Dynamic Mechanical Properties of Self-Assembled Bottlebrush Polymer Networks

We systematically investigate the effects of composition on the dynamic mechanical properties of bottlebrush polymer networks self-assembled by linear–bottlebrush–linear triblock copolymers. We fix the molecular architecture of the bottlebrush, which consists of 51 poly(dimethyl siloxane) (PDMS) side chains of 5 kg/mol and has a molecular weight of 255 kg/mol, and increase only the volume fraction f of the linear poly(benzyl methacrylate) (PBnMA) blocks. As f increases from 0.05 to 0.41, the network shear modulus G at room temperature increases from ∼4 to ∼100 kPa. Yet, depending on the network morphology, the relation between G and f exhibits two regimes. (i) For sphere morphology, G is nearly a constant; yet, because of a large fraction of loops, the absolute value of G is about 40% of the stiffness Gm of the PDMS bottlebrush matrix. (ii) For cylinder morphology, G increases slowly with f but remains nearly 4 orders of magnitude lower than 109 Pa for the glassy cylinders formed by the end PBnMA blocks. We explain this remarkable behavior by modeling the polymer as a polycrystalline material consisting of randomly oriented grains, and each grain is a fiber-reinforced composite. We propose a modified Halpin–Tsai model to describe the shear modulus of such a polycrystalline material: G = Gm(1+ζf)/(1–f), in which ζ is an adjustable parameter that describes the grain size relative to the fiber diameter. Above the glass-transition temperature of end blocks, the reinforcement to network modulus from the glassy fibers diminishes, such that G becomes a constant of the matrix stiffness. Our results not only reveal previously unexplored molecule–structure–property relations of self-assembled bottlebrush polymer networks but also provide a new class of soft, solvent-free, and reprocessable polymeric materials with a wide range of controllable stiffness.