Effect of bamboo fiber length on mechanical property, crystallization

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Effect of bamboo fiber length on mechanical property, crystallization behavior and in vitro degradation of bamboo fiber/nano-hydroxyapatite/ poly(lactic-co-glycolic) composite Jiang Liuyun, Li Ye, Ma Bingli, Ding Haojie, Su Shengpei, and Xiong Chengdong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05354 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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Industrial & Engineering Chemistry Research

Effect of bamboo fiber length on mechanical property, crystallization behavior and in vitro degradation of bamboo fiber/nano-hydroxyapatite/ poly(lactic-co-glycolic) composite Jiang Liuyun1,2，3*, Li Ye 4, Ma Bingli1,2,3, Ding haojie1,2,3, Su Shengpei1,2,3 , Xiong Chengdong4 1 National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China 2 Key Laboratory of Sustainable Resources Processing and Advanced Materials, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China 3 Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Ministry of Education, China), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China 4 Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China

Abstract Nano-hydroxyapatite/poly-lactic-co-glycolic acid (n-HA/PLGA) composite reinforced by bamboo fiber (BF), and the effects of BF different lengths (≤1 mm, 4-5 mm and 9-10 mm) on the mechanical properties, crystallization behavior and in vitro degradation of the composite were investigated by electromechanical universal tester, SEM, DSC, POM and soaking in simulated body fluid (SBF) at 37 °C for 2, 4, 8 and 12 weeks, comparing with the n-HA/PLGA composite. The results showed that BF (4-5 mm) had the excellent comprehensive reinforce performance based on the best interface bonding and the best promotion crystallization effect. However, the in vitro degradation experiment revealed that the BF (≤1 mm) had the slightest tensile strength reduction percent, because the shortest size has difficult in escaping from

the

n-HA/PLGA

matrix.

The

present

work

would

provide

important guidance

significance for selecting appropriate bamboo fiber length to prepare an ideal PLGA-based composite used for bone materials. Keywords: Bamboo fibers; Hybrid composites; Mechanical properties; Thermal properties

1. INTRODUCTION Nano-hydroxyapatite/poly-lactic-co-glycolic acid (n-HA/PLGA) composite combined the advantages of PLGA and n-HA, which has been frequently investigated as bone materials, 1, 2 however, the mechanical property and degradation are still expected to be further improved, 1

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based on our previous study results.3-5 The design and preparation of fiber reinforced multi-component composite systems represent a viable strategy to develop innovative multifunctional biomaterials.6, 7 Natural plant fibers are renewable, and they have the advantages of abundant and cheaper over glass or carbon fibres.8,9 In order to pursue the biodegradable composites, so natural fibers are customarily used to reinforce polylactic acid (PLA), and they could achieve high mechanical properties.10-13 Among the various plant fibers, bamboo fiber (BF) is a natural degradable fiber, which is a good candidate to increase mechanical strength for polymer due to its excellent strength and flexibility.14-16 Accordingly, many researchers have reported considerable studies on different properties of polymer-based composites reinforced with BF, focusing on the mechanical properties, crystallizations kinetics and thermal resistance, and so on. For example, biodegradable poly (butylene succinate) (PBS) was blended with BF, whose tensile modulus and impact strength of the composites were enhanced with the increasing of bamboo fiber contents.17 Similarly, BF could greatly improve the impact property or tensile strength of PLA, and promote PLA crystallization.18, 19

Moreover, the effect of many parameters of BF on mechanical properties of composites

were investigated, including chemical treatment, composite preparation techniques, fiber length and fiber loading.20 For example, modification of BF could improve interface compatibility between polymer and BF, making it possible to improve mechanical strength of the composite.21 In addition, different scales of fiber fillers were reported to make different contributions to enhance the mechanical properties of PLA, for instance, mechanical properties of tamarind fruit fiber and glass fiber reinforced polyester composites was studied 2


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with different fiber lengths, such as 1, 2, and 3 cm, and it stated that the mechanical properties were optimally improved at 2 cm fiber length when compared with 1 and 3 cm fiber lengths.22 In our previous work, bamboo fiber was modified with the silane after alkali treated by us, which showed that it could lead to the best bending strength and modulus of the BF/n-HA/PLGA composite.23 However, the effect of BF length on the strength and crystallization behavior of the n-HA/PLGA composite has not been studied. In addition, as we know, degradation is very important for bio-absorbable bone materials. Our previous study showed the degradation rate of PLGA (95: 05 of LA: GA) was too slowly,24 and we studied the effect of nano-HA and whisker-HA on the degradation behavior of PLGA, and it was revealed that w-HA could maintain mechanical strength for a longer time than n-HA during the degradation owing to the bigger aspect ratio.25 Therefore, for the BF/n-HA/PLGA composite, the addition of BF would affect the degradation of n-HA/PLGA composite. Especially, whether the different lengths of BF would have different influences on degradation of the BF/n-HA/PLGA composite, which is necessary to be explored. Based on these, the purpose of the current paper is to prepare the n-HA/PLGA composite reinforced by different lengths of BF (≤1 mm, 4-5 mm and 9-10 mm, respectively), and the influence of BF different lengths on the mechanical properties, crystallization behavior and in vitro degradation of the composite were investigated and elucidated. The study is expected to provide guidance for selecting the bamboo fiber length to prepare an ideal PLGA-based composite used for bone materials.

2. EXPERIMENT SECTION 3



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2.1. Materials. Nano-hydroxyapatite (n-HA ) was prepared in our laboratory,26 whose size is 80-100 nm in length and 20-40 nm in width. PLGA (95:05) with intrinsic viscosity of 4.0-4.2 Pa/S was also prepared in our laboratory.27 Bamboo fiber (BF) was provided by Zhejiang A&F University, whose size is 6-10 cm in length and 0.03-0.2 mm in diameter treated by NaOH and KH550 in our previous mentioned reference. 23 Other agents were all of analytical grade. 2.2. Preparation of BF/n-HA/PLGA Composite. The BF/n-HA/PLGA composites with weight ratio of 5: 5: 90 were prepared by solution mixing method. Firstly, 0.5 g n-HA was highly dispersed in absolute ethanol and dichloromethane mixture solution (V/V= 1:1) at 800 W for 30 min by ultrasonic treatment (BILON-500DL, China). Then, the dispersed n-HA was slowly added into PLGA dichloromethane solution of 3 % (w/v) with the help of magnetic stirring and ultrasonic treatment for 4 h. Finally, the n-HA/PLGA solution were precipitated in absolute ethanol solution containing three different lengths of BF (≤1 mm, 4-5 mm and 9-10 mm, noted as S-BF, M-BF and L-BF, respectively), and scattered bamboo fibers were entangled in n-HA/PLGA composite, then washed three times with absolute ethanol and dried in a vacuum oven at 40 °C to remove the excess of solvent. Meanwhile, n-HA/PLGA composite with weight ratio 5: 95 was prepared according to the similar procedure for comparison. 2.3. Characterization for BF/n-HA/PLGA Composite. The mechanical properties of composites were evaluated using the Electromechanical universal testing machine (CMT6000, Sans, China). The samples were pressed to rectangular bars with the size of 4 mm × 6 mm × 60 mm and the small piece of 0.5 mm × 5 mm × 60 mm, annealed for 30 min 4


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at 110 ℃ in a vacuum oven, which was used to test the bending strength and tensile strength of the BF/n-HA/PLGA composites. The bending strength was measured at a crosshead speed of 20 mm/min in accordance with GB1042-79, and the tensile strength was measured with the crosshead speed of 2 mm/min in accordance with GB/T6569-1986, respectively. Five parallel specimen of each sample were tested, and the mean values were given. The microstructure the four composites were observed with scanning electron microscopy (SEM) (KYKY-2800 KYKY, China). The fracture surface was uniformly sputtered with a gold layer. The crystallization behaviors of n-HA/PLGA and three BF/n-HA/PLGA composites were performed with a differential scanning calorimetric (DSC) analyzer (Q20, TA Instruments-Waters, USA). The samples for DSC were equilibrium at 40 ℃, and heated to 190 ℃ at a rate of 10 ℃/min, holding at 190 ℃ for 5 min to eliminate previous thermal history, then cooled to 40 ℃ at a cooling rate of 10 ℃/min, finally heated to 190 ℃ at the same rate again. The isothermal crystallization samples were heated from room temperature to190 ℃with a rapidly heating rate of 200 ℃/min and kept at this temperature for 5min to eliminate previous thermal history, then cooled to predetermined crystallization temperature (Tc) (100, 110, 120 ℃) at cooling rate of 200 ℃/min, and maintained at Tc for necessary time till complete crystallization of the polymer matrix. The spherulitic morphology and growth process of the four composites were observed by polarized optical microscopy (POM) equipped with a hot stage (model XPN-203). The samples were heated to 200 ℃ from 25 ℃ till melt, kept for 10 min, and then cooled to predetermined crystallization temperature with a rate of 10 °C/min, and the spherulitic morphologies and growth process 5



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of samples were observed at 130 ℃ as an example. The in vitro degradation of n-HA/PLGA and three BF/n-HA/PLGA composites were carried out by soaking in simulated body fluid (SBF) at 37 ℃ for 2, 4, 8 and 12 weeks.28 The tensile strength reduction of the four composites were tested at interval time according to the previous procedure. The pH value of solution was measured with a pH meter. The water absorption was tested according to the following process. The specimens were removed from SBF at the prescribed time, and rinsed gently with distilled water three times, absorbed the water on the surface with filter paper, marked wet weight as W1, and dried completely in a vacuum oven at 40 ℃, marked dry weight as W2. The water absorption was calculated from the wet weight W1 and final dried weight W2 as following: Water absorption (%) =

%

The surfaces microstructure of the samples after being soaked for different time were observed with SEM, which was carried out according to the previous method.

3. RESULTS AND DISCUSSION 3.1. Mechanical performance analysis. Tensile and bending properties of n-HA/PLGA composite and three BF/n-HA/PLGA composites reinforced with different BF lengths are shown in Figure 1. As it can be seen, the mechanical properties of the BF/n-HA/PLGA composites were markedly improved comparing with the composite without BF, indicating that BF could improve the tensile and bending strength and modulus of n-HA/PLGA composite. However, different lengths of BF have various reinforce effects for n-HA/PLGA composite. For the composite with S-BF, whose bending strength and tensile strength was improved slightly than the composite without BF, while the composite with L-BF had the 6


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highest tensile but the lowest bending strength. However, it was observed that the composites with M-BF had superior tensile strength and modulus than the other two composite with S-BF or L-BF, for the composite with M-BF, whose tensile strength and modulus were also increased by 24.6 % and 48.9 %, and the same trend was repeated in bending strength and modulus, whose bending strength was increased from 131.4 MPa to 137.5 MPa, and the modulus from 1.59 GPa to 1.98 GPa, respectively. Obviously, fiber length had an important effect on mechanical properties for BF/n-HA/PLGA composite. Considering the tensile and bending properties comprehensively, the composite with M-BF showed the excellent mechanical properties, that is to say, the fiber length of 4-5 mm had the best reinforce effectiveness. The reasons should be ascribed from the microstructure and the promotion crystallization effect.29, 30 3.2. SEM observation. The microstructure of the fracture surface after the bending test are shown in Figure 2. As seen in Figure 2, BF was attached to n-HA/PLGA matrix, and there was no obvious interface peeling between bamboo fiber and matrix, which indicated good interfacial adhesion between the fiber and n-HA/PLGA matrix, and it is the one reason that mechanical properties of BF/n-HA/PLGA composites were higher than n-HA/PLGA composite. For the same BF content, the smaller BF size would have the larger contact area between BF and PLGA, so the tensile fractures of the composite with S-BF became rougher. The significant deformation of the composite materials indicated that the adding of BF increased the capability of energy absorption. For the composite with M-BF, the BF surface was stripped, or the PLGA matrix was stretched, indicating good interface bonding of the two phases, which is the main reason that the composite with M-BF displayed good 7



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mechanical properties. For the composite with L-BF, the length is too long to stretch fully in the matrix, so BF tangled together seriously. These images showed that excess long BF could not be dispersed well and stacked in the matrix and induced the poor interfacial adhesion. It was reported that the interfacial interaction affected the mechanical properties of the composite, 31-33 and it was confirmed by mechanical test results discussed in the foregoing section. 3.3. Crystallization of BF/n-HA/PLGA Composites. Thermal parameters and Avrami kinetic parameters of samples from the isothermal crystallization of the n-HA/PLGA and BF/n-HA/PLGA composites are shown in Table 1. Compared to the n-HA/PLGA, the glass transition temperature (Tg) and cold crystallization temperature (Tcc) of BF reinforced composites shifted to lower temperatures but shifted to higher temperatures for long fibers, which was ascribed to the induced chain segments motion and crystallization by the BF. The explanation was that BF promoted the melt-recrystallization, and HA promoted forming the small crystals, so the recrystallization model occurred during the annealing process, leading to higher crystallinity of PLGA.34 More importantly, fiber length had a great influence on the molecular chain movement and crystallinity. When the fiber length ≤1 mm, the smaller size of the reinforcement is, the lower temperature for glass transition and cold crystallization, but this made little sense for the composites with longer fiber (≥1 mm). |ΔHcc-ΔHm| is proportional to the degree of crystallinity, and the composite with M-BF had the highest value of crystallinity. In addition, according to the Avrami parameters in the Table 1, it can be seen that the value of K became larger and the Tmax got smaller with the addition of BF at the same TC. Moreover, the fiber length had important influence on K and Tmax, and the 8


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composite with M-BF had the maximum of K and the lowest of Tmax, while the longer fiber had more serious entanglement between the excess long fibers, which would further decrease the nucleation sites for crystallization.35 Therefore, M-BF displayed better promotion crystallization effect for PLGA than S-BF and L-BF did. 3.4. Polarized Optical Microscopy (POM) observation. The spherulite morphology and growth process of n-HA/PLGA and BF/n-HA/PLGA composites during the crystallization process were observed at 130 °C (shown in Figure 3). It is interesting to see that the transcrystal was formed on BF surface in the BF/n-HA/PLGA composites rather than in the n-HA/PLGA composite. Additionally, for BF/n-HA/PLGA composites, it took shorter time to form petty crystals and grow to full screen, comparing to n-HA/PLGA composite. The reason is that BF promoted crystallization of PLGA acted as a nucleation site instead of nucleation agent, and it also indicated that bamboo fiber and n-HA had a synergetic effect for PLGA crystallization.36 Moreover, for the three composites with different lengths of BF, the promotion crystallization effect obeyed the sequence of M-BF＞L-BF＞S-BF, which was consistent with the conclusion drew from the DSC analysis and mechanical test. In addition, it can be seen that the S-BF and M-BF had a better dispersion, while L-BF displayed serious entanglement, which was in accord with the SEM result, the entanglement of BF would decrease the nucleation site of crystallization, resulting in lower crystallinity and weaker mechanical strength. 3.5. In vitro degradation 3.5.1. Tensile strength change. To discuss the effect of fiber length on the degradation behavior of BF/n-HA/PLGA, the tensile properties of the BF/n-HA/PLGA composites with 9



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different lengths of BF during the soaking are shown in Figure 4, and n-HA/PLGA as a control. It can be seen that the BF/n-HA/PLGA composites had more prominent tensile strength decay than n-HA/PLGA composite, and the tensile strength of the three BF/n-HA/PLGA composite was less than half of the original value after 12 weeks degradation, which showed that BF promote the degradation of n-HA/PLGA composite. However, there was a little difference for the n-HA/PLGA composites reinforced with different lengths of BF. For the composite with S-BF, at 2 week, the tensile strength had a slight decrease, however, it had a great decrease from 8 week to 12 week. Instead, the composites with M-BF or L-BF had a great decay at the initial stage, then the decay became placid at the later degradation. In a word, the tensile strength decay of the composite with S-BF displayed a little smaller than the other two composites with M-BF or L-BF. The reason is that there was a more compact between the S-BF and n-HA/PLGA, so that it is difficult to escape from the n-HA/PLGA matrix.37 3.5.2 pH value change. Figure 5 shows the pH value of solution change during the soaking. As shown in the results, the pH decreased for four composites, due to the hydrolysis of PLGA. For n-HA/PLGA composite, it had a slight higher pH value than the three BF/n-HA/PLGA composites, which further showed that BF made n-HA/PLGA composite have faster degradation.38 Moreover, for the three BF/n-HA/PLGA composites, the different lengths of BF fiber exhibited subtle difference. For example, the composite with S-BF had higher pH value than the other two composites with M-BF or L-BF, indicating that the composite with S-BF had the slowest degradation, which was in accord with the result of the tensile strength reduce. 10


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3.5.3 Water absorption. Figure 6 shows the water absorption of the n-HA/PLGA and BF/n-HA/PLGA composites. It can be seen that the water absorption of the four composites increased with immersion time, moreover, they had similar trend, that is, at the initial degradation, the water absorption decreased a little, then, there was a great increase from 4 week to 8 week, showing that the materials had an obvious degradation at 8 week. Moreover, the BF/n-HA/PLGA composites had higher water absorption amount than n-HA/PLGA composite, and there was subtle difference among the three BF/n-HA/PLGA composite, and the composite with S-BF had a little lower water absorption amount than the other two composites with M-BF or L-BF, which further showed that the composite with S-BF had slower degradation. The reason was connected with the fact that it was difficult for the shortest fiber to detach from the n-HA/PLGA matrix, so as to there was the fewest cavities, which resulted in the relative lower water absorption amount. 3.5.4 Surfaces microstructure. To further make clear the process of n-HA/PLGA and BF/n-HA/PLGA composites degradation, the surface morphology variations of the four composites are shown in Figure 7. During the initial 4 weeks, the four composites kept the dense microstructure, after 8 weeks and 12 weeks degradation, the surface became microscopically porous or form macroscopic water pockets and voids, which showed the degradation rates of the composites were accelerated after 4 weeks soaking. Also, it can be seen that the n-HA/PLGA composite and the composite with S-BF had a few micropores, while the other two composites with M-BF or L-BF surfaces displayed several craterlets. Obviously, the craterlets would result in high water absorption amount, which was correspondent with the previous results, suggesting that the composite with S-BF had the 11



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slowest degradation among the three BF/n-HA/PLGA composites.

4. CONCLUSIONS Based on the above experimental results and analysis, it could be concluded that the BF with different lengths had different influences on mechanical property, crystallization behavior and in vitro degradation of BF/n-HA/PLGA composites, and M-BF with 4-5 mm length had the excellent comprehensive reinforce performance for n-HA/PLGA composite due to the best bonding interface and the best promotion crystallization effect for PLGA. In addition, the addition of BF could also accelerate the degradation of n-HA/PLGA composite, and S-BF had the least influence on the degradation of n-HA/PLGA composite based on the relative slightest tensile strength reduction percent, highest pH value, the relative lowest water absorption amount and fewest cavities, because it is difficult for S-BF to escape from the n-HA/PLGA matrix owing to the shortest length. The study would be of guidance to select the appropriate fiber length for n-HA/PLA or PLGA composites in manufacturing a novel biodegradable composite used as bone materials in the future.

■ AUTHOR INFORMATION Corresponding Authors

⃰ Tel./Fax: +86 0731 88873111: E-mail address: [email protected] ( L.-Y. Jiang). Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS The authors would like to acknowledge the support of Sichuan Provincial Youth Science and Technology Foundation (Grant No. 2014JQ0059), Natural Science Foundation of Hunan Province (Grant No. 2017JJ2179), the Innovation Platform Open Found of Hunan Provincial 12


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Education Department (Grant No.17K055), and Opening Found of Key Laboratory of C Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Ministry of Education of China), Hunan Normal University (No. KLCBTCMR201812).

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polyvinyl

alcohol,

nano-hydroxyapatite and cellulose nanocrystals. Cellulose 2014, 21, 3409-3426. (29) Aziz, S.H.; Ansell, M.P. The effect of alkalization and fibre alignment on the mechanical and thermal properties of kenaf and hemp bast fibre composites: part 1- polyester resin matrix. Compos. Sci. Technol. 2004, 64, 1219-1230.

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(30) Han, S.O.; Karevan, M.; Sim, I.N.; Bhuiyan, M.A.; Jang, Y.H.; Ghaffar, J.; Kalaitzidou, K. Understanding the reinforcing mechanisms in kenaf fiber/PLA and kenaffiber/PP composites: a comparative study. Int. J. Polym. Sci. 2012, 1–8. (31) Xie, X.L.; Zhou, Z.M.; Jiang, M.; Xu, X.L.; Wang, Z.Y.; Hui, D. Cellulosic fibers from rice straw and bamboo used as reinforcement of cement-based composites for remarkably improving mechanical properties. Compos Part B- Eng 2015, 78, 153-161. (32) Wang, Y.N.; Weng, Y.X.; Wang, L. Characterization of interfacial compatibility of polylactic acid and bamboo flour (PLA/BF) in biocomposites. Polym Test 2014, 36, 119-125. (33) Ramamoorthy, S.K.; Bakare, F.; Herrmann, R.; Skrifvars, M. Performance of biocomposites from surface modified regenerated cellulose fibers and lactic acid thermoset bioresin. Cellulose 2015, 22, 2507-2528. (34) Yu, F.M.; Liu, T.; Zhao, X.L.; Yu, X. J.; Lu, A.; Wang, J.H. Effects of talc on the mechanical and thermal properties of polylactide. J Polym Sci. Part B: Polym Phys 2012, 125, 99-109. (35) Dai, X.; Zhang, Z.S.; Wang, C.G.; Ding, Q.; Jiang, J.; Mai, K.C. Nucleation effect of montmorillonite with β-nucleating surface on polymorphous of melt-crystallized isotactic polypropylene nanocomposites. Compos Sci Technol 2013, 89, 38-43. (36) Wang, Y.; Tong, B.; Hou, S.; Li, M.; Shen, C. Transcrystallization behavior at the poly(lactic acid)/sisal fibre biocomposite interface. Compos Part A-App 2011, l 42, 66-74. (37) Li, H.Y.; Chang, J. pH-compensation effect of bioactive inorganic fillers on the 17



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degradation of PLGA. Compos Sci Technol 2005, 65, 2226-2232. (38) Lee, S.H.; Song, W.S. Enzymatic hydrolysis of polylactic acid fiber. Appl Biochem Biotechnol 2011, 164, 89-102.

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Figures and Tables captions

Figure 1 The strength and modulus of tensile and bending of BF/n-HA/PLGA composites with different lengths of BF. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF. Figure 2 SEM morphology of BF/n-HA/PLGA composites with different lengths of BF. (A, a) without BF, (B, b) with S-BF, (C, c) with M-BF, (D, d) with L-BF. Figure 3. The POM morphology of BF/n-HA/PLGA composites with different lengths of BF. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF. Figure 4. The tensile strength of BF/n-HA/PLGA composites with different lengths of BF during the in vitro degradation. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF. Figure 5. The pH value change of BF/n-HA/PLGA composites with different lengths of BF during in vitro degradation. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF. Figure 6. The water absorption of BF/n-HA/PLGA composites with different lengths of BF during in vitro degradation. (A) without BF, (B) with S-BF,

(C) with M-BF, (D)

with L-BF. Figure 7. SEM photographs of BF/n-HA/PLGA composites with different lengths of BF during in vitro degradation. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF. Table 1. Thermal Parameters and Avrami Kinetic Parameters of n-HA/PLGA composite and BF/n-HA/PLGA composites with different lengths of BF. 19



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Figure 1. The strength and modulus of tensile and bending of BF/n-HA/PLGA composites with different lengths of BF. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF.

Figure 2. SEM morphology of BF/n-HA/PLGA composites with different lengths of BF. (A, a) without BF, (B, b) with S-BF, (C, c) with M-BF, (D, d) with L-BF.

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Figure 3. The POM morphology of BF/n-HA/PLGA composites with different lengths of BF. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF.

Figure 4. The tensile strength of BF/n-HA/PLGA composites with different lengths of BF during the in vitro degradation. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF.

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Figure 5. The pH value change of BF/n-HA/PLGA composites with different lengths of BF during in vitro degradation. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF.

Figure 6. The water absorption of BF/n-HA/PLGA composites with different lengths of BF during in vitro degradation. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF.

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Figure 7. SEM photographs of BF/n-HA/PLGA composites with different length of BF during in vitro degradation. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF.

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Table 1. Thermal Parameters and Avrami Kinetic Parameters of n-HA/PLGA composite and BF/n-HA/PLGA composites with different lengths of BF. Sample

Tg (℃)

n-HA/PLGA

BF/n-HA/PLGA

60.24

59.61

(S-BF)

BF/n-HA/PLGA ) 59.99 (M-BF)

BF/n-HA/PLGA (L-BF)

60.41

ΔHcc ΔHm |ΔHcc-ΔHm| Tc (℃) (℃) (J/g) J/g J/g 134.3 5.698 7.466 1.768 100 110 120 133.1 9.854 11.47 1.616 100 110 120 132.7 8.790 10.69 1.900 100 110 120 134.2 5.184 6.846 1.662 100 110 120 Tcc

n 3.05 3.01 2.91 2.86 2.80 2.84 2.63 2.59 3.00 2.81 2.73 3.06

K

Tmax

(10-4min-1)

(min)

2.13 4.26 0.31 5.54 13.1 0.77 13.0 27.1 0.46 7.92 18.1 0.34

14.0 11.5 30.8 11.9 9.16 24.1 10.4 8.12 24.4 10.8 8.55 25.3

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Graphical Abstract A bioactive material of nano-hydroxyapatite/poly-lactic-co-glycolic acid (n-HA/PLGA) composite reinforced by an environmentally friendly natural bamboo fiber (BF) was prepared, and the effects of BF different lengths (≤1 mm, 4-5 mm and 9-10 mm, noted as S-BF, M-BF and L-BF, respectively)

on the mechanical performance, fractured surface morphology,

crystallization parameter, spherulite growing and in vitro degradation behavior, including the tensile strength change, pH value change, water absorption and surfaces microstructure were investigated. The main aim is to elucidate the effect of BF different lengths on the mechanical properties, crystallization behavior and in vitro degradation of the BF/n-HA/PLGA composite. The study is expected to provide guidance to select the suitable BF fiber length for n-HA/PLA composite in manufacturing bone material in future.

The strength and modulus of tensile and bending of BF/n-HA/PLGA composites with different lengths of BF. (A) without BF, (B) with S-BF, (C) with M-BF, (D) with L-BF.


Effect of bamboo fiber length on mechanical property, crystallization

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