Role of Bottlebrush Additives on the Structure of Block Copolymers in the Bulk and Thin Films

Abstract

Blending block copolymers (BCP) with additives is a useful approach for controlling BCP morphology and properties. In athermal systems, blends of BCPs with polymer additives having very high molecular (M_n) mass generally result in macrophase separation. Bottlebrush polymers, which consist of a linear backbone and grafted side chains, present an interesting alternative where the overall M_n of the system can be very large but the low M_n side chains may drive miscibility with the BCP. In this study, a bottlebrush with a polynorbornene backbone and polystyrene (PS) side chains is blended with PS-b-poly(methyl methacrylate) (PS-b-PMMA) of varying M_n, and the resulting morphologies are examined in both the bulk and thin films. Two different M_n of PS-b-PMMA were used in the bulk study, and the analysis of small-angle X-ray scattering data shows that the blends were miscible and lamellar at all concentrations. This deviates from reference series of both low and high M_n linear polymer additives, which either showed morphological transitions from lamellae to cylinders (low M_n) or were immiscible at all mass fractions studied (high M_n). The relative molecular mass of the side chain (N_SC) and the corresponding component in the BCP (N_A) dictate the distribution of the bottlebrush throughout the BCP, analogous to BCP/linear blends or grafted nanoparticles in a homopolymer matrix. The studies on thin films show a thickness dependence for bottlebrush mass fractions at or above 0.17, a behavior which may be driven by conformational changes of the bottlebrush upon confinement.