Robert M. Johnsen

Quasielastic light scattering (QELS) in semi-dilute polymer solutions a comparison of “slow-mode” diffusion with self-diffusion from FT-pulsed field gradient NMR

SummaryQuasielastic light scattering measurements and pulsed field gradient NMR measurements have been made on poly(ethylene oxide) fractions in semi-dilute solutions. Similar measurements have been made on polystyrene in chloroform and toluene. Through suitable choice of sampling times, the QELS experiments allowed analysis of the “fast” and “slow” modes of relaxation in terms of the corresponding diffusion coefficients and their relative amplitudes. Comparison of the slow mode with self-diffusion shows that the former is between one and two orders of magnitude slower than self-diffusion which contrasts with a number of recent reports. Furthermore, the proportion of fast to slow modes is highly sensitive to temperature and concentration. It is concluded that the slow mode must involve a concerted pseudogel motion sensitive to polymer-polymer friction.

Dynamics in concentrated polymer solutions by polarized Rayleigh–Brillouin scattering and dynamic light scattering

Solutions of polystyrene in toluene in the concentration range 0.1–1.0 g/ml have been examined using polarized Rayleigh–Brillouin scattering and dynamic light scattering. The purpose was to extend measurements to the hitherto little studied interval between semidilute solutions and the bulk polymer. The Brillouin shift increases monotonically with polymer volume fraction (Φp) and a linewidth maximum is observed at about Φp=0.6. Up to Φp=0.56, a single cooperative diffusion mode is found, which passes through a broad maximum at Φp=0.5. For the sample with Φp=0.78, the nature of the main component depends sensitively on temperature. At 17 °C it is purely relaxational (q independent), while at 40 and 50 °C, it is diffusive. At intermediate temperatures, mixed behavior is noted where the dynamic processes overlap because the slower relaxational mode has a stronger temperature dependence. At Φp=0.89, the correlation functions are highly nonexponential and strongly temperature dependent. The bimodal correlation...

The dynamics of symmetric polystyrene-polybutadiene diblock copolymer melts studied above and below the order-disorder transition using dynamic light scattering

This communication describes dynamic light scattering measurements on three low molar mass symmetric polystyrene–polybutadiene diblock copolymer melts. Above but close to the order–disorder transition, four modes contribute to the time‐dependent correlation function. In the depolarized geometry there is a very fast, broad relaxation typifying the segmental reorientational dynamics and a slower, single‐exponential mode related to chain orientation and stretching. In the polarized experiments two additional processes are seen: the heterogeneity mode related to the self‐diffusion of individual copolymer molecules and a slow mode representing the translational diffusion of large (≂100 nm) clusters. Just below the order–disorder transition (ODT), the slowest mode is shifted to very long times. The heterogeneity mode gives rise to depolarized scattering below the ODT because of the anisotropy induced by the lamellar structure. With the lowest molar mass copolymer above the ODT, it could be seen that the amplitu...

Dynamic light scattering measurements on the polystyrene/ethyl acetate system at semi-dilute concentrations as a function of temperature

SummaryMeasurements of autocorrelation functions extending over a broad time range are reported for a sample of polystyrene in ethyl acetate as a function of temperature between −44°C (θ-temperature) and 70°C. The corresponding spectra of decay times are obtained by two mathematical methods. The existence of three dynamic processes is shown and their temperature and angular behaviour is studied.

Dynamic properties of concentrated solutions above the glass transition temperature

Concentrated solutions of polybutylacrylate (PBA) in dioxane and polybutadiene (PBD) in cyclohexane and carbon tetrachloride have been studied at 50 °C in the concentration range 0.1–0.9 g/ml using Rayleigh–Brillouin scattering (RBS) and static and dynamic light scattering. The purpose was to extend measurements to concentrated solutions of ‘‘soft’’ polymers in a temperature range well above the glass transition temperature. The high frequency modulus from RBS is much larger than the low frequency osmotic modulus, but they have identical power law behavior. Contrary to the pronounced maximum in Dc observed as a function of Φp in concentrated polystyrene/toluene solutions [Brown et al., J. Chem. Phys. 95, 8568 (1991); Macromolecules 24, 5484 (1991)], the concentration dependence of Dc increases strongly above Φp≊0.56 in the PBA/dioxane system. This change is attributed to a reduction in the effective solvent viscosity caused by the presence of the polymer and analogous to the effect noted by Lodge and co‐w...