Critical insights into the reinforcement potential of cellulose nanocrystals in polymer nanocomposites

•Cellulose nanocrystals are polar, negatively-charged hydrophilic nanoparticles.•CNCs have high crystallinity and stiffness.•CNCs can exceptionally reinforce certain polymer nanocomposites.•CNCs can act as effective nucleating agents, and improve cross-linking capacity.•CNCs have a promising potential for industrial, biomedical and sports applications.