Semiconductor nanocrystals were prepared for use as fluorescent probes in biological staining and diagnostics. Compared with conventional fluorophores, the nanocrystals have a narrow, tunable, symmetric emission spectrum and are photochemically stable. The advantages of the broad, continuous excitation spectrum were demonstrated in a dual-emission, single-excitation labeling experiment on mouse fibroblasts. These nanocrystal probes are thus complementary and in some cases may be superior to existing fluorophores.

We report the chain conformations of polymer molecules accommodated at the solid-polymer melt interfaces in equilibrium. Polystyrene "Guiselin" brushes (adsorbed layers) with different molecular weights were prepared on Si substrates and characterized by using x-ray and neutron reflectivity. The results are intriguing to show that the adsorbed layers are composed of the two different nanoarchitectures: flattened chains that constitute the inner higher density region of the adsorbed layers and loosely adsorbed polymer chains that form the outer bulklike density region. In addition, we found that the lone flattened chains, which are uncovered by the additional prolonged solvent leaching (∼120 days), are reversibly densified with increasing temperature up to 150 °C. By generalizing the chain conformations of bulks, we postulate that the change in probabilities of the local chain conformations (i.e., trans and gauche states) of polymer molecules is the origin of this densification process.

We report the origin of the effect of nanoscale confinement on the local viscosity of entangled polystyrene (PS) films at temperatures far above the glass transition temperature. By using marker x-ray photon correlation spectroscopy with gold nanoparticles embedded in the PS films prepared on solid substrates, we have determined the local viscosity as a function of the distance from the polymer-substrate interface. The results show the impact of a very thin adsorbed layer (~7 nm in thickness) even without specific interactions of the polymer with the substrate, overcoming the effect of a surface mobile layer at the air-polymer interface and thereby resulting in a significant increase in the local viscosity as approaching the substrate interface.

We report a mobility gradient of polymer chains in close proximity of a planar solid substrate in compressed carbon dioxide (CO 2 ) gas. A series of bilayers composed of bottom hydrogenated polystyrene (h-PS) and top deuterated PS (d-PS) layers were prepared on Si substrates. A high-pressure neutron reflectivity (NR) technique was used to study the diffusive motion at the h-PS/d-PS interface as a function of the distance from the substrate interface. The results reveal that the interdiffusive chain dynamics gets strongly hindered compared to the bulk when the distance from the substrate is less than 3R g (R g is the radius of polymer gyration of the h-PS). At the same time, by utilizing rapid quench of CO 2 and subsequent solvent leaching, we reveal the presence of the CO 2 -induced polymer adsorbed layer on the substrate. We postulate that loop components in the adsorbed polymer chains provide a structure that can trap the neighboring polymer chains effectively, hence reducing the chain mobility in the close vicinity of the solid substrate even in the presence of the effective plasticizer.

We report the structural heterogeneity of recrystallized linear low-density polyethylene (LLDPE) films (25, 50, and 100 nm in thickness) in the direction normal to the surface, based on in situ grazing incidence small-angle X-ray scattering (GISAXS) and X-ray diffraction (GID) measurements. The GID results have clarified the presence of the edge-on lamellae at the surfaces and in the interior of the LLDPE films prepared on Si substrates as thin as 25 nm in thickness. However, the degree of the crystallinity for the 25 nm thick film was almost half of those for the 50 and 100 nm thick films, while the melting temperature (T m ) for all the films remained unchanged relative to the bulk (T m = 117 °C). Moreover, the GISAXS results for the 25 nm thick film indicate the structural heterogeneity in the direction normal to the surface: (i) At the polymer/air interface, the presence of the disordered edge-on lamellae which lack well-defined long periods even at T ≪ T m ; (ii) At the polymer/substrate interface, the persistence of a substrate-bound edge-on lamellar layer even at T ≫ T m ; (iii) In between the two interfacial layers, the existence of the well-ordered edge-on lamellae with the long periods. These heterogeneous structures can be explained as a consequence of the nucleation initiated at the topmost surface of the substrate-bound lamellar layer.

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We report supercritical carbon dioxide (scCO2) technology used for forming a large degree of molecular scale porosity in semicrystalline polymer thin films. The following three steps were integrated: (i) pre-exposure to an organic solvent which melted crystalline structures but did not cause a decrease in thickness, (ii) scCO2 exposure under the unique conditions where the anomalous absorption of CO2 occurred, and (iii) subsequent quick evaporation of CO2 to preserve the swollen structures. This unified process resulted in homogenous low-density polyphenylene vinylene films (a 15% reduction in density) with the sustained structure for at least 6 months at room temperature.

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