*2.5. PLA*/*Humic Substances*

Humic substances (HS) are organic matter distributed in the natural environment. They comprise a complex mixture of humic acids (HA), fulvic acid (FA), and humins (Scheme 8) [65]. Humic substances can be considered as environmentally and biologically safe natural antioxidants. Blends of HA/FA and PLA are interesting because of their potential applicability as biomedical sca ffolds or packaging materials for oxidation-sensitive food. Owing to its biological origin, a black mixture of HA has an undefined composition that may vary depending on their origin and the process of extraction. The components generally contain aromatic and aliphatic structures with attached carbonyl, carboxylic, and phenolic groups, as well as a minor amount of amine and amide residues that may contribute as donors/acceptors for the formation of hydrogen bonds. Thus, HA may display interfacial activity, hydrophilicity, and cation exchange and complexation capacities. HA have an amphiphilic character and are soluble in aqueous alkaline media. They are not soluble in neutral to acidic media and form micelle-like structures under such conditions. Fulvic acids are the yellowish brown, low molecular weight fraction of the amorphous polymeric mixture of humic acids. FA are soluble in aqueous media of di fferent pH as well as in some organic solvents (acetone and ethanol).

**Scheme 8.** Exemplary components of Humic Acids (HA) and Fulvic Acids (FA).

The polycondensation of L-lactic acid was performed at 150 ◦C in the presence of HA (0.01% *w*/*v*), resulting in a 93% yield of a hybrid polymer of molecular weight 6.4 × 10<sup>5</sup> g/mol [66]. The incorporation of HA slightly enhanced the thermal stability of the polyester matrix. The glass transition temperature and the temperature of melting were reduced, although the elongation at break and ductility were enhanced. The presence of HA in the blend significantly improved the hydrophilicity of the polyester film, total content of phenolics (0.075 μmol GAE/g film), and water absorption capacity (90.65%). An improvement in the antioxidant activities and radical scavenging properties was also observed.

Humic acids may be also derivatized to increase their compatibility with the PLA matrix in the hybrid blends. For example, the amount of amide groups was increased by the amidation of HA using dodecylamine in the presence of carbonyl diimidazole as the coupling agen<sup>t</sup> (HA-amide-1) [67] or aniline in the presence of a phosphorus trichloride catalyst (HA-amide-2) [68]. The non-isothermal crystallization kinetics showed that HA amides incorporated during the thermoplastic processing may serve as nucleating agents that enhance the crystallization rate of PLA. Inclusions of HA amide provided a large number of heterogeneous nucleation sites thus increasing the degree of the crystallinity of the PLA/HA-amide composites (Figure 10). The crystallization behaviour varied under different cooling conditions. The mechanical properties of the blends were also improved after the introduction of HA amides that increased the rigidity of the network structure in the PLA matrix (Figure 11).

**Figure 10.** DSC thermograms of non-isothermal crystallization for PLA (**a**) and PLA containing 0.3 wt% of HA-amide-1 (**b**) [68]. Reprinted with permission from Springer Nature. Copyright (2017).

**Figure 11.** Effects of HA-amide-1 on mechanical performance of PLA blends: (**a**) tensile strength and elongation at break; (**b**) impact strength (insets: impact fracture SEM images) [67]. Reprinted with the permission of the publisher (Taylor & Francis Ltd., http://www.tandfonline.com).

Analogously to HA, FA structural units consist of polycyclic aromatic structures and functional groups (e.g., -COOH, -OH, R-CH=CH-OH, and C=O). They can thus be modified through sulfonation, nitration, etc. reactions in order to broaden the application scope. Fulvic acids have also been transformed into amide derivatives in order to augmen<sup>t</sup> their interactions with polylactide chains. Fulvic acid amide (FAA) was synthesized with fulvic acids and urea [69]. FA derivatives were also prepared by coupling FA with benzhydrazide (FA-BH) [70] or p-phenylenediamine (MFA) [71].

Polylactide blends containing the derivatives of fulvic acids were obtained upon melt blending. FAA accelerated the PLA crystallization rate and improved toughness of the PLA/FAA composites [69]. FAA acted as a heterogeneous nucleation agen<sup>t</sup> and enhanced the three-dimensional spherulitic crystal growth in PLA. The nucleating mechanism of PLA/FAA proposed to explain the observed phenomena was based on the hydrogen bonding between C=O in the polyester backbone and N-H residues in FA derivatives. The rheological behaviour of the PLA/FAA blends showed an increase in the storage modulus induced by FAA. The apparent viscosities and thermal stability of the composites were much higher after the blending. The rheological behaviour indicated good interfacial compatibility between PLA and FA grafted with *p*-phenylenediamine (MFA) [71]. Mechanical tests showed that the impact strength of PLA admixed with 0.5 wt% MFA was improved by 97.2% and the impact destruction resulted in the characteristic ductile fracture. Moreover, the rate of nucleation was improved and the crystallinity of the PLA increased from 4.9% to 36.9% in the presence of MFA. Interestingly, MFA also influenced the mechanism of enzymatic degradation, increasing the Km of proteinase K and somewhat inhibiting the degradation. The mechanical performance of composites containing 0.1 wt% FA-BH was improved [70]. The network structure of the PLA/FA-BH blend was more rigid. The tensile strength, tensile modulus, elongation at break, and impact strength increased, respectively, by 6.38%, 27.47%, 28.75% and 74.56% (Figure 12). However, larger amounts of FA-BH deteriorated the properties of the PLA. FA-BH acted efficiently as nucleating agents (Figure 13). The crystallization rate of the PLA matrix was improved by 0.1 wt% of FA-BH. The degree of crystallinity increased to 41.88% (Figure 14) and the temperature of crystallization increased from 97.2 to 116.4 ◦C upon cooling from melt.

**Figure 12.** Effects of fulvic acid–benzhydrazide (FA-BH) on mechanical properties of PLA-based composites: (**a**) maximum tensile strength and elongation at break; (**b**) impact strength; (**c**) tensile modulus [70]. Reproduced with permission of John Wiley & Sons Ltd. from Liu P., Zhen W., Bian S., Wang X., Advances in Polymer Technology, 2018; 37, 2788–2798.

**Figure 13.** Evolution of spherulitic structures in PLA (**c**) and PLA3 (0.1 wt%) (**f**) melted at 200 ◦C and then crystallized at 130 ◦C for 15 min, observed with polarized optical microscopy [70]. Reproduced with permission of John Wiley & Sons Ltd.

**Figure 14.** Thermal characteristics of PLA and PLA/FA-BH hybrid blend (PLA3) containing 0.1 wt% of the additive upon cooling at 2 ◦C/min [70]. Reproduced with permission of John Wiley & Sons Ltd. Copyright (2018).

Polylactide has been also modified with an FA-based hybrid macromolecular nucleator—a poly(lactic acid)-fulvic acid graft copolymer (PLA-FA) [72]. PLA-FA was synthesized with lactic acid monomer and FA as shown in Scheme 9. The obtained hybrid polymers had molecular weight (Mw) of about 14,300 g/mol and polydispersity index (PDI) = 1.3. The narrow molecular weight distribution implies that the grafted PLA chains were rather short and the PLA-FA macromolecules

had a compact quasi-spherical structure. The presence of polyester components improved the compatibility of FA with the PLA matrix. The structure characterization and tests demonstrated that the PLA-FA used as a hybrid filler effectively enhanced the performance of the PLA composites prepared by melt blending. A plasticization effect of PLA-FA was indicated by the results of rheological analysis. Various plasticizers of low or high molecular weight are well known for the improvement of ductility, flexibility, and processability of PLA due to the increased number of independently moving segments [6]. Plasticisation can also be beneficial for the growth of crystals in amorphous matrices. Consequently, the PLA-FA additive promoted not only the nucleation (Figure 15) but also the rate of the non-isothermal crystallization of the PLA composites and improved their thermal stability and toughness.

**Scheme 9.** Synthesis of copolymer PLA-FA.

**Figure 15.** (**a**) The spherulite growth rate of PLA and 0.5 wt% of PLA-FA (PLA3) and (**b**) cooling DSC curve of PLA and PLA3 [72]. Reprinted by permission of the publisher Taylor & Francis Ltd. Copyright (2019).

The effect of PLA-FA on the properties of polylactide blends can be related to the amount of the macromolecular additive. It affected not only the crystallization process but also the thermal and mechanical properties of the composite. The toughness of the PLA matrix was improved, while its strength and rigidity were enhanced. The sample containing 0.5 wt% of PLA-FA (PLA3) had the best properties (Figure 16). A significant increase in ductility and flexibility was noted for PLA3. The impact strength of this sample was improved by almost 200% compared with pure PLA. This is larger than the increase in impact strength induced by amidated FA (FAA, FA-BH, or MFA). However, when the amount of PLA-FA exceeded 0.5%, the mechanical properties of the blends gradually decreased. The effect was attributed to the agglomeration of the additive in the polylactide matrix, which led to stress concentration.

**Figure 16.** The impact strength (inset: impact fracture SEM images) of PLA/PLA-FA composites (PLA1: 0.1 wt%; PLA2: 0.3 wt%; PLA3: 0.5 wt%; PLA4: 0.7 wt%; PLA5: 1 wt% of PLA-FA) [72]. Reprinted by permission of the publisher Taylor & Francis Ltd. (http://www.tandfonline.com).
