Diagram of States of Bottlebrush Polymers under Poor Solvent Conditions

Introduction Bottlebrush polymers are macromolecules which consist of a long chain serving as “backbone” onto which flexible side chains are densely grafted. There is a lot of recent interest in both the synthesis of these macromolecules and characterization of physical properties by various experimental techniques (light and neutron scattering, atomic force microscopy, electron microscopy, etc); some of this work is motivated by the use of these cylindrical brushes as building blocks in functional supramolecular structures. The backbone length, the side chain length and the grafting density of bottlebrushes are parameters that allow the control of structure and eventually their function and properties. However, the most important structures occur when one varies external parameters (i.e. pH of the solution, temperature) allowing the use of bottlebrushes as molecular actuators, sensors, etc. For a single chain under poor solvent conditions one expects its collapse leading to the formation of a globule or alternatively crystallization. In the case of bottlebrush polymers at intermediate grafting densities and under poor solvent conditions the interplay of grafting density σ and the length N of the flexible side chains grafted onto the much stiffer backbone leads to the formation of clusters, which consist of a number of neighboring side chains. For higher grafting densities and poor solvent conditions the formation of homogeneous cylindrical structures is expected. Apart from qualitative scaling predictions on the diagram of states that one should expect for the bottlebrush and qualitative observations for a lattice model with very short side chains, this type of lateral microphase separation under poor solvent conditions and at intermediate grafting densities has not yet been characterized. In our study we fill this gap and describe large scale Molecular Dynamics (MD) simulations of an off-lattice model to provide a first test of the proposed diagram of states for bottlebrush homo-polymers.