Lidia M. Quinzani

Rheological study of linear high density polyethylenes modified with organic peroxide

Abstract Four high density polyethylenes were modified using different concentrations of an organic peroxide in order to change their molecular structure. The effects of the presence of vinyl groups in the original polymer molecules and of the peroxide concentration used in the modification process were analyzed. All the concentrations of peroxide used in this study were below the critical concentration that produces a macroscopic molecular network. The weight-average molecular weight of all the polyethylenes augments and the molecular weight distribution gets wider as the concentration of peroxide increases. These results support the general belief that the chain-linking reactions dominate the modification process. Evidence of the important role played by the vinyl groups is found not only in the change of the width of the chromatograms but also in the position of their maximums. The vinyl-containing polymers display the largest molecular changes for a given peroxide content. The magnitude of the viscous and elastic moduli of the polyethylenes goes up as the concentration of peroxide used increases showing the effect of the generated large molecules. The linear viscoelastic response of the modified polymers is thermo-rheologically complex. This complexity can be associated with the generation of branched molecules. For similar molecular weights and peroxide concentration, the flow activation energy displayed by the polyethylenes with larger concentration of vinyl groups is larger. This result suggest that a much more complex molecular structure is formed in the presence of vinyl groups. The dynamic moduli of the polymers were analyzed using the generalized viscoelastic model. The spectrum of relaxation times was determined for each polymer and analyzed as a function of the peroxide concentration.

Rheological and barrier properties of nanocomposites of HDPE and exfoliated montmorillonite

Polyethylene (PE)/clay nanocomposites were prepared by melt mixing using PE grafted with maleic anhydride (PEg) as compatibilizer. Concentrations between 2 and 15 wt% of an organophilic montmorillonite (MMT) and concentration ratios of 1:1, 2:1 and 3:1 of PEg/MMT were employed. The materials were characterized using X-ray diffraction, scanning electron microscopy (SEM) and thermogravimetry. The SEM images show that the presence of PEg results in a large degree of exfoliation at all clay concentrations. For 5 wt% MMT, the best degree of exfoliation is obtained for a 2:1 ratio of PEg/MMT. This ratio results in higher increase in the elastic modulus, mainly at low frequencies, with respect to that of the corresponding matrix. As the clay concentration increases, for a 2:1 ratio of PEg/MMT, the dynamic moduli increase showing pseudo solid-like behavior at clay concentrations higher than 8 wt%. Moreover, the nanocomposites show rheological properties that are affected by annealing at 200°C signaling further exfoliation or improved platelet and tactoid distributions. The oxygen permeability of PE decreases gradually with the clay concentration, reaching a maximum reduction of ∼30% for 15 wt% MMT.

Structure of partially hydrogenated polybutadienes

Abstract An analysis of the molecular structure of partially hydrogenated fractions of a monodisperse polybutadiene has been carried out in order to study the nature of the heterogeneous hydrogenation process. The polybutadiene was synthesized by polymerization of butadiene with t -butyl lithium in a non-polar medium. This reaction produces linear polibutadiene chains with about seven 1,2 addition units every 100 structural units. The catalytic hydrogenation of the polybutadiene fractions was done either in cyclohexane with Pd/CaCO 3 catalyst or in heptane with Pd/BaSO 4 catalyst. The reaction time was varied in order to obtain materials with different amount of unsaturations. The resulting polymers were characterized by gel permeation chromatography, differential scanning calorimetry, and infra-red spectroscopy. Solvent extraction using n -hexane was also performed over thin films of several samples of hydrogenated polybutadienes. The resulting fractions of the non-extracted material, and of some of the extracted ones, were analysed using infra-red spectroscopy. The results show that the hydrogenation reaction generates a bimodal distribution of polymer species for global conversions lower than approximately 89%. The catalytic reaction takes place in such a way that, in each adsorption step, approximately 89% of the unsaturated groups of each adsorpted molecule are hydrogenated. This process generates a mixture of polymers that are either highly hydrogenated poly(ethylene-1-butene-butadiene) copolymer or non-hydrogenated polybutadiene. The resulting blends are immiscible at all the studied global conversions (12–89%).

Effect of montmorillonite on the crystallization and thermal degradation of poly(propylene-co- ethylene-co-1-butene) nanocomposites

Polymeric nanocomposites based on poly(propylene-co-ethylene-co-1-butene) (PEBC) were elaborated by melt mixing using an organophilic montmorillonite (o-MMT) and maleated PEBC (PEBCg) as compatibilizer. The effect of clay concentration, PEBCg:o-MMT ratio, and grafting degree of the compatibilizer were studied. X-ray diffraction and scanning electron microscopy show formation of partially intercalated structures in all compatibilized composites with well-distributed small tactoids. According to the differential scanning calorimetry results, the anhydride groups of the compatibilizer have a marginal nucleating effect, while the o-MMT causes a slight decrease in the crystallization temperature of the polymer. PEBC presents the largest activation energy of crystallization (Eα), while the composites show lower Eα than their matrices. It is also observed that the rate of degradation of PEBC is not affected by the presence of PEBCg. The nanoclay, on the other hand, retards the decomposition process of the polymeric matrix in about 40°C and augments its rate of degradation approximately four times.

Effect of the Thermal History on the Thermal and Rheological Behavior of a Thermotropic Polyester

The biphasic behavior and phase transitions of a thermotropic main-chain liquid crystalline polyester, the poly(oxytrimetilen-etilen glycol p,p′-bibenzoate), was studied by X-ray diffractometry, differential scanning calorimetry (DSC), and rotational rheometry. The liquid crystalline mesophase structure of the polymer evolves from Smectic C to Smectic A at around 100°C and changes to the isotropic state at about 200°C. The annealing of polymer samples at different temperatures within the smectic–isotropic transition gives rise to double endotherm and exotherm peaks on the heating and cooling DSC traces. These peaks are attributed to the preferential segregation of the lower molecular weight molecules from the liquid crystalline to the isotropic phase in the biphasic zone. The rheological studies are consistent with this result. The dynamic moduli measured at constant frequency in temperature ramps using polymer samples with different thermal histories reveal that the type of annealing processes applied in the biphasic zone generates different evolution of the rheological data.

Linear viscoelasticity of blends of polybutadiene and highly hydrogenated polybutadiene

The linear viscoelastic behavior of a series of partially hydrogenated polybutadienes (PHPB) obtained by catalytic hydrogenation of an almost monodisperse polybutadiene (PB) was studied experimentally at temperatures above 110 °C. The molecular characterization of the polymers used in this study showed that the catalytic hydrogenation of PB produces materials with a bimodal distribution of polymer species for global conversions lower than approximately 90%. All the PHPBs, even the less hydrogenated, are a mixture of a highly hydrogenated PB (with approximately 89% saturation) and unreacted PB. The rheological behavior of the fluids was studied measuring the elastic and viscous moduli at different temperatures in small-amplitude oscillatory shear flow. The two-phase polymer blends show thermorheological complex behavior and an increase in elasticity at low frequencies which may be associated with long relaxation time processes. The dynamic moduli of the pure components in the terminal region is modeled usi...