Perspectives of polylactic acid from structure to applications

Introduction

From the invention in late 1800’s, which brought revolutionary changes in the field of polymers, plastics play an integral role in industries, mainly as packaging solutions. The abundant dependence on these non degradable plastic materials that are mainly derived from non-renewable crude oil and petroleum based sources, had made lumps of plastic wastes being dumped, and they remain in the earth as such for long. This eventually results as a threat to the environment. Environmental protection agency in USA, conducted a study on this issue and found out that about a rate lower than 10% of plastics were recovered. ¹ There arises the demand for materials that are biodegradable and at the same time, posses beneficial properties to the industries. Biopolymers introduced in 1980’s, ² which can be derived from natural resources are the best solution to this problem. Biodegradable polymers are described as those polymers that can mineralize with the aid microbially induced chain scission. ³ Rising consciousness about the protection of the environment had resulted in an increased consumption of these biopolymers. As these polymers rely on natural sources, depletion of non renewable sources can be reduced to an extent. Natural polymers, synthetic polymers from natural resources, and microbial fermentation assisted polymers are the main three different categories of biopolymers. ⁴ Among these different biopolymers, polylactic acid(PLA) is a synthetic biopolymer that is being researched widely because of its potential to substitute the conventional petroleum based plastics. PLA, an aliphatic polyester derived from natural sources such as corn and sugar beets ⁵ falls under the category of thermoplastics and possess properties such as biodegradability, biocompatibility, transparency, high modulus and strength, ⁶ good mechanical property, thermoplastic processibility, low environmental impact ⁷ and good optical and barrier properties. ¹ Since PLA is compostable and derived from sustainable sources, it has been viewed as a promising material to reduce the societal solid waste disposal problem. PLA was first discovered by Carothers in the year 1932 (at DuPont). ⁸ Lactic acid(2-hydroxy proponoic acid) is the building block of PLA. Depending on the proportion of optically active D- and L-enantiomers of lactic acid(2-hydroxy proponoic acid), PLA with different properties can be obtained ¹ accordingly with the performance requirements. Different Polylactic Acid categories include Racemic PLLA (Poly-L-lactic Acid), Regular PLLA (Poly-L-lactic Acid), PDLA (Poly-D-lactic Acid), and PDLLA (Poly-DL-lactic Acid). PLA is classified as a “thermoplastic” polyester (as opposed to “thermoset”), and the name has to do with the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (150–160 degrees Celsius in the case of PLA). A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like polylactic acid liquefy, which allows them to be easily injection moulded and then subsequently recycled. Until the last decade, the main uses of PLA have been limited to medical applications such as implant devices, tissue scaffolds, and internal sutures, because of its high cost, low availability and limited molecular weight. Recently, new techniques which allow economical production of high molecular weight PLA polymer have broadened its uses. ³ Today PLA is being used as a major candidate in packaging industry, ^{5,8 -10} film applications in agriculture, ¹¹ in fibers and fabrics, electronics, furniture and house wares. ⁸ Also PLA can be combined with other materials to form composites with enhanced properties. Also, PLA can be blended with other biopolymers to prepare polymer blends with required properties that suit various applications. Brittleness of PLA limits its applications, and study reports states that the mechanical and thermal properties of PLA can be enhanced with the modification using other polymers or materials. ^{12 -14}

A better understanding of a material is an unavoidable factor to understand the role of the properties possessed by it and to make it readily available to modifications for further improvements as per the application requirements. There have been several reviews conducted on PLA. Here in this work we focuses on the application oriented property studies, and the applications of these properties on several platforms. Through this paper we go through an in-depth review on PLA. Starting from the production techniques, the various properties, its limitations, property enhancements via modification with other materials are discussed here. The implicational area where PLA can be utilized, their benefits as well as potential capability of the material have been studied deeply.

Synthetic route to PLA

PLA is linear aliphatic thermoplastic polyester. Figure 1 shows the molecular structure of PLA.

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Figure 1.

Molecular structure of PLA.

The single monomer of PLA, lactic acid (2-hydroxyl proponoic acid), obtained via chemical synthesis or carbohydrate fermentation is the simplest hydroxyl acid with an asymmetric carbon and it exists in two optically active configurations, the L(+) and D(−) isomers. These stereoisomers are produced by bacterial fermentation of carbohydrates. The production of lactic acid is mainly from fermentation technique rather than chemical synthesis because of the major limitations of the synthetic route, which includes high manufacturing costs, limited capacity, and its inability to make only desired L(+) enantiomer. ^8,15 The chemical structure of L and D lactic acid is shown in the Figure 2.

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Figure 2.

Chemical structure of (a): L lactic acid; (b): D-lactic acid.

The current expansion in the uses of PLA is attributed to the economical production of higher molecular weight PLA polymers. ¹ Hartmann ¹⁶ had discussed about various synthesis methods for the production of high molecular weight PLA (about 100,000 Da). The main three methods include direct condensation polymerization; azeotropicdehydrative condensation and polymerization through lactide formation as represented in Figure 3. Among these three routes, direct condensation through polymerization is comparatively the less expensive one. But the difficulty to obtain solvent free PLA restricts this route from being industrially acceptable, and the addition of coupling agents and adjuvants so as to overcome this difficulty in turn add cost and complexity to the process. ^{16 -19}

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Figure 3.

Synthesis method for high molecular weight PLA, adapted from Hartmann.

Azeotropic dehydration condensation techniques can produce high molecular weight PLA without the addition of any adjuvants or coupling agents. The general procedure includes the removal of condensation water by the reduced distillation pressure of lactic acid. The resultant polymer after the addition of catalyst along with diphenyl ether can be isolated as is or can be dissolved and precipitated for further purification. ^16,20,21 The residual catalyst is one of the major drawbacks of this technique. Polymerization through lactide formation is the commonly used method to obtain polylactide polymers of high molecular weight for commercial applications. In this method lactic acid obtained from dextrose fermentation is prepolymerized to get an intermediate low molecular weight PLA. It is then treated with heating catalyst which results in the production of a mixture of lactide stereo isomers. High molecular weight PLA is then obtained by ring opening polymerization of vacuum distilled lactides. This scheme adapted from Gruber is represented in Figure 4. ⁶⁵

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Figure 4.

Current production process for PLA, adapted from Gruber.

Properties of PLA

Along with the benefit of bio compatibility and production from renewable sources, PLA has got many other properties that make it suitable for various applications. The glass transition temperature of PLA is 55⁰C and the melting temperature is around 170⁰C. PLA is considered to be hydrophobic, compared with that of other polyesters. Dioxane and choloroform are the best suited solvents for PLA.The permeability coefficients of CO₂, O₂, N₂, and H₂O for PLA are lower than for polystyrene (PS), but higher than PET (poly-ethylene-terephthalate). PLA excels in crease retention and crimp properties along with, grease and oil resistance, easy low-temperature heat seal ability, and good barrier to flavors and aromas. ⁵ In this section, the structural, crystalline, mechanical, thermal and degradation properties of PLA are discussed.

Mechanical properties

The mechanical properties along with biocompatibility had made PLA an excellent candidate in biomedical and packaging industries. Depending on different parameters like, crystallinity, molecular weight, structure of the polymer and material composition (amount of plasticizers, blends etc.), the mechanical behavior of PLA ranges from soft and elastic to stiff and high strength materials. PLA is normally a stiff and brittle polymer. By controlling the stereochemical structuring, the speed and the crystallinity rate can be monitored, thus by allowing a precise control over mechanical properties and the processing temperatures of the material. Superior mechanical properties have been achieved by stereocomplexation of enantiomeric PLAs, which was ascribed to formation of stereocomplex crystallites giving intermolecular crosslinks. Use of plasticizers such as glycerin, enthylene glycol, sorbitol, etc. in the film formulations or composites is advantageous to impart pliability and flexibility, which improves handling. ²⁸ The hydrogen bonding with lipid and hydrocolloid molecules are interfered with the addition of plasticizers, which in turn reduces the brittleness of the composite. Jacobsen et al. ²⁹ had reported that the mechanical properties of PLA make it a good substitute for petroleum based polymers. Commercial PLA, such as poly (92% L-lactide, 8% meso-lactide), has a modulus of 2.1 GPa and an elongation at break of 9%. Sangeetha et al. ²³ had reported the reduction of young’s modulus to 0.7 GPa and a 200% rise in elongation at break with the addition of plasticizers. Balakrishnan et al. ¹³ had done a comparative study on mechanical properties such as elongation at break, tensile strength, and young’s modulus of PLA with that of other petroleum based plasticis such as polypropylene (PP) polystyrene (PS), high density polyethylene (HDPE) and polyamide (PA6). Graphical representation of their results is illustrated in the Figure 5.

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Figure 5.

Mechanical properties of PLA and other commodity plastics. ¹³

The Figure 5 clearly shows that the elongation at break is comparatively less for PLA, while as the Young’s Modulus is greater. Lesser the elongation at break, more brittle the material is. The use of Poly ethylene glycol (PEG), glucomonoesters, and partially fatty acid esters to improve the brittle nature of PLA have been studied by Jacobsen and Fritz. ³⁰ They had reported an increase from 3.3 to 40% in the elongation at break with 10 wt% addition of PEG. Kulinski and Piorkowska ³¹ had also conducted a similar study on PEG plasticized PLA. About 20% increase in elongation at break was reported with the addition of 10 wt % plasticizer. At the same time a decrease in the stiffness and yield strength was observed. In another study report ^10,32 with the annealing of L-PLA, the tensile strength was seen to be enhanced, due to the increased stereoregualarity in the chain and increased impact resistance due to the cross linking effects on the crystalline domains. Bindhu et al. ³³ reported 2.3 times increase in tensile strength of boron nitride PLA composites than plain PLA matrix. Iwatake et al. ³⁴ had found out that microfibrillated cellulose(MFC) could act as a good reinforcement agent to PLA thereby encouraging the production of sustainable green-composites. They had developed a technique to attain uniform dispersion of MFC in PLA matrix by premixing the fibers and the matrix in an organic solvent medium, followed by kneading.

Thermal properties and degradation

Most of the thermoplastic polymers exhibit glass transition temperature (Tg) and possess a melting point (Tm). PLA also falls in the same category and obviosly exhibits both Tg and Tm.A ruuberlybehaviour is seen for PLA above its Tg(∼58⁰C), and below this temperature, it becomes a glass which is still capable to creep until it is cooled to its β transition temperature at approximately∼45⁰C, below which it behaves as a brittle polymer. ³⁵ The glass transition temperature and melting point of different polymers are tabulated in Table 1.

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Table 1.

Thermal properties of different polymers. ³⁵

Polymer	Tg (0C)	Tm (0C)
Polylactic Acid (PLA)	55	170
Plyglycolide (PGA)	35–40	220–225
Polycarpolactone (PCL)	−60	60–65
Poly(butylene succinate) (PBS)	−35	90–120
High Density Polyetrhylene (HDPE)	−125	135
Polypropylene (PP)	−10	75
Poly styrene (PS)	100	No true Tm; Gradually softens until melt
Polyamide 6 (PA6)	50	215
Acrylonitrile Butadiene Styrene (ABS)	110	No true Tm; Gradually softens until melt

As shown in the table, PLA has got relativley low Tm and high Tg than other thermoplasts. The molecular weight and the optical purity of the polymer are reported to have effect on Tg of PLA. ¹ Optical purity is found to have an effect on Tm also. The Tg is related to molecular weight. Furthermore, PLA with higher content of l-lactide has higher Tg values than the same polymer with the same amount of d-lactide. ³⁶ Flory-Fox equation (eqn.1) describes the relation between Tg and molecular weight.

T g = T g ∞ − K M n

1

where T g ∞ is the Tg at the infinite molecular weight, K is a constant representing the excess free volume of the end groups for polymer chains, and M_n is the number average molecular weight. Low crystalline PLA polymers are reported to show a tendency to undergo rapid aging in a couple of days under prominent conditions. ^37,38 Oligomeric lactic acid have been used as an efficient plasticizer in PLA matrix ¹⁴ to enhance the thermal behavior. A reduction in glass transition temperature 58⁰C to 18⁰C was found along with a non negligible depression of the melting point temperature of about 10–15⁰C.

One of the most important factors in food packaging polymers is their barrier or permeability performance against transfer of gases, water vapor, and aroma molecules. Barrier properties of semi-crystalline polymers are influenced with the amount of by the crystallinity, since crystalline zones are excluded volumes where molecules cannot sorb or diffuse. PLA has high barrier to aromatic compounds and most non-polar solvents and comparatively low barrier to water vapor, oxygen, and carbon dioxide. The crystallinity is observed to have a great influence on the barrier properties and the reason being the higher resistance of restricted amorphous regions to water vapor permeation compared with that of the free amorphous regions. PLA have better water permeability than oriented polystyrene and are comparable with PET.

Degradation of the polymers is as important as their properties. PLA products rapidly degrade in both aerobic and anaerobic composting conditions. After several months of exposure to moisture, PLA degrades primarily due to hydrolysis. The hydrolytic degradation of PLA is an important property for a variety of applications, such as drug delivery and food packaging. ³⁹ The rate of degradation is dependent on diffusion water and temperature. Many works ^1,5,10,39 had reported the two step degradation process of PLA. In the initial phase, high molecular polyester chain is hydrolyzed to oligomers (low molecular weight). Acids or base can accelerate this step, and moisture level and temperature can affect this phase. ⁵ This phase also leads to embrittlement of PLA. In the next phase, low molecular weight PLA, is diffused out of the bulk matrix and is converted to carbon dioxide, water and humus with the aid of microorganisms present in the environment. The process is also dependent upon the chemical and physical characteristics of the polymer. These include diffusivity, porosity, morphology, crosslinking, purity, chemical reactivity, mechanical strength, thermal tolerance, and resistance to electromagnetic radiation . PLA degradation has been found to be dependent on a range of factors, such as Molecular weight, crystallinity, purity, temperature, pH, presence of terminal carboxyl or hydroxyl groups, water permeability, and active additives.

PLA blends and composites

For the extension of applications via enhancement of properties, several modifications can be made in the polymer matrix. PLA also can be blended with other materials, to form composites with enriched properties. These polylactic acid blends and composites have occupied major applications in different fields such as, green packaging, medical device packaging, and as biomedical devices. Chitosan, starch, hydroxyapatite, cellulose etc are the major materials being blended with PLA. The basic nature of chitosan neutralizes the acidic nature of PLA resulting in more biocompatible composite. Due to the difference in types of solvents (dilute acidic solution for Cs and organic solvents for PLA), solvent aided blending is difficult. Wu and Wu ⁴⁰ had prepared PLA/chitosan modified montmorrillonite nanocomposites with significant improvements in mechanical and thermal stability and the reason was attributed to the presence of inorganic layers of silicates. The better dispersion rates of PLA into m-MMT were seen in XRD studies. Another work by Cai et al. ⁴¹ reports the preparation of nanocomposites of hydroxyapatite and chitosan in the presence of PLA. Hydroxyapatite nanoparticles were homogeneously distributed in chitosan-PLA matrix. Significant improvements in elastic modulus and compressive strength were seen for the composites. Kasuga et al. ⁴² prepared composites with fibrous hydroxyapatite and PLA. Modulus of elasticity was improved with the addition of HAF content. Almost no change in the structure of PLA and no degradation in bending strength was seen with addition of HAF in to the matrix. Even though starch can play as nucleating agent in PLA, its hydrophilic nature makes it difficult to get homogeneous dispersion to be made with hydrophobic PLA. Since a composite with starch and PLA would reduce the cost of production and results in much environment friendly product, many studies are going on to make blend with PLA and starch. Park et al. ⁶⁴ found an increase in the crystallization rate and in the enthalpies of crystallization and melting for PLA with starch contents above 5%. Zhang et al. ⁴³ investigated a layer-like microstructure in PLA/starch blends. Pressure-induced flow (PIF) processing was facilitated to form PLA/starch blends. It was found that the impact and tensile strength for PLA/starch blends with the layer-like microstructure can be enhanced by 200 and 40%, respectively.

When cellulose microfibrils were introduced to the PLA matrix, both young’s modulus and tensile modulus were increased by 40% and 25% respectively with 10 wt% microfibrils. ^25,34 Another work by Janoobi et al. ⁴⁴ used nanofibers separated from kenalf pulp as reinforcing phase for PLA matrix, with less aggregation for cellulose fibers. A 24% increase in modulus and a 21% increase in tensile strength was observed with addition of 5 wt% nanofibers. PLA/Cellulose nanocrystals(CNC) were synthesized by Khoo et al. ⁶ via solution casting technique. CNC were reported to act as nucleating agent for PLA, by enhancing the thermal stability of the composite and slows down rate of thermal degradation. Kim et al. ⁴⁵ found that light transmission of the PLA/bacterial cellulose nanocomposites was quite high due to the size effect of the nanofibrillar bacterial cellulose. An increase in tensile strength by 203% and young’s modulus by 143% was also observed for the composite compared to neat PLA.

Oksman et al. ⁴⁶ studied the effect of natural fibers as reinforcement agents in PLA matrix and found that composite strength doubles and stiffness increased by 147% with the addition of 30 wt% flax fibers, which are higher than conventionally used thermoplastics. Moran et al. ⁴⁷ studied PLA/Polyglycolic acid for cartilage tissue engineering. Composites with 0 to 68% PLA were synthesized by coating nonwoven meshes of PGA via solvent evaporation technique. The compressive modulus of scaffolds increased linearly with the addition of PLA. The degradation time of the scaffolds were seen to be dependent on the amount of PLA, this in turn initiate to vary the PLA content so as to get the required scaffold property as per the needs. These composites were reported to contribute crucial information for the design of scaffolds for cartilage tissue engineering. Nanocomposites of PLA with coir/banana fibers were prepared using molding method by Bahalek et al. ²⁶ These fibers acted as nucleating agents, to have effect on crystallization.

Jaratrotkamjorn et al. ⁶⁶ toughened PLA by blending with natural rubber (NR), epoxidized natural rubber (ENR) and natural rubber grafted with poly(methyl methacrylate). PLA/NR blend excelled in the case of Young’s modulus and was reported as the best toughening agent compared with ENR and NR-g-PMMA. With 5% of ENR, and 10% of NR, the yield stress of PLA dropped. The lower stress at break was attributed to their lower ductility. PLA/Acrylonitile Butadiene Styrene (ABS) blends were studied by Shimizu et al. ⁴⁸ They utilized melt blending method and introduced styrene acrylonitrile GMA copolymer (SAN-GMA) as a reactive compatibilizer and ethyltriphenylphosphonium bromide (ETPB) as a catalyst. When compared to the pristine values, elongation and impact strength was improved while as modulus and tensile strength was slightly reduced.

Applications

PLA is one of the first commodity plastic with reduced energy consumption, waste generation, and emission of green house gases. Hence PLA is currently the most promising and popular material with the brightest development prospect and is considered as the “green” eco-friendly material. PLA as fibers are being widely studied for its various applications. The unique properties like its nat ural soft feel, ease of processing, and resistance to stain and soil, makes PLA fibers to be made utilized in different fiber based products. Some of the current fiber uses include hollow fiberfill for pillows and comforters, bulk continuous filament for carpet, filament yarns, and spun yarns for apparel, spun bond, and other nonwovens and bicomponent fibers for binders and self-crimping fibers. Also PLA fibers find uses in non-woven textiles like upholstery, awnings, hygiene products and diapers. PLA can be used instead of PET in these applications because of its excellence in performance and the fact that the disposable products can be produced from fibers that are from 100% renewable resources and fully biodegradable. PLA as films also have gained more attention in various applications, as they are almost transparent when stress crystallized and have acceptance by customers for food contact.

Polylacticacid were used commercially for the first time as resorbable sutures. Various fields of applications were explored for PLA after this. The properties such as biocompatibility, biodegradability and bioresorbability ^10,48 makes PLA an excellent candidate in medical applications. PLA has been studied for wound healing, tissue engineering, and drug delivery. ⁴⁹ Kricheldrof et al. ¹⁷ had studied the usefulness of PLA as resorbable wound dressing and they succeeded with more than 30 patients being successfully treated with the PLA rich wound dressing method. Li et al. ⁵⁰ had developed PLA stents that can be used in the treatment of ureteral war injuries. Biodegradable membrane by blending PLA with polytrimethylene Carbonate could act as a effective physical barrier to reduce the post operative adhesion formation. ⁵¹ PLA as a surgical implant could facilitate dental extraction wound healing by reducing the destructive effects of fibrinolysis by providing an alternative medium for granulation tissue support in the event that the original fibrin clot fails. ⁵² Since PLA undergoes scission in the body to monomeric units of lactic acid as a natural intermediate in carbohydrate metabolism, ⁵³ they can be effectively made use in drug delivery system. They have been used in the encapsulation of many drugs including savoxepine, ⁵⁴ tyrphostins, ⁵⁵ hormone, ⁵⁶ oridonin, ⁵⁷ fluorescent dyes ⁵⁸ and protein (BSA). PLA along with its copolymers like PLA-polyethylene glycol block copolymer (PLA-PEG) are being used as carriers for bone morphogenetic proteins. ^58,59 The enhanced mechanical properties achieved via copolymerization of PLA enables to make tissue scaffolds. ^46,48,60,61 Narayanan et al. ⁶² had explored PLA for 3D bioprinting applications. Other than the medical uses, PLA is widely used in the area of packaging. PLA has a medium permeability to water vapor and oxygen. ⁸ An improvement in the permeability property can make PLA a substitute to poly(ethylene terephthalate) (PET) in packaging, as high barrier protection is important. ¹ PLA is a growing alternative as a “green” food packaging polymer in two forms: high value films and rigid thermoformed containers. Dannon and McDonalds in Germany, uses PLA in yoghurt cups and cutlery. ¹⁰ Also PLA find use as trays and bowls for fast food, films and trays for fruits and vegetables by companies like McDonalds, Treophan, Natura, IPER. ⁸ In the agricultural fields PLA is suitable as mulch films and bags. ⁵ PLA as a fiber product posses many beneficial characteristics like natural soft feel, easy processing, and resistance to stain and soil. ³⁵ Fila had launched apparels made of PLA. PLA fiber is also being used in sports clothes ² and is resistant to UV radiation. PLA and kenaf are used as composite in electronics applications. Sanyo had used PLA in compact disks and Fujistu had launched computer case made of PLA. ⁶³ The commercially available PLA being used in major fields promises to bring new changes in the polymer industry as a green product.

Conclusions

Bio polymers had received much attention in the last decades due to their potential applications in the fields related to environmental protection and the maintenance of physical health. These categories make a good substitute to the finitely available petroleum products, on which most of the industries are highly dependent. Among the biopolymers, Polylactic acid (PLA) is a prominent one, that has got good mechanical, thermal, and barrier properties, which can further be enhanced with the addition of plasticizers, or blending with other polymers. PLA find applications in biomedical devices, packaging industry, and electronics and so on. The unique properties make PLA a potential green polymer with future scope in different aspects. It can be combined with other materials/polymers to make it suitable for 3D printing, packaging applications in biomedical fields, as coverings in automobiles, by modifying the properties as per the requirements. Undoubtdly, these composites and blend of PLA will nourish the industries both economically and ecologically. The illustration for the study is given in Figure 6 and the summary of the discussions are given in the Table 2.

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Figure 6.

Illustration of the study.

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Table 2.

Summary of the study.

Study	Result
Synthetic route	The main three methods include direct condensation polymerization; azeotropicdehydrative condensation and polymerization through lactide formation.
Structural and crystalline properties	The stereo chemical architecture of lacties (L-levo, D-dextro and meso-lactic acid) determines the molecular architecture of unmodified PLA linear macromolecules. Based on the structural composition of optically active enantiomers of lactic acid(L- and D, L enantiomers), PLA crystals can grow in three forms: α, β and γ.
Mechanical properties	PLA is normally a stiff and brittle polymer. By controlling the stereochemical structuring, the speed and the crystallinity rate can be monitored, thus by allowing a precise control over mechanical properties.
Thermal properties	A ruuberlybehaviour is seen for PLA above its Tg(∼580C), and below this temperature, it becomes a glass which is still capable to creep until it is cooled to its β transition temperature at approximately ∼450C, below which it behaves as a brittle polymer.
PLA blends/composites and its applications	It can be combined with other materials/polymers to make it suitable for 3D printing, packaging applications in biomedical fields, as coverings in automobiles, by modifying the properties as per the requirements.