Poster: Improved long-chain branch detection in LDPE via exteded Mark-Houwink calibration

By Esther López, Alberto Ortín, Laura Santonja-Blasco. Polymer Char, Valencia, Spain.

Improved Long-Chain Branch detection in LDPE via extended Mark-Houwink calibration with a broad linear HDPE

Abstract:

To analyze polyethylene with different structures, gel permeation chromatography with triple detection is the method of choice. The viscometer and light scattering detectors allow direct measurement of the true molecular weight of the sample and study the presence of long-chain branches (LCB) by means of the Mark-Houwink plot.
To quantify the presence of LCB, several requirements must be met: 1) A linear reference material is needed. 2) Only materials of equal chemistry should be compared. 3) The molar mass distributions of the branched and linear polymers should overlap in the region of interest.
From a practical point of view, before analyzing a branched polymer (e.g., LDPE), the common methodology involves analyzing a material with no branches (linear PE) to construct the reference Mark-Houwink plot. Unfortunately, the molar mass range of the extendedly used certified linear PE sample is rather low, and its distribution is relatively narrow—contrary to requirement 3)—making it unsuitable. Therefore, it is of interest to find an alternative linear reference sample to expand the applicable range.
This study addresses a real need by presenting molecular weight moments and distributions of several linear references to illustrate the available options, including a sample with broader molar mass distribution and a significant high molar mass fraction. The absence of branching was demonstrated by the close overlay of the Mark-Houwink plots of such linear references. Furthermore, for most accurate long chain branching detection in LDPE samples it is advantageous to select as linear reference the one with broadest molar mass distribution due to the largest overlap of their Mark-Houwink plots.
In addition to the accurate characterization of LCB that this proposed linear reference offers, extended characterization of other properties was conducted, including parameters such as intrinsic viscosity in various solvents and short-chain branch content, to support its use as reference material for other calibrations in GPC or related techniques.