Jaan Noolandi

Theory of photogeneration and fluorescence quenching

We give the exact solution of the geminate recombination probability of a pair of oppositely charged particles, corresponding to the boundary condition of a partly absorbing sphere of finite radius at the origin. In the limit of an infinite recombination velocity (κ→∞) and a vanishing radius (a→0) we recover the well‐known result of Onsager. We use the solution in the formulation of a model of photogeneration and fluorescence quenching in organic solids, with thermalization lengths which are comparable to the lattice spacing. As an illustration we analyze fluorescence quenching and quantum efficiency data for x‐metal‐free phthalocyanine, assuming the extreme case of complete internal conversion and no thermalization length. We discuss the form of the slope‐to‐intercept ratio for small applied fields determined from the solution for the generalized escape probability.

Solution of the time‐dependent Onsager problem

We obtain the Green’s function solution of the Smoluchowski equation with a Coulomb potential and an electric field, corresponding to a general boundary condition at the origin. In the low‐field limit the solution exhibits diffusive behavior for long times, but above a critical field the long‐time decay becomes purely exponential. We derive expressions for the time evolution of quantities such as the rate of geminate recombination and the survival probability, and for t→∞, a→0 (where a is the radius of a perfectly absorbing sphere at the origin) we recover Onsager’s results. We consider a variety of initial conditions, such as a δ function, isotropic and constant distributions, as well as a more complicated distribution applicable to fluorescence quenching in the presence of an electric field. For small values of the electric field and the Laplace transform variable we obtain an expression for the probability of a scavenging reaction taking place in competition with geminate recombination. Using the presc...

Solution of the Smoluchowski equation with a Coulomb potential. II. Application to fluorescence quenching

The rate of fluorescence quenching is derived for the continuum model of a diffusion‐controlled reaction, using the exact solution to the Smoluchowski equation with a Coulomb potential derived earlier. The first two terms in the long‐time expansion of the reaction rate are formally the same as the result obtained by Smoluchowski for no long‐range forces. However, the sum of the hard‐core radii in Smoluchowski’s expression is replaced by an effective encounter distance. Our expression for the reaction rate with the Coulomb interaction constitutes an extension of Debye’s steady‐state result to the transient case.

Theory of inhomogeneous weakly charged polyelectrolytes

A theory of inhomogeneous multicomponent systems containing weakly charged polyelectrolytes is developed. The theory treats the polymer conformation and the electrostatics simultaneously using a functional integral representation of the partition function. A mean-field approximation to the theory leads to two sets of coupled mean-field equations: a Poisson-Boltzmann type equation describing the electrostatic potential, and a set of self-consistent field equations describing the equilibrium densities. Asymptotic forms of the theory at weak and strong segregation limits are derived. The theory can be used to study the interfacial properties, microphase structures, and adsorptions of a variety of weakly charged polyelectrolyte systems. As a simple example, the interface between the polymer-rich and polymer-poor phases of a polyelectrolyte solution is studied.

Stiff chain model—functional integral approach

We present a modified probability distribution for a single stiff chain. This probability distribution includes the usual Wiener measure that accounts for the bond length distribution and a term that associates an explicit energy with the bending of the chain. The modification includes additional factors that suppress the end fluctuations to make the chain homogeneous. Our result for the mean squared end‐to‐end distance is that for the Kratky–Porod chain but the analysis clarifies a number of points in the literature.

Self-consistent theory of block copolymer blends : neutral solvent

We present a theoretical study of the lamellar structure of diblock copolymers blended with neutral solvent. The calculations are based on numerical solutions of the equations of the mean field self‐consistent theory of incompressible polymer/solvent blends, from the weak through to the strong segregation limits. For idealized model systems of symmetric copolymers and perfectly nonselective solvent, we examine the variation of the equilibrium lamellar thickness with molecular weight, Flory χ parameter, solvent quality, and copolymer volume fraction, as well as the distribution of solvent and polymer within the subdomains. We also examine a real system which has been extensively studied experimentally, namely PS‐b‐PI with the (nearly) nonselective solvent toluene.

Polymeric nanostructured material for high-density three-dimensional optical memory storage

The unique properties of a polymer photonic crystal are examined with respect to applications as a medium for high-density three-dimensional optical data storage media. The nanocomposite material was produced from core-shell latex particles, in which the latex cores contained dye-labeled polymer. Nonfluorescent latex shells were attached to the core particles. Upon annealing, the close-packed core-shell particles formed a nanostructured material with the fluorescent particles periodically embedded into the optically inert matrix in a hexagonal close-packed structure. A two-photon laser scanning microscope was used to write bits of information into the material by photobleaching the optically sensitive particles and, under much lower fluence, read out the resulting image. Relative to conventional homogeneous storage media, the nanostructured periodic material is shown to increase the effective optical storage density by at least a factor of 2 by spatially localizing the optically active region and imposing...

The surface science of xerography

Over the past four decades xerography, the dry ink marking process developed by the photocopy industry, has grown from nothing into a

A model of the DNA transient orientation overshoot during gel electrophoresis

170 billion industry worldwide. This amazing commercial success is due to the fact that during this period, xerographic technology experienced constant and often-dramatic improvement created by sustained industrywide research and development. Indeed, the development of the xerographic copying and printing industry is one of the great applied surface science successes of all time. In this article we outline the story of the advances in xerographic technology during the past four decades, describe the profound dependence on these advances of the control of surface and interface properties of increasingly sophisticated multi-component materials systems, and indicate the potential impact on the industry of the continuing development of the surface and interface science of the multi-component materials packages used in xerographic technology.

A geminate recombination model for photoluminescence decay in plasma-deposited amorphous Si:H

Linear dichroism and electric birefringence measurements show that when an electric field is applied to a DNA molecule at equilibrium in an agarose gel, the isotropic molecular conformation quickly orients in the field direction, reaching first a maximum ‘‘overshoot’’ orientation before it relaxes towards a somewhat less oriented but still anisotropic steady‐state conformation. We present here a simple analytical model of this overshoot effect together with numerical results from a computer simulation of gel electrophoresis. The predicted dependence of the overshoot time and orientation upon field intensity and molecular size are in good agreement with experimental results. The dynamics of the overshoot involves U‐shape conformations that disappear only after the internal elastic forces completely dominate the electric forces. It is also predicted that a different overshoot regime takes place for low electric fields and small molecular sizes, and that a primary and a secondary overshoot may appear for ver...