n-Alkanes are often an important ingredient of waxes, oils, gasoline, and a byproduct in polyolefins. Separation and identification of alkanes in commercial high-density polyethylene (HDPE) are of vital importance from various points of view. Currently used chromatographic techniques require extraction and laborious preconcentration. Porous graphite Hypercarb as a stationary phase in highperformance liquid chromatography (HPLC) in combination with a mobile phase gradient n-decane → 1,2-dichlorobenzene (ODCB) enables a selective separation of C 40 −C 160 n-alkanes at temperatures between 100 and 160 °C. Thus, it is possible to resolve the individual alkanes in HDPE of 1 kg/mol. An unambiguous assignment of the individual homologues in the chromatogram was possible by doping the HDPE with a C60 standard, and the results were also confirmed by MALDI TOF. Using this method, the presence of alkanes in a HDPE sample with a weight-average molar mass of 70 kg/mol could be shown. n-Hexane as isocratic mobile phase enables to separate and identify n-alkanes in the range C 18 −C 36 in a mixture at a temperature of 50 °C while alkanes with lower carbon numbers are not detected with the evaporative light scattering detector used in this study. The newly developed HPLC system allows separation and identification of alkanes in model blends as well as in high molecular weight HDPE without any prior extraction and work-up. The upper limit of detection with regard to carbon number is superior to the traditionally used gas chromatography.

Thermal gradient interactive chromatography (TGIC) is an emerging analytical tool to determine the chemical composition distribution of polyolefins. TGIC separates polyolefins based on their interaction with graphitic carbon at different temperatures in an isocratic mobile phase. In the case of copolymers of ethylene and 1-olefins, TGIC offers a larger commoner range of separation than crystallization based techniques. However, currently TGIC shows approximately only half of the resolution compared to crystallization based techniques for an ethylene content higher than 92 mol % when 1,2-dichlorobenzene (ODCB) and 1,2,4-trichlorobenzene (TCB) are used as the mobile phase. We conducted a systematic study exploring the potential of using different binary solvent mixtures as the mobile phase to increase the resolution in TGIC of poly(ethylene-stat-1-octene). Apart from the well-established chlorinated aromatic solvents like ODCB and TCB, for the first time, binary solvent mixtures containing alkanes and aliphatic alcohols have been evaluated as components of the mobile phase using an evaporative light scattering detector. As an outcome, optimized binary solvent mixtures could be identified that enabled an improvement in resolution. This was exemplarily verified by separating a model blend of two poly(ethylene-stat-octene) samples with varying 1-octene content

Comprehensively characterizing the relationship between the distributions with regard to chemical composition (CCD) and molar mass (MMD) of polyolefins is vital to establish structure–property relationships. Two‐dimensional high‐temperature liquid chromatography (2D HT‐LC), which couples a high‐temperature solvent gradient interactive chromatography (HT‐SGIC) step for compositional separation with a high‐temperature size exclusion chromatography (HT‐SEC) step for MMD determination in an on‐line manner is emerging as a tool of choice to satisfy this necessity. A serious problem commonly associated with multidimensional chromatography is the very low detector response (DR) due to the dilution in the HT‐SEC step. Using higher sample concentrations or larger sample loops cannot address this appropriately because injections may become irreproducible, or poorly shaped peaks may result due to concentration effects. This paper shows that stacked injections (repeatedly injecting a polymer sample and then starting the desorption step) is a unique way to overcome these difficulties. 2D HT‐LC analysis of blends of ethylene/1‐octene copolymers using stacked injections proves that the DR can be significantly enhanced without co‐elution of the blend components or overloading of the column.