2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Sustainability, recently, is getting intensive attention from research communities due to critical environmental concerns. Lignin is the second most abundant renewable polymers on earth, after cellulose. However, utilization of lignin as a feedstock for value added products is very limited. We report a method to control the macromolecular structure of lignin to prepare thermo-responsive shape memory composites. Lignin was reacted with a nitrile butadiene rubber in the melt. The resulted composites exhibited excellent shape memory characteristics. The shape of materials can be programmed by external stress at selected temperatures. Physical crosslinks induced by hydrogen-bonding between plentiful hydroxyl groups of lignin and nitrile groups of rubber fix the pre-determined (temporary) shape of the materials. The chemical crosslinks within the composites recover the permanent (original) shape of the materials by applying an external stimulus, temperature. The crosslinked density and molecular rigidity of these materials were tuned by a thermal annealing process. The presence of unstable ether linkages within β-O-4’, β-5’ phenylcoumaran and β-β’ resinol structural units of lignin promoted the generation of free-radicals for crosslinking reactions. Thermal activation of the lignin based composites indicated significant improvement of the elastic work density and the shape memory characteristics. The chemistry of various lignin molecules, their corresponding thermo-responsivity and programmable shape characteristics of the lignin based composites are discussed.