For elastomer networks above the glass transition temperature Tg, a unified approach is presented to relate the residual dipolar couplings in various independent NMR experiments to the crosslink density. This is demonstrated on a series of cross-linked poly(styrene-co-butadiene) elastomers. The presence of dynamic physical and permanent chemical cross-links leads to a nonzero average of the homonuclear and heteronuclear dipolar couplings, which results in a solid-like NMR relaxation behavior. The residual dipolar couplings are expressed as a function of the effective number of statistical segments Ne between the physical and N e X between the chemical cross-link points, using a simplified network model with Gaussian statistics. These effective numbers are extracted for each sample of the series from the 13 C-edited transverse 1 H magnetization relaxation of the CH group. It is shown that the respective Ne values can be used to scale the time domain of various NMR experiments such as (a) the free induction decay, (b) the 13 C-edited 1 H transverse magnetization relaxation, (c) the cross-polarization curves, and (d) the 1 H magnetization exchange between the CH and CH2 groups. This proves the validity of the unified view on the dipolar interactions in elastomer networks and provides a way to estimate the cross-link density.
Solid-state two-dimensional proton magnetization-exchange NMR is used to investigate intergroup residual dipolar couplings in a cross-linking series of poly(styrene-cobutadiene) elastomers. The magnetization-exchange process between the CH and the CH2 group in the regime of short mixing time provides valuable insight regarding molecular order. A three-spin model is employed, in which the CH and CH2 protons are considered to be coupled by residual dipolar interactions. The spin-system response reflects well-localized dipolar interactions. The time scale in which the exchange process takes place justifies these assumptions as well as the interpretation of various NMR relaxation experiments previously performed on this class of polymers. The residual intergroup dipolar couplings are measured along the average polymer-chain direction and correlated with the dynamic storage modulus. It is shown that they are sensitive to both cross-link points and nonpermanent geometric constraints of the chain motion. The dynamic order parameter along the chains is evaluated. It exhibits the remarkably high value of 〈P2〉≊0.13 for the carbon–carbon bond connecting the CH and the CH2 groups in polybutadiene in the uncross-linked copolymer melt and only slightly increases with the cross-link density.
Honeybees form societies in which thousands of members integrate their behaviours to act as a single functional unit. We have little knowledge on how the collaborative features are regulated by workers’ activities because we lack methods that enable collection of simultaneous and continuous behavioural information for each worker bee. In this study, we introduce the Bee Behavioral Annotation System (BBAS), which enables the automated detection of bees’ behaviours in small observation hives. Continuous information on position and orientation were obtained by marking worker bees with 2D barcodes in a small observation hive. We computed behavioural and social features from the tracking information to train a behaviour classifier for encounter behaviours (interaction of workers via antennation) using a machine learning-based system. The classifier correctly detected 93% of the encounter behaviours in a group of bees, whereas 13% of the falsely classified behaviours were unrelated to encounter behaviours. The possibility of building accurate classifiers for automatically annotating behaviours may allow for the examination of individual behaviours of worker bees in the social environments of small observation hives. We envisage that BBAS will be a powerful tool for detecting the effects of experimental manipulation of social attributes and sub-lethal effects of pesticides on behaviour.
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