Strained Bottlebrushes in Super-Soft Physical Networks

ABA triblock copolymers composed of a poly(dimethylsiloxane) (PDMS) bottlebrush central block and linear poly(methyl methacrylate) (PMMA) terminal blocks self assemble into a physical network of PDMS bottlebrush strands connected by PMMA spherical domains. A combination of small- and ultrasmall-angle X-ray scattering techniques was used to concurrently examine dimensions of PMMA spherical domains and PDMS bottlebrush strands both in the bulk and at the PMMA PDMS interface. In agreement with scaling model predictions, the degrees of polymerization of the bottlebrush backbone (n(bb)) and PMMA block (n(A)) correlate with the measured PMMA domain size and area per molecule at the PMMA-PDMS interface as D-A proportional to (n(bb)n(A))(1/3) and S proportional to n(A)(2/3)n(bb)(-1/3), respectively. In the bulk, bottlebrush strands are extended due to steric repulsion between the side chains and unfavorable interactions between the different blocks. At the PMMA-PDMS interface with large curvature, packing constraints require additional bottlebrush backbone extension and alignment of side chains along the backbone in the direction perpendicular to the interface.