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Review

The Development of Polylactide Nanocomposites: A Review

by
Purba Purnama
1,2,
Zaki Saptari Saldi
3,4 and
Muhammad Samsuri
5,*
1
School of Applied Science Technology Engineering and Mathematics, Universitas Prasetiya Mulya, Tangerang 15339, Banten, Indonesia
2
Vanadia Utama Science and Technology, PT Vanadia Utama, Jakarta 14470, Indonesia
3
Department of Product Design, Universitas Pembangunan Jaya, Tangerang Selatan 15413, Banten, Indonesia
4
Center for Urban Studies, Universitas Pembangunan Jaya, Tangerang Selatan 15413, Banten, Indonesia
5
Chemical Engineering Department, Universitas Bhayangkara Jakarta Raya, Bekasi 17121, West Java, Indonesia
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2024, 8(8), 317; https://doi.org/10.3390/jcs8080317
Submission received: 16 June 2024 / Revised: 21 July 2024 / Accepted: 9 August 2024 / Published: 10 August 2024
(This article belongs to the Special Issue Sustainable Biocomposites, Volume II)
Browse Figures
Figure 1
<p>The chemical structure of lactide stereoisomers.</p> "> Figure 2
<p>FTIR spectra of PDLLA/hydroxyapatite nanocomposites at various weight ratios (3:1, 2:1, 1:1) (<b>a</b>); magnified FTIR spectra at section A (<b>b</b>) and section B (<b>c</b>); schematic model of hydrogen bonding between PDLLA and hydroxyapatite particles (<b>d</b>). Adapted with permission [<a href="#B31-jcs-08-00317" class="html-bibr">31</a>]. Copyright 2007 American Chemical Society.</p> "> Figure 3
<p>WAXD diffractogram of various organoclays with C16 organic modifiers. Adapted with permission [<a href="#B9-jcs-08-00317" class="html-bibr">9</a>]. Copyright 2002 American Chemical Society.</p> "> Figure 4
<p>The most commonly used nanofillers in polymer nanocomposites and their properties. Adapted with permission [<a href="#B7-jcs-08-00317" class="html-bibr">7</a>]. Copyright 2012 American Chemical Society.</p> "> Figure 5
<p>The schematic illustration structure of polymer nanocomposities using layered silicate nanoparticles [<a href="#B55-jcs-08-00317" class="html-bibr">55</a>]. (Copyright and permission, Elsevier 2003).</p> "> Figure 6
<p>Transmission electron micrograph of acetylated cellulose nanowhiskers (<b>a</b>). FT-IR spectra of acetylated cellulose nanowhiskers, PDLA and PDLA-g-cellulose nanowhiskers (<b>b</b>) [<a href="#B77-jcs-08-00317" class="html-bibr">77</a>]. (Copyright and permission, Springer Nature 2014).</p> ">
Versions Notes

Abstract

:
Polylactide materials present a promising alternative to petroleum-based polymers due to their sustainability and biodegradability, although they have certain limitations in physical and mechanical properties for specific applications. The incorporation of nanoparticles, such as layered silicate (clay), carbon nanotubes, metal or metal oxide, cellulose nanowhiskers, can address these limitations by enhancing the thermal, mechanicals, barriers, and some other properties of polylactide. However, the distinct characteristics of these nanoparticles can affect the compatibility and processing of polylactide blends. In the polylactide nanocomposites, well-dispersed nanoparticles within the polylactide matrix result in excellent mechanical and thermal properties of the materials. Surface modification is required to improve compatibility and the crystallization process in the blended materials. This article reviews the development of polylactide nanocomposites and their applications. It discusses the general aspect of polylactides and nanomaterials as nanofillers, followed by the discussion of the processing and characterization of polylactide nanocomposites, including their applications. The final section summarizes and discusses the future challenges of polylactide nanocomposites concerning the future material’s requirements and economic considerations. As eco-friendly materials, polylactide nanocomposites offer significant potential to replace petroleum-based polymers.

1. Introduction

The consumption of polymer materials has increased significantly since 1950 and it is predicted to generate up to 25,000 million metric tons of plastic waste by 2050 [1]. Polypropylene (PP), polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC) are examples of petroleum polymers commonly used in various applications [2]. However, the disposal of non-degradable petroleum polymer wastes leads to numerous environmental problems, contributing significantly to global pollution. Furthermore, petroleum resources are non-renewable, limiting their long-term sustainability. In response, global environmental regulations have promoted the development of biodegradable polymers as alternatives to petroleum-based polymers.
Numerous biodegradable polymers, such as polylactide (PLA), polycaprolactone (PCL), polylactide-co-glycolic acid (PLGA), polyhydroxy butyrate (PHB), and polyethylene glycol (PEG), have been developed as future material alternatives. Among these, PLA stands out as the most promising replacement for petroleum-based polymers due to its biodegradability and sustainability. Technological advancement in polymeric materials have addressed issues related to material degradability, sustainability, and tunability. Nevertheless, while PLA exhibits good mechanical and thermal properties, further improvements are necessary for its application in a wider range of advanced uses [3].
Nanocomposites represent a promising method for improving polymeric materials, including PLA biopolymers, via the addition of nanoscale inorganic nanoparticles (<100 nm). Blending PLA with nanomaterial fillers to produce PLA nanocomposites is an effective approach to overcome some of PLA’s inherent property limitation [4]. In such PLA nanocomposites, the size and dispersion of the nanoparticles are crucial factors that enhance PLA properties, including its mechanical, thermal, optical, and degradation characteristics [5,6]. PLA nanocomposite development has enabled the modification of biodegradable materials to create eco-friendly products with high-performance and specifically engineered properties. Numerous studies have been conducted on the processing and characterizations of PLA nanocomposites derived from various nanoparticles [6,7,8]; some reports have focused on nanoparticle modification to improve their dispersion in the PLA matrix, thereby enhancing PLA properties [9,10,11,12,13].
This review article aims to present the most significant developments in the area of PLA nanocomposites. It begins with a general discussion of PLA and nanoparticle development, followed by an exploration of the challenges and recent advancements in PLA nanocomposite processing, properties, and prospective applications. We believe that this review will provide meaningful insights into the recent development and future potential of PLA nanocomposite materials.

2. Polylactide (PLA)

PLA is an important biodegradable polymer due to its biocompatibility and biodegradability, making it a viable substitute for conventional polymers in various applications. As a green polymer, it decomposes into naturally occurring metabolites through hydrolysis or enzymatic processes [14]. The basic constituents in PLA synthesis are agricultural raw materials such as sugar, corn, and starch, which are converted to lactic acid through bacterial fermentation. This lactic acid is then transformed into lactide via oligomerization and cyclization, and subsequently polymerized through ring-opening polymerization [15].
PLA is an aliphatic polyester consisting of a lactide-repeating unit, with three different stereoisomers—L-lactide, D-lactide and meso-lactide (D,L-lactide)—as depicted in Figure 1. PLA can be synthesized via two methods: condensation polymerization of lactic acid and ring-opening polymerization of lactide. The former poses the typical limitation of step polymerization, which requires water removal to achieve high yield and polymerization conversion due to the equilibrium process involved. The latter technique, ring-opening polymerization, is generally used to produce high-molecular-weight PLA with stereoregularity via coordination–insertion mechanisms [16]. The purity of L-, D-, and DL-lactide used affects the degree of crystallinity in PLA and mixtures of D- and L-lactide, typically leading to the synthesis of amorphous PDLLA.
PLA is a thermoplastic biocompatible biopolymer that can be either amorphous or semicrystalline. It is stiff and brittle at room temperature, with a glass transition temperature of ~55 °C and a melting temperature of ~180 °C. PLA has good mechanical and thermal properties, thermal plasticity and biocompatibility, making it suitable as an alternative to petroleum-based polymers, such as polyethylene terephthalate (PET), PVC, and high-impact polystyrene (HIPS), in some packaging roles. The mechanical properties of polylactide are summarized in Table 1 [7].
As biodegradable polymers with good mechanical properties, PLA can be used as a substitute for petroleum polymers in various applications, including electronic and automotive [17], packaging [18], textile [19,20], scaffold [21], implant [22], fixation device, and drug delivery system applications [23]. However, for some specific and advanced applications, the properties of PLA may not completely meet the requirements. Therefore, improving the characteristics of PLA is necessary to create high-performance materials with specific and tunable properties.

3. Nanomaterials as Nanofillers

In the field of polymers, composite materials, produced by combining polymer matrices with filler materials, are highly attractive for various applications. The addition of various types and sizes of fillers to polymeric materials aims to improve their final properties. However, while increasing the filler concentration can enhance physical and mechanical properties, it can also lead to issues such as reduced flexibility, increased brittleness, and altered processing characteristics. Consequently, optimizing the content of fillers in polymer blends is crucial.
In recent decades, researchers have extensively studied the effect of nanometer-sized fillers on the formation of polymeric blends. Nano-sized materials offer advantages in interfacial area and particle density per unit volume, and sufficiently small quantities can significantly enhance the physical and mechanical properties of the neat polymer without causing processing problems.
Nanofillers, which have dimensions of less than 100 nm, can be classified based on their shape and size, as follows [7]:
  • Zero-dimension (0D) nanoparticles all have dimensions less than 100 nm.
  • One-dimension (1D) nanofibers or nanowhiskers, such as carbon nanotubes and nanocellulose/cellulose nanowhiskers, have diameters less than 100 nm.
  • Two-dimension (2D) layered nanomaterials, such as clays and graphene, have plate-like or layered structures with thickness of approximately <100 nm.
  • Three-dimension (3D) interpenetrating networks, such as polyhedral oligomeric silsesquioxane (POSS), have interpenetrating network dimensions approximately <100 nm in size; a common example is.
A wide range of materials can be prepared as nanofillers in polymer composites, with the selection of nanofillers depending on the targeted applications. For example, inorganic nanoparticles can improve mechanical and antimicrobial properties in packaging applications [24], while organic cellulose nanowhiskers enhance material strength while maintaining its biodegradability [25].
Inorganic nanoparticles commonly used to improve PLA properties include silver nanoparticles, zinc oxide, magnesium hydroxide, silver–copper nanoparticles, titanium dioxide, calcium hydroxyapatite, tricalcium phosphate, calcium carbonate, silica, and clays. Silver nanoparticles possess antimicrobial activity due to their ability to release Ag+, making them suitable for tissue engineering and biomedical applications [26]. Zinc oxide nanoparticles have been used in packaging applications [27,28], while magnesium hydroxide nanoparticles enhance the mechanical properties of PLA-based materials and inhibit an inflammatory response in biomedical applications by neutralizing the acidic degradation process [29,30]. Tricalcium phosphate and hydroxyapatite are widely used in bone implant and fixation devices [30,31]. Organically modified layered silicates significantly improved the mechanical properties and biodegradability of PLA [8]. In addition, organically modified nano-clays can drive morphological changes towards intercalation and exfoliation structures, enhancing PLA matrix properties [5]. Furthermore, the addition of multifunctional nanodiamonds to the PLA matrix can increase the strain failure by approximately 280% and accelerate bone mineralization [32].
Nanoparticles based on bioresources, such as chitosan, protein, peptides, starch, polysaccharides, lipid, cellulose and carbon-based materials, have also been developed as nanofillers for PLA-based materials. The combination of PLA and chitosan improves hydrophilicity, mechanical properties, and porosity, leading to slower in vitro degradability, which is suitable for 3D scaffold materials [33]. Improved hydrophilicity of PLA/chitosan systems also benefits membrane applications [34]. PLA/collagen hybrid materials exhibit higher porosity, interconnected pores, and greater mechanical strength for use in articular cartilage tissue engineering [35]. Carbon-based nanomaterials, such as carbon nanotubes and graphene nanosheets, enhance the mechanical and thermal properties of PLA-based materials [36,37,38].
Inorganic nanoparticles such as nanofillers for polymeric matrices often face limitations in miscibility due to their chemical structures and characteristics [39,40]. Surface modification is necessary to improve their miscibility in polymeric matrix [41,42]. For instance, the presence of organic modifiers on the nanoparticle surface enhances interaction and miscibility with the polymeric matrix [31]. Molecular interactions between nanoparticle surfaces and polymer chains can form percolation networks, well-dispersed nanoparticles, and strong molecular bonding, resulting in improved physical and mechanical properties [43]. The presence of –OH functional groups on nanoparticle surfaces affects their hydrophilicity and interaction in PLA-based nanocomposites. The number of –OH functional groups correspond to interfacial compatibility with hydrophobic PLA molecules. For example, cellulose nanowhiskers with a small number of –OH functional groups have limited compatibility with the PLA matrix, while a high number of –OH groups lead to hydrophilicity and break interfacial compatibility [44]. The abundance of –OH functional groups in cellulose nanowhiskers can be reduced via esterification reactions [45]. In PLA–hydroxyapatite nanocomposites, the formation of hydrogen bonds between the C=O groups in PLA chains and the –OH groups on nanoparticle surfaces improves nanoparticle miscibility and material properties [31]. Hydrogen bonding between the PLA matrix and hydroxyapatite has been evaluated through Fourier transform infrared (FTIR) spectroscopy, with evidence shown by the splitting of the C=O stretching vibration and P–OH vibration peaks, as shown in Figure 2 [31]. The –OH functional groups on nanoparticle surfaces can also serve as co-initiators of lactide ring-opening polymerization, enabling direct PLA grafting. These methods allow one to control the hydrophobic/hydrophilic compatibilities and improve interfacial interactions between fillers and the PLA matrix [45].
In layered clay materials, which consist of several stacked layers, organic modifiers play an important role in dictating the gallery spacing and dispersibility in the polymer chains [9]. The chain length of organic modifiers and the size (length) of the layered clay affect the gallery spacing, thereby influencing the dispersion of the layers into the PLA matrix to form intercalated or fully exfoliated structures. Changes in stacking order and dispersion can be observed by shifting and broadening peaks in wide-angle X-ray diffraction (WAXD) patterns, as shown in Figure 3 [9]. For instance, smectite clay with a length of ~50 nm is easier to distort compared to mica with a length of ~200–250 nm, resulting in less-ordered stacking of the layered clay (exfoliation). The presence of a small number of –OH functional groups in the organic modifier also facilitates the possibility of grafting PLA chains onto the layered clay materials via an in situ polymerization process [46,47,48]. The dispersibility, molecular chain interaction, and stacking order of the layered clay nanoparticles in the polymer matrix significantly enhance the physical and mechanical properties of polymer nanocomposite materials. The addition of nanomaterials (nanoparticles or layered clay) to polymer matrices aims to improve the physical, mechanical, and thermal properties of PLA. Specific improvements include reduced gas permeabilities, an increased crystallization rate, enhanced mechanical strength, and elevated glass transition temperature. Toxicity and environmental concerns arise from the use of nanoparticles. A toxicity evaluation for humans directly using inorganic nanoparticles is crucial. However, incorporating a small portion of nanoparticles into blended biopolymers can reduce both toxicity and the environmental impact. In biomedical applications, nanoparticles with low toxicity or derived from biologically based sources are preferred for combining with biocompatible and biodegradable polymers.

4. Polylactide (PLA) Nanocomposites

PLA nanocomposites represent an advanced class of hybrid materials comprising PLA and nanoparticles. The addition of nanofiller to the PLA matrix serves to improve its physical and mechanical properties, including its optical characteristics, mechanical strength, modulus, heat resistance, lower gas/moisture permeation, reduced flammability, increased crystallization rate, biocompatibility, and biodegradability. A wide range of nanofillers with various properties have been employed in polymer nanocomposites. Generally, the most common nanofillers are classified as metal nanoparticles, metal oxide ceramic nanoparticles, silicates, and non-oxide ceramics. These nanofillers are applied to the polymer matrix to improve its properties, as shown in Figure 4 [7].
Researchers have developed various methods to prepare PLA nanocomposites [6,8,9,47,48,49,50,51,52,53,54]: the solution method [49], the melt process [6,8,9,50], in situ polymerization [47,51,52], master batch procedures [53], and the supercritical fluid method [48,53,54]. In the solution method, PLA and nanomaterials are simply mixed in suitable solvents, with solubility in organic solvents being critical for obtaining highly dispersed PLA nanocomposites. The melt process involves directly mixing PLA and nanomaterials at high temperatures, with processing temperature and time being the critical factors in minimizing the thermal degradation of the polymer matrix. In situ polymerization of PLA onto a nanoparticle’s surface can be performed using –OH functional groups from organic modifiers or co-initiators, resulting in the grafting of PLA onto nanoparticles surfaces [47,48]. The masterbatch method starts with in situ polymerization, followed by mixing with high-molecular-weight PLA to improve compatibility between the nanomaterials and the PLA matrix [53]. Supercritical fluid can be used for the direct mixing of nanomaterials into PLA homopolymers. Each method follows a different mechanism to incorporate nanomaterials into the PLA matrix, depending on their compatibility, chain mobility, conformation, and miscibility during processing.
Various types of nanoparticles have been used in the development of PLA nanocomposites including layered silica/clay, cellulose nanowhiskers, metal oxides, and polyhedral oligomeric silsesquioxanes (POSSs) to enhance the thermomechanical properties of PLA. The incorporation of layered silicates into PLA has been developed over 25 years due to their ability to improve these properties at low contents. The different characteristics of hydrophilic-layered silicate and organophilic polymer chains affect the dispersion level and galley exfoliation of the layered silicate structure, which correlates with the enhancement of the thermomechanical properties of PLA. Three nanocomposite structures are possible depending on the interaction between PLA and organically modified layered silicate particles, as illustrated in Figure 5 [55]:
  • Intercalated structure: PLA chains are inserted into the layered silica structure in a regular crystallographic fashion, with a repeating layer distance of a few nanometers.
  • Flocculated structure: PLA chains are inserted into the silicate layer, though the stacked layer flocculates at times due to the hydroxylated edge–edge silicate interactions.
  • Exfoliated structure: Individual silicate layers are completely separated and are distributed homogeneously in the PLA matrix.
PLA–layered silicate nanocomposites can be produced via melt blending [5,8,9,50], solution casting [49], and in situ polymerization [5,47,48,51,52,53,54], including methods involving supercritical fluids [48,53,54]. The dispersion of layered silicates in the PLA matrix via melt blending and solution casting is limited due to their suitability and compatibility. In these methods, the polymer chains diffuse from bulk into the galleries between silicate layers, leading to dispersion ranging from intercalated to exfoliated structures [56,57]. To increase the affinity between layered silicate and the PLA matrix, inorganic cations inside silica galleries are substituted by organo-modifying agents. Among processing methods, in situ intercalative polymerization is optimal for generating highly exfoliated PLA–layered silicate nanocomposites. The –OH groups in layered silicate can initiate PLA polymerization and graft PLA chains onto layered silicate/nanoparticle surfaces [58]. In supercritical fluid methods, monomer solubility and the presence of –OH functional groups on organic modifiers in layered silicates act as co-initiators of PLA polymerization, with supercritical conditions helping to expand the galleries of layered silicates [48]. Research has shown that the presence of layered silicate nanoparticles in the PLA matrix leads to improved mechanical properties, including high storage modulus (solid and melt states), enhanced flexural properties, higher heat distortion temperature, decreased gas permeability, and an increased rate of biodegradability compared to neat PLA [58]. Table 2 provides a comparison between PLA and its nanocomposites [57].
Carbon nanotubes (CNTs) are a carbon-based material with excellent mechanical properties and can be used as reinforcing agents in polymeric matrices. CNTs are an allotropic form of carbon, like diamond, graphite, and fullerene, and can be classified into single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs). PLA–carbon nanotube (PLA-CNT) nanocomposites can be produced via solvent evaporation, in situ polymerization and the melt blending process [59,60,61,62]. Due to their lack of solubility, the surface modification/functionalization of CNTs is required to increase their dispersion in the PLA matrix. Specific functional groups, such as –COOH and –OH, can be grafted onto the CNT surface, which can then interact with PLA functional groups via hydrogen bonding [63], resulting in a significant improvement in thermal and mechanical properties, as shown in Table 3 [64]. The excellent electrical conductivity properties of CNTs are potentially useful in biomedical applications. For instance, conductive PLA–CNTs have been developed to support cell growth and tissue healing; however, while CNTs possess specific characteristics to support these processes, their toxicity and biocompatibility need further evaluation. Other carbon-based nanoparticles, such as graphene oxide, can be dispersed homogeneously in the PLLA matrix after grafting with PLLA chains, thereby enhancing mechanical and thermal properties [65].
Metal-based or metal oxide nanoparticles can also be used as reinforcing agents in PLA nanocomposites. Several types of metal/metal oxide nanoparticles exist, such as alumina (Al2O3), titanium oxide (TiO2), calcium hydroxyapatite, magnesium hydroxide, zinc oxide (ZnO), iron oxide (Fe2O3 and Fe3O4), gold, silver, copper, etc. Some nanoparticles have antimicrobial properties; for example, silver nanoparticles exhibit excellent antimicrobial activity via the formation of Ag+ ions, which then complex with sulfur-, nitrogen-, or oxygen-containing functional groups from bacterial enzymes to destabilize the cell walls [66]. Researchers have also combined TiO2 with the PLA matrix [67,68,69,70]. However, the direct mixing of TiO2 with the PLA matrix tends to lead to aggregation due to compatibility issues. To increase compatibility with the PLA matrix, surface modification of TiO2 by propionic acid and alkyl amine can be carried out, with additional advantages in photodegradability [67].
In another study, the addition of calcium hydroxyapatite into PLA blends enhanced their mechanical and gas barrier properties, exhibiting potential for orthopedic implant applications [71]. Modified magnesium hydroxide nanoparticles have demonstrated the ability to prevent acid-induced inflammation of poly(lactide-co-glycolide) by neutralizing the acidic environment [32,33]. Researchers have also shown that silane-treated ZnO can be blended with and dispersed homogeneously in the PLA matrix, improving its mechanical performance and thermal stability [72]. Additionally, PLA–silver nanocomposites show strong mechanical properties and antibacterial activity [29]. In another study, grafting long-chain PLAs onto SiO2 improved the interfacial interaction between the two, changed the crystalline morphology, and improved nucleating effectiveness [73].
The size and compatibility of nanoparticles are critical factors in enhancing PLA properties. Smaller nanoparticle sizes increase surface area contact to PLA chains. An optimum nanoparticle concentration is necessary to achieve the best performance in PLA blends, including mechanical and thermal properties. Exceeding this level can lead to reduced flexibility, increased brittleness, and diminished mechanical properties. The compatibility of nanoparticles is related to its dispersion in PLA matrices. In some cases, nanoparticles require surface modification to ensure uniform dispersion within PLA matrices. Poor compatibility can result in particle aggregation, thereby compromising the mechanical properties of the PLA-blended materials. Proper compatibility ensures nanoparticles are highly and uniformly dispersed in the PLA matrices, facilitating effective nucleation to enhance crystallinity, mechanical, and thermal properties.
For future applications, bio-based nanoparticles have attracted attention due to their sustainability and environmental friendliness. Cellulose nanowhiskers are promising candidates in this regard and exhibit excellent mechanical properties [74]. They are generated from bioresource materials, with the diameters of whisker-like regions of the polysaccharide chain ranging from 5 to 30 nm. In the PLA–cellulose nanowhisker nanocomposites, the hydrophilic nature of the nanowhiskers can be modified by surface functionalization [45,75,76] and polymer grafting [44,45]. Additionally, surface modifiers for cellulose nanowhiskers can also be investigated for their function as PLA polymerization co-initiators. Acetylated cellulose nanowhiskers have been successfully isolated via hydrolysis and acetylation with sizes of 200–300 nm in length observed by transmission electron microscope (TEM) analysis. Grafting PDLA onto cellulose nanowhiskers was confirmed by FTIR spectra, as depicted in Figure 6 [77]. In terms of property improvements, organic surface modifiers enhanced the crystallinity and thermomechanical properties of PLA materials by acting as a reinforcing agent, as shown in Table 4 [76]. Furthermore, other bio-based nanoparticles, such as stereocomplex PLA, can be utilized as a nucleating agent to enhance the thermomechanical properties of PLA [78]. Stereocomplex PLA exhibits good miscibility, as it consists of lactide fragments and enhances the mechanical properties of high-molecular-weight PLA by increasing its crystallinity.

5. PLA Nanocomposite Applications

PLA has important characteristics for future development: sustainability, as it is produced from renewable resources, and degradability, as its waste can be composted and degraded completely. Based on these features, PLA-based materials are the best candidates for replacing petroleum-based polymers. The addition of nano-sized inorganic or organic materials to the PLA matrix enhances its mechanical, thermal, barrier, and physicochemical properties compared to the properties of neat PLA. These improvements make PLA nanocomposites ideal green plastics for a wider range of applications with specific technical properties such as packaging, automotives, biomedical, and high-performance materials.
In packaging applications, ductility, gas and water vapor permeability, and antibacterial activity are critical properties to consider. The improvement in permeability is achieved through the high dispersion of nanoparticles, which can postpone the molecular pathway and force the diffusion path to become more tortuous. The dispersion of nano-clays in the PLA matrix provides excellent barrier properties through tortuous path effects [79,80]. Certain metal oxides exhibit advantages in antibacterial activity, making them suitable for food packaging applications. Some property modifications and improvements in packaging applications are summarized in Table 5.
The thermal and mechanical properties of automotive and high-performance materials must meet a required performance specification regarding, for example, processability, tensile and flexural modulus, ductility, impact resistance, heat distortion temperature, and long-term performance. The combination of PLA and organoclay up to a loading of 4% has shown a significant improvement in the heat distortion temperature, rising by approximately 18 °C, and a notable enhancement in the tensile and flexural moduli [8,55,93]. In another study, the excellent dispersion of 10% cellulose nanowhiskers in the PLA matrix increased the heat distortion temperature to 120 °C and promoted excellent crystallinity [45]. The surface modification of graphene oxide via the grafting of PLLA chains also increased the interfacial interaction between the nanoparticles and PLA matrix, resulting in an improved reinforcement effect and enhanced crystallization kinetics of the PLA matrix [65].
PLA nanocomposites can also be applied in clothing and disposable garments, upholstery, nappies, etc. In these applications, PLA is converted into fibers or non-woven textiles. For over 30 years, Dupont (E.I. du Pont de Nemours and Company, Wilmington, DE, USA) has developed materials and their processing with two basic types of sheets: surface-bonded materials, which are stiffer and have the feel of a paper-based structure, and spot-bonded materials, which are more flexible and have the feel of a fabric-based structure [94]. PLA nanocomposites have been implemented in common applications, such as furniture, suitcases, grinding discs, and automotive parts, where mechanical and thermal properties are the main focus of PLA material development.
Biomedical applications represent one of the most advanced uses of PLA nanocomposites. Materials for such applications should exhibit biocompatibility, cellular interactions, tunable thermal and mechanical properties, and a controlled degradation rate. As advanced materials, the combination of nanomaterials and PLA presents the advantage of controlling these properties while maintaining the biocompatibility and biodegradability of PLA. Thus, PLA nanocomposites are suitable for biomedical applications despite some limitations in their physical and mechanical properties.
The selection of nanomaterials is significant in generating appropriate biomaterials with tunable properties and has a notable impact on the quality of newly formed tissues. In the tissue engineering of artificial support, PLA has been combined with calcium hydroxyapatite and tricalcium phosphate (TCP) to improve its surface biocompatibility to support cell growth [95,96,97,98,99]. Hydroxyapatite is the most famous nanomaterial utilized in PLA nanocomposites due to its chemical similarity to mineralized human bone tissue [96,99]. The presence of hydroxyapatite in bone scaffold materials supports the induction of bone regeneration, self-repair, the differentiation of osteoblasts, and osteoclastogenesis [100,101]. Research has shown that PLLA and highly dispersed grafted nano-hydroxyapatite exhibit higher cell compatibility and promote new bone formation [102,103]. Hydroxyapatite-grafted PDLA increases the dispersibility of hydroxyapatite in the PLLA matrix, thus improving its physical and mechanical properties for applications in orthopedic implants [71]. Pietrzykowska et al. reported that a PLA–nanohydroxyapatite composite with a 1:1 volume ratio achieved Young’s modulus and compressive strength values similar to those of natural bone [104]. Tricalcium phosphate, employed incorporated PLA scaffolds, supported the proliferation and osteogenic differentiation of human adipose-derived stem cells [105]. In bone tissue engineering, PLA/beta-TCP scaffolds induced neo-vessel formation, which can be exploited for therapeutic factor deliveries [106]. The presence of beta-TCP also showed potential to induce osseointegration during bone regeneration [107]. Bioactive glass and resorbable PLA have shown potential in bone regeneration with uniform morphology and dispersion [108]. Roether’s group successfully developed a highly bioactive bioresorbable scaffold for tissue engineering [109], and another researcher created scaffold materials for bone marrow mesenchymal stem cell culture from PLLA and calcium-deficient nanohydroxyapatite [110].
Other nanomaterials have also been utilized with PLA for biomedical applications. Collagen and chitosan have been combined with PLA to produce scaffolds in cartilage tissue engineering based on their microstructure, water uptake, viability, and mechanical properties [35]. The nano- and macro-sized pore scaffold materials synthesized from PLLA and exfoliated montmorillonite clay have also promoted cell growth and led to reinforced properties [111].
The presence of nanoparticles in the PLA matrix provides enhanced barrier properties, which can be utilized in drug delivery systems to minimize material swelling and promote the controlled release of the active compound. Chen et al. reported on the development of self-assembled drug molecules on PLA–TiO2 nanocomposites [70]. In another study, rhodamine was completely released in less than 10 h from PLA and hollow gold nano shells, serving as a photo-triggered drug release system [112]. A smart drug delivery system has also been developed for the oral delivery of doxorubicin using graphene oxide/poly(2-hydroxyethylmethacrylate)-g-poly(lactide)-b-polyethyleneglycol-b-poly(2-hydroxyethylmethacrylate)-g-poly(lactide), which exhibited antitumor activity and biocompatibility [113]. The nanocomposite of PLA and montmorillonite intercalated with gentamicin and neomycin has also been utilized as a drug delivery system due to its antibacterial properties and improved degradation rate [114,115,116,117].
Recently, PLA nanocomposites have been applied for sensors [118,119,120] and high-performance flame-retardant materials. Blending PLA with 1D CNTs and carbon black results in an interconnected CNT network, which enhances electrical conductivity and the mechanical properties of PLA nanocomposites [118]. Various nanofillers such as CNTs, quantum dots, nano-clays, nanofibers, and graphene can be utilized to improve the electrical properties, thermal conductivity, and dielectric constant of PLA, which are essential for various sensing applications [119]. BaTiO3 nanosheets significantly enhance the piezoelectric properties of PLA nanocomposites, showing potential for applications in human motion monitoring [120]. Incorporating bio-based additive nanosheets (PP-Fe) into PLA materials improves specific properties such as fire safety, ultraviolet resistance, and flame retardancy [121]. PLA blended with bio-inspired halloysite nanotubes exhibits excellent fire safety and overall properties of bio-composite materials [122].

6. Conclusions

The development of future materials necessitates the consideration of environmental issues, material sustainability, and the tunability of material properties. PLA nanocomposites are promising materials in this regard and can fulfil these criteria. PLA is sustainable and environmentally friendly as it can be produced from plant-based and degradable materials. The challenge of PLA nanocomposite development is to meet the specific properties required by certain applications. PLA has different characteristics from some nanoparticles, which limits their processing and compatibility. The surface modification of nanoparticles by organic molecules helps to increase their compatibility and the possibility for them to act as co-initiators to graft PLA molecules. The compatibility of nanoparticles relates to their dispersion in the PLA matrix. Highly dispersed nanoparticles in the PLA matrix enhance its properties up to a certain level, which can be controlled depending on specific requirements. The main applications of PLA nanocomposites include packaging, high-performance materials, and biomedical applications. The selection of nanoparticles will expand the applications of PLA nanocomposites. From another perspective, the molecular modification of PLA polymers will widen their applications with specific developments in their properties. The main limitation in competing with conventional materials is their higher price. This may lead to a future direction to lower the cost and identify suitable processing methods for PLA nanocomposites. The optimization of the number and compatibility of nanoparticles is the option to minimize the cost of PLA nanocomposites.

Author Contributions

Conceptualization, M.S. and P.P.; resources, Z.S.S. and P.P.; data curation, P.P. and Z.S.S.; writing—original draft preparation, P.P.; writing—review and editing, P.P. and M.S.; visualization, Z.S.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The chemical structure of lactide stereoisomers.
Figure 1. The chemical structure of lactide stereoisomers.
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Figure 2. FTIR spectra of PDLLA/hydroxyapatite nanocomposites at various weight ratios (3:1, 2:1, 1:1) (a); magnified FTIR spectra at section A (b) and section B (c); schematic model of hydrogen bonding between PDLLA and hydroxyapatite particles (d). Adapted with permission [31]. Copyright 2007 American Chemical Society.
Figure 2. FTIR spectra of PDLLA/hydroxyapatite nanocomposites at various weight ratios (3:1, 2:1, 1:1) (a); magnified FTIR spectra at section A (b) and section B (c); schematic model of hydrogen bonding between PDLLA and hydroxyapatite particles (d). Adapted with permission [31]. Copyright 2007 American Chemical Society.
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Figure 3. WAXD diffractogram of various organoclays with C16 organic modifiers. Adapted with permission [9]. Copyright 2002 American Chemical Society.
Figure 3. WAXD diffractogram of various organoclays with C16 organic modifiers. Adapted with permission [9]. Copyright 2002 American Chemical Society.
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Figure 4. The most commonly used nanofillers in polymer nanocomposites and their properties. Adapted with permission [7]. Copyright 2012 American Chemical Society.
Figure 4. The most commonly used nanofillers in polymer nanocomposites and their properties. Adapted with permission [7]. Copyright 2012 American Chemical Society.
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Figure 5. The schematic illustration structure of polymer nanocomposities using layered silicate nanoparticles [55]. (Copyright and permission, Elsevier 2003).
Figure 5. The schematic illustration structure of polymer nanocomposities using layered silicate nanoparticles [55]. (Copyright and permission, Elsevier 2003).
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Figure 6. Transmission electron micrograph of acetylated cellulose nanowhiskers (a). FT-IR spectra of acetylated cellulose nanowhiskers, PDLA and PDLA-g-cellulose nanowhiskers (b) [77]. (Copyright and permission, Springer Nature 2014).
Figure 6. Transmission electron micrograph of acetylated cellulose nanowhiskers (a). FT-IR spectra of acetylated cellulose nanowhiskers, PDLA and PDLA-g-cellulose nanowhiskers (b) [77]. (Copyright and permission, Springer Nature 2014).
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Table 1. The physical properties of Poly L-lactide (PLLA) [7].
Table 1. The physical properties of Poly L-lactide (PLLA) [7].
PropertiesUnitTypical Value
Densityg/cm31.25–1.36
Melting temperature°C170–190
Glass transition temperature°C50–65
Heat of fusionJ/g93–203
Tensile modulusGPa6.9–9.8
Tensile strengthGPa0.12–2.26
Elongation at break%12–26
Table 2. Comparison of properties between neat polylactide and polylactide nanocomposites (montmorillonite) [57].
Table 2. Comparison of properties between neat polylactide and polylactide nanocomposites (montmorillonite) [57].
PropertiesNeat PolylactidePolylactide Nanocomposites (Montmorillonite)
Storage Modulus1.63 GPa2.32 GPa
Flexural strength4.8 GPa5.5 GPa
Heat Distortion Temp.76.2 °C94 °C
O2 gas permeability coef.200 mL mm m−2 day−1 MPa−1173 mL mm m−2 day−1 MPa−1
Table 3. Thermal and mechanical properties of PLA and PLA/MWNT-g-PLA nanocomposites [64].
Table 3. Thermal and mechanical properties of PLA and PLA/MWNT-g-PLA nanocomposites [64].
MaterialsTg (°C)Tm (°C)ΔHm (J/g)Modulus (MPa)Yield Strength (MPa)
PLA53.4165.934.91928 ± 15649.3 ± 2.2
PLA/MWNT-g-PLA0.156.7167.336.92250 ± 5959.4 ± 3.7
PLA/MWNT-g-PLA0.256.4166.836.12357 ± 8864.0 ± 4.0
PLA/MWNT-g-PLA0.556.5166.935.32387 ± 6665.7 ± 5.3
PLA/MWNT-g-PLA1.056.2166.038.82541 ± 19972.3 ± 3.7
PLA/MWNT-g-PLA5.055.9165.039.12504 ± 22148.0 ± 8.9
Note: Tg is glass transition temperature, Tm is melting temperature, and ΔHm is enthalpy of melting.
Table 4. Mechanical properties of PLA–cellulose nanowhisker nanocomposites with different surface modifiers [76].
Table 4. Mechanical properties of PLA–cellulose nanowhisker nanocomposites with different surface modifiers [76].
MaterialsCellulose Nanowhisker TypeStorage Modulus (MPa)
Neat PLA-1300
PLA–cellulose nanowhiskersAmino-based3740
PLA–cellulose nanowhiskersn-propyl-based4250
PLA–cellulose nanowhiskersMathacrylic-based4360
PLA–cellulose nanowhiskersAcrylic-based4130
Table 5. PLA nanocomposite development for packaging applications: nanoparticles and properties.
Table 5. PLA nanocomposite development for packaging applications: nanoparticles and properties.
MaterialsNanoparticlesSpecific Properties ImprovementsRef.
PLA-ClayNanoclayIncreasing Cloisite 20A improves the water vapor barrier properties but reduces the tensile properties[81]
PLA-CuCoper nanoparticlesThe presence of copper nanoparticles supports antibacterial characteristics[82]
PLA-AgAg nanoparticlesLower WVTR (4%) at 1% Ag loading
Lower OTR value (22%) at 1% Ag loading		[83]
PLA-ZnOZnOImprovement in thermomechanical, barrier properties, and antibacterial activity[84,85]
PLA-TiO2TiO2 nanoparticlesLow WVTR (51%) at 5% TiO2 loading [86]
PLA-grapheneGraphene oxide87.6% reduction in water vapor permeability and good processability[87]
PLA-Ag-Cu-CEOAg-Cu NP and cinnamon essential oilIncreasing cinnamon essential oil reduces the barrier properties
The Ag-Cu NP supports the antibacterial inhibition		[88]
PLA-MgOMgO nanoparticlesLow OTR (25) at 2% MnO loading[89]
PLA-MgOMgO nanoparticles2% content affects good transparency, UV radiation screening, antibacterial efficacy, mechanical and oxygen barrier properties[90]
PLA-ZnOZnO nanoparticlesLow WVTR (40%) at 9% ZnO loading
Low OTR (33.5%) at 9% ZnO loading		[91]
PLA-ZnO
PLA-MgO		Metal oxide ZnO and MgOZnO and MgO showed antimicrobial inhibition, but a decrease in mechanical strength[92]
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Purnama, P.; Saldi, Z.S.; Samsuri, M. The Development of Polylactide Nanocomposites: A Review. J. Compos. Sci. 2024, 8, 317. https://doi.org/10.3390/jcs8080317

AMA Style

Purnama P, Saldi ZS, Samsuri M. The Development of Polylactide Nanocomposites: A Review. Journal of Composites Science. 2024; 8(8):317. https://doi.org/10.3390/jcs8080317

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Purnama, Purba, Zaki Saptari Saldi, and Muhammad Samsuri. 2024. "The Development of Polylactide Nanocomposites: A Review" Journal of Composites Science 8, no. 8: 317. https://doi.org/10.3390/jcs8080317

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