Percorrer por autor "Davis, Fred"
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- New Tools for Understanding Complex Polymer BehaviourPublication . Mitchell, Geoffrey R.; Maetus, Artur; Davis, Fred; Mohan, Saeed; Gkourmpis, THomas; Youngs, TristanThe process of manufacturing with polymers usually involves shaping in the melt followed by a transition to the solid to preserve that shape and provide the required mechanical properties. The development of an understanding of those transitions is critical to the optimisation of materials and manufacturing technology. For synthetic polymers there are three key length scales in any phase transition such as crystallisation: the first involves the thin (~10nm) lamellar crystals, the second is the crystal planes in the unit cell (~1nm) and the third the regular local chain conformation (~0.1nm). We are using the extended Q range available with NIMROD at the ISIS Facility in the UK to obtain neutron scattering data which follows the transformation pathways of these three length scales simultaneously. We are using computational modelling procedures to analyse these data to develop a firm understanding of the multiscale processes involved in crystallisation. This paper describes the methodology and some of the experimental challenges using data drawn from this study. This work is part of the FCT funded programme UC4EP.
- SANS/WANS Time-resolving Neutron Scattering Studiesof Polymer Phase Transitions Using NIMRODPublication . Mitchell, Geoffrey Robert; Bowron, Daniel; Mateus, Artur; Bártolo, Paulo; Gkourmpis, Thomas; Phomphrai, Khamphee; Lopez, Daniel; Davis, FredABSTRACT We use new neutron scattering instrumentation to follow in a single quantitative time-resolving experiment, the three key scales of structural development which accompany the crystallisation of synthetic polymers. These length scales span 3 orders of magnitude of the scattering vector. The study of polymer crystallisation dates back to the pioneering experiments of Keller and others who discovered the chain-folded nature of the thin lamellae crystals which are normally found in synthetic polymers. The inherent connectivity of polymers makes their crystallisation a multiscale transformation. Much understanding has developed over the intervening fifty years but the process has remained something of a mystery. There are three key length scales. The chain folded lamellar thickness is ∼ 10nm, the crystal unit cell is ∼ 1nm and the detail of the chain conformation is ∼ 0.1nm. In previous work these length scales have been addressed using different instrumention or were coupled using compromised geometries. More recently researchers have attempted to exploit coupled time-resolved small-angle and wide-angle x-ray experiments. These turned out to be challenging experiments much related to the challenge of placing the scattering intensity on an absolute scale. However, they did stimulate the possibility of new phenomena in the very early stages of crystallisation. Although there is now considerable doubt on such experiments, they drew attention to the basic question as to the process of crystallisation in long chain molecules. We have used NIMROD on the second target station at ISIS to follow all three length scales in a time-resolving manner for poly(e-caprolactone). The technique can provide a single set of data from 0.01 to 100Å-1 on the same vertical scale. We present the results using a multiple scale model of the crystallisation process in polymers to analyse the results.