Controlling the mechanical behavior of novel supramolecular materials is of the utmost importance and requires a fundamental understanding of the underlying physical processes. We present a multimethods approach to the dynamics of entangled transient polyisoprene networks. Small-angle neutron scattering (SANS) on randomly functionalized chains shows homogeneous supramolecular melts with Gaussian chain conformations. The H-bond lifetimes (dielectric α*-process) and the rheological response in terms of the loss modulus G″ differ by 2 orders of magnitude in time. Within the concept of a compact random walk (RW), where the random walker (urazole group acting as a sticker) undergoes multiple returns to its starting point and following the concept of theoretical proposed renormalized sticky bond lifetimes, we quantitatively solve this longstanding and unexplained large discrepancy: While the bond opening gives rise to the dielectric response, for rheological relaxation the association with a new partner is relevant. This takes place only after multiple returns to the original binding partner.
C-C coupling reactions between arylsilanes and alkylfluorides are efficiently catalyzed by disilyl cation 1. Primary as well as secondary alkylfluorides were quantitatively coupled with arylsilanes; however, in the case of tertiary fluorides, the hydrodefluorination reaction predominated. Primary alkylfluorides were found to give arenes with mostly rearranged alkyl substituents. In all cases subsequent Friedel-Crafts-type chemistry occurred.
Supramolecular groups in polymeric systems lead to responsive materials which are ideally suited for applications in dynamic environments. The key to their advanced properties such as shape-memory or self-healing is the reversibility of secondary interactions which can be triggered by external stimuli such as temperature, light, or pH-value. Controlling the (mechanical) behavior of such systems requires a precise understanding of intrinsic properties. We present a multimethod study of transient polyisoprene networks that were functionalized with different amounts of hydrogen bonding urazole groups. This work aims at understanding rich rheological features on the basis of their microscopic origin. First, the thermorheological simple behavior is validated experimentally. Subsequently, we characterize the underlying microscopic processes by broadband dielectric spectroscopy (a-process and a Ã-process), differential scanning calorimetry (glass transition behavior), and Fourier-transform infrared spectroscopy (thermodynamics of group association/dissociation). Based on these results, the influence of the supramolecular groups on the rheological response is analyzed. The observed features such as the onset of elastomeric properties in the flow regime, a drastic increase in the chain relaxation time with an increasing amount of functional groups, and the occurrence of a second rheological relaxation process, which is the most prominent effect, are discussed and related to their physical origin. V
Amyloid-β (Aβ) oligomers represent a promising biomarker for the early diagnosis of Alzheimer's disease (AD). However, state-of-the-art methods for immunodetection of Aβ oligomers in body fluids show a large variability and lack a reliable and stable standard that enables the reproducible quantitation of Aβ oligomers. At present, the only available standard applied in these assays is based on a random aggregation process of synthetic Aβ and has neither a defined size nor a known number of epitopes. In this report, we generated a highly stable standard in the size range of native Aβ oligomers that exposes a defined number of epitopes. The standard consists of a silica nanoparticle (SiNaP), which is functionalized with Aβ peptides on its surface (Aβ-SiNaP). The different steps of Aβ-SiNaP synthesis were followed by microscopic, spectroscopic and biochemical analyses. To investigate the performance of Aβ-SiNaPs as an appropriate standard in Aβ oligomer immunodetection, Aβ-SiNaPs were diluted in cerebrospinal fluid and quantified down to a concentration of 10 fM in the sFIDA (surface-based fluorescence intensity distribution analysis) assay. This detection limit corresponds to an Aβ concentration of 1.9 ng l-1 and lies in the sensitivity range of currently applied diagnostic tools based on Aβ oligomer quantitation. Thus, we developed a highly stable and well-characterized standard for the application in Aβ oligomer immunodetection assays that finally allows the reproducible quantitation of Aβ oligomers down to single molecule level and provides a fundamental improvement for the worldwide standardization process of diagnostic methods in AD research.
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