Addition-fragmentation of 5-bromopenta-1,3-diene in free radical polymerization of methyl methacrylate

5-Bromopenta-1,3-diene (BPD) was examined as an addition-fragmentation chain transfer agent (AFCTA) in the free radical polymerization of methyl methacrylate (MMA). Studies of the kinetics of polymerization in the presence of this compound showed it to be a very effective chain transfer agent and that retardation was not significant, implying efficient reinitiation by the expelled Br radical. Analysis of the resulting polymers showed that the intermediate radical formed by the addition of the propagating radical to the C₁ carbon of BPD underwent exclusive fragmentation. However, addition on the C₄ carbon, with a relative probability of 0·6, led to its copolymerization with MMA. Kinetic studies showed BPD to be a better chain-end functionalization agent than its 5-t-butyl thio derivative for deriving pentadiene-functional macromonomer. © 1998 SCI.     

Sequential interpenetrating polymer networks from bisphenol A based cyanate ester and bimaleimide: Properties of the neat resin and composites

Blends of varying composition of a bisphenol A based cyanate ester—viz., 2,2‐bis‐(4‐cyanatophenyl) propane (BACY)—and a bisphenol A based bismaleimide—viz., 2,2‐bis[4‐(4‐maleimido phenoxy) phenyl] propane (BMIP)—were cured together in a sequential manner to derive bismaleimide–triazine network polymers. Enhancing the bismaleimide content was conducive for decreasing the tensile properties and improving both the flexural strength and fracture toughness of the cyanate ester‐rich neat resin blends. Although DMA analyses of the cured blend indicated a homogeneous network for the cyanate ester dominated compositions, microphase separation occurred on enriching the blend with the bismaleimide. Addition of bismaleimide did not result in any enhancement in T_g of the blend. Interlinking of the two networks and enhancing crosslink density through coreaction with 4‐cyanatophenyl maleimide impaired both the mechanical and fracture properties of the interpenetrating polymer network (IPN), although the T_g showed an improvement. Presence of the bismaleimide was conducive for enhancing the mechanical properties of the composites of the cyanate ester rich blend, whereas a higher concentration of it led to poorer mechanical properties due to the formation of a brittle interphase. The IPNs showed reduced moisture absorption and low dielectric constant and dissipation factor, the latter properties being independent of the blend composition. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2737–2746, 1999     

Studies on antioxidants. III. 2[(3,5-di-tert-butyl-4-hydroxyphenyl)methylene]hydrazine carbothioamide as a melt stabilizer for unstabilized polypropylene

Polypropylene's high processing temperatures initiate thermal oxidative degradation and loss of physical properties. Antioxidants protect polymers through the severe conditions of processing and fabrication and also through end use. Small amounts of antioxidants like 2,6-di-tert-butyl-4-methylphenol and 2[(3,5-di-tert-butyl-4-hydroxyphenyl)methylenel]-hydrazine carbothioamide are shown to be effective in protecting isotactic polypropylene (IPP) against thermooxidative degradation. 2[3,5-Di-tert-butyl-4-hydroxyphenyl)-methylene]hydrazine carbothioamide was melt compounded with IPP using a twin screw extruder. The melt flow index of the extruded sample was measured. The extruded granules were injection molded to get specimens suitable for testing mechanical properties, such as heat deflection temperature, Izod impact strength, tensile strength, tensile modulus, flexural strength, and flexural modulus. 2[(3,5-Di-tert-butyl-4-hydroxyphenyl)methylene]hydrazine carbothioamide was found to be an effective melt stabilizer for unstabilized polypropylene. © 1995 John Wiley & Sons, Inc.     

Evaluation by various experimental approaches of the crosslink density of urethane networks based on hydroxyl‐terminated polybutadiene

Crosslink density (CLD) is an important characteristic for elastomeric polymer networks. The mechanical and viscoelastic properties of the elastomers are critically dependant on the CLD. Several methods have been adopted for its determination, but swelling and stress–strain methods continue to be more popular because of the convenience associated with these techniques. In this article, the determination of CLD of allophanate–urethane networks based on hydroxyl‐terminated polybutadiene and toluene diisocyanate with swelling and stress–strain methods is reported. The Flory–Rhener relationship was applied to calculate CLD from the swelling data. CLDs were also calculated from the initial slope of the stress–strain curve (Young's modulus), Mooney–Rivlin plots, equilibrium relaxation moduli, and dynamic mechanical properties. A comparison was drawn among the values obtained with the various methods. Although the CLD values obtained from Mooney–Rivlin plots were slightly lower than those obtained from swelling data, the values obtained with Young's modulus and storage modulus were considerably higher. The values obtained with swelling and equilibrium relaxation moduli data were very close to each other. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3129–3133, 2007     

Engaging zwitterions in carbon-carbon and carbon-nitrogen bond-forming reactions: A promising synthetic strategy

An Account of carbon-carbon and carbon-nitrogen bond-forming reactions mediated by zwitterions generated by the addition of organic nucleophiles to activated unsaturated systems highlighting their synthetic potential is presented.     

A scanning electron microscopic study to observe the changes in the growth morphology of the α phased Alumina-13 wt.% titania coatings during plasma spraying

Plasma spraying of Alumina-13 wt.% titania coatings shows preferential stability of α phase, in the as sprayed conditions, to an extent of 15-40%. As reported in the literature, the quantity of this α phase can be considered to be closely related to the processing parameters of the air plasma spray process, such as arc current, plasma gas flow rate etc. Normally, a lower value of arc current, results in larger quantity of preferentially stabilized α phase. In this paper, we have attempted to measure the orientation image mapping (OIM) of these α phased Alumina-13 wt.% titania coatings with Electron Back Scattered Diffraction. From the OIM, the pole figures for the α phase were established and from appropriate pole figures, such as (0001), (0006), (01ī1) and (10ī1) the growth morphology of the α phase could be determined. For larger values of arc current, at which the growth rate is faster and due to the imposed rapid solidification conditions, the hexagonal basal plane of (0001)-H type of the α phase tends to align with the (001) type FCC faces of the metastable γ phase. This alignment can be considered to be similar to a directionally solidified chill cast texture. On the other hand, at slower growth rate and at lower arc current value not only a larger quantity of α phase forms, but for accommodation of these larger number of α grains in the thinner section of splats there appears to be a need for a change in the growth morphology. (100)-R type rhombohedral faces, with shorter axial dimensions were found to align in preference to the directional solidified (0001)-H type basal planar arrangement. This type of change in growth morphology can possibly accommodate more α grains for the same thickness of the splat as compared to a basal plane (0001)-H type arrangement.     

HEMA‐grafted chitosan for dialysis membrane applications

Chitosan was graft copolymerized with HEMA (2‐Hydroxyethylmethacrylate) for the development of blood‐compatible dialysis membranes. The permeation characteristics of HEMA‐grafted chitosan films for four different solutes creatinine, urea, glucose, and albumin was studied in vitro at 37°C for assessment of the suitability as dialysis membranes. The grafted film CH‐12.5 composition (425% grafting) showed very high permeation to creatinine by reaching the equilibrium within 45 min. The compositions CH‐7.5 and CH‐12.5 showed excellent permeation to glucose when compared to virgin chitosan films. In the case of urea permeation, all the grafted compositions exhibited higher percent permeation than the virgin chitosan films. The copolymer films CH‐7.5 and CH‐12.5 showed enhanced permeability for the high molecular weight solute, albumin. The other grafted copolymer compositions followed almost the same trend as that of chitosan for the low molecular weight solutes as well as the high molecular weight solute. The copolymer films were also found to be highly blood compatible, noncytotoxic, and biodegradable. Hence, the need for developing blood‐compatible chitosan membranes with desirable permeability properties is achieved by the graft copolymerization of HEMA onto chitosan. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2960–2966, 2006     

Characterization of multilayer pvd nanocoatings deposited on tungsten carbide cutting tools

The present trends in the coating technologies are gradient coatings, metastable coatings, multicomponaent coatings and multilayer or super lattice coatings. The physical vapour deposition (PVD) process is well-suited technology for these advanced coating technologies. The performance of the coated tools can be improved considerably using multi-layer micro and nanocoatings. The present paper discusses the deposition and characterization of multilayer TiN/Al₂O₃ coatings on cemented tungsten carbide cutting tools using reactive sputtering. The characterization of the coatings was carried out using X-ray diffraction (XRD) for phase analysis, chemical composition using EDAX, adhesion and toughness evaluation using Rockwell indentation test and surface roughness. It was observed that with decrease in thickness of each alumina layer to nanolevel in multilayer coating system results considerable improvement in final surface finish, adhesion and toughness of the coating. The experimental results are presented and analyzed in this paper.     

Physical and rheological characteristics of liquid natural rubber modified bitumen

Polymer modification of commercially available bitumen has been attempted by the incorporation of liquid natural rubber (LNR) of medium viscosity. Both soft and blown bitumens have been studied. Physical and rheological characteristics of the samples were investigated. Improvement in physical properties such as shear strength and ductility in the case of blown bitumen and resistance to flow in the case of soft bitumen were observed. It was also found that as a result of addition of LNR, the activation energy of flow increases in the case of soft bitumen and decreases in blown bitumen. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:53–61, 1998     

Segmented block copolymers of natural rubber and 1, 3-butanediol–toluene diisocyanate oligomers

A series of segmented block copolymers of NR and 1,3-butanediol–toluene diisocyanate oligomers have been synthesized with varying hard segment content. The synthesis has been carried out by one-shot and two-shot processes in solution. The products were characterized by spectral analysis, thermal and mechanical analysis, SEM and optical microscopy. They are found to be amorphous materials having no potential for hydrogen bonding between the ‘hard' and ‘soft' segments. Their two-phase morphology has been deduced from SEM and optical micrographs and established by DMA and thermal studies. DSC analysis shows a soft segment glass transition temperature at −62±2°C and hard segment glass transitions between 70° and 100°C, depending on the polyurethane content. The T_g values determined by the dynamic mechanical analysis are significantly higher than these values.

The thermogravimetric analysis indicates a two-stage thermal decomposition of the materials by random nucleation mechanism and corresponds to the two phases present in the block copolymer. Depending on the proportion of the continuous and dispersed phases, the block coolymers behave like quasi-elastomers at lower hard segment concentrations and brittle plastics at higher hard segment contents. This variation in mechanical behaviour is consistent with the sample morphology. Materials synthesized by the two-shot process are found to possess better mechanical properties than the one-shot products, presumably due to a more systematic ordering of the different segments in the former.