Showing papers by "James A. Forrest published in 1997"

Interface and chain confinement effects on the glass transition temperature of thin polymer films

Abstract: We have used Brillouin light scattering and ellipsometry to measure the glass transition temperature ${T}_{g}$ of thin polystyrene (PS) films as a function of the film thickness $h$ for two different molecular weights ${M}_{w}.$ Three different film geometries were studied: freely standing films, films supported on a ${\mathrm{SiO}}_{x}$ surface with the other film surface free (uncapped supported), and films supported on a ${\mathrm{SiO}}_{x}$ surface and covered with a ${\mathrm{SiO}}_{x}$ layer (capped supported). For freely standing films ${T}_{g}$ is reduced dramatically from the bulk value by an amount that depends on both $h$ and ${M}_{w}.$ For $h\ensuremath{\lesssim}{R}_{\mathrm{EE}}$ (the average end-to-end distance of the unperturbed polymer molecules), ${T}_{g}$ decreases linearly with decreasing $h$ with reductions as large as 60 K for both ${M}_{w}$ values. We observe a large ${M}_{w}$ dependence of the ${T}_{g}$ reductions for freely standing films which provides the first strong evidence of the importance of chain confinement effects on the glass transition temperature of thin polymer films. For both the uncapped and capped supported films, ${T}_{g}$ is reduced only slightly $(l10\mathrm{K})$ from the bulk value, with only small differences in ${T}_{g}$ $(l4\mathrm{K})$ observed between uncapped and capped supported films of the same thickness. The results of our experiments demonstrate that the polymer-substrate interaction is the dominant effect in determining the glass transition temperature of PS films supported on ${\mathrm{SiO}}_{x}.$

Phase separation morphology of spin-coated polymer blend thin films

Abstract: We present the results of a study of the morphology of phase separation in thin films of two different polymer blend systems: polystyrene/polyisoprene and polystyrene/poly(methyl methacrylate). For each blend system, the two polymer components are dissolved in a common solvent. Spin coating of the ternary solutions (polymer blend/solvent) is used to confine the blends to a thin film geometry and to produce phase separation because of rapid evaporation of the solvent (solvent quench). As a quantitative measure of the phase separation morphology the average domain area of the minority component is measured as a function of the polystyrene mass fraction. For both blend systems we identify a small range of composition corresponding to a large increase in the average domain area. We show that the strong dependence of the average domain area on spin speed allows control over the quench time of the polymer blend thin films.

Manufacturing clad products by spray forming

Abstract: Whilst it has been feasible to spray form clad products for several years, it has been difficult to generate a high integrity metallurgical bond at the interface of the mandrel and the sprayed alloy. Consequently, roll preforms have generally been produced in the form of rings which are subsequently fitted to the roll arbor or mandrel. However, a joint project in progress between Forged Rolls (UK) Ltd, British Rollmakers Corporation, Osprey Metals Ltd and Sheffield University (with part funding from the UK, Department of Trade and Industry, under the LINK enhanced engineering materials programme) has demonstrated that direct bonding of roll alloys onto steel arbors can be achieved using arbor preheating in conjunction with multi-atomisers and other special techniques. The equipment to achieve such results is described, together with an outline of the deposition method. Initial metallographic and mechanical property results are also presented with a comparison made between conventionally processed and spray formed roll alloys, where a considerable refinement in the spray formed microstructure is apparent. These encouraging results have led to the spray forming of narrow, clad hot and cold strip mill rolls for actual field trials.

Post-irradiation flashes and continuous emission from solid deuterium

Abstract: Optical emission from proton-beam-irradiated solid deuterium near 800 nm has been studied after termination of the proton beam. The continuous red emission shows a residual intensity that persists over 30 min from termination of the beam. Optical flashes can also be thermally triggered over 10 min after termination of irradiation. Such triggered flashes are shown to quench the infrared absorption of Stark-shifted charge-induced features. Ultraviolet photons can stimulate this red emission after termination of irradiation with no measurable decrease in intensity for 40 min. The cause of this continuous emission and optical flashes is discussed in the light of these results. {copyright} {ital 1997} {ital The American Physical Society}