1. Introduction
Extracted from Radix Paeoniae Alba,
Bletilla striata polysaccharide (BSP), also known as
Bletilla hyacinthina gumis, is a polymer with low toxicity and high safety. The main components of BSP are glucose and mannose [
1]. The naturally extracted BSP is a powder with a slight sweetness. The powder can dissolve in water to form a viscous solution, which can be used as a film-forming material. As a natural polymer material, BSP exhibits good biocompatibility and biodegradability [
2], having excellent application prospects [
3,
4]. BSP has high safety as a food additive or ingredient, and its unique properties make it widely used in the food industry [
5]. Related research shows that using the excellent film-forming properties of BSP, the preparation of fruit coating film preservative can reduce the evaporation of water and achieve the purpose of preservation [
6]. BSP has good anti-inflammatory and acid-resistance ability, and is less affected by factors such as pH, inorganic ions and temperature, and can improve the stability of the emulsified product. With the development and clinical needs of modern pharmaceutical technology, BSP is also widely used in pharmacology and clinical practice as mediation therapy, coupling agent, etc. Cai et al. [
7] developed a white ultrasonic medical couplant, and compared it with paraffin oil emulsion and Japanese ultrasonic coupling agent, found that the main quality index of white enamel ultrasonic coupling agent exceeds paraffin oil emulsion, which is superior to Japanese products.
Polylactic acid (PLA) is a natural polymer material with a wide range of sources. PLA can be regenerated, is completely biodegradable and nontoxic and it exhibits good biocompatibility and biodegradability [
8,
9,
10]. In addition, PLA has a wide range of applications and is easy to process. It is a green polymer with excellent performance and can blend with other natural polymer materials to form fully biodegradable composites [
11,
12,
13]. With the theme of environmental protection and sustainable development, PLA has attracted increasing attention and has been extensively studied in the fields of industry and agriculture [
14,
15], biomedicine [
16,
17] and food packaging [
18,
19,
20]. At present, PLA is considered one of the materials with the most promising development prospects [
21]. Ma et al. [
22] obtained the PLA/Fe
3O
4-AZM microspheres by emulsification-solvent evaporation technology, and the sustained release effect was obvious. Tan et al. [
23] obtained a PLA/PCL-PVA-CS-Ag nanofiber auxiliary material by electrospinning technology and achieved good antibacterial effects. Swaroop et al. [
24] used polylactic acid as a raw material to prepare magnesium oxide nanoparticle-enhanced biofilms by a solvent casting method. The prepared film is transparent, can shield ultraviolet radiation, and has excellent antibacterial properties. It is an excellent food packaging material. However, the high price of PLA has constrained its development [
25]. In addition, other factors such as low heat-resistance, poor hydrophilicity, high brittleness, insufficient elasticity, low degradation rate, varied degradation cycle, low mechanical strength, poor moisture vapor barrier property, low crystallization rate and low thermal stability [
26,
27,
28,
29] greatly limit the application scope of PLA. Therefore, physical modification, chemical modification and composite preparation have been carried out to modify PLA materials to improve their performance and reduce cost.
Wu et al. [
30] used elastomers and PLA to prepare composites. The obtained materials have the advantages of a short plasticizing time, high melt fluidity, and high tensile elongation at break, but they have the disadvantage of significantly decreased tensile strength. Pan et al. [
31] prepared a polyacrylic acid grafted starch/PLA composite using a graft copolymerization and blending technique. The obtained products have significantly increased tensile strength and hydrophilicity but exhibit decreased degradation and thermal performance. Similarly, Wu et al. [
32] observed increased tensile strength and elongation at break for a polylactic acid grafted starch/PLA composite but it had a significantly decreased initial thermal decomposition temperature and deteriorated thermal stability. Tao et al. [
33] prepared aliphatic polycarbonate (PPC)/PLA composites by solution casting. The obtained materials exhibit an increased biodegradation rate that improves the biodegradability of PLA but also exhibit a lower tensile strength and modulus compared with PLA. As such, the thermal properties of the materials are not improved. Zhang et al. [
34] modified PLA with biodegradable hyperbranched polyester amide ester (HBP) by melt blending. The obtained PLA composites have increased tensile strength and elongation at break, excellent toughness but poor thermal stability. Li et al. [
35] used tetrabutyl titanate to modify a PLA/starch composite. The obtained materials exhibit enhanced flexibility and degradation performance but show no improvement in thermal properties. Zhang et al. [
36] prepared and characterized PLA/cellulose nanocrystalline composites. The obtained materials have increased tensile strength but decreased elongation at break and deteriorated toughness. Feng et al. [
37] blended polyurethane with PLA. The obtained materials have the advantages of high elongation at break, high impact strength and high tensile strength but exhibit no improvement in thermal properties. Thus, research has shown that using natural polymer materials such as starch or cellulose to modify PLA can only improve either mechanical properties or thermal properties but not both. Therefore, finding a suitable material that can improve both the mechanical properties and thermal properties of PLA is important. Compared with other natural polymer materials, BSP has the advantages of wide availability, low cost, excellent toughness, and natural degradability. The blending of BSP and PLA can not only reduce the cost but also improve the performance of PLA materials. In this study, BSP is selected as the material for modifying PLA. The results show that the obtained composite exhibits improved thermal properties and improved mechanical properties, which is a new highlight in PLA blend modification research.
In this study, BSP was used as the modified material, PLA was used as the raw material, and 1,4-dioxane and ultrapure water were used as solvents to physically blend PLA and BSP to prepare BSP/PLA composite films. The optimum ratio was deduced to achieve the best modification effect. The glass-transition temperature (Tg) of the composite film was determined by dynamic thermomechanical analysis (DMA). The crystal morphology was determined by X-ray diffraction (XRD). The thermal decomposition temperature was determined by thermogravimetry (TG). The thermal stability was determined by differential scanning calorimetry (DSC) and TG. The morphologies of the cross section and surface were observed by scanning electron microscopy (SEM). In addition, the mechanical properties of the composite film were tested on a tensile testing machine.
3. Materials and Methods
3.1. Materials
BSP was purchased from Xi’an Tianrui Biotechnology Co., Ltd. (Xi′an, China). Activated carbon was purchased from Jiangsu Agnes Environmental Technology Co., Ltd. (Jiangsu, China). Ethanol and 1,4-dioxane were purchased from Tianjin Damao Chemical Reagent Co. (Tianjin, China). PLA was prepared in our own laboratory, and the average molecular weight was 11.0 × 104. A glass plate with an inner diameter of 20 cm × 20 cm was prepared in our laboratory.
3.2. Purification of BSP
Fifty grams of extracted BSP was dissolved in 1000 mL of distilled water and continuously stirred with a glass rod. After BSP was completely dissolved, the mixture was filtered with gauze 3 times and the filtrate was collected by vacuum filtration. A small amount of activated carbon was added to the collected filtrate, and the mixture was heated with stirring for 1 h. After hot suction filtration, the obtained BSP solution was heated and concentrated to 400 mL. After the solution cooling, ethanol was slowly added with stirring until the glue was completely precipitated. After setting overnight, the precipitate was collected by suction filtration and dried at 80 °C to yield 35 g of BSP.
3.3. Preparation of BSP/PLA Composite Films
The BSP/PLA composite film was prepared by a solvent volatilization technique. Two grams of PLA was added to each of two 100 mL conical flasks. After 50 mL 1,4-dioxane was added, the mixture was stirred for 12 h until complete dissolution of PLA. BSP in amounts of 0%, 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4% and 100% of the amount of PLA (referred to hereafter as PLA1, PLA2, PLA3, PLA4, PLA5, PLA6, PLA7, PLA8 and BSP, respectively) was accurately weighed into 1 mL centrifuge tubes, and 0.6 mL of ultrapure water was added to each tube. After BSP was completely dissolved, the solution from one of the centrifuge tubes was added into the conical flask drop by drop and the resultant solution was stirred for 4 h. Afterwards, the mixture was poured onto a flat glass plate sitting on a flat platform. The plate was dried in a wind-free dry environment for 36 h until the solvent had completely evaporated. The dried film, with a thickness range of 50–55 μm, was collected for measurement and subsequent performance evaluation.
3.4. Dynamic Thermomechanical Analysis (DMA) Analysis
A dynamic mechanical analyzer (DMA 242 E, Netzsch, Selb, Germany) was used to collect DMA data. First, samples were made into 5 mm × 15 mm specimens. The experiments were conducted via the dynamic stretching method; the initial frequency was 1 Hz, the absolute amplitude was 120 μm, the maximum dynamic force was 1.5 N, the additional static force was 0.05 N, the heating rate was 2 °C/min, and the temperature range was 25–180 °C.
3.5. Different Scanning Calorimeter(DSC) Analysis
DSC data were collected using a differential scanning calorimeter (DSC 214 Polyma, Netzsch). A sample of 3–5 mg was weighed and sealed in an aluminum crucible. The N2 flow rate was 30 mL/min, the heating rate was 5 °C/min, the temperature range was 25–200 °C. TG-DSC was used to identify and compare the starting temperature where peaks appeared in the thermogram. The crystallinity formula [
43] is as follows:
where
is the crystallinity of the sample,
is the enthalpy of fusion, and
is the enthalpy of fusion for the sample with complete PLA crystallization, which is 93.0 J/g.
3.6. Thermogravimetric(TG) Analysis
Thermogravimetric analysis (TGA) data were obtained using a thermogravimetric analyzer (STA449F31, Netzsch). The heating rate was controlled at 10°C/min under a nitrogen atmosphere. The TGA analysis was carried out between 25 and 500 °C.
3.7. X-Ray Diffraction Characterization (XRD)
The samples were subjected to X-ray analysis using a wide-angle X-ray diffractometer (Bruker, Karlsruhe, Germany). A Cu target (λ = 0.154 nm) operated at a working voltage 45 kV and working current 150 mA was used; the scanning-angle range was 5–60°, and the scanning rate as 5°/min.
3.8. Scanning Election Microscopy (SEM )Analysis
SEM (NOVA NANOSEM-450, FEI, Hillsboro, OR, USA) was used to observe the surface and cross-section of the sample. To effectively observe the surface and cross-section, the sample was first quenched in liquid nitrogen and the cross-section was subjected to a gold spray treatment. The accelerating voltage was 5 kV, and the magnification was 5000×. The pore size was analyzed using the Nano-measurer software.
3.9. Mechanical Performance Test
The mechanical properties of the samples were tested using a microcomputer-controlled electronic universal testing machine (CMT 4104, MTS Industrial Systems Co., Inc., Shenzhen, China). First, the sample was made into a 45 mm × 10 mm specimen, and a 50 N sensor was selected for the measurement. The tensile performance test of a plastic film was selected as the measurement mode. The test speed was 100 mm/min. Parallel experiments (seven each) were conducted, and average values were taken.
3.10. Contact Angle Test
Measurements were made using a contact angle measuring instrument model OCA200 (Dataphysics, Stuttgart, Germany). A 2 cm diameter sample was mounted on a clean, smooth glass slide and the drop was dropped onto the surface of the film by hanging drop. The contact angle of water on the surface of the film was measured by a contact angle meter, and each sample was measured twice.