Article pubs.acs.org/IECR
Effect of Different Surface Treatment for Bamboo Fiber on the Crystallization Behavior and Mechanical Property of Bamboo Fiber/ Nanohydroxyapatite/Poly(lactic-co-glycolic) Composite Ye Li,†,§ Liuyun Jiang,*,†,‡ Chengdong Xiong,† and Wanjia Peng† †
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China Key Laboratory of Sustainable Resources Processing and Advanced Materials, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China § University of the Chinese Academy of Sciences, Beijing 100039, PR China ‡
ABSTRACT: Bamboo fiber was treated by two different surface treatment methods of alkali treatment, silane modification after alkali treatment, and SEM, FTIR, XRD, and XPS were used to analyze bamboo fiber before and after treatment. The results showed that the surface morphology, crystallization degree, and chemical elements of bamboo fiber were changed after being treated with different surface treatment methods. Then, the treated bamboo fibers were designed to separately incorporate into the nanohydroxyapatite/PLGA (n-HA/PLGA) composite, and the crystallization behavior and mechanical properties of the bamboo fiber/n-HA/PLGA ternary composites were evaluated by DSC, POM, XRD, SEM, and the electromechanical universal tester. The results indicated that surface-treated bamboo fiber has a greater potential in promoting the crystallization of PLGA, and the interfacial adhesion and compatibility were improved, so that the tensile strength and elongation of the composite with alkali treated bamboo fiber were increased by 47.6% and 64.3%, respectively, and the bending strength and flexural modulus of the composite with silane modification after alkali treatment BF were also enhanced nearly 10 and 400 MPa, respectively, compared to the untreated bamboo fiber. All the above results suggested surface-treated bamboo fiber might develop a novel biodegradable bamboo fiber/n-HA/PLGA ternary composite used as bone materials in the future.
1. INTRODUCTION To avoid the possible risks of retained metallic implants for bone fracture internal fixation, such as corrosion and stressprotection weakening of bone,1 polylacticacid (PLA), polyglycolic acid (PGA), and their copolymer (PLGA) have led to increasing interest in biodegradable materials, especially PLGA, which has some important advantages, including excellent biocompatibility, tunable chemical composition, good biological reorganization, and adjustable biodegradation.2,3 However, conventional single-component polymer materials cannot satisfy the requirement of bone fracture internal fixation materials, such as relatively poor mechanical strength and the lack of osteogenic activity. Therefore, the design and preparation of PLGA-based multicomponent systems can provide a viable strategy to develop innovative multifunctional biomaterials. Nanohydroxyapatite (n-HA) is an ideal candidate for orthopedic and dental implants because of its excellent biocompatibility and bone integration ability. 4 So the introduction of n-HA into PLGA polymer will be a promising approach, which would endow PLGA with osteoconductive and form a strong bond to natural bone in vivo.5−7 However, the mechanical strength of n-HA/PLGA composite still needs to be improved, which was concluded from our previous publications.8,9 Bamboo fiber (BF) has good mechanical properties comparable to the high-performance fibers, such as glass fibers and carbon fibers, which can be used to reinforce polymer matrix as novel alternative inorganic material, such as epoxy resin (EP), poly(butylene succinate) (PBS), polypropylene © XXXX American Chemical Society
(PP), and poly(lactic acid) (PLA), and so on. So the utilization of bamboo fibers as reinforcement in polymers has been attracting much attention, and it has increased tremendously and has undergone a high-tech revolution in recent years as a response to the increasing demand for developing biodegradable, sustainable, and recyclable materials.10−13 Moreover, compared to synthetic fibers, bamboo fibers also have the advantage of low cost, low density, recyclability, biodegradability, antibacterial, anti-UV, antistatic, and no skin irritations.14,15 Therefore, it is promising to choose bamboo fiber as reinforcement for n-HA/PLGA composite, which might achieve a novel biodegradable PLGA-based composite with higher mechanical strength. However, bamboo fiber, a kind of an uneven anisotropic material, is composed of cellulose, lignin, hemicellulose, and various extractives, in which an abundant hydroxyl group exists, so the surface of bamboo fiber presents strong hydrophilicity,16,17 while the PLGA polymer usually displays hydrophobicity. Inevitably, microscopic phase separation interface occurred when materials were mixed, thus, the poor interfacial adhesion between the bamboo fiber and PLGA would lead to an inefficient stress transfer under load, resulting in low mechanical strength. In our previous study, we concluded that the adding of BF could increase the capability Received: July 24, 2015 Revised: October 16, 2015 Accepted: November 4, 2015
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DOI: 10.1021/acs.iecr.5b02724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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stirred for 30 min to ensure its complete hydrolysis. Then, alkali-treated bamboo fiber was added to the above mixed solution, heated to 70 °C, and kept for 1 h. Finally, modified bamboo fiber was washed with distilled water until neutral and dried as above. The above method was called silane modification after alkali treatment. 2.3. Preparation of Bamboo Fiber/n-HA/PLGA Composite. Ternary composites consisting of PLGA, n-HA, and bamboo fiber were prepared by solution mixing method. The weight ratio of PLGA:n-HA:bamboo fiber was 8.5:0.5:1. First, precalculated dispersed n-HA was added dropwise into the PLGA dichloromethane solution of 3% (w/v) with the help of magnetic stirring and ultrasonic treatment simultaneously. Next, a certain amount of bamboo fiber was dispersed in enough absolute ethanol by stirring, and the mixture solution containing PLGA and n-HA was precipitated in the excess absolute ethanol. Then, the precipitate was washed three times with absolute ethanol and dried in a vacuum oven at 40 °C to remove the excess solvent. 2.4. Characterization for Bamboo Fibers. The two surface treated and untreated bamboo fibers were identified using Thermo Niclet 670 spectrometer, and the spectrum was analyzed between 400 and 4000 cm−1. The morphologies and structure of three kinds of bamboo fibers were observed by scanning electron microscope SEM (KYKY-2800 KYKY, China). Element analysis was performed by XPS using a Kratos Axis Ultra system with monochromatized Al Kα radiation to illustrate the effect of different fiber surface treatment. The crystallization of BFs was investigated by X-ray diffraction (XRD; X’ pert Pro MPD). The scanning range and rate was 5−50° and 2°/min, respectively. 2.5. Characterization for Bamboo Fiber/n-HA/PLGA Composites. The mechanical properties of the bamboo fiber/ n-HA/PLGA composites prepared by different kinds of bamboo fiber and were measured by using electromechanical universal testing machine (CMT6000, Sans, China) in accordance with GB1042-79. Three samples of each specimen were tested. The morphology and structure of fractured specimens obtained after a bending experiment was studied by SEM. The crystallization behavior morphologies of PLGA and the three bamboo fiber/n-HA/PLGA ternary composites was researched by X-ray diffraction (XRD; X’ pert Pro MPD), differential scanning calorimetric (DSC) analyzer (Q20, TA Instruments-Waters), and polarized optical microscopy (POM) equipped with a hot stage (model XPN-203). The XRD scanning range and rate was 5−60° and 2°/min, respectively. The samples for DSC were equilibrium at 40 °C then heated to 190 °C at a rate of 10 °C/min, holding at 190 °C for 5 min to eliminate previous thermal history then cooled to 40 °C at a cooling rate of 10 °C/min, and finally heated to 190 °C at the same rate again. The isothermal crystallization behavior samples were equilibrium at 190 °C and kept at 190 °C for 5 min to eliminate previous thermal history then cooled to a predetermined crystallization temperature (Tc) (100, 110, 120 °C) at a cooling rate of 200 °C/min, and maintained at Tc for enough time to ensure complete crystallization of the polymer matrix. The crystal morphology samples were melted at 190 °C for 5 min and then cooled to a predetermined crystallization temperature at a rate of 30 °C/min. The spherulitic morphologies and growth process of samples were observed at 130 °C.
of energy absorption so as to increase the mechanical properties of n-HA/PLGA composite. However, when the loading of untreated bamboo fiber exceeded 5 wt %, obvious aggregation of bamboo fiber and poor interface compatibility between bamboo fiber and PLGA was found, which was a disadvantage for stress transfer and caused poor mechanical properties of composites. It has been suggested that surface treatment for bamboo fiber plays an important role in improving the interface compatibility of bamboo fiber and polymer, such as associating polar groups on to the polymer backbone, different kinds of cellulose fiber treatment including mercerization, acetylation, etherification, silanization, benzoylation, and the introduction compatibilizing agent, and so on. Among the above-mentioned methods, mercerization and silanization of bamboo fiber are the most simple and effect means to improve interface compatibility in bamboo fiber/PLA composite, which showed that lowconcentration alkali solution treatment could increase the amount of cellulose exposed on the fibrous surface, thus increasing the number of possible reaction sites. It also showed that 3-aminopropyltriethoxysilane improved the tensile strength of PLA/bamboo fiber composites as best it could.18−22 However, for the surface-treated bamboo fiber, there is no report on the crystallization, interfacial adhesion, and compatibility between bamboo fiber and PLGA, not to mention bamboo fiber/n-HA/PLGA composite with surface-treated bamboo fiber. In the current study, two surface treatment methods of alkali treatment or pre-sailane modification after alkali treatment for bamboo fiber were carried out, and the surface-treated bamboo fiber was introduced into n-HA/PLGA composite as reinforcement. The effect of different surface treatments of bamboo fiber on the mechanical property and crystallization behavior of bamboo fiber/n-HA/PLGA composites were systematically studied, and the reinforcement mechanisms of different surface treatments were analyzed. The main purpose of the study is to investigate whether the surface treatment for bamboo fiber would produce better mechanical strength of bamboo fiber/nHA/PLGA composite, compared to the untreated bamboo fiber, so as to provide an ideal method to develop a novel biodegradable bamboo fiber/n-HA/PLGA ternary composite used as bone materials in the future.
2. EXPERIMENTAL SECTION 2.1. Materials. Nanohydroxyapatite (n-HA) with a mean size of 120 nm in length and 30 nm in width and PLGA with a composition of LA:GA is 95:05 (mol:mol) were prepared in our laboratory. Bamboo fiber (BF) with crystallinity of 70%, 6− 10 cm in length and 0.03−0.2 mm in diameter, was provided by Zhejiang A&F University. BF contains cellulose, hemicellulose, and lignin mainly, and its degree of polymerization is approaching 1000. BF was cut into 4−5 mm and dried at 105 °C in an oven at least 8 h before use. All other agents were analytical grade. 2.2. Bamboo Fiber Surface Treatment. Bamboo fiber was added into 1% (w/w) NaOH solution, and heated to 90 °C, maintained at this temperature for 1 h, and then further washed several times with distilled water until neutral. Finally, the fiber was dried at 80 °C for 24 h in a vacuum oven. The above method was called alkali treatment. Silane coupling agent (KH550, FW = 221.37, 99%) was added dropwise into an ethanol/water solution with the volume ratio of 95:5, the pH of the silane/ethanol/water mixture was fixed at pH = 9 using NaOH solution, and the solution was B
DOI: 10.1021/acs.iecr.5b02724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. SEM photographs of bamboo fibers: (A, a) Untreated, (B, b) NaOH treated, and (C, c) NaOH+silane treated.
3. RESULTS AND DISCUSSION 3.1. Characterizations of Surface-Treated Bamboo Fiber. SEM was applied to observe surface structure and morphology of bamboo fibers before and after surface treatment before preparing the composite. As shown in Figure 1, the untreated bamboo fibers aligned together closely and amorphous cellulose and impurities appeared on the surface of the untreated bamboo fiber (shown in Figure 1A, a), which might have some negative effect on the properties of the composite. While the two surface-treated bamboo fibers had obvious differences in structure and morphology, for the alkali treated bamboo fiber (shown in Figure 1B, b), the surface was clear and some arranged in parallel deep grooves were formed, showing that alkali treatment can remove the impurities, and it may also transform amorphous cellulose into crystalline cellulose.23 Furthermore, alkali treatment destroyed the interaction between the bamboo fibers so that the majority of the bamboo fibers displayed a fibril form shape and approximately 10 μm in diameter, and the wrinkled surface could increase the specific surface area, which would expand the effective contact area and provide more bonding sites for the matrix.24 Compared to the alkali-treated bamboo fiber corresponding to the silane modified after alkali treatment bamboo fibers, no impurities were adsorbed on the bamboo fibers surface, and the bamboo fibers were neat and dispersed into fibrillary, finer grooves appearing on the surface, which can be seen in Figure 1c. Additionally, a thin film attached on the surface, which would help to change the hydrophilic surface of the bamboo fiber and increase interfacial interaction between the polymer matrix and bamboo fibers. FTIR spectrum was performed to analyze chemical structure of the three bamboo fibers, as shown in Figure 2. The absence of a signal at 1730 cm−1 for carbonyl stretching in all three spectra might be because the ester bond from the hemicelluloses was completely cleaved.25 The wave number range of 1500−1750 cm−1 were the characteristic peaks of lignin, except for 1640 cm−1.26 For alkali-treated bamboo fiber, no new functional groups were introduced to the untreated cellulose structure. The short peak at 1626 cm−1 shifted to 1639 cm−1 after alkali treatment, indicating an absorption peak of water, while the −CH3 stretching vibration absorption peak at 2922.8 cm−1 had a right shift by 20 cm−1. For the silane modification after alkali-treatment bamboo fibers, peaks at 1031 and 1113 cm−1 are stronger after silane modification, as shown in the illustration, which is related to Si−O−Si and Si−O−C
Figure 2. FTIR spectra of the different bamboo fibers. (a) Untreated, (b) NaOH treated, and (c) NaOH+silane treated.
stretching vibration, respectively, demonstrating that new chemical bonds had been formed between bamboo fibers and the silane coupling agent after the bamboo fiber being silane coupling agent modified.27 However, the peaks at 765 and 465 cm−1 assigned as a Si−C symmetric stretching bond and Si− O−C asymmetric bending, respectively, were not seen in this FTIR analysis. It is presumably due to the grafting ratio of the silane coupling agent being too low to show all the peak changes in the FTIR spectrum.28 Chemical elements content on the surface of the three bamboo fibers were analyzed by XPS, seen in Table 1. Three Table 1. Surface Composition (Atom Fractions and Atom Ratios) Determined Using XPS for Bamboo Fibers condition
C
Si
O
Si/C
O/C
KH550 untreated NaOH treated NaOH+silane treated
69.23% 64.94% 56.79% 51.41%
7.69% 7.52% 5.56% 5.19%
23.08% 27.54% 37.65% 43.40%
0.111 0.116 0.098 0.101
0.333 0.424 0.663 0.844
elements C, O, and Si in the silane coupling agent (KH550) and bamboo fibers were discussed. It is known that the main ingredient of bamboo fiber is cellulose (C6H9O5), the O/C ratio of which is 0.833. Compared to the untreated bamboo fibers, the values of Si/C ratio of the alkali-treated bamboo fibers decreased and O/C increased, indicating that some alkalisoluble substance was removed, which was consistent with the conclusion of SEM obtained. Additionally, the value of the Si/C C
DOI: 10.1021/acs.iecr.5b02724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 3. X-ray diffraction patterns of the different bamboo fibers.
Figure 4. Mechanical strength of bamboo fiber/n-HA/PLGA composites prepared with different kinds of bamboo fibers.
diffraction peaks between 15°∼17° and a sharp high peak at 2θ = 22.7°, which are assigned to cellulose I. The diffraction peak at an angle of 15−17° of the bamboo fibers is the result of the overlapping of the two peaks at angles of 14° and 16°. It is because of a high percentage of amorphous hemicellulose and lignin in the bamboo fiber.25,30 The crystallinity (calculated using the XRD analysis software Jade 6.5) of the alkali-treated fiber is 72.5%, increased slightly compared to the untreated BF (70.5%), while it decreased to 71.3% after silane treatment. The explanation was that only the amorphous regions and crystal surfaces in the cellulose structure can react with alkali and be removed. Thus, the interfibrillar regions are likely to be less dense and less rigid and thereby make the fibrils more capable of rearranging by themselves. Consequently, the crystallinity index of fibers increases at lower NaOH concentration.31 3.2. Effects of Surface-Treated Bamboo Fiber on BF/nHA/PLGA Composite. The mechanical properties of three bamboo fiber/n-HA/PLGA composites were shown in Figure 4. It can be seen that the bending strength and bending modulus of the composite with alkali-treated BF were increased obviously, compared to the composite with the untreated bamboo fiber. Especially, the values of the composite with silane modification after alkali treatment of bamboo fiber were further increased than that of the composite with alkali treatment of bamboo fiber and enhanced nearly 10 and 400 MPa, respectively, compared to the composite with untreated bamboo fiber, which may have arisen from the improvement of the interfacial interaction between the hydrophobic polymer matrix and the hydrophilic bamboo fibers, with the thin film formation of silane coupling agent observed by surface morphology. Moreover, the surface-treated bamboo fibers also caused significant improvements in tensile strength and elongation of the composites compared with the composite with untreated bamboo fibers. For example, silane modified after alkali treatment provided slight improvement over untreated bamboo fiber, while only alkali-treated BF provided
better tensile property of the bamboo fiber/n-HA/PLGA, whose tensile strength and elongation were increased by 47.6% and 64.3%, respectively, compared to the composite with untreated bamboo fiber. The reason for the different effectiveness may be connected with the difference of interfacial adhesion and crystallization behavior.32 The fractured surface of the samples after the bending test was observed by SEM, which was shown in Figure 5. It can be seen that clear phase separation existed between bamboo fiber and the n-HA/PLGA matrix when untreated bamboo fiber was added; moreover, bamboo fiber obviously aggregated together (Figure 5A, a), which may be the result of poor compatibility, and lignin as an adhesive tied cellulose together firmly.33 However, after the bamboo fiber was treated with alkali, the cellulose fiber swelled, thereafter converting the cellulose structure to a thermodynamically more stable structure than the original one, which was one of the reasons that the mechanical properties of the composites increased after loaded alkali-treated fiber.34 Meanwhile, obvious grooves were also present after removal of the soluble substances on the surface, and the bigger specific surface area of reinforcement would have greater probability of matrix infiltration. Moreover, PLGA matrix was stretched on the bamboo fiber surface due to good bonding of the two phases at the interface (seen in Figure 5B, b), which was another main reason for the composite material to improve the mechanical strength.35 For the composites loaded with silane modification successively after alkali-treated bamboo fiber (Figure 5C, c), dispersion of fibers were most even in a polymer matrix, and the polymers were also attached to bamboo fiber surface simultaneously, showing that the combination of alkali treatment and silane modification could achieve the best results, which is the reason for the highest bending strength of those composites. The crystallization behavior of bamboo fiber/n-HA/PLGA composites were characterized by X-ray diffraction (XRD), D
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Figure 5. SEM images of the flexural fracture surfaces of bamboo fiber/n-HA/PLGA composites. (A, a) untreated bamboo fiber, (B, b) NaOHtreated bamboo fiber, and (C, c) NaOH+silane-treated bamboo fiber.
differential scanning calorimetry (DSC), and polarized optical microscopy (POM). In order to clearly illustrate the role of bamboo fiber played in the ternary composites, PLGA and nHA/PLGA composite were also analyzed. Figure 6 depicts the XRD pattern of PLGA, n-HA, and three BF/n-HA/PLGA composites. Peaks at 16.5° corresponded to
Figure 7. Second heating DSC thermograms of samples. (a) PLGA, (b) n-HA/PLGA, (c) bamboo fiber/n-HA/PLGA composites with untreated bamboo fiber, (d) NaOH-treated bamboo fiber, and (e) NaOH+silane-treated bamboo fiber.
melting endotherm. The glass transitions temperature (Tg) signifies that the segments of the polymer starts to move, and the higher the Tg, the more the surroundings hinder the segments. As shown in Table 2, compared to PLGA, the n-HA/ PLGA composite had the lower Tg because of incorporation of n-HA nanoparticles, and Tg decreased further when n-HA and bamboo fiber were both added. Additionally, the Tg of surfacetreated bamboo fiber reinforced n-HA/PLGA composites shifted to lower temperature, but decreased only slightly, indicating that surface-treated bamboo fiber could promote movement of molecular chains and also implying that the compatibility between fiber and PLGA was improved after bamboo fiber was treated. Similarly, the melting temperature (Tm) had a slight decrease with the incorporation of n-HA, and the Tm shifted to lower temperatures for bamboo fiber/n-HA/ PLGA composite, especially the composite with the modified bamboo fiber, which showed that the effectiveness is more prominent when the modified bamboo fiber and n-HA were simultaneously added into PLGA. In terms of crystallinity, which can be represented by ΔHCC − ΔHm,36 the n-HA/PLGA composite had a higher degree of crystallization than PLGA. Furthermore, the bamboo fiber/n-HA/PLGA composites had higher crystallinity than the n-HA/PLGA composite, showing bamboo fiber also played an important role in the PLGA crystallization process presumably. Moreover, it also can be
Figure 6. X-ray diffraction patterns of the different samples: (a) PLGA; (b)n-HA/PLGA; BF/n-HA/PLGA composites with (c) untreated bamboo fiber, (d) NaOH-treated BF, (e) NaOH+silanetreated BF.
PLGA, which is very flat and wide, the peaks at 26°, 27.8°, 31.2°, 34.3° agrees with the characteristic peaks of n-HA, 15.0°∼17° and 22.7° were assigned to BFs as mentioned above, while the peak at 15.0°∼17° of BF and PLGA overlap in one area. Intensity of PLGA was strengthened when it was reinforced by n-HA and shows another increase with loading n-HA and BFs, showing that bamboo fiber and n-HA might have a synergetic effect for PLGA crystallization. As expected, the intensity of PLGA of treated BF reinforced BF/n-HA/ PLGA composites was obviously increased compared to untreated BF/n-HA/PLGA. The DSC curve of PLGA and the composites were shown in Figure 7, and their thermal properties were given in Table 2. After eliminating the thermal history, the composites exhibited three main transitions: glass transitions, cold crystallization, and E
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Table 2. Thermal Parameters and Avrami Kinetic Parameters of Samples from the Avrami Equation for the Isothermal Crystallization of Samples sample
Tg (°C)
Tm (°C)
ΔHcc (J/g)
ΔHm (J/g)
|ΔHcc − ΔHm| (J/g)
Tc (°C)
n
K (10−4min−1)
Tmax (min)
PLGA
61.4
165.6
1.379
2.078
0.699
n-HA/PLGA
61.2
166.6
1.735
2.70
1.005
bamboo fiber/n-HA/PLGA (untreated)
61.0
163.8
3.205
4.294
1.089
bamboo fiber/n-HA/PLGA (NaOH treated)
60.2
162.4
6.801
8.338
1.537
bamboo fiber/n-HA/PLGA (NaOH+silane treated)
60.9
162.6
7.495
9.054
1.559
100 110 120 100 110 120 100 110 120 100 110 120 100
2.40 2.62 2.65 2.50 2.83 2.64 2.63 2.83 2.27 2.88 2.66 2.61 2.44
2.71 1.77 0.90 5.66 3.55 1.71 6.32 7.55 3.49 3.52 15.4 2.04 1.39
24.4 22.4 31.3 16.2 14.2 22.2 13.8 10.8 25.9 13.7 9.59 21.4 11.9
110 120
2.56 2.83
26.0 1.48
8.44 19.3
Figure 8. POM photographs of samples at 130 °C. (a) PLGA, (b) n-HA/PLGA, (c) bamboo fiber/n-HA/PLGA composites with untreated bamboo fiber, (d) NaOH-treated bamboo fiber, and (e) NaOH+silane-treated bamboo fiber.
found that the surface-treated bamboo fiber could improve the crystallinity than the untreated bamboo fiber, owing to the increased binding site and improved compatibility. Especially, silane modified after alkali-treated bamboo fiber have more obvious effects on the crystallinity of the corresponding composite than the composite with alkali-treated bamboo fiber, which might be attributed to the better interfacial compatibility.37 To further elucidate the crystallization behavior of bamboo fiber/n-HA/PLGA composites, Avrami kinetic parameters obtained from the isothermal crystallization were also listed
in Table 2. In Table 2, it is found that all Avrami exponential values n, between 2.2 and 3.0, indicated a three-dimension spherulite growth of heterogeneous nucleation, and the minimum values of Tmax were all shown at 110 °C for all the samples, suggesting that the introduction of bamboo fiber did not change the crystallization mode.38 However, for PLGA and the n-HA/PLGA composite, the crystallization rate constant K decreased with the rise of temperature, while the K value of bamboo fiber/n-HA/PLGA composites were maximum at 110 °C, and the values of Tmax diminished greatly, especially the bamboo fiber/n-HA/PLGA composites with surface-treated F
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bamboo fiber, which further indicated that bamboo fiber also had promotion crystallization effectiveness. Combining silane modification with alkali treatment for bamboo fiber had a better effectiveness than alkali treatment, and the conclusion is in accord with the above discussion. To understand the spherulite growing during the crystallization process of the bamboo fiber/n-HA/PLGA composites, POM micrographs of PLGA, n-HA/PLGA composite, and bamboo fiber/n-HA/PLGA composites with untreated or two different surface-treated bamboo fiber were observed clearly at 130 °C (shown in Figure 8). It is interesting to see that the crystals appeared around a single bamboo fiber in a short time when bamboo fiber was introduced into the n-HA/PLGA composite, and the crystals were grown in the direction perpendicular to the fiber surface because of too many nuclei sites and fast crystal growing, which formed cylindrical crystal structure around fibers finally. This phenomenon was called transcrystallization.39 However, no transcrystallization was formed in the PLGA or n-HA/PLGA composite. Different composites had the different sizes of transcrystalline, while the size of monocrystalline had no difference until spherulites filled the entire space. Additionally, the time of petty crystals formed and full screen coved of bamboo fiber/n-HA/PLGA was faster than PLGA, n-HA/PLGA composite, which also indicated that bamboo fiber and n-HA might have a synergetic effect for PLGA crystallization. Moreover, fillers of n-HA and bamboo fiber might play a different role simultaneously in forming trans-crystalline, and crystals were formed due to n-HA as a nucleating agent, while growth of crystals was promoted by bamboo fiber filler.40 Comparing the three bamboo fiber/nHA/PLGA composites, the untreated bamboo fiber reinforced composite materials had a longer crystallization time, while it became shorter obviously after the alkali treatment of the bamboo fiber, and it extended slightly by the bamboo fiber after NaOH and silane treatment. This might be due to the different surface structure of bamboo fiber after modification. Alkali treatment could remove impurities and improve the specific surface area of the fiber, thus providing more binding sites for n-HA/PLGA, so crystals grew earlier and faster. However, due to the similar chemical structure and properties between the silane coupling agent and PLGA, the crystal on or around the modified bamboo fiber grew nearly at the same time and had a similar growth rate. It was also concluded that modification of bamboo fiber had a significant effect on the spherulite crystallization rate.
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AUTHOR INFORMATION
Corresponding Author
*Tel./Fax: +86 0731 88873111. E-mail:
[email protected] (L.-Y. Jiang). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors would like to acknowledge the support of Sichuan Provincial Youth Science and Technology Foundation (Grant 2014JQ0059), the Talent Training Project of West Light Foundation of Chinese Academy of Science, and National Natural Science Foundation of China (Grant 31000440).
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4. CONCLUSIONS On the basis of the above experimental results and analysis, it could be concluded that the two different surface-treated bamboo fibers had different effectiveness on the mechanical properties of the bamboo fiber/n-HA/PLGA composite. The alkali-treated bamboo fiber endowed the ternary composite with the highest tensile strength, while silane modification after alkali-treated bamboo fiber could lead to the best bending strength and modulus. The reinforcement mechanisms of different surface treatments contributed to the difference of the surface morphology of bamboo fiber, interfacial adhesion, compatibility between bamboo fiber and n-HA/PLGA matrix, and the promotion crystallization behavior of PLGA. The study provides significant guiding in designing novel biodegradable PLGA-based composite with higher mechanical properties used as bone fracture internal fixation materials in the future. G
DOI: 10.1021/acs.iecr.5b02724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
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DOI: 10.1021/acs.iecr.5b02724 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
