**3. PLA**/**Nanocellulose Composites**

Nanocellulose (NC) is an organic homopolymer, constituted of 1,4-anhydroglucopyranose repeating units (Figure 17). Cellulose nanomaterials are cost-effective, renewable, thermally stable up to 200 ◦C, and lightweight and provide high strength and stiffness. As a biomaterial of anisotropic shape, good biocompatibility, excellent mechanical properties, and tailorable surface chemistry, it is of high interest for material science and biomedical engineering (wound dressing, nanocarriers for drug delivery, and scaffolds for tissue engineering) [73–76]. Nanocellulose of defined nano-scale structural dimensions is derived from cellulosic extracts or processed materials. The term covers cellulose nanocrystals (CNCs; also called nanocrystalline cellulose (NCC) or cellulose nanowhiskers (CNWs)), cellulose nanofibrils (CNFs; also known as nano-fibrillated cellulose (NFC)), and bacterial cellulose (BC). They have different crystallinities, surface chemistries, and mechanical properties. Moreover, the rod-like CNCs show concentration-dependent liquid crystalline self-assembly behaviour. Micrometre-long entangled fibrils of CNFs contain both amorphous and crystalline cellulose domains, and their entanglement results in highly viscous aqueous suspensions even at relatively low concentrations (<1 wt%). BC is pure cellulose produced extracellularly by microorganisms.

The development of green nanocomposites based on polylactide and bio-based cellulose nanofillers for different applications, particularly for packaging and biomedical materials, has attracted significant attention [77]. The excellent properties of renewable and bio-based PLA/NC nanocomposites, their cytocompatibility and biodegradability, and their relatively low cost make those materials suitable for biomedical devices with enhanced mechanical performance. The earliest reports can be found in the cited reviews concerning polylactide containing various nanocellulose fillers. The most recent trends, illustrated by the examples given below, include porous and fibrous materials.

Most frequently, the preparation of PLA/nanocellulose composites involves melt processing, wet processing, or their combination. However, the hydrophilic nature of the surface hydroxyl groups on NC particles results in the poor dispersion of nanocellulose in nonpolar media, poor interface adhesion, and agglomeration in the PLA matrix. Thus, the surface modification of reinforcing nanocellulose fillers is often employed to improve their interfacial compatibility with the hydrophobic

polylactide matrix. The -OH residues provide abundant active sites for covalent bond formation (oxidation, esterification, etherification, acetylation, carboxymethylation, silylation, and polymer grafting onto the polysaccharide backbone) as well as noncovalent binding. The e fficiency of the surface modification strongly depends on the nature of the nanoparticles. Surface treatment may improve both the interfacial adhesion and dispersibility of NC. A significant increase in both mechanical and thermomechanical properties was observed with the addition of surface-treated cellulose. Moreover, hydrophobic nanocellulose can act as a barrier agent. Interfacial bonding and adhesion mechanisms determine the final properties of the polymer composites. Various factors should be taken into account to predict the overall e ffect. Molecular dynamics simulations characterized the interfacial structure and adhesion behaviour of crystalline nanocellulose in contact with polylactic acid [78]. It was found that adhesion between PLA and the surface of cellulose nanofibers is a ffected not only by the polarity of the functional groups and hydrogen bonds between CNC and PLA but also the surface roughness. PLA macromolecules can adopt conformations that accommodate the cellulose surface. The rougher the surface of the cellulose planes, the stronger the adhesion. Interestingly, among the four available interfaces, the best adhesion can be achieved at the (010) one due to the greater number of hydrogen bonds formed between the polar (010) surface and the polymer chain. However, the polar (110) and (1−10) surfaces produced similar adhesion work to the nonpolar (100) one (van der Waals effect), which illustrates the amphiphilicity of cellulose.

**Figure 17.** Structural characteristics of cellulosic fibre [75]. Reprinted from International Journal of Biological Macromolecules. Copyright (2019), with permission from Elsevier.

The functionalization not only enhanced the dispersion of modified NC in polymer composites but also may improve their mechanical properties, supporting specific interactions at the interface between the particles and the matrix. The improved association between the polymer matrix and the filler facilitates stress-transfer and enhances the nanocomposite strength. The amount of cellulose nanofillers determines the crystallinity and mechanical properties of the composite. Well-dispersed nanocellulose fillers act as nucleation sites in PLA, improving the degree of crystallinity and increasing the temperatures of phase transitions. The changes depend on the filler content and its surface chemistry, as well as on the technique used for the preparation of the nanocomposites. For example, ultra-strong PLA/CNC nanocomposites (an ultimate strength of 353 MPa and a toughness of 107 MJ/m3) of increased Tg (93–95 ◦C) can be manufactured at a large scale through the surface modification of the nanocrystals, liquid-assisted extrusion, and solid-state drawing [79]. Silylation was applied to improve the surface compatibility between hydrophobic PLA and hydrophilic bamboo cellulose nanowhiskers

(BCNWs) [80,81]. The glass transition temperature, crystallinity, tensile strength, and tensile modulus decreased irrespectively of the silane used. However, the maximum elongation at break for the samples containing silanized cellulose increased to 213.8% (16 wt% triethoxyvinylsilane), 111.3% (8 wt% aminopropyltriethoxysilane), 255.3% (8 wt% methacryloxypropyltrimethoxysilane), and 209.8% (8 wt% mercaptopropyltrimethoxysilane) with respect to 12.4% of the untreated PLA/BCNW composite. The results sugges<sup>t</sup> interactions between the ester groups of the PLA backbones and both the functional organic groups and silanol residues of the coupling agents. Many crazes were produced and absorbed most of the tensile energy during mechanical tests (wire-drawing and formation of thin necks) (Figure 18).

**Figure 18.** Tensile properties of PLA/composites containing bamboo cellulose nanowhiskers (BCNWs) treated with vinyltriethoxysilane (**a**), and exemplary stress-strain curves (**b**) [80]. Reprinted with permission from Springer Nature: Journal of Materials Science. Copyright (2018).

It was also shown that the dispersion of the cellulose nanocrystals and formation of a percolation network can be influenced by the molecular weight of the PLA and its crystallizability [76]. Lower CNC percolation concentrations could be obtained in PLA matrices of low molecular weight. CNCs more easily interpenetrate shorter PLA chains dissolved in a good solvent. The CNC percolation concentration could be lowered even more, providing high enantiomeric purity of polylactide chains. Upon solvent evaporation, PLA chains crystallized around the dispersed CNCs, which could prevent the further re-agglomeration of the latter. The rheological properties and thermal stability of such blends were improved.

3-Methacryloxypropyltrimethoxysilane was also used to modify the surface of cellulose nanofibrils (CNF) that were then used as fillers in poly(lactic acid) [82]. It was found that the silanization slightly decreased the thermal stability of the CNF, but their morphological integrity and rod-like morphology were retained. The treated nanofibrils dispersed well and crossed with each other as a percolated network in the PLA matrix. The tensile strength and elongation of the blends changed with the content of modified CNF. The highest tensile strength was shown for PLA with 1 wt% of the nanocellulose filler (Figure 19). The results were much better than those obtained with BCNW, which confirms the role of percolation for the properties of the composites.

**Figure 19.** Tensile properties of PLA containing cellulose nanofibrils (CNF) modified with 3-methacryloxypropyltrimethoxysilane [82]. Reproduced with permission of John Wiley & Sons Ltd. Copyright (2012).

A similar concentration dependence was observed when epoxidized microfibrillated cellulose (MFC-EPI) was employed as an interfacial compatibilizer as well as a reinforcement filler in PLA/polybutylene succinate (PBS) blends [83]. The tensile strength and elongation at break of the composite containing 2% MFC-EPI reached 71.4 MPa and 273.6%, respectively. The toughening mechanism was explained by a "bridge" effect of the filler that contributes to energy transfer and dissipation during deformation (Figure 20). The entanglement of long nanofibers of MFC may enhance the load transfer between MFC and the polymer matrix during crack propagation. The interactions were enhanced by chemical cross-linking between the epoxy groups of MFC-EPI and hydroxyl/carboxyl groups of PLA and PBS. MFC-EPI acted as both the compatibilizer and reinforcement filler and endowed the PLA-based materials with high tensile strength and toughness. However, larger amounts of MFC-EPI agglomerated in the polymer matrix, deteriorating the properties of the blend.

**Figure 20.** The fracture mechanism explaining the behaviour of PLA/polybutylene succinate (PBS)/epoxidized microfibrillated cellulose (MFC-EPI) composites [83]. Reprinted with permission from ACS Sustainable Chemistry & Engineering. Copyright (2020), American Chemical Society.

The surface of nanocellulose particles can be also rendered with a high content of carboxylic groups through the esterification of the native hydroxyl moieties with citric acid in a solid phase reaction [84]. The modified cellulose nanoparticles were further fibrillated via friction grinding. It strengthened the hydrogen bonding between NC and PLA. The resulting cellulose/PLA composites exhibited a desirable filler-polymer compatibility. The dispersion of the modified cellulose in PLA matrix was good, and the flexural properties of the prepared PLA composites were improved with regard to those of the pristine PLA resin. Maleic anhydride-grafted poly(lactic acid) (PLA-*g*-AMS/MAH) was used as a compatibilizer for the microcrystalline cellulose (MCC)/poly(lactic acid) (PLA) composites [85].

The modification of the nanocellulose filler may also change the gas barrier properties of the PLA nanocomposites [86]. PLA containing well-dispersed lauryl-functionalized cellulose nanocrystals (LNC) exhibited lower gas permeability due to the formation of a rigidified PLA interfacial region with a size comparable to the LNC diameter. The solvent molecules trapped in those regions were released at much higher temperatures than those locked in phase-separated blends.

Biocompatible PLA/nanocellulose composites with good absorbent properties and high mechanical strength can be also used for the preparation of artificial networks and three-dimensional sca ffolds with a structure mimicking that of extracellular matrix (ECM). The high porosity of such 3D networks is desirable and beneficial for cell migration as well as nutrient input and metabolite output. Electrospinning technology can be used to create nanofibrous networks, although it is not possible to control their pore size and shape. Well-defined 3D sca ffolds made of poly(lactic acid)/regenerated cellulose (PLA/RC) were thus formed using a method that combined freeze-drying and crosslinking [87]. Citric acid was applied as a non-toxic chemical cross-linker to RC nanofibers through esterification with their -OH groups. The resulting bioactive PLA/RC nanofiber-crosslinked sca ffolds exhibited a dual pore structure and dimensional stability. Their high water absorption, hierarchical cellular structure, fast recovery from 80% strain, and apatite nucleating capacity indicate good osteogenic potential for bone tissue engineering. Other PLA-based open-pore porous blends suitable for this purpose, with high porosity and interconnectivity as well as superabsorbent ability, were prepared by melt-blending using crosslinked superabsorbent sodium polyacrylate particles (SAP) as a porogen [88]. The SAP particles were leached out from the PLA matrix in an aqueous environment. This generated high and tuneable porosity. The sca ffolds allowed the good cell adhesion and proliferation of mouse embryo fibroblasts. It was also shown that similar 3D cellulose/PLA porous bio-composite templates can significantly facilitate PLLA/PDLA stereocomplex crystallization by accelerating the nucleation of SC crystals without the suppression of their growth (Figure 21) [89]. The interfacial hydrogen bonding between cellulose templates and polylactide molecules promoted the formation of precursor racemic helical pairs.

**Figure 21.** Formation of 3D cellulose/stereocomplex structure (SC) PLA porous biocomposites with improved thermal and mechanical properties [89]. Reprinted with permission from Springer Nature: Cellulose. Copyright (2020).

PLA-based blends containing nanocellulose as a reinforcing phase can be also used as an excellent feed material for 3D printing by means of fused deposition moulding (FDM). Unfortunately, the molecular weight of PLA typically becomes reduced after recycling, which limits the reuse of PLA in FDM-based 3D printing. The addition of an epoxy-based chain extender and a reinforcing phase of microcrystalline cellulose (MCC) to regenerated PLA improved its processability and mechanical performance [90]. The addition of 1 wt% nanocellulose enhanced the thermal stability of PLA, increased its crystallization rate, and shortened the crystallization half-time, which is vital for the solidification of a 3D printed object [91]. Polylactic acid/cellulose acetate (PLA/CA) mixtures loaded with antiseptic 1-chloro-2,2,5,5-tetramethyl-4-imidazolidinone were used as biodegradable, low-cost, antibacterial sca ffolds prepared by a direct ink writing (DIW) technique [92]. The printability of the DIW inks was improved with the addition of CA due to the formation of a hydrogen-bonded 3D network between PLA and CA.

## **4. PLA Composites with Cyclodextrins**

Cyclodextrins (CD) are natural cyclic oligosaccharides produced from starch and consisting of six (α-CD), seven (β-CD), or eight (γ-CD) d(+)-glucose units joined by α-1,4-linkages (Scheme 10) [93]. They have a shallow truncated cone shape with a hydrophobic cavity and hydrophilic outer surface. γ-cyclodextrin is the most flexible of the CD molecules, whereas α-cyclodextrin is the most rigid one. The solubility of β-CD in water (18.4 g/L) is low when compared to that of α- and γ-CD (129.5 and 249.2 g/L, respectively) [94]. It appears that intramolecular hydrogen bonding in β-CD results in a tighter crystal structure. Cyclodextrin molecules are amphiphilic, with -CH2OH groups linked to the narrower rim and the wider rim displaying -OH groups connected to the glucose rings. The hydrophilic groups are situated on the outside of the molecular cavity whereas the hydrophobic inner surface is lined with ether-like anomeric oxygen atoms and methine units. Thus, cyclodextrin molecules may bind non-polar and suitably-sized aliphatic or aromatic compounds. The binding is driven by the enthalpic and entropic gain related to the reduction in the hydrophobic-aqueous surface and the release of water molecules from the CD cavities. Those specific features make CD e ffective hosts for a variety of gues<sup>t</sup> molecules (small organic species and macromolecules), resulting in the formation of inclusion complexes (IC) [95–97]. The IC formation mechanism involves fitting the gues<sup>t</sup> molecules in the CD cavity through non-covalent interactions (van der Waals interactions and the hydrophobic e ffect).

**Scheme 10.** Structural, space-filling, and graphical representations of cyclodextrin molecules: α-CD, β-CD, and γ-CD [96]. Reprinted with permission from Symmetry (MDPI) 2019.

Cyclodextrins of all types were used for the formation of "green" IC with polylactide chains. Such polymeric IC may have an important role for constructing supramolecular architectures (molecular tubes or poly(pseudo)rotaxanes). The formed nanometre-scale ordered structures can also be used as nucleating agents to enhance the crystallization of the gues<sup>t</sup> PLA due to the restriction of macromolecular chain motions inside the CD cavities. The introduction of water-soluble cyclodextrins threaded on polymer chains not only a ffects the crystallization but also modifies the hydrophilicity of polymer surfaces, degradation, and thermal performances. Recently, a significant interest may be also observed in PLA/CD-based materials for the delivery of encapsulated drugs [98–104] and other medical applications [105–108].
