Epoxidation and grafting route to prepare thin and flexible polymer film with high lignin content

Abstract

Recently, lignin has become one of the most important materials of interest in polymer development owing to its various properties, such as mechanical strength, renewability, and availability. However, a few well-known drawbacks, such as self-aggregation, low reactivity, low solubility, etc., have been limiting the large-scale application of lignin. Out of these drawbacks, the low reactivity of lignin has been stabilized as the primary cause behind the limited application. Therefore, in order to improve the lignin's reactivity, in this work, we have first incorporated the epoxy group by reacting with epichlorohydrin (ECH) to prepare epoxy lignin (EAcL). Thereafter, EAcL was grafted with lactic acid (LA) for synthesizing lignin-grafted polylactic acid copolymer (Lig-g-PLA). The incorporation of epoxy in EAcL and grafting of LA in Lig-g-PLA copolymer was characterized by FT-IR, Solid-state 13C NMR, and XPS analysis. TGA and XRD analysis were used to study the effect of epoxidation and grafting reaction on the crystallinity behavior and thermal stability of the EAcL and Lig-g-PLA copolymer. Excellent thin polymer film (≤250 μm thin) was successfully prepared from the Lig-g-PLA copolymers synthesized with higher EAcL (50 wt%) using the hydraulic hot press method. Smooth surfaces of the copolymer films were confirmed through FE-SEM analysis. The DMA analysis showed that Lig-g-PLA copolymer films synthesized with higher LA concentration (EAcL:LA 1:4) possess higher mechanical strength and elastic response.