A characteristic correlation peak in small-angle X-ray scattering (SAXS) profiles of brush-like polymers, commonly referred to as the “bottlebrush peak”, provides key insights into molecular packing in melts and networks. The peak position, intensity, and width are known to depend on brush architecture and chemical composition. However, a clear understanding of the origin of the peak is still lacking, frequently leading to its assignment to an unrelated structural length scale. To resolve this ambiguity, we conducted a systematic investigation of brush macromolecules spanning comb and bottlebrush regimes using a combination of SAXS, rheology, and coarse-grained molecular dynamics simulations. In densely grafted bottlebrushes, side chains effectively suppress backbone fluctuations, resulting in semiflexible filament-like conformations. In this regime, the peak position q* corresponds to an effective brush diameter d = 2π/q*, scaling with grafting density as d∼ng–1/2, where ng corresponds to backbone spacing between side chains. In contrast, comb polymers with lower grafting densities exhibit distinct scaling behavior, depending on chemical composition. For homopolymer combs, where backbones and side chains are chemically identical, the peak shifts to lower q, following q*∼ng–1/4, reflecting coupled composition fluctuations of the interlinked backbones and side chains. A similar behavior is observed in heteropolymer combs composed of weakly immiscible backbones and side chains. For heteropolymer combs with strongly immiscible components, the peak exhibits a low-q shift, q*∼ng–α with a scaling exponent 1/4 < α ≤ 2/3. This behavior is a signature of a microphase separation between incompatible backbones and side chains and is associated with a characteristic domain size, L = 2π/q*∼ngα. Together, these results identify distinct physical origins of the bottlebrush peak across diverse brush systems.
Moses et al. (Thu,) studied this question.