Controllable synthesis of the Al₄B₂O₉ nanowhisker and their applications in reinforced polyhydroxyalkanoate composites

Abstract

Large quantities of aluminum borate (Al₄B₂O₉) nanowhiskers have been successfully synthesized by sol–gel and post-thermal-treatment methods. A series of experiments have been conducted to obtain an optimum condition for generating nanoscale precursor for the synthesis of Al₄B₂O₉ nanowhiskers with an average diameter of 20–40 nm and length of several hundred nanometers, respectively. With decreasing boron content, the crystallized temperature is reduced, and the structure of Al₄B₂O₉ phase is different in the Al₂B₃ and Al₂B system. In addition, the effects of Al₄B₂O₉ nanowhiskers on the mechanical properties of biodegradable polyhydroxyalkanoate (PHA) have been studied. It is worth noting that with adding the 0.6 wt% Al₄B₂O₉ nanowhiskers, the mechanical properties of PHA composites are improved, the elongation to break increased by 13.5%, the ultimate tensile increased by 106.8% and yield strength increased by 190% compared with the pure PHA.

---

1. Introduction

Reinforcing polymer by incorporating some whiskers with structural strength and chemical inertness provides the possibility to obtain composite materials with excellent mechanical properties.¹ Whisker reinforcement plays an important role and has been successfully used in the automobile and aerospace industries.² Scaling the whisker materials down to the nanometer range appears to be a characteristic trend in order to further enhance the mechanical proper ties of the nanowhisker-reinforced composites.^3,4

Due to aluminium borate whiskers Al₁₈B₄O₃₃ (abbreviated as Al₁₈) low thermal expansion coefficient, high strength, low density and creep resistance, which are similar to those of SiC whiskers.^5,6 Many attempts have been focused on the aluminium borate composite metal alloys, although uncertainties remain concerning the interfacial reaction in composite metal alloys and the corrosion resistance to environmental vapours.^7,8 The Al₁₈ reactions take place at a temperature over 1000 °C, and only the Al₄B₂O₉ (abbreviated as Al₄) phase totally disappeared and the Al₁₈ became the main production, until the temperature increased to 1150 °C.^9,10 The property of Al₄ phase is similar to Al₁₈ phase, although the Al₄ phase is unstable and will be decomposed at high temperature. The unstable temperature (1100 °C) of Al₄ phase is so high for the polymer composite that the unsteadiness will does not affect the property of polymer. Recently, the development of polymeric nanocomposites with high mechanical property and thermal stability has become a hot issue in related fields.^11–18

Polyhydroxyalkanoate (PHA) has prospective potential for biological engineering and protective packaging to replace conventional plastics due to its advanced properties such as complete biodegradability, good biocompatibility, environmental protection, and plastic thermal process ability.^19–22 Nevertheless, it is not fully competitive compared to conventional plastics because there are some drawbacks to limiting its further application. Polymer nanocomposite is possible an value way to enhance its mechanical properties and thermal stability, which is an important class of polymers that have applications in society.

In the present work, we synthesized large quantities of Al₄B₂O₉ nanowhiskers with typical 20–50 nm in diameter and length of several hundred nanometers in length, and the structures of Al₄B₂O₉ phase considerably depend on reactant composition. In addition, Al₄ nanowhisker not only increases the toughness but also improves the rigid of the PHA polymers. But for the PHA composites heated 50 °C for one week, only the rigid was enhanced, the toughness was not improved.

2. Experimental

2.1 Synthesis of Al₄B₂O₉ nanowhiskers

The starting materials, aluminum isopropoxide (ATSB), isopropanol (IPA), ethyl acetoacetate (EAA) and trimethyl borate ((CH₃O)₃B), were of analytical reagents grade and used directly without further purification. ATSB was mixed with IPA (molar ratio 1 : 10) and stirred for 10 h at 60 °C. Next, some EAA was slowly added and stirred for 6 h. Then (CH₃O)₃B was added into the organic solution and stirred for another 1 h. The three molar ratios of ATSB to (CH₃O)₃B were used: 2.0, 1.0 and 2/3 (denoted Al₂B, AlB, Al₂B₃ in their stoichiometric ratios). Finally, deionized water was carefully added into the ATSB–EAA–IPA–(CH₃O)₃B organic solution until a homogeneous gel was developed. After, the gel was dried to xerogel at 40 °C. The xerogel was heated at 1000 °C for 3 h in an alumina tube which was mounted in a tube furnace to obtain Al₄.

2.2 Synthesis of Al₄B₂O₉ nanowires reinforced PHA

The Al₄ nanowires were dispraised into 20 ml chloroform by stirred and ultrasound treatment. Then PHA was put into the mixed solution and the stirring was continued until a translucent and homogeneous colloidal solution was obtained at 50 °C. Finally, films were cast at ambient atmosphere for several hours. PHA nanocomposite films containing 0–0.8 wt% Al₄ nanowhiskers were produced.

2.3 Characterization methods

The structure and morphology of the samples were identified by X-ray powder diffraction (XRD, BRUKER D8 FOCUS), a field emission scanning electron microscopy (SEM, HITACHI S-4800) and a high resolution transmission electron microscopy (HRTEM, JEM-2010). The thermal decomposition behavior of the sample was detected by differential thermal analysis (TG-DTA, SDT22960, TA Instruments, USA), carried out in dynamic air from room temperature to 1200 °C at a heating rate 10 °C min⁻¹.

For the mechanical measurement, the as-prepared films were stretched on a Micro Bionix equipped with a 5 N load cell (MTS Systems Corporation) at a strain rate of 30 mm min⁻¹. Test Star IIV 4.0 (MTS Systems Corporation) was used as the data collection and equipment control software. Test samples were trimmed to 4 mm wide by 25 mm long and casting resulted in films that were approximately 0.1 mm thick. All the measurements were carried out at room temperature.

3. Results and discussion

3.1 Controllable synthesis of Al₄ nanowhisker

Thermal analyses for the starting xerogel were carried out under air atmosphere. A typical DTA-TG curve taken from boron-rich Al₂B₃ is shown in Fig. 1. The weight loss of ∼35% up to ∼300 °C was associated with two endothermal peaks, which relates to the pyrolyses of some boric ester and organic groups. A broad exothermal peak from 300 to 600 °C and a little weight loss were observed, which may be corresponding to the combustion of carbon. A constant weight was maintained from 600 °C to 1050 °C, but there was a shape exothermal peak at 750 °C, which is the crystallization of aluminium borate (Al₄) progresses. The exothermal peak at 750 °C is asymmetry, which may be caused by the crystallization of boron oxide, because there is redundant boron after crystallization of Al₄ progresses. A broad exothermal peak from 1000 °C to 1200 °C suggested the Al₄ phase is unstable and will be decomposed to Al₁₈ and boron oxide at high temperature. The boron oxide evaporation occurs just when the reaction temperature is higher than 1000 °C.

Fig. 1 TG and DTG profiles of the thermolysis of precursor.

Further examination of the structure modification by calcination temperature and reaction products can be evaluated by XRD. The samples were obtained by heating the various xerogels in a tube furnace at different temperatures for 2 h. The corresponding XRD patterns are shown in Fig. 2.

Fig. 2 The XRD pattern of the samples synthesized in different proportioning (Al₂B, AlB, Al₂B₃) at different temperature.

For the xerogel with composition Al₂B₃, the pattern of the sample calcined at 600 °C, 800 °C, 900 °C and 1000 °C, as shown in Fig. 2(a). An amorphous phase was observed below the crystallization temperature (750 °C), as the treatment temperature at 600 °C. The Al₄ phase can be recognized from the pattern of the sample calcined at 800 °C, and the crystallinity was improved with increasing the treatment temperature. The XRD result shows a highly crystallized phase at 1000 °C, diffraction peaks could be indexed as orthorhombic Al₄ structure with lattice constants of a = 14.746 nm, b = 15.268 nm, c = 5.557 nm. It matches well with those assigned to the Al₄ phase concerning both profile and intensity (JCPDS 29-0010). But, it also reveals other small peaks, which could be assigned to B₂O₃ impurities. The impurities were fully removed by washing with methanol at 60 °C for two times, only Al₄ phase could be identified. The XRD result indicates an effective Al₄ crystallization at 1000 °C.

With decreasing boron content, only needed a relatively low temperature for the samples to crystallize into the Al₄ structure. For the composition AlB, the XRD result shows a highly crystallized phase at 900 °C and it also observed the impurities B₂O₃. Although the Al₄ phase was still a major phase, Al₁₈ phase could be identified from the patterns when the reaction temperature was higher than 1000 °C.

For the reactant Al₂B system, the pattern of the sample calcined at 800 °C, 900 °C and 1000 °C, as shown in Fig. 2(c). The XRD pattern of the crystallized phase could be appropriately indexed in peak positions as an orthorhombic (Pbam 55) Al₄ structure with lattice parameter a = 0.7617 nm, b = 0.7617 nm, and c = 0.2827 nm. The pattern matches well with those assigned to the Al₄ phase concerning both profile and intensity (JCPDS 47-0319).²³ When the treatment temperature was set at 900 °C, an Al₁₈ phase began to appear but the starting Al₄ phase was still a major phase. The Al₄ phase totally disappeared until the temperature was increased to 1000 °C and the Al₁₈ became the main production. The crystallization of the Al₁₈ phase could be indexed as an orthorhombic structure with the lattice parameters as: a = 0.7687 nm, b = 1.5013 nm, c = 0.5664 nm, which are in good agreement with that reported previously.¹⁵

It is worth noting, the structures of orthorhombic Al₄ phase were different in the reactant Al₂B₃ and Al₂B system. Meanwhile, the existence of the Al₄ phase can be maintained at a higher temperature with increasing boron content, because the redundant boron can be a certain degree to impede the decomposition of Al₄ in the process of reaction. Additionally, the Al₄ phase is a low-temperature stable phase and will be decomposed at high temperature, so the Al₄ and Al₁₈ phases can co-existence in a certain temperature range. We previously reported the decomposition temperature of Al₄ phase should be within the range of 1000–1150 °C for Al₂B₃ system.⁹

Extensive SEM and TEM examinations were carried out to exhibit the morphological evolution depending on the reactant composition and the temperature. An SEM image of the product calcined from the Al₂B₃ source is shown in Fig. 3. Irregular particles were the commonly observed morphology at 600 °C as shown in Fig. 3(a). With increasing the temperature, the Al₄ phase was synthesized, the one dimension nanostructure gradually formed, and the length became longer. Fig. 3(b)–(d) clearly display some nanowhiskers accompanied by a great number of irregular residue which should be the excessive boron oxide. After the impurities were fully removed by washing with methanol at 60 °C for two times, only Al₄ nanowhiskers with smooth surface could be identified, as shown in Fig. 3(e) and (f). The Al₄ nanowhiskers display a uniform diameter distribution from 20 to 40 nm and the length over 500 nm. TEM observation confirms the diameter distribution of the nanowhisker structures, and to further understand the structure details. Fig. 3(g) shows the typical low-magnification TEM image of the Al₄ phase synthesized by the calcination of Al₂B₃ xerogel at 1000 °C, which exhibits a uniform diameter distribution from 20 to 40 nm. And, the tip morphology implies that the Al₄ nanowires grow within the framework of the self-assembly wire growth, or self-catalytic growth mechanism. A high-resolution TEM image for a single nanowhisker is shown in Fig. 3(h). In the AlB system, the statistical diameter for the Al₄ phase was still the same as that of the AlB system. The average diameter is several tens of nanometres (∼30 nm), which does not changed with increasing the temperature. But the length increased from ∼150 nm (for AlB at 800 °C) to several hundred of nanometres (∼600 nm for AlB at 900 °C) as shown in Fig. 4(a) and (b). However, the morphological was changed into little short rod with uneven length by calcining AlB at 1000 °C as shown in Fig. 4(c). It maybe because Al₁₈ phase began to appear at 1000 °C (the XRD has confirmed in Fig. 2(b)) and some B₂O₃ vapor was evaporated of from the Al₄ nanowhiskers which led to the disruption of nanowhiskers. Additionally, all the products contained a considerable amount of amorphous boron oxide were observed in the SEM. Fig. 5(a) shows no nanowhisker-like structure could be found from the reactant Al₂B calcined at 800 °C. When calcined at 900 °C, nanoparticles were observed as shown in Fig. 5(b). However, the products synthesized at 1000 °C tend to aggregate together and consequently increase their diameters (Fig. 5(c)). With the decomposing of Al₄ nanowhiskers, much B₂O₃ vapor was evaporated, which promote the nanowhiskers to aggregate together.

Fig. 3 SEM images of the products heat-treated Al₂B₃ at 600 °C (a), 800 °C (b), 900 °C (c) and 1000 °C (d); the products (heat-treated 1000 °C) were washed by methanol at 60 °C (e) and (f); low-magnification TEM images showing a uniform diameter distribution (g); high-resolution TEM image for a single nanowhisker (h).

Fig. 4 SEM images of the products heat-treated AlB at 800 °C (a), 900 °C (b) and 1000 °C (c).

Fig. 5 SEM images of the products heat-treated Al₂B at 800 °C (a), 900 °C (b) and 1000 °C (c).

The length to diameter ratio and synthesized temperature increases with increasing boron oxide content from Al₂B to AlB, exhibiting a considerably obvious change dependence on reactant composition. The excessive boron oxide addition does not only help to increase the nanowhisker length further, but also redundant boron oxide well coated on the surface of Al₄ nanowhisker. For the low boron, the gas of B₂O₃ is relatively low, which lead to Al₄ nanowhisker hard to grow up, in the process of reaction. Therefore, the optimal synthetic condition for the Al₄ phase with a fine aspect ratio should use the xerogel precursor with the composition AlB and the calcined temperature ∼900 °C.

3.2 Application of aluminum borate nanowhiskers in biodegradable PHA

In order to obtain PHA polymers with high mechanical strength, the as-prepared Al₄ nanowhisker were used as additive in PHA polymers to form composites. Then we studied the mechanical properties of the composites with various contents of Al₄. Fig. 6 illustrates representative stress–strain curves for the polymer samples. The elongation at failure is a measurement of toughness and reflects the total deformation that the polymer can withstand before fracture. The results shown in Fig. 6(a) indicate that the PHA films could achieve elongation to break statistics in 700%, ultimate tensile strength in 28 MPa and yield strength in 26.3 MPa when the composition of Al₄ nanowires was 0.2%. When the content of Al₄ nanowhisker was used as 0.6%, the elongation to break increased by 13.5%, the ultimate tensile increased by 106.8% and yield strength increased by 190% compared with the pure PHA. However, the excessive Al₄ nanowhisker addition increases the rigid but do not help to further improve the toughness. Therefore, we can conclude that the usage of Al₄ nanowhisker not only increases the toughness but also improves the rigid of the PHA polymers. Generally speaking, the toughness will decrease with increasing of the rigid, but both of the rigid and the toughness increased by the incorporation of Al₄ nanowhisker. However, stress–strain curves were completely changed as the polymer samples heated 50 °C for one week, the toughness absolutely disappear, which belong to friability break as shown in Fig. 6(b). When the content of Al₄ nanowhisker was used as 0.8%, the ultimate tensile strength is 26.5 MPa, which increased by 28.2%. For the heated PHA composites, the toughness was not improved, only the rigid was enhanced, with increasing the content of Al₄ nanowhisker, so the optimal reinforcement condition should use 0.6%.

Fig. 6 (a) The stress–strain curves for polymer samples tested in uniaxial extension with strain ratio (30 mm min⁻¹) at room temperature (20 °C); (b) the stress–strain curves of polymer samples heated 50 °C for one week tested in uniaxial extension.

The Al₄ nanowhisker strength is greater than the PHA, which has a large elastic modulus. The load and stress along the substrate can be passed to the nanowhisker, so it can bear more stress. In addition, when the crack extend to the interface of Al₄ nanowhisker and PHA, the crack is generally difficult to continue to expand through the Al₄ nanowhisker and according to the original direction. So it can disperse the crack tip stress concentration and change the crack, ultimately termination crack. Therefore, the mechanical properties of the biodegradable PHA composites can be improved by added Al₄ nanowhisker.

4. Conclusion

In summary, large quantities of Al₄B₂O₉ nanowhiskers with typical 20–50 nm in diameter and several hundred nanometers in length have been successfully synthesized by controlling the reaction conditions. With increasing the boron content from Al₂B to Al₂B₃, the length to diameter ratio and synthesized temperature was increased, and the structure of Al₄B₂O₉ phase is different, which exhibits a considerably obvious change dependence on reactant composition. But the excessive boron oxide addition does not help to increase the nanowhisker length further. In addition, we studied the mechanical properties of the biodegradable PHA composites with various contents of Al₄. The Al₄ nanowhisker not only increases the toughness but also improves the rigid of the PHA polymers. It is worth noting that the addition of 0.6 wt% Al₄B₂O₉ nanowhiskers, the mechanical properties is improved, the elongation to break increased by 13.5%, the ultimate tensile increased by 106.8% and yield strength increased by 190% compared with the pure PHA. But, when the PHA composites heated 50 °C for one week, only the rigid was enhanced, the toughness was not improved.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No.11504266; No. 51271192; No. 51322605) and National High Technology Research and Development Program 863, No. 2013AA014201 and the National key foundation for exploring scientific instrument of China, No. 2014YQ120351.

References

1. O. Gain, E. Espuche, E. Pollet, M. Alexandre and P. Dubois, J. Polym. Sci., Part B: Polym. Phys., 2005, 43(2), 205–214.
2. M. Y. Zheng, K. Wu, H. C. Liang, S. Kamado and Y. Kojima, Mater. Lett., 2002, 57, 558.
3. Y. N. Xia, P. D. Yang, Y. G. Sun, Y. Y. Wu, B. Mayers, B. Gates, Y. D. Yin, F. Kim and H. Q. Yan, Adv. Mater., 2003, 153, 53.
4. Z. Pan, Z. Dai and Z. Wang, Science, 2001, 2911947.
5. J. Pan, G. Sasaki, L. J. Yao, M. Yoshida and H. Fukunaga, Mater. Sci. Technol., 1999, 15, 1044.
6. M. Y. Zheng, K. Wu, H. C. Liang, S. Kamado and Y. Kojima, Mater. Lett., 2002, 57, 558.
7. G. Sasaki, M. Yoshida, O. Yanagisawa, N. Fuyama and T. Fujii, Mater. Sci. Forum, 2003, 777, 419.
8. M. Shibata and Y. Takemoto, J. Jpn. Inst. Met., 2000, 64, 1.
9. Z. J. Mo, J. Lin, Y. Fan, X. H. Zhang, Y. M. Xue, C. Y. Liang, H. S. Wang, X. W. Xu, L. Hu, Z. M. Lu and C. C. Tang, Adv. Mater. Res., 2012, 509, 125–131.
10. J. Zhang, Y. Huang, J. Lin, X. X. Ding, Z. X. Huang, S. R. Qi and C. C. Tang, J. Phys. Chem. B, 2005, 109, 13060.
11. W. Pan, X. W. He and Y. Chen, Int. J. Polym. Mater., 2011, 60, 223.
12. T. K. Das and S. Prusty, Polym.-Plast. Technol. Eng., 2013, 52, 319.
13. N. P. S. Chauhan, R. Ameta, P. B. Punjabi and S. C. Ameta, Int. J. Polym. Mater., 2012, 61, 1102.
14. A. S. Chandran and S. K. Narayanankutty, Polym.-Plast. Technol. Eng., 2011, 50, 443.
15. G. Rajasudha, A. P. Nancy, T. Paramasivam, N. Boukos, V. Narayanan and A. Stephen, Int. J. Polym. Mater., 2011, 60, 877.
16. Y. Z. Zhou, H. Ren, H. B. Lu and X. K. Meng, Polym.-Plast. Technol. Eng., 2011, 52, 581.
17. A. J. Ma, H. C. Li, W. X. Chen and Y. G. Hou, Polym.-Plast. Technol. Eng., 2013, 52, 295.
18. M. Ravi, Y. Pavani, S. Bhavani, A. K. Sharma and V. V. R. N. Rao, Int. J. Polym. Mater., 2012, 61, 309.
19. S. Kan, T. Mokari, E. Rothenberg and U. Banin, Nat. Mater., 2002, 2, 155.
20. W. J. M. Christopher and S. Friedrich, J. Biotechnol., 2007, 132, 296.
21. S. Yun and G. E. Gadd, Polym. Bull., 2008, 61, 267.
22. Y. Parulekar and A. Mohanty, Macromol. Mater. Eng., 2007, 292, 1218.
23. D. Mazza, M. Vallino and G. Busca, J. Am. Ceram. Soc., 1992, 75, 1929.

---