**1. Introduction**

The properties of polymers can be manipulated through an alteration in their chemical structure and preparation process [1]. The polymers are being applied in different areas such as packaging [2], automobile manufacturing [3], computer [4], clothing, etc., and also they are being used as additives in dyes, glues, coatings [5] and concrete [6]. Despite the fact that they are now an inseparable part of our lives, their low degradation rate makes those polymers a threat to the environment [7–9]. Degradation refers to any irreversible process—light, heat, moisture, chemical conditions and biological activity—leading to

**Citation:** Momeni, S.; Rezvani Ghomi, E.; Shakiba, M.; Shafiei-Navid, S.; Abdouss, M.; Bigham, A.; Khosravi, F.; Ahmadi, Z.; Faraji, M.; Abdouss, H.; et al. The Effect of Poly (Ethylene glycol) Emulation on the Degradation of PLA/Starch Composites. *Polymers* **2021**, *13*, 1019. https://doi.org/ 10.3390/polym13071019

Academic Editor: José Miguel Ferri

Received: 27 February 2021 Accepted: 23 March 2021 Published: 25 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

a change in the polymers molecular structure followed by influencing the physical and chemical properties [10–12].

Biodegradation is a phenomenon by which organic substances are cleaved by living organisms, followed by being degraded with oxygen (aerobic) or without oxygen (anaerobic) [13,14]. The biodegradation process is vital because it causes the removal of polymer products from the environment, or in other words, to return to the life cycle [15]. Microorganisms such as bacteria and fungi are involved in the degradation of natural and synthetic plastics [16]. The key parameters affecting the degradation kinetic are the polymer, organism type and environmental conditions. To penetrate a microorganism's cell, most polymers are too large. Therefore, they need to depolymerize into smaller monomers to be uptaken and degraded [13,17]. The chemical structure is a decisive factor showing whether a polymer is biodegradable. There are some practical ways to promote biodegradation, including changing the polymers' molecular structure and developing some microorganisms capable of consuming a particular carbon source [18].

The degradation process is caused in polymers when a change in their physical and/or chemical properties occurs [19]. Physical changes include reducing the molecular weight, tensile strength, elongation at break and teardrop in the level of transparency [20,21]. Chemical changes are referred to the alterations in the chemical structure of a material [22]. Taking sunlight as an example into account, the physical and chemical changes in polymers as the result of that show themselves as discolored cracks, loss of mechanical properties and loss of gloss [23]. Generally, when the degradation is caused by an optical source like sunlight, UV, etc., the polymer will be converted into a low molecular weight polymer followed by turning into carbon dioxide [24–26].

Biodegradable plastics can be referred to the thermoplastic materials such as polylactic acid (PLA), polycaprolactone (PCL), polyhydroxybutyrate (PHB), starch and polyvinyl alcohol (PVA) [27,28]. PLA is a linear aliphatic polyester obtained from renewable sources such as corn and sugar cane, making it an appropriate biological polymer [29]. The amounts of lactic acid entered into the human body through packaging are less than the lactic acid usually taken through eating food. Therefore, lactic acid-based polymers as environment-compatible materials are suitable alternatives for food packaging [30]. Considering the good processability of PLA, it has better potential than other biodegradable polymers such as PCL and it can be processed through injection molding, blow molding and thermoforming [31]. The diffusion of water, oxygen and carbon dioxide is weak through PLA [32]. The addition of an emulsifier to PLA increases the transient molecule's solubility by increasing the free volumes and as the result, the toughness will be improved. To reach a biodegradable PLA with a desirable toughness, alloying with other biodegradation polymers is required, but it is costly [33].

To increase the biodegradability and decrease costs, PLA is often mixed with starch [34]. The PLA and starch mixtures are used in thermal products such as drinking cups, food trays, dishes and boxes, food packaging, children's clothing and medical applications dental implants, suture and bone screws [14,35]. Starch is a carbohydrate belonging to the polysaccharides family and it is composed of many glucose units. Starch has two molecular architectures: linear amylose molecules and branched amylopectin molecules [36]. Natural starch is not thermoplastic and has limited processability due to the large size of the particles (5–100 µm). In the presence of a plasticizer, thermoplasticity is provided at a high temperature and sheared/cut by overcoming the strong inter-molecular and intra-molecular hydrogen bonds of starch [37]. During this process called gelatinization, the melting temperature and glass transition temperature of starch decrease and become injectable, just like the traditional synthetic plastics [38,39]. It is worth mentioning that the branches with short chains decrease the crystallinity degree of aliphatic polyesters and the branches with long chains decrease the melting viscosity yielding a rigid starch [40,41]. Hence, the brittleness of PLA and starch mixtures is the main problem in most of the applications. A few plasticizers with low molecular weights, such as glycerol and sorbitol, are used in the mixtures [42]. Polyethylene glycol (PEG) is a common plasticizer and emulsifier

used to reduce crystallinity and improve the mechanical properties of PLA/starch [43]. PEG is among the most applicable stabilizers and intermediates used in different industrial processes [44]. In addition, to decreasing the crystallinity degree, this plasticizer not only decreases Tm but also provides more thermal stability by producing sufficient interactions with the polymer's structure and it is more efficient than other plasticizers—sorbitol, glycerol, etc. [45].

The present study aimed to investigate the physical, chemical and biological properties of PLA/TPS when different contents of PEG had been added. Under laboratory conditions, the degradation of PEG-added PLA/TPS was tested and tracked-in soil. The weight loss, molecular weight change and surface erosion were assessed. Moreover, the effects of time and surface morphology on the degradation rate were studied.
