Chemical Modification and Plasma-Induced Grafting of Pyrolitic Lignin. Evaluation of the Reinforcing Effect on Lignin/poly( L -Lactide) Composites

Pyrolitic lignin was modified through two methods. First, it was grafted with polylactide chains via a solvent-free process by ring-opening polymerization of l-lactide using calcium hydride as a catalyst. The efficiency of grafting was determined by infra-red, nuclear magnetic resonance and time-of-flight secondary ion mass spectrometry analyses. Then, lignin particles were oxygen plasma-treated and immersed in l-lactide solution. Infra-red and X-ray photoelectron spectroscopy revealed that chains bearing ester groups similar to that of lactide were covalently grafted onto the lignin. Composite cast films based on poly(l-lactide) matrix containing ungrafted lignin (lignin/PLLA), chemically-grafted lignin copolymer (PLA-g-lignin/PLLA) and plasma-treated lignin (plasma-treated lignin/PLLA) were investigated. Differential scanning calorimetry and dynamical mechanical analyses revealed that plasma-treated lignin preserved the crystalline structure of PLLA matrix and had a significant reinforcing effect compared with lignin and PLA-g-lignin. Results were compared with literature data.