The fluorescent probe dimethylaminonaphthylsulfonamide is covalently bound to the ends of the pol(ethylene glycol) chains of the swellable block copolymers poly(ethylene glycol)–polystyrene (PEG-PS) and poly(ethylene glycol)–poly(ethylene imine) (PEG-PEI) to investigate the molecular mobility inside the polymers, swollen by different liquids. Steady-state and time-resolved studies of the Stokes shift between absorption and fluorescence spectra reveal that the probe is solvated by both the polymer matrix and the liquid phase. The extent of solvation by the liquid and the mobility of the microenvironment of the probe depend on both the swelling volume of the polymer and the solubility of the probe in this liquid. Steady-state and time-resolved fluorescence depolarisation measurements show that the polymer matrix forms a very rigid solvent cage, which almost completely immobilizes the probe. Upon solvation of the probe by the liquid, the mobility of the probe increases. In PEG-PEI swollen by polar solvents, the mobilities of the probe itself and of its microenvironment, although not reaching the values observed in homogeneous solution, are significantly higher than in PEG-PS, due to the hydrophilic nature of the polymeric backbone in PEG-PEI.

Robert Solow's quip that see computers everywhere but in the productivity statistics has prompted a large and growing literature examining the Information Productivity Paradox. Some aggregate and industry-level studies have failed to detect a positive contribution to productivity growth from investments in computer technology.' More recent studies utilizing firm-level data, however, have detected a significant contribution.' This paper confirms the results of these latter studies using U.S. firm-level computer asset and financial data for nonagricultural firms during the period 1977-1993 from a variety of data sources including the U.S. Census Bureau's Enterprise and Auxiliary Establishment Surveys, Compustat, and Computer Intelligence Infocorp, a market research firm. Our principal finding, that computers--especially personal computers--do contribute positively to productivity growth, suggests that the traditional Information Productivity Paradox is largely a measurement problemn. This is closely related to the more general problem of assessing service sector productivity, because computers are used most intensively in the service sector and in the 'service' functions of non-service sector firms (e.g., payroll and purchasing). As Zvi Griliches (1994) noted, these activities pose the greatest problems for output and productivity measurement. Moreover, although we may see computers everywhere, they represent only a tiny fraction of capital stock (on about 2% of plant, property and equipment), so aggregate effects may be hard to detect (Oliner and Sichel 1993). Our data indicate that computers not only contributed to productivity growth during the period 1977-1993, but they yielded excess returns relative to other types of capital. The data also suggest that computer productivity peaked around 1986/1987,3 that computers are complementary with skilled labour (Krueger 1993 and Autor, Katz and Krueger 1997), and that use of computers may permit reductions in inventory levels. Finally, and perhaps most interesting, the evidence

Polyethyleneglycol (PEG) chain mobility in gelatineous microbeads is investigated by means of dynamic excimer formation. Pyrenebutyric acid (PYB) is covalently linked to the chain ends as probe molecule. Excimer formation is monitored by steady-state and time-resolved fluorescence spectroscopy in the presence of a series of liquid phases and in the dry state. PYB concentration in the beads is varied over three orders of magnitude up to c = 6·10 −2 M . The concentration is derived from absorption measurements in stirred bead suspensions, considering the deviations from Lambert–Beer's law in heterogeneous systems. Excimer formation is found to be a dynamic process in the presence of liquid phases which solvate both the polymer and the fluorophore. The collisional rate constant, k DM , is of the order of k DM -values of PYB in homogeneous solutions, indicating a high translational mobility. Excimer-to-monomer intensity ratios are in general accordance with the solvation capacity of the liquid phase. In the dry state excimer formation is found only at high PYB concentrations, c ≈ 3·10 −2 M . It is concluded that this excimer emission arises from aggregated PYB, since corresponding fluorescence response curves show no rise time.

Porous microbeads of low cross-linked polystyrene (PS) grafted with poly(ethylene glycol) chains (PEG) are labeled at the free chain ends with 3-(1,6-diphenyl-1,3,5-hexatrienyl)propionic acid (DPH-PA) and 1-(dimethylamino)naphthalene-5-sulfonic acid (DANS), which probe the polarizability, polarity, and viscosity of their environments. The beads are investigated in a series of pure liquid phases and in acetonitrile/water mixtures by electronic absorption and steady state as well as time-resolved fluorescence spectroscopy. The extent of solvation in the polymer/liquid interphase is characterized from spectral shifts by introducing a solvation fraction which quantifies the relative amount of polymer and liquid in the solvation shell. The rotational mobility of DPH-PA is derived from time-resolved fluorescence anisotropy measurements. Depending on the liquid phase, the rotational correlation times vary strongly between tau(R)=500 ps and 100 ns. The shortest tau(R)-values are found in liquids which are able to solvate both the fluorophore and the polymer, e.g. toluene and acetonitrile. Long correlation times are observed in the presence of aliphatic hydrocarbons, which do not solvate the bead and therefore cannot penetrate into the polymeric frame, and in presence of water, which solvates PEG but not PS and DPH. In contrast, water is able to weakly solvate DANS chemically bound to the polymeric frame.

Chromatographic surfaces are investigated by means of fluorescent probes (aza-aromatic compounds and anthroic acids). From absorption and fluorescence spectra the acid-base equilibria and types of binding at the interfaces are determined. The different surface acidities are given in terms of pH equivalents, which cover a range of more than three ΔpH units. The rotational mobility of probe molecules on reversed phase materials is measured to get a more detailed understanding of the chromatographic separation mechanism. The rotational correlation time is calculated from fluorescence anisotropy and decay time measurements.

Using multiple-beam interferometry in evaporated wedge-shaped metal films and movable beams of pulsed hypersound, the phase velocity and attenuation of longitudinal and transverse hypersound was measured for the first time over the broad frequency range from 1 GHz to 24 GHz. We investigated polycrystalline films of silver, copper and constantan. The bulk and shear moduli can be deduced from the measured phase velocities. The observed attenuation is due to sound scattering at grain boundaries and other defects whereas the interaction with electrons is too weak to contribute significantly.

By means of movable beams of coherent pulsed phonons it is possible to record multiple-beam interferences in wedge-shaped evaporated films as a function of thickness. By fitting calculated interferograms to these phonon interferograms the attenuation and phase velocity of longitudinal and transverse hypersound can be determined for these films. Details of the experimental setup used at 1 GHz, 3 GHz, 9 GHz and 24 GHz as well as the preparation of wedge-shaped metals films are described, too.