Forensics of Brush Networks

We apply a forensics-like approach to the analysis of the mechanical properties of brush-like poly(n-butyl acrylate) (PBA) networks with systematically varied degree of polymerization (DP) of side chains n_sc=0-41, number of repeat units between them n_g=1-10, and DP of the backbone between crosslinked side chains n_x =25-1200 . This approach is based on the analysis of a network’s non-linear response to deformation and allows us to quantify the DP between crosslinks, the strand flexibility, and the density of stress-supporting strands, including the contribution from trapped entanglements. For networks with comb-like strands, we obtain the strand Kuhn length to be 1.91 nm, which is in excellent agreement with literature values 1.79 -1.90 nm of the bare PBA backbones. For networks of bottlebrush strands with densely grafted side chains, forensics approach recovers the Kuhn length dependence on the crowding parameter. The obtained Kuhn length values are compared with the distance between adjacent brush backbones where the intrinsic contrast of electron density at the backbone results in a distinct scattering peak corresponding to the brush diameter. The determined values of the Kuhn lengths are used in calculations of the DP between crosslinks.