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Post-doctoral Associate Office: 226 Mason Lab Phone: (203) 436-4059 |
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Ph.D. in Applied Physics from University of Greifswald, Germany (Aug 2007) M.Sc. in Physics from Cochin University of Science and Technology (CUSAT), Kerala, India (Nov 2001) B.Sc. in Physics from University of Kerala, India (June 1998) I joined the Osuji lab in March 2008 after completing my PhD in Germany in the lab of Prof. Christiane Helm at Greifswald and at Hahn-Meitner Institute (Neutron reflectometer (V6)), Berlin. Originally, I am from India. My research interests include surface and interfacial properties of polyelectrolyte systems and X-ray scattering applied to macromolecular systems. |
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Reversible hydrogen bonding between a surfactant and a polymer having complementary sites for the surfactant can lead to phase separation and block copolymer-like morphology. Our studies are focused on the structure and dynamics of polymer-surfactant complexes, with efforts paid to explicitly address the effect of the strength of the interaction between the polymer and surfactant. Above a critical stoichiometry, i.e. the ratio of ligand concentration to that of binding sites, these polymers may display liquid crystalline behavior in the melt state. In solution, there is a coil-globule collapse that is precipitated by the hydrophobic interactions between adjacent ligands. The weak interaction between the ligands means that the binding equilibrium is dynamic, and so ligands are thought to change their sequence distribution along the polymer chain in response to changes in temperature, stoichiometry and the chemistry of the ligand. Characterizing this behavior is the central theme in our research. We study the thermal properties of the mesophases via DSC, and correlate this with polarized optical microscopy and small angle X-ray scattering studies in solution and in the melt. Figure 2 depicts temperature dependent SAXS measurements on a melt sample showing the emergence of a bilayer liquid crystalline structure at elevated temperatures. The structure transitions back to a monolayer at lower temperatures. |
Figure 1:SAXS data as a function of
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The surfactant is designed such that specific environmental stimuli such as temperature can induce changes in the polymer chain-level structure by modifying the interaction strength. We use a combination of NMR (Figure 2), FTIR and equilibrium dialysis measurements to carefully quantify the weak, reversible hydrogen bonding interactions between hydrophilic materials such as poly(acrylic acid) or poly(vinyl alcohol) and conjugate small molecular ligands. |
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Figure 2: NMR spectra detail the interaction between PAA and mesogen. Inset: van't Hoff plot with the extracted binding energy (-6.3 kJ/mol) and the entropy of binding (-0.3 kJ/mol). |
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Reflectivity of Polyelectrolyte Multilayers |
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For my PhD, we did a detailed invesitigation of the the temperature effect and the hydrophobic (nonelectrostatic) interactions on the polyelectrolyte multilayer growth (X-ray reflection), surface structure (AFM, X-ray reflection), layer interpentration and hydration. We found that above a critical salt concentration, indicating that attenuation of the electrostatics (range 1nm) is necessary to allow secondary forces to contribute. The thickness of PAH/PSS bilayers from 1 M NaCl or KCl solution increases on increase in the temperature of the deposition solution (5 to 60 °C) by a factor of 3. We found that the bilayer thinkness is independent of the kind of salt, yet different composition (hydration). On heating the preparation solution, eventually the amount of bound water decreases. We also found a correlation between the amount of bound water and the layer interdigitation (less bound water leads to roughening of the interfaces). i.e, the interdigitation is higher at high temperatures. We also observed that the onset of roughening depends on the kind of salt (50 or 35 °C for films made from 1 M NaCl or KCl, respectively). on increase of temperature, the hydrophobic force dominates the changes in polyelectrolyte multilayer growth and composition.
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Figure 1: Top: Neutron reflectivity curves normalized relative to the Fresnel reflectity of polyelectrolyte multilayers with selectively deuterated layers. The films are prepared from 1 M NaCl solution at the temperatures indicated. Always, the relative humidity is 0%. Bottom: The deduced scattering length density profiles. |
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Figure 2: Schematic of contrast decrease due to larger layer interpenetration caused by polyelectrolytes adsorbing in a pearl-necklace or a partly globular structure. 
Last modified: February 10, 2009