Javier Martínez-Salazar

Physical ageing and glass transition in amorphous polymers as revealed by microhardness

Microhardness (MH) data as a function of temperature for two amorphous polymers [poly(methylmethacrylate) and poly(vinylacetate)] and two semicrystalline polymers [poly(ethyleneterephthalate) and poly(arylether ether ketone)] quenched into the amorphous state are presented. It is shown that MH can conveniently detect the glass transition temperature (Tg) for the above mentioned polymers. Molecular rearrangements taking place above and belowTg, such as physical ageing leading to a more compact molecular packing, and thermal expansion can also be followed by means of MH measurements. Finally, the presence of a crystalline phase in these materials has been shown to shift theTg value towards higher temperatures.

Crystallization kinetics and morphology of poly(propylene-stat-ethylene) fractions

The crystallization and melting behaviour and the morphology of fractions of poly(propylene-stat-ethylene) with 2.7–11.0 mol% ethylene were studied by differential scanning calorimetry, wide-angle X-ray scattering, polarized light microscopy and transmission electron microscopy, after etching with permanganic acid. The inclusion of ethylene co-repeating units in isotactic polypropylene (0–11.0 mol% ethylene) caused approximately linear decreases in kinetic and equilibrium melting temperatures and in the glass transition temperature with increasing ethylene content. X-ray scattering showed that the content of the γ form increased with increasing ethylene content, increasing crystallization temperature and decreasing cooling rate from the molten state. It was shown for one of the copolymers (8.7 mol% ethylene) that during heating approximately 50% of the γ form was converted to the α form before the final melting of the sample. The rest of the γ crystals melted without transformation to the α form. The multimodality of the crystal melting above the crystallization temperature in the polymers with a more uniform crystal structure was caused by recrystallization during heating, whereas polymers with appreciable contents of both α and γ forms exhibited multimodal melting at all the heating rates adopted. The size of the low temperature melting peak as assessed at a heating rate of 40 K min−1 was approximately proportional to the initial content of the γ form. By comparison with the spherulitic structure of homopolymers, that of the copolymers was coarser with internal and peripheral pockets of molten material during spherulite growth. The crystal lamellae exhibited more curvature in the copolymer samples than in the homopolymer.

New aspects on the rheological behaviour of metallocene catalysed polyethylenes

Abstract In this article, the linear viscoelastic properties of a family of metallocene-catalysed ethylene homopolymers and ethylene/1-hexene copolymers near the ‘plateau’ and in the terminal regions have been measured. The zero shear viscosity and relaxation time both scale to 3.41 power of the molecular weight, as is observed for most of the linear polymers. At the same time, no rheological signs associated with the presence of long chain branching are detected. However, an unexpected low value of the plateau modulus has been found. This result is discussed in terms of a possible decrease in the entanglement density and/or an increase in chain flexibility with respect to other model polymers. A simple method, previously tested in the literature, based on simplified molecular dynamics, enables a correct description of the dynamic modulus from the knowledge of the form of the molecular weight distribution and the material rheological constants GNo and ηo.

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.

Crystal structure and morphology of melt-crystallized poly(propylene-stat-ethylene) fractions

Abstract Crystallinity, crystal structure and lamellar thickness in melt-crystallized samples of poly(propylene-stat-ethylene) fractions with 2.7–11.0 mol% ethylene comonomer and of approximately constant tacticity were assessed by wide- and small-angle X-ray scattering, differential scanning calorimetry and infra-red spectroscopy. Most of the samples were crystallized under isothermal conditions at 373 K. In comparison with an isotactic homopolymer of polypropylene, the copolymers showed lower crystallinity, melting enthalpy and average length of 3 1 helices, a slightly larger unit cell, a longer long period and an invariant lamellar thickness. The X-ray crystallinity of the copolymers remained approximately constant with increasing ethylene content, whereas the γ-crystallinity increased and the heat of fusion decreased moderately. It is suggested that the ethylene units are partially included in the crystals, and that this causes the invariance in crystallinity and crystal thickness. The observed gradual decrease in average 3 1 helix length with increasing ethylene content as assessed by infra-red spectroscopy is in accordance with this suggestion.

A theoretical study of the comonomer effect in the ethylene polymerization with zirconocene catalytic systems

The PM3(tm) semiempirical method has been used to optimize the structures for the reactants and transition states of the first and second ethylene insertion processes into zirconocene catalytic systems. The results obtained for these reactions are compared with calculations published in the literature performed at different ab-initio theoretical levels. The agreement between our calculations and those reported in the literature is satisfactory. Taking advantage of the reduced computational effort required in semiempirical calculations two additional processes related with the so-called comonomer effect were also studied: ethylene/1-hexene copolymerization, and chain termination reaction, both in the homopolymerization and in copolymerization of ethylene with 1-hexene comonomer. The calculated activation energies support some experimental findings such as the higher polymerization activities in the presence of comonomers and also the molecular weight reduction of the copolymers due to the more favorable β-elimination reactions.

Percolation threshold of conductive polycarbonate/carbon composites as revealed by electron microscopy

Several reports have appeared recently on conductive polymeric composites based on mixtures of nonconductive polymers with conductive solid microadditives [1-4]. Among the various combinations, polymer-carbon-black composites are of special interest because they combine the good application properties of commerical polymers (processability, low cost) with relatively high conductivity, thus offering an interesting type of light material. The low concentration of the additive required to form a conductive network is specially attractive. In a previous letter we have reported [5] on the electrical conductivity (a) variation ofpolycarbonate (PC)/carbon composites as a function of the concentration of the microadditive. The influence of film thickness and temperature dependence on the conductivity level reached was also examined [5]. In these systems the supermolecular structure of the polymer matrix has only a minor influence on the carrier mobility [6, 7]. The conductivity mainly depends on the percolation threshold for the conducting particles. In this system, carriers are transported above the percolation threshold through the carbon network predominantly involving a tunnelling conduction mechanism [6, 8]. The specific type of initial chain-like carbon-black structure, defined through the aggregation of primary particles (few nanometres in size) in strongly bonded aggregates of several hundreds of nanometres and weakly bonded agglomerates of these aggregates of several micrometres, presumably plays an important part in defining the onset of the conductivity threshold [9]. The purpose of the present letter is twofold: first, to report on the influence of the structure of carbon-black particles when transferred over into the polymer matrix on the conductivity level; secondly, to attempt to explain the change observed in the percolation threshold. We have used two types of carbon-black materials: XE2 from Philips Petroleum (A) having a welldeveloped chain-like structure (dibutyl phthalate absorption (DBP) - 400 cc/100 g) and acetylene carbon black from S.E.A. Tudor (B) with a lessdeveloped structure (DBP-~ 140 cc/100 g). Several mixtures of polycarbonate (bisphenol A) from Bayer with different carbon-black concentrations were prepared using a plastograph. Polycarbonate was melted at the working temperature of 230 ° C. Thereafter the filler was added and finally the mixture was mechanically stirred at 50 r.p.m, for 10 m until homogeneous materials were obtained. Compressionmoulded films (T = 230°C, p = 50bar), 500#m thick and 2 x 2cm of lateral dimensions, were

Water-mediated conformations of the alanine dipeptide as revealed by distributed umbrella sampling simulations, quantum mechanics based calculations, and experimental data.

An exhaustive umbrella sampling simulation of the alanine dipeptide in solution is reported. The presence of stable Y conformations in solution is assessed by both quantum calculations and experimental data from X-ray and NMR protein-solved structures available in the protein coil library. The agreement between experimental and simulation Ramachandran plots of the dipeptide in solution is excellent. A suitable explanation of the stabilization of the Y conformation mediated by water for the alanine dipeptide is discussed on the basis of Car-Parrinello MD calculations of the dipeptide in water.

Effect of crystallization temperature on the cocrystallization of hydroxybutyrate/hydroxyvalerate copolymers

Measurements on the effect of crystallization temperature on the crystallinity, long period and unit cell dimensions of P(HB/HV) copolymers with 0–27% HV crystallized isothermally at 23, 52 and 81°C are presented. The crystal composition has been estimated from the unit cell expansion and the relationship between crystallinity and density. The amount of cocrystallization of HV in the PHB crystal structure is found to decrease as the crystallization temperature is increased: the amount of HV in the crystals formed at 81°C is reduced to 23 of the value for crystallization at room temperature. The results are consistent with cocrystallization being an effect of the kinetics of crystallization which becomes less significant as equilibrium crystallization conditions are approached.

On phase separation in high- and low-density polyethylene blends: 1. Melting-point depression analysis

A melting-point depression analysis of high-density polyethylene crystals embedded in molten branched polyethylene is presented. The results indicate that the melting temperature of the crystals is influenced by both the branching content of the low-density polyethylene and the relative volume concentration of the two polymers. The results are critically discussed in terms of the Flory-Huggins approximation for polymer-polymer mixtures.