Materials with switchable mechanical properties are widespread in living organisms and endow many species with traits that are essential for their survival. Many of the mechanically morphing materials systems found in nature are based on hierarchical structures, which are the basis for mechanical robustness and often also the key to responsive behavior. Many "operating principles" involve cascades of events that translate cues from the environment into changes of the overall structure and/or the connectivity of the constituting building blocks at various levels. These concepts permit dramatic property variations without significant compositional changes. Inspired by the function and the growing understanding of the operating principles at play in biological materials with the capability to change their mechanical properties, significant efforts have been made toward mimicking such architectures and functions in artificial materials. Research in this domain has rapidly grown in the last two decades and afforded many examples of bioinspired materials that are able to reversibly alter their stiffness, shape, porosity, density, or hardness upon remote stimulation. This review summarizes the state of research in this field.
Cellulose nanocrystals (CNCs) are widely studied as reinforcing fillers for polymers. In many cases the mechanical properties of polymer/CNC nanocomposites do not match the theoretical predictions, arguably on account of CNC aggregation. This problem can be mitigated through the addition of a small amount of a judiciously selected polymeric dispersant that also serves as a binder among the CNCs. We show that the addition of 1–5% w/w poly(vinyl alcohol) (PVA) has a very significant impact on the mechanical properties of poly(ethylene oxide-co-epichlorohydrin)/CNC nanocomposites. Remarkable improvements of the stiffness and strength were observed at a PVA content as low as 1% w/w, and the extent of reinforcement increased up to a PVA content of 5% w/w, where Young’s modulus, storage modulus, and strength increased by up to 5-fold vis-à-vis the PVA-free nanocomposites. Similar effects were observed for CNC nanocomposites made with polyurethane or poly(methyl acrylate) matrices, demonstrating that the approach is broadly exploitable. Laser scanning microscopy based resonance energy transfer experiments that involved nanocomposites made with CNCs and PVA that had been labeled with rhodamine and fluorescein, respectively, confirmed that the enhanced mechanical properties of the three-component nanocomposites are indeed related to an improved dispersion of the CNCs.
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 which 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.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 f...
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