The effect of molecular weight and crystallite structure on yielding in ethylene copolymers

Nominal stress-strain curves of a series of random ethylene-hexene copolymers having narrow composition and most probable molecular weight distributions were investigated. A series of such molecular weight copolymers with a constant concentration of branches were crystallized under a variety of conditions. In each molecular weight series the level of crystallinity was approximately constant. Particular attention was focused on the yield region and the nature of the yielding process. It was found, quite surprisingly, that the yield stress was not solely dependent on the crystallinity level. Moreover, the shape of the force-elongation curve in the yield region was very dependent on the molecular weight and the crystallization mode. These changes in yielding correlated quite well with the overall crystallite structure that was characterized by thin section transmission electron microscopy. The orthorhombic unit cell of polyethylene was maintained in all the samples despite the changes that occurred in the overall crystallite structure. © 1997 John Wiley & Sons, Inc.     

Role of matrix crystallinity in carbon nanotube dispersion and electrical conductivity of iPP‐based nanocomposites

A homopolymer iPP and a series of propylene‐ethylene random copolymers with a content of ethylene from 7 to 21 mol % were used as matrices to prepare single‐walled carbon nanotube (SWCNT) nanocomposites in a range of SWCNT concentration from 0.15 to 1 wt %. The solution blending and melt‐ compression molding procedures were kept identical for all nanocomposites. The poly(propylenes) have crystallinities ranging from 70 to 10%, and serve to test the role of SWCNTs acting as nucleants to preserve in the nanocomposites the uniform dispersion of SWCNTs after sonication. The major role of polymer crystallinity is to mediate toward a more open and more connected SWCNT network structure. Fast nucleation and growth of high crystalline matrices on multiple sites along the surface of the nanotubes prevents SWCNT clustering, and entraps the SWCNT network between the semicrystalline structure reducing the driving force of nanotubes to curl and twist. Depletion of crystallites in the less crystalline matrices (<35% crystallinity) leads to curled and poorly connected nanotubes. A consequence of the gradual loss of SWCNT connectivity is a decreased electrical conductivity; however, the change with crystallinity is not linear. Conductivity decreases sharply with decreasing crystallinity for SWCNT contents near the percolation region, while for contents approaching the plateau region the electrical conductivity is less sensitive to matrix crystallinity. The percolation threshold decreases rapidly for polymers with <～30% crystallinity and slowly levels off at crystallinities >～40%. At SWCNT concentrations of 0.15 wt %, SEM images of nanocomposites with the highest crystallinity matrix indicate debundled and interconnected nanotubes, whereas more disconnected and curled SWCNTs remain in the lowest crystallinity nanocomposites. Electrical conductivity in the former is relatively high, whereas the latter are insulators. Also discussed is the nucleating effect of nanotubes and restrictions of the filler to polymer chain diffusion in the crystallization of the polymers. SEM images and Raman spectra in the radial breathing modes region (100–400 cm^?1) are complementary tools to extract the quality and details of the SWCNT dispersion in the nanocomposites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2084–2096, 2010     

Zirconium phospho-sulfates with NaZr2(PO4)3-type structure

Two new crystalline zirconium phospho-sulfates, α- and β-Zr₂(PO₄)₂(SO₄) were prepared by the gel method followed by drying and calcining at controlled temperatures. Their X-ray patterns were indexed and their infrared spectra interpreted. They belong to the NaZr₂P₃O₁₂ or [NZP] structural family, and constitute the first example with S⁶⁺ in the tetrahedral site.     

Crystallization and Melting of Model Polyethylenes with Different Chain Structures

The crystallization and melting of three model polyethylenes of different chain structures have been studied. The polymers studied were a linear copolymer, hydrogenated poly(butadiene); a hydrogenated poly(butadiene)-atactic poly(propylene) diblock copolymer; and a three-arm star hydrogenated poly(butadiene). An important feature of this work was that the crystallizing portions of the copolymers all have the same molecular lengths.It was found that the overall crystallization rate decreases steadily from a linear to a diblock to the star copolymer. The differences in crystallization rates are related primarily to the activation energy for segmental transport. The non-crystallizable structure affects the segmental mobility to different degrees. An estimation of this effect is presented from the analysis of the overall crystallization rates using classical nucleation theory. In spite of thedifferences in their molecular structure, there are no major differences in the supermolecular structure of samples crystallized rapidly or slowly cooled.The melting process followed by DSC of the isothermally crystallized linear and star copolymers shows two endothermic peaks at intermediate undercoolings. The double melting is associated with a partitioning of crystallizable ethylene sequences during crystallization. The longest sequences are preferentially selected in the early stages of the crystallization. Single melting peaks are obtained for high and very low undercoolings for the linear and the star copolymers as well as for the diblock in the whole range of temperatures. The lack of the second, lower melting endotherm in the diblock could be associated with the influence in the crystallization process of the amorphous block in the microphase segregated melt.This revised version was published online in November 2005 with corrections to the Cover Date.     

Fissile core and Tritium-Breeding Blanket: structural materials and their requirements

High radiation resistant structural materials for fusion and fission nuclear power plants are a key issue for the development of both types of reactors. Selection criteria, elements of metallurgy of the selected materials, and the major issues as they are revealed by the results of the present development programmes, are presented. At low temperature (300 °C) ferritic/martensitic steels are suffering from He-embrittlement, associated with possible hardening due to α/α^′ unmixing. The kinetics of hardening and embrittlement versus dose, especially saturation with dose, are still open key issues, difficult to settle on the basis of a purely experimental programme. Important progress is still to be made in mastering the initial microstructure, inclusion cleanness and joining techniques of oxide dispersion strengthened steels for higher heat resistance. Physics modeling as presented in this issue should promote guidance to the understanding of the mechanisms involved, provide solutions to master the initial microstructure and phase stability, and mitigate the in-service property degradation. To cite this article: J.-L. Boutard et al., C. R. Physique 9 (2008).     

Relativistic Kinematic Honeycombs

In this paper, we study the honeycombs of the relativistic velocities space using the fact that this space has a hyperbolic space structure. For the three-dimensional case there are only four of these honeycombs with bounded cells and, since in this space there is a characteristic length, the cells have a fixed size. This fact, strongly limit the possible relative velocities between the vertices of the honeycombs. We consider as a possible application to cosmology the interpretation of these honeycombs as discrete large scale mass distribution.     

Separation of short-chain branched polyolefins by high-temperature gradient adsorption liquid chromatography

A new separation principle was recently introduced into the analytical characterization of polyolefins by researchers from the German Institute for Polymers in Darmstadt. It was demonstrated that polyolefins can be selectively separated via high-performance liquid chromatography on the basis of their adsorption/desorption behaviours at temperatures as high as 160 °C. A Hypercarb? column packed with porous graphite gave the best results. The mobile phase consisted of a mixture of 1-decanol and 1,2,4-trichlorobenzene. In this work, the same chromatographic system is applied to the separation of ethylene/alkene and ethylene/norbornene copolymers. It was found that the elution volumes of the samples correlate linearly with the average chemical composition of samples. The elution volume is indirectly proportional to the concentration of branches in the ethylene/alkene copolymer. Branching shortens the length of continuous methylene sequences of the polymer backbone, thus decreasing the probability of orientation of a methylene sequence in a flat conformation on the graphite surface, which enables the most intensive van der Waals interactions between the methylene backbone and the carbon surface. An opposite trend in the elution order has been found for ethylene/norbornene copolymers. The elution volume of the ethylene/norbornene copolymers increased with the concentration of norbornene. It indicates pronounced attractive interactions between graphite and the cyclic comonomer.     

Solution and thermal properties of poly(1,3-dioxocane)

Poly( 1,3-dioxocane) was synthesized by cationic ring-opening polymerization with triphenyl-methane hexafluoroantimoniate as the initiator and was studied with regard to its solubility, unperturbed chain dimensions, and thermal transitions. The intrinsic viscosity and Flory-Huggins interaction parameter were used to determine the solubility parameter, δ_p = 9.6 cal^1/2cm^?3/2, a value that agrees with that calculated empirically. Fractions were obtained from the solvent/non-solvent system benzene/methanol at 25°C. The number-average molecular weight M_n and intrinsic viscosity [η] were measured in toluene at 25°C. The relation [η] = 1.45₉. 10^?4 M_n^0.79 was found. A value of 5.3 was obtained for the characteristic ratio 〈r²〉₀/nl². Results are correlated with the main thermal transitions of this polyformal.