Polymer nanocomposites, due to combining constituent properties, can fulfil a wide variety of applications, ranging from structural materials to filtration membranes to battery separators. However, nanofiller aggregation can pose detrimental to best maximizing the nanocomposite properties. Polymer-grafted nanoparticles, which when cast into films without additional matrix material, limit nanofiller aggregation due to the covalent bonding between nanofiller and polymer matrix, and although the class of materials has seen increasing study, gaps remain particularly in anisotropic nanofiller and ionically conductive matrices. This dissertation will focus on investigating polymer-grafted cellulose nanocrystals to establish structure/property relationships, particularly between the grafted polymer conformation and composite mechanical properties, ion conductivity enhancement in hydrated, polyelectrolyte grafted systems, as well as explore the rationale for ion conductivity enhancement at their interfaces. To this end polystyrene-grafted cellulose nanocrystals will be used as a model system to investigate the polymer graft conformation and its impact on the thermally dependent elastic modulus and material fracture toughness. Hydrated, poly(2-vinylpyridine)-grafted cellulose nanocrystals treated with iodomethane will then be used to probe the ionic conductivity enhancement (compared to ungrafted polymer) by altering the polymer molecular weight, grafting density, and architecture. Finally, the interfacial contribution to ionic conductivity will be explored using thin film, model polymer brushes on interdigitated electrodes to differentiate competing hypotheses of surface hydrophilicity and functional group content.