Vikram Sarabhai Space Centre

About: Vikram Sarabhai Space Centre is a facility organization based out in Thiruvananthapuram, India. It is known for research contribution in the topics: Aerosol & Ultimate tensile strength. The organization has 2092 authors who have published 3058 publications receiving 47975 citations. The organization is also known as: VSSC.

Phenolic resins bearing maleimide groups : Synthesis and characterization

Abstract: Novel phenolic novolac resins, bearing maleimide groups and capable of undergoing curing principally through the addition polymerization of these groups, were synthesized by the polymerization of a mixture of phenol and N-(4-hydroxy phenyl)maleimide (HPM) with formaldehyde in the presence of an acid catalyst. The polymerization conditions were optimized to get gel-free resins. The resins were char- acterized by chemical, spectral, and thermal analyses. Differential scanning calorime- try and dynamic mechanical analysis revealed an unexpected two-stage curing for these systems. Although the cure at around 275°C was attributable to the addition polymer- ization reaction of the maleimide groups, the exotherm at around 150 to 170°C was ascribed to the condensation reaction of the methylol groups formed in minor quantities on the phenyl ring of HPM. Polymerization studies of non-hydroxy-functional N-phenyl maleimides revealed that the phenyl groups of these molecules were activated toward an electrophilic substitution reaction by the protonated methylol intermediates formed by the acid-catalyzed reaction of phenol and formaldehyde. On a comparative scale, HPM was less reactive than phenol toward formaldehyde. The presence of the phenolic group on N-phenyl maleimide was not needed for its copolymerization with phenol and formaldehyde. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 641- 652, 2000

Diglycidyl ether of bisphenol‐A epoxy resin modified using poly(ether ether ketone) with pendent tert‐butyl groups

Abstract: Bisphenol-A-based difunctional epoxy resin was modified with poly(ether ether ketone) with pendent tert-butyl groups (PEEKT). PEEKT was synthesized by the nucleophilic substitution reaction of 4,4'-difluoro benzophenone with tert-butyl hydroquinone in N-methyl-2-pyrrolidone. Blends with various amounts of PEEKT were prepared by melt-mixing. All the blends were homogeneous in the uncured state. The glass transition temperature of the binary epoxy/PEEKT blends was predicted using several equations. Reaction-induced phase separation was found to occur upon curing with a diamine 4,4'-diaminodiphenyl sulfone. The phase morphology of the blends was studied using scanning electron microscopy. From the micrographs, it was found that PEEKT-rich phase was dispersed in a continuous epoxy matrix. The domain size increased with the amount of PEEKT in the blends. The increase in domain size was due to the coalescence of the domains after phase separation. Dynamic mechanical analysis of the blends gave two peaks corresponding to epoxy-rich phase and thermoplastic-rich phase. The tensile strength and modulus of the blends remained close to that of the unmodified resin, while the flexural properties decreased with the addition of PEEKT to epoxy resin. The fracture toughness of the epoxy resin increased with the addition of PEEKT. Investigation of the fracture surfaces revealed evidences for local plastic deformation of the matrix, crack pinning, crack path deflection, and ductile tearing of PEEKT-rich phase. Thermogravimetric analysis revealed that the initial decomposition temperature of the blends were close to that of the unmodified resin. Finally, the properties of the blends were compared with other modified PEEK/epoxy blends.

Effect of equatorial ionization anomaly on the occurrence of spread-F

Abstract: The unique geometry of the geomagnetic field lines over the equatorial ionosphere coupled with the E- W electric field causes the equatorial ionization anomaly (EIA) and equatorial spread-F (ESF). Ionosonde data obtained at a chain of four stations covering equator to anomaly crest region (0.3 to 33 °N dip) in the Indian sector are used to study the role of EIA and the associated processes on the occurrence of ESF. The study period pertains to the equinoctial months (March, April, September and October) of 1991. The ratios of critical frequency of F-layer ( f0F2) and electron densities at an altitude of 270 km between Ahmedabad (33 °N dip) and Waltair (20 °N dip) are found to shoot up in the afternoon hours on spread-F days showing strengthening of the EIA in the afternoon hours. The study confirms the earlier conclusions made by Raghava Rao et al. and Alex et al. that a well-developed EIA is one of the conditions conducive for the generation of ESF. This study also shows that the location of the crest is also important in addition to the strength of the anomaly.

Potential Source Regions Contributing to Seasonal Variations of Black Carbon Aerosols over Anantapur in Southeast India

Abstract: Continuous measurements of black carbon (BC) mass concentration performed at Anantapur [14.62°N, 77.65°E, 331 m asl], a suburban location in southeast India, using an Aethalometer from January to December, 2010, are analyzed and discussed here. The annual mean BC mass concentration ([BC]) was 3.03 ± 0.27 µg/m 3 for the above study period. The sharp morning (fumigation) peak occurs between 07:00 and 08:00 h almost an hour after the local sunrise while a broad evening (nocturnal) peak is at ~21:00 h with a minimum in noon hours (14:00–16:00 h). The seasonal mean values of [BC] are 5.05 ± 0.51 μg/m 3 in the winter, 3.77 ± 1.23, 1.55 ± 0.51, and 2.33 ± 0.82 µg/m 3 in the summer, monsoon and postmonsoon seasons, respectively. High BC values tend to occur when the wind is directed from the 180–225° sector, which may be well defined by the geographical location of the observation site. During the winter, the trajectory air mass pathways originated through north or central India with significant advection of continental aerosols arriving before the measurement region, results in an enhanced [BC]. Whereas in the monsoon season, the pristine marine air mass from the oceanic environment led to decrease in the concentration of BC. Comparison of monthly mean variations in AOD at 500 nm and black carbon aerosols is observed to be positive with poor correlation coefficient of 0.42. The ratio of BC/PM2.5 varied from 1.3% to 7.2% with a mean value of 4.6% at Anantapur during the observation period and this ratio decreased with decreasing Angstrom exponent (alpha).