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

Abstract

SynopsisLow-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 90 min, 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 45 min. 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 90 min. 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 a) Author to whom correspondence should be addressed; electronic mail: victor.h.rolongarrido@tu-berlin.de

A hierarchical multi-mode MSF model for long-chain branched polymer melts part I: elongational flow

“…5 Comparison of elongational viscosity data (symbols) of LDPE III to LDPE VI measured by RME rheometer (Bastian 2001), as well as LDPE 1840D (Rolón-Garrido et al 2013), Escorene and Bralen (Pivokonsky et al 2006) measured by SER rheometer and characterized in Tables 2, 3 and 4 with predictions of HMMSF model (continuous lines) using the RME rheometer, as well as LDPE 1840D (Rolón-Garrido et al 2013), Escorene and Bralen (Pivokonsky et al 2006) measured by the SER rheometer. The elongational viscosity data of LCB-PP (Wagner et al 2006) measured by the MTR are shown in Fig.…”

“…The comprehensive aspects of this commercial rheometer, as well as the rheological theories behind it, have been explained elsewhere (Meissner and Hostettler 1994;Narimissa et al 2015). Rolón-Garrido et al (2009, 2013 and Pivokonsky et al (2006) conducted their uniaxial elongational measurements of Lupolen 1840D, Escorene and Bralen through Sentmanat Extensional Rheometer (Sentmanat 2004) universal testing platform from Xpansion Instruments, i.e. models SER2-P and SER-HV-A01, respectively.…”

“…The elongational rheological modellings of this study were conducted on different grades of low-density polyethylene (LDPE) melts previously characterized and tested by Bastian (2001), Rolón-Garrido et al (2013) and Pivokonsky et al (2006), as well as long-chain branched polypropylene (LCB-PP) melt characterized by Wagner et al (2006). Tables 1, 2 and 3 display the weight average molecular weight (M w ), number average molecular weight (M n ), polydispersity index (M w /M n ), melting temperature (T m ), room temperature density (ρ RT ), testing temperature density (ρ T ), activation energy (E a ), dilution modulus G D obtained by modelling of the elongational data and the linear viscoelastic relaxation spectrum of LDPE I to LDPE VI at their testing temperature.…”

Size exclusion chromatography of photo-oxidated LDPE by triple detection and its relation to rheological behavior

“…The experimental techniques applied were the solvent extraction method, rheology and Fourier transform infrared (FTIR) spectroscopy. In contrast to thermo- [9] and thermo-oxidative degradation [9,10], the linear viscoelastic response of the samples, measured in small amplitude oscillatory shear flow, was found to be strongly affected by the photooxidative degradation [7]. The rheological data obtained in the nonlinear regime, i.e.…”

Investigation of thermal degradation of branched polypropylene via rheology

“…In order to stabilize the polymer processing, it is very important to understand all possible structural changes of the particular polymer due to thermal degradation. It has been shown that rheology can be used as the powerful tool to understand degradation of different polymers due to its sensitivity to very small changes in the molecular structure of polymers as well as to understand their molecular structure via utilization of advanced constitutive equations, which are based on the molecular arguments [1][2][3][4][5][6][7][8][9][10]. With the aim to extend the knowledge in this area, thermal stability of branched polypropylene was investigated in this work by using basic rheological tool and methodologies.…”