Lignin-Grafted Poly(<i>n</i>-Butyl Acrylate) Copolymers as Soft Materials for Adhesive Applications: A Comparison of Hardwood, Softwood, and Straw Lignin

Lignin from birch (BLN), spruce (SLN), and wheat straw (WSLN) was first converted into bromoisobutyrate-based macroinitiators (LNBr) and subsequently grafted with <i>n</i>-butyl acrylate (LNBA) copolymers via atom transfer radical polymerization (ATRP). The influence of hardwood, softwood, and straw lignin on the properties of the copolymers was investigated in terms of thermal behavior, rheology, and tack test. Structural analysis by FTIR, NMR, and SEC/GPC confirmed successful grafting and an increased and broad molecular weight distribution. Thermal analysis (DSC and TGA) showed increased glass transition temperatures (<i>T</i>_g) and different thermal degradation compared to the homopolymer poly(<i>n</i>-butyl acrylate). Additionally, BLN-based copolymers with varying degrees of polymerization (2, 4, and 10) were synthesized to optimize the material properties. The copolymers showed adhesive properties for all BLNBA and SLNBA variants, with BLNBA2 and BLNBA4 meeting the Dahlquist criteria for pressure-sensitive adhesives.