Separation of ethylene-vinyl acetate copolymers by high-temperature gradient liquid chromatography

Ethylene-vinyl acetate (EVA) copolymers can be semicrystalline or amorphous materials, depending on their chemical composition. A variety of different methods were used for the analysis of the chemical composition distribution of these copolymers, which in general were time- and labor-consuming and could be applied only for a limited range of compositions. In the present work a novel chromatographic method is presented that can be used for chemical composition analysis regardless of the composition of the copolymer. High-temperature gradient HPLC has been found to be suitable for chemical composition separation of semicrystalline and amorphous EVA copolymers. In addition, separation is achieved from the respective homopolymers. We have found that gradients of 1,2,4-trichlorobenzene/cyclohexanone, decalin/cyclohexanone, and decalin/1-decanol enable the selective elution of the copolymers from silica gel at 140 degrees C. The EVA copolymers elute in dependence of their content of the polar vinyl acetate comonomer. Full adsorption and desorption of the samples controlled by the gradient could be achieved for all compositions. Coupling of the gradient HPLC system with FTIR spectroscopy through a LC-transform interface confirmed the chemical composition separation and revealed the chemical heterogeneity of the copolymers. Conditions for liquid chromatography at critical conditions (LCCC) have been established for poly(vinyl acetate) at 140 degrees C.