Percorrer por autor "Mitchell, Geoffrey Robert"
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- 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.
- Structure–property relationships in polyethylene based films obtained by blow molding as model system of industrial relevancePublication . Duraccio, Donatella; Mauriello, Amalia; Cimmino, Sossio; Silvestre, Clara; Auriemma, Finizia; Rosa, Claudio de; Pirozzi, Beniamino; Mitchell, Geoffrey RobertA method for the study of structure-property relationships of polyethylene (PE)-based films of potential use in food packaging has been set up. The approach has been demonstrated in the case of films obtained by blow-molding using model mixtures of two different polyethylenes, namely a metallocene-made grade of linear low density polyethylene (mLLDPE) and a low density polyethylene (LDPE). We show that physical properties important for numerous applications of these films, such as water vapor and oxygen permeability, toughness and stress strength, are related to the molecular structure and structural and morphological parameters of the blends, in particular, thickness of amorphous layers in lamellar stacks, lamellar thickness, lamellar twisting, and degree and type of branching.
- Time-resolving study of stress-induced transformations of isotactic polypropylene through wide angle X-ray scattering measurementsPublication . Auriemma, Finizia; De Rosa, Claudio; Di Girolamo, Rocco; Malafronte, Anna; Scoti, Miriam; Mitchell, Geoffrey Robert; Esposito, SimonaThe development of a highly oriented fiber morphology by effect of tensile deformation of stereodefective isotactic polypropylene (iPP) samples, starting from the unoriented form, is studied by following the transformation in real time during stretching through wide angle X-ray scattering (WAXS) measurements. In the stretching process, after yielding, the initial form transforms into the mesomorphic form of iPP through mechanical melting and re-crystallization. The analysis of the scattering invariant measured in theWAXS region highlights that the size of the mesomorphic domains included in the well oriented fiber morphology obtained at high deformations increases through a process which involves the coalescence of the small fragments formed by effect of tensile stress during lamellar destruction with the domain of higher dimensions.
- Ultrasound assisted synthesis of polylimonene and organomodified-clay nanocomposites: A structural, morphological and thermal propertiesPublication . Derdar, Hodhaifa; Mitchell, Geoffrey Robert; Cherifi, Zakaria; Belbachir, Mohammed; Benachour, Mohamed; Meghabar, Rachid; Bachari, Khaldoun; Harrane, AminePolylimonene-clay nanocomposites (PLM-Mag 2, 3, 6 and 10% by weight of clay) were prepared by mix-ing Maghnite-CTA+ (Mag-CTA+) and polylimonene (PLM) in solution using ultrasonic irradiation. The catalyst preparation method were studied in order to determine and evaluate their structural, morpho-logical and thermal properties. The Mag-CTA+ is an organophylic montmorillonite silicate clay pre-pared through a direct exchange process, using green natural clay of Maghnia (west of Algeria) called Maghnite. The Algerian clay was modified by ultrasonic-assisted method using cetyltrime-thylammonuim bromide (CTAB) in which they used as green nano-reinforcing filler. Polylimonene was obtained by the polymerization of limonene, using Mag-H+ as a catalyst. The morphology of the ob-tained nanocomposites was studied by X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electronic microscopy (TEM) and infrared spectroscopy (FT-IR). Thermogravimetric anal-ysis (TGA) shows that the nanocomposites have a high degradation temperature (200−250 °C) com-pared with the pure polylimonene (140 °C). The analyses confirmed the chemical modification of mont-morillonite layers and their uniformly dispersion in the polylimonene matrix. Exfoliated structures were obtained for low amounts of clay (2 and 3% by weight), while intercalated structures and immisci-ble regions were detected for high amounts of clay (6 and 10% by weight).