Fabrication and Characterization of Lignin-Based, Thermoresponsive Soft Composites Containing Fractionated and Cleaned Lignin

Herein, we present a systematic investigation of the influence of lignin molecular weight (MW), lignin concentration, and accelerator concentration on the mechanical and transport properties of soft composites containing lignin, poly(N-isopropylacrylamide) (PNIPAm), and poly(vinyl alcohol) (PVA). Specifically, two series of membranes were fabricated, where the mass ratio of lignin, PNIPAm, and PVA was varied from 1:1:1 to 2:2:1 (lignin/PNIPAm/PVA). Each series of membranes was fabricated with both bulk softwood Kraft lignin and lignin fractionated and cleaned via the Aqueous Lignin Purification with Hot Agents (ALPHA) process, which generated both low MW ultraclean (LMW UCL) and high MW ultraclean (HMW UCL) lignins. Within each series, the concentration of tetramethylethylenediamine (TMEDA) was varied to probe the influence of radical accelerator concentration on final membrane performance properties. Most notably, the addition of lignin was seen to significantly decrease methylene blue (MB) permeability, while also increasing Young's modulus by as much as 45% when compared to the PVA-PNIPAm control. The MB permeability decreased by over an order of magnitude, when compared to the control, for 2:2:1 membranes containing HMW UCL and a TMEDA concentration of 10 wt %. At higher lignin concentrations, a clear trend of increasing Young's modulus with increasing lignin MW was observed. Furthermore, the thermoresponsive nature of these soft composites materialized as increases in elastic moduli as large as 15 MPa at temperatures (∼40 °C) above the volume phase-transition temperature of PNIPAm. Results from this study highlight how both lignin concentration and lignin MW can be leveraged to mechanically improve the network structure, as well as to tune the transport properties of thermoresponsive soft composites.