Cellulose nanocrystals (CNCs) are widely used as reinforcing filler in polymers, due to their exceptionally high stiffness and strength and because the biological species from which they are isolated represent renewable resources. However, aggregation of the CNCs, which is concomitant with limited reinforcement, is often difficult to avoid. One-component nanocomposites (OCNs) based on polymer-grafted nanoparticles can solve this problem, because this approach affords, by design, materials in

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... ch no such aggregation is possible. At the same time, chain entanglements between the CNC-grafted polymer chains provide stress-transfer among the particles. To demonstrate this, we investigated OCNs based on polymethacrylate-grafted CNCs. A previously un-accessed compositional space, i.e., OCNs with a CNC content of 10 or 20 wt%, was explored. Cotton linter-based CNCs were modified via surface-photoinitiated free radical polymerization, which involved the functionalization of the CNC surfaces with benzophenone moieties as photo-radical initiator species, and the subsequent surface-photoinitiated polymerization of methyl or hexyl methacrylate under UVirradiation at 365 nm. The resulting particles readily dispersed in THF. Solvent-casting and compression-molding afforded films of homogeneous appearance, which display remarkable 2 improvements in stiffness or toughness and strength in comparison to conventional twocomponent nanocomposites of unmodified CNCs and the respective polymers. significantly enhance the mechanical properties of the final material. In this context, we surmised that OCNs based on polymer-grafted cellulose nanocrystals (CNCs) would be an interesting materials family to investigate. To the best of our knowledge, only two experimental studies 7, 8 and two computational simulations, 9, 10 have been published on OCNs based on hairy CNCs. CNCs are rod-like NPs that are readily isolated through selective hydrolysis from renewable bio-sources, such as higher plants, bacteria, and tunicates, among others. 11, 12 CNCs consist of linear homopolysaccharides composed of anhydroglucopyranose repeat units that are connected via  (1-4') glycosidic bonds. 11 CNCs are moderately to highly crystalline (54-88%) with an aspect ratio (10-170) that depends mainly on the raw material, but also on the hydrolysis conditions. 12 CNCs are frequently used as reinforcing filler in polymer nanocomposites, due to their low density, high tensile strength (7.5 -7.7 GPa) 13 and stiffness (57-151 GPa), 12 and the renewable nature of the sources from which they are isolated. 14 Data also suggest that unlike other nanofillers, CNCs have a limited associated toxic potential when investigated under realistic exposure scenarios and doses. 15 The use of CNCs as reinforcement agent in a polymeric matrix was pioneered by Favier et al., who observed an improvement of the mechanical properties of poly(styrene-co-butyl acrylate) when a latex of this polymer was combined with CNCs extracted from tunicates and processed into films. 16, 17 Since then, CNC-polymer nanocomposites have been widely investigated and their properties are now well-understood. 14 The mechanical reinforcement in polymer/CNC nanocomposites is generally accepted to take place through a hydrogen-bonded CNC network that percolates within the polymer matrix, which is responsible for efficient stress transfer. 17 The reinforcement achieved is typically well predicted by a percolation model, which considers that the CNC percolating network is formed over a certain filler content (i.e., 4 percolation threshold). 18, 19 However, in many cases the actual reinforcement does not match with the prediction of this model. 20, 21 One possible reason is that it does not take into account CNC aggregation, which we have recently shown to be important. 22 The remaining challenge is thus to enhance the dispersion of CNCs within the polymer matrix. Many efforts have been directed to address this problem by increasing the hydrophobicity of the CNCs via surface modification. 23 For instance, stable dispersions of CNCs/poly(lactic acid) in chloroform 24 or CNCs/atactic poly(propylene) in toluene 25 have been reported after physically coating the CNC surface with surfactants. The physical adsorption of polymers on CNCs was also shown to improve their dispersibility. 26 For example, after adsorbing poly(ethylene oxide), the CNCs dispersed more readily in (hydrophobic) low-density polyethylene and the resulting nanocomposites could be processed via melt-extrusion into objects of homogeneous appearance. Another strategy to enhance the dispersibility of CNCs is to chemically modify their surface with motifs that are similar to the polymer matrix or sites that display specific interactions with the matrix. 23 Examples include the TEMPO-mediated oxidation of the surface OH-groups, 27 the attachment of small molecules such as organic acid chlorides, 28 or acetic anhydride, 29 and the introduction of long polymer chains. 30, 31 As discussed above, one-component nanocomposites (OCNs) based on polymer-grafted nanoparticles promise to be another solution to the dispersion problem, because their architecture prevents macrophase separation. At the same time, chain entanglements between the CNC-grafted polymer chains can enable stress-transfer among the particles. Indeed, the polymer chain length, the grafting density and the polymer structure also should influence the mechanical properties of CNC-based OCNs according to the recent computational predictions reported by Hansoge et al. 9, 10 5 Polymer-grafted CNCs can be synthesized via "grafting-from", "grafting-to", or "graftingthrough" approaches involving functional groups on the CNCs surface. 3, 32 The grafting-from approach, which was utilized here, involves the functionalization of the CNCs with polymer brushes by way of surface-initiated polymerization from initiator groups immobilized on the NPs' surface. This framework generally leads to polymer grafts with a well-controlled length and high polymer grafting density. 3 The first example of OCNs based on polymer-grafted CNCs, using a grafting-from approach, was reported by Chen et al., 7 who functionalized CNCs with semicrystalline poly(ε-caprolactone) via surface-initiated ring-opening polymerization. OCNs with a CNC content of between 4 and 8 wt% proved to be melt-processable and the authors reported a non-linear dependence between the CNC content and the OCN mechanical properties. Unfortunately, no comparison with conventional composites was made and no correlation between the PCL graft structure and the properties of the OCNs could be established. In another study, Chang et al. reported the synthesis of CNCs-g-poly(ethynylenefluorene) through Sonogashira coupling via a grafting-from approach, 8 but the mechanical properties of the material were not investigated. Here, we report the synthesis of amorphous, polymethacrylate-grafted CNCs through a synthetically undemanding free radical polymerization protocol. It involves the surface functionalization of CNCs with a benzophenone derivative that serves as radical photoinitiator for the surface-initiated photopolymerization of methyl and hexyl methacrylate upon UVirradiation. The morphology and thermomechanical properties of the OCNs made from these "hairy" CNCs were investigated and the materials were found to display remarkable improvements in stiffness or toughness and strength in comparison to conventional twocomponent reference nanocomposites consisting of unmodified CNCs and the respective polymethacrylate. Notes The authors declare no competing financial interest nanocrystal sulfate half-ester content by conductometric titration. Cellulose 2015, 22 (1), 101-116. 34. Biyani, M. V.; Jorfi, M.; Weder, C.; Foster, E. J., Light-stimulated mechanically switchable, photopatternable cellulose nanocomposites. Polym. Chem. 2014, 5 (19), 5716-5724. 35. Dorman, G.; Prestwich, G. D., Benzophenone Photophores in Biochemistry. Biochemistry 1994, 33 (19), 5661-5673. 36. Majoinen, J.; Walther, A.; McKee, J. R.; Kontturi, E.; Aseyev, V.; Malho, J. M.; Ruokolainen, J.; Ikkala, O., Polyelectrolyte Brushes Grafted from Cellulose Nanocrystals Using Cu-Mediated Surface-Initiated Controlled Radical Polymerization.