Percorrer por autor "Olley, Robert H."
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- Controlling Morphology Using Low Molar Mass NucleatorsPublication . Mitchell, Geoffrey; Wangsoub, Supatra; Nogales, Aurora; Davis, Fred J.; Olley, Robert H.Crystallisation is a hugely important process in physical sciences and is crucial to many areas of, for example, chemistry, physics, biochemistry, metallurgy and geology. The process is typically associated with solidification, for example in the purification of solids from a heated saturated solution familiar to all chemistry undergraduates. Crystalline solids are also often the end result of cooling liquids, or in some cases gases, but in order to form require nucleation, in the absence of nucleation supercoiling of liquids well below the melting point is possible (Cava-gna, 2009). The quality of crystals, as gauged by size and levels of order is highly variable, and may depend on factors such as material purity and the rate of cool-ing; rapid cooling may result in poor crystallisation, or even the formation of amorphous materials with no long range order. In geological systems rates of cooling may vary over many orders of magnitude, for example obsidian is a large-ly amorphous material produced when lava is rapidly cooled (Tuffen, 2003), while the gypsum crystals found in the Cueva de los Cristales in Chihuahua, Mexico can reach 10 metres in length (Figure 1) and are formed over hundreds of thousands of years. In this latter case the formation of such large spectacular structures as shown in Figure 1 can only be explained by a low nucleation rate (García-Ruiz, 2007; Van Driessche, 2011).
- On row-structures in sheared polypropylene and a propylene–ethylene copolymerPublication . Olley, Robert H.; Mitchell, Geoffrey R.; Moghaddam, YasminThe crystallisation of polymers such as polyethylene or polypropylene from the melt is greatly influenced by the flow-thermal history prior to the crystallisation. We explore the influence of the chemical configuration of polypropylene based chains on the formation of row structures on crystallisation. We use a combination of in situ time resolved smallangle X-ray scattering, ex situ wide angle X-ray scattering with optical and scanning electron microscopy to show that row nuclei are formed in random copolymers of propylene with a limited amount of ethylene subjected to modest shear flow fields. We contrast observations performed using two homopolymers of isotactic polypropylene and one random copolymer of propylene and ethylene. We propose that it is not strictly necessary to argue that the row nuclei are already crystalline nor to invoke epitaxial crystallisation as the mechanism for the nucleation of lamellae, as similar structures can be formed on carbon nanotubes and fibrils of dibenzylidene sorbitol. The combination of microscopy and scattering provides a powerful approach to investigating these phenomena, especially as compared to either technique used in isolation.
- Orthogonal templating control of the crystallisation of Poly("-Caprolactone)Publication . Mitchell, Geoffrey R.; Olley, Robert H.The crystal growth of poly("-caprolactone) can be very effectively directed through the use of small amounts of dibenzylidene sorbitol in conjunction with modest flow fields to yield extremely high levels of the preferred polymer crystal orientation. We show that by introducing small quantities of a terpolymer, based on polyvinyl butyral we can switch the symmetry axis of the final lamellar orientation from parallel to perpendicular to the melt flow direction. During shear flow of the polymer melt, the dibenzylidene sorbitol forms highly extended nanoparticles which adopt a preferred alignment with respect to the flow field and on cooling, polymer crystallisation is directed by these particles. The presence of the terpolymer, based on polyvinyl butyral, limits the aspect ratio of the dibenzylidene sorbitol (DBS) particles, such that the preferred orientation of the particles in the polymer melt changes from parallel to normal to the flow direction. The alignment of lamellar crystals perpendicular to the flow direction has important implications for applications such as scaffolds for tissue engineering and for barrier film properties.