Elution behavior of polyethylene and polypropylene standards on carbon sorbents

The elution behavior of linear polyethylene and isotactic, atactic and syndiotactic polypropylene was tested using three different carbon column packings: porous graphite (Hypercarb), porous zirconium oxide covered with carbon (ZirChrom-CARB), and activated carbon TA 95. Several polar solvents with boiling points above 150°C were selected as mobile phases: 2-ethyl-1-hexanol, n-decanol, cyclohexylacetate, hexylacetate, cyclohexanone, ethylene glycol monobutyl ether and one non-polar solvent, n-decane. Polyethylene standards were completely or partially adsorbed in all tested sorbent/solvent systems. Polypropylene standards were partially adsorbed on Hypercarb and carbon TA95, but did not adsorb on ZirChrom-CARB. ZirChrom-CARB retained polyethylene pronouncedly when 2-ethyl-1-hexanol, cyclohexylacetate or hexylacetate were used as mobile phases at temperature 150 or 160°C, while all three basic stereoisomers of polypropylene eluted in size exclusion mode in these sorbent/solvent pairs. This is very different from the system Hypercarb/1-decanol, which separated polypropylene according to its tacticity. The opposite elution behavior of polyethylene and polypropylene in system ZirChrom-CARB/2-ethyl-1-hexanol (polypropylene eluted, polyethylene fully adsorbed) enabled to realize separation of blends of polyethylene and polypropylene. Ethylene/1-hexene copolymers were separated according to their chemical composition using system Hypercarb/2-ethyl-1-hexanol/1,2,4-trichlorobenzene.