Showing papers by "James A. Forrest published in 1998"

Brillouin light scattering studies of the mechanical properties of thin freely standing polystyrene films

Abstract: We have used Brillouin light scattering (BLS) to measure the room-temperature, high-frequency mechanical properties of thin freely standing polystyrene (PS) films. We have investigated the effects of chain confinement and the free surface on the mechanical properties by measuring the velocity of film-guided acoustic phonons in films with thicknesses ranging from less than to greater than the average end-to-end distance of the unperturbed molecules ${R}_{\mathrm{EE}}.$ We find that the measured, room-temperature sound velocities are the same, to within $\ifmmode\pm\else\textpm\fi{}1%,$ for all films that have glass transition temperatures that differ by as much as 65 K. Our results, which differ markedly from those of recent picosecond acoustic measurements, are discussed in terms of models proposed to explain anomalous glass transition behavior in thin polymer films. A careful analysis of the BLS data reveals that, at room temperature, the mechanical stiffness, mass density, and thermal expansion of thin, freely standing PS films in the glassy state are consistent with bulk values for all film thicknesses.

Relaxation dynamics in ultrathin polymer films

Abstract: We have used photon correlation spectroscopy and quartz crystal microbalance techniques to examine the relaxation dynamics of ultrathin $(hl400\AA{})$ polystyrene films in both supported and freely standing geometries. These studies probe relaxation dynamics of polymer films in which the glass transition temperature ${(T}_{g})$ is reduced below the bulk value. Both the shape of the relaxation function and the dependence of relaxation time on temperature above the glass transition are remarkably similar to that of the bulk polymer, though the range of relaxation times is shifted according to the shift in ${T}_{g}.$ The results indicate that the microscopic relaxation dynamics of thin films remain similar to that of the bulk polymer even, in the extreme case in which the ${T}_{g}$ value is shifted more than 70 K below the bulk value.

Mechanical confinement effects on the phase separation morphology of polymer blend thin films

Abstract: We have measured the phase separation morphology of polystyrene–poly ~methyl methacrylate! ~PSPMMA! blend films of thickness h on a silicon oxide ~SiO x) substrate with a SiOx capping layer. We observe a novel phase separation morphology for small capping layer thicknesses L and a transition from lateral to lamellar morphology as L is increased. We present a simple model that explains the observed lateral morphology and the transition in morphology in terms of a balance between the free energy increase associated with forming the interfaces between PS-rich and PMMA-rich domains and the free energy increase associated with the elastic bending of the SiOx capping layer. The simple model reveals the dependence of the transition capping layer thickness Lc on the polymer blend film thickness h , and gives a reasonable quantitative prediction of Lc . @S1063-651X~98!11505-2#

The structure and properties of spray formed cold rolling mill work roll steels

Abstract: A study of the effect of spray casting on the microstructure and properties of cold rolling mill work roll steels has been undertaken. Industry standard qualities have been spray formed and a comparison of the resulting properties and microstructures has been made with those of the conventionally processed counterparts. The influence of process variables on the spray formed product has also been investigated. The results indicate that cold mill work roll steels which have comparable properties and structures to conventionally cast and forged products can be produced successfully in a single processing step. Furthermore, a high integrity metallurgical bond between the deposit and the substrate can be achieved.