Sommaire:

Single chain nanoparticles (SCNPs) are an emergent class of synthetic nano-objects, of size 5-20 nm, obtained through intramolecular cross-linking of single polymer chains. They are the basis of the so-called single-chain technology, a rapidly growing research area of enormous possibilities and applications in fields as biosensors, catalysts, drug delivery or rheological agents. In this presentation we summarize the state-of-the art and our most recent contributions to the field through the combination of synthesis, simulations, scattering (neutron and X-ray) and relaxation techniques (dielectric spectroscopy and rheology), and discuss the main challenges for the next years.

SCNPs are topologically polydisperse. The standard synthesis protocols produce SCNPs with open sparse topologies consisting of tightly linked domains connected by flexible linkers, displaying analogies with intrinsically disordered proteins (IDPs) and suggesting SCNPs as a model system -free of specific interactions- to discriminate steric effects on IDPs in crowded cellular solutions As opposite to the gaussian conformations adopted by linear chains, sparse SCNPs in concentrated solutions and bulk exhibit an entropically driven collapse to fractal globular conformations. This observation is independent of the specific architecture of the macromolecular crowders (linear or complex) surrounding the SCNP [3,4] and on the lifetime of the SCNP bonds (irreversible or dynamic).

Simulations can be used to envisage a broad set of realistic synthesis routes to control the compactness of SCNPs, leading to globular cross-linked objects much smaller than microgels. Concentrated solutions of globular SCNPs exhibit, in contrast to hard spheres, structural and dynamic anomalies, as soft caging, reentrant behaviour (ordering and speeding up by increasing concentration) and absence of dynamic heterogeneities. This constitutes a realization in a real macromolecular system of the qualitative scenarios proposed for a long time by generic models of structureless ultrasoft particles.

Finally we explore the possibility of using SCNPs as nanofillers for linear matrixes in fully-polymeric nanocomposites and even for designing bulk materials based only in SCNPs. We characterize several dynamic scales going from the alpha-relaxation to the reorientation of the SCNPs. Whereas the glass transition is just slightly affected with respect to their linear counterparts, sparse SCNPs exhibit a much faster relaxation of the chain modes, reflecting a strong suppression of entanglements. When SCNPs are used in nanocomposites, their specific topology has a different effect on the dynamics of the linear chains. By characterizing the entanglement network in the nanocomposite we find that sparse and globular SCNPs lead respectively to mild dilation and strong narrowing of the tube for the reptational motion of the linear chains.

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