Federica Lo Verso

Spherical polymer brushes under good solvent conditions: Molecular dynamics results compared to density functional theory

A coarse grained model for flexible polymers end-grafted to repulsive spherical nanoparticles is studied for various chain lengths and grafting densities under good solvent conditions by molecular dynamics methods and density functional theory. With increasing chain length, the monomer density profile exhibits a crossover to the star polymer limit. The distribution of polymer ends and the linear dimensions of individual polymer chains are obtained, while the inhomogeneous stretching of the chains is characterized by the local persistence lengths. The results on the structure factor of both single chain and full spherical brush as well as the range of applicability of the different theoretical tools are presented. Finally, a brief discussion of the experiment is given.

Star Polymers with Tunable Attractions: Cluster Formation, Phase Separation, Reentrant Crystallization

We consider a model to describe starlike polymers featuring a steric repulsion accompanied by a dispersion- or depletion-induced tunable attraction. The range and depth of the latter can be controlled by suitable choices of the solvent, salt concentration and/or depletant size and type, whereas the strength of the steric repulsions is set by the arm number f of the stars. We focused on star polymers with arm number f = 32. Depending on the choice of the attraction characteristics and on the temperature, the system exhibits, in addition to the usual ultra-soft repulsion, a relatively short range attraction and a secondary repulsive barrier at longer distances. Our results show a variety of structurally distinct states. In the fluid phase we find evidence for cluster formation which is accompanied by fluid-phase separation. Moreover the system presents unexpected fluid-solid transitions which are completely absent for the purely repulsive case. The dependence of the cluster and solid regions, and the location of the critical point on the potential parameters is quantitatively analyzed.

Liquid–vapour transition of the long range Yukawa fluid

Two liquid state theories, the self-consistent Ornstein–Zernike equation (SCOZA) and the hierarchical reference theory (HRT) are shown, by comparison with Monte Carlo simulations, to perform extremely well in predicting the liquid–vapour coexistence of the hard-core Yukawa (HCY) fluid when the interaction is long range. The long range of the potential is treated in the simulations using both an Ewald sum and hyperspherical boundary conditions. In addition, we present an analytical optimized mean field theory which is exact in the limit of an infinitely long-range interaction. The work extends a previous one by C. Caccamo, G. Pellicane, D. Costa, D. Pini, and G. Stell, Phys. Rev. E 60, 5533 (1999) for short-range interactions.

Phase behavior of low-functionality, telechelic star block copolymers.

We apply state-of-the-art, Grand Canonical Monte Carlo simulations to determine the self-organization and phase behavior of solutions of block copolymer stars. The latter consist of f AB-block copolymers with N monomers each, which contain a solvophilic block A and solvophobic block B, and which are tethered on a common center on their A-side. We vary the degree of polymerization N and the relative composition of the block copolymer arms and investigate the interplay between macrophase and microphase separation in the system. Preliminary results of the effect of increasing the number of arms, f of the stars are also presented.

Self-assembly scenarios of block copolymer stars

We examine the self-organization scenarios of star-shaped AB-block copolymers, consisting of a solvophilic A-block and a solvophobic B-block, in which f such blocks are chemically anchored on a common centre on their A-parts, leaving the B-blocks exposed on their exterior. We employ a lattice model and we perform Grand Canonical Monte Carlo simulations for the case fu2009=u20096, varying thereby the percentage of attractive monomers as well as the concentration of stars. In agreement with previous studies on the low-functionality case fu2009=u20093 [F. Lo Verso, A.Z. Panagiotopoulos, and C.N. Likos, Phys. Rev. E 79, 010401(R) (2009)], we find that when the majority of monomers in the star are attractive, macrophase separation between two fluid phases at different concentrations of stars occurs below a system-dependent critical temperature. When the majority of monomers is repulsive, novel forms of self-organization arise, which include not only well-defined spherical micelles but also the coexistence of a multiply-connected percolating cluster with smaller micelles having a well-defined size. The morphological characteristics and the sizes of the ensuing aggregates are quantitatively analysed and a critical comparison with the case fu2009=u20093 and fu2009=u200910 is presented.

Effect of attraction on the dynamical arrest of soft colloids

We consider various scenarios leading to molecular control of the rheological state of concentrated solutions of soft colloids, based on the star polymer prototype system. At high values of star functionality, where a dynamically arrested state exists, we discuss the mechanisms that bring about a restoration of ergodicity upon addition of polymer chains, and the theoretical evidence for this effect which is supported by previously published experimental results. For intermediate functionalities, a systematic shift of the glass transition to higher star densities upon addition of additive-induced attractions is established. Finally, for low functionalities, we put forward evidence for a novel phenomenon that is specular to that appearing at high functionalities: a glassy state, which is absent for the pure star system, and that can be induced by the introduction of suitable additives. The latter phenomenon takes place when the effective star–star interaction is modified to display short-range attractions and longer-range repulsions.

Ground states of ultrasoft particles with attractions: a genetic algorithm approach

We analyse in detail the ground-state structure of model systems of athermal star polymers with an additional, tunable attraction that may result from dispersion or depletion forces. To perform a free, unbiased search in the space spanned by the crystal parameters, we employ genetic algorithms, which are enhanced with respect to previous versions in their ability to find stable structures that occupy very narrow basins of attraction in the energy landscape. Application of this method brings about a very large variety of ground states for star polymers with attractions, in particular for the case of intermediate functionalities and strong, short-range attractive forces.