Juan Francisco Vega

Phase morphology and melt viscoelastic properties in blends of ethylene/vinyl acetate copolymer and metallocene-catalysed linear polyethylene

Abstract The aim of this study was to determine the linear viscoelastic properties of a series of ethylene/vinyl acetate copolymer/metallocene-catalysed polyethylene (mPEs) blends. Newtonian viscosity showed a pronounced positive deviation from the double reptation model, which assumes miscibility or, at least, cooperative relaxation between the mixed species. Enhanced values of steady-state compliance and elastic indices with respect to those of the pure components were also noted. These features are typical of emulsion-like polymer blends and are thought to arise from additional relaxation processes associated with dispersed phase deformability. Application of the Palierne model for emulsions of two viscoelastic liquids showed good agreement with our experimental dynamic results at both ends of the phase diagram. However, the model failed at intermediate compositions. Through the application of several rheological criteria we were able to locate the phase inversion concentration at a weight fraction of w =0.60 in the mPEs. It is suspected that, in this composition range, a fully co-continuous phase develops due to the phase inversion mechanism, which has considerable effects on the viscoelastic properties of the blends.

Structural Insights on the Plant Salt-Overly-Sensitive 1 (SOS1) Na+/H+ Antiporter

The Arabidopsisthaliana Na(+)/H(+) antiporter salt-overly-sensitive 1 (SOS1) is essential to maintain low intracellular levels of toxic Na(+) under salt stress. Available data show that the plant SOS2 protein kinase and its interacting activator, the SOS3 calcium-binding protein, function together in decoding calcium signals elicited by salt stress and regulating the phosphorylation state and the activity of SOS1. Molecular genetic studies have shown that the activation implies a domain reorganization of the antiporter cytosolic moiety, indicating that there is a clear relationship between function and molecular structure of the antiporter. To provide information on this issue, we have carried out in vivo and in vitro studies on the oligomerization state of SOS1. In addition, we have performed electron microscopy and single-particle reconstruction of negatively stained full-length and active SOS1. Our studies show that the protein is a homodimer that contains a membrane domain similar to that found in other antiporters of the family and an elongated, large, and structured cytosolic domain. Both the transmembrane (TM) and cytosolic moieties contribute to the dimerization of the antiporter. The close contacts between the TM and the cytosolic domains provide a link between regulation and transport activity of the antiporter.

Model linear metallocene-catalyzed polyolefins: Melt rheological behavior and molecular dynamics

The linear viscoelastic properties of two families of metallocene-catalyzed polyolefins were determined in the “plateau” and terminal regions. Low values of the plateau modulus GN0 were observed for linear polyethylene relative to values cited for conventional and model hydrogenated polyolefins in several studies. This diminished GN0 value is discussed in terms of a possible decrease in entanglement density compared to that of other polymers synthesized through different polymerization processes [Aguilar et al. (2001)]. For atactic polypropylene the situation becomes more complex, but the values obtained are within the broad range observed in the literature for isotactic and atactic species (0.30–0.86 MPa) [Aguilar et al. (2003)]. When the viscoelastic response is inferred from molecular data, the result may vary depending on the physical entanglement state. It would thus appear that great care should be taken to ensure that measurements are performed on samples in suitably entangled reference states. The...

Viscoelastic behaviour of metallocene-catalysed polyethylene and low density polyethylene blends: Use of the double reptation and Palierne viscoelastic models

The linear viscoelastic behaviour of a series of mPE/LDPE blends was evaluated. Newtonian viscosity showed slight positive deviation from the double reptation model, which assumes miscibility or, at least, cooperative relaxation between the mixed species. This feature is typical of emulsion-like polymer blends and is attributable to additional relaxation processes that occur at low frequencies associated with deformability of the dispersed phase. Close agreement between data obtained by applying the Palierne model for emulsions of two viscoelastic liquids and experimental data was only shown for the blend of lowest LDPE content. Good predictions were derived for the higher LDPE contents, when partial miscibility of the components was assumed. In these cases, the viscoelastic response can be modelled through a hybrid model that takes into account the double reptation approach in the miscible phase models. Contrary to other heterogeneous blends, the blends examined here do not show an enhanced elastic character. This behaviour is probably due both to the extremely different relaxation times of the phases in the blends rich in mPE, and to the partial miscibility of the components in the blends of high LDPE content.

Effect of molecular weight distribution on Newtonian viscosity of linear polyethylene

A strong effect of the third moment of the molecular weight distribution on the melt linear viscoelastic response of linear polyethylene samples is reported. The Newtonian viscosity of samples with broad molecular weight distributions shows a noticeable increase with respect to model linear samples with the same weight-average molecular weight but symmetric and narrower molecular weight distributions. These experimental results are in agreement with previous empirical and theoretical studies based on reptation concepts that predict a dependence of the viscosity in terms of the Mz/Mw ratio. In addition, it is possible to obtain a description of the whole linear viscoelastic fingerprint of the samples by mapping the entanglement parameters obtained from atomistic simulations together with the molecular features into the reptation model. The results shown here are of great importance, as the Newtonian viscosity–molecular weight relationship has become a widely used tool, not only to test the molecular weight dependence, but also to assess the possible presence of long-chain branching in polyolefins.

Effect of short chain branching in molecular dimensions and Newtonian viscosity of ethylene/1-hexene copolymers: matching conformational and rheological experimental properties and atomistic simulations

The molecular dimensions in dilute solution and the linear viscoelastic melt properties of a series of linear ethylene/1-hexene random copolymers with variable short chain branching content are reported. The results obtained from size exclusion chromatography and viscosimetry in dilute solution show a molecular contraction as the branching level increases. Additionally, a clear dependence of the Newtonian viscosity with the short chain branching content at T = 463 K is obtained. Both experimental observations are in agreement with previous experimental results found in ethylene/α-olefin copolymers, but more interestingly with recent full atomistic simulations made in our group in this type of macromolecular systems. The dependences observed can be related to the changes observed in the macromolecular conformational features and also in the equilibration entanglement time and the molecular weight between entanglements as the number of short chain branches increases.

Assessment of entanglement features and dynamics from atomistic simulations and experiments in linear and short chain branched polyolefins

We report on full atomistic computer simulations of the molecular dimensions and dynamics of molten entangled linear and short chain branched polyethylene chains of the same length with butyl branches content up to 55 per 1000 carbon atoms. This has been achieved by using a number of molecules equilibrated in all length scales by advanced Monte Carlo moves and Molecular Dynamics simulations. The simulations have been compared with published experimental results. With the increase in branching the simulations show: a chain contraction and higher flexibility of polymer chains; a higher effective step length of the primitive path and a slowing down of the chain dynamics. Furthermore the time evolution of the mean-square inner segments displacement is in quantitative agreement with the results obtained by incoherent neutron spin echo experiments for a monodisperse polyethylene sample with a similar molecular architecture. Finally, an anomalous subdiffusive behaviour of the centre of mass displacement is also predicted by the simulation, in accordance with recent theoretical approaches.

Size and conformational features of ErbB2 and ErbB3 receptors: a TEM and DLS comparative study

ErbB2 and ErbB3 receptors belong to the epidermal growth factor receptor family. The members of this family are able to form homo- and heterodimers that trigger diverse downstream signalling concerned to multiple cellular events. In the absence of a ligand, ErbB3 adopts a characteristic tethered conformation, which differs from ErbB2 extended conformation. In this work, transmission electron microscopy (TEM) and dynamic light scattering (DLS) have been used to characterize the conformational features and the size of ErBb2 and ErbB3 receptors. Two main objectives are presented. The first one is to evaluate the use of TEM as a tool for structural studies for this family of receptors. The low molecular weight of these proteins represents a challenging purpose for TEM studies. The other one is to search for a relationship between the results obtained by TEM and those obtained for the hydrodynamic size measured by DLS. This comparison has allowed us to identify the conformational differences of the receptors and to anticipate the use of these experimental techniques for the study of the ligand activated heterodimerization, a process related to a significant number of human malignancies.

Temperature and branching dependence of surface extrusion instabilities in metallocene catalysed polyethylene

We have found a branching dependence of critical extrusion temperature in metallocene-catalysed linear and branched polyethylene, below which the commonly found surface distortion instabilities disappear. The elimination of these extrusion instabilities has been observed in a wide range of temperatures up to 23°C above the melting point of each polymer. However, in contrast to previous observations, a window of minimum flow resistance, i.e., extrusion pressure, is not detected. This low temperature effect is discussed in terms of a flow ordered phase at the wall that induces local chain orientation. This so-called mesophase would prevent the mechanism for the formation of distortions to operate below a critical temperature.

Eliminating sharkskin distortion in polyethylene extrusion via a molecular route

This study examines the linear viscoelastic properties and processing of blends of a metallocene polyethylene with different ultrahigh molecular weight polyethylenes (UHMWPEs). Blend compositions were prepared such that they were below and above the UHMWPE coil overlap concentration, c∗. Linear rheology and processing were found to be very sensitive to molecular weight and concentration, suggesting an effective macromolecular network threshold. The linear viscoelastic behavior of the blends could be described using a simple model based on reptation, implying a segmental interaction of the macromolecules. Extrusion flow curves were identical for the pure material and blends. However, for a given blend composition, the amplitude of the sharkskin distortion diminished as the UHMWPE molecular weight in the blend increased, and completely vanished at a value corresponding to the coil overlap condition. For a given molecular weight of the UHMWPE, it was also possible to obtain distortion free extrudates simply ...