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R Dhamodharan
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R Dhamodharan
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R Dhamodharan
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Dhamodharan, Raghavachari
Raghavachari, Dhamodharan
Dhamodharan, R.
Iyengar, Dhamodharan R.
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5 results
Now showing 1 - 5 of 5
- PublicationStrengthening polymer interfaces With triblock copolymers(10-02-1997)
;Dai, Chi An ;Jandt, Klaus D.; ;Slack, Nelle L. ;Dai, Kevin H. ;Davidson, William B. ;Kramer, Edward J.Hui, Chung YuenWe have measured the fracture toughness, Gc, of an interface between polystyrene (PS) and poly (2-vinylpyridine) (PVP) reinforced with triblock copolymers (PVP-b-dPS-b-PVP) as a function of the areal chain density, Σ, of the copolymers at the interface. The failure mechanisms of the interface are studied by transmission electron microscopy and forward recoil spectrometry. For triblock copolymers with long PVP blocks (NPVP > NePVP, where NePVP is the entanglement polymerization index of PVP), a transition from chain scission at low Σ to crazing at high Σ* is observed. By comparing the areal chain density Σ* for the transition from chain scission to crazing for the triblock copolymers (Σ* = 0.015 chains/ nm2) to that for the diblock copolymers, PVP-dPS (Σ* = 0.03 chains/nm2), we show that most of the triblock copolymers form a "staple" structure at the interface with the dPS block making a loop on the PS side of the interface and the PVP ends anchoring the "staple" in the PVP side. As a result of the "staple" structure, the saturation areal chain density of the triblock copolymer (Σsat) at the interface is half of that for the diblock copolymer of similar molecular weight. For Σ< Σsat in the crazing regime, the fracture toughness of the interface is controlled by the areal joint density, Σcross, where Σcross is the number density of the copolymer excursions across the interface. For Σ> Σsat, the triblock copolymers appear to reinforce the craze fibrils at the crack tip better than the corresponding diblock copolymers, leading to an interface fracture toughness approaching that of the PS homopolymer itself. For a triblock copolymer with short PVP blocks NPVP < NePVP), there is a transition in the fracture mechanism from pull out of the PVP block to crazing with increasing Σ. Short triblock copolymers can form two chain conformations: one in which two PVP blocks anchor the copolymer on the homopolymer PVP side (staple structure) and one in which one PVP block anchors the copolymer on the PVP side (tail structure) of the interface. Comparison of Gc between the triblock copolymer and the corresponding diblock copolymer is made. The larger Gc values of the triblock copolymer reinforced interface in the crazing regime are observed as a result of enhancement in entanglements between the dPS loops of the triblock copolymer and the homopolymer PS. - PublicationPhotochemical synthesis and characterization of hypercrosslinked polystyrene, a novel porous organic material(01-01-1999)
;George, Beena ;Mohammed Nasrullah, J.Benzoylated polystyrene (BPS) was subjected to UV radiation in solution in the presence of isopropanol where upon, it was observed to undergo essentially quantitative crosslinking to form polystyrene resin with tertiary hydroxyl functionality. The crosslinking kinetics was followed by IR spectroscopy. The desired degree of crosslinking was accomplished by controlling the extent of benzoylation of the polymer. Following crosslinking, the gels were freeze-dried and suitably characterized by solid state NMR and IR spectroscopic techniques. Scanning electron microscopic studies reveal the formation of porous network. Some of the crosslinked resins, dried suitably, exhibit high surface area as confirmed by BET adsorption isotherm studies. - PublicationAdsorption of alginic acid and chondroitin sulfate-A to amine functionality introduced on polychlorotrifluoroethylene and glass surfaces(15-06-1999)
; McCarthy, Thomas J.The amine functionality is introduced onto a glass surface by using 3-(aminopropyl)-triethoxysilane (3-APTS)-a silane coupling agent-as well as by the adsorption of poly(L-lysine). The amine functionality is introduced onto the hydroxyl-functionalized polychlorotrifluoroethylene surface (PCTFE-OH) using 3-APTS as well. The amine-functionalized surfaces were thoroughly characterized by X-ray photoelectron spectroscopy (XPS). Selected polysaccharides, involved in fouling processes, such as alginic acid and chondroitin sulfate-A were allowed to adsorb onto amine-functionalized surfaces. It is shown that alginic acid adsorbs from calcium-free artificial seawater onto amine-functionalized glass surface prepared by the adsorption of poly(L-lysine) and onto amine-functionalized polychlorotrifluoroethylene surfaces (PCTFE-NH2). The adsorption of alginic acid to PCTFE-NH2 is very rapid, resulting in high-affinity isotherms and approx. 5 angstrom thick dry overlayer of the polysaccharide. In addition, we observe that the adsorbed alginic acid protects the PCTFE-NH2 surface from hydrolysis by phosphate buffer 2 and from hydrolysis/oxidation by copper(II) acetate monohydrate solution. It is also shown that chondroitin sulfate-A adsorbs from potassium hydrogen phthalate buffer to the amine-functionalized glass surface prepared by the reaction between silanol groups on glass and 3-APTS. It is observed that alginic acid and chondroitin sulfate-A do not adsorb to the unmodified glass surface, which is polar and negatively charged, and the PCTFE surface, which is nonpolar, from aqueous solutions under various solution conditions. This study clearly indicates the importance of surface functional groups on the adsorption of polysaccharides, an important preliminary step in the biofouling of surfaces. - PublicationDielectric studies of hydrogen bonded polar binary mixtures of ethyl benzoate with aromatic amines(01-12-1999)
;Chitra, M.; ;Madhurima, V. ;Subramanyam, B.Murthy, V. R.K.The chosen liquids: ethyl benzoate, aniline and n-methyl aniline are high boiling point liquids for possible microwave applications. Dielectric measurements are carried out at 300 KHz, 5.59 GHz, 7.95 GHz, 33.7 GHz, 72 GHz and at optical frequency on the above polar liquids and their equimolar binary mixtures diluted in non-polar solvent (benzene). Cole-Davidson plot is obtained for the binary mixture of ethyl benzoate and aniline whereas Debye plot is observed for the binary mixture of ethyl benzoate and N-methyl aniline in dilute solutions of benzene. The binary mixture of an associated liquid aniline and non-associated liquid ethyl benzoate leads to formation of an extensive tree like hydrogen bonded system. In N-methyl aniline, where one hydrogen atom of the -NH2 group is replaced by a methyl group, it acts as a terminator to prevent hydrogen bonding. The experimentally determined dipole moments are compared with computational results obtained from semiempirical molecular orbital package MOPAC Version 6.0 and ab initio calculations using the Gaussian 92W program. The results are discussed. - PublicationSelective hydrogenation of phenylacetylene using block copolymer additional poisoning agent(01-01-1996)A block copolymer, poly (styrenc-block-4-vinylpyridine) is used as an additional poisoning agent in combination with Lindlar's catalyst to drastically afreet the selectivity in the semi-hydrogenation of phenylacetylene. Selectivity is demonstrated from gas chromatography data. The extra advantage in terms of preparing poisoning agent free, very pure and dry monomer are illustrated.