Development of High Temperature Liquid Chromatography for Chemical Composition Separation of Polyolefins

Polyolefins are the world’s most widely used synthetic polymers. Their global production has grown at a substantial rate over the past few years and is expected to continue to do so in future as new technologies and synthesis procedures are constantly developed to satisfy commercial demands. Polyolefins, like any other synthetic polymer, can display various types of molecular heterogeneities. Measuring these is the key to develop structure property relationships and thus to tailor the properties of the end products. Molar mass distribution (MMD) and chemical composition distribution (CCD) are the two basic molecular parameters which are of pivotal interest in the case of polyolefins, as they have the strongest impact on the properties of the final products. Liquid chromatography is an important tool to characterize molar mass and chemical composition heterogeneities present in copolymers or polymer mixtures which are soluble at ambient temperature. While size exclusion chromatography (SEC) is routinely used to measure the MMD, interactive high performance liquid chromatography (HPLC) has been widely used to characterize the CCD of polymers. HPLC may potentially provide a fast approach to characterize the CCD of polyolefins without some of the drawbacks existing in established techniques like Temperature Rising Elution Fractionation (TREF) or Crystallization Analysis Fractionation (CRYSTAF) which are based on crystallization. However, many polyolefins require temperatures > 100 °C to be dissolved due to their semi-crystalline nature. In addition to the limited choice of solvents to dissolve polyolefins, there is insufficient understanding about solute interactions with the sorbent and solvents. All these constraints make it very challenging to identify new sorbent/solvent systems for HPLC of polyolefins. With the aim to identify new sorbent/solvent systems for high temperature (HT)-HPLC of polyolefins new mobile and stationary phases were tested in this work. Considering the ability of Hypercarb to fractionate polyolefins with regard to their composition and microstructure the focus was to probe alternative carbon-based sorbents in combination with polar solvents (aromatic solvents and alcohols) as mobile phase regarding their capability to separate polyethylene (PE) and polypropylene (PP) standards. The adsorption properties of carbon based sorbents for polyolefins were compared in a systematic way and quantitative information about the adsorption of the polymer on to a sorbent was collected to better understand the influence of solvent, molar mass and microstructure of the polymer on its adsorption. Particular attention was given to the influence of molar mass on the elution of polymer and the capability of chromatographic system to discriminate the stereoisomers in the case of PP, as these are the most prominent features of carbon based sorbents. It was found that PE and/or PP adsorb to different extent, depending on the sorbent/solvent pair used. The nature of the carbon sorbent strongly influences the extent of adsorption with Hypercarb showing a stronger affinity for adsorption than other carbon-based sorbents. When a solvent gradient alcohol (C7 – C9) →1,2,4-trichlorobenzene (TCB) was used at 160 °C all stereoisomers of PP were selectively adsorbed and desorbed and the retention of the stereoisomers increased with the polarity of the alcohol. The developed new chromatographic systems were used for the chemical composition separation of several olefin copolymers for some of which HPLC was never used before. Separation of ethylene/1-olefin copolymers demonstrated that short chain branches (SCB) play an important role in adsorption of polyolefins. It was observed that the SCB hinder the retention of the polymer molecule. Interestingly, in the case of 1-octadecene as comonomer the drop in adsorption is relatively less compared to the copolymers with equivalent comonomer containing carbon units C8 or less. This indicates that the SCB itself also contributes to adsorption of polymer when it is sufficiently long. For the first time amorphous ethylene-propylene (EP) copolymers and ethylene propylene diene terpolymers (EPDM) were separated according to their chemical composition using HT-HPLC. Hypercarb in combination with a solvent gradient 1-decanol TCB selectively interacted with the ethylene units and consequently the EP copolymers elute in the gradient according to their ethylene content. In the case of EPDM, the retention volume was a function of the content of both ethylene and ethylidene-2-norbornene (ENB). The contribution of ENB alone to the chromatographic retention was quantified by calculating the difference in elution volume between the EPDM and an EP-copolymer having an equivalent content of ethylene. Studying the separation of EPDM with other dienes indicated that the contribution of the diene to the retention varies with its nature. Complete molecular heterogeneity, i.e. the relationship between CCD and MMD of EPDM was unravelled for the first time by hyphenating the HPLC separation with SEC. HT-HPLC is a new technique for the separation of PE, PP and other polyolefins according to the chemical composition. The presented work enlarges substantially the number sorbent/solvent systems suitable to realize HT-HPLC of polyolefins. Some sorbent/solvent pairs described in this work have a potential to be applied for a routine analytical and/or preparative LC separation of polyolefins. For example, the system Hypercarb/2-octanol→TCB enables to separate ethylene-propylene copolymers according to their chemical composition in a range of 0 – 100 wt. % of ethylene. In contrast to the separation techniques TREF and CRYSTAF, HT-HPLC requires a smaller amount of samples, solvents and short time for analysis. Moreover, as HT-HPLC is based on selective adsorption and desorption, amorphous as well as semicrystalline polyolefin samples can be selectively separated.