Microphase separation in bottlebrush polymers under poor-solvent conditions

Abstract

Molecular-dynamics simulations are used to study the structure of bottlebrush polymers with rigid backbones, for various grafting densities, side chain lengths, and varying solvent quality. While we confirm different states of the bottlebrush proposed by Sheiko et al. (Eur. Phys. J. E, 13 (2004) 125) we find that the transition between stretched and collapsed brushes occurs in a rather gradual manner. The pearl-necklace structure occurring at intermediate grafting densities and rather low temperatures has a pronounced medium-range order along the backbone.

Molecular dynamics simulations of single-component bottle-brush polymers with flexible backbones under poor solvent conditions

“…On the other hand, as N increases to 40, we obtain a behavior that is similar to the case of a cylindrical bottle-brush with a rigid backbone [29]. That is an overlap of the curves at low enough temperatures.…”

“…Furthermore, as the temperature increases, there is a significant change in the shape of the bottle brush from a globular coil to an elongated object. Although we can see that we depart from globular structure, we can say very little on whether we obtain a coil-like structure or a homogeneous bottle brush similar to a collapsed brush with a rigid backbone [29] from this quantity. Furthermore, it is worth noting (figure 10) that such non spherical objects possess a cylindrical symmetry, even at low temperatures, and this cylindrical symmetry is only distorted at intermediate temperatures, i.e., within the crossover regime between collapsed chains (globules) and coils or cylindrical brushes.…”

“…When the grafting density, or the side-chain length increases, chains start forming clusters which consist of more than one side chain. Further increase of the grafting density (or very long side chains) to the so-called brush regime, leads to an homogenously cylindrical structure [29,30]. When we study the case of a single-component bottle-brush polymer with a fully flexible backbone under poor solvent conditions, we can basically distinguish two regimes: At low temperatures, or poor solvent conditions, relatively far from the Θ point, the chain obtains an almost spherical structure, like those in the left part of figure 1 for either lower or higher grafting density, shorter or longer backbone, and shorter or longer side chains (we shall discuss the shape of this spherical form later in this section).…”

“…At low temperatures, the bottle-brush chain collapses, forming sphericallike objects with a constant density close to the center of mass of the macromolecule, irrespective of the choice of parameters σ, N b , and N. Thus, the density profile showed a dependency only on the total number of monomers. For higher temperatures, there is a crossover from such collapsed configurations to coil-like structures or cylindrical brushes depending on the choice of the parameters σ, N b , and N. For small grafting densities, backbone, and chain lengths, coil-like structures are favored, whereas, as the values of the above properties increase, we smoothly obtain brushes with monomers homogeneously distributed around the backbone, similar to a bottle-brush polymer with a rigid backbone [29]. An interesting aspect regarding the shape of bottle-brush polymers with a flexible backbone is that the increase of the side-chain length overwhelms the stretching of the side chains, which is caused by the steric repulsions between monomers, as the side-chain length increases.…”

“…Such an analysis is relevant for bottle brushes with a rigid backbone, whereas for bottle-brush polymers with a flexible backbone, only two regimes of rather homogenous structures occur (see figure 1): the collapsed chain state for temperatures further from the Θ regime, and the coil/brush regime, for temperatures close to the Θ temperature. Therefore, pearl-necklace-like structures are fully attributed to the backbone stiffness [29].…”