*Article* **Improvement in Thermal Stability of Flexible Poly(L-lactide)-***b***-poly(ethylene glycol)-***b***-poly(L-lactide) Bioplastic by Blending with Native Cassava Starch**

**Yaowalak Srisuwan and Yodthong Baimark \***

Biodegradable Polymers Research Unit, Department of Chemistry and Centre of Excellence for Innovation in Chemistry, Faculty of Science, Mahasarakham University, Mahasarakham 44150, Thailand **\*** Correspondence: yodthong.b@msu.ac.th

**Abstract:** High-molecular-weight poly(L-lactide)-*b*-poly(ethylene glycol)-*b*-poly(L-lactide) triblock copolymer (PLLA-PEG-PLLA) is a promising candidate for use as a biodegradable bioplastic because of its high flexibility. However, the applications of PLLA-PEG-PLLA have been limited due to its high cost and poor thermal stability compared to PLLA. In this work, native cassava starch was blended to reduce the production cost and to improve the thermal stability of PLLA-PEG-PLLA. The starch interacted with PEG middle blocks to increase the thermal stability of the PLLA-PEG-PLLA matrix and to enhance phase adhesion between the PLLA-PEG-PLLA matrix and dispersed starch particles. Tensile stress and strain at break of PLLA-PEG-PLLA films decreased and the hydrophilicity increased as the starch content increased. However, all the PLLA-PEG-PLLA/starch films remained more flexible than the pure PLLA film, representing a promising candidate in biomedical, packaging and agricultural applications.

**Keywords:** poly(lactic acid); block copolymer; cassava starch; biocomposites; thermal stability

### **1. Introduction**

Among bioplastics, poly(L-lactic acid) or poly(L-lactide) (PLLA) is a promising substitute for traditional petroleum-based plastics in many fields, such as biomedical, tissue engineering, drug delivery and packaging applications [1–4]. This is due to its excellent biodegradability, biocompatibility and compostability, as well as being environmentally friendly and having good processability and feasibility for an increased production scale [5,6]. However, the low flexibility of PLLA, due to its high glass-transition temperature (Tg, around 60 ◦C), limits its use in many applications [7–9]. High-molecular-weight PLLA*b*-poly(ethylene glycol)-*b*-PLLA triblock copolymers (PLLA-PEG-PLLA) have exhibited more flexibility than PLLA because of the high flexibility of PEG middle blocks [10–12].

Unfortunately, these pure PLLA-PEG-PLLAs had high melt flow ability (low melt strength), which is not appropriate for many processing applications, such as injection molding, blow film molding and extrusion molding [11,12]. The melt flow properties of PLLA-PEG-PLLA can be controlled by reaction with a chain extender to form long-chain branching structures. However, our previous works reported that the thermal stability of PLLA-PEG-PLLA decreased after the chain extension reaction [11–13]. The branching structures of the chain-extended PLLA and PLLA-PEG-PLLA suppressed its thermal stability by preventing molecular interactions. This resulted in a narrower window of processing for the chain-extended PLLA-PEG-PLLA (the range between its melting and thermal decomposition temperatures was narrower). The 60/40 chain-extended PLLA-PEG-PLLA/thermoplastic starch (TPS) blends were co-continuous phase structures, indicating that they had good phase compatibility [14]. The PEG middle blocks enhanced the phase compatibility between PLLA end blocks and TPS phases. Moreover, the TPS blending

**Citation:** Srisuwan, Y.; Baimark, Y. Improvement in Thermal Stability of Flexible Poly(L-lactide)-*b*-poly(ethylene glycol)-*b*-poly(L-lactide) Bioplastic by Blending with Native Cassava Starch. *Polymers* **2022**, *14*, 3186. https:// doi.org/10.3390/polym14153186

Academic Editor: Evgenia G. Korzhikova-Vlakh

Received: 14 June 2022 Accepted: 2 August 2022 Published: 4 August 2022

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improved the thermal stability of PLLA-PEG-PLLA. However, the effect of native starch on the thermal stability of PLLA-PEG-PLLA has not yet been explored in detail. the thermal stability of PLLA-PEG-PLLA has not yet been explored in detail. Native starches are low-cost and non-scarce biopolymers and have been blended

compatibility between PLLA end blocks and TPS phases. Moreover, the TPS blending improved the thermal stability of PLLA-PEG-PLLA. However, the effect of native starch on

*Polymers* **2022**, *14*, x FOR PEER REVIEW 2 of 14

Native starches are low-cost and non-scarce biopolymers and have been blended with various biodegradable plastics, such as PLLA [15–17], poly(ε-caprolactone) [18] and polyhydroxybutyrate [19], for reducing their production costs and for maintaining their biodegradability. However, the highly brittle PLLA/native starch composites are limited in practical applications due to the poor mechanical properties of native starch and poor phase compatibility between PLLA and native starch particles [15,20]. Therefore, the aim of this work is to investigate the influence of native cassava starch (5, 10 and 20 wt%) on the thermal stability, phase morphology, mechanical properties and hydrophilicity of PLLA-PEG-PLLA compared to PLLA/native cassava starch blends. with various biodegradable plastics, such as PLLA [15–17], poly(ε-caprolactone) [18] and polyhydroxybutyrate [19], for reducing their production costs and for maintaining their biodegradability. However, the highly brittle PLLA/native starch composites are limited in practical applications due to the poor mechanical properties of native starch and poor phase compatibility between PLLA and native starch particles [15,20]. Therefore, the aim of this work is to investigate the influence of native cassava starch (5, 10 and 20 wt%) on the thermal stability, phase morphology, mechanical properties and hydrophilicity of PLLA-PEG-PLLA compared to PLLA/native cassava starch blends.

#### **2. Experimental Section 2. Experimental Section**

#### *2.1. Materials 2.1. Materials*

PLLA and PLLA-PEG-PLLA were synthesized through ring-opening polymerization of L-lactide monomer (96% L-enantiomer content) at 165 ◦C in bulk under a nitrogen atmosphere, as described in our previous works [11,12,14]. Stannous octoate (95%, Sigma, St. Louis, MO, USA) was used as a catalyst. Moreover, 1-dodecanol (98%, Fluka, Buchs, Switzerland) and poly(ethylene glycol) with molecular weight of 20,000 g/mol (Sigma, St. Louis, MO, USA) were used as initiators for the synthesis of PLLA and PLLA-PEG-PLLA, respectively. Number-average molecular weight (Mn) and dispersity index (*Ð*) of PLLA, analyzed by gel permeation chromatography (GPC, e2695 separations module, Waters, Milford, MA, USA), were 88,400 g/mol and 2.3, respectively, while the M<sup>n</sup> and *Ð* values of PLLA-PEG-PLLA were 89,900 g/mol and 2.8, respectively. Melt flow index (MFI) of obtained PLLA was 24 g/10 min at 190 ◦C under 2.16 kg load. However, the MFI of obtained PLLA-PEG-PLLA was too high. The melt flow property of PLLA-PEG-PLLA was then adjusted to be close to the value for PLLA obtained by chain extension reaction with 4 parts per hundred of resin by weight (phr) of Joncryl® ADR4368 chain extender (BASF, Bangkok, Thailand) [11,14]. The MFI of the obtained chain-extended PLLA-PEG-PLLA was 23 g/10 min. Native cassava starch (food-grade, 22% amylose content) was supplied by Kriangkrai Co., Ltd. (Nakornprathom, Thailand). A scanning electron microscope (SEM, JSM-6460LV, JEOL, Tokyo, Japan) image of native cassava starch is presented in Figure 1. The starch particles were in the range of 5–20 µm. PLLA and PLLA-PEG-PLLA were synthesized through ring-opening polymerization of L-lactide monomer (96% L-enantiomer content) at 165 °C in bulk under a nitrogen atmosphere, as described in our previous works [11,12,14]. Stannous octoate (95%, Sigma, St. Louis, MO, USA) was used as a catalyst. Moreover, 1-dodecanol (98%, Fluka, Buchs, Switzerland) and poly(ethylene glycol) with molecular weight of 20,000 g/mol (Sigma, St. Louis, MO, USA) were used as initiators for the synthesis of PLLA and PLLA-PEG-PLLA, respectively. Number-average molecular weight (Mn) and dispersity index (*Đ*) of PLLA, analyzed by gel permeation chromatography (GPC, e2695 separations module, Waters, Milford, MA, USA), were 88,400 g/mol and 2.3, respectively, while the Mn and *Đ* values of PLLA-PEG-PLLA were 89,900 g/mol and 2.8, respectively. Melt flow index (MFI) of obtained PLLA was 24 g/10 min at 190 °C under 2.16 kg load. However, the MFI of obtained PLLA-PEG-PLLA was too high. The melt flow property of PLLA-PEG-PLLA was then adjusted to be close to the value for PLLA obtained by chain extension reaction with 4 parts per hundred of resin by weight (phr) of Joncryl® ADR4368 chain extender (BASF, Bangkok, Thailand) [11,14]. The MFI of the obtained chain-extended PLLA-PEG-PLLA was 23 g/10 min. Native cassava starch (food-grade, 22% amylose content) was supplied by Kriangkrai Co., Ltd. (Nakornprathom, Thailand). A scanning electron microscope (SEM, JSM-6460LV, JEOL, Tokyo, Japan) image of native cassava starch is presented in Figure 1. The starch particles were in the range of 5–20 µm.

**Figure 1.** SEM image of cassava starch (bar scale = 20 µm). **Figure 1.** SEM image of cassava starch (bar scale = 20 µm).

*2.2. Preparation of PLLA/Starch and PLLA-PEG-PLLA/Starch Composites*

Chain-extended PLLA-PEG-PLLA and cassava starch were dried in a vacuum oven at 50 ◦C for 24 h before fabricating the composites. PLLA-PEG-PLLA/starch composites were prepared using a Rheomix batch mixer (HAAKE Polylab OS, Thermo Fisher Scientific, Waltham, MA, USA) at 180 ◦C with a rotor speed of 100 rpm for 5 min. The composites with PLLA-PEG-PLLA/starch ratios of 100/0, 95/5, 90/10 and 80/20 (*w*/*w*) were investigated. PLLA/starch composites were also prepared using the same process for comparison.

Before film forming, the composites were dried at 50 ◦C in a vacuum oven for 24 h. Composite films (0.2 mm in thickness) were prepared using a compression molding machine (Auto CH, Carver, Inc., Savannah, GA, USA). The composite pellets were filled into a mold and preheated at 180 ◦C for 3 min without compression force, followed by hot pressing at the same temperature under 5.0 MPa load for 1 min. Afterwards, the mold was rapidly cooled in a water-cooled press under 5.0 MPa load for 1 min. The obtained film samples were stored in a desiccator for 24 h before characterization.
