In recent decades, synthetic organic polymers, commonly known as plastics, have become ubiquitous in everyday life. Their ease of handling, physicochemical, mechanical, and elastic properties have enabled the production of a wide range of products, including packaging, containers such as bottles and bags, clothing, medical supplies, and materials for construction and transportation [1], [2]. However, petroleum-based plastics are neither renewable nor degradable, posing risks to wildlife and contributing to environmental pollution [3], [4], [5], [6], [7], [8], [9], [10]. The growing concerns surrounding the high demand for synthetic plastics have prompted the consideration of natural polymer-derived plastics as a viable alternative [11], [12], [13], [14], [15]. Although bioplastics may not possess the optimal mechanical characteristics of conventional plastics [16], [17], they offer biocompatibility, non-toxicity, sustainability, and, in most cases, biodegradability, all while deriving from renewable resources, making them environmentally friendly.

Numerous sources of biopolymers exist, including plant-based (starch, cellulose, lignin, hemicellulose) [18], [19], [20], [21], [22], animal-derived (eg. proteins like chitin and chitosan) [19], [23], [24], [25], [26], and bacterial resources [19], [27], [28], [29]. Starch, in particular, has recently emerged as the primary raw material for producing biodegradable plastics due to its low processing cost, abundance in plants, and thermoplastic and mechanical properties that can be significantly enhanced when combined with other materials. For instance, starch has been blended with plasticizers to increase flexibility by reducing intramolecular hydrogen bonds [30], [31], [32]. Common plasticizers include hydrophilic polyols like glycerol or sorbitol [33], [34], amides [35], [36], [37], amines [38], [39], fatty acids such as oleic and linoleic acids [40], [41], [42], [43], [44], and certain sugars like maltose or glucose [45], [46]. When plasticizers are combined, particularly with water, the manipulation and processability of starch improve, resulting in bioplastics with increased ductility [32], [33], [35], [47], [48].

Starch is a polysaccharide that serves as energy storage in plants and is composed of amylose (15%–35%) and amylopectin (65%–85%). Both components consist of α(1–4)-linked D-glucose with branches of α(1–6) linkages [47], [48], [49]. Amylose, unlike amylopectin, is only slightly branched and considered an ordered linear chain [50], [51]. Amylose’s structure can form single or double helical conformations, which influence starch crystallinity [47], [49], [52], [53], [54]. Compared to amylopectin, amylose produces stronger films and gels, tends to retrograde, and interacts with various compounds to form inclusion complexes [17], [48], [55], [56], [57].

Amylopectin and amylose have been employed as model systems for starch in molecular dynamics simulations [31], [58], [59], [60], [61], [62], [63], [64], [65]. Computational methods have provided valuable information and insights into the plastic properties of starch in the presence of plasticizers. For water as a plasticizer, studies investigating the vitrification process reveal that the glass transition temperature increases as the hydration level of starch decreases [58], which aligns with experimental data. The stability and flexibility of amylopectin are also limited by its degree of hydration, as its structure fits well into the surrounding water [59]. Mechanical properties have been explored when both amylopectin and amylose are mixed with biodegradable polymers such as polyvinyl alcohol, polycaprolactone, or polybutylene succinate [60]; when starch/biopolymer weight fractions exceed 40%, the crystallinity of amylopectin/biopolymer compounds is higher than that of amylose/biopolymer due to amylopectin’s branched structure.

Molecular dynamics simulations have also examined the amylose /linoleic acid complex immersed in water [61], [62]. The inclusion of linoleic acid within the helical cavity of amylose occurs when it adopts a V-shaped conformation [62]; this inclusion is thermodynamically stable at temperatures above room temperature and up to 373 K [61]. Other studies demonstrate that fatty acids surround amylose, as seen in the case of Brazilian Cerrado oil, which encapsulates amylose while most water molecules are displaced away from the complex. Only a small number of water molecules form hydrogen bonds with the exposed parts of amylose [63]. As hydrogen bond formation is the critical mechanism underlying plasticization [35], [36], [66], [67], it has been rigorously studied using molecular dynamics simulations for the starch/glycerol complex [31], [64], [65]. According to J. Yang et al. [31], the starch/glycerol system exhibits strong hydrogen bonding interactions, which increase with decreasing temperature or increasing glycerol content. When glycerol content increases from 20% to 40%, the inter- and intramolecular hydrogen bonds of starch are disrupted, resulting in a lower glass transition temperature. Using the same computational methods, H. D. Özeren et al. [64] confirmed that the transition in the degree of plasticization occurs between 20% and 30% plasticizer. A comparative study of plasticizing effects on starch using glycerol, sorbitol, xylitol, triethanolamine, diethanolamine, and glucose, conducted through molecular dynamics simulations, showed that glycerol is the most efficient plasticizer, as it forms the fewest hydrogen bonds [65]. This finding is validated by analyzing the mechanical properties of starch, as indicated by elastic modulus and tensile strength.

Despite extensive knowledge of starch plasticization mechanisms gained through experimental and computational efforts, the process is not yet fully understood. As such, it remains unclear which plasticizer, or combination thereof, achieves the best thermoplastic and mechanical properties. Since understanding starch-plasticizer interactions at a molecular level is essential for practical applications, but can be time-consuming and costly for experimental analysis, our study uses molecular dynamics simulations to investigate the effects of polymer length on the mechanical and structural properties of amylose when combined with water, glycerol, or oleic acid.

Water and glycerol were selected because they are the most common and effective plasticizers known for biopolymers. The amylose/water and amylose/glycerol complexes are used as reference points to compare variations in results due to the difference in lengths of the amylose chain studied. Oleic acid (C18:1), the most abundant fatty acid in lipids along with linoleic acid (C18:2), is chosen as it is one of the most effective plasticizers known for zein [43], [68], [69] and, when combined with another plasticizer in small amounts, can enhance both plasticization and thermal stability of starch [63], [70], [71]. However, the interaction between oleic acid and starch has not yet been described using computational methods. In this work, the amylose/oleic-acid complex is investigated for the first time using molecular dynamics simulations to analyze oleic acid’s plasticizing effects and potential formation of inclusion complexes, as well as the effect of amylose length.

Additionally, the amylose/water system with 40% glycerol or oleic acid content is analyzed, as this concentration is suggested for plasticization according to previously cited literature and experimental studies mentioned by Faisal et al. [72]. A temperature of 363 K is considered for all systems, as it is above the glass transition temperature for both amylose/glycerol and amylose/glycerol/water composites (complexes) under the (co)plasticizer concentrations considered in this work [64], [65], [73], [74]. In addition to studying the effect of size, the impact of temperature changes in systems containing only glycerol or oleic acid is examined at 363 K and 400 K. Physical and structural properties, such as root mean square deviation, radial distribution function, diffusion coefficients, hydrogen bond formation, and Young’s modulus, are calculated over extended simulation times.

Our results revealed that the polymer solubility increases proportionally to chain length. Temperature changes above the glass transition significantly impact the diffusivity of plasticizers and the elastic properties of starch. Oleic acid may be a suitable alternative as a plasticizer, and mixing glycerol or oleic acid with water enhances the elastic properties of amylose.