Tensile Mechanical Properties of Linear High Density Polyethylenes Modified with Organic Peroxide

CLAUDIO JAVIR PÉREZ; GUILLERMO CASSANO; ENRIQUE M. VALLÉS; LIDIA MARÍA QUINZANI; MARCELO DANIEL FAILLA

POLYMER ENGINEERING AND SCIENCE

JOHN WILEY & SONS INC

Lugar: New York; Año: 2003 vol. 43 p. 1624 - 1633

0032-3888

The molecular structure of several high-density polyethylenes having different molecular weight and vinyl content was modified using an organic peroxide in concentrations that did not alter their thermoplastic character.  Chain linking was the main chemical event that occurred during the modification process.  Samples of these polymers were crystallized from the melt generating materials with different morphologies.  Two crystallization procedures were followed: slow cooling and quenching.  The density and crystallinity of the polymers were determined using a density gradient column and a differential scanning calorimeter.  Both properties were found to be slightly dependent on the molecular structures generated by the modification process.  Tensile tests were performed at room temperature to evaluate the mechanical behavior of all the polymers.  The mechanical response of some of the slowly cooled samples changed from brittle to ductile behavior when increasing concentrations of peroxide were added to the formulation.  All the quenched samples displayed ductile behavior.  The elastic modulus and yield stress were found to increase linearly with the crystallinity of the polymers. Furthermore, the relationship between these mechanical properties and the crystallinity is independent of the molecular structure generated by the modification process.  The total crystallinity of the polymers seems to be much more important in terms of its incidence on the modulus and yields stress than other characteristics of the molecular structure generated by the crosslinking process.  In the range of experiments covered in this work the molecular weight of the modified polymers appears to be the main structural property that influences the draw ratio after break and the ultimate tensile stress of the samples.  The draw ratio diminishes, while the ultimate tensile stress increases with the molecular weight of the polymers, irrespective of the evolution of other molecular parameters that are also changed during the chemical modification of the polymers.