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Development of materials for use in medicine and industries is one of the biggest challenges in  research in materials science. Today, absorbable, biocompatible and biodegradable polymers have  been identified to be used as alternative materials for biomedical applications. Among the biomaterials  used in medical fields, polylactic acid (PLA) have been considered widely. This polymer obtained  from completely renewable sources and due to their good physicochemical properties in various  industries it is highly recommended. The purpose of this study was to introduce PLA mechanical  properties using molecular dynamics (MDs) simulation. The special advantage of PLA over other  polymers is its biodegradation, which is generally degraded by a single-step process involving a  bacterial attack on the polymer itself. It is easily degraded in atmospheres with high humidity and  temperatures of 50-70°C. However, at lower temperatures or lower humidity, the stability of PLA products is higher. The MD analysis indicates that the density value corresponds well to the  experimental values. Based on the molecular model, the elastic modulus properties of the model were investigated. In this study, the average elastic modulus of the molecular PLA model was calculated to  be about 2.2 GPa. 

Keywords: Mechanical engineering; Molecular dynamic; Mechanical simulation; Elastic modulus 

In recent years, due to increasing  environmental awareness, increasing oil  prices and the challenges associated with  global warming, attention to biopolymers  has increased [1-4]. Because these  polymers are obtained from renewable  sources and their use has minimal  environmental petroleum effects compared  to conventional polymers. Today, these compounds are used in various fields such  as physiotherapy, pharmacy, medicine,  tissue engineering, food products and  packaging materials. Body-compatible  synthetic polymers are degradable and  adsorbable. These polymers are easily  converted to three-dimensional (3D)  matrices with various structures [5-8].  Prominent compatibility and non-toxic  nature and biodegradability are the end products of degradation of these polymers,  which are the basis for their use in areas related to human health [6-9]. Synthetic  polymers used for tissue engineering  include poly-alpha-hydroxy esters [10-14]. The examples of poly-alpha-hydroxy esters is polylactic acid (PLA) [15-21]. Today, these polymers are used separately  or in copolymer form from this polymer [21-24]. In this article, we introduce this  polymer as one of the important polymers and after describing the chemical structure  and manufacturing method, we refer to  their mechanical properties and physical  properties using MD and its application. An extensive consumption of lactic acid  and its derivatives in food, textile,  pharmaceutical, cosmetic, chemical and  especially polymer industries have led to  many studies in recent years to produce  this organic acid. In addition, lactic acid  can be converted to beneficial substances  such as propylene oxide, propylene glycol  and ester lactate through chemical  reactions [25-34]. The manufacture of PLA and acrylic biodegradable acids has also  created an important market for pure lactic  acid. PLA is a linear aliphatic  thermoplastic polyester that can be extracted %100 from renewable sources  such as corn. Today, this polymer is widely  used in packaging, textiles and plastic  containers [35-38]. PLA belongs to the  family of aliphatic polyesters, which are  usually made from alpha-hydroxy acids.  PLA is a thermoplastic with high strength  and modulus which its raw material is  produced from renewable sources such as  potatoes and corn. Subsequently, major advances in process technology coupled  with the reduction in the price of  biologically produced lactic acid led to the  commercial production of biodegradable  lactic acid polymers in non-medical applications. This integration into  biotechnology and chemistry was an  important strategy that improved many  processes in the following years. There are  two chemical methods for converting lactic  acid to high molecular weight PLA. Toatsu et al. was able to convert lactic acid  directly into high molecular weight PLA in  a basic solvent process with azeotropic  removal of water by distillation [25-36]. PLA is one of the few polymers whose  molecular structure can be controlled by  the ratio of L and D isomers to obtain a  high-weight crystalline or amorphous  polymer. PLA is known as a food safety agent and can come into contact with food.  This polymer decomposes without the need  for a catalyst by hydrolysis of ester bonds.  The rate of decomposition depends on the  shape and size of the polymer object, the  isomer ratio and the hydrolysis temperature [37-44]. PLA a crystalline substance with a  regular chain structure; Poly L is lactic  acid (PLLA), which is a heme crystal, and  so with a regular chain structure; Lactic Side Poly DL (PDLLA) which is  amorphous. The raw materials for the  production of PLA are agricultural  products such as corn, sugar beet, wheat and other products rich in starch. It is  noteworthy that it is not necessary to use  the main agricultural products to produce  PLA, but agricultural waste can also be  widely used in the production of PLA. In  major industrial-to-industrial countries,  due to the inclusion of agricultural waste in  the production cycle of other products, there is virtually no large waste disposal to  be used in the production of polylactic  acid. For this reason, the raw materials  needed to produce PLA in these countries  are cultivated industrially. A variety of  carbohydrates and nitrogenous substances  have been used to produce lactic acid. The choice of raw material depends on the  microstructure and the desired product. Sucrose, maltose, glucose, mannitol, etc.  have been used commercially. Molasses is  cheap, but has little efficiency in producing  lactic acid. Corn, straw, cotton husk, etc.  have also been studied [41-47]. It is vital  for the agricultural industry to find  profitable use for these biodegradable  materials that are discarded as residual  sugar extraction. This bioplastic is made  from sugar beet pulp and another  biodegradable polymer called PLA by  extrusion process [47-51]. Whey is one of  these wastes and disposables, because  whey requires a great deal of biochemical  oxygen, about 30,000 to 50,000 ppt, to be  absorbed into nature, the amount of  environmental pollution is much higher  than the virtual limit set by the  environmental protection agency for  industrial effluents. 

However, most of the whey from the  cheese factories now enters nature in the  form of waste. Interestingly, whey is more  efficient than other raw materials that can  be used to produce PLA. Whey has the  highest production efficiency among other  raw materials. However, production  efficiency is not the only criterion for  selecting the raw material, but the quantity  and price of the raw material are the owner of the choice for industrial production.  The process of producing PLA and fibers  from corn is studied exclusively. In  general, the production process of PLA polymer is produced in three main stages,  which are conversion of corn to dextrose. 

PLA is a semi-crystalline polymer with a  glass transition temperature of about 55 to  59°C and a melting point of 174-184°C. It  represents good mechanical strength, high  elastic modulus, good thermal plasticity,  good processability and this relatively  hydrophobic polyester, unstable in wet  conditions, which can cause chain  disorders in the human body to non-toxic  by-products, and lactic acid. Converts  carbon and water, which is subsequently  eliminated through the Krebs cycle in the urine [18]. In general, it can be said that  this polymer is biocompatible, biodegradable, absorbable, semi-crystalline  polymer, glass transition temperature  around 55-59°C, melting point 174-184°C,  high water vapor permeability, High  tensile strength 50-70 MPa, low impact resistance, high hardness, thermal ductility,  good processability, resistant to fat and  water penetration. PLA is generally known  for its good mechanical properties, with an  elastic modulus of 3000-4000 MPa and tensile strength of 50-70 MPa, elongation  at break point is 2-10%, flexural strength is 100 MPa and the flexural modulus is 4000- 5000 MPa. Therefore, it can be an  alternative to ordinary polymers in many  applications, such as packaging, extrusion  and heating containers [26-39]. However,  there is a slight increase in length at rest,  which is an example of this polymer  limiting some of its applications. Fragile at  room temperature, breaking through the  grazing mechanism. Efforts have been made to improve the properties of PLA by  copolymerization, combined with other  biodegradable polymers. It should be noted  that the mechanical properties of PLA do  not significantly affect its synthesis  methods [40-43]. It is interesting to note  that material processing has a significant  effect on PLA impact resistance [32-40].  Therefore, since PLA is a material  characterized by a relatively low value of  impact resistance, the effect of crystallization and molecular weight has  been considered in scientific applications. Rockwell hardness of PLA is usually  between the range of 70 and 90 according to the scale [41-47]. Rockwell hardness of  PLLA is affected by very little  crystallization, which for amorphous  PLLA ranges from 83-88 H and 82-88H are evident for the semi-crystalline PLLA.  The Rockwell hardness dependence on  molecular weight also appears to be  negligible. However, the effect of glass  transition temperature (TG) is more  pronounced. PDLLA is characterized by a  lower hardness in the range of 72-78 H [41-46]. The lower hardness for PDLLA  than PLLA can be explained in terms of  lower triglyceride than PDLLA [48-52].  Among biodegradable polymers, PLA is  characterized by high elastic modulus and  high hardness. The mechanical property,  which penetrates the applications of these  materials, strongly depends on its chemical  composition. The presence of the polar  ester group in the neighborhood and its  regular distribution, affect physical  interactions. Finally, chain interactions  between polarity and high glass  temperatures are the source of high PLA  hardness. 

Molecular dynamics (MDs) method is one  of the numerical simulation methods that is  used to simulate and obtain the physical  and mechanical properties of various  materials, including nanomaterials and  polymers [29-39]. In this method, the  numerical model of atoms and molecules is  made according to the interatomic forces  and after equilibrium, the physical and  mechanical properties of the material are  extracted using statistical mechanical  equations. In many studies, this method is  known as a relatively accurate method for predicting the properties or factors  affecting the properties of materials. In this  study, molecular dynamics simulation was  used to model the PLA atomic model. In  this study, the COMPASS force field is  used. This force field has been successfully  used to predict the properties of polymer based materials. The molecular structure of  PLA monomers is shown in Fig. 1. By  creating a chain structure and random  distribution of PLA molecules in an  amorphous cubic simulation box with  periodic boundary conditions, the PLA  atomic model is constructed according to  Fig. 2. The initial density of the PLA  model was 1 (g/cm3). After geometry  optimization and energy minimization,  molecular dynamics simulation was  performed using NPT at 298 K at 40 ps.

The interaction forces of Van Der Waals  and Columbus are taken into account in  these calculations. The cut-off distance for  atomic modeling is 12.5 A [47-52]. Fig. 3 shows the energy changes of the atomic  model in the NPT simulation representing  the equilibrium of the system. Fig. 4 shows  the density of the molecular model in the NPT simulation converges to 1.2 (g/cm3).  This density value corresponds well to the  experimental values. Based on the  molecular model, the elastic properties of  the model are investigated. In this study,  the average elastic modulus of the  molecular PLA model is calculated to be  about 2.2 GPa.

The main advantages of using PLA are prevention of environmental pollution,  wastes from the consumption of goods  made of polylactic acid are decomposed  and metallized by being in the vicinity of  the soil. Preventing the increase of  greenhouse gases, industrial production of  polylactic acid requires the cultivation of  plants used as raw material on a large  scale. Compatibility with living tissue,  since the raw materials for the production  of PLA are plants, in packaging  applications, they do not transfer to  packaged goods, especially food,  chemicals and unnatural substances.  Reducing the use of oil resources in  production in the production of PLA not  only is oil not used as a raw material, but  its production process also requires less  fuel resources. Using renewable sources,  plants can be produced and recycled as raw  materials for the production of PLA [41- 48]. Each of these properties alone can be a strong reason for the expansion of  production and use of PLA compared to  today’s plastics [39-52]. Despite the many  advantages that PLA has over other  biopolymers, its use in industry and  competition with industrial polymers faces  several major challenges, including crisp  and brittle that needs to be improved and  permeability high to water vapor and  gases, low glass transition temperature  (Tg) and poor thermal stability [24, 25]. To  overcome these problems can be solutions  such as using suitable softeners, combining  with other polymers, optimizing conditions  crystallization and the use of suitable  additives to produce a variety of  composites. The use of nanoparticles and  the production of nanocomposites to  improve the properties of polymers is very  important due to the great variety and  effectiveness of these particles.

PLA also has non-polymeric applications,  including its conversion to ethyl esters for use as natural derivatives. PLA prices will  fall further as new markets for lactic acid reach new markets. PLA is first destroyed  by hydrolysis, not by microbial attack.  Therefore, even at high humidity, it is  uncommon for high molecular weight PLAs to be contaminated by fungi, mold,  or other microbes. This unusual property of  the biodegradable polymer for applications  where there is direct contact with food over  a long period of time. This density value  corresponds well to the experimental values.  Based on the molecular model, the elastic  properties of the model are investigated. In this  study, the average elastic modulus of the  molecular PLA model is calculated to be about  2.2 GPa. 

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