Self-Assembly of Molecular Brushes with Responsive Alternating Copolymer Side Chains

Molecular brushes having alternating AB copolymer side chains composed of soluble (A) and responsive (B) monomer units were studied using dissipative particle dynamics simulations. As for thermoresponsive polymers, responsiveness means a variation of solubility of monomer units upon changing external conditions, leading to a variation of solvent quality. In other words, the solvent has a variable selectivity changing from non-selective to strongly selective for the side chains of the brush. The collapse of single brushes (regime of infinite dilution) and their self-assembly in the solution were examined. The effects of grafting density, backbone length and its solubility, as well as the sequence of monomer units in the side chains (alternating and random) were considered. The results demonstrated a similar behavior in the collapse of single brushes regardless of the considered comonomer sequences. Meanwhile, macromolecular aggregation in the solution showed the presence of a gel phase in a particular range of the solvent selectivity, and for strong selectivities, precipitation occurs. This range mainly depends on the grafting density of the side chains: the higher it is, the lower is the solvophobicity of the responsive segments needed for gelation. At the same time, this effect diminishes in the case when the backbone is sufficiently long. Conversely, for brushes with a short or solvophobic backbone, no gelation was observed, neither for the linear copolymers of the same composition as the side chains.