Effect of Environmental Exposures on the Aramid Fibre Kevlar

The effects of exposure of Kevlar 49 fibres to thermal, ultrasonic and chemical environments have been analysed. Both the tensile strength and the tensile modulus deteriorate with thermal exposures . However, the former is more sensitive than the latter. The prominent crystal structural changes induced by thermal exposures are reduction in crystallinity, molecular misalignment about the fibre axis, increase in the inter layer distance, changes in crystallite size and/or microstrain. Thermally induced macro changes include introduction of surface holes, partial hollowness near the core, localised thinning, material loss in the vicinity of surface impurities etc . The structural changes could be satisfactorily correlated with the corresponding changes in tensile properties . All the thermally induced effects are controlled by two parameters, viz ., the temperature (T) and the duration of the cumulative exposure to T, te,,,,,(T). The T-tcam(T) effect leads to the observation of isothermal decomposition of Kevlar over a range of temperatures which include T's, far below the reported decomposition temperature of 500/550 °C. A hitherto unknown direct correlation between X-ray diffraction peak positions and the tensile strength has also been identified . Exposure to ultrasonic waves introduces axial compression, evinced by the formation of kink bands, fibrillation and macro buckling . The unit cell of axially compressed fibres manifests an anisotropic deformation. The process of moisture uptake by Kevlar 49 fibres is controlled by the relative humidity (RH) of ambient atmosphere . The saturation moisture content is, however, determined by structural characteristics such as the degree of crystallinity, molecular alignment etc . The uptake of common organic solvents by Kevlar is dependent on the molar volume of the former

Women in Aerospace in India: Aerowoman

The main objective of the seminar was to bring together the Indian women in the field of aerospace and identify their scientific and technical contributions. In this context, the seminar was perhaps the first of its kind held in India. It was also aptly conducted in the x2018;Womenx2019;s Empowerment Year x2013; 2001x2019; declared by the Govt. of India.13

Effect of thermal aging on the crystal structural characteristics of poly(tetra fluoro ethylene)

The residual effects of cumulative thermal aging on the crystal structural characteristics of the fluoro carbon poly(tetra fluoro ethylene) (PTFE) have been studied by X-ray diffraction methods. The initial hexagonal arrangement of the PTFE chains in a 157 helical conformation is left unaffected by the exposures to temperatures (T), up to and beyond its melting point, Tm. The unit cell registers a residual anisotropic volume expansion. The anisotropy arises from the enhanced enlargement of the basal plane dimension a compared with the axial dimension c. Conformational changes contributing to the observed increase in the chain length have been examined. Enhancement of residual crystallinity of samples aged at T's lt; Tm suggests that the selective thermal aging could be used as an effective tool to improve the initial crystallinity of commercially available PTFE. The activation energy for 50% enhancement in initial crystallinity has been estimated as 53.9 kJ mol-1. Aging at 400xB0;C, a temperature above Tm, is accompanied by markedly different features viz., deterioration in crystallinity and other structural characteristics. The overall behavior of thermally aged PTFE bears a marked similarity to many polyamides. POLYM. ENG. SCI., 47:1724-1729, 2007. xA9; 2007 Society of Plastics Engineer

Thermally induced structural changes in Nomex fibres

Thermally aged Nomex fibres manifest several residual effects viz. reduction in X-ray crystallinity, weight loss and deterioration in tensile characteristics. Surface damages in the form of longitudinal openings, holes, material deposits etc have also been observed. Based on the data from thermally exposed fibres, the time needed for states of zero tensile strength and modulus have been predicted

Investigations on the Problem of Moisture Absorption13; by Kevlar Fibres

Kevlar fibres are know, to have affinity for moisture. We have investigated (i) the effect of relative humidity (RH) of ambient atmosphere and ( ii ) the effect of crystallinity of fibres on the process of moisture uptake.13; For RH values ranging fran 3 to 80% variation of moisture content of initially dry fibres with time has been measured. It is found that saturation moisture content varies with RH value. Specimens in which crystallinity has been reduced by apropriate treatmrent exhibit a marked increase in moisture content.Experiments on the effect of soaking the fibres in water at 26xB0;C and 98xB0;C have also been carried out. The site of ITOisture absorption has been studied using X-ray of dry Kevlar 49 fibres and those with clifferent levels13; of misture content. The results suggest that water molecules do not enter the unit cell

Decomposition behaviour of Kevlar 49 fibres:Part I. At TTd

The structural changes which accompany decomposition of Kevlar 49 fibres at T= 500xB0;C: and 550xB0;C, respectively, have been elucidated. At both these temperatures, cumulative thermal exposures of specific durations, tcum{T}, are required to result in decomposition. The conspicuous features which characterize isothermal decomposition of the fibres are: (i) progressive reduction and an eventual total loss in fibre crystallinity, (ii) progressive loss in weight, (iii) introduction of surface damages, (iv) introduction of hollowness, and (v) deterioration in tensile properties

Crystal and molecular structure of the discogen bis[1,3-di(p-n- Decylphenyl)propane-1,3-dionato]paliadium(ll)

The transition metal containing discogen, bis[1,3-di(p-n-decylphenyl)propane-1 ,3-dionato ]paliadium(II)crystallizes in the triclinic space group pI with one molecule in a unit cell of dimensions a = 10.260(2), b = 12.961(2), c = 13.403(2)A., ex= 110.54(1), f3= 101.75(1) and 'I = 98.44(lt. The crystal structure was solved by direct methods and refined using full matrix least squares procedure. The palladium atom is lying on an inversion center. The decyl chains are fully extended in an all trans conformation. In the crystal structure, the molecules form a tilted columnar arrangement. The angle between the normal to the core and the column axis is 68xB0;. Each column is surrounded by six others. Along the13; column axis, adjacent palladium atoms are separated by 10.26A

3-(5,6,7,8-Tetra­hydro-2-naphth­yl)iso­benzofuran-1(3H)-one

The title compound, C18H16O2, was prepared by reduction of 2-(5,6,7,8-tetra­hydro-2-naphtho­yl)benzoic acid with zinc dust. The benzene ring in the tetra­hydro­naphthyl substituent is nearly perpendicular to the plane of the isobenzofuran-1(3H)-one ring [87.15 (4)°]. The cyclo­hexane unit has a half-chair conformation in which two methylene groups in the tetra­methyl­ene bridge are disordered over two positions; the site-occupancy factors are 0.838 (4) and 0.162 (4). The crystal structure exhibits alternating isobenzofuran-1(3H)-one and tetra­hydro­naphthalene layers

X-ray analysis of stacking faults in ytterbium

At room temperature ( z 300 K), ytterbium has a predominantly f c c structure [l, 21. However, a small13; amount of the h c p structure corresponding to the low- as well as the high-temperature phases [3-61 is also known to coexist [7, 81 with the room-temperature f c c structure. We have earlier reported [8] that mechanical deformation of 99.2% pure ytterbium specimens at room temperature, induces a partial structural transformation of the type f c c -+ h c p, leading to an enhancement of the h c p component in the deformed material. I t has also been found [9] that surfaces of ytterbium specimens subjected to polishing with emery or to filing also exhibit similar enhancement of the h c p component. These observations suggest that in 99.2% pure ytterbium, the f c c -+ h c p transformation which readily accompanies different types of13; mechanical treatments, is probably associated with the stacking faults in the material

Use of radical scavengers in X-ray data collection form protein crystals

Occurance of radiation damage is a feature normally encountered while collecting X-ray intenslty data 'from single crystals of proteins. It is wel1known that prote! crystals contain large . solvent regions consisting 13; mainly of water. The protein crystals,' like the molecules themselves, are stable only in'aqueous medium and are therefore mounted with tlie solvent in a capillary for data.collection. According to the indirect action theory of radiation effects, the damage to the protein13; molecule is associated with the production of free13; radicais from the aqueous medium. These radicals,in turn13; transfer the energy acquired from the Jncident radiation to the protein molecule. The protein molecple is thus left in an unstable,pnized state leading to significant changes in its.structure and properties. The onset of radiation13; damage is detected by reduction in the intensity13; of the diffraction pattern which proceeds on to its13; total disappearance on prolonged irradiation. The13; amount of radiation dosage required to cause the13; damage varies with substances, some affected within13; hours of irradiation and some others within few13; days. Hence most often, in protein crystaIlography,13; one is compelled to use several crystals to13; collect a complete set of data. Most proteins are13; available only in smaIl quantities and it is often13; difficult to grow a large number of good crystals.13; Also, correlation of different data sets sometimes13; poses problems when several crystals are used for13; data coIlection