A Characteristic Study of Polylactic Acid/Organic Modiﬁed Montmorillonite (PLA/OMMT) Nanocomposite Materials after Hydrolyzing

: In this study, the montmorillonite (MMT) clay was modiﬁed with NH 4 Cl, and then the structures were exfoliated or intercalated in a polylactic acid (PLA) matrix by a torque rheometer in the ratio of 0.5, 3.0, 5.0 and 8.0 wt%. X-ray diffraction (XRD) revealed that the organic modiﬁed-MMT(OMMT) was distributed successfully in the PLA matrix. After thermal pressing, the thermal stability of the mixed composites was measured by a TGA. The mixed composites were also blended with OMMT by a co-rotating twin screw extruder palletizing system, and then injected for the ASTM-D638 standard specimen by an injection machine for measuring the material strength by MTS. The experimental results showed that the mixture of organophilic clay and PLA would enhance the thermal stability. In the PLA mixed with 3 wt% OMMT nanocomposite, the TGA maximum decomposition temperature (T max ) rose from 336.84 ◦ C to 339.08 ◦ C. In the PLA mixed with 5 wt% OMMT nanocomposite, the loss of temperature rose from 325.14 ◦ C to 326.48 ◦ C. In addition, the elongation rate increased from 4.46% to 10.19% with the maximum loading of 58 MPa. After the vibrating hydrolysis process, the PLA/OMMT nanocomposite was degraded through the measurement of differential scanning calorimetry (DSC) and its T g , T c, and T m1 declined. Author Contributions: Data curation, J.-J.H. and H.-J.L.; Investigation, S.-M.H. and J.-J.H.; Methodol-ogy, J.-J.H. and A.-M.Z. All authors have read and agreed to the published version of the manuscript.


Introduction
Recently, biodegradable polymers have been intensively studied due to the depletion of fossil energy and its impacts on the environment [1][2][3]. Biodegradable polymers, defined by the American Society for Testing and Material (ASTM) as being degradable by reacting with microbes, such as germs and fungi, can re-nourish soils during composting. In other words, these polymers possess sustainability and eco-efficiency. Polylactic acid (PLA), one of the polymers, represents the best environmentally friendly product. It has a hydrolyzable ester functional group, enabling its waste to be naturally decomposed to H 2 O and CO 2 during composting [4]. Therefore, PLA is so far the most economical and competitive biodegradable polymer [5][6][7][8][9][10].
PLA is a long-chained thermoplastic polyester, including α, β and γ structures [11,12], as shown in Figure 1. However, compared with traditional polymers, the applications of pure PLA is limited due to its rigid and less flexible characteristics [13][14][15][16][17]. In the industry, the procedures for manufacturing PLA are (1) hydrolysis of corn and wheat, (2) transfer of starch to glucose or maltose, (3) biological fermentation to obtain lactic acid monomer, (4) condensation or ring opening polymerization [18,19]. Organic modified montmorillonite (OMMT) has a large interlayer distance and lipophilicity. The properties not only decrease the surface energy of silicate, but also increase the affinity between OMMT layers and polymer molecular chains, and result in the uniform distribution of OMMT in the polymer substrate to form polymer/layered silicate (PLS) nanocomposites [20][21][22]. In the practical application, even the weight percentage of OMMT in PLS is less than 10 wt% (usually 3-5%), and the product's stiffness, strength, heat resistance and mechanical properties can compete with conventional glass fibers or other mineral filling enhanced composites (filler content ≧ 30%) [23][24][25]. Furthermore, OMMT has outstanding brightness and transparency, because its nano-particle is smaller than the wave length of visible light [26,27].
The manufacture of PLS nanocomposites is called intercalation compounding, including intercalative polymerization and in-situ polymerization [28]. Intercalative polymerization can be categorized as monomer addition polymerization and monomer condensation, while in-situ polymerization can be classified into polymer solution intercalation and polymer melting intercalation. The purpose of manufacturing PLS is to accomplish intercalation dispersion, which keeps the ordered structure of layered silicate, or exfoliation dispersion. Because of that, the disordered structure causes the structure's discrepancy and various properties [29].
So far, several researches have focused on the manufacture and properties investigation of PLA nanocomposites [30][31][32]. Bandyopadhyay et al. found that layered silicate, prepared by polymer melting intercalation, could be used as the barrier for blocking the gas, increasing the decomposition temperature (Tmax) of PLA/organic modified fluorine clay nanocomposites [33]. Sinha et al. investigated the heat deformation test (HDT) of PLA/organic modified synthesized fluorine mica nanocomposites and indicated that heat deformation was substantially improved by a small change of Tm [34]. Paul et al. studied the heat stability of PLA nanocomposites and found that the thermal stability increased primarily but then decreased as the content of OMMT increased [35,36]. The bio-degradation of PLA is usually complex. At first, the unstable bonds on the main chain are hydrolyzed to develop oligomers. Then, oligomers are further decomposed to H2O and CO2 when an appropriate enzyme exists. The above chain reaction of the hydrolysis of the polymer makes a huge impact on bio-degradation [4,9].
It is a major challenge in the chemical process to produce biodegradable nanocomposites with good quality and homogeneous blending quickly and in large quantities. Therefore, in this study, PLA/OMMT nanocomposites were prepared by a torque rheometer and a co-rotating twin screw extruder method-instead of the conventional solution blending method, in which a co-rotating twin screw extruder pelletizing system-was used for mechanical blending and molding by an injection mold to produce PLA/OMMT nanocomposites and specimens quickly and in large quantities. The nanocomposites were subsequently subjected to thermogravimetric analysis to measure their thermal stability. PLA/OMMT nanocomposites were immersed in phosphate solution at 37 °C to monitor the thermal properties before/after hydrolysis by differential scanning calorimetry (DSC), Organic modified montmorillonite (OMMT) has a large interlayer distance and lipophilicity. The properties not only decrease the surface energy of silicate, but also increase the affinity between OMMT layers and polymer molecular chains, and result in the uniform distribution of OMMT in the polymer substrate to form polymer/layered silicate (PLS) nanocomposites [20][21][22]. In the practical application, even the weight percentage of OMMT in PLS is less than 10 wt% (usually 3-5%), and the product's stiffness, strength, heat resistance and mechanical properties can compete with conventional glass fibers or other mineral filling enhanced composites (filler content 30%) [23][24][25]. Furthermore, OMMT has outstanding brightness and transparency, because its nano-particle is smaller than the wave length of visible light [26,27].
The manufacture of PLS nanocomposites is called intercalation compounding, including intercalative polymerization and in-situ polymerization [28]. Intercalative polymerization can be categorized as monomer addition polymerization and monomer condensation, while in-situ polymerization can be classified into polymer solution intercalation and polymer melting intercalation. The purpose of manufacturing PLS is to accomplish intercalation dispersion, which keeps the ordered structure of layered silicate, or exfoliation dispersion. Because of that, the disordered structure causes the structure's discrepancy and various properties [29].
So far, several researches have focused on the manufacture and properties investigation of PLA nanocomposites [30][31][32]. Bandyopadhyay et al. found that layered silicate, prepared by polymer melting intercalation, could be used as the barrier for blocking the gas, increasing the decomposition temperature (Tmax) of PLA/organic modified fluorine clay nanocomposites [33]. Sinha et al. investigated the heat deformation test (HDT) of PLA/organic modified synthesized fluorine mica nanocomposites and indicated that heat deformation was substantially improved by a small change of Tm [34]. Paul et al. studied the heat stability of PLA nanocomposites and found that the thermal stability increased primarily but then decreased as the content of OMMT increased [35,36]. The bio-degradation of PLA is usually complex. At first, the unstable bonds on the main chain are hydrolyzed to develop oligomers. Then, oligomers are further decomposed to H 2 O and CO 2 when an appropriate enzyme exists. The above chain reaction of the hydrolysis of the polymer makes a huge impact on bio-degradation [4,9].
It is a major challenge in the chemical process to produce biodegradable nanocomposites with good quality and homogeneous blending quickly and in large quantities. Therefore, in this study, PLA/OMMT nanocomposites were prepared by a torque rheometer and a co-rotating twin screw extruder method-instead of the conventional solution blending method, in which a co-rotating twin screw extruder pelletizing system-was used for mechanical blending and molding by an injection mold to produce PLA/OMMT nanocomposites and specimens quickly and in large quantities. The nanocomposites were subsequently subjected to thermogravimetric analysis to measure their thermal stability. PLA/OMMT nanocomposites were immersed in phosphate solution at 37 • C to monitor the thermal properties before/after hydrolysis by differential scanning calorimetry (DSC), and finally, the hydrolyzed PLA/OMMT nanocomposites were subjected to a tensile strength test to determine the material strength.

Preparation of Hot Pressed Film Specimen
40 g of PLA was blended with various amounts of OMMT (0, 0.5, 3, 5, 8 wt%) by a torque rheometer (Brabender, Model PL 2000, Hsinchu, Taiwan), and then cut into pellets. The sample was first poured into the feed tank according to the formula ratio in Table 1, and the mixing temperature (200 • C) was set for hot melting with a fixed speed of 50 rpm/min. After 2 min, the OMMT was poured into the feed tank for mixing, and the mixing sample was taken out after 6 min of mixing. After that, the nanocomposite was pressed (200 • C) to form the 0.5-mm thick specimen by a compression molding machine (HAS-100 TON, Hsinchu, Taiwan). The operation procedure of the hot press is as follows: raise the lower heating plate to a distance of about 1 cm from the upper heating plate and preheat the material for 2 min. Next, raise the lower heating plate pressure to 650-675 psi and hold for 30 s; then, raise the lower heating plate pressure to 1400-1450 psi and hold for 10 s. Relieve the pressure of the lower heating plate to an appropriate distance to take the object, remove the covered Teflon cloth and take out the film-like specimen. Finally, this specimen is sealed by aluminum foil and left in a dry cabinet.

Preparation of Injection Molding Specimen
4000 g of PLA was blended with various amounts of OMMT (0, 0.5, 3, 5, 8 wt%) by a co-rotating twin screw extruder pelletizing system (SHJ-36), and the parameters and conditions of the machine operation are shown in Table 2. The nanocomposite was then molded as a specimen by the injection molding machine (FS-90) according to the criterion of ASTM_D638 (Type 1), as shown Figure 2. The specimen was dried by the hot air oven under the temperature setting of 60 • C for 24 h, before being sealed by aluminum foil and left in a dry cabinet. The tensile strength variation of the tested specimen would be measured by the standard material testing system (810 MTS) under the setting conditions of a pulling speed of 5 mm/min and a clamping distance of 115 mm.

Decomposition Test
The pressed PLA/OMMT nanocomposite film with various OMMT amounts (0, 0.5, 3, 5, 8 wt%) was cut into specimens with the size of 30 mm in length, 5 mm in width and 0.5 mm in thickness. These specimens were soaked into the 20 mL phosphate buffer solution (PBS, pH = 7.4) and then put in the constant low temperature shaking bath (Dengyng DKW-40, Hsinchu, Taiwan) under the temperature setting of 37 • C for the designed periods (0, 3, 6, 9 weeks) for the decomposition test by DSC. The specimens were taken out before 3, 6 and 9 weeks later since the beginning of decomposition for the DSC test.

Property Analysis
The value of X-ray diffraction 2θ could be determined through the Thermo ARL X'tra X-ray diffractometer. The interlayer distance (d) could be calculated according to Bragg's equation (nλ = 2d sinθ). For analyzing the PLA/OMMT nanocomposite, the scanning range (2θ) was within 2~15 • , and the scanning speed was 0.04 • /min.

Transmission Electron Microscopy (TEM)
The samples for TEM analysis were prepared by placing the films of PLA/OMMT nanocomposites into epoxy resin capsules and by curing these capsules at 70 • C for 24 h in a vacuum oven. Then, the cured epoxy resin that contained the PLA/OMMT nanocomposites was a microtome with a Reichert-Jung Ultracut-E to form 60~90 nm-thick slices (Optische Werke AG Wien, Austria). Subsequently, one layer of carbon around 10 nm thick was deposited onto the slices, which were placed on 100-mesh copper nets for TEM observation using a JEOL 2010 instrument (Tokyo, Japan) that was operated at an accelerating voltage of 200 kV. The variation of decomposition temperature of the specimen would be measured by the thermogravimetric analyzer, Perkin Elmer TGA 7. The experimental conditions: sample volume was 5 mg; temperature ranged from 30 • C to 500 • C, heating rate was 10 • C/min, N 2 and air flow rate were 20 mL/min.

Differential Scanning Calorimetry (DSC)
The behavior analysis of specimens during the melting and crystallizing processes could be practiced through the differential scanning calorimetry (Perkin-Elmer Pyris 6). The procedures comprised the following six stages.
1st stage: The specimen was kept at 30 • C for 1 min to reach the balanced condition. 2nd stage: The specimen was heated from 30 • C to 170 • C with the speed of 5 • C/min. 3rd stage: The specimen was kept at 170 • C for 1 min to eliminate the thermal history. 4th stage: The specimen was decreased from 170 • C to 30 • C with a speed of 20 • C/min. 5th stage: The specimen was kept at 30 • C for 1 min to reach the balanced condition. 6th stage: The specimen was heated from 30 • C to 170 • C with the speed of 5 • C/min.

Crystalline of PLA/OMMT Nanocomposites
The dispersion of OMMT in PLA could be measured by the XRD, and the result is shown in Figure 3. In Figure 3, the diffraction peak of modified OMMT was located at 2θ = 4.59 (d = 19.3 nm), and there was no significant diffraction peak for the pure PLA (Curve A-0.0%), indicating the non-crystal structure of pure PLA. After mixing PLA with various amounts of OMMT, the primary diffraction peak appeared at 2θ = 2.52 (d = 34.9 nm), indicating that the interlayer distance of OMMT was extended by 15.6 nm and the exfoliation in the PLA substrate had occurred. However, when the amount of OMMT was over 3.0 wt%, the diffraction peak of OMMT at 2θ = 4.59 also appeared. The intensity of this peak increased with the amount of OMMT, indicating that the dispersion of OMMT in PLA was not complete when its amount was larger than 3.0 wt%.

The Morphology of the PLA/OMMT Nanocomposites
After mixing with the torque rheometer, the PLA/OMMT nanocomposites can be examined by TEM to understand the change in the morphology of the composite material.

The Morphology of the PLA/OMMT Nanocomposites
After mixing with the torque rheometer, the PLA/OMMT nanocomposites can be examined by TEM to understand the change in the morphology of the composite material. The original OMMT of about 1 mm in diameter exhibited mostly an exfoliated silicate structure (100~200 nm) by the intercalation effect of PLA polymer chains, and the obvious black filaments or strips can be observed in the TEM image of Figure 4. Whether by adding 3%wt OMMT-(A), 5%wt OMMT-(B) or 8%wt OMMT-(C), it can be dispersed evenly. As the ratio of OMMT increases, the content of layered silicate also increases proportionally. As shown in Figure 5, the TEM image shows the morphology of the PLA/OMMT nanocomposites obtained by co-rotating the twin screw extruder, and the exfoliated silicate of the clay is evenly dispersed in the PLA to become a homogeneous composite material, which is helpful for the improvement of physicochemical and mechanical properties.

Heat Resistance
The heat resistance of the materials could be measured by a thermogravimetric analyzer (TGA). The thermogravimetry analysis results of pure PLA and PLA/OMMT nanocomposites with various OMMT amount are shown in Figure 6 and Table 3. In Figure 6, the degraded temperature of PLA (Tdeg) increased with the content of OMMT. The maximum Tmax (339.08 °C) appeared at the PLA/3 wt %OMMT nanocomposite. However, further increasing the wt% of OMMT would decrease the Tmax of PLA/OMMT nanocomposite. For instance, the Tmax of PLA/8 wt %OMMT was even lower than that of pure PLA. In Table 1, T5 wt% (the temperature at which the 5% weight loss occurred) increased with the adding amount of OMMT in PLA, and it reached the peak value of 326.48 °C at the PLA/3%OMMT nanocomposite. Nevertheless, T5% remained at 326.27 °C when the wt% of OMMT was larger than 3 wt%. This may be due to the fact that the nanocomposite

Heat Resistance
The heat resistance of the materials could be measured by a thermogravimetric analyzer (TGA). The thermogravimetry analysis results of pure PLA and PLA/OMMT nanocomposites with various OMMT amount are shown in Figure 6 and Table 3. In Figure 6, the degraded temperature of PLA (Tdeg) increased with the content of OMMT. The maximum Tmax (339.08 °C) appeared at the PLA/3 wt %OMMT nanocomposite. However, further increasing the wt% of OMMT would decrease the Tmax of PLA/OMMT nanocomposite. For instance, the Tmax of PLA/8 wt %OMMT was even lower than that of pure PLA. In Table 1, T5 wt% (the temperature at which the 5% weight loss occurred) increased with the adding amount of OMMT in PLA, and it reached the peak value of 326.48 °C at the PLA/3%OMMT nanocomposite. Nevertheless, T5% remained at 326.27 °C when the wt% of OMMT was larger than 3 wt%. This may be due to the fact that the nanocomposite

Heat Resistance
The heat resistance of the materials could be measured by a thermogravimetric analyzer (TGA). The thermogravimetry analysis results of pure PLA and PLA/OMMT nanocomposites with various OMMT amount are shown in Figure 6 and Table 3. In Figure 6, the degraded temperature of PLA (T deg ) increased with the content of OMMT. The maximum T max (339.08 • C) appeared at the PLA/3 wt %OMMT nanocomposite. However, further increasing the wt% of OMMT would decrease the T max of PLA/OMMT nanocomposite. For instance, the T max of PLA/8 wt %OMMT was even lower than that of pure PLA. In Table 1, T 5 wt% (the temperature at which the 5% weight loss occurred) increased with the adding amount of OMMT in PLA, and it reached the peak value of 326.48 • C at the PLA/3%OMMT nanocomposite. Nevertheless, T 5% remained at 326.27 • C when the wt% of OMMT was larger than 3 wt%. This may be due to the fact that the nanocomposite structure was in the state of complete exfoliation with the small amount of OMMT (≤3 wt%), and this structure effectively improved the thermal stability of nanocomposites. When the OMMT amount was over the threshold limitation, however, the thermal stability could not be further improved. This suggested that the specific shape in limited regions of the PLA substrate causing the incomplete exfoliation of OMMT was the main reason [35,36].

Tensile Strength
The tensile test is an important index for indicating the stiffness of a material. The MTS testing results of PLA and PLA/OMMT nanocomposites with various OMMT amounts are shown in Figures 7 and 8. Table 2 illustrated the details. In Figure 8 and Table  4, when the amount of OMMT increased from 0 to 3.0 wt%, the elongation of PLA/OMMT nanocomposite samples rose from 4.46 to 10.19%. However, if the amount of OMMT was larger than 3.0 wt%, the elongation of PLA/OMMT nanocomposite samples would decline. This may be due to the fact that the overweight OMMT in PLA would cause incomplete exfoliation. Furthermore, the overweight OMMT would create more nuclei and defects in the crystals, accordingly, during cooling process, and result in the decrease of the tensile strength [37]. The pulling strength at breaking point is the so-called tensile strength. The specimen C-3.0 wt% had the lowest tensile strength, and its color around the breaking point clearly changed. This may be due to the fact that the larger tensile extension led to a serious shrinkage of the specimen, and thus reduced the tensile strength.

Tensile Strength
The tensile test is an important index for indicating the stiffness of a material. The MTS testing results of PLA and PLA/OMMT nanocomposites with various OMMT amounts are shown in Figures 7 and 8. Table 2 illustrated the details. In Figure 8 and Table 4, when the amount of OMMT increased from 0 to 3.0 wt%, the elongation of PLA/OMMT nanocomposite samples rose from 4.46 to 10.19%. However, if the amount of OMMT was larger than 3.0 wt%, the elongation of PLA/OMMT nanocomposite samples would decline. This may be due to the fact that the overweight OMMT in PLA would cause incomplete exfoliation. Furthermore, the overweight OMMT would create more nuclei and defects in the crystals, accordingly, during cooling process, and result in the decrease of the tensile strength [37]. The pulling strength at breaking point is the so-called tensile strength. The specimen C-3.0 wt% had the lowest tensile strength, and its color around the breaking point clearly changed. This may be due to the fact that the larger tensile extension led to a serious shrinkage of the specimen, and thus reduced the tensile strength.

Hydrolysis
In general, the heat flow change during the process, such as specific heat, reaction heat, conversion heat, etc., could be measured directly by the DSC. The sample purity, reaction rate and crystallinity of a specimen could also be measured. In this research, the variation of heat properties before and after hydrolysis, including T g , T c and T m of pure PLA and PLA/OMMT nanocomposites, were measured by DSC.
3.5.1. Before Hydrolysis Figure 9 and Table 5 demonstrated the variations of T g , T c and T m of pure PLA and PLA/OMMT nanocomposites before/after the hydrolysis process measured by DSC. In Table 5, the addition of OMMT would not affect the T g of PLA. If the amount of OMMT was less than 3 wt%, the T c of PLA/OMMT nanocomposites remained at 116.3 • C. However, if the amount of OMMT was higher than 5.0 wt%, the T c of PLA/OMMT would ascend to 121.3 • C. Similar variations appeared for T m at 148.2 • C and 149.5 • C, respectively. This may be due to the fact that the dispersion of OMMT in PLA would depend on the amount of OMMT. The structure of OMMT in PLA was the incomplete exfoliation, also affected by the thermal properties of OMMT.

Hydrolysis
In general, the heat flow change during the process, such as specific heat, reaction heat, conversion heat, etc., could be measured directly by the DSC. The sample purity, reaction rate and crystallinity of a specimen could also be measured. In this research, the variation of heat properties before and after hydrolysis, including Tg, Tc and Tm of pure PLA and PLA/OMMT nanocomposites, were measured by DSC.
3.5.1. Before Hydrolysis Figure 9 and Table 5 demonstrated the variations of Tg, Tc and Tm of pure PLA and PLA/OMMT nanocomposites before/after the hydrolysis process measured by DSC. In Table 5, the addition of OMMT would not affect the Tg of PLA. If the amount of OMMT was less than 3 wt%, the Tc of PLA/OMMT nanocomposites remained at 116.3 °C. However, if the amount of OMMT was higher than 5.0 wt%, the Tc of PLA/OMMT would ascend to 121.3 °C. Similar variations appeared for Tm at 148.2 °C and 149.5 °C, respectively. This may be due to the fact that the dispersion of OMMT in PLA would depend on the amount of OMMT. The structure of OMMT in PLA was the incomplete exfoliation, also affected by the thermal properties of OMMT.    3.5.2. After Hydrolysis Figure 10 and Tables 5 and 6 demonstrated the variations of T g , T c and T m of the PLA/3.0 wt% OMMT nanocomposite, measured by DSC, passing through the hydrolysis process for 0, 3, 6 and 9 weeks. The T g of the PLA/3.0 wt % OMMT nanocomposite decreased slightly with the period of the hydrolysis process. The total variation of T g was low (−0.43 • C) after 9 weeks of hydrolysis process. The variation of T c was −3.7 • C in this period. Figure 8-c indicated that the DSC testing results of PLA/3.0 wt% OMMT demonstrated the double melting peaks, and the strength of T m2 was intensified with the increasing period of hydrolysis process. The lower melting peak, T m1 , decreased gradually, and the higher melting peak, T m2 , increased obviously. The presence of double melting peaks might indicate the existence of two crystals [37]. The higher melting peak, T m2 , might indicate the α-type crystalline with a denser and ordered structure.   Figure 10 and Tables 5 and 6 demonstrated the variations of Tg, Tc and Tm of the PLA/3.0 wt% OMMT nanocomposite, measured by DSC, passing through the hydrolysis process for 0, 3, 6 and 9 weeks. The Tg of the PLA/3.0 wt % OMMT nanocomposite decreased slightly with the period of the hydrolysis process. The total variation of Tg was low (−0.43 °C) after 9 weeks of hydrolysis process. The variation of Tc was −3.7 °C in this period. Figure 8-c indicated that the DSC testing results of PLA/3.0 wt% OMMT demonstrated the double melting peaks, and the strength of Tm2 was intensified with the increasing period of hydrolysis process. The lower melting peak, Tm1, decreased gradually, and the higher melting peak, Tm2, increased obviously. The presence of double melting peaks might indicate the existence of two crystals [37]. The higher melting peak, Tm2, might indicate the α-type crystalline with a denser and ordered structure.  Tables 5 and 6 presented the DSC testing results of PLA/OMMT nanocomposites after several weeks of hydrolysis process. In general, after hydrolysis, the T g , T c and T m1 of the PLA/OMMT nanocomposites would decrease, and the T m2 would increase. In Table 5, except for pure PLA, the T m1 of the same PLA/OMMT nanocomposites decreased with the increasing period of the hydrolysis process. This might be due to the fact that the ester function groups broke randomly on the polymer surfaces at the beginning of the hydrolysis process. With the elongation of the hydrolysis process, the polymer structure of PLA/OMMT nanocomposites was degraded, and its molecular weight decreased. The reason for the stronger T m2 peak is that the crystalline phase of PLA becomes untidy when it is hydrolyzed, and the defective part will be decomposed first, while the undecomposed part will be rearranged more neatly when it reaches a higher temperature by DSC measurement, thus producing a larger crystalline peak and a slightly higher melting point. Moreover, in Table 6, the T m1 of pure PLA increased after the hydrolysis process. The total variation of T g , T c and T m1 of PLA/OMMT nanocomposites with higher OMMT wt% (≥ 5.0%) was larger than others after 9 weeks of hydrolysis process. It prevented the amount of OMMT from benefiting from the hydrolysis process. If the amount of OMMT was larger than 8%, the total variation of T g and T c decreased inversely, due to the weaker thermal properties and uneven dispersion of OMMT.

Conclusions
PLA contains the hydrolyzable ester function group, which could be hydrolyzed into lactic acid, and then metabolized into CO 2 and H 2 O via Kreb's cycle. These properties make PLA a biodegradable, bioresorbable and biocompatible product, which has become the most economical and competitive bio-degradable plastic. This research adopted PLA and OMMT as the substrate and reinforcing material, respectively, to prepare the PLA/OMMT nanocomposites by using a torque rheometer, a hot embossing machine, a co-rotating twin screw extruder, a pelletizing system and an injection molding machine. It was proved by XRD that the interlayer distance of OMMT could be expanded to 34.9 nm. However, the doping effect became worse if the amount of OMMT was larger than 5.0 wt%. For the heat resistance of the material, it was proved from the TGA testing results that the T max and T 5% of the PLA/0.3 wt% OMMT nanocomposite were the highest values, of 339.08 and 326.48 • C, respectively. Meanwhile, for the stiffness of the material, the elongation of the same nanocomposite was 10.19%, the highest value among others, and the tensile strength remained optimal at this ratio. Therefore, doping the specific amount of OMMT in PLA could uplift the thermal stability, elongation and processability of the material.
The PLA/OMMT nanocomposites were soaked in the phosphate buffer solution of 37 • C for a certain period. After the hydrolysis process, the soaked nanocomposites passed the DSC test. The testing results indicated the double melting peak phenomenon of the nanocomposites, and the intensity of the double melting peak (T m2 ) increased with the hydrolysis period, regardless of the amount of OMMT in the nanocomposites. In general, T g , T c and T ml declined, and T m2 ascended after the hydrolysis process. This was due to the fact that the OMMT would benefit from the hydrolysis process, and the hydrolysis period was proportional to the decomposition level. In conclusion, the addition of OMMT in PLA could improve the performance of PLA, reduce the processing period and intensify the decomposition of waste. This may extend its application scopes on green materials.