**1. Introduction**

With the increasing tension in petroleum resources and environmental issues, several bio-based degradable resources have been drawing wide attention in recent years. Poly(lactic acid) (PLA), which is the most promising bio-based degradable material, can be obtained through the polymerization [1] of lactic acid by fermenting plant starch. PLA has broad application prospects in daily lives, packaging industry, and the biomedical field [2–4] due to its good mechanical properties and biocompatibility. However, the high brittleness, low impact strength [5], and high cost of this material limit its development and applications. An effective way to overcome these drawbacks is to add other biodegradable materials to PLA [6]. Blending PLA with starch [7], which is a widely derived

biodegradable material, yields high-performance and low-cost PLA/starch blends and expands the application range of renewable resources.

Starch contains numerous hydroxyl groups and has high polarity. However, when starch and PLA are blended, using the former as a filler cannot improve and even reduces the toughness of the PLA/starch blends. The layered microstructure along the flow direction is formed in the PLA/starch blends through the pressure-induced flow (PIF) process proposed by Zhang et al. [8]. During the compression process, PLA is oriented, whereas starch is deformed. This process increases the uniformity of the thickness of the PLA crystal layer. The layered structure allows the PLA/starch blend to absorb high impact energy, thereby improving the toughness of the material. However, the use of PIF to improve the toughness of the material is limited, and high pressure causes material damage. Small molecular plasticizers [9,10], such as polyethylene glycol (PEG) and citrate esters, can improve the ductility of PLA/starch blend materials but cannot change the immiscibility of PLA and starch. Plasticizers migrate with time and do not stabilize in the blends. Starch can be chemically modified through esterification, oxidation, crosslinking, and grafting [11] to promote the compatibility of the PLA/starch blend materials. To weaken the hydrogen bonding force among the starch molecular chains, the starch particle size or the polarity of the starch surface can be reduced to enhance the interaction force between PLA and starch. Wang et al. [12] prepared core–shell starch nanoparticles (CSS NPs). By esterifying the starch and subsequently polymerizing the emulsion, the CSS NPs utilized starch as the core to ensure that the material achieves a strong rigidity and polyethyl acrylate (PEA), which has a good a ffinity with PLA, as shell. Given the synergistic e ffect of starch and PEA, the PLA/CSS NP blends exhibited excellent toughness. Thielemans et al. [13] modified nanosized single crystal starch particles by using stearoyl chloride and PEG methyl ether. The crystal structure of the modified starch did not change, but the graft crystallized on the surface of the modified starch did. Appropriate grafts can weaken the hydrogen bonding force among starches and reduce the polarity of the starch surface. These changes help improve the compatibility of PLA/starch blends. In addition to the two-step method to modify PLA/starch composite materials, the chemical modification of starch can also be achieved through one-step reactive blending, and at the same time the compatibility of the PLA/starch composite system can be enhanced. Xiong et al. [14] synthesized a bio-based monofunctional epoxy compound Epicard and used it for the reactive compatibilization of PLA/starch composite materials. The epoxy groups on the Epicard can react with starch and form a hydrophobic layer on the surface of the latter, which will enhance the adhesion of the PLA/starch blends and improve the compatibility between PLA and starch. Jariyasakoolroj et al. [15] used chloropropyl trimethoxysilane (CPMS) modified starch (CP-starch) to melt blend with PLA to prepare PLA/starch composites. During the melt-blending process, CP-starch reacted with PLA to form a block copolymer, which acts as a nucleating agen<sup>t</sup> and can significantly increase the crystallinity of the blend and improve the compatibility between PLA and starch. Modifying starch can e ffectively improve its surface polarity and increase the interaction force between starch and PLA. Reactive blending, which is a simple and practical method, can directly establish the compatibility of PLA/starch composite materials.

In this study, PLA/premixed starch (PSt)/PEG/citric acid (CA) blends were prepared through reactive blending to examine the e ffects of CA and PEG on the toughness of PLA/PSt (wt%/wt%, 70/30) blends. CA with high carboxyl underwent an esterification reaction with starch and consequently enhanced the interface adhesion between starch and PLA. As a plasticizer of PLA and starch, PEG e ffectively dispersed in the matrix and enhanced the mobility of the molecular chains. When the PEG that entered the matrix participated in the esterification reaction between CA and starch, a branched/crosslinked polyester was easily formed at the interface. This polyester further improved the compatibility of the PLA/starch blend materials. This PLA/starch composite material uses biodegradable raw materials, and at the same time, a highly filled PLA/starch composite material with excellent performance is obtained through reactive blending. Therefore, this composite material has broad application prospects in the medical field and packaging field.

## **2. Materials and Methods**
