Characterization of NCMC complex
The alkoxysilane groups are activated by hydrolysis resulting in -Si-OH, which can react with each other and also with other -OH by condensation. As illustrated in Fig. 1b, silane coupling agents have a long chain and at the other side, there are three silicon hydroxyl groups. In the reaction, the –Si-O-R group would transform to silanol groups (Liu et al., 2018), and then it will form -Si-O-Si- bond by condensing with the -OH which is on the surface of the nanocellulose and microcrystalline cellulose (Xing et al. 2018; Wei et al. 2018). The hydrolysis-condensation produces siloxane bond cross-links and consequently leads to the formation of a polysilsesquioxane networks (Battirola et al. 2018). All these reactions allow the anchoring of nanocellulose to the polysilsesquioxane network.
Infrared spectroscopy (IR)
The IR spectra were carried out to characterize the chemical structure of cellulose and to elucidate the changes after chemical treatments. Absorbance peaks located in the range of 3300–3450 cm− 1 are attributed to the stretching of -OH groups, and 2900 cm− 1 are attributed to aliphatic saturated -CH. The absorbance peak of 897 cm− 1 is associated with the cellulose β-glycosidic linkage. Additionally, the spectra absorption peak at 1429 cm− 1 is attributed to a symmetric -CH2 bending vibration and is known as the “crystallinity band” (Zhao et al. 2018; Patel et al. 2016). Spectra of modified NCMCs are shown in Fig. 2 along with spectrum of unmodified MCC for comparison. A significant increase in the intensity of the large band at 3300–3450 cm− 1 is observed in curve b. The terminal group of KH-550 is –NH2, and its stretching vibration peak is also at 3300 cm− 1, leading to enhancement of the characteristic peak in curve b. This result indicates that KH-550 can be condensed into the surfaces of MCC. For curve c, KH-560 has ether bond (-O-), but MCC itself also has ether bond, so the infrared curve of 560 NCMC does not show other unique peaks. For curve d, a new band appeared around 1720 cm− 1, which corresponds to the characteristic peak of carbonyl group (-C = O). The presence of this peak band indicates the KH-570 molecules have been condensed into the surfaces of MCC. However, the bands corresponding to -Si-O-Si- and- Si-O-C- are overlapping with -C-O-C- and are hidden by the large bands from 1000 to 1200 cm− 1, and thus it is difficult to see the existence of -Si-O-Si- and -Si-O-C- via IR (Hajlane et al. 2017).
Scanning electron microscopy (SEM)
The SEM images of different nanocelluloses are shown in Fig. 3. Nanocellulose prepared via 64 wt% sulphuric acid is long fibrous or acicular (Fig. 3a). The long fibrous nanocellulose was interwoven into a single layer network structure with KH-550 (Fig. 3b). Due to the further hydrolysis of silanols which were produced by KH-560 at high temperature, a small amount of silica appears (Shchipunov et al. 2018) and the nanocellulose was bound to the surface of the silica spherical aggregates (Fig. 3c). Each of the spherical aggregates was connected by long fibrous nanocellulose. Similarly, KH-570 treated nanocellulose exhibited a single layer nanocellulose network with a small amount of irregular silica impurities (Fig. 3d).
The comparison of SEM images of MCC and NCMC complex in Fig. 4 supports statements about reaction procedure. Any change in the microstructure would influence the cellulose and overall mechanical properties of corresponding 3D printing material.
MCC is a kind of purified and partially degenerated cellulose, which is hydrolyzed by dilute inorganic acid solution. The amorphous cellulose is destroyed through acid hydrolysis and thus the specific surface area and the reactivity of the resulting MCC increased. Untreated MCC has a long rod shape in the longitudinal direction. There are obvious cracks vertically and irregular grooves. NCMCs were prepared in order to create hierarchical structure combining the MCC and nanocellulose. Its surface is relatively smooth and clean at high magnification (80000x), as it is shown in Fig. 4(a). After modification via nanocellulose and silane coupling agents (KH-550, KH-560, or KH-570), the specific surface area and the reactivity enhanced further. Figure 4(b) exhibits that KH-550 lead to the appearance of reticular structure on the surface of NCMC, which is due to the connection of silicon oxygen bonds making strip nanocellulose and strip nanocellulose link together, and cohesive force making cellulose form a network in layer by layer growth mode. Unlike KH-550 treated nanocelluloses, Fig. 4(c) exhibits that KH-560 treated nanocelluloses are wrapped into the surfaces of impurity spheres. The diameter of the spheres ranges from 200–1000 nm, and different nano spheres and MCC are linked via strip nanocellulose. KH-570 can link strip nanocellulose and rods impurity to form reticular structure and make nanocellulose network grow in the surfaces of NCMC, as it is shown in Fig. 4(d). Nanocellulose net and nanocellulose spheres in the surfaces of NCMC increased the specific surface area and the reactivity of cellulose substantially. NCMC is more active with PLA in high temperature extrusion blending than MCC and other nature cellulose due to its special microstructures.
X-ray diffraction (XRD)
X-ray analyses were performed in order to study the effect of the chemical treatments on the microstructure of MCC and NCMC complex.
The main reflections at 2θ = 14.9°,16.4°, 22.2° and 34.1°,were observed for the sample indicating that MCC were cellulose I type, corresponding to the crystallographic plane (1–10), (110), (200) and (004) (Ko et al. 2018; French, 2014). In the XRD spectra of different samples, the position of the characteristic peaks did not change significantly. This result show that the cellulose crystalline zones of the NCMC were not destroyed and the crystal structures did not change. The treatment methods had little effect on the crystalline zone. The similar patterns of XRD for several curves demonstrated that silane treatment did not change the cellulose structure significantly, which was in accordance with the results of FT-IR.
The relative crystallinity of cellulose is calculated by Segal method according to the follow formula:
$${C}_{\text{r}}=\frac{{I}_{\left(200\right)}-{I}_{\text{a}\text{m}}}{{I}_{\left(200\right)}}\times 100\text{\%}$$
In formula, Cr is the relative crystallinity, I(002) is maximum intensity of (200) lattice diffraction angle, and Iam is scattering intensity of non-crystalline background diffraction when the diffraction angle is 18 degrees (Agarwal et al. 2018).
As shown in Fig. 5 and Table 1, the effect of different silane coupling agents on the relative crystallinity of cellulose in NCMC is significantly different. KH-550 could increase the relative crystallinity of cellulose, while KH-560 and KH-570 decreased the relative crystallinity. Contrast to the previous SEM, KH-550 treatment made the surface of microcrystalline cellulose full of relatively pure and clean nanocellulose.
Table 1
Relative crystallinity of untreated MCC and NCMC prepared via different silane coupling agents and different quantity of nanocellulose
Sample | I(002) | Iam | Cr/% |
MCC | 59550 | 14975 | 74.85 |
550 NCMC (NCMC-50) | 39550 | 9117 | 76.95 |
560 NCMC | 25691 | 10975 | 57.28 |
570 NCMC | 39908 | 14958 | 62.52 |
NCMC-100 | 62550 | 14242 | 77.23 |
NCMC-150 | 48942 | 10500 | 78.55 |
Nanocellulose was made from concentrated acid, and the amorphous cellulose was hydrolyzed preferentially during acid treatment under concentrated acid conditions. The crystal zone had higher resistance to acid and maintained integrity during acidification. The obtained nanocellulose was mostly nanocrystalline, and its crystallinity is higher than that of MCC. The surface of 550 NCMC was filled with filamentous nanocrystals with high crystallinity, resulting in higher crystallinity. Although there was a large amount of nanocellulose on the surface of 560 NCMC, the proportion of cellulose in NCMC decreased due to the presence of a large number of s impurity spheres. Thus, the relative crystalline content decreased. 570 NCMC had a small amount of rods impurity on its surfaces, and the proportion of cellulose was also reduced. The relative crystalline content decreased as well, and the degree of relative crystallinity was relatively lower than that of 560 NCMC because of less impurities.
Figure 5 and Table 1 indicated that, the relative crystallinity of NCMC increased gradually with the increase of nanocellulose sol. According to SEM, under the action of KH-550, filamentous nanocrystals with high crystallinity could grow layer by layer on the surfaces of NCMC, and the relative crystallinity also increased gradually.
Thermogravimetric analysis (TG)
Thermal stability of the untreated MCC and 550 NCMC were studied by TGA as shown in Fig. 6. The initial weight loss of MCC and NCMC below 100 oC was attributed to the release of water. MCC had water absorbability due to its surface defects such as irregular grooves and cracks vertically. Since the surface defect was covered by nanocellulose, NCMC showed reduced water absorption. NCMC had a much lower initial pyrolysis temperature due to the surface sulfate half esters (R-O-SO3H) groups introduced by sulphuric acid in acidolysis process (Sheikhi et al. 2019). The existence of sulfonate decreased the thermal stability of cellulose and thus NCMC was easy to decompose in hot atmosphere (Gonçalves et al. 2018). The decreased thermal decomposition rate of NCMC was attributed to the substitution of some hydroxyl groups by silicon-oxygen bonds. The decomposition residue of NCMC was also significantly increased due to the same reason. In general, sulfuric acid hydrolysis decreased the thermal stability of the cellulose, while silane coupling agent played a certain role in flame retardant.
Specific surface area (SSA)
The BET surface area and Langmuir surface area of different NCMCs are shown in Table 2. MCC was a heterogeneous porous substrate with both external and internal surfaces. Due to the removal of some amorphous cellulose by dilute acid (Liu et al. 2018), MCC had a large number of cracks and irregular grooves on its surface. The inital BET surface area of MCC was 1.8317 m2/g.
The specific surface area of different NCMC was visible increased compared with MCC. Generally, high specific surface area indicates higher reactivity. The BET surface area of 550 NCMC was 2.1549 m2/g. After modification with long acicular sulfuric acid-nanocellulose and KH-550, the surface was covered with nanocellulose network, which increased the specific surface by about 17.64%. The specific surface area of NCMC modified by KH-560 can reach 2.8722 m2/g, which is about 56.80% higher than untreated MCC. The increase of specific surface area was attributed to the existence of a large number of nano-silica spheres, surface nanocellulose bands and nanocellulose networks. The specific surface area of 570 NCMC was 2.4573 m2/g, which was between 550 NCMC and 560 NCMC since the surface of cellulose network was only covered with a small amount of fine nano spherical rod-like silicon impurities.
As can be seen from Table 2, the specific surface area of NCMC did not continue to increase with the increase of nanocellulose quantity. The specific surface of 100-NCMC was 3.2908 m2/g, which was about 79.66% higher than that of untreated MCC. However, the specific surface of 150-NCMC was 2.9499 m2/g. Excess nanocellulose would form a denser network which was the main factor causing the reduction of surface porosity and the specific surface area of NCMC.
Table 2
Specific surface area of untreated MCC and NCMC prepared via different silane coupling agents and different quantity of nanocellulose
samples | MCC | 550 NCMC (NCMC-50) | 560 NCMC | 570 NCMC | 100- NCMC | 150- NCMC |
BET Surface Area (m2/g) | 1.8317 | 2.1549 | 2.8722 | 2.4573 | 3.2908 | 2.9499 |
Characterization of NCMC/PLA composites for 3D printing
Mechanical properties for 3D printing
During high temperature extrusion, not only simple physical blending between PLA and cellulose occurred, but possibly chemical reactions could also happen as Fig. 7a (Zuo et al. 2018; Spinella et al. 2015) and Fig. 8. The theoretical calculation of the combination of cellulose and PLA can be seen in supporting information.
To understand the nature of the composites, we further explored the molecular interactions between a cellulose unit and a PLA fragment by the theoretical method.
The local minimum of the supermolecule was fully optimized by analytic gradient techniques. The methods used were the density functional theory (DFT) with Becke’s three-parameter (B3) exchange functional along with the Lee-Yang-Parr (LYP) nonlocal correlation functional (B3LYP). The standard valence double-ζ basis set augmented with d-type polarization functions and s- and p-type diffuse functions, 6–31 + + G (d, p), was used. Basis set superposition error (BSSE) was corrected for the calculation by applying Boys and Bernardi’s counterpoise procedure (CP). The Gaussian 09 program was used in the calculations. The interaction energy (ΔE) was calculated from the expression:
ΔE = E(ABC)abc—E(A)abc—E(B)abc—E(C)abc
where E(A)abc denoted the energy of the fragment A with the enlarged basis abc.
Considering the computational efficiency, we selected five units of PLA polymer and one unit of cellulose (French, 2017) to investigate the interaction between cellulose and PLA. We constructed three H-bonding models to optimize. Three hydroxyls on C2, C3 and C6 atoms (shown in scheme 1) in cellulose interact with a carbonyl on PLA polymer, respectively.
After optimization, three interaction modes were obtained and the optimized structures of the supermolecules were illustrated in Fig. 8. It was clear that hydrogen bonds were formed. In mode 1, the H…O distances between two hydroxyl hydrogen atoms of cellulose and two carbonyl oxygen atoms of PLA fragment were 1.989 Å and 1.973 Å, respectively, and the total hydrogen bonding energies were calculated to be -9.06 kcal·mol− 1. In mode 2, the H…O distances between two hydroxyl hydrogen atoms of cellulose and a carbonyl oxygen atom of PLA fragment were 1.884 Å and 2.188 Å, respectively, and the total hydrogen bonding energies were calculated to be -6.81 kcal·mol− 1. In mode 3, the H…O distance between the hydroxyl hydrogen atom of cellulose and the carbonyl oxygen atom of PLA fragment was 1.937 Å, and the hydrogen bonding energy was calculated to be -5.14 kcal·mol− 1.
These results indicates that the H-bonding between cellulose molecules and PLA polymers can be formed, which makes these molecules combine steadily in the system.
The results showed that, when only MCC was added to the PLA, a decrease in the mechanical properties including the tensile properties and bending properties of the 3D printing test samples was promoted. Cellulose could act as a reinforcing phase in PLA, however, MCC was difficult to bond closely with the PLA matrix due to its poor interfacial compatibility with PLA and larger average molecular. Thus the orderly arrangement of polymer chains was hindered. As a result, addition of a small amount of MCC had opposite effect, and the tensile strength and bending strength of the composite declined by 11.6% and 3.3% respectively. Also, MCC was a rigid molecule, which hindered the flexible activity of the polymers and led to the decrease of elongation at break.
For NCMC, due to the relatively large specific surface area and a large amount of unreacted hydroxyl groups (-OH), silicon hydroxyl groups (-Si-OH) remaining on the surface, it would have more thorough reaction with PLA and integrate the two components more closely, as shown in Fig. 7b. The existence of silane coupling agents layer also builded up bridges between the NCMC and PLA molecular chains via hydrogen bonding and physical entanglements. Besides, due to the compatibilization effect of the different silane coupling agent, the surface wetting ability and interfacial interaction of PLA molecules and cellulose improved, as did the combination of the two phases (Wang et al. 2019).
As shown in Fig. 9a-9b, NCMC prepared by three silane coupling agents can greatly improve the mechanical properties of the composites at NCMC loading of 0.5 wt%. Nanocellulose covered the surface of MCC had high specific surface area and high reactivity, which can make NCMC react with PLA molecule completely and become a three-dimensional network molecule via high temperature extrusion blending. Based on the FT-IR results, coupling agent grafted onto the MCC surface through C-O-Si linkages and hydrogen bonding possibly had interactions with the PLA backbone. The silylated nanocellulose which is on the surfaces of NCMC linked the PLA molecular chains in amorphous regions and crystalline regions, and it forced high-elastic deformation of the tangled PLA molecules and the slippage of spherical crystallites in stretching process, respectively (Qian et al. 2017). Elongation at break of NCMC/PLA was also improved. 550 NCMC can improve tensile strength, bending strength and the elongation at break of the composites by 20.2%, 60.2% and 31.1% compare with pure PLA, respectively. For 560 NCMC/PLA and 570 NCMC/PLA materials for 3D printing, higher specific surface area and the existence of nano impurity nano silicon can further improve the properties. 560 NCMC had the highest specific surface area and much more nano-silicon spheres with regular shapes, and thus the properties of corresponding materials were higher than those of 570 NCMC/PLA.
As shown in Fig. 9c-9d, different quantities of nanocellulose in NCMC had a significant impact on the mechanical properties. Corresponding to the specific surface of NCMC measured previously, NCMC-100/PLA had the best mechanical properties since nanocellulose on NCMC and PLA can form more complex three-dimensional network molecule. Tensile strength of the composite was increased by 40.3% and bending strength increased by 84.1% compare with pure PLA. However, with the increase of the content of -C-O-Si-, -O-Si-O-Si-O- linkages and hydrogen bonding, the elongation at break increased gradually. NCMC-150/PLA exhibited better elasticity, with the elongation at break increased by 34.3%.
In fact, in test samples composed of orthotropic layers, the principal direction of highest stiffness is the printing orientation direction (Ferreira et al. 2017), which means changing the printing orientation along the direction of tension or bending can further improve the corresponding test performance.
Application in 3D printing.
Cellulose and PLA are biodegradable materials, and NCMC/PLA composites prepared from cellulose and PLA is also environmentally friendly to develop biodegradable and biomass-based 3D printing materials. To illustrate the potential of the NCMC/PLA for 3D printing, different 3D objects of NCMC/PLA were fabricated. MCC/PLA 3D printing wire rods are opaque yellow. The presence of the unmodified MCC was detrimental to 3D printing. During the continuous printing process, because the residual large particles of MCC in the matrix are easy to agglomerate, the printing head was blocked and causing printing interruption which was not conducive to the continuous production of printed products. Warping and expansion at the edge of the samples were observed since the precision of the MCC/PLA 3D objects was slightly lower and their appearance line feeling was obvious and completely opaque. The NCMC/PLA 3D printing wire rods are light yellow and translucent with smooth surfaces, and the printed products had wax texture and their line feeling was not relatively obvious without further polishing. The appearances of 3D printed objects with MCC/PLA and NCMC/PLA composites are shown in the Fig. 8e. Due to low addition and more thorough reaction of NCMC with PLA during production processes, no blockage of printer nozzle was found in continuous production of NCMC/PLA. The thermoplastic composite had the ability to rapidly manufacture into complex shapes, which was beneficial for high accurate 3D printing. Besides, higher mechanical properties enable the materials to be used for printing complex and practical products such as furniture. Relatively simple production process also show that NCMC/PLA is a kind of 3D printing materials with good effect, ideal performances and practical value.
Scanning electron microscopy (SEM)
The surface of pure PLA should be relatively smooth, which can be attributed to the relative low crystallization ability of pure PLA (Qian et al. 2018). Figure 10a shows that there were some voids at the interface of PLA as well as tiny holes left by the pullout MCC, revealing the weak interfacial adhesion between the MCC and PLA. A few thin fragments and micro-cracks appeared in the fracture surface of 550 NCMC/PLA. 560 NCMC made the fracture surface of materials rough and a few uniform round holes appeared. The surface of the 570 NCMC/PLA was uneven and rough, with many nano grains. Generally speaking, the fracture surfaces of NCMC/PLA were relatively regular and smooth. It is believed that the orientation of silane coupling agents forced the slippage or deformation of PLA spherulites and thereby guided the alignment of PLA crystals during the forming and drawing process (Wang et al. 2019). Besides, the hydrophilic segment such as –OH or –Si-OH grafted onto the MCC interacted with PLA through hydrogen bonding, improving the compatibility between the cellulose and PLA and thus NCMC can be well-dispersed in the PLA matrix. As shown in Fig. 10e and 10f, along with the increase of the nanocellulose quantity, the regularity of the obtained material fracture surfaces was significantly improved due to more –OH or –Si-OH on the surface of NCMC.
Thermogravimetric analysis (TG)
TG curves of pure PLA and different methods treated NCMC/PLA are displayed in Fig. 11. It is clear from the figure that the NCMC prepared from different methods significantly affected the thermal decomposition of PLA matrix in a positive way.
Although the concentrated sulfuric acid used in the preparation of nanocellulose can introduce sulfonic acid group with low thermal stability, the extrusion temperature was higher than 160 oC during the preparation process of NCMC/PLA 3D printing materials. Sulfonates affecting thermal stability have been basically decomposed completely, while the silicon-oxygen bond in the material can play the role in flame retardant. Due to the high dissociation energy of Si-O bonds (ca. 460 kJ/mol) in comparison with typical organic bonds C-O (ca. 350 kJ/mol) or C-C (ca. 340 kJ/mol), polymers containing silicon-oxygen bonds are commonly known to be more thermally and chemically resistant materials than typical organic polymers (Januszewski et al. 2018). Improved thermal stability of different NCMC followed the order of 560 NCMC/PLA, 570 NCMC/PLA, and 550 NCMC/PLA. According to the previous analysis of SEM and XRD of NCMC, more surface impurity silicon resulted in better thermal stability.
By analyzing the two TG curves of NCMC-50/PLA and NCMC-150/PLA, it is concluded that with the increase of nanocellulose on the surface of NCMC, the steadily improved thermal stability of cellulose was attributed to more silicon-oxygen bonds and nanocellulose with high crystallinity.
X-ray diffraction (XRD)
XRD patterns of PLA and different methods treated NCMC/PLA samples obtained from the experiments are shown in Fig. 12a-12b. Two sharp diffraction peaks located at 2θ = 16.6o and 19.1o were correlated to the (200) (110) and (203) planes of PLA α-crystals (Lizundia et al. 2016; Mangin et al. 2018). The positions of the diffraction peaks of the samples were basically unchanged, which indicates that the crystal form of the composites did not change. The intensity of the diffraction peak often reflects the crystallinity of the measured samples, and it is evident that the addition of different NCMC caused a gradual increase of crystallinity.
Nanocellulose and MCC can form hydrogen bonds with PLA chains, and they could act as nucleating agents for crystallization of the PLA (Dos Santos et al., 2017; Frone et al. 2013). Improved crystallinity for NCMC/PLA followed the order of 560 NCMC/PLA > 570 NCMC/PLA > 550 NCMC/PLA and NCMC-100/PLA > NCMC-150/PLA > NCMC-50/PLA.NCMC/PLA composites containing NCMC with higher specific surface area exhibited higher crystallinity since more hydroxyl groups and silicon hydroxyl groups were exposed on the surface of those NCMC and contributed to increased contact area between the surfaces of NCMC and PLA. At the same time, a small amount of silicon can also be acted as a nucleating agent in PLA since the hydroxyl groups in silica can interact with the PLA matrix. The mechanical properties of PLA with high crystallinity tended to be better.
Differential scanning calorimetry (DSC)
As shown in Fig. 12c-12d, DSC was used to investigate the glass transition, melting phenomena and crystallization of PLA in composites to study the effect of different NCMCs on the thermal properties of the produced PLA, as shown in Fig. 11c-11d. The pure PLA showed an endothermic peak corresponding to glass-transition (Tg) and an endothermic peak due to melting (Tm) at 65.92 and 165.88 oC, respectively. With the addition of NCMC, the melting peak of Tg moved forward, and the peak became smooth at the same time Tg of the composites was associated with cooperative motion of long-chain segments (Zhou et al., 2018). The composites had Tg from 52.3 to 56.1 oC, which were lower than that of PLA. Nanocellulose and -O-Si-O-Si-O- increased free volume and flexibility of polymeric chains. As a result, NCMC-150/PLA had relatively minimum Tg and maximum elongation at break. Figure 11 also shows that the Tm increases slightly (166.1 oC -166.9 oC) with different NCMC/PLA composites. These results indicated that NCMC did not affect the processing parameters of the materials and 3D printing parameters.
Table 3
DSC results of PLA and NCMC/PLA composites for 3Dprinting
Sample | Tg/ oC | Tm/ oC | ΔHm/(J/g) | Xc/% |
PLA | 65.92 | 165.88 | 38.86 | 41.47% |
550 NCMC/PLA | 53.70 | 166.10 | 41.82 | 44.86% |
560 NCMC/PLA | 56.11 | 166.91 | 42.89 | 46.01% |
570 NCMC/PLA | 52.33 | 166.24 | 42.03 | 45.09% |
NCMC-50/PLA | 53.70 | 166.10 | 41.82 | 44.86% |
NCMC-100/PLA | 54.40 | 166.82 | 43.84 | 47.02% |
NCMC-150/PLA | 53.81 | 166.55 | 42.94 | 46.06% |
The crystallinity (Xc) of PLA in the materials was calculated using the formula.
$${X}_{\text{c}}\left(\text{\%}\right)=\frac{{\varDelta H}_{\text{m}}}{{\varDelta H}_{\text{m}}^{0}}\times \frac{100}{\omega }$$
In the formula, \({\varDelta H}_{\text{m}}\) is the experimental melting enthalpy (J/g), \({\varDelta H}_{\text{m}}^{0}\) is the melting enthalpy of 100% crystalline PLA (93.7 J/g) and ω is the weight fraction of PLA matrices in the composites (Ho et al. 2015).
T g, Tm, \({\varDelta H}_{\text{m}}\) and Xc of PLA and the NCMC/PLA were summarized in Table 3. The crystallization trends of PLA in composites are in good agreement with those previously measured by XRD.