Circular dichroism—LXIII : Third-sphere contribution to the R-band cotton effect of lactones

The Cotton effect within the n→π* band of the lactone chromophore of the halo lactones 3a-c, prepared from (1S)-2-bromo adamantanedione-4.8 (1) is positive and can be used to determine unequivocally the sign of third-sphere contributions of the appropriate sector rule. An “axial” halogen (Cl, Br, I) substituent in a β-position to the keto group of these halogeno oxa-homoadamantanediones gives an antioctant contribution to the Cotton effect of the carbonyl chromophore as observed earlier for the corresponding halogeno-adamantanones.     

Thermal contraction and extension in fibrous crystals of polyethylene

Summary Solution grown polyethylene shish-kebabs have a core, diameter 20–30 nm, which is highly superheatable. When a mat of shish-kebabs is heated to 132 °C the lamellar material melts, leaving birefringent fibres in an isotropic matrix as seen under the polarizing microscope. Microscopic observations reveal that these fibres, aggregates of many shish-kebabs, contract as they are melted and extend on subsequent recrystallization from the partially molten state as the temperature is lowered. This extension is completely reversible on temperature cycling. For fibres originally crystallized at below 96 °C this effect results in crimping on recrystallization and straightening from the crimped state on remelting. This is because these latter fibres are isolated from each other by the lamellar overgrowth and they buckle rather than extending the sample. Analogous reversible length changes and crimping effects were observed also in fibrous crystals produced from the melt.The above observations allow certain conclusions to be reached as regards the nature of the shish-kebab fibres. Thus an estimate of the forces involved in the bucking indicates that the basic unit which buckles is less than 50 nm in diameter in agreement with the single shish-kebab core when this is directly observable. Similar buckling in the melt crystallized fibrous entities indicates a core of similar size, which is not so easily seen. Further, the observed shrinkage behaviour, together with the associated birefringence and calorimetric information collected in this study, can only be accounted for if melting is visualised as the formation of disordered amorphous regions which alternate with perfect crystal regions along the shish-kebab core. This leads to a model equivalent to a linked row of fringed micelles within a given fibre. The chain straightening of the amorphous sections on recrystallization would then be responsible for the fibre extension. It follows further that for the amorphous sections to form in the first instance there must be preexisting centres of imperfections along the fibre. A model of randomly distributed imperfections (visualised as interlocking chain loops) can account for the present observations in a quantitative manner with realistic predictions as regards the lengths of the undisturbed crystal regions.Finally there is an obvious analogy with the crystallization of oriented polymer networks (e.g. stretched rubber) as regards the observed extension —contraction behaviour, providing a link between recent findings and long established experience.With 12 figures and 1 table     

A note on sorption measuring instruments

Gas sorption phenomena can be used to characterise porous solids and dispersed materials. Usually isotherms of nitrogen and noble gases like He, Ar, Kr are measured at low temperatures (77–90 K). Other gases and vapours like water, CO₂ and benzene are used at near ambient conditions of pressure and temperature. From the amount of gas adsorbed on the (external or internal) surface characteristic quantities like the specific surface area, specific pore volume and pore spectrum of the material are derived by standardised methods. Experimental techniques most often used are the carrier gas, the volumetric/manometric and the gravimetric method. A comprehensible overview of today's available instruments, their advantages and drawbacks is given.This revised version was published online in November 2005 with corrections to the Cover Date.     

The reaction of secondary perfluoroalkyl iodides with zinc

Evidence for the existence of two mechanistic pathways for the reaction of secondary perfluoroalkyl iodides with zinc has been obtained. At high concentration of secondary iodides, a bimolecular homolytic type reaction of the iodide on the zinc surface to yield dimers appears to be preferred. When the reaction is carried out under dilute conditions, an ionic type mechanism appears to predominate. This paper also shows that the substrate itself can influence the reaction route.     

Oligomeric bisphenol A‐based PEEK‐like phthalonitrile‐cure and polymer properties

The cure behavior and properties of oligomeric bisphenol A‐based PEEK‐like phthalonitrile (PN) are thoroughly examined in this article. The resin is easily processed from the melt at a relatively low temperature (150–200 °C) and the monomer cure occurs in a controlled manner as a function of the amine content and processing thermal conditions. Dynamic mechanical measurements and thermogravimetric analysis show that the polymer properties improve as the maximum PN postcure temperature is increased to 415 °C. The effects of the amine and polymer postcure conditions on the flexural and tensile properties of the PN polymer are investigated. The mechanical properties of the polymer are maximized after postcuring to moderate temperatures (330–350 °C). The polymer exhibits an average flexural strength and tensile strength at break of 117 and 71 MPa, respectively. After oxidative aging at 302 °C for 100 h, the polymer retains excellent mechanical properties. The average flexural and tensile strength retention of the polymers are 81 and 75%, respectively. Microscale calorimetric measurements reveal that the flammability parameters of the oligomeric PN are low compared to other thermosets. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3769–3777     

Practical key recovery attacks on FlexAEAD

Designs, Codes and Cryptography - FlexAEAD  is a block cipher candidate submitted to the NIST Lightweight Cryptography standardization project, based on repeated application of an...     

On the structure and the chain conformation of side-chain liquid crystal polymers

Abstract

Backbone anisotropy and the structure of the mesophases of a series of side-chain liquid crystal polymers have been studied in the bulk by neutron scattering. The backbone conformation is obtained by small-angle neutron scattering on mixtures of hydrogenous polymers with deuteriated backbones. The components of the radius of gyration parallel, R _⊥ and perpendicular, R ∥ to the magnetic field are determined as a function of temperature for both the nematic phase and the smectic phase. It is shown that the polymer backbone is deformed in both phases. When the polymer exhibits only a nematic phase, it adopts a prolate conformation, where the average backbone direction is more or less parallel to the aligned mesogenic groups. Upon transition from the smectic phase to a nematic phase, the backbone in the nematic phase assumes a slightly oblate shape. This tendency towards oblate shape is due to the smectic fluctuations which are always present in such nematic phases. The exentricity of the oblate backbone conformation in the smectic phase is always larger than in the nematic phase. This is attributed to a periodic distribution of the backbone between the mesophase layers. Then, the backbone anisotropy depends not only on the smectic structure (S_A1, S_Ad), but also on the temperature dependence of the density of aligned mesogenic groups in the layers. On the other hand, it is shown that the isotopic mixtures are no longer ideal when polymers deuteriated in the mesogenic moieties are mixed with the corresponding hydrogenous polymers.