Anna Cavallo

Single chain structure in thin polymer films: corrections to Flory's and Silberberg's hypotheses

Conformational properties of polymer melts confined between two hard structureless walls are investigated by Monte Carlo simulation of the bond fluctuation model. Parallel and perpendicular components of chain extension, bond–bond correlation function and structure factor are computed and compared with recent theoretical approaches attempting to go beyond Florys and Silberbergs hypotheses. We demonstrate that for ultrathin films where the thickness, H, is smaller than the excluded volume screening length (blob size), ξ, the chain size parallel to the walls diverges logarithmically, R2/2N≈b2+clog(N) with c~1/H. The corresponding bond–bond correlation function decreases like a power law, C(s) = d/sω with s being the curvilinear distance between bonds and ω = 1. Upon increasing the film thickness, H, we find—in contrast to Florys hypothesis—the bulk exponent ω = 3/2 and, more importantly, a decreasing d(H) that gives direct evidence for an enhanced self-interaction of chain segments reflected at the walls. Systematic deviations from the Kratky plateau as a function of H are found for the single chain form factor parallel to the walls in agreement with the non-monotonic behaviour predicted by theory. This structure in the Kratky plateau might give rise to an erroneous estimation of the chain extension from scattering experiments. For large H the deviations are linear with the wavevector, q, but are very weak. In contrast, for ultrathin films, H

Phase behaviour of quasi-block copolymers: A DFT-based Monte-Carlo study

We develop a mesoscopic density functional theory (DFT)-based Monte-Carlo approach for studying the phase behaviour of multi-component systems comprised of irreversibly bonded, conventional macromolecules and supramolecular entities. The latter can reversibly associate with each other and the conventional components to “living”, equilibrium polymers. The computational approach can be applied to a broad class of supramolecular systems and we focus here on quasi-block copolymer systems that contain conventional, “dead” AB-copolymers with a supramolecular B-terminus and supramolecular B-units. The simulations show that, by properly selecting the architecture of the “dead” copolymers and by varying the supramolecular association constant and the incompatibility between the segment species, A and B, one obtains a variety of different microphase-separated morphologies and macrophase separations. Two representative phase diagrams are reported as a function of the association constant, Eb, and the Flory–Huggins parameter, χ, quantifying the repulsion between A and B segments. The simulation results are qualitatively rationalised by considering the dependence of the stoichiometry on the systems parameters, and fractionation effects between coexisting phases are illustrated.

Polymer mixtures in confined geometries: Model systems to explore phase transitions

While binary (A,B) symmetric polymer mixtures ind = 3 dimensions have an unmixing critical point that belongs to the 3d Ising universality class and crosses over to mean field behavior for very long chains, the critical behavior of mixtures confined into thin film geometry falls in the 2d Ising class irrespective of chain length. The critical temperature always scales linearly with chain length, except for strictly two-dimensional chains confined to a plane, for whichTc ∝N5/8 (this unusual exponent describes the fractal contact line between segregated chains in dense melts in two spatial dimensions,d = 2). When the walls of the thin film are not neutral, but preferentially attract one species, complex phase diagrams occur due to the interplay between capillary condensation and wetting phenomena. For ‘competing walls’ (one wall prefers A, the other prefers B) particularly interesting interface localization-delocalization transitions occur, while analogous phenomena in wedges are related to the ‘filling transition’.

Chain Conformations and Phase Behavior in Confined Polymer Blends

We investigate the chain conformations and phase separation in binary polymer blends. Using large scale semi-grandcanonical Monte Carlo simulations and finite size scaling, we investigate the molecular extension and the intermolecular paircorrelation function in thin films with hard, non-preferentially adsorbing surfaces. The interplay between chain conformations, demixing and the validity of mean field theory is investigated for a large variation of chain lengths 16 ≤ N ≤ 512. Three regimes of film thickness D can be distinguished: (i) For film thicknesses much larger than the unperturbed chain extension R e, bulk behavior is observed, i.e., the critical temperature of demixing T c increases linearly with chain length, and the mean field theory becomes asymptotically correct for large N. (ii) For D ∼ R e, the critical temperature scales linearly, T c ∼ N, but the mean field theory overestimates the prefactor even in the limit N → ∞ (iii) For ultrathin films, the chain conformations are quasi-two-dimensional, T c ∼ √N and mean field theory completely fails.