Moldable and Humidity-Responsive Self-Healable Complex from Lignosulfonate and Cationic Polyelectrolyte

We have developed a moldable and self-healable material from lignosulfonate by taking advantage of the ionic interactions between lignosulfonate and various cationic polyelectrolytes. Three lignosulfonate complexes with different cationic polyelectrolytes were prepared. The complex of sodium lignosulfonate (L-SO3Na) and poly(diallyldimethylammonium chloride) (PDADMACl) was deemed a promising moldable material based on its mechanical toughness in tensile tests (>3 MJ/m(3)). The mechanical properties of the L-SO3Na/PDADMACl complex were further characterized with regard to the effects of composition, the molecular weight of PDADMACl, and relative humidity. Our findings were as follows: (i) the addition of adequate amounts of lignosulfonate to PDADMACl improved the toughness of the complex (from 1.8 to 3.6 MJ/m(3) at maximum), (ii) using higher-molecular-weight (>200,000) PDADMACl enhanced the mechanical properties of the complex, and (iii) increasing the relative humidity softened the complex through water absorption. The self-healing properties of the L-SO3Na/PDADMACl complex were also evaluated. The healing efficiency, in terms of toughness recovery, was only 14% at 50% relative humidity but increased to almost 100%, i.e., the sample did not break at the healed position, at 60% relative humidity. This study proposes the use of lignosulfonate as a component of humidity-responsive and completely self-healable materials.