Biomedical subjects

Vivek M Prabhu

Publications and source records attributed to Vivek M Prabhu.

4 recordsLinked to original sources

PubMed2006

Effect of deprotection extent on swelling and dissolution regimes of thin polymer films.

The response of unentangled polymer thin films to aqueous hydroxide solutions is measured as a function of increasing weakly acidic methacrylic acid comonomer content produced by an in situ reaction-diffusion process. Quartz crystal microbalance with energy dissipation and Fourier transform infrared spectroscopy measurements are used to identify four regimes: (I) nonswelling, (II) quasiequilibrium swelling, (III) swelling coupled with partial film dissolution, and (IV) film dissolution. These regimes result from chemical heterogeneity in local composition of the polymer film. The acid-catalyzed deprotection of a hydrophobic group to the methacrylic acid tends to increase the hydrophilic domain size within the film. This nanoscale structure swells in aqueous base by ionization of the methacrylic acid groups. The swollen film stability, however, is determined by the hydrophobic matrix that can act as physical cross-links to prevent dissolution of the polyelectrolyte chains. These observations challenge current models of photoresist film dissolution that do not include the effects of swelling and partial film dissolution on image quality.

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PubMed2005

Direct measurement of the counterion distribution within swollen polyelectrolyte films.

The depth profile of the counterion concentration within thin polyelectrolyte films was measured in situ using contrast variant specular neutron reflectivity to characterize the initial swelling stage of the film dissolution. We find substantial counterion depletion near the substrate and enrichment near the periphery of the film extending into the solution. These observations challenge our understanding of the charge distribution in polyelectrolyte films and are important for understanding film dissolution in medical and technological applications.

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PubMed2005

Control of moisture at buried polymer/alumina interfaces through substrate surface modification.

Moisture absorption in poly(4-tert-butoxycarbonyloxystyrene) (PBOCSt) films supported on Al(2)O(3) sputter coated silicon wafers is measured using neutron and X-ray reflectivity. Accumulation of water at the interface during moisture exposure results in an apparent film-thickness-dependent swelling for ultrathin PBOCSt films. The swelling of a film on Al(2)O(3) is less than the swelling of a film of the same thickness on SiO(x) for films thinner than 20 nm. This is due to comparatively less moisture accumulation at the Al(2)O(3)/PBOCSt interface. A simple, zero adjustable parameter model consisting of a fixed water-rich layer at the interface and bulk swelling through the remainder of the film describes the thickness-dependent swelling quantitatively. The influence of four different Al(2)O(3) surface treatments on the moisture distribution within PBOCSt films was examined: bare Al(2)O(3), tert-butylphosphonic acid, phenylphosphonic acid, and n-octyltrichlorosilane. Both the phenyl and the octyl surface treatments reduce the accumulation of water at the polymer/substrate interface. The tert-butyl treatment does not reduce the interfacial water concentration, presumably due to insufficient surface coverage.

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PubMed2004

Counterion associative behavior with flexible polyelectrolytes.

At low ionic strength, organic counterions dress a flexible charged polymer as measured directly by small-angle neutron scattering and neutron spin-echo spectroscopy. This dressed state, quantified by the concentration dependence of the static correlation length, illustrates the polymer-counterion coupled nature on the nanometer length scale. The counterions, made visible by selective hydrogen and deuterium labeling, undress from the polymeric template by addition of sodium chloride. The addition of this electrolyte leads to two effects: increased Debye electrostatic screening and decoupled organic counterion-polymer correlations. Neutron spin-echo spectroscopy measures a slowing down of the effective diffusion coefficient of the labeled counterions at the length scale of 8 nm, the static correlation length, indicating the nanosecond counterion dynamics mimics the polymer. These experiments, performed with semidilute solutions of tetramethylammonium poly(styrene sulfonate) [(h-TMA(+)) d-PSS], apply to relevant biopolymers including single and double stranded DNA and unfolded proteins, which undergo orchestrated dynamics of counterions and chain segments to fold, unfold, and assemble.

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