Effects of heat exposure and volumetric compression on Poisson's ratios, Young's moduli and polymeric composition during thermo-mechanical conversion of auxetic open cell polyurethane foam

The effects of thermo-mechanical auxetic foam conversion parameters on the Young's modulus and Poisson's ratio of open-cell polyurethane foam are related to changes in chemical bonding and cell structure. Applied volumetric compression, conversion temperature and duration are reported on foam Young's modulus, Poisson's ratio and structural stability. Fourier transform infrared spectral analysis on samples converted at and above 160°C strongly indicates a hydrogen bond interaction increase in urea groups (C=O---H-N) and an increase in hydrogen bonding population. Spectral changes inferred soft segment degradation following extensive heat exposure (200°C, 180 minutes), specifically a shift and intensity change in CH2 and C-O-C polyol bands and a broad baseline increase between 3600 and 2400 cm-1. These changes are linked to: i) resistance to dimensional recovery over time and during re-heating, ii) Poisson's ratio becoming negative, then zero in tension or marginally positive in compression, iii) Young's Modulus reducing then increasing, iv) mass loss, evidenced by thermogravimetric analysis increasing from 150°C. The minimum mean values of Poisson's ratio of ~-0.2 (to 10% compression/tension) are comparable to other studies. All samples that retain their imposed compression over time are isotropic, with near constant Young's moduli and Poisson's ratio (to 10 % compression/tension)

A novel keratin 13 variant in a four-generation family with white sponge nevus

Dermatology-oncolog

Rapid binding and release of Hfq from ternary complexes during RNA annealing

The Sm protein Hfq binds small non-coding RNA (sRNAs) in bacteria and facilitates their base pairing with mRNA targets. Molecular beacons and a 16 nt RNA derived from the Hfq binding site in DsrA sRNA were used to investigate how Hfq accelerates base pairing between complementary strands of RNA. Stopped-flow fluorescence experiments showed that annealing became faster with Hfq concentration but was impaired by mutations in RNA binding sites on either face of the Hfq ring or by competition with excess RNA substrate. A fast bimolecular Hfq binding step (∼108 M−1s−1) observed with Cy3-Hfq was followed by a slow transition (0.5 s−1) to a stable Hfq–RNA complex that exchanges RNA ligands more slowly. Release of Hfq upon addition of complementary RNA was faster than duplex formation, suggesting that the nucleic acid strands dissociate from Hfq before base pairing is complete. A working model is presented in which rapid co-binding and release of two RNA strands from the Hfq ternary complex accelerates helix initiation 10 000 times above the Hfq-independent rate. Thus, Hfq acts to overcome barriers to helix initiation, but the net reaction flux depends on how tightly Hfq binds the reactants and products and the potential for unproductive binding interactions