22−24 Additionally, the melting behavior of PBS varies with crystallization temperature.25 Here, the exothermal peak should be connected with the overall performance of melt-recrystallization of crystal formed at lower Tc (see Figure 3a), and the double endothermal peaks

of PBF-nucleated PBS are assigned to the melt-recrystallizationremelting mechanism: the lower temperature peak (Tm1) is attributed to the melting of the initial crystal; the higher one (Tm2) corresponds to the melting of recrystallized crystals. Tc values of PBF-nucleated PBS are higher than that of PBS, leading to formation of a perfect initial crystal. So Tm1 of the specimens shifts toward higher temperature with increasing amount of PBF, and the thermal enthalpy around Tm2 decreases. The melting point of PBF in the PBS matrix was about 138 °C, which was quite difficult to be noted because of the low content of the PBF. The melting enthalpy was 31.2 J/g, much lower than that of neat PBF (77.3 J/g).18 Besides Tc and Tm, more thermal parameters including crystallization enthalpy during cooling (ΔHc), melting enthalpy during heating (ΔHm), and relative crystallinity (Xc) are obtained, as shown in Table 1. Xc is calculated by comparing ΔHm to the theoretical value of 100% crystallized crystal (ΔHm0), taken as ΔHm0 = 110.5 J/g;26 and Xc = ΔHm/ΔHm0/φ × 100%, φ is the weight percentage of PBS in the matrix. The increase of ΔHc and ΔHm of PBFS with the increasing addition of PBF reveals that PBF not only raises the Tc of PBS, but also enhances the relative crystallinity during the limited time of measurement. Xc risess from 68.6% for neat PBS to 80.6% with the presence of 4 wt % PBF. Both aspects, which are helpful in enhancing the crystallization ability, greatly benefit the industrial processing of PBS and its copolymers. Isothermal Crystallization. Figure 4 shows the DSC curves of different specimens isothermally crystallized at 100 °C. Obviously, at such a low supercooling degree, the crystallization rate of neat PBS is rather low. It takes longer than 120 min before finishing the crystallization process. When 0.1 wt % PBF is added, the crystallization time is sharply shortened to about 10 min; and further shortened with more PBF. The crystallization time span is 4 min for PBS with 2 wt % PBF, which is just a thirtieth of neat PBS. The relative crystallinity (Xt) at a given time (t) can be obtained from the 10684

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Table 2. Avrami Exponents of Neat PBS and PBF-Nucleated PBS from 100 to 103 °C samples neat PBS

PBS+0.1 wt % PBF

Figure 4. DSC curves of isothermal crystallization at 100 °C for (a) neat PBS, (b) PBS+0.1 wt % PBF, (c) PBS+0.5 wt % PBF, (d) PBS+1 wt % PBF, (e) PBS+2 wt % PBF.

PBS+0.5 wt % PBF

integrated area of the DSC curve from t = 0 to t divided by the integrated area of the whole exothermal curve. A horizontal line from a point after the crystallization exotherm was used as the baseline for integration, and then the typical sigmoid shape conversion curves of Xt versus t were attained. Figure 5 displays the Xt−t plotting of neat PBS and PBF-nucleated PBS crystallized at 100 °C.

PBS+1 wt % PBF

PBS+2 wt % PBF

The well-known Avrami equation is employed to analyze the overall isothermal melt-crystallization kinetics of PBS. The equation is given as27−29 (1)

where k is the overall rate constant composed of nucleation and growth part, n is the Avrami exponent related to nucleation mechanism and growth dimension of crystal. The logarithmic form of eq 1 is written as log[− ln(1 − X t )] = log k + n log t

n

100 101 102 103 100 101 102 103 100 101 102 103 100 101 102 103 100 101 102 103

2.78 2.34 2.15 1.98 2.89 2.83 2.96 2.89 3.15 2.99 3.17 3.09 2.95 3.12 3.21 3.25 2.86 2.88 2.94 3.05

k (min−n) 2.97 2.37 2.72 2.81 7.40 3.01 9.16 1.70 4.12 1.92 4.81 1.58 1.57 4.75 1.81 6.54 2.04 6.30 3.01 1.11

× × × × × × × × × × × × × × × × × × × ×

10−5 10−5 10−5 10−5 10−3 10−3 10−4 10−4 10−2 10−2 10−3 10−3 10−1 10−2 10−2 10−3 10−1 10−2 10−2 10−2

t0.5 (min) 37.3 75.2 112.1 165.3 4.8 6.8 9. 4 17.8 2.5 3.3 4.8 7.2 1.7 2.4 3.1 4.2 1.5 2.3 2.9 3.9

and the heterogeneous nucleation is independent of time for the instantaneous nucleation close to athermal nucleation.30 n values of PBS between 80 and 95 °C were reported to be around 3.0 (we also get the similar result, and data is not shown here for brevity), suggesting a three-dimensional growing mode with heterogeneous nucleation.31 An increase of crystallization temperature would strengthen the ability of heterogeneous primary nucleation, thus the decrease of n value of neat PBS in this study should be attributed to the transition from threedimensional to two-dimensional growth. This phenomenon was somewhat confusing, because the PBS crystal shows spherulitic growth. However, other literature data show similar experimental results,32 and the reason is still not clear at present. It is of particular interest that n values of PBFnucleated PBS vary within the range of 2.83−3.25. This indicates that the crystal growth mechanism of PBS might be different after PBF was added. The addition of PBF increases the growth dimensionality of the PBS matrix. t0.5 values of 2 wt % PBF nucleated PBS are approximately 4.0% and 2.4% of those of neat PBS at 100 and 103 °C, respectively. The equilibrium melting point (Tm0) of the samples can be determined by the Hoffman−Weeks equation.33,34 The equilibrium melting point Tm0 is obtained by extrapolation of the resulting straight line between the experimental Tm and the isothermal crystallization temperature Tc to intersect the line Tm = Tc. Thus, as shown in Figure 6, Tm0 values of 128.0, 130.7, 135.2, and 137.7 °C for samples of neat PBS, 0.1 wt % PBFnucleated PBS, 0.5 wt % PBF-nucleated PBS, and 1 wt % PBFnucleated PBS were obtained, respectively. All values of fitting variance (R2) are larger than 0.999. The change of Tm at the same Tc would lead to the change of Tm0. It also can be seen that the Tm0 value increases with the addition of PBF, suggesting that the crystalline phase in PBF-nucleated PBS is more perfect than that of neat PBS. Similar phenomena of nucleating agents affecting the Tm0 value of polymer matrix were also reported in the literature.35,36 For example, Ge et al. found that barite improved the Tm0 value of poly(ethylene

Figure 5. Development of relative crystallinity with crystallization time at 100 °C for (a) neat PBS, (b) PBS+0.1 wt % PBF, (c) PBS+0.5 wt % PBF, (d) PBS+1 wt % PBF, (e) PBS+2 wt % PBF.

1 − X t = exp( −kt n)

crystallization temperature (°C)

(2)

The crystallization parameters n and k are calculated from the slopes and intercepts of fitting lines, respectively, through plotting log[−ln(1 − Xt)] versus log t. For comparison, n, k, and the crystallization half-time (t0.5) are summarized in Table 2. The n value decreases from 2.78 at 100 °C to 1.98 at 103 °C for neat PBS. The Avrami index n is composed of the time dimension which is related to the nucleation mode (homogeneous or heterogeneous) and growth dimension of crystal. The homogeneous nucleation is dependent on time, with the number of nuclei continuously increasing with time, 10685

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is quite few at 80 and 104 °C, the spherulitic diameter can reach as large as 200 and 800 μm respectively before impinging with each other. As to the PBF-nucleated PBS, very small spherulites with blurry boundaries are observed when crystallized at 104 °C, and the amount of nuclei increases and the size of spherulite decreases dramatically with increasing PBF content, which displays directly that PBF has greatly enhanced the primary nucleating ability of PBS. POM results further prove that PBF can be utilized as a highly efficient nucleating agent for PBS, which is in accord with the aforementioned DSC results. Figure 8 shows the spherulitic radial growth rate (G) of neat PBS and PBF-nucleated PBS at different temperatures. Because Figure 6. Hoffman−Weeks plots of apparent melting temperature versus crystallization temperature. The insert is the enlarged image of measurement range.

terephthalate) (PET) in PET/barite nanocomposites,35 while Zhou et al. found that silane functionalized multiwalled carbon nanotubes (SFMWCNT) depressed the T m 0 value of polypropylene in their composites.36 All these results suggest that the nucleation activity of nucleating agents on the matrix could change the Tm0 value sometimes, which might be due to the change of crystal perfection. Spherulitic Morphology and Growth. Usually, a nucleating agent provides a surface that reduces the free energy barrier of the primary nucleation process, which induces the increase of the nucleation density and reduction of the size of spherulites. Figure 7 shows the POM micrographs of neat PBS and PBF-nucleated PBS isothermally crystallized at the specified temperatures. Since the nuclei amount for neat PBS

Figure 8. Spherulitic radial growth rate of PBS and PBF-nucleated PBS at high temperatures.

of the high nuclei density with high PBF content, only the G value of specimens with rather low PBF content at a narrow temperature range could be measured. The G value increases as the amount of PBF increases. The increase of G could be attributed to the increase of secondary nucleation behavior, because the diffusion of PBS chains should not be affected by so little PBF (for example 0.5%). In our lately published paper,18 PBF and PBSF showed much higher spherulite growth rate than PBS, which is due to the decrease of the secondary nucleation barrier with an increase of butylene fumarate content. DSC data showed that the melting enthalpy of additive PBF in the PBS matrix was much lower than that of neat PBF. Thus, it is speculated that part of additive PBF crystallized before PBS to serve as nucleating reagent, and another part of PBF assisted to raise the radial growth rate of PBS spherulites through reduction of the secondary nucleation barrier. The G values at different temperatures are analyzed by the secondary nucleation theory, which is expressed by ⎡ ⎤ ⎡ Kg ⎤ U* G = G0 exp⎢ − ⎥ exp⎢ − ⎥ ⎣ R(Tc − T∞) ⎦ ⎣ Tc(ΔT )f ⎦

(4)

where G0 is a rate constant, U* is the activation energy for transporting the polymer chains to the crystallization site, R is the gas constant, T∞ is the temperature below which the polymer chain movement ceases, ΔT is the supercooling degree given as (Tm0 − Tc), f is a factor accounting for the variation in the fusion enthalpy Δhf, given as 2Tc/(Tm0 + Tc), and Kg is nucleation constant. Before the kinetic analysis, the values of U* and T∞ have to be determined to start fitting the experimental data. Generally, two sets of values are utilized. One is the universal empirical values of U* = 1500 cal/mol, and T∞ = Tg − 30 K; the other is Williams−Landel−Ferry (WLF)

Figure 7. Spherulitic morphology of PBS crystallized at (a) 80 °C and (b) 104 °C; (c) PBS+0.1 wt % PBF, (d) PBS+0.5 wt % PBF, (e) PBS +1 wt % PBF, and (f) PBS+2 wt % PBF at 104 °C. The length of inserted bar in each image is 100 μm. 10686

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values of U* = 4200 cal/mol, and T∞ = Tg − 51.6 K. The Tg of PBS was measured as −42.0 °C by DSC. Figure 8 shows the plots of lnG + U*/R(Tc − T∞) as a function of 105/T(ΔT)f for neat PBS and PBF-nucleated PBS by using the empirical values. It was found that the data of each PBF-nucleated PBS specimen were fitted well by two straight lines with different slopes, R2 > 0.999, (Figure 9). The calculated results by using the secondary

Figure 9. Plots of ln G + U*/R(Tc − T∞) as a function of 105/T(ΔT)f for neat PBS and PBF-nucleated PBS by using the empirical values of U* = 1500 cal/mol and T∞= Tg − 30 K.

Figure 10. AFM images of neat PBS and PBF-nucleated PBS isothermally crystallized at 104 °C: (a) neat PBS, (b) PBS+0.1 wt % PBF, (c) PBS+0.5 wt % PBF, and (d) PBS+4 wt % PBF.

nucleation theory are summarized in Table 3. The slope ratio of the two straight lines is about 1.5, and these results indicate that there are two regimes. Regime II and regime I exist in such a higher crystallization temperatures range, with the regime transition temperature at 108 °C. Therefore, the isothermal crystallization behavior of PBF-nucleated PBS measured between 100 and 103 °C occurred in regime II. Gan et al. reported that PBS has a transition temperature from regime III to regime II at about 96 °C,37 and the crystallization of neat PBS (Mn = 7.7 × 104, PDI = 2.0) above 100 °C is located in regime II. After adding PBF as a nucleating agent, the crystallization regime does not change, but the Kg and G0 values with the content of PBF at 0.1 wt % and 0.5 wt % significantly increase compared with that of neat PBS, respectively. These directly suggest that PBF has greatly accelerated the secondary nucleation ability and enhanced the crystallization rate, and the driving force may originate from the enlarged supercooling by the addition of PBF, exhibited as the rise of Tm0. Ultrathin Film Morphology. In this part, we will try to present more visual supporting information about the effect of PBF on the PBS matrix by AFM. It is quite difficult to get further results based on complex spherulitic structure observed by AFM compared with POM, but the ultrathin film method provides a convenient way to study the lamellar structure. Figure 10 shows the morphologies of ultrathin films of neat PBS and PBF-nucleated PBS isothermally crystallized at 104 °C. The neat PBS film forms a one-layer flat-on lamellar structure with a thickness of about 7 nm; but for the PBFnucleated PBS film, there are many wrinkly structures

(indicated by arrows) on the flat-on lamellae with the same thickness of about 7 nm, and the amount of wrinkly structure increases drastically with the increase of PBF content, which demonstrates there exists a close relationship between the wrinkly structure and PBF. The profile of wrinkle is carefully measured at many different positions, and the highest height ranges from about 9 to 16 nm. The appearance of wrinkles visually reveals that PBF indeed affects the crystallization behavior of PBS at the lamellar structure level, and the uniform distribution of the particular structure also illustrates at some level that PBF is well dispersed in the PBS matrix. Further reasons for the formation of a wrinkle structure are under investigation.

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CONCLUSION In this research, the nucleation mechanism of PBF on a PBS matrix are systematically studied. DSC results show that PBF can improve Tc and Xc of PBS during the crystallization process with a cooling rate of 10 °C/min, and the Tc value can reach as high as 94.4 °C when the PBF content is 4 wt %. Both crystallization time span and spherulitic size of PBS decrease drastically with a small amount of PBF, because PBF cannot only enhance the primary nucleation of PBS by an epitaxy mechanism, but also greatly accelerate the secondary nucleation process, which is exhibited as the significant improvement of the Kg and G0 with a small amount of PBF. Further, the appearance of wrinkles in the PBF-nucleated PBS ultrathin film

Table 3. Crystallization Parameters for Neat PBS and PBF-Nucleated PBS universal empirical

Williams−Landel−Ferry

specimens

KgII

G0II

KgI

G0I

KgII

G0II

KgI

G0I

PBS PBS+0.1 wt % PBF PBS+0.5 wt % PBF

0.64 × 105 1.09 × 105 1.44 × 105

0.2 14.7 98.1

1.73 × 105 2.02 × 105

1.7 × 103 2.5 × 103

0.70 × 105 1.16 × 105 1.54 × 105

2.5 × 102 1.9 × 104 6.0 × 104

1.76 × 105 2.09 × 105

1.7 × 107 2.9 × 107

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confirms PBF indeed visually affects the subsequent growing behavior, besides the primary nucleation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-62784550. Fax: +86-10-62784550. Present Address ‡

Department of Materials Science and Engineering, China University of Petroleum, Beijing, 102249, China. Author Contributions #

H.-M.Y. and Y.-R.T. contributed equally to the paper.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was 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, 20974060), and the Science Foundation of China University of Petroleum (Beijing) (No. YJRC-2013-14).

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