Separation of short-chain branched polyolefins by high-temperature gradient adsorption liquid chromatography

Abstract

A new separation principle was recently introduced into the analytical characterization of polyolefins by researchers from the German Institute for Polymers in Darmstadt. It was demonstrated that polyolefins can be selectively separated via high-performance liquid chromatography on the basis of their adsorption/desorption behaviours at temperatures as high as 160 °C. A Hypercarb® column packed with porous graphite gave the best results. The mobile phase consisted of a mixture of 1-decanol and 1,2,4-trichlorobenzene. In this work, the same chromatographic system is applied to the separation of ethylene/alkene and ethylene/norbornene copolymers. It was found that the elution volumes of the samples correlate linearly with the average chemical composition of samples. The elution volume is indirectly proportional to the concentration of branches in the ethylene/alkene copolymer. Branching shortens the length of continuous methylene sequences of the polymer backbone, thus decreasing the probability of orientation of a methylene sequence in a flat conformation on the graphite surface, which enables the most intensive van der Waals interactions between the methylene backbone and the carbon surface. An opposite trend in the elution order has been found for ethylene/norbornene copolymers. The elution volume of the ethylene/norbornene copolymers increased with the concentration of norbornene. It indicates pronounced attractive interactions between graphite and the cyclic comonomer.

High‐Temperature Solvent Gradient Liquid Chromatography of Model Long Chain Branched Polyethylenes

“…In Figure 3b the data for LCB samples are scattered without indicating a correlation with the average molar mass of the samples. This is in agreement with our findings [27] that SCB has a stronger adsorption effect than the molar mass on the retention behavior of LLDPE samples in the system 1-decanol!TCB.…”

“…[26,27] The long chain branches in the model LCB PE are much longer (M n per arm ¼ 10-24 kg/mol $ 700-1700 carbons, Table 1). If the LCB of the model LCB polyethylenes studied here were unbranched, this effect could probably be seen.…”

Recent Advances in High-Temperature Fractionation of Polyolefins

“…Independence of the elution volumes on the molar mass >15-20 kg/mol was also found for PE [164] and EP, EB, EO and ethylene/1-decene copolymers [161]. It is typical for adsorption phenomena that adsorption depends on the shape of a molecule, on positions and numbers of functional groups, on polarizability of the molecule, as well as on the type and nature of the sorbent and the solvents.…”

“…It was found that both comonomers, ethylene and diene, are adsorbed. On the other hand, adsorption of EP [164], ethylene-butene (EB), ethylene-hexene (EH), ethylene-octene (EO) or ethylene/1-decene copolymers depends linearly on the average content of ethylene [161]. This is very practical from the point of evaluation of chromatograms.…”

Ethylene/1‐heptene copolymers as interesting alternatives to 1‐octene‐based LLDPE: Molecular structure and physical properties

“…Though the combination of solvents used in HT-HPLC may differ, the mechanism of separation when the Hypercarb V R stationary phase is used is commonly agreed. 9,[33][34][35] Copolymer chains interact with the stationary phase via van der Waals forces. The strength of the interactions is based upon the length of the ethylene sequences (nE) in the copolymer chain in contact with the stationary phase.…”