High temperature size exclusion-liquid adsorption chromatography (HT-SEC LAC): Full isocratic separation of parent isotactic polypropylene homopolymer from ethylene-propylene copolymers

A simple and highly selective chromatographic separation method named high temperature size exclusion liquid adsorption chromatography (SEC-LAC) was developed for the separation of the parent isotactic polypropylene (iPP) homopolymer from either segmented or random ethylene-propylene co-polymers (EPCs). The compositions of the adsorption promoting and desorption promoting solvents were carefully optimized, so that an intermediate elution mode between total liquid adsorption (LAC) and liquid chromatography at critical conditions (LCCC) of iPP was established. At the same time selectivity in the separation of PPs of different tacticity was not altered. This was encouraging and showed that this approach should be applicable across samples of different stereoregularity. The main advantage of the SEC-LAC method over LAC methods is the completely independent first step SEC analysis of the iPP homopolymers and the subsequent LAC analysis of all other olefin homopolymers and copolymers over a wide range of chemical compositions. Therefore, SEC-LAC combination can overcome the major drawbacks in LAC such as loss of separation selectivity depending on the molar masses of iPP homopolymers. In this way, the present study clearly proves that this separation protocol can discriminate the molar mass heterogeneity of iPP homopolymers present in complex EP copolymer systems without using multidimensional analytical approaches such as high temperature two-dimensional liquid chromatography (HT-2D LC). FTIR measurements were performed to obtain structural information on the separated components for the proper understanding of the separation mechanism. In conclusion, this study clearly shows that SEC-LAC coupled to FTIR is a powerful analytical method that is ideal for quantifying small amounts of iPP homopolymer contaminations in propylene/alpha-olefin copolymers and other multicomponent polyolefin blends. (C) 2015 Elsevier Ltd. All rights reserved.