Design of new routes for the synthesis of unimolecular soft nanoparticles: from sparse to compact nanoobjects

Abstract

Single-chain nanoparticles are the smallest unimolecular nano-objects (size<15nm) that can be prepared starting from a linear polymer chain through intrachain folding/ collapse. They represent a new class of nano-materials displaying unique properties with potential applications in different fields as rheological agents, enzyme mimics, catalytic systems, nano-carriers, sensors and smart gels. By means of Molecular Dynamics simulations and experiments we design new experimental protocols and functionalization strategies for the synthesis of compact unimolecular soft nanoparticles (SNP). The precursor chains are modeled as linear backbones with short side groups, some of them randomly selected as linker groups in order to mimic real synthesis conditions. We systematically characterize the nano-particle structure in good solvent as a function of molecular weight and fraction of linker groups.

This includes the determination of the scaling exponents for the radii of gyration, as well as a characterization of the shape (e.g., asphericity and prolateness) of the SNPs. In agreement with SAXS experiments, simulation results reveal sparse nanoobjects. In order to obtain more spherical and compact structures we explored two different new synthesis routes. First of all we tuned the chemical composition of the SNPs by using binary copolymers (i.e. two different types of linkers "A" and "B") as precursors, and we investigated the effect of the so-called orthogonal chemistry which allow to select different groups which cross-link in solution by different protocols (simultaneous or sequential cross-linking of A and B). Amazingly, this route leads to more compact objects than those obtained from precursors with a single type of cross-linker. As a second route we investigate the cross-linking under bad solvent conditions and by fixing the precursor to a wall in order to prevent intermolecular aggregations. The swelling of the obtained SNPs results in highly spherical objects (see figure). This protocol is very promising for designing SNPs of tunable density and softness by varying the fraction of cross-linkers. The effective interactions between macromolecules with different topologies and the effects on the phase behavior is discussed.