With the increase in the quantity of synthetic plastic, the damaging effects of plastic waste on the environment have also intensified, and the scale of worldwide plastic pollution has become one of the most persistent public concerns. The underlying reasons for this are the affordability, convenience, and accessibility of synthetic plastics. Of the 6300 metric tons (mt) of plastics discarded in 2015, ~550 mt (~9%) have been recycled, ~750 mt (~12%) incinerated, and as much as ~5000 mt have been accumulated in the environment [1]. With the current production and recycling rate of plastics, ~12,000 mt of plastics is expected to accumulate by 2050 [1]. Emphasizing the interest in polymer degradability, several excellent comprehensive review articles on biodegradable polymers have been published and have critically emphasized their effective use in various areas, including packaging (e.g., coating films, food containers, wrapping), agriculture (e.g., mulching films), and biomedical/biotechnology (e.g., tissue engineering scaffolds, drug and gene delivery matrices, wound healing hydrogels, dental materials) ^{[2][3][4][5][6]}[2,3,4,5,6].

After realizing the environmental impact of fuel-based plastics, many governments have instigated rigorous measures to endorse the notion of creating materials from renewable resources and to increase the adoption of eco-friendly biodegradable plastics from Nature-based materials. These materials are designed to biodegrade in the natural environment over time, reducing the long-term environmental impact of plastic waste. In addition to initiatives taken by governments and global companies, there is also a growing awareness from consumers who are paying attention to the global climate crisis. Furthermore, the circular economy action plan was endorsed by leaders from the World Economic Forum, the European Parliament, and Fortune 500 companies as a tactic to restore the environment [7]. A circular economy is restorative by intention [8] and aims for “the elimination of waste through the strategic design of materials, products, and processes”. This leads to finding suitable sustainable solutions, preferentially from renewable feedstocks. Waste streams represent an even better alternative, as their usage allows us to fully remove waste from the industrial chain [9].

2. Ionic Liquids and Biopolymers

Ionic Liquids (ILs) are loosely defined as salts that melt below 100 °C ^[10][33]. Unlike conventional organic compounds, many of them exhibit negligible volatility, non-flammability, high thermal stability, and high conductivity. Since the discovery of cellulose dissolution in ILs ^[11][34], it has been shown that ILs are excellent solvents for the dissolution of a wide range of biopolymers comprising carbohydrates, proteins, and enzymatically produced polymers. Given the ability of ILs to dissolve virtually every renewable biomass to some extent, this capability results in the perfect platform for the preparation of high-value products for new and existing industrial applications that will substitute current plastics. This would result in new uses of unmodified biopolymers in technologies that currently use synthetic polymers, rather than breaking them down into platform chemicals. The research in this area takes advantage of the IL-based strategy, which avoids chemical transformations and allows the utilization of the existing functions and properties of the biopolymers “as is” in virtually any architecture. Using ILs, biopolymers can be cast, molded, spun (using dry-/wet-jet extrusion) or electrospun, 3D printed, etc. to produce functional materials ^[12][13][35,36]. The addition of new functionality by forming composites with organic or inorganic solutes, nanoparticles, etc., is also possible during this process and further flexibility is available through the ability to produce biopolymer composites with tunable properties by blending them with other polymers. Finally, due to the insolubility of biopolymers in most solvents, they can be surface-modified through either covalent or ionic functionalization. Materials already made from biopolymers using this approach include hydrogels for drug delivery, chitin-calcium alginate composite fibers for wound care, electrospun chitin nanomaterials with specific chemical functionality such as catalysts, sorbents, filters, or sensors, films for drug delivery, and beads for water purification, to name a few ^[12][14][35,37]. Ion constituents of the IL affect the inter- and intra-molecular interactions in biopolymeric systems. Because anions in the ILs act as hydrogen bond acceptors, upon dissolution of the biopolymer, the hydrogen bonding network of biopolymers naturally present in these systems is disrupted and new bonds are formed between the anion of the IL and the OH- groups of the polymer (Figure 1). Simultaneously, the cation associates with the ether oxygen atoms or -CH group of the biopolymer ^[11][15][16][34,38,39]. It is known that biopolymeric solutions with the same polymer load are able to produce materials with different topological and mechanical properties upon dissolution/regeneration ^[17][40]. The reasons behind this are the steric and electronic effects of anions (e.g., bulkier [MeSO₃]⁻ vs. small Cl⁻) and cations (e.g., longer alkyl chain butyl- vs. shorter ethyl-), resulting in different interaction strengths between IL ions and biopolymers. For instance, the cellulose-silk films prepared using 1-allyl-3-methylimidazolium chloride ([Amim]Cl), 1-ethyl-3-methylimidazolium chloride ([C₂mim]Cl), 1-butyl-3-methylimidazolium chloride ([C₄mim]Cl), or 1-ethyl-3-methylimidazolium acetate ([C₂mim][OAc]) are clearer, stronger, and less brittle than the same films prepared from 1-butyl-3-methylimidazolium bromide ([C₄mim]Br) or 1-butyl-3-methylimidazolium methanesulfonate ([C₄mim][MeSO₃]) ^[17][40]. Morphological changes based on IL are also evident and a larger extent of IL interactions with natural polymers causes more significant changes in crystallinity. The films regenerated from [Amim]Cl, [C₂mim]Cl, and [C₄mim]Cl were smooth, while the film made using [C₂mim][OAc] possessed a somewhat porous structure caused by the formation of channels during the IL-removal process. The films regenerated from [C₄mim]Br and [C₄mim][MeSO₃] had a fibrous string-like morphology. Respectively, the crystallinity of the material is also a function of the IL ^[17][40]. As determined by crystal fraction calculation (using FTIR spectra deconvolution and analysis of amide I absorbance band), the films prepared with chloride anion-containing ILs were the lowest in crystallinity. Thus, β-sheet crystal fraction for the films made using [Amim]Cl was 31.0%, [C₄mim]Cl—37.5%, [C₂mim]Cl—37.1% whereas bromide and methanesulfonate-containing ILs showed much higher β-sheet crystal fraction: [C₄mim]Br—58.6%, [C₄mim][MeSO₃]—58.9%). These differences are correlated with the extent of disruption to the hydrogen bonding network. The X-ray scattering study confirmed that the prepared films were either amorphous or semicrystalline, and the spacing differences in the ILs correlated with the extent of intermolecular interactions.

3. Ionic Liquids as Plasticizers

From the above, it is evident that ILs can disrupt a hydrogen-bonding network of biopolymers, which makes them suitable to act as plasticizers. In fact, the ILs most reported as plasticizers are also reported to dissolve biopolymers (Figure 2). The plasticization and dissolution of biopolymers by ILs proceeds by the same mechanism, namely, the disruption of the biopolymer’s hydrogen bonding network ^{[18][19][20][21][22][23][24]}[41,42,43,44,45,46,47]. The primary property of ILs that can be used to understand this phenomenon is the ILs’ polarity ^[25][48]. The empirical solvent descriptors (often called coefficients, derived from the Kamlet–Taft equation) α- and β-, where α- is hydrogen bond acidity ^[26][49] and β- is hydrogen bond basicity ^[27][28][50,51], can provide a quantitative comparison between ILs. It was found that the basicity of the IL-anion controls β-coefficient whereas the IL “as a whole entity” controls the α-coefficient. For the full disruption of the hydrogen bonding network (i.e., for complete biopolymer dissolution), the ILs are required to have β-values > 0.8 (e.g., β > 0.5 is required for chitin dissolution, and β > 0.8 is required for cellulose dissolution) ^[29][30][31][52,53,54]. β-values increase when the anion has a hydrogen-bonding acceptor with a high electron density ^[32][55]. Hence, anion–cellulose interactions decrease in the order Cl⁻ > [OAc]⁻ > [(CH₃O)₂PO₂]⁻ > [SCN]⁻ > [PF₆]⁻. While (lower) β-values control the dissolution and plasticization of the biopolymers, indicating the greater importance of anions ^[33][56], α-values appear not to be as important. Although the exact mechanism of how cations are involved in the disruption of the hydrogen bonding network of the biopolymer is still under discussion ^[18][34][35][41,57,58], the cations also play a significant role in this process ^[36][59]. Molecular dynamics (MD) simulations confirmed these findings ^[37][38][60,61] and suggested that, in the presence of IL, hydrogen bonding interactions between the anion and the biopolymer arise, with the anions strongly interacting with the polar domains of the biopolymer. In contrast, the [C_nmim]⁺ cations interact with the nonpolar domains of the biopolymer via dispersion forces. In addition to a high degree of compatibility with biopolymers and the ability to affect the morphology and crystallinity of the materials, many ILs demonstrate negligible vapor pressure, which is important since plasticizers should not evaporate from a bioplastic material, otherwise they will revert to their original brittle condition. This also results in reduced human exposure through evaporation. Many ILs exhibit low-temperature lubricity, high-temperature stability, enhanced stability to UV light, and reduced flammability ^[39][62]. In addition, the viscosity of the ILs can be tuned, an important property for ILs when they are used as plasticizers. Since ILs have a higher viscosity than conventional organic solvents due to hydrogen bonding and van der Waals interactions within the liquid, they make more preferential low-leaching plasticizers than organic compounds because of limited leaching and migration. To prepare ILs with the required viscosity (which depends on the polymer being plasticized), different strategies have been used, such as modifying the alkyl chain length of the cation, using different cationic cores, and changing the anion of the ILs ^[40][63]. The choice of the specific IL and its concentration can be tailored to achieve biopolymeric materials with specific properties, such as improved elasticity and reduced brittleness. This task involves adjusting both the type and concentration of the ionic liquid, as well as exploring different processing techniques to achieve the desired properties in the resulting biopolymer-based materials. As plasticizers, the IL-modified biopolymeric materials show changes in mechanical properties, i.e., a decrease in tensile strength and an increase in elongation at break are generally observed (Table 1).