Prominent Nucleating Effect of Finely Dispersed Hydroxyl-Functional

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Prominent Nucleating Effect of Finely Dispersed Hydroxyl-Functional Hexagonal Boron Nitride on Biodegradable Poly(butylene succinate) Yi-Ren Tang, Da-Wei Lin, Yang Gao, Jun Xu,* and Bao-Hua Guo* Advanced Materials Laboratory of Ministry of Education, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Hydroxyl-functional hexagonal boron nitride nanosheets (OH-BNNSs) were prepared successfully by ultrasonic treatment, and then homogeneous dispersions of OH-BNNSs in 1,4-butanediol (BDO) were obtained. Various techniques were applied to characterize the OH-BNNSs and their hydroxyl functional groupd. Furthermore, biodegradable poly(butylene succinate) (PBS)/OH-BNNS nanocomposites were prepared by in situ polymerization of succinate acid (SA) and BDO containing well-dispersed OH-BNNS. The nucleating effect of low OH-BNNS loadings on PBS in the PBS/OH-BNNS nanocomposites was investigated. It is found that OH-BNNSs significantly improve the melt-crystallization temperature and degree of crystallinity of PBS during the nonisothermal crystallization process without changing the crystal structure of PBS. Moreover, the Avrami exponent n calculated for isothermal crystallization increased from 2 to 3 after the addition of 0.05 wt % OH-BNNSs. The nucleation density of PBS spherulites in the nanocomposites increased dramatically. These results demonstrate that OH-BNNSs function as an outstanding nucleating agent of PBS. isolated as exfoliated boron nitride nanosheets (BNNSs).21 The similar lattice structure of BNNSs leads to extraordinary thermal conductivity and mechanical properties analogous to those of graphene, whereas the transparency, thermal stability, insulation, and prominent nucleation ability are all unique advantages of BNNSs in certain applications such as optical components, transparent composites, and nucleating agents.22 BNNS-containing nanocomposites could find potential applications as flexible optoelectronic, heat-conductive, and lubricating materials. However, the poor dispersion of h-BN and BNNSs in polymer matrixes restricts their application. To our knowledge, research on PBS/BNNS nanocomposites has not previously been reported. In this work, hydroxyl-functional h-BN nanosheets (OHBNNSs) that can be finely dispersed in 1,4-butanediol were obtained by Lin et al.’s method,23 and then PBS/OH-BNNS nanocomposites were prepared by in situ polycondensation. The hydroxyl groups and dispersity of the OH-BNNSs were carefully verified. Moreover, the effects of OH-BNNSs on the crystal structure, spherulitic morphology, and nucleation mechanism of PBS were clarified in detail.

1. INTRODUCTION Presently, significant research attention is being addressed toward biodegradable polymers, motivated by their potential applications in substituting petroleum-based materials and the urgency of protecting the environment to build a sustainable world. Among various biodegradable polymers, poly(butylene succinate) (PBS), chemically synthesized by the polycondensation of 1,4-butanediol and succinic acid, plays an important role in the biodegradable polymer field because of its biomass-based origin, appropriate degradation rate, thermal stability, mechanical properties, and good processability.1−3 However, the low crystallization rate of neat PBS restricts its end-use applications. Forming polymer nanocomposites is one effective way to improve the physical properties of the polymer matrix. A significant number of investigations have been carried out on PBS/filler nanocomposites system, using fillers such as organoclay,4−6 carbon nanotubes,7 attapulgite,8 silica,9 and graphene.10−12 Although most inorganic fillers can improve the mechanical properties of the matrix, some of them can act as nucleation obstacles. Since its first discovery in 2004 as a novel two-dimensional material, graphene has been under a spotlight owing to its exceptional thermal conductivity, high Young’s modulus, and high electrical conductivity.13−15 Thus, polymer/graphene nanocomposites continue to be a focus in research on polymer/nanocomposite systems.10−12,16−20 Unfortunately, there are still some problems with PBS/graphene nanocomposites:10−12 The crystallization ability of the nanocomposites is sometimes inhibited by graphene or graphene oxide (GO), the molecular weight of PBS decreases upon in situ polymerization, and applications in industry are restricted because of the black color of the nanocomposites even with low loadings of graphene or graphene oxide (GO). Hexagonal boron nitride (h-BN), which is isostructural with the hexagonal lattice of graphene, exists as a layered structure of alternating boron and nitrogen atoms in a hexagonal lattice, and it can be © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Hexagonal boron nitride (h-BN) was purchased from Aladdin Industrial Corporation. Hydroxyl-functional h-BN nanosheets (OHBNNSs) were obtained by ultrasonic treatment in deionized water.23 The as-obtained “milky” solution was centrifuged at 8000 rpm for 30 min, and the top two-thirds of the supernatant was collected. The freeze-drying method was used to obtain Received: Revised: Accepted: Published: 4689

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Figure 1. (a) AFM height image of individual few-layer OH-BNNSs and (inset) corresponding height profile (bar length = 500 nm). (b) HRTEM image of OH-BNNSs (bar length = 20 nm). (c) HRTEM image of sample P-0.3 (bar length = 200 nm). (d) HRTEM image of sample P-0.3 and (inset) enlarged image of the dotted area (bar lengths = 50 and 10 nm, respectively).

AFM samples were prepared by solution casting 50 μL of the centrifuged mixture on mica and dried in a vacuum oven for 8 h before AFM measurements. UV−vis spectra were recorded on a Pgeneral TU-1810 twinbeam spectrophotometer from 200 to 900 nm. FTIR spectra were obtained on a Nicolet-560 IR spectrometer by signal averaging over 32 scans at a resolution of 4 cm−1 in the wavenumber range of 4000−400 cm−1, using samples that had been thoroughly dried. X-ray photoelectron spectroscopy (XPS) studies were conducted with an ESCALAB 250 instrument using a monochromatic Al Kα source (1253.6 eV). Wide-angle X-ray diffraction (WAXD) analysis was carried out at room temperature using a Rigaku D/max2550HB +/PCX-ray diffractometer with Cu Kα radiation. Scanning was performed with 2θ from 14° to 30° at a rate of 4°/min with a step of 0.02°. The samples were annealed at 50 °C for 2 h after their thermal histories had been eliminated by being held at 160 °C about 3 min. The morphologies and radial growth rates of spherulites in specimens isothermally crystallized at different temperatures were observed under a Leica polarized optical microscope (DM2500P) equipped with a Linkam THMS600 hot stage. The isothermal and nonisothermal crystallization behavior of specimens were investigated by differential scanning calorimetry (DSC) using a Shimadzu DSC-60 instrument. Indium and

OH-BNNS powder. Various amounts of OH-BNNS powder were dispersed in butanediol (BDO) once again with vigorous agitation and ultrasonic treatment for 1 h at room temperature before polycondensation. Poly(butylene succinate) (PBS) and PBS/OH-BNNS nanocomposites were synthesized by the in situ polycondensation method, as reported previously in the literature.25 In our experiments, all samples were prepared under the same conditions, and the contents of BNNSs were 0.03, 0.05, 0.1, and 0.3 wt % with respect to BDO. For brevity, PBS composites with 0, 0.03, 0.05, 0.1, and 0.3 wt % OH-BNNS loadings are denoted as P-0, P-0.03, P-0.05, P-0.1, and P-0.3, respectively. In contrast, 0.3 wt % pristine h-BN was directly mixed in solution with P-0, giving a sample denoted as h-0.3. The viscosityaverage molecular weights of samples P-0, P-0.03, P-0.05, P-0.1, P-0.3, and h-0.3 were 9.4 × 104, 8.6 × 104, 11.7 × 104, 11.4 × 104, 8.1 × 104, and 9.4 × 104 g/mol, respectively. All composites were dried in a vacuum at 50 °C for 48 h before use. 2.2. Characterization. High-resolution transmission electron microscopy (HRTEM; JEOL, JEM-2010) and atomic force microscopy (AFM; Shimadzu SPM-9700) were employed to measure the thickness, perfection, and dimensions of the BNNS layers. The HRTEM samples were prepared by drying a droplet of the centrifuged mixture on a lacy carbon grid. The 4690

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zinc standards were used for calibration before measurements. A specimen of about 2−5 mg was held in an aluminum seal during each process at an appropriate heating or cooling rate under a nitrogen atmosphere. During the nonisothermal crystallization process, specimens were melted at 160 °C for 3 min to eliminate their thermal history, cooled to 30 °C at different cooling rates ranging from 20 to 2.5 °C/min, and then reheated to 160 °C. The endothermic and exothermal peak temperatures were taken as the melting point and crystallization temperature, respectively. While, for the isothermal crystallization process, the specimens were first melted, then quickly cooled to a preset isothermal temperature from the molten state, and maintained at this temperature until the crystallization process was complete. After that, the specimens were reheated to 160 °C.

3. RESULTS AND DISCUSSION 3.1. Preparation of Individually Dispersed OH-BNNSs in BDO. To determine the exfoliation degree and dimensions

Figure 4. FTIR spectra of h-BN and OH-BNNSs.

Figure 2. (a) Tyndall effects of nanosheet dispersions: (left) OHBNNSs in BDO, (right) pure water. (b) (Left) 0.3% h-BN direct sonication in BDO solution and (right) 0.3% OH-BNNS redispersion in BDO.

Figure 5. O 1s and N 1s peaks in the XPS spectra of h-BN and OHBNNSs.

Figure 3. UV−vis spectra of OH-BNNSs deionized water solution and BDO solution.

of the OH-BNNSs, AFM was used. The AFM height image in Figure 1a provides convincing evidence that individual nanosheets with lateral sizes of ≤500 nm and thicknesses of ≤2 nm could be prepared by sonication in deionized water, which is consistent with Lin et al.’s23 results. In addition to dimensional information, HRTEM images could also be used to determine the crystal perfection of the OH-BNNSs. Figure 1b

Figure 6. Variation of Tc with cooling rate for neat PBS and PBS/NAs.

shows that sonication did not destroy the hexagonal crystals of OH-BNNSs. An intact crystal structure is very important for the nucleation capability and mechanical properties of composites. To further investigate whether polycondensation 4691

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Figure 7. DSC curves of PBS with different amounts of OH-BNNSs during (a) cooling from the melt at a rate of 5 °C/min and (b) subsequent heating at a rate of 10 °C/min.

ethylene glycol, propylene glycol, and other liquid diol solution. For simplicity, the results are not shown here. Figure 3 shows that the UV−vis spectrum of the OH-BNNSs in water is very similar to that of OH-BNNSs in butanediol solution at the same concentration. These phenomena confirm that OHBNNSs can disperse as well in butanediol as in deionized water. The outstanding dispersity of OH-BNNSs in diols can be attributed to its hydrophilic property, which is mainly due to hydrogen bonding between hydroxyl groups. FTIR spectroscopy (Figure 4) was used to characterize the presence of hydroxyl groups. The O−H stretching signal at 3425 cm−1 and the symmetric B−O stretching (v1) band at 1036 cm−1 confirm the presence of B−OH groups in OH-BNNSs.26 The XPS spectra of OH-BNNSs and h-BN are shown in Figure 5. In the spectra, the OH-BNNSs shows a huge increase in the signal intensity of the O 1s peak compared with that for h-BN because of the hydroxyl groups covalently grafted onto the BNNSs.27,28 On the other hand, the O 1s peak of h-BN is centered at 532.4 ev, whereas for OH-BNNSs, the O 1s peak is located at 531.9 ev. The shift of the O 1s peak is consistent with covalently bound B−OH functional groups in the system.29 3.2. Nonisothermal Crystallization. The nucleation performance of OH-BNNSs was detected by DSC nonisothermal crystallization, and the crystallization temperature (Tc) values of PBS and PBS/nucleation agents (NAs) under different cooling rates are summarized in Figure 6. It can be seen that the Tc value of PBS is lower than that of PBS/NAs regardless of the cooling rate, which means that the crystallization ability of PBS has been improved by the NAs.

Table 1. Thermal Parameters of Neat PBS and OH-BNNSNucleated PBS sample

Tc (°C)

ΔHc (J/g)

Tm1 (°C)

Tm2 (°C)

ΔHm (J/g)

Xc‑DSC (%)

Xc‑XRD (%)

P-0 P-0.03 P-0.05 P-0.1 P-0.3 h-0.3

76.2 82.4 87.2 89.4 91.9 82.4

58.6 56.9 58.8 60.2 63.7 59.3

− 100.7 104.0 105.6 106.7 −

112.5 112.7 112.8 113.7 113.8 113.3

67.9 66.9 67.3 71.2 77.4 70.9

61.4 60.5 60.9 64.4 70.0 64.2

65.0 57.6 62.4 65.6 73.0 −

affected the dispersity of OH-BNNSs, HRTEM was used on an ultrathin section of sample P-0.3. In Figure 1c, one can see randomly distributed and well-dispersed OH-BNNSs in the PBS matrix (the arrows point to OH-BNNSs that are vertical with respect to the base plane). The large van der Waals forces among OH-BNNSs will cause restacking. However, the similarity in the numbers of layers in panels c and d of Figure 1 confirms that the OH-BNNSs are stable during the polycondensation process. According to a previous report, dispersion of a nucleating agent affects its efficiency, and the best effect is obtained by molecular-level dispersion.24 The Tyndall effect (Figure 2a) indicates the scattering effect of the nanosheets in solution. The suspended material can preserve a fine dispersion for a few additional days without any observable precipitation. In contrast, Figure 2b reveals that an h-BN/BDO dispersion prepared by direct sonication for 8 h mostly precipitated after a few minutes. Even more striking is that the OH-BNNSs enabled the production of homogeneous dispersions in

Figure 8. Plots of relative crystallinity versus crystallization time for (a) P-0 and (b) P-0.1 during isothermal melt crystallization. 4692

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Figure 9. Avrami plots of (a) P-0 and (b) P-0.1 during isothermal melt crystallization.

Table 2. Isothermal Crystallization Kinetics of PBS and PBS/OH-BNNSs from 96 to 102 °C crystallization temperature (°C) 96 98 100 102 96 98 100 102 96 98 100 102 96 98 100 102 96 98 100 102

n

Sample P-0 2.35 2.12 1.98 1.82 Sample P-0.03 2.61 2.46 2.23 2.01 Sample P-0.05 3.01 2.93 3.14 2.79 Sample P-0.1 3.20 3.24 3.56 3.16 Sample P-0.3 2.91 3.10 3.37 3.09

k (min−n)

t0.5 (min)

1.22 1.03 5.16 4.08

× × × ×

10−4 10−4 10−5 10−5

39.3 62.4 123 207.4

7.40 1.61 5.12 1.36

× × × ×

10−3 10−3 10−4 10−4

10.2 19.3 36.6 69.3

1.83 3.87 3.37 1.63

× × × ×

10−2 10−3 10−4 10−4

3.4 5.9 11.4 20.0

4.81 7.17 4.40 1.25

× × × ×

10−2 10−3 10−4 10−4

2.4 4.1 8.0 14.3

4.03 6.15 5.93 9.09

× × × ×

10−1 10−2 10−3 10−4

1.2 2.1 4.0 7.6

Figure 10. WAXD patterns of PBS and PBS/NAs isothermally crystallized at 50 °C.

apparent cold crystallization peak that represents a lower crystallization rate30 that disappeared after the addition of NAs. The two endothermic peaks can be explained by the mechanism of melting, recrystallization, and remelting of PBS crystals: The melting of the initial crystals causes the lower endothermic peak (Tm1), and the melting of recrystallized lamellar crystals causes the higher one (Tm2).31−33 With the added NAs, PBS could form more perfect crystals at a higher crystallization rate; thus, Tm1 shifted to higher temperatures. These two phenomena indicate that the crystallization ability of PBS could be markedly improved by the addition of a small content of OH-BNNSs. Thermal parameters including crystallization enthalpy during cooling (ΔHc), melting enthalpy during heating (ΔHm), and relative crystallinity (Xc) were obtained, as shown in Table 1. As reported by Miyata and Masuko,34 the theoretical melting heat of 100% crystallized PBS (ΔHm0) is 110.5 J/g, and Xc‑DSC can be calculated by the function Xc = [(ΔHm/ΔHm0)/φ] × 100%, where φ is the weight percentage of PBS in matrix. For comparison, Xc‑XRD was calculated from the WAXD data by comparing the area of the crystallization peak to the sum of the areas of the crystallization and amorphous peaks, where the peak areas were obtained using XPSPEAK software.35 The crystallinities of P-0.03 and P-0.05 were lower than that of neat PBS, and the crystallinity increased with increasing OH-BNNS content. Zhou et al. found similar results in the poly(butylene succiante)/TiO2 nanofiber system.36 Liang et al. reported that

The other trend is that Tc decreases with increasing cooling rate for the reason of insufficient time to crystallize. To further illustrate the nonisothermal crystallization behavior of neat PBS and PBS/NAs, we measured the curves of the melt-crystallization and reheating process of neat PBS and its nanocomposites at a cooling rate of 5 °C/min, as shown in Figure 7. With a trace amount of OH-BNNSs (0.03 wt %), the crystallization temperature (Tc) of PBS increased from 76.2 to 82.4 °C, which is identical to the value for the direct solution mixture of 0.3 wt % h-BN with PBS. Tc increased with increasing amount of OH-BNNSs and reached 91.9 °C at 0.3 wt % content, which confirms that the excellent dispersion and, therefore, the large nucleation surface of OH-BNNSs is the crucial factor for nucleation performance. Among various nucleating agents for PBS,4−12 OH-BNNSs exhibit the highest efficiency among inorganic materials, even approaching that of the nucleating agent poly(butylene fumarate) based on isomorphism.24,25 As shown in Figure 7b, PBS exhibits an 4693

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Figure 11. Spherulitic morphologies of (a) P-0 at 90 °C and (b) P-0.03, (c) P-0.1, and (d) P-0.3 at 100 °C. Bar length in each image = 100 μm.

log[−ln(1 − Xt)] versus log t. All linear correlation factors R2 were greater than 0.999, suggesting that the Avrami equation is suitable for the isothermal melt-crystallization process. For comparison, n, k, and the crystallization half-time (t0.5) are summarized in Table 2. The physical meaning of t0.5 is the time when the sample has achieved 50% relative crystallinity compared to the final crystallinity. t0.5 can be calculated by the equation

the crystallinity of graphene oxide/poly(vinyl alcohol) (PVA) decreased dramatically.37 In contrast, Jiang et al. found that the crystallinity of graphene/PVA increased.38 These results can be attributed to the combined effects of the heterogeneous nucleation role of OH-BNNSs and their lamellar structure, which restricts the regular arrangement of the molecular chains of PBS. 3.3. Isothermal Crystallization. Because of the high isothermal crystallization rate of PBS/NAs, the isothermal crystallization window was chosen from 96 to 102 °C, and the interval was 2 °C. Figure 8 shows plots of the relative degree of crystallinity versus the crystallization time for neat PBS and PBS/NAs at various values of Tc. The conversion curves exhibit the typical sigmoid shape. With increasing crystallization temperature, the crystallization time increased for the samples. This means that nucleation control rather than diffusion control occurred at the selected temperatures. In addition, the crystallization time decreased considerably after addition of 0.1 wt % NAs. For example, neat PBS required 300 min to complete crystallization, whereas sample P-0.1 required only 24 min, demonstrating that the OH-BNNSs have a prominent nucleation ability for PBS. To investigate the overall isothermal melt-crystallization kinetics of PBS, the well-known Avrami equation39−41 was employed 1 − X t = exp( −kt n)

t0.5 =

(3)

It is interesting to investigate the Avrami exponent n, which relates to the nucleation mode (homogeneous or heterogeneous) and growth dimension of the crystal for conjecture on the nucleation mechanism of OH-BNNSs. As the isothermal crystallization temperature increased, the value of n tended to decrease for samples P-0 and P-0.03, whereas the value for the other nanocomposites experienced minor changes around 3. Owing to the higher nucleation energy barrier at a lower degree of supercooling, fewer spherulites appeared because of the decreasing homogeneous primary nucleation, and the growth was mainly two-dimensional; thus, the n values of samples P-0 and P-0.03 decreased to around 2. However, the n value increased to 3 with increasing content of OH-BNNSs and remained almost unchanged at various crystallization temperatures. This indicates a typical three-dimensional growth and once again demonstrates the outstanding heterogeneous nucleation ability of OH-BNNSs. 3.4. Crystal Structure and Spherulite Morphology. In addition to the thermal properties, mechanical, electrical, and degradation properties are affected profoundly by the solidstate structure of polymer. Thus, it is very important to investigate the effect of the OH-BNNS content on the crystal structure of PBS. Figure 10 shows WAXD diffractograms of neat PBS and PBS/NAs with different contents of OH-BNNSs isothermally crystallized at 50 °C. Three main diffraction peaks

(1)

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

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

(2)

Figure 9 shows Avrami plots of neat PBS and P-0.1 composite. The Avrami parameters n and k were obtained from the slope and intercept of the fitting line for a plot of 4694

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are observed at about 19.6°, 22°, and 22.8°, corresponding to the (020), (021), and (110) planes, respectively, for neat PBS.24 Moreover, all of the PBS/NAs exhibit similar diffraction peaks at almost the same locations as for neat PBS. The deviations are due to the variance of the cell dimensions. In brief, the incorporation of NAs does not change the crystal structure of PBS. Generally, the heterogeneous nucleation rate enhances after the addition of nucleation agents, because a nucleation agent can provide extra surface that reduces the free energy barrier of the primary nucleation process and then makes the nucleation density increase and reduces the size of spherulites. As shown in Figure 11, for neat PBS at 90 °C, the spherulite diameter reached 400 μm before the spherulites impinged on each other. After addition of OH-BNNSs, the amount of nuclei increased tremendously, and very small spherulites with blurry boundaries appeared. This phenomenon confirms visually that the OH-BNNSs greatly improved the heterogeneous primary nucleating ability of PBS. Combined with the previous analysis, our results show that OH-BNNSs can be utilized as a highly efficient nucleating agent for PBS.

In this work, we have developed a facile strategy to fabricate well-dispersed OH-BNNSs in BDO and other diol solutions. An in situ polycondensation method was applied to fabricate PBS/OH-BNNS composites with well-dispersed OH-BNNSs. A low loading of OH-BNNSs exhibited a tremendous nucleation efficiency compared to other nanofillers. DSC results showed that OH-BNNSs can improve the Tc and Xc values of PBS during the nonisothermal crystallization process and the Tc value can reach as high as 91.9 °C when the OHBNNS content is 0.3 wt %. At specified temperatures, the crystallization time span and spherulite size of PBS decreased with increasing content of OH-BNNSs. WAXD results revealed that OH-BNNSs do not alter the crystal structure of PBS. The main reason for the enhanced nucleation ability of OH-BNNSs is most probably the tremendous surface area for chain adsorption, which decreases the energy barrier for crystallization nucleation. Thus, the good dispersity of OH-BNNSs in diol solution provides a new way to prepare highly efficient nucleating agents for polyester.

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

This work was financially supported by the National High-tech R&D Program of China (863 Program) (Grant 2011AA02A203), the National Natural Science Foundation of China (Grants 21274077, 21374054), and the Sino-German Center for Research Promotion. 4695

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dx.doi.org/10.1021/ie403915j | Ind. Eng. Chem. Res. 2014, 53, 4689−4696