Polyolefins Microstructure Characterization by Automated Cross-Fractionation Chromatography (CFC) - - Chromatography Online
Polyolefins Microstructure Characterization by Automated Cross-Fractionation Chromatography (CFC)


The Application Notebook


Alberto Ortín Benjamín Monrabal and Pilar del Hierro, Polymer Char, Valencia, Spain.

Introduction

Polyolefin technology has made remarkable progress during the last few decades through the development of new technologies, which allow more control of the polymer structures by the product designers, so high value products are produced with varying and increasing microstructure complexity. Polymer characterization provides information about the molecular structure, which builds a bridge between polymer properties and polymerization conditions.1

In most industrial polyolefins there are essentially two molecular parameters of interest: the molar mass distribution (MMD) and the chemical composition distribution (CCD), which are manipulated to impart desired properties to the final product. The MMD tells how much material of a certain molar mass is present. The CCD tells how much material of a certain composition, or comonomer content, is present.

Separate information on MMD and CCD, which are routinely determined by size exclusion chromatography/gel permeation chromatography (SEC/GPC) and temperature rising elution fractionation (TREF) or crystallization analysis fractionation (CRYSTAF)2 respectively, although important and in many cases sufficient, are not enough to fully characterize a Polyolefin resin and the full bivariate distribution is required.3


Figure 1: Contour plot of the CCD × MMD and individual distributions as measured by TREF and GPC/SEC.
Hyphenated techniques, such as SEC-FTIR and TREF-LS that only perform the fractionation along one molecular axis and determine some average of the other, have become popular over the last few years,1 although they result in some information loss, the extent of which depends on the method employed and on the shape of the CCD × MMD surfaces of the materials (Figure 1).4 Thus, it is generally preferable to access the full CCD × MMD, which has been traditionally a challenging and time consuming task.

With the introduction by Polymer Char of the CFC automated analytical-scale instrument for polyolefin cross-fractionation, it is now possible to fully characterize complex comonomer/molar mass distributions on the order of hours instead of days and without the need of intermediate manual steps in a benchtop instrument.5 Very high resolution can be achieved in both dimensions by using optimized analytical columns, analysis conditions and sample sizes.

Experimental

The instrument is based on a high resolution analytical TREF apparatus combined with a dedicated GPC/SEC columns oven and equipped with five vessels for sample preparation so up to five can be analysed sequentially. Polymer detection is made through an IR4 infrared detector for maximum sensitivity in polyolefin applications, which also provides excellent long-term baseline stability.

The polymer sample is placed in solid form (40–200 mg) into a stainless steel vessel, where it is dissolved in a proper solvent, o-DCB or TCB, at temperatures of 140–160 °C for 60–90 minutes under stirring. The instrument works by loading the polymer solution into the TREF column where it is crystallized by cooling down the TREF oven and then eluting the fractions in step-wise temperature increments towards the GPC columns.


Figure 2: GPC/SEC chromatograms collected at 23 different fractionation temperatures, showing both the molar mass and the compositional heterogeneity of the analysed PE blend.
A series of chromatograms are collected at the different fractionation temperatures (Figure 2). Those chromatograms describe the molar mass distribution of every compositional fraction. The relative area of the individual chromatograms reflects the compositional heterogeneity while their retention time and shape relates to the molar mass dimension.

Experimental conditions may be varied to optimize resolution and signal quality depending on the chemical composition distribution, by setting any number and arrangement of temperature fractionation points as required: evenly distributed in all the range every 2 °C or 3 °C for a typical Z-N LLDPE sample having broad CCD, or concentrating more fractionation points every 1 °C in a narrow range around 85–95 °C for HDPE, which tends to have quite narrow composition distribution. Sample size is also optimized according to the fractionation conditions to prevent overloading of the SEC/GPC columns while keeping a good enough signal from the IR detector.

An SEC/GPC columns calibration curve is established so that MMD and molar mass moments are calculated at every elution temperature. The relative weight fraction is obtained from the area under each chromatogram and it is combined with the individual fractions MMD to obtain the full CCD × MMD distribution for the sample. All the data processing and calibration is performed within a dedicated and comprehensive software package, which additionally provides the TREF profile and MMD for the whole sample.


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