Increase of long-chain branching by thermo-oxidative treatment of LDPE: Chromatographic, spectroscopic, and rheological evidence

Low-density polyethylene was thermo-oxidatively degraded at 170°C, i.e., degraded in the presence of air, by a one thermal cycle (1C) treatment during times between 30 and 90min, and by a two thermal cycles (2C) treatment, i.e., after storage at room temperature, an already previously degraded sample was further degraded during times between 15 and 45min. Characterization methods include gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, as well as linear and nonlinear rheology. A reduction of molar mass was detected for all degraded samples by GPC, as well as an increase of the high molar mass fraction of the 1C sample degraded for the longest time. Intrinsic viscosity measurements indicate also a reduction of molar mass with increasing degradation times for both 1C and 2C samples. Thermo-oxidation is confirmed for 1C and 2C samples by analyzing specific indices in FTIR. Linear viscoelasticity seems to be in general only marginally affected by thermo-oxidative exposure, while the enhanced strain-hardening effect observed in uniaxial extension experiments presents a clear evidence for an increased long-chain branching (LCB) content in both 1C and 2C samples. Elongational viscosity data were analyzed by the molecular stress function (MSF) model as well as the Wagner-I model, and for both models, quantitative description of the experimental data for all samples was achieved by fit of only one nonlinear model parameter. Time-deformation separability was confirmed for all samples degraded, 1C as well as 2C, for cumulative degradation times of up to 90min. The characterization by GPC was confronted with the characterization obtained from nonlinear rheology. It can be stated that elongational rheology is a powerful method to detect structural changes due to thermo-oxidative degradation, especially the formation of enhanced LCB. It has the further advantage that experimental data can be quantified by a single nonlinear model parameter of constitutive equations like the MSF or the Wagner-I model. © 2013 The Society of Rheology

Rheological characterization and constitutive modeling of two LDPE melts

V této praci byly analyzovány experimentální reologické data dvou tavenin LDPE pomocí MSF modelu. Bylo zjištěno, že pro kvantitativní modelování obou toků bylo nutné použít pouze tři nelineární viskoelastické materiálové parametry.Experimental data of two low-density polyethylene (LDPE) melts at 200 degrees C for both shear flow (transient and steady shear viscosity as well as steady first normal stress coefficient) and elongational flow (transient and steady-state elongational viscosity) as published by Pivokonsky et al. [1] were analyzed by use of the Molecular Stress Function (MSF) model for broadly distributed, randomly branched molecular structures. For quantitative modeling of melt rheology in both types of flow and in a very wide range of deformation rates, only three nonlinear viscoelastic material parameters are needed: While the rotational parameter, a(2), and the structural parameter, beta, are found to be equal for the two melts considered, the melts differ in the parameter f(max)(2), describing maximum stretch of the polymer chains.SIP-IPN [20082971]; Mexico, by Grant Agency of the Czech Republic [A200600703]; Ministry of Education of the Czech Republic [MSM 7088352101]; German Science Foundation (DFG

Evaluate the nonlinear viscoelasticity of polyolefin melts using the Larson model

[[abstract]]Nonlinear rheological properties of polyolefins have long been studied and predicted by using the Larson model with the damping function generally obtained from the stress relaxation measurements. In this study, we investigate the nonlinear rheological properties of high density polyethylene (HDPE) and polypropylene (PP) using the Larson model with damping functions obtained from either the dynamic frequency sweep or the stress relaxation test. Experimental measurements and their corresponding model predictions for the rheological parameters were then compared to evaluate the applicability of the Larson model to the nonlinear rheology, and the following conclusions could be achieved. The steady shear viscosity could be satisfactorily described by the Larson model with the damping functions obtained from the two different methods, except at shear rates higher than 103 s-1. The predicted first normal stresses also account for the measured data, except for those using the stress relaxation data showing a little deviation for the PP sample. In addition, the predictions for elongation viscosity are also in good agreement with the experimental results within the short range of elongation rate achieved in this work.[[notice]]補正完畢[[incitationindex]]SCI[[incitationindex]]EI[[booktype]]紙本[[booktype]]電子

Psicopatología en el cine paraguayo: una primera aproximación

Introducción: el cine puede ser una herramienta útil para la educación médica. En la Psiquiatría, específicamente, el cine permite explorar las representaciones de diferentes trastornos mentales, cómo es vivir con una enfermedad mental, el estigma, la ética médica, la relación psiquiatra/paciente, entre otros. El objetivo de este estudio fue determinar la presencia de algún tipo de psicopatología en películas producidas en Paraguay, con el fin de que las mismas puedan ser utilizadas como herramientas didácticas en la enseñanza de la Psicopatología. Metodología: el diseño de esta investigación fue de tipo cualitativo exploratorio. Se seleccionaron, por conveniencia, 10 películas paraguayas, producidas entre los años 2010 y 2019. Las películas se eligieron tomando en cuenta su disponibilidad de visualización en las plataformas Netflix, Amazon Prime Video, HBO GO, Apple TV+, YouTube, y en formato DVD. Resultados: se analizaron en busca de Psicopatología las siguientes películas Universo Servilleta (2010), Cuchillo de Palo (2010), 7 cajas (2012), La Enamorada (2012), Latas Vacías (2014), La Chiperita (2015), Los Buscadores (2017), Leal (2018), Las Herederas (2018), y El Supremo Manuscrito (2019). En las películas seleccionadas se encontraron personajes con signos y síntomas que pueden resultar útiles para enseñar algunos de los principales síndromes psiquiátricos: trastornos depresivos, ansiedad, intentos de suicidio, rasgos obsesivo-compulsivos, trastornos de la personalidad, ideas delirantes, alteraciones del control de los impulsos y adicciones. Conclusión: el cine paraguayo, cada vez más rico y diverso, además de ser una fuente de inspiración y entretenimiento, puede ayudar en la enseñanza de la Medicina en general, y de la Psiquiatría en particular

From reactor to rheology in industrial polymers

This article reviews current efforts towards quantitative prediction of rheological properties of industrial polymer resins, based upon their polydisperse branched molecular structure. This involves both an understanding of how reactor and reaction conditions influence the distribution of chain lengths and branch placement (which is the province of reactor engineering) and an understanding of how the molecular structures in turn give rise to the rheology (the province of polymer physics). Both fields are reviewed at an introductory level, focussing in particular on developments in theoretical prediction of rheology for both entangled model polymers and industrial polymers. Finally, we discuss three classes of reaction for which the fields of reactor engineering and polymer physics have been truly combined to produce predictions from reactor to rheology