**2. Experimental**

#### *2.1. Experimental Materials*

A 2002D grade of PLA was used in this present study. It was from Natureworks LLC (Minnesota, MN, USA), with a number molecular weight of 100,000 g/mol. The modification method to form MPLA was similar to that in previous studies [27,28], and it involved grafting maleic anhydride to PLA (thus, maleic anhydride-grafted PLA was produced). The grafting rate of MPLA was about 1.2%, and the tensile strength of MPLA was 37.08 ± 0.7 MPa.

### *2.2. Preparation of Composites*

PLA/CSDG and MPLA/CSDG composites were prepared, with the CSDG concentration varied from 10 to 50%. The proportion of components in each material is shown in Table 1. The steps of preparing the composite materials are as follows:


The prepared composite was hot-pressed using a vulcanizer at a temperature of 180 ◦C for 10 min. The material was taken out of the vulcanizer to left to cool. A custom-made cutter was used to cut the material and form a dumbbell-shaped sample, which was used for testing and characterization.


**Table 1.** Composition of PLA/CSDG and MPLA/CSDG composites.

#### *2.3. Fourier Transform Infrared Spectroscopy*

The infrared absorption spectrum of a material can be obtained by detecting infrared absorption, also known as molecular vibration. Before the test, samples were dried at 80 ◦C and were cut into pieces. The spectral analysis of the samples was characterized by Fourier transform infrared (FTIR) (NICOLET 6700, Thermo Scientific, Waltham, MA, USA) spectroscopy to obtain the characteristic peaks of the samples.

For the test parameter setting, the test wavelength range was 4000–500 cm−<sup>1</sup> .

#### *2.4. Mechanical Properties*

A microcomputer-controlled electronic universal testing machine (FBS-10KNW, Xiamen Forbes Testing Equipment Co. Ltd., Xiamen, China) in plastic-film tensile test mode was used to determine the mechanical properties of the PLA/CSDG and MPLA/CSDG composites. Tensile strength referred to the ratio of the maximum load P before the test sample broke to the cross-sectional area of the test sample under a specific test temperature and humidity. A tensile load was applied along the axial direction. Tensile strength is usually expressed as *δ<sup>t</sup>* , and its calculation formula is:

$$
\delta\_l = P/(b \times d) \tag{1}
$$

where *P* = maximum breaking load, N; *b* = sample width, mm; *d* = sample thickness, mm.

The elongation at break was the relative elongation of the test sample when it broke, and was calculated by the following formula:

$$
\varepsilon\_l = (F - G) / G \times 100\% \tag{2}
$$

where *G* = distance between marking lines of sample, mm; *F* = distance between marking lines when sample broke, mm.

The characterization parameter setting is as follows: sample in dumbbell shape; 1000 N sensor range; 2 mm/min set pulling speed.

#### *2.5. X-ray Diffraction*

X-ray diffraction (XRD) technology is the most important structural test method. The prepared sample was analyzed using XRD (D2 PHASER, Bruker, Germany), and corresponding peaks for PLA/CSDG and MPLA/CSDG composites were obtained.

The test parameter setting is given as follows: working voltage = 40 kV, working current = 30 mA, scanning area = 0.02◦/s, and step length = 10–90◦ .

#### *2.6. Morphology Characterization*

Scanning electron microscopy (SEM) was used to observe the fracture surface morphology of tensile samples taken from composite materials. SEM is an excellent technique for obtaining morphological information by scanning the surface with electron beams. It can generate high-resolution 3D surface images to describe the surface structure of samples. The generated SEM images clearly depicted the morphology of PLA/CSDG and MPLA/CSDG composites and the distribution of CSDG in them. Before SEM tests were conducted, samples were attached to a support and sputtered with a coating or layer of gold. Then, SEM (VEGA3SBU, TESCAN, Brno, Czechia) was operated to characterize the cross-sectional morphology of PLA/CSDG and MPLA/CSDG composites.

The SEM test parameter setting is indicated as follows: time of spraying gold on samples, 30 s; working current, 5 mA; scanning or acceleration voltage, 3 kV.

#### *2.7. Thermal Analysis*

Differential scanning calorimetry (DSC) or a thermal analyzer (DSC200 F3, NETZSCH, Selb, Germany) operated under nitrogen gas was used to test the crystallization temperature and crystallinity of composite films. The sample to be tested was placed in a crucible, and a reference material was placed in another crucible. Then, the two crucibles were heated at the same rate. The sample had to undergo melting, crystallization, oxidation, and degradation processes, so as to obtain the following parameters: T crystallization, T oxidation, T melting, and T decomposition.

$$X\_{\mathbb{C}} = (\frac{\Delta H}{\Delta H\_0 \times wt\%}) \times 100\% \tag{3}$$

In Equation (3), *X<sup>C</sup>* = degree of crystallinity, ∆*H* = test sample heat of fusion, ∆*H*<sup>0</sup> = heat of fusion of a pure PLA substrate when the crystallinity is 100%, and *wt*% = percentage of PLA in the sample. PLA with 100% crystallinity has a theoretical enthalpy of 93.7 J/g.

The test parameter setting is given as follows: heating rate of 25 ◦C/min (heating was from room temperature to 200 ◦C); maintaining 200 ◦C constant for 10 min; reduction of temperature to 2 ◦C; finally, increase of temperature to 200 ◦C at a rate of 10 ◦C/min. The sample crystallization temperature was measured.

### *2.8. Thermogravimetric Analysis*

Thermogravimetric analysis (TGA) is a method used to study the composition and thermal stability of materials. Results were obtained by measuring changes in the sample weight. After data were obtained, DTG (differential TG) could be analyzed to determine the degree of change in the sample weight as the temperature increased (i.e., mass loss rate). A thermal analyzer (STA 409PC, Netzsch Company, Erlangen, Germany) was used, wherein both the test samples and the balance were under nitrogen flow. Subsequently, TGA curves were obtained.

The test parameter setting is given as follows: nitrogen was used as shielding gas with a flow rate of 70 mL/min; starting from room temperature, the temperature was increased at a rate of 10 ◦C /min, and the heating was stopped when it reached 600 ◦C. Origin software was used to make a diagram for temperature–mass loss ratio and for temperature–DTG data. At the same time, the following were listed: initial degradation temperature, maximum temperature, and total weight loss rate.

#### *2.9. Oxygen Barrier Properties*

The penetration of small gas molecules through defect-free films is a molecular diffusion process. First, small gas molecules would be adsorbed and dissolved on the film. Under the action of concentration gradient, the gas molecules would then diffuse from a high concentration to a low concentration, when the gas concentration increases to a certain level. Finally, they would diffuse out on the other side of the film. A differential pressure gas permeation meter (VAC-V2, Labthink Instrument Co. Ltd., Jinan, China) in proportional mode was used to test PLA/CSDG and MPLA/CSDG composites in terms of their oxygen barrier performance, which is expressed as oxygen permeability. Its unit was cm3/m<sup>2</sup> d Pa.

The test parameter setting was as follows: temperature of test chamber was set to 25 ◦C, film thickness = 0.08 mm, test environment contained oxygen + nitrogen, judgment ratio = 10%, and relative humidity was 100%.

#### *2.10. Water Vapor Barrier Properties*

A water vapor transmission (WVT) rate test system (W3/060, Labthink Instrument Co. Ltd.; Jinan, China) in standard mode was used to test the water vapor barrier properties of PLA/CSDG and MPLA/CSDG composites. The WVT rate was calculated using the equipment software. The WVT performance includes two meanings: water vapor permeability (WVP) and WVT coefficient. These two meanings are different, but they can both be used to indicate the ability of water vapor to pass through a certain material. The WVT rate indicates the weight of water vapor passing through the material in a certain period of time, under certain temperature and humidity conditions. The WVT coefficient is the standard value of WVT calculated by the system. It is used for comparing different test results and standard values of different samples. The result was expressed as WVP in g/m2/d.

The test parameter setting was as follows: temperature of test chamber, 25 ◦C; film thickness, 0.08 mm; test interval, 30 min; humidity, 50%.

#### *2.11. Contact Angle Test*

A contact angle measuring instrument (JC2000D, Shanghai Zhongchen Digital Technology Equipment Co. Ltd., Shanghai, China) was used to test and characterize PLA/CSDG and MPLA/CSDG composites in terms of their hydrophilic properties. Contact angle is a measure of the degree of wetting a solid with a liquid. If the test value is less than 90◦ , the surface of the sample is hydrophilic, and the smaller the value, the better the wettability of the liquid to the sample. If the test value is greater than 90◦ , the sample surface is hydrophobic, and the larger the value, it means that it is not easy for the liquid to wet the sample.

A microsyringe was used to extract 2 µL of distilled water, which was dropped on a sample surface. Contact angles were recorded following a five-point fitting method. For all samples, three measurements were taken, and the average was calculated.

The test parameter setting was as follows: sample thickness, 0.08 mm; test time was 0, 3, and 5 s; repetition for each test, 3 times.

## *2.12. Water Absorption*

Water uptake (WU) was measured according to standard methods [29]. Samples were cut into 2 cm × 2 cm and dried to constant weight in an oven at 105 ◦C. Before tests were done, samples were weighed and soaked in distilled water for 24 h at room temperature. Then, they were taken out of the water, rid of adhering water drops, and weighed. Water absorption or *WU (g/g)* was calculated using Equation (4). The sample weight before soaking was represented as *m*<sup>0</sup> (g), and *m<sup>f</sup>* was the sample weight after soaking (g).

$$\text{WLI}(\text{g/g}) = \frac{m\_f - m\_0}{m\_0} = 100\% \tag{4}$$

#### *2.13. Degradation Performance Test*

The dimension of samples was 2 cm × 2 cm. They were dried in an oven at 105 ◦C until constant weights were recorded. Before the degradation test was conducted, samples were weighed and buried in soil at room temperature. The test cycle was 180 days. Samples were retrieved from the soil every 30 days, and they were washed, dried, and weighed. The degradation rate was calculated according to Equation (5). *C*<sup>0</sup> was the initial sample weight (g), and *C<sup>t</sup>* was the sample weight after the degradation (g).

$$\text{degradation rate } \left(\%\right) = \frac{\mathcal{C}\_0 - \mathcal{C}\_t}{\mathcal{C}\_0} \times 100\% \tag{5}$$
