A rigid
and brittle cross-linking structure was introduced into the flexible
poly(n-hexyl methacrylate) (PHMA) network by the
mechanochromic cross-linker difluorenylsuccinonitrile-containing methacrylate
(DFMA), whose central C–C bond acted as a dynamic covalent
bond and could generate pink radicals when fractured. PHMA with DFMA
showed a remarkable hysteresis loop and mechanical enhancement. After
deformation, the reassociation of dynamic covalent bonds and the reorganization
of the network structure slowed down the recovery of the polymer to
its initial state. The correlations between tension stimulation, energy
dissipation, and mechanoresponsive color change were discussed. Under
stress, the polymer changed from light gray to pink. The broad distribution
of red channel intensity under large deformation detected on the surface
confirmed that the rupture of dynamic covalent bonds occurred evenly
throughout the polymer and suppressed stress concentration. The color
showed a strong dependence on stress, which started to appear at around
1.5 to 2.0 MPa. The incorporation of DFMA promised mechanical enhancement
and noncontact stress detection ability of the PHMA soft material.
Stimuli-recovery polymer networks with enhanced mechanical performance were designed and synthesized through UV-curing photo-polymerization. Thanks to a mechanochromic cross-linker difluorenylsuccinonitrile-containing dimethacrylate (DFMA), poly(stearyl methacrylate-co-N,N-dimethyl acrylamide) (P(SMA−DMAA)) networks visualized the stress, showing improvements in toughness and higher energy dissipation. The molar ratios determined the transition temperatures, crystal structures, and mechanical performance of the polymer networks. A more efficient and scientific analysis based on achromatic gray-scale colorspace was first proposed to evaluate the mechano-responsive color quantitatively. Uniform evaluation criteria were expected to be established based on the method. The stress distribution and dissociation of dynamic covalent bonds were recorded precisely and expressed clearly on gray-scale color maps, providing a clear warning for when materials were threatening to break. Mechanochromic P(SMA−DMAA) networks showed a prolonged recovery at room temperature. While under heat stimulation, they presented excellent recovery ability with 90% strength and 95% energy. Additionally, the mechano-responsive color changes repeated, showing a similar changing trend to that in the first cycle. The mechanochromic stimuli-recovery P(SMA−DMAA) networks had enhanced mechanical performance and a reliable visual fracture warning function.
Joint analysis of the energy spectrum of ultra-high-energy cosmic rays measured at the Pierre Auger Observatory and the Telescope Array Yoshiki Tsunesada , * on behalf of the Pierre Auger and the Telescope Array Collaboration
The sources of ultra-high-energy cosmic rays are still unknown, but assuming standard physics, they are expected to lie within a few hundred megaparsecs from us. Indeed, over cosmological distances cosmic rays lose energy to interactions with background photons, at a rate depending on their mass number and energy and properties of photonuclear interactions and photon backgrounds. The universe is not homogeneous at such scales, hence the distribution of the arrival directions of cosmic rays is expected to reflect the inhomogeneities in the distribution of galaxies; the shorter the energy loss lengths, the stronger the expected anisotropies. Galactic and intergalactic magnetic fields can blur and distort the picture, but the magnitudes of the largest-scale anisotropies, namely the dipole and quadrupole moments, are the most robust to their effects. Measuring them with no bias regardless of any higher-order multipoles is not possible except with full-sky coverage. In this work, we achieve this in three energy ranges (approximately 8-16 EeV, 16-32 EeV, and 32-∞ EeV) by combining surface-detector data collected at the Pierre Auger Observatory until 2020 and at the Telescope Array (TA) until 2019, before the completion of the upgrades of the arrays with new scintillator detectors. We find that the full-sky coverage achieved by combining Auger and TA data reduces the uncertainties on the north-south components of the dipole and quadrupole in half compared to Auger-only results.
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