In accordance with some embodiments of the present invention, a composite material is prepared by blending a bio-derived filler into a polymer, wherein the filler includes a diene-modified cellulosic nanomaterial (e.g., cellulose nanocrystals (CNCs) and/or cellulose nanofibrils (CNFs) functionalized to contain a diene) and a dienophile-modified cellulosic nanomaterial (e.g., CNCs and/or CNFs functionalized to contain a dienophile). The modulus of the composite material is reversibly controllable by adjusting a degree of crosslinking between the diene-modified cellulosic nanomaterial and the dienophile-modified cellulosic nanomaterial. This degree of crosslinking is thermally reversible. On one hand, the degree of crosslinking may be increased via a Diels-Alder (DA) cycloaddition reaction at a first temperature, thereby increasing the modulus of the composite material. On the other hand, the degree of crosslinking may be decreased via a retro-DA reaction at a second temperature higher than the first temperature, thereby decreasing the modulus of the composite material.