Influence of sample inhomogeneity on the linear elongational viscosities of two low density polyethylenes

TL;DR

This study examines how sample inhomogeneity affects linear elongational viscosity measurements of two low-density polyethylenes, introducing a novel in-situ diameter measurement method to improve accuracy.

Contribution

It presents a new in-situ measurement technique and analyzes the impact of sample non-uniformities on elongational viscosity data for LDPEs.

Findings

Deviations up to 40% in viscosity measurements due to inhomogeneity.

Verification of Trouton relationship for Lupolen 1840 H.

Inhomogeneous deformation linked to initial sample non-uniformity.

Abstract

An experimental investigation of the impact of sample inhomogeneity on measurements of the linear elongational viscosity of two low-density polyethelenes is presented. A novel method of in-situ measurements of the diameter of samples under extension has recently been implemented to properly account for the sample non-uniformities during elongation. Two types of low density polyethylenes (LDPE's) have been investigated: Lupolen 1840 D and LPDE 1840 H. Whereas in the case of Lupolen 1840 H the Trouton relationship is verified, in the case of Lupolen 1840 D the deviations can be as large as 40%, depending on the magnitude of the initial sample non-uniformity and the experimental conditions. Based on real time visualization of the sample, these deviations are associated with an inhomogeneous deformation of the specimen. Differences in the homogeneity of deformation between the H and D samples are explained by significantly different maximal retardation times. The experimental investigation is complemented by a simplified theoretical estimation of the error induced by the sample inhomogeneity in the case of measurements of elongational viscosities in the linear range. A fair level of agreement is found with the experimentally measured error.