*3.4. Poly(ε-caprolactone)*

Poly-ε-caprolactone (also called, polycaprolactone, PCL) is a biodegradable, hydrophobic, semicrystalline synthetic aliphatic polyester whose monomer unit is built from 6 hydroxyhexanoic acid (6-hydroxycaproic acid) [89,98]. PCL can be synthesized by polycondensation of 6-hydroxyhexanoic acid, but because of the equilibrium nature of this process and the need to remove the water formed during the reaction, obtaining a polymer with a high degree of polymerization and molecular weight values above 10,000 is challenging [90]. Thus, the most promising method is the ROP of ε-caprolactone, which allows the production of PCL with low dispersity and high molecular weights. However, this method of synthesis requires the use of catalysts, often based on metals, which can possess toxic effects [89]. The ε-caprolactone monomer may be obtained by oxidation of cyclohexanol by peracetic acid (industrial method of production) [11], and also by microorganisms as one of the intermediate products [89]. PCL can be synthesized in a wide range of molecular weights. The crystallinity degree of PCL can be up to 69%, and it decreases with the growth in molecular weight [89,98]. It was reported that the nonenzymatic hydrolytic degradation process increased the crystallinity of the PCL sample from the initial 45% to nearly 80% [113]. As with other crystalline aliphatic polyesters, the molecular weight and corresponding degree of crystallinity of PCL have a significant impact on its physical, mechanical, thermal, and degradation properties [89,98]. For example, PCL with *M<sup>n</sup>* below 15,000 has a low viscosity and forms very brittle materials, while polymer with *M<sup>n</sup>* from 25,000 to 90,000 is characterized by desirable mechanical and rheological properties [90]. A rapid thermal degradation is observed for PCL at temperatures above 170 ◦C, but its low melting (55 to 70 ◦C) and glass-transition temperatures (−65 to −54 ◦C) allow its easy processing, which distinguishes PCL from other biodegradable aliphatic polyesters with higher *Tm*, such as PLA, PGA, their copolymers, PHB and its copolymers [79,90]. In addition to higher thermal stability, PCL in comparison with its biodegradable analogues is characterized by higher viscoelastic properties [98]. Thus, the listed characteristics along with variable viscosity make PCL very technological, suitable, and promising for melt processing such as melt extrusion, electrospinning, injection molding, and 3D printing [90].

PCL is strongly inferior in its mechanical properties to other aliphatic polyesters, and the materials made from it—depending on the architecture, and especially porous ones are characterized by an even lower load-bearing capacity. This disadvantage imposes significant limitations on the use of PCL materials. This problem can be solved by obtaining copolymers with PCL or composites based on it [90,98]. It should be noted that PCL can be blended with many polymers (PVC, polycarbonates, PLA, PLGA, and several others to form mechanically compatible composites. This property of PCL opens up many opportunities to regulate mechanical, biodegradation, and biological properties for a variety of tasks [89,90,98]. Finally, in comparison to PLA, the low degradation rate of PCL also contributes to the minimal formation of physiological problems caused by pH shifts in the environment during the biodegradation of PCL [90,98].

#### *3.5. Poly(butylene succinate)*

Poly(butylene succinate) (PBS) is a synthetic, biocompatible, semicrystalline, thermoplastic, biodegradable aliphatic polyester [80,91]. PBS is prepared by the polycondensation

of succinic acid (or dimethylsuccinate) with 1,4-butanediol. The monomers (succinic acid and 1,4-butanediol) can be obtained from renewable or fossil-based resources [91]. Safe and accessible microwave radiation can be used to synthesize PBS, resulting in reduced reaction times and increased yields. PBS can be synthesized with high molecular weights and is a commercial product, but its cost is higher compared to common petrochemical plastics. PBS is characterized by two types of crystalline structures (α- and β-form). A β-crystalline structure is observed for the material in the state of deformation [80]. The degree of crystallinity for PBS is 35–45% [91]. Mechanical and thermal properties, as well as biodegradation, depend on the molecular weight of the polymer and its crystallinity [80,91]. PBS is characterized by flexibility and tensile strength close to that of PE and PP [91]. Some physical properties of PBS are comparable to PET. PBS is characterized by good thermal properties. The glass-transition temperature of this polymer is considerably lower than room temperature and ranges from −45 to −10 ◦C. The melting point of PBS is higher than for PCL but lower in comparison with PLA, PHB, PHBV, and PGA, and varies in the range of 90–120 ◦C. In view of the above, for PBS various methods of processing are applicable: extrusion, thermoforming, injection molding, etc. These properties, along with biodegradation, distinguish PBS from polyolefins [91]. However, PBS is characterized by disadvantages such as low melt viscosity, slow crystallization rate, gas tightness, and relative brittleness [80,91]. To improve these characteristics, as well as to vary the rate of decomposition and reduce the cost of materials, it is possible to obtain copolymers and composites based on PBS [91]. Furthermore, the introduction of plasticizers in the PBS matrix can improve the rheological properties of this polymer and reduce brittleness [80]. Biodegradability, environmental friendliness, chemical resistance, transparency, physical and mechanical properties, recyclability, and processability allow its application in various fields, first of all in packaging and disposable tableware, but also in textiles, automotive industry, agriculture, forestry, fisheries, medicine, etc.

#### **4. Modification of Cellulose Micro- and Nanomaterials**

#### *4.1. Adsorption*

Physical modification, or adsorption, is one of the simplest and oldest techniques to modify cellulose nanomaterials [114]. To date, there are a lot of publications describing the modification of cellulose nanomaterials with small molecules and polymers. Selected examples are summarized in Table 3. Basically, the modifying agents are surface-active molecules, or surfactants. Being amphiphilic, they serve as intermediates between hydrophilic cellulose and hydrophobic polyesters. The hydrophilic fragments of surfactants interact with cellulose hydroxyls, while other parts of the (macro)molecule surround the surface like "brushes", preventing the direct interaction between cellulose fibers/particles in nonpolar surroundings.

Among the small molecules, ethoxylated nonylphenol phosphate ester and cetyltrimethylammonium bromide (CTAB) are the most widely used compounds. Li et al. showed that the modification of CNC with low-molecular surfactants did not affect the size of nanocrystals and their distribution [115]. As for modification with polymeric surfactants, such polymers as lignin, poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), poly(Nvinylpyrrolidone) (PVP), and poly(ethylene glycol) (PEG) are widely applied to enhance the compatibility of cellulose nanomaterials with aliphatic polyesters.

The most common technique for the physical modification of cellulose is the adding of surfactants to aqueous cellulose dispersion, followed by freeze-drying [116–118]. The success of modification of cellulose via adsorption was testified by several groups by such methods for structure characterization as Fourier transform infrared (FTIR) spectroscopy [115,118–120] and solid-state <sup>13</sup>C NMR spectroscopy [116]. Besides these, a decrease in turbidity also can indirectly testify the surface modification. In particular, Gois et al. also evaluated the turbidity of the dispersions of the neat CNC and CNC modified with different PEGs or Pluronic VR L44 in chloroform [117]. The authors found that pure CNC began to precipitate after 2 min, while the modified CNC started to reduce its turbidity only after 3–5 min, depending on surfactant.

*Polymers* **2022**, *14*, 1477


spectroscopy (transparency); XPS: X-ray photoelectron spectroscopy; WVP: water-vapor permeability; WAXS/WAXD: wide-angle X-ray scattering/diffraction; MTT-test: (3-[4,5 dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) test; NMR: nuclear magnetic resonance; SBS: short beam shear; ATR-FTIR: attenuated total reflection FTIR; DLS: dynamic light scattering; BET: Brunauer–Emmett–Teller; POM: polarized optical microscopy. Abbreviations: Pluronic: triblock copolymer of poly(ethylene oxide) and poly(propylene oxide); rGO:

reduced graphene oxide; CTAB: cetyltrimethylammonium bromide; PEG: poly(ethylene glycol).

Modification of the cellulose nanomaterials with surfactants leads to diminishing the agglomeration of cellulose nanomaterials, and as result to their better distribution in the matrix of hydrophobic polyesters. The homogenous distribution of the filler in the matrix polymer has been observed by many authors [114,116,127].

The properties of modified cellulose and its composite materials are strongly influenced by the chemical nature of the modifying agent. For example, cellulose fibers modified with lignin had no effect on the degradation temperature, while coating with tannin led to a decrease in the fiber degradation temperature [116]. In the latter case, such behavior can be explained by the lower intrinsic thermal resistance of tannin molecules compared to cellulose and lignin. No effect on degradation temperature was observed either for PLA composites filled with CNC bearing adsorbed CTAB [115]. A discussion of the dependence of the mechanical properties of composites on cellulose modification is presented below (see Section 5.2).

In general, the advantages of adsorption as a modification tool are its simplicity and its ability to vary the modifier over a wide range. In turn, the possibility of a leakage of adsorbed molecules from the surface when dispersing the modified cellulose in nonpolar solvents to prepare composites by solution casting or precipitation is a main disadvantage of this approach. As a result, leakage of the modifier from the cellulose surface may affect the properties of the prepared composites, for example, not improving the mechanical properties.

#### *4.2. Covalent Modification with Small Molecules*

Cellulose covalent modification is limited by the reactions of its functional groups, namely hydroxyls. In particular, depending on modifier agents, ester, urethane, or silyl ether bonds can be formed due to chemical reactions of cellulose hydroxyls (Figure 5).

**Figure 5.** Scheme of cellulose-modification pathways with small molecules (esterification, acylation, silanization, modification with isocyanates).
