Sreekumar, K(Japaneese society of Applied physics, November 8, 2006)
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Abstract:
Copper doped methylene blue sensitized poly(vinyl alcohol) (MBPVA)–acrylamide films were fabricated to improve the
storage life of recorded gratings. The films were fabricated using gravity settling method and the copper chloride
concentration was optimized as 3:18 10 3 mol/l for a dye concentration of 6:2 10 4 mol/l. The gratings recorded on the
optimized film constitution could be stored for months with stable diffraction efficiency (24%) without any chemical or
thermal fixing techniques. The resolution of the material is found to be unaffected with the addition of copper chloride.
The effect of chromium doping on methylene
blue sensitized Poly (vinyl alcohol)/Acrylamide was carried
out by varying the ratios of Ammonium dichromate
and methylene blue. In the case of films without ammonium
dichromate, the diffraction efficiency was found to
decrease on storage. On chromium doping the storage life
was improved. Interestingly, a self-enhancement in efficiency
was observed for a particular ratio of methylene
blue and ammonium dichromate.
The differaction efficiency,sensitivity and storage life of Methylene blue sensitized poly(vinyl chloride) film was improved by the addition of an electron donor in the matrix. The
addition of pyridine enhanced the diffraction efficiency by two times and storage life of the
gratings was increased to 2-3 days.
Sreekumar, K(Optical society of America, June 20, 2004)
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Abstract:
A methylene-blue-sensitized polymer blend of polyvinyl alcohol and polyacrylic acid is fabricated and
tested for holographic recording. It was found to have good characteristics such as high sensitivity,
storage stability, ease of fabrication, and environmental stability. Optimization of the ratio of polyvinyl
alcohol polyacrylic acid, the sensitizer concentration, pH, energy, diffraction efficiency measurements,
etc., have been done. pH is found to have a great influence on the recovery of the dye in this matrix.
The results of experimental investigations into the properties of this new material are reported.
Sreekumar, Krishnapillai; Rajesh Krishnan, G(Eur. J. Org. Chem, August , 2008)
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Abstract:
This paper describes the first use of polystyrene-supported
poly(amidoamine) (PAMAM) dendrimers as heterogeneous
basic organocatalysts for carbon–carbon bond formation.
Polystyrene-supported PAMAM dendrimers of first, second
and third generations have been used as reusable base catalysts
in Knoevenagel condensations of carbonyl compounds
with active methylene compounds. The reactions proceed in short periods of time and with 100% selectivity. This novel
catalyst eliminates the use of aromatic and halogenated solvents,
as well as complex purification processes. The catalysts
can be recycled ten times.
Sreekumar, K; Elizabeth, Chirackal Varkey(Springer, January 13, 2010)
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Abstract:
A new class of chiral polyurethanes containing
amido linkages in the polymer backbone have been synthesized
by reacting toluene diisocyanate with isosorbide
(IS) chiral moiety and the chromophores [N,N0-ethane-
1,2-diyl bis(6-hydroxy hexanamide), N,N0-butane-1,4-diyl
bis(6-hydroxy hexanamide) and N,N0-hexane-1,6-diyl bis
(6-hydroxy hexanamide)]. The corresponding chromophores
were obtained by the aminolysis of e-caprolactone by
using the diamines, diaminoethane, diaminobutane and
diaminohexane, respectively. All the polymers were synthesized
according to the symmetry conditions so as to
obtain the non-centrosymmetric environment. A series of
polyurethanes were synthesized by varying the chiral–
chromophore composition. The polyurethanes developed
were characterized by optical and thermal methods.
Description:
J Mater Sci (2010) 45:1912–1920
DOI 10.1007/s10853-009-4177-1
Sreekumar, K(Taylor and francis, October 20, 2003)
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Abstract:
Polymer materials find application in optical storage technology,
namely in the development of high information density and fast access type
memories. A new polymer blend of methylene blue sensitized polyvinyl alcohol
(PVA) and polyacrylic acid (PAA) in methanol is prepared and characterized
and its comparison with methylene blue sensitized PVA in methanol and
complexed methylene blue sensitized polyvinyl chloride (CMBPVC) is presented.
The optical absorption spectra of the thin films of these polymers
showed a strong and broad absorption region at 670-650 nm, matching the
wavelength of the laser used. A very slow recovery of the dye on irradiation was
observed when a 7:3 blend of polyvinyl alcohol/polyacrylic acid at a pHof 3.8
and a sensitizer concentration of 4.67 10 5 g/ml were used. A diffraction
efficiency of up to 20% was observed for the MBPVA/alcohol system and an
energetic sensitivity of 2000 mJ/cm2 was obtained in the photosensitive films
with a spatial frequency of 588 lines/mm.
Sreekumar, K; Elizabeth, Chirackal Varkey(Wiley, July 20, 2010)
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Abstract:
The toluene diisocyanate based optically
active chiral polyurethanes were synthesized according to
the symmetry conditions. The noncentrosymmetric (both
charge asymmetry and spatial asymmetry) environment
were attained by the incorporation of the chiral units
(diethyl-(2R,3R)(þ)-tartrate) and donor-acceptor building
blocks in the main chain which induce a helical conformation
in the macromolecular chain. A series of optically
active polyurethanes containing chiral linkages in the
polymer back bone have been synthesized by using
DBTDL catalyst by incorporating the amido diols which
were obtained by the aminolysis of e-caprolactone by
using the diamines, diaminoethane, diaminobutane, and
diaminohexane respectively. The effect of incorporation of
the chiral molecule diethyl-(2R,3R)(þ)-tartrate on the properties
of polyurethanes was studied by changing the chromophores
and also by varying the chiral-chromophore
composition. Various properties of polyurethanes were
investigated by UV, Fluorescence, TG/DTA, XRD, polarimetric
techniques, Kurtz-Perry powder techniques, etc.
Description:
Journal of Applied Polymer Science, Vol. 119, 111–119 (2011)
Sreekumar, K(Indian academy of science, February 24, 2003)
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Abstract:
Significant results of our experimental investigations on the dependence of pH on real time
transmission characteristics on recording media fabricated by doping PVC with complexed methylene blue
are presented. The optimum pH value for faster bleaching was found to be 4×5. In typical applications, the
illumination from one side, normal to the surface of this material, initiates a chemical sequence that records
the incident light pattern in the polymer. Thus direct imaging can be successfully done on this sample. The
recorded letters were very legible with good contrast and no scattering centres. Diffraction efficiency
measurements were also carried out on this material.
Knoevenagel condensation between carbonyl compounds and active methylene compounds as well as three component Mannich reaction between aldehydes, ketones and amines proceeded smoothly in water with good to excellent yield and high selectivity in the presence of zero and first generation poly(amidoamine) (PAMAM) dendrimers. The products and the catalyst were separated by simple biphasic extraction. The catalyst was found to be reusable.
Poly(6-tert-butyl-3,4-dihydro-2H-1,3-benzoxazine) was synthesized by
thermally activated cationic ring opening polymerization. The structure of the
polymer was confirmed by spectral and thermal studies. The highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were
estimated using cyclic voltammetry and optical absorption. Modulated photocurrent
measurement technique was employed to study the spectral and field dependence of
photocurrent. Photocurrent of the order of 1.5 micro A/m2 was obtained for polymer at a
biasing electric field of 40 V/mico m.
Sreekumar, K; Rani, Joseph; Pramitha, V; Sudha Kartha, C(Indian Academy of Sciences, December , 2010)
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Abstract:
Plane-wave transmission gratings were stored in the same location of silver-
doped photopolymer ¯lm using peristrophic multiplexing techniques. Constant and vari-
able exposure scheduling methods were adopted for storing gratings in the ¯lm using
He{Ne laser (632.8 nm). The role of recording geometry on the dynamic range of the ma-
terial was studied by comparing the results obtained from both techniques. Peristrophic
multiplexing with rotation of the ¯lm in a plane normal to the bisector of the incident
beams resulted in better homogenization of di®raction e±ciencies and larger M/# value.
A new photopolymerizable recording media is introduced based on poly
(vinyl alcohol) and vinyl acetate sensitized with methylene blue. It is observed
that this MBPVA/VAc system can be reused a number of times without
significant decrease in diffraction . The PVA-VAc ratio was optimized
at 2:1. Diffraction efficiency of 6.3% was obtained without any fixing at a dye
concentration of 9.3 10 4 mol/l at an exposure of 750 mJ/cm2. The material is
attractive on account of its reusability.
A new photopolymerizable recording media is introduced based on poly
(vinyl alcohol) and vinyl acetate sensitized with methylene blue. It is observed
that this MBPVA/VAc system can be reused a number of times without
significant decrease in diffraction efficiency. The PVA-VAc ratio was optimized
at 2:1. Diffraction efcienc4y of 6.3% was obtained without any fixing at a dye
concentration of 9.3 x 10- mol/l at an exposure of 750mJ/cm2. The material is
attractive on account of its reusability.
Sreekumar, K; Rajesh Krishnan, G(Taylor & Francis, February 22, 2010)
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Abstract:
Poly(amidoamine) dendrimers were synthesized on cross-linked aminomethyl polystyrene. Palladium
complexes of supported dendrimers prepared by ligand exchange method were reduced to dendrimernanoparticle
conjugates supported on polystyrene resin. The supported nanoparticles were used as
heterogeneous catalysts for the Suzuki coupling between aryl boronic acids and aryl halides. Various factors
affecting the catalysts performance were studied. Higher generation dendrimers gave well-defined
nanoparticles without agglomeration and these particles showed good catalytic performance
Sreekumar, K; Sudheesh Kumar, K(Taylor & Francis, June 1, 2007)
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Abstract:
Various polyurethanes containing photoactive bis(azo) and bis(o-nitrobenzyl)
groups in the main chain were synthesized by polyaddition reactions of diols such
as bis(4-hydroxyphenylazo)-2,20-dinitrodiphenylmethane, 4-hydroxy-3-methylphenylazo-
40-hydroxyphenylazo-2,20-dinitrodiphenylmethane and bis(4-hydroxy-3-
methylphenylazo)-2,20-dinitrodiphenylmethane with hexamethylene di-isocyanate
(HMDI), in dimethyl acetamide (DMAc) in the presence of di-n-butyltin dilaurate
(DBTDL) as catalyst. All of them were characterized by IR, UV-vis, 1H NMR and
13C NMR spectra and also by thermogravimetric analysis (TGA), differential scanning
calorimetry (DSC) and gel permeation chromatography (GPC).
Description:
International Journal of Polymeric Materials, 58:160–166, 2009
This manuscript describes the first example of silver ion complex of a dendritic tetranitrile ligand catalyzed
one-pot three component Mannich reaction and 1,5-benzodiazepine synthesis. The catalyst can be separated from the
products by a change in the solvent. The catalyst is reusable.