Polyvinyl alcohol (PVA) is a widely used polymer that replaces traditional non-degradable plastics due to its water solubility, biodegradability and biocompatibility [1, 2]. However, PVA is rarely used alone in the manufacture of products due to its low yield strength, high cost and poor thermal stability [3]. In general, PVA is blended with fillers or polymers, such as cellulose, chitosan and lignin to decrease cost or improve mechanical properties [4–6]. Lignin contains up to 30% by weight of lignocellulose and is considered as the second most abundant neutral carbon source [7, 8]. Lignin is a promising raw material for the production of composites due to its low cost and abundant structure [9]. Over the past few decades, the preparation of bio-based composites using different types of lignin has been extensively researched [10, 11]. Incorporating lignin into PVA for the fabrication of composites decreased the cost of PVA materials and increased the value of the lignin. However, there were unavoidable problems in balancing the mechanical strength of the composites with the lignin content when producing PVA/lignin composites for practical applications [12, 13]. Previous studies of PVA/lignin composites have shown that the distribution of lignin content ranges from 1–60% [14, 15]. The mechanical strength of the polymer matrix is unaffected by the addition of lignin at modest levels and may even be enhanced. The mechanical properties of lignin-based materials gradually decreased with increasing lignin content due to self-aggregation resulting from excessive lignin content, which reduces the interaction and inter-solubility with the polymer [12, 16].
Two effective approaches have been proposed to address the above conundrums. On one hand, functional additives, such as compatibilizers, plasticizers and crosslinkers, are added during the manufacturing process to improve the toughness, compatibility and interfacial bonding of the films [17–19]. For example, poly[ethylene-co-(glycidyl methacrylate)] (PEGMA) was used as a compatibilizer to blend lignin-g-PMMA copolymer with low-density polyethylene (LDPE) [20]. It was found that the obtained blends exhibited improved tensile strength, ultimate elongation and thermal stability during the processing. On the other side, lignin is chemically modified and incorporated into the polymer matrix [21]. The addition of additives to composites greatly improves the performance of the material, but at the cost of increased impact on human health and the environment. In general, the thermal stability of lignin treated by hydromethylation, demethylation, phenylation and amination is improved, and the glass transition temperature (Tg) as well as the self-aggregation of lignin are lowered and reduced [22]. The purpose of these chemical modification strategies is to enhance the miscibility and interfacial interactions of lignin by adjusting its macromolecular structure. However, the addition of functional additives or the modification of lignin will inevitably involve the use of organic reagents, which are difficult to be degraded in air and are not conducive to the ecological environment as well as green sustainable development. In other words, lignin is rich in hydroxyl groups, which form hydrogen bonds with the hydroxyl or carbonyl groups of polymers [23–25]. Accordingly, we are committed to investigating the use of potential intermolecular forces to prepare lignin-based composites with excellent properties without chemical modification or the addition of additives.
Furthermore, the valorization of lignin is severely limited by its heterogeneous properties in terms of structural diversity, high polydispersity, and uneven reactivity, etc. [11, 26]. Therefore, there are few reports on the preparation of PVA/lignin composites from untreated lignin. The organic solvent method has been widely reported as an effective way to increase lignin homogeneity and reduce molecular weight for lignin separation. Acetone is commonly employed as a fractionating solvent to purify lignin during the lignin extraction process, where the polysaccharides in the lignin can be removed to further increase its purity [27]. As previously reported, lignin fractionated with acetone revealed a lower molecular weight, a higher total phenolic content, better structural homogeneity, and more 1,1-diphenyl-2-trinitrophenylhydrazine (DPPH) radicals for scavenges as well as greater antioxidant activity [28]. Accordingly, acetone plays an essential role in the field of lignin separation and stimulates the potential economic benefits of lignin.
In this study, the graded lignin fractionated with acetone displayed more hydroxyl groups, then the lignin-based composite films were fabricated by means of hydrogen bonding between the fractionated lignin and PVA (Scheme 1). In particular, the influence of lignin structure on the properties of lignin-based composite films was discussed in detail. Additionally, to verify the practicality of the composite film, the mechanical properties, thermal properties, degradability, UV resistance and other experimental tests were systematically investigated. Notably, the lignin-based composite film in this work had a high lignin content up to 70%, which could no doubt make it possible to enhance lignin valorization and reduce costs. More importantly, this helps to overcome the problem of the physical and chemical properties of the soil being affected by the build-up of traditional agricultural mulching films over a long period of time.