Leonor Pérez-Trejo

Inlet instabilities in the capillary flow of polyethylene melts

Inlet instabilities in the capillary flow of polyethylene melts were studied in this work. Extrudate distortions in branched polyethylenes, produced by unstable upstream flow, were found to be accompanied by pressure oscillations that do not have their origin in the slip phenomenon, but on polymer compressibility. The absence of slip was clearly evidenced in the experiments, and the differences between pressure oscillations occurring in linear and branched polymers are shown.Pressure oscillations in the capillary flow of branched polyethylenes were found to be made up of at least two components of different frequency and amplitude. These two components were identified with different bulk defects appearing in the extrudates. Information about the dynamics of vortices upstream of the contraction and extrudate distortions is obtained from the analysis of pressure oscillations.The influence of capillary entrance angle on flow curves was also investigated. From the results, it is concluded that the extensional component of the flow in the contraction is the main factor responsible for the slope change usually found in the log-log flow curves of both linear and branched polyethylenes.

About the determination of the steady state flow for polymer melts in capillary rheometers

The different parameters influencing the time required to reach steady state flow in polymer melts in a capillary rheometer were investigated. The parameters of the rheometer considered were the contraction ratio, the capillary diameter and its length to diameter ratio. The influence of polymer compressibility and viscosity were also taken into account. It was found that the time required to reach steady state flow for the studied polymers increased when the amount of material in the reservoir, the viscosity of the melt and the contraction ratio for a given L/D capillary ratio were increased and vice versa. It is shown that the main characteristic of the rheometer that determines the time to reach a steady flow is the contraction ratio and not the capillary diameter. This result is explained on the basis of polymer compressibility, its viscosity and the mechanical work done on the system, which depends on the amount of material in the barrel.

JERARQUIZACIÓN DE COMPETENCIAS ESPECÍFICAS EN EL PROGRAMA DE FÍSICA DE LA ESCUELA SUPERIOR DE FÍSICA Y MATEMÁTICAS DEL IPN-MÉXICO UTILIZANDO LA MATRIZ DE MORGANOV-HEREDIA

In this paper, the so-called Morganov-Heredia matrix was employed to obtain a hierarchy of competencies for physics graduates in Physics at the Higher Scholl of Physics and Mathematics of the National Polytechnic Institute in Mexico. The study is done in the context of the effort of the institution to implement Competence-Based Educational Model on its curricula. The Morganov-Heredia matrix allowed identifying and grouping competences by their characteristics, such as labor disciplinary and social competences. Also, the matrix allowed organizing the proposed competences from the most basic one to the final competence goal.