title: Development of new chromatographic methods for the characterization of the chemical composition distribution of ethylene propylene diene terpolymers
Creators: name: Deshmukh, Subrajeet
Publisher: name: UNSPECIFIED
Year: 2023

description: Ethylene-propylene-diene terpolymers (EPDM) belong to the class of polyolefin elastomers and are, by volume, the most important elastomers for non-tire applications. In EPDM terpolymers, the monomers are randomly incorporated, resulting in their amorphous and elastic character. The extremely versatile end-use properties, such as elasticity, filler acceptance, ozone and UV resistance, are complemented by a good cost/performance ratio. These benefits are driving the use of EPDM in a number of novel and diverse applications. The versatility described can be achieved because the molecular heterogeneities can be controlled through advances in catalyst and process technology. At the same time, however, it also requires the development of comprehensive, precise analysis techniques for their molecular characterization. The most commonly used dienes are 5-ethylidene-2-norbornene (ENB), dicyclopentadiene (DCPD) and vinylnorbornene (VNB). When incorporated into the polymer chain by coordination polymerisation, they provide a double bond which then allows the material to be vulcanised (eg with sulphur, peroxides or phenolic resins) after polymerisation. According to their commercial importance, the dienes can be classified as follows: ENB &gt; DCPD &gt; VNB. ENB is the most widely used diene because it is commercially most efficiently cured with sulfur. VNB shows outstanding peroxide vulcanization efficiency, while DCPD, the cheapest of all non-conjugated dienes used in EPDM production, shows slightly higher peroxide vulcanization efficiency than ENB. Size exclusion chromatography (size exclusion chromatography, SEC) is an important tool for determining the molar mass distribution (molar mass distribution, MMD) and can, with the use of a suitable detector, also be used to determine the distribution of a comonomer along the polymer molar mass. The results can provide insight into the polymerization mechanism and help to develop an understanding of structure-property relationships. As shown in various works, the influence of the catalyst structure on the comonomer distribution in linear low-density polyethylene could be elucidated by coupling SEC and FTIR. EPDM's SEC-FTIR has also been introduced. SEC-IR is a standard method for determining EP ratio, but is not suitable for determining diene content. The LC transform approach for SEC-IR is basically suitable for determining the double bond content, but its use is tedious and not conceivable in a routine context. This opens a technological gap that could potentially be closed by -SEC-UV. As far as is known, there are very few studies on the UV absorption of polymers containing isolated and non-conjugated double bonds. In the first chapter it is shown how a UV detector coupled to the SEC can be used to study the behavior of EPDM terpolymers. For this purpose, the influence of important experimental parameters on the UV absorption of ethylidene norbornene (ENB), vinyl norbornene (VNB) and dicyclopentadiene (DCPD) is evaluated. The results are then used to determine the ENB content along the MMD. be adjusted by varying the average molecular weight (MM) and the chemical composition (CC), as well as varying the distributions on which the two variables are based (MMD and CCD). In the past, the CCD of polyolefins was often analyzed using crystallization-based methods such as elution fractionation with temperature increase (temperature rising elution fractionation, TREF), analytical crystallization fractionation (crystallization analysis fractionation, CRYSTAF) and crystallization elution fractionation (crystallization elution fractionation, CEF). These methods are based on the crystallization of macromolecules from a dilute solution, which in turn is related to the polymer composition, and therefore cannot be applied to amorphous polymers. Liquid adsorption chromatography (liquid adsorption chromatography, LAC) using porous graphitic carbon (porous graphic carbon, PGC) as a stationary phase, has been developed into a reliable method for the determination of the CCD of polyolefins and has also proven valuable for the characterization of amorphous, olefin-based elastomers. LAC separates macromolecules based on their selective adsorption on a graphitic surface at a specific mobile phase temperature and composition. The selective adsorption can either be performed isothermally (by varying the composition of the mobile phase, so-called solvent gradient interaction chromatography interactive chromatography, SGIC) or isocratic (by varying the temperature, so-called interaction chromatography with thermal gradient interactive chromatography, TGIC). Several research groups have been looking for solvents for the LAC of polyolefins. Nevertheless, there is no comprehensive knowledge of the influence of the chemical structure of the adsorption and desorption promoting solvents on the chromatographic behavior of polyolefins. In addition, the LAC of EPDM terpolymers is still a largely unexplored area. Therefore, in the second part of this work, a rational approach for the selection of solvents for the adsorption chromatography of EPDM terpolymers is developed. First, a series of benzene derivatives are used as the mobile phase for the separation of EPDM terpolymers on PGC. Connections are made between the molecular structure of the aromatic solvents and their influence on the retention of EPDM on the graphitic surface. Finally, a method is developed to make solvent selection for EPDM liquid chromatography more efficient. Between the SEC and LAC limiting cases, entropic effects and enthalpic interactions cancel each other out at some point. This is known as (liquid chromatography at critical conditions, LCCC) and macromolecules containing identical repeating units elute independently of their molar mass, so that a separation according to other molecular parameters can be achieved. Various research groups have looked at the use of LCCC to analyze block copolymers by using CC for one of the two homopolymers. However, there are no studies that present a holistic approach to the identification of CC for random copolymers. In the third part of this work, CC for random ethylene-propylene copolymers (EP copolymers) of different chemical composition are developed. To do this, suitable solvent candidates are selected using a recently published approach using structure-retention relationships and Hansen solubility parameters. As the CCD of EPDM terpolymers remains a largely unexplored area, it is expected that the determined CC will provide a helpful tool for the diene content -based separation and characterization of EPDM terpolymers The relationship between the MMD and the other molecular heterogeneities of polymers (e.g. CCD x MMD) can be studied by coupling HPLC and SEC. This concept, also known as two-dimensional liquid chromatography (2D - LC), was developed to characterize polymers and has been presented in detail in other works. In most cases, 2D LC results were presented as contour plots and compared qualitatively. The relative volumes of areas in different contour plots were compared to provide quantitative information on the composition of samples. However, this approach does not provide quantitative information on proportions present in both samples (i.e., species that have identical molecular weight and chemical composition), nor on areas present in only one of the two samples being compared (different or unique proportions). In the case of 2D-LC-NMR, information on the molecular heterogeneities in different polymers was also obtained in the form of contour plots. In the last chapter of this work, EPDM copolymers with different average chemical composition will be analyzed using HT 2D-LC/IR and a method to quantify the identical and unique proportions in the samples will be described. The contour plots corresponding to these EPDM copolymers are prepared and the matrices corresponding to these contour plots are used for quantification.