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Studying the interactions of polyethylene with graphite in the presence of solvent by high temperature thermal gradient interactive chromatography, thermal gradient nuclear magnetic resonance spectroscopy, and solution differential scanning calorimetry

: Mekap, D.; Malz, F.; Brull, R.; Zhou, Z.; Cong, R.; Groo, A.W. de; Parrott, A.R.


Macromolecules 47 (2014), Nr.22, S.7939-7946
ISSN: 0024-9297
ISSN: 1520-5835
Fraunhofer LBF ()

High temperature thermal gradient interactive chromatography (HT-TGIC or TGIC) has emerged as an important analytical tool to determine the comonomer content distribution (CCD) present in copolymers of polyethylene (PE). The method separates the macromolecules according to the differences in their interactions, in solution, with a graphite stationary phase using temperature as a variant. It is vital to gain insight into the nature of these interactions in order to further improve the separation resolution and increase the accuracy of CCD measurement. Nuclear magnetic resonance (NMR) spectroscopy has been routinely applied for quantitative determination of comonomer content in olefinic copolymers. In this paper NMR has been adapted in a unique manner to monitor the temperature-dependent concentration of PE in solution in the presence of nanographite (NG). It was found that in the presence of NG a decrease in polymer concentration can be observed for both a semicrystalline PE homopolymer and a highly amorphous poly(ethylene-stat-1-octene) with 37 wt % 1-octene content (E/O-37), upon reducing the temperature. The reduction observed for the homopolymer started at a temperature significantly above its crystallization temperature from solution. A similar reduction was also observed for the highly amorphous E/O-37. This decrease was found to be reversible upon heating, with the absence of any hysteresis which gives an evidence for an adsorption/desorption mechanism in the system polyolefin/graphite/ODCB. Solution DSC experiments carried out at comparable conditions showed no exothermic event at the temperature where the decrease was observed in TG-NMR, thus excluding induced crystallization as the underlying mechanism. As a result, the mechanism of interaction between the graphite surface and the macromolecules can be concluded to be based on adsorptive interactions. The temperature of the reduction in concentration of PE homopolymer and E/O-37 observed in TG-NMR matched the elution temperature in TGIC under similar experimental conditions which confirmed the TGIC separation mechanism as based on adsorption without any influence of induced crystallization. Another interesting finding was the mobility of PE macromolecules observed at temperatures significantly below the crystallization point in dilute solution by H-1 NMR.