Barbara Capone

Telechelic Star Polymers as Self-Assembling Units from the Molecular to the Macroscopic Scale

By means of multiscale molecular simulations, we show that telechelic-star polymers are a simple, robust, and tunable system, which hierarchically self-assembles into soft-patchy particles and mechanically stabilizes selected, open crystalline structures. The self-aggregating patchy behavior can be fully controlled by the number of arms per star and by the fraction of attractive monomeric units at the free ends of the arms. Such self-assembled soft-patchy particles while forming, upon augmenting density, gel-like percolating networks, preserve properties as particle size, number, and arrangement of patches per particle. In particular, we demonstrate that the flexibility inherent in the soft-patchy particles brings forward a novel mechanism that leads to the mechanical stability of diamond and simple cubic crystals over a wide range of densities, and for molecular sizes ranging from about 10 nm up to the micrometer scale.

Core-Shell Structure of Monodisperse Poly(ethylene glycol)-Grafted Iron Oxide Nanoparticles Studied by Small-Angle X-ray Scattering.

The promising applications of core–shell nanoparticles in the biological and medical field have been well investigated in recent years. One remaining challenge is the characterization of the structure of the hydrated polymer shell. Here we use small-angle X-ray scattering (SAXS) to investigate iron oxide core–poly(ethylene glycol) brush shell nanoparticles with extremely high polymer grafting density. It is shown that the shell density profile can be described by a scaling model that takes into account the locally very high grafting density near the core. A good fit to a constant density region followed by a star-polymer-like, monotonously decaying density profile is shown, which could help explain the unique colloidal properties of such densely grafted core–shell nanoparticles. SAXS experiments probing the thermally induced dehydration of the shell and the response to dilution confirmed that the observed features are associated with the brush and not attributed to structure factors from particle aggregates. We thereby demonstrate that the structure of monodisperse core–shell nanoparticles with dense solvated shells can be well studied with SAXS and that different density models can be distinguished from each other.

Design and folding of colloidal patchy polymers

The creation of functional nanoscale materials with complex 3D structures has been achieved by biological systems e.g. proteins, but remains a daunting challenge in materials science. Recent progress in this direction has been made with patchy particles, which can be made to self-assemble into specific structures by fine tuning the numbers, locations and interactions of the patches. Here, we present a different, bio-inspired approach to create 3D objects from chains of patchy particles that fold into structures determined by the particle sequence along the chain. The particles linked in the chains are spherical with homogeneous weak repulsive or attractive potentials and symmetry-breaking patches that provide attractive directional interactions. We show, using computer simulations, that particle sequences along the string can be designed to steer the folding into specific target structures. Moreover, we introduce a scheme to discriminate configurations that present a golf-hole like free energy landscape, which inhibits folding, from target structures that are easy to design.

Coarse graining of star-polymer – colloid nanocomposites

We consider mixtures of self-avoiding multiarm star polymers with hard colloids that are smaller than the star polymer size. By employing computer simulations, and by extending previous theoretical approaches, developed for the opposite limit of small star polymers [A. Jusufi et al., J. Phys.: Condens. Matter 13, 6177 (2001)], we coarse-grain the mixture by deriving an effective cross-interaction between the unlike species. The excellent agreement between theory and simulation for all size ratios examined demonstrates that the theoretical approaches developed for the colloidal limit can be successfully modified to maintain their validity also for the present case of the protein limit, in contrast to the situation for mixtures of colloids and linear polymers. We further analyze, on the basis of the derived interactions, the non-additivity parameter of the mixture as a function of size ratio and star functionality and delineate the regions in which we expect mixing as opposed to demixing behavior. Our results are relevant for the study of star-colloid nanocomposites and pave the way for further investigations of the structure and thermodynamics of the same.

Hierarchical self-assembly of telechelic star polymers: from soft patchy particles to gels and diamond crystals

The design of self-assembling materials in the nanometer scale focuses on the fabrication of a class of organic and inorganic subcomponents that can be reliably produced on a large scale and tailored according to their vast applications for, e.g. electronics, therapeutic vectors and diagnostic imaging agent carriers, or photonics. In a recent publication (Capone et al 2012 Phys. Rev. Lett. 109 238301), diblock copolymer stars have been shown to be a novel system, which is able to hierarchically self-assemble first into soft patchy particles and thereafter into more complex structures, such as the diamond and cubic crystal. The self-aggregating single star patchy behavior is preserved from extremely low up to high densities. Its main control parameters are related to the architecture of the building blocks, which are the number of arms (functionality) and the fraction of attractive end-monomers. By employing a variety of computational and theoretical tools, ranging from the microscopic to the mesoscopic, coarse-grained level in a systematic fashion, we investigate the crossover between the formation of microstructure versus macroscopic phase separation, as well as the formation of gels and networks in these systems. We finally show that telechelic star polymers can be used as building blocks for the fabrication of open crystal structures, such as the diamond or the simple-cubic lattice, taking advantage of the strong correlation between single-particle patchiness and lattice coordination at finite densities.

Limiting the valence: advancements and new perspectives on patchy colloids, soft functionalized nanoparticles and biomolecules

Limited bonding valence, usually accompanied by well-defined directional interactions and selective bonding mechanisms, is nowadays considered among the key ingredients to create complex structures with tailored properties: even though isotropically interacting units already guarantee access to a vast range of functional materials, anisotropic interactions can provide extra instructions to steer the assembly of specific architectures. The anisotropy of effective interactions gives rise to a wealth of self-assembled structures both in the realm of suitably synthesized nano- and micro-sized building blocks and in nature, where the isotropy of interactions is often a zero-th order description of the complicated reality. In this review, we span a vast range of systems characterized by limited bonding valence, from patchy colloids of new generation to polymer-based functionalized nanoparticles, DNA-based systems and proteins, and describe how the interaction patterns of the single building blocks can be designed to tailor the properties of the target final structures.

A systematic coarse-graining strategy for semi-dilute copolymer solutions: from monomers to micelles

A systematic coarse-graining procedure is proposed for the description and simulation of AB diblock copolymers in selective solvents. Each block is represented by a small number, n(A) or n(B), of effective segments or blobs, containing a large number of microscopic monomers. n(A) and n(B) are unequivocally determined by imposing that blobs do not, on average, overlap, even if complete copolymer coils interpenetrate (semi-dilute regime). Ultra-soft effective interactions between blobs are determined by a rigorous inversion procedure in the low concentration limit. The methodology is applied to an athermal copolymer model where A blocks are ideal (theta solvent), B blocks self-avoiding (good solvent), while A and B blocks are mutually avoiding. The model leads to aggregation into polydisperse spherical micelles beyond a critical micellar concentration determined by Monte Carlo simulations for several size ratios f of the two blocks. The simulations also provide accurate estimates of the osmotic pressure and of the free energy of the copolymer solutions over a wide range of concentrations. The mean micellar aggregation numbers are found to be significantly lower than those predicted by an earlier, minimal two-blob representation (Capone et al 2009 J. Phys. Chem. B 113 3629).

Competing micellar and cylindrical phases in semi-dilute diblock copolymer solutions

We develop a “first principles” coarse-graining procedure based on a soft effective segment representation of an athermal AB diblock copolymer model in selective solvent, to map out the self-assembly phase diagram for several asymmetry ratios f using Monte Carlo free energy calculations. We find that the free energy per unit volume is surprisingly insensitive to the aggregation number of monodisperse cubic and cylindrical phases for a given polymer volume fraction. The cylindrical phase is found to pre-empt the cubic micellar phases for nearly symmetric (f ≃ 0.6) copolymers.

Rescaling of structural length scales for “soft effective segment” representations of polymers in good solvent

It is shown by simple scaling arguments that the structural length scales of semi-dilute polymer solutions, calculated from coarse-grained “soft effective segment” representations of long polymers, need to be rescaled by a single, well-defined scaling factor to match the corresponding properties of the underlying microscopic polymer model. The validity of this rescaling is illustrated by extensive Monte Carlo simulations of the density profiles and average height of stretched polymer brushes. PACS numbers: 68.47.Mn, 68.47.Pe, 61.25.Hq, 82.35.Lr

Crystal stability of diblock copolymer micelles in solution

We investigate the relative stability of the disordered phase and of four crystal structures of micelles resulting from the self-assembly of AB diblock copolymers in semi-dilute solutions. Starting from the micelle–micelle pair distribution functions determined previously in the disordered fluid phase by Monte Carlo simulations of a coarse-grained model of diblock copolymers, we extract effective pair potentials v(r) between micelle centres of mass by a novel extrapolation/inversion technique. These v(r) are used in extensive Monte Carlo simulations of micellar assemblies to determine the structures, mean-square displacements, and free energies of four ordered phases including FCC, BCC, diamond and the less common A15 crystals. For micelle densities close to melting, we predict the most stable structures to be FCC and A15, with the latter phase having the lowest free energy for micelles with small cores and large coronae, in agreement with recent predictions for micelles forming in copolymer melts [G.M. Grason et al., Phys. Rev. Lett. 91, 058304 (2003)].