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Effects of bamboo cellulose nanowhisker content on the morphology, crystallization, mechanical, and thermal properties of PLA matrix biocomposites
Composites Part B 133 (2018) 203e209
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Effects of bamboo cellulose nanowhisker content on the morphology, crystallization, mechanical, and thermal properties of PLA matrix biocomposites Shaoping Qian a, b, *, Huanhuan Zhang b, Wenchao Yao b, Kuichuan Sheng b, ** a b
School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou, 310058, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 November 2016 Received in revised form 17 September 2017 Accepted 17 September 2017 Available online 19 September 2017
To improve the mechanical and thermal properties of poly (lactic acid) (PLA) composites, bamboo cellulose nanowhiskers (BCNW) were extracted and introduced into PLA composites as fillers. PLA/BCNW biofilms were fabricated by solution casting with different BCNW contents (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 wt%). The characteristics of the biofilms were investigated by scanning electron microscopy (SEM), mechanical measurements, synchrotron radiation wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). The results showed that the maximum tensile modulus and elongation at break of 427.72 ± 19.32 MPa and 22.27 ± 3.50% were reached at 2.5 wt% and 1.0 wt% loadings, respectively. Both homogeneity and stereocomplexed crystallites were observed, and heterogeneous nucleation effect was confirmed. With the addition of BCNW, the crystallite size of PLA/BCNW composites increased remarkably, and the largest crystallinity was 30.7 ± 0.9% with 2.5 wt% BCNW. These results provided data support for enlarging the application of PLA. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Particle-reinforcement Polymer-matrix composites (PMCs) Mechanical properties Thermal analysis
1. Introduction Poly(lactic acid) (PLA) is a green bioplastic that can be produced from natural sources such as corn and cassava. In addition, PLA degrades naturally into H2O and CO2 through composting [1e3]. With an ever-increasing public consciousness promoting environmental protection, the serious problem of “white pollution” is a cause for much concern; thus, PLA exhibits a wide degree of potential applications [4e6]. So far, the applications of PLA have involved biomedical devices, textiles, films, decorative panels, electrical components, food packages etc. [7e10]. However, the poor thermal stability and high brittleness of PLA leads to difficulties in processing and decreases the product's performance [3,11], limiting the wide application of PLA [12]. Thus, new effective methods to improve the thermal and mechanical properties of PLA are of great interest. The employment of bio-nanoparticles as reinforcements in PLA
* Corresponding author. 818 Fenghua Road, Ningbo 315211, China. ** Corresponding author. 866 Yuhangtang Road, Hangzhou 310058, China. E-mail addresses: [email protected] (S. Qian), [email protected] (K. Sheng). https://doi.org/10.1016/j.compositesb.2017.09.040 1359-8368/© 2017 Elsevier Ltd. All rights reserved.
composites has been proved to be a valid method [2]. Recent studies reported that cellulose nanowhiskers (CNW), with a large aspect ratio, high strength, and elastic modulus reinforced PLA composites and presented excellent thermal and mechanical properties [13e18]. Moso bamboo (Phyllostachys heterocycla) grows abundantly in many tropical and subtropical regions of the world, especially in Zhejiang Province, China. It has been widely used in furniture manufacturing, construction materials and household items due to the advantages of fast growth, high strength, surface hardness and easy machinability [19e22]. However, a large amount of moso bamboo processing residue is underused. The cellulose content of bamboo is 40%e65%, which is comparable to wood [23]. Thus, bamboo residue is a good resource for renewable nanobiobased filler. Brito et al. [24] and Lu et al. [25] prepared cellulose nanocrystals with a length of approximately 100 nm by the hydrolysis of bamboo fibers in the presence of sulfuric acid and phosphoric acid. Visakh et al. [26] employed bamboo cellulose nanowhiskers (BCNW) as reinforcements in rubber composites and found that the BCNW with high length and aspect ratios improved the performance of composites. Our group prepared bamboo cellulose nanowhisker from bamboo residues, and investigated the mechanism of acid hydrolysis [27]. BCNW may act as a nucleating
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agent or plasticizer, and has been used as an applicable nanofiller in PLA composites [28,29]. In this regard, the crystallization performance, thermal property, and mechanical properties of the composites would be improved. Lee et al. [30] found that all properties of thermology, rheology and mechanics increased with the addition of CNW (0.1 wt% and 0.5 wt%) into PLA composites. Additionally, acetylated-CNW with different contents (1 wt%, 2 wt%, 3 wt%) reinforced PLA composites were also reported; the crystallization behavior and mechanical properties of the composites were improved. These composites can be used in artificial bone and biomembranes, etc. [31]. Arias et al. [32] prepared PEO-coated nanocrystals, with the cellulose nanocrystals (CNC) evenly dispersed in the PLA matrix, and studied the performance of composites with different CNC contents. Arjmandi et al. [28] investigated the reinforcement mechanism of a montmorillonite/CNW/PLA ternary system and found that the three compositions with proportions of 5:1:10 presented the highest tensile properties. Sanchez-Garcia et al. [33] concluded that PLA/CNW composites had better mechanical properties with a CNW loading below 3 wt%. Although it was previously reported that the thermal and mechanical properties of a PLA composite would be enhanced to some extent by adding CNW, the agglomeration of CNW would decrease the thermal and mechanical properties of PLA in some cases due to the abundant hydroxyl groups [34e36]. Thus, the optimal content of CNW incorporated into a PLA matrix remains to be explored. In addition, the unique morphology of BCNW and the effects of different BCNW contents on the crystallization behavior and macroscopic property of PLA biocomposites still needs to be further investigated. In this paper, BCNW were extracted from bamboo residue through hydrolysis in the presence of sulfuric acid. The morphology of the BCNW was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Nanocomposite films were prepared through incorporating different contents of BCNW into a PLA matrix using a solution casting method. The reinforcement mechanism of BCNW in the PLA composites was studied. The interface characteristics of the composites were observed by SEM. The crystallite structures of the composites were investigated by synchrotron radiation wide angle X-ray scattering (WAXS). The crystallization behavior and thermal performance were characterized by differential scanning calorimetry (DSC). The tensile strength, tensile modulus, and elongation at break of the composites were investigated. On the basis of these results, the optimal BCNW content in a PLA matrix was obtained. These results provided basic reference data for further improving the properties of PLA biocomposites and achieving extensive applications in engineering fields. 2. Materials and methods 2.1. Materials Moso bamboo residues were kindly supplied by a local moso bamboo processing factory, Lin'an, Zhejiang Province, China. The particles were screened through a mesh size of 100 and dried at 105 C to constant quality for further use. PLA (4032D) was produced by the NatureWorks Corporation (USA). The density was 1.25 g/cm3, and the molecular weight was 52,000 g/mol. All other reagents and solvents were used as received from the commercial source. 2.2. Preparation and characterization of bamboo cellulose nanowhiskers (BCNW) Bamboo particles (BP) (<150 mm) were mercerized with a NaOH solution followed by bleaching with a NaClO2 solution before
hydrolyzing. Then, the bleached cellulose was hydrolyzed in a 65 wt% sulfuric acid solution at 45 C for 3 h. The procedure was in accordance with previous work [27]. Nevertheless, different acid solution concentrations and hydrolysis times were tried and it was observed that lower concentration led to insufficient hydrolysis, but higher treatment times led to carbonization and darkening of the product. BCNW was checked for micromorphology by transmission electron microscopy (TEM) using a JEM 1230 (JEOL, Japan) equipped with a digital Image Tools software (UTHSCSA, USA). A BCNW aqueous suspension of 0.05 wt% was ultrasonically treated for 30 min. Samples were then stained in a 2 wt% solution of uranyl acetate for 3 min. Drops of the stained samples were deposited on copper TEM grids, and the excess water was absorbed with a tissue. The surface morphology of the raw bamboo particles and BCNW were observed using a field launch scanning electron microscope (S-8010, Hitachi, Japan). All samples were coated with gold before observation. The launching voltage of the electron microscope was 4.0 kV. 2.3. Fabrication and observation of PLA/BCNW composites film Approximately 5.0 g PLA was added to 60 mL chloroform and stirred in a 50 C water bath until PLA dissolved completely. The solution was blended with different contents of dried BCNW, and ultrasonic stirring was used for 30 min to disperse the solution. The blended solution was cast in a self-made PTFE mold (diameter of 80 mm) and dried at room temperature for 48 h. This resulted in PLA/BCNW biocomposite films with a thickness of 0.3e0.5 mm. The surface morphology of composites with different BCNW contents was also observed using a scanning electron microscope (S-8010, Hitachi, Japan). 2.4. Tensile test Tensile tests of PLA/BCNW composites were performed on a universal testing machine at room temperature (CMT4503, MTS, Inc.). A self-made knife was used to cut the films into dumbbell shaped specimens with a length of 50 mm, a cross-sectional width of 4.0 mm and an initial gauge length of 15 mm [37]. A fixed crosshead rate for tension of 20 mm/min was utilized in all cases and the results were calculated based on the average results from five specimens. 2.5. Differential scanning calorimetry (DSC) DSC (200F3, Netzsch) was adopted to study the thermal properties of pure PLA and the biocomposites. Approximately 10.0 mg sample were weighed and hermetically sealed. The samples were heated from room temperature to 180 C at a rate of 10 C/min, maintained for 3 min, cooled to 0 C at a rate of 10 C/min and heated again to 180 C at a rate of 10 C/min. The second DSC thermograms were recorded for further evaluation. Nitrogen was used as a purging gas at a rate of 50 mL/min. An empty aluminum pan was used as a reference. Thermal property parameters were calculated based on the average of three specimens. Cold crystallization degree (Xcc) was estimated according to the following equation,
Xcc ð%Þ ¼
DHcc 100% DH0 XPLA
(1)
where, DHcc refers to the cold crystallization enthalpy of PLA/BCNW composites; DH0 refers to the enthalpy value during 100% crystallization of PLA, which is 93.6 J/g [38]; and XPLA refers to the weight
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ratio of PLA in PLA/BCNW composites. 2.6. Wide angle X-ray scattering (WAXS) analysis WAXS analysis was conducted using a special synchrotron radiation wide angle X-ray scattering at BL16B1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF). Biofilms were cut into square pieces of 10 mm 10 mm. The wavelength of the radiation source was 0.124 nm. The sample-to-detector distance was 137 cm, and the irradiation time was 10 s. The scattering patterns were recorded using a SX-165 CCD detector (Rayonix, Illinois, USA), with a resolution was 2048 2048 pixels with size of 80 80 mm2. All data was corrected for the background and air scattering. Fit2D software was used to convert the 2D data into 1D data. 3. Results and discussion 3.1. Morphology of bamboo cellulose nanowhisker Fig. 1 illustrates the micro-morphology of raw bamboo particles and BCNW. We previously reported that the BCNW exhibited a large length-to-diameter ratio (L/D) and had rod-like shapes and network-like structures [27], which were different with whiskers from jute, coconut husk, rice husk and cotton [39]. The fibrillated BCNW had a higher surface area and better cross-linking characteristics when used as nano-fillers. Two types of aggregation of BCNW, namely, closely spaced crystal and isolated single whiskers were observed (Fig. 2). The BCNW had an average length, diameter and L/D ratio of 455 nm, 12 nm and 37, respectively [27]. 3.2. Tensile properties of PLA/BCNW composites Tensile properties of the composites with different BCNW contents and the appearance of untested samples are shown in Fig. 3. With the increase in BCNW content, tensile strength of the composites reduced gradually, similar to the results shown by the research group of Qu [40]. This was mainly caused by a poor
Fig. 2. TEM image of BCNW (65 wt% H2SO4 3 h).
interfacial compatibility between the BCNW and PLA. The hydrogen bonds between polar hydroxyl on BCNW and ester group on PLA might destroy the dense arrangement of PLA molecules in the amorphous region, resulting in PLA molecular segments slipping under tension [41]. In other words, it is of great significance to improve the interfacial adhesion between PLA and BCNW in order to reinforce the mechanical properties of the composites [42]. In addition, further increases of BCNW content caused BCNW agglomeration and a poor BCNW dispersion in PLA matrix, which prevents the formation of cross-linking networks of PLA molecular chains and the rod-like BCNW. As a result, the stress resistance capability of the composites decreased. As to the appearance of the sample, the pure PLA film was more transparent than the rest of the samples. Similar film transparency was observed when the BCNW contents were 0.5 wt%, 3.5 wt% and 4.0 wt%, which indicated a
Fig. 1. SEM images of bamboo cellulose with different treatment. (a) Raw BP; (b) Alkali treated; (c) 55 wt% H2SO4 4 h; (d) 65 wt% H2SO4 3 h.
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to the high rigidity of crystalline region, the latter may be due to the stress concentration caused by the uneven distribution of BCNW. 3.3. WAXS analysis of PLA/BCNW composites
Fig. 3. Tensile properties of PLA/BCNW composites. Note: The different normal letters indicate significantly difference by Duncan's multiple range test at 0.05 level (p < 0.05) (n ¼ 5).
similar crystallization behavior. This is further evidenced by the thermal analysis. The transparency of the film decreased with increasing BCNW content in the cases exceeding 2.5 wt%. The tensile modulus and elongation at break of pure PLA were 411.51 MPa and 17.7%, and they showed an upward trend to 427.72 MPa and 22.27% with increasing BCNW contents of up to 2.5 wt% and 1.0 wt%, respectively. However, both the values of the tensile modulus and elongation at break decreased when increasing the BCNW contents above 2.5 wt% and 1.0 wt%. This trend is different from the previous report which shown that tensile modulus and elongation at break of PP or PE composites decreased steadily when reinforced by natural fiber or other cellulose whisker (without coupling agent) [43]. Tensile modulus represents the stiffness of material and reflects the ability to withstand deformation. For pure PLA film, PLA molecules and molecular segments in the amorphous region were stress relaxed without a BCNW steric effect during the solvent evaporation, which endowed the pure PLA with a relatively high molecular density. The bonding mechanism between PLA and BCNW mainly included the diffusion of BCNW into the PLA amorphous area, the hydrogen bonding, and the Van der Waals force between BCNW and PLA. With the addition of fewer BCNW, interactions between PLA molecular chains were destroyed, leading to a decrease in tensile modulus. The highest tensile modulus of the composites was achieved with 2.5 wt% BCNW content due to the strong heterogeneous nucleation effect. Therefore, the PLA molecules crystallized to form stable and compact crystalline regions, and the deformation resisting ability was enhanced. The tensile modulus dropped largely when the BCNW content exceeded 3.0 wt%, as mentioned previously, as the agglomeration of BCNW hindered the crystallization behavior of PLA. Sanchez-Garcia also reported a similar result; the proper filling proportion of CNW was less than 3 wt% [33]. Notably, the tensile modulus of PLA/BCNW composites showed varying trends with the increase in BCNW content, but other using other CNW biocomposites showed a steady decrease. The reason could be that the intrinsic tensile modulus of bamboo fiber is high, close to 32e34.6 GPa [44]. In addition, BCNW presented a higher length-to-diameter ratio than other nanocrystals [24,45e47]. Thus, there might be more cross-linking network structures consisted of BCNW and PLA macromolecules in the composites, and these structures were not destroyed easily under tension. Tensile deformation mainly took place in the amorphous region of PLA, at the interface between BCNW and PLA, and the small deformation of BCNW, which can be neglected. When the BCNW contents were 2.5 wt% and 4.0 wt%, the elongation at break values of the composites were low. The former decrease may be due
Crystalline structures of PLA and the biocomposites were characterized by synchrotron radiation Wide-Angle X-ray Diffraction (WAXD) (Fig. 4). As the diffractogram described, a-form homocrystallites showed four crystalline diffraction peaks at 2q of 12.2 , 13.8 , 15.7 and 18.4 corresponding to 010, 110/200, 203 and 015 crystallographic planes, respectively [48,49]. In pure PLA and the composites, the crystalline region was mainly composed of homogeneous crystal, especially in the 110/200 crystallographic plane. The BCNW-added diffraction peak became narrow and intense, which gave evidences that the BCNW produced a heterogeneous nucleation effect in the PLA matrix and sped up the crystallization behavior. It was worth mentioning that the X-ray wavelength used in this experiment was 0.124 nm, which is different from the traditional wavelength of 0.154 nm. Thus, the characteristic diffraction peaks of PLA were different from other studies [49,50], even though the lattice dimensions calculated by the Bragg equation were consistent. In the case of 2q at 10.3 , the heterogeneous stereocomplex crystallite of the PLLA and PDLA molecular chain segments were observed in accordance with the reported literature [48]. It could assume that a little amount of PDLA in PLA raw material formed stereocomplex crystallite in the 110 crystallographic plane during solution casting. It is known that a stereocomplex crystallite structure has better thermal stability than homogeneous crystallite. However, the addition of BCNW barely influenced the formation of stereocomplex crystallite in 110 crystallographic plane, and the stereocomplex crystallite diffraction peak at 2q of 17.2 in 300/030 crystallographic plane was not found [51]. When 2q was 13.8 , the average crystallite size was estimated by the Scherrer's equation as follows:
D¼
kl b cos q
(2)
where D refers to the average crystallite size (nm), b presents the full width at half maxima, l is the wavelength of the X-ray (0.124 nm), q is half of the angle for the corresponding peak (2q), and k is the Scherrer crystal shape constant, k ¼ 0.89 [52].
Fig. 4. WAXS patterns of PLA and PLA/BCNW composites. HC and SC indicate the characteristic diffractions of PLA homogeneous and heterogeneous crystallites, respectively. Subscript numbers of HC and SC denote the Miller indices of crystal lattice.
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The size of the homogeneous crystalline grain increased remarkably at 2q of 13.8 after adding BCNW to PLA (Fig. 5), but no obvious change can be seen with the increase in BCNW content. This indicated that the lamella crystallite of pure PLA grew slowly after the formation of crystal nucleus, because the crystallization behavior was influenced by the formation of crystal nucleus and the growth of grain. When BCNW was added, a heterogeneous nucleation effect was induced so that the number of crystal nucleuses increased. In addition, the PLA molecular chains were guided to gather in an orderly manner by the strong negative charge of hydroxyl. Consequently, the crystallization properties of the composites improved through the grain growth. 3.4. Thermal property of PLA/BCNW composites DSC thermograms of pure PLA and PLA/BCNW composites with different proportions are shown in Fig. 6. The thermal parameters, including glass transition temperature (Tg), cold crystallization temperature (Tcc), melting temperature (Tm), cold crystallization enthalpy (DHcc), and the degree of cold crystallinity (Xcc) are tabulated in Table 1. The addition of BCNW to PLA resulted in increases in Tg, Tcc and Tm when compared to pure PLA, suggesting an improvement in its thermal properties. This might be because the presence of BCNW restrained the macromolecular mobility of the PLA chains. As a result, the molecules required more energy to start moving, and needed higher temperatures to convert the PLA from a glassy state to a rubbery state [19]. When the BCNW content increased from 0.5 wt% to 1.5 wt%, Tg decreased from 45.2 C to 41.1 C. This could be explained by the fact that the polar hydroxyl groups on the BCNW surface reduced the cross-linking density of the interfacial region, resulting in a decrease in the total crosslinking density of the PLA matrix. However, when the crystalline region dominated the composites (BCNW content >2.0 wt%), the improved crosslinking density of PLA increased the glass transition temperature. A decrease in Tcc was observed with increasing BCNW content, which denoted that the BCNW induced nucleation of PLA and initiated crystallization at relative low temperatures. In addition, the abilities of nucleation and crystal growth were increased by the heterogeneous nucleation; thus, the crystallinity increased noticeably. As seen in Fig. 6, the pure PLA exhibited a single crystalline melting peak, whereas the composites showed small double melting peaks, especially the highest peak which was achieved with 2.5 wt% BCNW. This was probably because the crystallization behavior of the composites was enhanced, which caused imperfect cold crystallization crystallites to form at low temperatures that formed more perfect crystallites upon heating, causing the melting
Fig. 6. DSC thermograms of pure PLA and PLA/BCNW composites.
peak to shift to higher temperatures [53,54]. Notably, the supercooling degree of pure PLA and the composites with 0.5e1.5 wt% BCNW contents were lower than those with 2.0 wt%. The nucleation rate increased with an increasing super-cooling degree, which meant that the composites with over 2.0 wt% BCNW had a better heterogeneous nucleation effect. However, when the BCNW content exceed 2.5 wt%, agglomeration occurred, and the crystal nucleus failed to increase. These results can be in accordance with WAXS analysis. 3.5. SEM analysis of PLA/BCNW composites Fig. 7 shows the SEM images of pure PLA and PLA/BCNW composites with a 2.5 wt% BCNW content (before and after fracture). The morphological surface of pure PLA was relatively smooth, which could be attributed to the low crystallization ability of pure PLA; most PLA molecular chains remained in an amorphous state during solvent evaporation. However, the surface of the PLA/BCNW composites was uneven and rough, with many grains and gullies. This could be due to the improvement of crystallization after the addition of BCNW, as some PLA molecule chains crystallized to regular orientation while the rest of PLA chains were still in an amorphous state. As to the fractural surface observation, pure PLA exhibited a smooth, clean, brittle rupture as no wire-drawing appearance was observed. This indicated that pure PLA had high tensile strength; when the stress exceeded its yield stress, rupture occurred immediately. However, the fractural surface of PLA/BCNW composites exhibited a wire-drawing appearance while tensile analysis showed no strength improvement. This implied that BCNW toughened the PLA composites, but poor interface interaction limited the efficient toughness [55]. Fig. 7e and f show the combinations of the BCNW and PLA matrix. The embedded (e) form and fiber-bridged (f) form represented the good and poor interaction, respectively. Thus, it can be deduced that the improvement of interfacial characteristic between BCNW and PLA will largely improve the tensile ductility of the composites. 4. Conclusions
Fig. 5. Size of crystallites when 2q ¼ 13.8 .
In this paper, bamboo cellulose nanowhiskers were employed as nano-filler for PLA biocomposites. The rod-like BCNW had an average length of 455 nm, diameter of 12 nm and length-todiameter ratio (L/D) of approximately 37. PLA/BCNW composites with 2.5 wt% BCNW content had a relatively high level of transparency and tensile modulus; excessive BCNW contents lowered
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Table 1 Thermal behaviors of PLA and PLA/BCNW composites. Samples
Tg ( C)
PLA 0.5 wt% 1.0 wt% 1.5 wt% 2.0 wt% 2.5 wt% 3.0 wt% 3.5 wt% 4.0 wt%
40.9 45.2 45.3 41.1 43.6 45.7 40.9 43.2 45.9
± ± ± ± ± ± ± ± ±
0.2a 0.1b 0.2b 0.2c 0.1bc 0.3b 0.3a 0.2bc 0.4b
Tcc ( C)
DHcc (J/g)
96.8 ± 1.2a 103.7 ± 0.9b 101.7 ± 0.7c 100.3 ± 1.1c 94.5 ± 0.6d 96.1 ± 0.6bc 94.3 ± 0.8e 95.4 ± 0.3bc 94.5 ± 0.9d
15.8 27.4 24.5 25.6 24.1 27.7 23.2 27.0 26.9
± ± ± ± ± ± ± ± ±
0.1a 0.1b 0.3c 0.2bc 0.2c 0.5b 0.4d 1.0b 0.9b
16.7 29.8 26.5 27.4 26.5 30.7 25.3 29.9 29.7
± ± ± ± ± ± ± ± ±
DHm (J/g)
Tm ( C)
Xcc (%) 0.8a 1.4b 0.9c 0.8cd 1.0c 0.9bc 1.1d 0.7b 1.4b
156.1 163.4 163.9 162.5 163.6 168.1 162.1 163.5 163.7
± ± ± ± ± ± ± ± ±
1.5a 1.8bc 0.9c 0.9d 1.0c 1.2e 0.9d 0.8c 1.3b
34.67 32.57 32.77 33.46 34.32 34.86 34.61 34.66 34.77
± ± ± ± ± ± ± ± ±
1.6a 1.8b 1.1b 1.5c 0.9a 1.0d 2.1cd 0.7bd 1.8d
Note: The different normal letters (a,b,c,d,e after the values) in the same row indicate significant difference among all treatments at 0.05 level.
Fig. 7. SEM graphs of pure PLA and PLA/BCNW composites with 2.5 wt% content, a: PLA surface, b: PLA fractural surface, c: PLA/BCNW composites surface, d: PLA/BCNW composites fractural surface, eef: micromorphology of BCNW in PLA matrix.
the tensile property. The addition of BCNW mainly affected the homocrystallites of PLA and enlarged the crystallite size. Pure PLA showed a single melting peak whereas PLA/BCNW composites showed a double melting peak. Additionally, the highest glass transition temperature and cold crystallinity of PLA/BCNW composites were found with the BCNW content of 2.5 wt%. However, due to the insufficiency of interfacial compatibility, surface modification between BCNW and PLA is needed to further improve the reinforcement effect.
Conflict of interest The authors declare no competing financial interest. Acknowledgments This work was financially supported by the Natural Science Foundation of Zhejiang Province (No. LY16E030003), the Applied Research Project on Public Welfare Technology of Zhejiang Province
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Effects of bamboo cellulose nanowhisker content on the morphology, crystallization, mechanical, and thermal properties of PLA matrix biocomposites Shaoping Qian a, b, *, Huanhuan Zhang b, Wenchao Yao b, Kuichuan Sheng b, ** a b
School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou, 310058, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 November 2016 Received in revised form 17 September 2017 Accepted 17 September 2017 Available online 19 September 2017
To improve the mechanical and thermal properties of poly (lactic acid) (PLA) composites, bamboo cellulose nanowhiskers (BCNW) were extracted and introduced into PLA composites as fillers. PLA/BCNW biofilms were fabricated by solution casting with different BCNW contents (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 wt%). The characteristics of the biofilms were investigated by scanning electron microscopy (SEM), mechanical measurements, synchrotron radiation wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). The results showed that the maximum tensile modulus and elongation at break of 427.72 ± 19.32 MPa and 22.27 ± 3.50% were reached at 2.5 wt% and 1.0 wt% loadings, respectively. Both homogeneity and stereocomplexed crystallites were observed, and heterogeneous nucleation effect was confirmed. With the addition of BCNW, the crystallite size of PLA/BCNW composites increased remarkably, and the largest crystallinity was 30.7 ± 0.9% with 2.5 wt% BCNW. These results provided data support for enlarging the application of PLA. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Particle-reinforcement Polymer-matrix composites (PMCs) Mechanical properties Thermal analysis
1. Introduction Poly(lactic acid) (PLA) is a green bioplastic that can be produced from natural sources such as corn and cassava. In addition, PLA degrades naturally into H2O and CO2 through composting [1e3]. With an ever-increasing public consciousness promoting environmental protection, the serious problem of “white pollution” is a cause for much concern; thus, PLA exhibits a wide degree of potential applications [4e6]. So far, the applications of PLA have involved biomedical devices, textiles, films, decorative panels, electrical components, food packages etc. [7e10]. However, the poor thermal stability and high brittleness of PLA leads to difficulties in processing and decreases the product's performance [3,11], limiting the wide application of PLA [12]. Thus, new effective methods to improve the thermal and mechanical properties of PLA are of great interest. The employment of bio-nanoparticles as reinforcements in PLA
* Corresponding author. 818 Fenghua Road, Ningbo 315211, China. ** Corresponding author. 866 Yuhangtang Road, Hangzhou 310058, China. E-mail addresses: [email protected] (S. Qian), [email protected] (K. Sheng). https://doi.org/10.1016/j.compositesb.2017.09.040 1359-8368/© 2017 Elsevier Ltd. All rights reserved.
composites has been proved to be a valid method [2]. Recent studies reported that cellulose nanowhiskers (CNW), with a large aspect ratio, high strength, and elastic modulus reinforced PLA composites and presented excellent thermal and mechanical properties [13e18]. Moso bamboo (Phyllostachys heterocycla) grows abundantly in many tropical and subtropical regions of the world, especially in Zhejiang Province, China. It has been widely used in furniture manufacturing, construction materials and household items due to the advantages of fast growth, high strength, surface hardness and easy machinability [19e22]. However, a large amount of moso bamboo processing residue is underused. The cellulose content of bamboo is 40%e65%, which is comparable to wood [23]. Thus, bamboo residue is a good resource for renewable nanobiobased filler. Brito et al. [24] and Lu et al. [25] prepared cellulose nanocrystals with a length of approximately 100 nm by the hydrolysis of bamboo fibers in the presence of sulfuric acid and phosphoric acid. Visakh et al. [26] employed bamboo cellulose nanowhiskers (BCNW) as reinforcements in rubber composites and found that the BCNW with high length and aspect ratios improved the performance of composites. Our group prepared bamboo cellulose nanowhisker from bamboo residues, and investigated the mechanism of acid hydrolysis [27]. BCNW may act as a nucleating
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agent or plasticizer, and has been used as an applicable nanofiller in PLA composites [28,29]. In this regard, the crystallization performance, thermal property, and mechanical properties of the composites would be improved. Lee et al. [30] found that all properties of thermology, rheology and mechanics increased with the addition of CNW (0.1 wt% and 0.5 wt%) into PLA composites. Additionally, acetylated-CNW with different contents (1 wt%, 2 wt%, 3 wt%) reinforced PLA composites were also reported; the crystallization behavior and mechanical properties of the composites were improved. These composites can be used in artificial bone and biomembranes, etc. [31]. Arias et al. [32] prepared PEO-coated nanocrystals, with the cellulose nanocrystals (CNC) evenly dispersed in the PLA matrix, and studied the performance of composites with different CNC contents. Arjmandi et al. [28] investigated the reinforcement mechanism of a montmorillonite/CNW/PLA ternary system and found that the three compositions with proportions of 5:1:10 presented the highest tensile properties. Sanchez-Garcia et al. [33] concluded that PLA/CNW composites had better mechanical properties with a CNW loading below 3 wt%. Although it was previously reported that the thermal and mechanical properties of a PLA composite would be enhanced to some extent by adding CNW, the agglomeration of CNW would decrease the thermal and mechanical properties of PLA in some cases due to the abundant hydroxyl groups [34e36]. Thus, the optimal content of CNW incorporated into a PLA matrix remains to be explored. In addition, the unique morphology of BCNW and the effects of different BCNW contents on the crystallization behavior and macroscopic property of PLA biocomposites still needs to be further investigated. In this paper, BCNW were extracted from bamboo residue through hydrolysis in the presence of sulfuric acid. The morphology of the BCNW was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Nanocomposite films were prepared through incorporating different contents of BCNW into a PLA matrix using a solution casting method. The reinforcement mechanism of BCNW in the PLA composites was studied. The interface characteristics of the composites were observed by SEM. The crystallite structures of the composites were investigated by synchrotron radiation wide angle X-ray scattering (WAXS). The crystallization behavior and thermal performance were characterized by differential scanning calorimetry (DSC). The tensile strength, tensile modulus, and elongation at break of the composites were investigated. On the basis of these results, the optimal BCNW content in a PLA matrix was obtained. These results provided basic reference data for further improving the properties of PLA biocomposites and achieving extensive applications in engineering fields. 2. Materials and methods 2.1. Materials Moso bamboo residues were kindly supplied by a local moso bamboo processing factory, Lin'an, Zhejiang Province, China. The particles were screened through a mesh size of 100 and dried at 105 C to constant quality for further use. PLA (4032D) was produced by the NatureWorks Corporation (USA). The density was 1.25 g/cm3, and the molecular weight was 52,000 g/mol. All other reagents and solvents were used as received from the commercial source. 2.2. Preparation and characterization of bamboo cellulose nanowhiskers (BCNW) Bamboo particles (BP) (<150 mm) were mercerized with a NaOH solution followed by bleaching with a NaClO2 solution before
hydrolyzing. Then, the bleached cellulose was hydrolyzed in a 65 wt% sulfuric acid solution at 45 C for 3 h. The procedure was in accordance with previous work [27]. Nevertheless, different acid solution concentrations and hydrolysis times were tried and it was observed that lower concentration led to insufficient hydrolysis, but higher treatment times led to carbonization and darkening of the product. BCNW was checked for micromorphology by transmission electron microscopy (TEM) using a JEM 1230 (JEOL, Japan) equipped with a digital Image Tools software (UTHSCSA, USA). A BCNW aqueous suspension of 0.05 wt% was ultrasonically treated for 30 min. Samples were then stained in a 2 wt% solution of uranyl acetate for 3 min. Drops of the stained samples were deposited on copper TEM grids, and the excess water was absorbed with a tissue. The surface morphology of the raw bamboo particles and BCNW were observed using a field launch scanning electron microscope (S-8010, Hitachi, Japan). All samples were coated with gold before observation. The launching voltage of the electron microscope was 4.0 kV. 2.3. Fabrication and observation of PLA/BCNW composites film Approximately 5.0 g PLA was added to 60 mL chloroform and stirred in a 50 C water bath until PLA dissolved completely. The solution was blended with different contents of dried BCNW, and ultrasonic stirring was used for 30 min to disperse the solution. The blended solution was cast in a self-made PTFE mold (diameter of 80 mm) and dried at room temperature for 48 h. This resulted in PLA/BCNW biocomposite films with a thickness of 0.3e0.5 mm. The surface morphology of composites with different BCNW contents was also observed using a scanning electron microscope (S-8010, Hitachi, Japan). 2.4. Tensile test Tensile tests of PLA/BCNW composites were performed on a universal testing machine at room temperature (CMT4503, MTS, Inc.). A self-made knife was used to cut the films into dumbbell shaped specimens with a length of 50 mm, a cross-sectional width of 4.0 mm and an initial gauge length of 15 mm [37]. A fixed crosshead rate for tension of 20 mm/min was utilized in all cases and the results were calculated based on the average results from five specimens. 2.5. Differential scanning calorimetry (DSC) DSC (200F3, Netzsch) was adopted to study the thermal properties of pure PLA and the biocomposites. Approximately 10.0 mg sample were weighed and hermetically sealed. The samples were heated from room temperature to 180 C at a rate of 10 C/min, maintained for 3 min, cooled to 0 C at a rate of 10 C/min and heated again to 180 C at a rate of 10 C/min. The second DSC thermograms were recorded for further evaluation. Nitrogen was used as a purging gas at a rate of 50 mL/min. An empty aluminum pan was used as a reference. Thermal property parameters were calculated based on the average of three specimens. Cold crystallization degree (Xcc) was estimated according to the following equation,
Xcc ð%Þ ¼
DHcc 100% DH0 XPLA
(1)
where, DHcc refers to the cold crystallization enthalpy of PLA/BCNW composites; DH0 refers to the enthalpy value during 100% crystallization of PLA, which is 93.6 J/g [38]; and XPLA refers to the weight
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ratio of PLA in PLA/BCNW composites. 2.6. Wide angle X-ray scattering (WAXS) analysis WAXS analysis was conducted using a special synchrotron radiation wide angle X-ray scattering at BL16B1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF). Biofilms were cut into square pieces of 10 mm 10 mm. The wavelength of the radiation source was 0.124 nm. The sample-to-detector distance was 137 cm, and the irradiation time was 10 s. The scattering patterns were recorded using a SX-165 CCD detector (Rayonix, Illinois, USA), with a resolution was 2048 2048 pixels with size of 80 80 mm2. All data was corrected for the background and air scattering. Fit2D software was used to convert the 2D data into 1D data. 3. Results and discussion 3.1. Morphology of bamboo cellulose nanowhisker Fig. 1 illustrates the micro-morphology of raw bamboo particles and BCNW. We previously reported that the BCNW exhibited a large length-to-diameter ratio (L/D) and had rod-like shapes and network-like structures [27], which were different with whiskers from jute, coconut husk, rice husk and cotton [39]. The fibrillated BCNW had a higher surface area and better cross-linking characteristics when used as nano-fillers. Two types of aggregation of BCNW, namely, closely spaced crystal and isolated single whiskers were observed (Fig. 2). The BCNW had an average length, diameter and L/D ratio of 455 nm, 12 nm and 37, respectively [27]. 3.2. Tensile properties of PLA/BCNW composites Tensile properties of the composites with different BCNW contents and the appearance of untested samples are shown in Fig. 3. With the increase in BCNW content, tensile strength of the composites reduced gradually, similar to the results shown by the research group of Qu [40]. This was mainly caused by a poor
Fig. 2. TEM image of BCNW (65 wt% H2SO4 3 h).
interfacial compatibility between the BCNW and PLA. The hydrogen bonds between polar hydroxyl on BCNW and ester group on PLA might destroy the dense arrangement of PLA molecules in the amorphous region, resulting in PLA molecular segments slipping under tension [41]. In other words, it is of great significance to improve the interfacial adhesion between PLA and BCNW in order to reinforce the mechanical properties of the composites [42]. In addition, further increases of BCNW content caused BCNW agglomeration and a poor BCNW dispersion in PLA matrix, which prevents the formation of cross-linking networks of PLA molecular chains and the rod-like BCNW. As a result, the stress resistance capability of the composites decreased. As to the appearance of the sample, the pure PLA film was more transparent than the rest of the samples. Similar film transparency was observed when the BCNW contents were 0.5 wt%, 3.5 wt% and 4.0 wt%, which indicated a
Fig. 1. SEM images of bamboo cellulose with different treatment. (a) Raw BP; (b) Alkali treated; (c) 55 wt% H2SO4 4 h; (d) 65 wt% H2SO4 3 h.
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to the high rigidity of crystalline region, the latter may be due to the stress concentration caused by the uneven distribution of BCNW. 3.3. WAXS analysis of PLA/BCNW composites
Fig. 3. Tensile properties of PLA/BCNW composites. Note: The different normal letters indicate significantly difference by Duncan's multiple range test at 0.05 level (p < 0.05) (n ¼ 5).
similar crystallization behavior. This is further evidenced by the thermal analysis. The transparency of the film decreased with increasing BCNW content in the cases exceeding 2.5 wt%. The tensile modulus and elongation at break of pure PLA were 411.51 MPa and 17.7%, and they showed an upward trend to 427.72 MPa and 22.27% with increasing BCNW contents of up to 2.5 wt% and 1.0 wt%, respectively. However, both the values of the tensile modulus and elongation at break decreased when increasing the BCNW contents above 2.5 wt% and 1.0 wt%. This trend is different from the previous report which shown that tensile modulus and elongation at break of PP or PE composites decreased steadily when reinforced by natural fiber or other cellulose whisker (without coupling agent) [43]. Tensile modulus represents the stiffness of material and reflects the ability to withstand deformation. For pure PLA film, PLA molecules and molecular segments in the amorphous region were stress relaxed without a BCNW steric effect during the solvent evaporation, which endowed the pure PLA with a relatively high molecular density. The bonding mechanism between PLA and BCNW mainly included the diffusion of BCNW into the PLA amorphous area, the hydrogen bonding, and the Van der Waals force between BCNW and PLA. With the addition of fewer BCNW, interactions between PLA molecular chains were destroyed, leading to a decrease in tensile modulus. The highest tensile modulus of the composites was achieved with 2.5 wt% BCNW content due to the strong heterogeneous nucleation effect. Therefore, the PLA molecules crystallized to form stable and compact crystalline regions, and the deformation resisting ability was enhanced. The tensile modulus dropped largely when the BCNW content exceeded 3.0 wt%, as mentioned previously, as the agglomeration of BCNW hindered the crystallization behavior of PLA. Sanchez-Garcia also reported a similar result; the proper filling proportion of CNW was less than 3 wt% [33]. Notably, the tensile modulus of PLA/BCNW composites showed varying trends with the increase in BCNW content, but other using other CNW biocomposites showed a steady decrease. The reason could be that the intrinsic tensile modulus of bamboo fiber is high, close to 32e34.6 GPa [44]. In addition, BCNW presented a higher length-to-diameter ratio than other nanocrystals [24,45e47]. Thus, there might be more cross-linking network structures consisted of BCNW and PLA macromolecules in the composites, and these structures were not destroyed easily under tension. Tensile deformation mainly took place in the amorphous region of PLA, at the interface between BCNW and PLA, and the small deformation of BCNW, which can be neglected. When the BCNW contents were 2.5 wt% and 4.0 wt%, the elongation at break values of the composites were low. The former decrease may be due
Crystalline structures of PLA and the biocomposites were characterized by synchrotron radiation Wide-Angle X-ray Diffraction (WAXD) (Fig. 4). As the diffractogram described, a-form homocrystallites showed four crystalline diffraction peaks at 2q of 12.2 , 13.8 , 15.7 and 18.4 corresponding to 010, 110/200, 203 and 015 crystallographic planes, respectively [48,49]. In pure PLA and the composites, the crystalline region was mainly composed of homogeneous crystal, especially in the 110/200 crystallographic plane. The BCNW-added diffraction peak became narrow and intense, which gave evidences that the BCNW produced a heterogeneous nucleation effect in the PLA matrix and sped up the crystallization behavior. It was worth mentioning that the X-ray wavelength used in this experiment was 0.124 nm, which is different from the traditional wavelength of 0.154 nm. Thus, the characteristic diffraction peaks of PLA were different from other studies [49,50], even though the lattice dimensions calculated by the Bragg equation were consistent. In the case of 2q at 10.3 , the heterogeneous stereocomplex crystallite of the PLLA and PDLA molecular chain segments were observed in accordance with the reported literature [48]. It could assume that a little amount of PDLA in PLA raw material formed stereocomplex crystallite in the 110 crystallographic plane during solution casting. It is known that a stereocomplex crystallite structure has better thermal stability than homogeneous crystallite. However, the addition of BCNW barely influenced the formation of stereocomplex crystallite in 110 crystallographic plane, and the stereocomplex crystallite diffraction peak at 2q of 17.2 in 300/030 crystallographic plane was not found [51]. When 2q was 13.8 , the average crystallite size was estimated by the Scherrer's equation as follows:
D¼
kl b cos q
(2)
where D refers to the average crystallite size (nm), b presents the full width at half maxima, l is the wavelength of the X-ray (0.124 nm), q is half of the angle for the corresponding peak (2q), and k is the Scherrer crystal shape constant, k ¼ 0.89 [52].
Fig. 4. WAXS patterns of PLA and PLA/BCNW composites. HC and SC indicate the characteristic diffractions of PLA homogeneous and heterogeneous crystallites, respectively. Subscript numbers of HC and SC denote the Miller indices of crystal lattice.
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The size of the homogeneous crystalline grain increased remarkably at 2q of 13.8 after adding BCNW to PLA (Fig. 5), but no obvious change can be seen with the increase in BCNW content. This indicated that the lamella crystallite of pure PLA grew slowly after the formation of crystal nucleus, because the crystallization behavior was influenced by the formation of crystal nucleus and the growth of grain. When BCNW was added, a heterogeneous nucleation effect was induced so that the number of crystal nucleuses increased. In addition, the PLA molecular chains were guided to gather in an orderly manner by the strong negative charge of hydroxyl. Consequently, the crystallization properties of the composites improved through the grain growth. 3.4. Thermal property of PLA/BCNW composites DSC thermograms of pure PLA and PLA/BCNW composites with different proportions are shown in Fig. 6. The thermal parameters, including glass transition temperature (Tg), cold crystallization temperature (Tcc), melting temperature (Tm), cold crystallization enthalpy (DHcc), and the degree of cold crystallinity (Xcc) are tabulated in Table 1. The addition of BCNW to PLA resulted in increases in Tg, Tcc and Tm when compared to pure PLA, suggesting an improvement in its thermal properties. This might be because the presence of BCNW restrained the macromolecular mobility of the PLA chains. As a result, the molecules required more energy to start moving, and needed higher temperatures to convert the PLA from a glassy state to a rubbery state [19]. When the BCNW content increased from 0.5 wt% to 1.5 wt%, Tg decreased from 45.2 C to 41.1 C. This could be explained by the fact that the polar hydroxyl groups on the BCNW surface reduced the cross-linking density of the interfacial region, resulting in a decrease in the total crosslinking density of the PLA matrix. However, when the crystalline region dominated the composites (BCNW content >2.0 wt%), the improved crosslinking density of PLA increased the glass transition temperature. A decrease in Tcc was observed with increasing BCNW content, which denoted that the BCNW induced nucleation of PLA and initiated crystallization at relative low temperatures. In addition, the abilities of nucleation and crystal growth were increased by the heterogeneous nucleation; thus, the crystallinity increased noticeably. As seen in Fig. 6, the pure PLA exhibited a single crystalline melting peak, whereas the composites showed small double melting peaks, especially the highest peak which was achieved with 2.5 wt% BCNW. This was probably because the crystallization behavior of the composites was enhanced, which caused imperfect cold crystallization crystallites to form at low temperatures that formed more perfect crystallites upon heating, causing the melting
Fig. 6. DSC thermograms of pure PLA and PLA/BCNW composites.
peak to shift to higher temperatures [53,54]. Notably, the supercooling degree of pure PLA and the composites with 0.5e1.5 wt% BCNW contents were lower than those with 2.0 wt%. The nucleation rate increased with an increasing super-cooling degree, which meant that the composites with over 2.0 wt% BCNW had a better heterogeneous nucleation effect. However, when the BCNW content exceed 2.5 wt%, agglomeration occurred, and the crystal nucleus failed to increase. These results can be in accordance with WAXS analysis. 3.5. SEM analysis of PLA/BCNW composites Fig. 7 shows the SEM images of pure PLA and PLA/BCNW composites with a 2.5 wt% BCNW content (before and after fracture). The morphological surface of pure PLA was relatively smooth, which could be attributed to the low crystallization ability of pure PLA; most PLA molecular chains remained in an amorphous state during solvent evaporation. However, the surface of the PLA/BCNW composites was uneven and rough, with many grains and gullies. This could be due to the improvement of crystallization after the addition of BCNW, as some PLA molecule chains crystallized to regular orientation while the rest of PLA chains were still in an amorphous state. As to the fractural surface observation, pure PLA exhibited a smooth, clean, brittle rupture as no wire-drawing appearance was observed. This indicated that pure PLA had high tensile strength; when the stress exceeded its yield stress, rupture occurred immediately. However, the fractural surface of PLA/BCNW composites exhibited a wire-drawing appearance while tensile analysis showed no strength improvement. This implied that BCNW toughened the PLA composites, but poor interface interaction limited the efficient toughness [55]. Fig. 7e and f show the combinations of the BCNW and PLA matrix. The embedded (e) form and fiber-bridged (f) form represented the good and poor interaction, respectively. Thus, it can be deduced that the improvement of interfacial characteristic between BCNW and PLA will largely improve the tensile ductility of the composites. 4. Conclusions
Fig. 5. Size of crystallites when 2q ¼ 13.8 .
In this paper, bamboo cellulose nanowhiskers were employed as nano-filler for PLA biocomposites. The rod-like BCNW had an average length of 455 nm, diameter of 12 nm and length-todiameter ratio (L/D) of approximately 37. PLA/BCNW composites with 2.5 wt% BCNW content had a relatively high level of transparency and tensile modulus; excessive BCNW contents lowered
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Table 1 Thermal behaviors of PLA and PLA/BCNW composites. Samples
Tg ( C)
PLA 0.5 wt% 1.0 wt% 1.5 wt% 2.0 wt% 2.5 wt% 3.0 wt% 3.5 wt% 4.0 wt%
40.9 45.2 45.3 41.1 43.6 45.7 40.9 43.2 45.9
± ± ± ± ± ± ± ± ±
0.2a 0.1b 0.2b 0.2c 0.1bc 0.3b 0.3a 0.2bc 0.4b
Tcc ( C)
DHcc (J/g)
96.8 ± 1.2a 103.7 ± 0.9b 101.7 ± 0.7c 100.3 ± 1.1c 94.5 ± 0.6d 96.1 ± 0.6bc 94.3 ± 0.8e 95.4 ± 0.3bc 94.5 ± 0.9d
15.8 27.4 24.5 25.6 24.1 27.7 23.2 27.0 26.9
± ± ± ± ± ± ± ± ±
0.1a 0.1b 0.3c 0.2bc 0.2c 0.5b 0.4d 1.0b 0.9b
16.7 29.8 26.5 27.4 26.5 30.7 25.3 29.9 29.7
± ± ± ± ± ± ± ± ±
DHm (J/g)
Tm ( C)
Xcc (%) 0.8a 1.4b 0.9c 0.8cd 1.0c 0.9bc 1.1d 0.7b 1.4b
156.1 163.4 163.9 162.5 163.6 168.1 162.1 163.5 163.7
± ± ± ± ± ± ± ± ±
1.5a 1.8bc 0.9c 0.9d 1.0c 1.2e 0.9d 0.8c 1.3b
34.67 32.57 32.77 33.46 34.32 34.86 34.61 34.66 34.77
± ± ± ± ± ± ± ± ±
1.6a 1.8b 1.1b 1.5c 0.9a 1.0d 2.1cd 0.7bd 1.8d
Note: The different normal letters (a,b,c,d,e after the values) in the same row indicate significant difference among all treatments at 0.05 level.
Fig. 7. SEM graphs of pure PLA and PLA/BCNW composites with 2.5 wt% content, a: PLA surface, b: PLA fractural surface, c: PLA/BCNW composites surface, d: PLA/BCNW composites fractural surface, eef: micromorphology of BCNW in PLA matrix.
the tensile property. The addition of BCNW mainly affected the homocrystallites of PLA and enlarged the crystallite size. Pure PLA showed a single melting peak whereas PLA/BCNW composites showed a double melting peak. Additionally, the highest glass transition temperature and cold crystallinity of PLA/BCNW composites were found with the BCNW content of 2.5 wt%. However, due to the insufficiency of interfacial compatibility, surface modification between BCNW and PLA is needed to further improve the reinforcement effect.
Conflict of interest The authors declare no competing financial interest. Acknowledgments This work was financially supported by the Natural Science Foundation of Zhejiang Province (No. LY16E030003), the Applied Research Project on Public Welfare Technology of Zhejiang Province
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