*Review* **Modification of Cellulose Micro- and Nanomaterials to Improve Properties of Aliphatic Polyesters/Cellulose Composites: A Review**

**Mariia Stepanova and Evgenia Korzhikova-Vlakh \***

Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoy pr. 31, 199004 St. Petersburg, Russia; maristepanova@hq.macro.ru

**\*** Correspondence: vlakh@hq.macro.ru

**Abstract:** Aliphatic polyesters/cellulose composites have attracted a lot attention due to the perspectives of their application in biomedicine and the production of disposable materials, food packaging, etc. Both aliphatic polyesters and cellulose are biocompatible and biodegradable polymers, which makes them highly promising for the production of "green" composite materials. However, the main challenge in obtaining composites with favorable properties is the poor compatibility of these polymers. Unlike cellulose, which is very hydrophilic, aliphatic polyesters exhibit strong hydrophobic properties. In recent times, the modification of cellulose micro- and nanomaterials is widely considered as a tool to enhance interfacial biocompatibility with aliphatic polyesters and, consequently, improve the properties of composites. This review summarizes the main types and properties of cellulose micro- and nanomaterials as well as aliphatic polyesters used to produce composites with cellulose. In addition, the methods for noncovalent and covalent modification of cellulose materials with small molecules, polymers and nanoparticles have been comprehensively overviewed and discussed. Composite fabrication techniques, as well as the effect of cellulose modification on the mechanical and thermal properties, rate of degradation, and biological compatibility have been also analyzed.

**Keywords:** microcrystalline cellulose; nanocrystalline cellulose; cellulose fibers; cellulose modification; aliphatic polyesters; polyhydroxyalkanoates; poly(lactic acid); poly(ε-caprolactone); poly(glycolic acid); poly(lactic acid-co-glycolic acid); poly(hydroxybutyrate); poly(butylene succinate); (bio)composites; "green" materials; mechanical properties; thermal properties; degradation; biocompatibility

### **1. Introduction**

In recent decades, aliphatic polyesters have attracted enormous interest as an alternative to plastics derived from petroleum [1]. Aliphatic polyesters are biocompatible, biodegradable, and have an excellent ability to a number of processing techniques allowing the production of electrospun nanofibers, films, filaments, nonwoven materials, 3D-printed materials of different shapes, molded and pressed materials, nanocomposite bulk materials, etc. [2–4]. Degradation to nontoxic products, the possibility of recycling, thermoplasticity, nontoxicity, comparability of some parameters with poly(ethylene terephthalate) (PET) and polypropylene (PP) [5–7], low flammability, smoke and refractive index, and dyeability [8] are among other positive features of aliphatic polyesters. In sum, these advantages make aliphatic polyesters very attractive polymers for obtaining biomedical (drug-delivery systems, suture threads, scaffolds for tissue engineering, etc.) [5,9–11] and environmentally friendly materials (packaging and disposable items such as clothing, tableware, etc.) [8,12,13]. However, their high hydrophobicity, insufficient thermal stability, and mechanical and barrier properties limit their wide application for technical purposes. The most powerful way to modify the properties of aliphatic polyesters is to obtain various

**Citation:** Stepanova, M.; Korzhikova-Vlakh, E. Modification of

Cellulose Micro- and Nanomaterials to Improve Properties of Aliphatic Polyesters/Cellulose Composites: A Review. *Polymers* **2022**, *14*, 1477. https://doi.org/10.3390/ polym14071477

Academic Editor: Debora Puglia

Received: 28 February 2022 Accepted: 31 March 2022 Published: 5 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

composites [14]. In this case, the properties of the matrix polymer can be adjusted by the selection of a certain filler. For example, metals [15], carbon nanotubes [16], graphene [17] and its derivatives [18], ceramics [19], and different organic nanoparticles [20–22] are considered to improve the properties of interest.

Despite the variety of potential fillers, the most attention is paid to micro- and nanomaterials that are nontoxic and inexpensive, which makes it possible to produce "green" biocomposites on an industrial scale. Cellulose micro- and nanomaterials are among the most potential fillers for producing such environmentally friendly and biocompatible composites [23–26]. Excellent mechanical properties, a large specific surface area of cellulosic materials, the possibility to obtain them from the wastes of various industries, as well as biodegradability and biocompatibility make their application as reinforcing materials for a variety of areas, including biomedicine and obtaining "green" materials, relevant. However, the hydrophilicity of cellulose impairs significantly its dispersion in hydrophobic polyesters, which leads to cellulose aggregation, poor dispersion in the matrix polyester, and as a consequence, unsatisfactory material properties [27,28]. This obstacle can be overcome by modifying cellulose materials to improve their compatibility with hydrophobic polymer matrices, and as a result, to provide a more homogeneous dispersion of the filler.

In recent years, much attention has been paid to improving the compatibility of cellulose with aliphatic polyesters by covalent and noncovalent modification [29–38]. Modification of the cellulose surface, in turn, affects the properties of the cellulose filler and allows the properties of aliphatic polyester/cellulose composites to be adjusted in a wide range. Recently, several review articles devoted to the composites based on poly(lactic acid) and cellulose [39,40] with special focusing on the processing techniques [41,42] and biofiber's properties [43] have been published. Some reviews have partially discussed cellulose modification [39,44]; however, progress in this area has not been extensively overviewed.

In this comprehensive review, we have summarized the progress on the various approaches reported for the modifications of cellulose micro- and nanomaterials and the further preparation of composites with aliphatic polyesters. The different techniques such as adsorption, covalent modification with small molecules, grafting with polymers, and modification with inorganic and organic nanoparticles have been discussed. Unlike most reviews that consider only poly(lactic acid) (PLA), we have also included other aliphatic polyesters used to produce composites with modified cellulose, e.g., poly(glycolic acid), poly(ε-caprolactone), poly(hydroxybutyrate), poly(butylene succinate) and their copolymers. Furthermore, the effect of modification on various properties of composites, such as mechanical, thermal, degradation and biological ones, have been analyzed.

#### **2. Cellulose Micro- and Nanomaterials**

It is known that cellulose is the most abundant polysaccharide on our planet. Its main sources are primarily plants, including wood, as well as algae, tunicate, and metabolic products of some bacteria [45,46]. The highest cellulose content (more than 90%) is characteristic of "bacterial cellulose" (BC), while for other sources this value does not exceed 80% (plant—30–80%, tunicate—about 60%, algae—8–47%) [46–49]. The exception is mature cotton fibers, which consist almost entirely of cellulose (88.0–96.5%) [50]. Accordingly, BC and mature cotton are characterized by fewer impurities, such as lignin and hemicellulose, which are present in large amounts in plant and algae cellulose [46,50]. Another feature of BC is the presence of a finer mesh structure [46]. Furthermore, the degree of crystallinity for cellulose from different sources also varies quite a lot. Regardless of the source, cellulose is a linear homopolysaccharide and consists of β-1,4-glycosidic bonded anhydro-D-glucose units [49,51]. A large number of Van der Waals and hydrogen interactions are formed between and within the polymeric cellulose chains, which lead to the formation of three-dimensional hierarchical structures, the structural unit of which is an elementary fibril [52]. Elementary fibrils, also called elementary nanofibrils, are threadlike bundles of cellulose molecules consisting of alternating crystalline and a number of amorphous domains providing fiber flexibility [26]. Elementary fibrils due to aggregation are packed

into microfibrils, which, in turn also aggregate, and this leads to the formation of cellulose fiber [53]. The widths of elementary fibrils and microfibrils range from 1.5 to 5 nm [54,55] and from 10 to 30 nm [49,54], respectively, and the width and length of microfibril aggregates can reach the order of 100 nm and tens of micrometers [49,51,53,55], respectively. The large number of hydroxyl groups (three reactive groups in each monomeric unit) and the supramolecular structure of cellulose determine its physical and chemical properties (insolubility in water and basic solvents, semicrystallinity, good mechanical properties, relative reactivity) [26,55–57].

Depending on the origin and the method of isolation, the degree of polymerization (DP) of cellulose and the molecular orientation of its chains can be different. For native cellulose, the most common crystalline structure is cellulose I, which under the influence of sodiumhydroxide solution or recrystallization changes to the most stable crystalline state, cellulose II. More details about the different forms of cellulose can be found elsewhere [49,51,55,58,59]. The degree of polymerization of cellulose ranges from a few hundred to several tens of thousands [46,49,58]. Given the structure of cellulose, cellulose objects can be produced as fibers, micro/nanofibrils, and micro/nanocrystals [26,44], which vary in degree of polymerization, crystallinity, and shape [60]. Figure 1 schematically demonstrates the general hierarchical structure and structure of a single polymer chain of cellulose with a list of the main cellulose micro- and nanomaterials obtained.

**Figure 1.** Cellulose from source to molecule and micro- and nanomaterials.

The nomenclature used to designate the various types of micro- and nanocellulose materials is currently ambiguous. Thus, cellulose nanocrystals (CNC) are called nanocrystalline cellulose (NCC), cellulose nanowhiskers (CNW), cellulose whiskers, nanocrystals, nanofibers, nanoparticles, nanorods, rod-like cellulose crystals, cellulose microcrystals (CMC), cellulose microcrystallites, cellulose microfibrils (CMF) [26,39,51,55]; cellulose microfibrils (CMF) are called microfibrillated cellulose (MFC), microfibrillar cellulose, nanofibrillated cellulose (NFC), cellulose nanofibrils (CNF) [26,55]; a synonym of microcrystalline cellulose (MCC) is whiskers [53]. Some time ago, the Technical Association of the Pulp and Paper Industry proposed to standardize the terminology used (nomenclature and abbreviation). According to the recommendations (WI 3021), depending on the dimensions (width (w) and length/width ratio (L/w)), cellulose materials are divided into: cellulose nanocrystals (CNC, w = 3–10 nm, L/w > 5), cellulose nanofibrils (CNF, w = 5–30 nm, L/w > 50), cellulose microcrystals (CMC, w = 10–15 µm, L/w < 2), cellulose microfibrils (CMF, w = 10–100 nm, L/w > 50) [61]. The main cellulose types found in the literature and used in the production of composite materials are shown in Figure 2.

**Figure 2.** Overview of cellulose micro- and nanomaterials commonly used as fillers to prepare composite materials. Electron micrographs of (**a**) sisal fiber (scanning electron microscopy (SEM), reproduced from [62] under the terms of the Creative Commons CC BY license), (**b**) tunicate whiskers (transmission electron microscopy (TEM), reproduced from [63] with permission of American Chemical Society), (**c**) sugar beet CMF (TEM, reproduced from [64] with permission of Elsevier), (**d**) CMC, commercial (SEM, reproduced from [65] with permission of John Wiley & Sons, Inc), (**e**) wood CNF (TEM, reproduced from [66] with permission of American Chemical Society), (**f**) CNC from ramie fibers (TEM, reproduced from [67] under the terms of the Creative Commons CC BY license).

The size, type, and consequently the physical and chemical properties of the resulting cellulose materials are influenced by the source of origin, processing, and extraction method [44]. For instance, the use of mechanical action alone or its combination with chemical treatment of previously purified cellulose pulp/fibers (e.g., carboxymethylation or TEMPO-mediated oxidation) and/or enzymatic hydrolysis results in thin long flexible micro- (CMF) or nanofibrillar (CNF) structures with alternating crystalline and noncrystalline domains. In turn, acidic hydrolysis produces stiffer particles with a high degree of crystallinity (CMC and CNC), which are the result of the action of acid on both amorphous and crystalline domains. Thus, in the first case, the obtained cellulose micro- and nanofibrils retain the inherent semicrystallinity and high aspect ratio (L/w, over 50 for CMF and CNF) [39,46,49,55], while acid exposure reduces the number of defects in the structure and results in more highly crystalline materials with much lower L/w values (8 to 67 for CMC and CNC) [26,53,68].

Despite the existing terminology recommendations for cellulosic micro- and nanomaterials (see above), the use of terminology in the practice of current publications varies. Nevertheless, we have attempted to generalize the available data on the size of the various

cellulose-based materials used. The preparation methods and summarized descriptions and characteristics of the obtained micro- and nanocellulose materials found in literature are presented in Table 1 [26–28,39,44,46,69,70].

**Table 1.** Cellulose micro- and nanomaterials.


Abbreviations: CNC: cellulose nanocrystals (nanocrystalline cellulose); CMC: cellulose microcrystals (microcrystalline cellulose); CNF: cellulose nanofibers; CMF: cellulose microfibers.

The degree of crystallinity and DP for all obtained materials depends largely on the source of the cellulose as well as the processing technique. The found values are very scattered. For example, DPs for micro- and nanoobjects in the literature range from 100 to 15,000 [46,70], and the degrees of crystallinity vary from a few dozen to more than 90% [26,27,55,70]. For instance, the degree of crystallinity for BC and tunicin (cellulose from tunicate) is 80–100%, for cellulose from algae it is more than 70%, and cellulose from plants it is 40–60% [39,46].

The source of cellulose also has a significant impact on its mechanical properties. Young's tensile modulus for cellulose fibers varies from 5 to 200 GPa [39,46,71]. The highest values from this range are typical for tunicin fibers (from 110 GPa), while for fibers from other origins the elastic modulus does not exceed 115–130 GPa [39,46]. Elongation at break and tensile strength of cellulose fibers are in the ranges of 1–30% and 0.2–1.2 GPa, respectively [26,71]. The application of cellulose-fiber treatments that help to reduce the amorphous components in the chain packing thereby leads to a decrease in DP, an increase in crystallinity and, as a result, an increase in the mechanical properties of the resulting cellulose material compared to the original fibers [39,55]. The theoretically calculated Young's modulus (E) of an ideal cellulose crystal (along the axis of the cellulose chain) is 167.5 GPa [72]. According to the published data, the practically identified Young's modulus values for micro/nanocrystals range from 60 to 220 GPa [26,39,44,46,49,73]; for micro/nanofibers from 14 to 84 GPa [39,46,73]; and for a single tunicin microfibril, a value of about 150 GPa has been found [74]. The established tensile-strength data are in the range of 1–10 GPa for CMC and CNC [39,44,46,58]; about 2–6 GPa for cellulose nanofibers [66]; and about 4–8% elongation at break for wood-cellulose CNF has been reported [46]. The above data indicate the excellent mechanical properties of these micro- and nanoscale cellulose materials. The characteristics of crystalline cellulose (density 1.5–1.6 g/cm<sup>3</sup> ) are close to—and in some cases significantly higher than—those of glass fibers used for composites (E about 70 GPa, density 2.6 g/cm<sup>3</sup> ), Kevlar (E 60–125 GPa, density 1.45 g/cm<sup>3</sup> ) and steel (E 200–220 GPa, density about 8 g/cm<sup>3</sup> ) [61].

In addition to the above subgroups, cellulose materials such as amorphous nanocellulose (ANC) and cellulose nanoyarn (CNY) are also mentioned in the literature. ANC are obtained from regenerated cellulose by acid hydrolysis and ultrasonic treatment and are generally elliptical particles 50–200 nm wide with DP of 60–70 with an amorphous structure

and hence poor mechanical properties. CNYs are electrospun nanofibers; their width and DP range from 500–800 nm and 300–600 nm, respectively [26]. CNFs can be made into cellulose filaments by flow-focusing, wet-extrusion, or spinning processes, but their mechanical properties are also inferior to the more highly crystalline forms of micro- and nanocellulose materials [46,75]. Figure 3 shows these three forms of cellulosic materials.

**Figure 3.** SEM images of (**a**) ANC from CMC (reproduced from [76] under the terms of the Creative Commons CC BY license), (**b**) CNY from carboxymethyl cellulose sodium salt with polyethylene oxide at ratio 1:1 (reproduced from [77] with permission of John Wiley & Sons), (**c**) cellulose filament from carboxymethylated CNF (reproduced from [78] under the terms of the Creative Commons CC BY license).

#### **3. Aliphatic Polyesters**

Aliphatic polyesters (polyhydroxyalkanoates) are biocompatible and biodegradable materials. Currently, they are of interest for the fabrication of biomedical and environmentally friendly materials to replace petroleum-based plastics. Polylactide, polyglycolide, polyhydroxybutyrate, polycaprolactone, poly(butylene succinate) and some copolymers based on them are among the widely considered aliphatic polyesters [79–83]. The chemical structures of key aliphatic polyesters are illustrated in Figure 4, while their key characteristics are summarized in Table 2.

**Figure 4.** Structures of PLA, PGA, PLGA, P3HB, PHBV, PCL, and PBS.



#### *3.1. Poly(lactic acid)*

Poly(lactic acid) (PLA) or polylactide is a synthetic, thermoplastic, biocompatible, and biodegradable aliphatic polyester derived from renewable lactic acid [7,11,84]. PLA can be produced by lactic-acid condensation or by the ring-opening polymerization (ROP) of lactide (lactic-acid cyclic dimer) [5,7,11,14,44]. Polycondensation of lactic acid allows the synthesis of only a low-molecular-weight polymer due to the side reaction of hydrolysis preventing the production of high-molecular-weight polymer chains. In contrast to polycondensation, ROP provides PLA with high molecular weight but requires the use of catalysts [7]. A combination of these methods is commonly used to produce PLA on an industrial scale [3]. In this case, a low-molecular-weight polymer is synthesized from lactic acid (2-hydroxypropionic acid) by polycondensation, then the formed polymer is depolymerized to form lactide, which is further used to produce PLA of high molecular weights by ROP [3,7].

The presence of optical isomers of lactic acid and lactide (L-lactic acid, D-lactic acid, L,L-lactide or L-lactide, D,D-lactide or D-lactide, D,L-lactide or meso-lactide) leads to obtaining PLA of four types: isotactic and optically active poly-L-lactide (PLLA) and poly-Dlactide (PDLA), syndiotactic and atactic optically inactive poly-D,L-lactide (PDLLA) [44,84,85]. Equimolar racemic mixture of L- and D-enantiomers of lactide (rac-lactide) is also designated as D,L-lactide [7,85]. Chemical structures of stereoisomers of monomers and PLA can be found elsewhere [44].

The molecular structure (enantiopure) of PLA and heat treatment affect its crystallinity [3,84]. Optically inactive PDLLA is amorphous, whereas stereoregular PLLA and PDLA are capable of homocrystallization [84], forming α- (highest thermodynamic stability), β- or γ-crystalline forms depending on composition [3,6]. Blending PLLA and PDLA leads to their cocrystallization and the formation of a stereocomplex with a different crystal structure, characterized by an increase in melting temperature (*Tm*) by about 50 ◦C relative to the homopolymer PLLA or PDLA [6,101]. The parameters of the crystalline forms of PLA can be found in detail elsewhere [6]. Stereochemistry has a tremendous influence on the supramolecular structure of PLA. The presence of more than 10 mol% of the links in the polymer chain different from the basic optical form leads to a significant decrease in crystallinity [84,102]. Branching of the polymer chain also leads to a rather significant decrease in the crystallinity and glass-transition temperature (*Tg*) of PLA [102]. The crystallinity of PLA is also affected by the molecular weight of the polymer, various treatments, the introduction of nucleation agents, plasticizers into the matrix, and for final products, PLA crystallization can be initiated by temperature annealing [3,6,103].

Crystallinity, as well as the parameters listed above, determine the physical (thermal, rheological, barrier, etc.) and mechanical properties, as well as the degradability of PLA [3, 7,44,104,105]. The high degree of crystallinity of polylactide leads to excellent thermal and mechanical properties [44]. However, a high degree of crystallinity is not always necessary and is determined by the application of the final polymer product. For example, the rapid crystallization can complicate stretching the product by blow molding, can diminish the optical transparency of the product, such as a bottle, and can increase the degradation time of the polymer, which may limit its use for some biomedical applications. At the same time, the presence of thermal stability due to high crystallinity is very important for products formed by injection molding [44]. It has been reported that the increase in molecular weight and crystallinity is accompanied by an increase in viscosity and softening point, so that the behavior of PLA in the melt becomes similar to polystyrene [85]. The thermal stability of PLA is similar to poly(vinyl chloride) (PVC), but significantly inferior to PP, polyethylene (PE), polystyrene (PS), and PET [85].

The glass-transition temperature and melting point of PLA are important parameters for predicting the material properties [84]. Both are influenced by the molecular weight of the polymer. *T<sup>g</sup>* and *T<sup>m</sup>* increase sharply when the number-average molecular weight (*Mn*) increases to 80,000 and 100,000, respectively, and then remain almost unchanged [102]. As optical purity decreases with a constant molecular weight, a decrease in glass-transition

temperature is observed. Moreover, PDLA is characterized by lower *T<sup>g</sup>* values than PLLA with the same molecular weight [3,102]. In turn, *T<sup>m</sup>* is more influenced by the amorphous state of PLA than *Tg*, due to the lack or complete absence of a crystalline phase [102].

The commercially available type of PLA produced on a large scale is mainly PLLA [6,84], since about 90% of all lactic acid is produced from renewable sources by microorganisms as L-isomer [84]. Thus, a commercial polylactide is a semicrystalline polymer with a *T<sup>g</sup>* of 55 to 65 ◦C and a *T<sup>m</sup>* of 140 to 180 ◦C depending on the amount of the D-enantiomer impurity [94]. In comparison with petrochemical-based plastics, PLA has a slow crystallization rate, low impact strength, low thermal resistance, and low glass-transition temperature and fragility [11,94]. For instance, to consider the substitution of PET by PLA for packaging fabrication, the barrier properties of PLA need to be improved. Typically, aliphatic polyesters with molecular weights greater than 60,000 are used for packaging, agricultural, and biomedical applications [11].

#### *3.2. Poly(glycolic acid)*

Poly(glycolic acid) or polyglycolide (PGA) and its copolymer with PLA (PLGA) is among the most widely studied and used polymers [84,86]. PGA is a semicrystalline, biodegradable, biocompatible aliphatic polyester that differs from PLA in the absence of a methyl group in the monomer unit (glycolic acid residue) [86,93]. PGA can be obtained by polycondensation of glycolic acid (difficult to obtain high molecular weights), ROP of glycolide (more economical, but pure monomer is required), and solid-phase polycondensation of halogen acetates (low degree of polymerization) [86]. The synthesis conditions determine PGA molecular weight, crystallinity, *T<sup>m</sup>* and *Tg*, and terminal groups. The growth of the molecular weight of PGA contributes to an increase in crystallinity, mechanical properties and a decrease in the biodegradation rate. Acceptable mechanical properties of PGA are achieved at molecular weights greater than 30,000 [86]. Due to the high degradation rate of PGA, its synthesis is more difficult and expensive than for PLA.

Mechanical, thermal, degradation properties and density of PGA are determined by molecular weights, dispersity, and crystallinity. PGA is characterized by high crystallinity. The most common crystallization degree is 45–55%, but 77% has also been reported. Due to the stabilized crystal cage, PGA has a high melting point (220–230 ◦C) and poor solubility (soluble only in highly fluorinated solvents such as hexafluoroisopropanol) [86]. The glasstransition temperature (*Tg*) of PGA is higher than the ambient temperature, but close to human body temperature (35–40 ◦C), which makes the material elastic when introduced into the human body (e.g., implantation) [86,93]. PGA is characterized by poor thermal stability because *T<sup>m</sup>* is very high and close to the degradation temperature [90,92]. The lack of solubility in conventional organic solvents and the narrow processing window of PGA melt create a problem in obtaining products based on it [93]. At the same time, the supramolecular structure of PGA provides excellent mechanical properties [86,93]. For example, the elastic modulus (*E*) of PGA is higher than that of other synthetic biodegradable polymers (PLLA, PDLLA, poly-ε-caprolactone) and is 6–7 GPa [86,93]. The high density of PGA (1.50–1.71 g/cm<sup>3</sup> ), due to the molecular-packing features, provides high gas-tight properties of the polymer, exceeding this parameter of polyethylene terephthalate (PET) by 100 times [86,92].

Currently, PGA is of great interest for renewable industry and biomedical applications due to its thermal properties, biocompatibility, biodegradation, nontoxicity, excellent mechanical characteristics, and low gas permeability. The obstacles of PGA application are overcome by making PGA-based copolymers and composites [86,93,106]. For example, by copolymerizing glycolide and various enantiomers of lactide and varying their ratios, the properties of the resulting PLGA (stiffness, crystallinity, melting point, and biodegradation rate) can be set. For example, PLGA demonstrates mechanical properties similar to those of human calcareous bone. In addition, PLGA is widely used as implants, micro- and nanoparticles for drug delivery [107,108].

### *3.3. Poly(hydroxybutyrate)*

Poly(3-hydroxybutyrate) (P3HB, PHB) is a thermoplastic, biodegradable, semicrystalline, linear microbial aliphatic polyester [5,8,79,87]. Biosynthesis in cells of natural/transgenic plants and bacterial fermentation, including the use of genetically modified microorganisms, are the main ways for the production of P3HB [8,79,87]. In particular, P3HB is produced by prokaryotic microorganisms from sugar-based media (agricultural industrial wastes, hydrolysates of some wood) and other carbon sources in the form of inclusion bodies, which serve as intracellular bacterial depots storing carbon and energy [5,79,87]. Microorganisms may accumulate up to 40–50% of P3HB from the dry-cell mass, and in the case of *Alcaligenes eutrophus* the accumulation may reach up to 96% of the dry cell mass [87]. Depending on the conditions and isolation forms, the resulting P3HB can have different characteristics (molecular weight, crystallinity, mechanical properties and ability to biodegrade) [8,79].

P3HB exhibits optical activity due to a chiral central carbon, and the main natural configuration is poly(D-3-hydroxybutyrate) [87]. The stereostructure and tacticity of P3HB can be specified by chemical synthesis, obtaining isotactic, syndiotactic, or atactic PHB [8,88,109]. The number of monomeric units in P3HB can vary in different range: (1) over than 10,000 for P3HB produced as cytosolic inclusions of bacteria, (2) 100–300 for P3HB from cell membranes and (3) up to 30 monomeric units for P3HB for other natural sources, including human tissues [8]. P3HB with a number of monomeric units greater than 1000 (molecular weight greater than 100,000) can be obtained chemically from β-butyrolactone [8,109].

The linear structure of the P3HB chain ensures its high crystallinity with the presence of an amorphous phase in addition to the crystalline phase [79]. The crystallinity of P3HB can vary in a wide range from 50 to 80%, and as with PLA, has a significant effect on the mechanical properties [79,87]. P3HB is generally characterized as a strong and stiff polymer with low thermal stability and low crystallization rate. Secondary crystallization of P3HB occurs at room temperature with the formation of amorphous lamellae, leading to polymer brittleness [87,110]. P3HB has piezoelectric properties and is also characterized by good resistance to acids, bases, and ultraviolet radiation [87]. In addition, P3HB has better barrier properties than PP, PE, PVC, and PET, and is characterized by some other properties similar to or superior to those of PP and PE [79]. In addition to these advantages, P3HB is a biocompatible and nontoxic polyester, which makes it a promising environmentally friendly alternative to petrochemical polymers, and also demonstrates suitability for various tissueengineering and other biomedical applications (scaffolds, surgical threads, drug-delivery systems, surgical mesh, etc.).

The low thermal stability is due to the close values of the melting and degradation temperatures of P3HB, which leads to a narrow heat-treatment window. Thus, thermal degradation of the polymer melt can occur during processing [110,111]. This problem can be partially solved by using a random copolymer of 3-hydroxybutyric and 3-hydroxyvaleric acids (poly(3-hydroxybutyrate-co-3-hydroxyvalerate) or poly(3-hydroxybutyric acid-co-3 hydroxyvaleric acid) abbreviated as PHBV or PHBHV. PHBV as well as P3HB homopolymer is characterized by high crystallinity, biocompatibility, biodegradability in different environments, good barrier properties, nontoxicity, UV stability, similarity to P3HB in solubility and chemical stability, hydrophobicity, and low impact resistance and fragility. Unlike P3HB, PHBV has a lower *Tm*, higher surface tension and flexibility [111]. Thus, PHBV appears to be technologically more attractive and of interest as materials for biomedical applications, agriculture, and packaging materials, and has been developed on an industrial scale [88]. PHBV copolymer can be produced by various microorganisms [111], including recombinant strains, in amounts up to 80% of dry-cell weight, and PHBV composition can vary over a wide range depending on substrate composition [111].

In addition to PHBV, lower melting-point values are observed for another copolymer, namely poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)). Thus, an increase in the number of 4-hydroxybutyrate units (from 0 to 38 mol%) provides a significant decrease in *T<sup>m</sup>* (from 176 to 54 ◦C), and with a further increase in the proportion of 4HB the melting temperature of copolymers practically does not change. P(3HB-co-4HB) is a

thermoplastic biodegradable aliphatic polyester produced by bacterial fermentation. The ratio of monomeric units is largely determined by the substrate used. Moreover, during biosynthesis the production of a mixture of copolymer compositions with a wide range of monomer compositions is observed, including the presence of P4HB, which significantly affects the characteristics of isolated polymers [112]. Regarding P3HB, P4HB is a relatively new material, which can also be obtained by polycondensation of 4-hydroxybutyric acid or ROP γ-butyrolactone. P4HB is also nontoxic, biocompatible, biodegradable, UV-resistant, demonstrates relatively good barrier properties, and exhibits optical activity, and therefore can be used for biomedical and packaging applications [79].
