Polymer chains attached by one end to an impenetrable surface at high coverage exemplify a tethered layer of mesoscopic dimensions. At equilibrium, thermal fluctuations of the segment density profile of the brushlike layer reflect the tethered chain dynamics; the probing of these fluctuations by evanescent-wave dynamic light scattering is reported. By utilizing a set of terminally attached layers with thicknesses (L 0 ) from 45 to 130 nanometers, it was found that there is a preferred wavelength of order L 0 of these fluctuations with a concurrent slowing down of their thermal decay rate. This technique could open the route for the investigation of the largely unexplored area of polymer surface dynamics.When polymer molecules are selectively attached by one end onto a solid surface at relatively high concentrations, the polymer chains avoid one another by extending away from the surface, forming a "brush" (1, 2). The formation of polymer brushes is of technological importance in colloid stabilization and in modifying bulk surfaces and interfaces for improved adhesion, wetting, and wear properties. Such brushes can be created by synthesizing diblock copolymers, in which one shorter block adsorbs strongly to the surface from a solution and the other longer block extends into the solvent. This stretched configuration of tethered chains is used as a model for a wide variety of confined polymer systems (3). Most theoretical and experimental studies (3-10) have addressed the static structure of brushes, such as the density profile above the surface. Although theoretical predictions have been made of their dynamical structure as well (4,5,11,12), direct relevant experimental observation, although much needed (7), is difficult because of the small size of the brush and its low scattering power.We have addressed this problem by performing evanescent-wave dynamic lightscattering (EWDLS) measurements of asymmetric poly(ethyleneoxide-b-styrene) (PEO-PS) copolymers (Fig. 1). Light is reflected from a high-refractive index prism; the polymer is attached at the glass surface at the base of the prism, and the evanescent wave propagates through the polymer brush under conditions of total internal reflection. The evanescent wave is then used as the incident beam in photon correlation spectroscopy (13), which allows fluctuations with wave vector q to be resolved on time scales from 10 Ϫ7 to 10 3 s. From these measurements, we obtained the time-correlation functions C(q, t) of the scatteredlight intensity that reflect concentration fluctuations in the brush. Our results show that long-lived fluctuations have a finite wavelength of order L 0 [O(L 0 )], where L 0 is the equilibrium thickness of the grafted PS layer. This result resembles that for the interal relaxation mode seen in bulk diblock copolymer solutions (14).Detailed material and surface characterization is essential for the interpretation of the results. The PEO-PS model system in toluene was selected on account of the extensive information on adsorbed amount, kinetics of...

We probe the dynamic structure factor of multiarm star polymer solutions in good solvent above the overlap concentration with photon correlation spectroscopy and NMR. With the aid of a mean-field theoretical approach, which accounts for the interplay between the topologically enhanced osmotic pressure and the arm entropic stretching, we identify three mechanisms for relaxing the concentration and density fluctuations: cooperative diffusion, self-diffusion and structural relaxation. These model spherical brushes exhibit a behavior intermediate between polymers and colloids.

No abstract

Determining whether the first step of photooxygenation is Type I or Type II is a necessary prerequisite in order to establish the mechanism of photodynamic action. But this distinction is not sufficient, because other processes, both consecutive and competitive, commonly participate in the overall mechanism. Thus, in both Type I and Type II reactions, the initial products are often peroxides that can break down and induce free radical reactions. These aspects of photosensitization are discussed and illustrated by sensitizer/substrate systems involving (1) only radical reactions (decatungstate/alkane) and (2) reactions of singlet oxygen occurring in competitive and consecutive processes and possibly followed by radical reactions (methylene blue/2′‐deoxyguanosine). Two other previously investigated systems involving, respectively, a Type II interaction followed by radical processes (methylene blue/alkene) and Type II reactions, some of which being competitive or consecutive (rose bengal/alkene), are briefly reconsidered.

No abstract