1,721,383 research outputs found
Characterization of Polymeric Materials using DSC, X-Ray Diffraction (XRD) and Nuclear Magnetic Resonance (NMR)
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Molecular Arrangements and Conformations of Liquid Unbranched Alkanes In Narrow Slits
No full textRealistic, atomistic models of liquid tridecane in broad slits (> 3 nm) and in narrow slits of thickness 1,2 nm and 1,0 nm have been obtained using the Monte Carlo technique. The setup of the models is such that the molecules in the slits are in equilibrium with the bulk liquid. The surfaces of the plates are modelled as two-dimensional arrays of hexagonally packed units having the same size and interaction parameters of a methylene group. The regions adjacent to the plates in slits with thickness > 3 nm are characterized by a well defined tendency to form partially ordered layer structures, while molecules at a distance from the plates larger than 1,5 nm are unperturbed. The simultaneous presence of two plates increases the tendency to form layer structures when their distance is 1,2 nm, while this tendency is almost totally destroyed when the slit is squeezed down to a thickness of 1,0 nm. This is also associated with a 10% decrease of the density in the latter slit
Tailoring crystallization and physical properties of polyolefins via organometallic catalysis.
No full textThe control of end-use properties of polymers via crystallization, processing and synthetic approac
New concepts in thermoplastic elastomers: The case of syndiotactic polypropylene, an unconventional elastomer with high crystallinity and large modulus
No full textSyndiotactic polypropylene (sPP), despite high crystallinity, shows unusual elastic properties, associated with a reversible crystal-crystal phase transition. The mechanism behind the crystal-crystal phase transition involved during cyclic elongation and recovery and the time scale of this process is here investigated, to unravel the origin of the elasticity of such an unconventional elastomer. The structural transformation of fibers of sPP during deformation has been studied by wide-angle X-ray diffraction measurements, using the high flux available on the beamline ID11 at the European Syncrotron Radiation Facility (ESRF), Grenoble, France, performing a time-resolved analysis of structural changes occurring while cyclically stretching and relaxing sPP fibers. This analysis indicates that the stress-induced phase transition is a martensitic transformation. As in martensitic phase transitions it occurs readily and directly, supporting the idea that elasticity in sPP is partially of enthalpic nature
Structure and properties of syndiotactic polypropylene: a highly crystalline thermoplastic elastomer.
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Structure of syndiotactic propylene-ethylene copolymers: effect of the presence of ethylene units on the structural transitions during plastic deformation and annealing of syndiotactic polypropylene.
No full textTime-resolved study of the martensitic phase transition in syndiotactic polypropylene
No full textThe structural transformations occurring in fibers of syndiotactic polypropylene (sPP) during stretching have been studied by wide-angle X-ray diffraction measurements, using syncrotron radiation. sPP samples were cyclically stretched and relaxed at controlled rate, while recording X-ray fiber diffraction patterns and stress-strain curves. Clear evidences that the polymorphic transition between the helical form II and the trans-planar form III of sPP is a fast process and occurs on the same time scale as the rate the material is stretched, are provided. The transition of form II into form III starts during the stretching in correspondence to critical values of the stress-strain parameters. As the strain increases, crystals of form 11 transform into form III and the inverse transition occurs, releasing the tension. Below a critical strain, the sample is almost completely in the helical form, as in the initial unstrained state. A complete recovery of the initial dimensions of the specimen is observed, upon releasing the tension. Moreover, the total amount of crystallinity does not change during cyclic elengation and recovery. This suggests that the reversible phase transition between form II and form III does not involve the formation of any disordered, intermediate phase, i.e., it is a direct and cooperative process, implying conformational and structural rearrangements of large bundles of close neighboring chains and occurs instantaneously. These data indicate that while the driving force leading the conventional elastomers to recover the initial dimensions is merely entropic, in the case of sPP elasticity is also assisted by the enthalpic gain achieved when the sample is relaxed. When the tension is removed, both the enthalpic factor, due to the structural transition in the crystalline region, and an entropic factor, due to the conformational transition of the chains in the entangled amorphous phase, contribute to the elastic recovery of the fiber. A possible mechanism for the martensitic crystal-crystal phase transition of sPP is suggested
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