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Abstract Network formation in elastomers with grafted side chains is investigated to understand the entanglement‐free nature of the bottlebrush architecture. Competition between elastically effective cross‐linkers and dangling side chains creates a unique environment where reaction kinetics and steric effects dictate network percolation. The evolving viscoelasticity of linear and bottlebrush networks with an equivalent number of cross‐linkers to backbones is measured during a catalytic curing reaction using time‐resolved rheology. The impact of reaction kinetics on network formation is addressed through the sol−gel transition by tuning catalyst concentration. Solidification falls into a rate‐limited regime where the modulus growth rate increases with increasing catalyst. The network formation process remains independent of the cure rate in the bottlebrush and linear systems. Side chains significantly decrease the fractal dimension of the critical gel cluster despite a comparable number of cross‐links. Time‐cure superposition is applied to quantify dynamics in the pre‐ and post‐gel states. The divergence of the shift factors around the gel point is independent of cure kinetics. The collapse of shift factors in the post‐gel region further suggests the universality of the network formation process. The approach to understanding internal structure development during processing will be critical as bottlebrush elastomers are utilized among a wide range of applications. 

Pulsed field gradient (PFG) NMR in combination with quasielastic neutron scattering (QENS) was used to investigate self-diffusion of water and acetone in Nafion membranes with and without immobilized vanillic acid (VA). Complementary characterization of these membranes was performed by small angle X-ray scattering (SAXS) and NMR relaxometry. This study was motivated by the recent data showing that an organic acid, such as VA, in Nafion can preserve its catalytic activity in the presence of water even at high intra-polymer water concentrations corresponding up to 100% ambient relative humidity. However, there is currently no clear understanding of how immobilized organic acid molecules influence the microscopic transport properties and related structural properties of Nafion. Microscopic diffusion data measured by PFG NMR and QENS are compared for Nafion with and without VA. For displacements smaller than the micrometer-sized domains previously reported for Nafion, the VA addition was not observed to lead to any significant changes in the water and/or acetone self-diffusivity measured by each technique inside Nafion. However, the reported PFG NMR data present evidence of a different influence of acetone concentration in the membranes with and without VA on the water permeance of the interfaces between neighboring micrometer-sized domains. The reported diffusion data are correlated with the results of SAXS structural characterization and NMR relaxation data for water and acetone. 

Abstract ObjectivesBlack older adults have a higher vascular burden compared to non‐Hispanic White (NHW) older adults, which may put them at risk for a form of depression known as vascular depression (VaDep). The literature examining VaDep in Black older adults is sparse. The current study addressed this important gap by examining whether vascular burden was associated with depressive symptoms in Black older adults. MethodsParticipants included 113 Black older adults from the Healthy Brain Project, a substudy of the Health, Aging, and Body Composition Study. In multiple regression analyses, clinical vascular burden (sum of vascular conditions) and white matter hyperintensity (WMH) volume predicted depressive symptoms as measured by the Center for Epidemiologic Studies Depression Scale, controlling for demographic variables. Follow‐up analyses compared the associations in the Black subsample and in 179 NHW older adults. ResultsHigher total WMH volume, but not clinically‐defined vascular burden, predicted higher concurrent depressive symptoms and higher average depressive symptoms over 4 years. Similar associations were found between uncinate fasciculus (UF) WMHs and concurrent depressive symptoms and between superior longitudinal fasciculus WMHs and average depressive symptoms. The association between depressive symptoms and UF WMH was stronger in Black compared to NHW individuals. ConclusionThis research is consistent with the VaDep hypothesis and extends it to Black older adults, a group that has historically been underrepresented in the literature. Results highlight WMH in the UF as particularly relevant to depressive symptoms in Black older adults and suggest this group may be particularly vulnerable to the negative effects of WMH.