Optimization and calibration of high-temperature liquid chromatographic separation of polypropylene and propylene based copolymers

Polyolefins are, by volume, the most significant group of polymers in the world, accounting for more than 50 % of the global polymer market, with Polypropylene (PP) and propylene based copolymers being the two significant representatives within the polyolefin family. Modern catalyst and reactor technologies have made it possible to control the molecular heterogeneities (i.e., Chemical Composition Distribution (CCD) and Molar Mass Distribution (MMD)) in polyolefins. Slight variations in these molecular metrics may lead to significant improvements in the final properties of these materials. It is state of the art to determine the MMD of the polymers by using Size Exclusion Chromatography (SEC). SEC separates the macromolecules according to their hydrodynamic volume in the solvent, which is an indirect measurement of the molar mass of these macromolecules. As polyolefins are generally not soluble at room temperature elevated temperatures are required to carry out SEC, which is referred to as High Temperature SEC (HT-SEC). Copolymers are made of more than one monomer, hence the CCD becomes a crucial molecular metric to derive structure-property relationships. It is state of the art to use crystallization based techniques i.e., Crystallization Analysis Fractionation (CRYSTAF), Crystallization Elution Fractionation (CEF) and Temperature Rising Elution Fractionation (TREF) to determine the CCD of semi-crystalline polyolefins. The crystallization based techniques utilize the relationship between comonomer content of macromolecules and their crystallizability from the hot dilute solution. However, crystallization based techniques have limitations to separate amorphous copolymers and also suffer from the effects of co-crystallization. Recently, High Temperature Liquid Adsorption Chromatography (HT-LAC) has emerged for the separation of less crystalline or even amorphous olefin based copolymers (e.g., Ethylene-Propylene (EP) copolymers). Porous Graphitic Carbon (PGC), commercially available as HypercarbTM, has been widely used as stationary phase in HT-LAC polyolefins. Liquid Chromatography at Critical Conditions (LCCC) is an intermediate mode of separation, existing between SEC (entropy driven process) and LAC (enthalpy driven process), where the entropic effects of the separation compensates the enthalpic effects of separation. In this work potential mobile phases for LCCC of Isotactic Polypropylene (it-PP) and Syndiotactic Polypropylene (st-PP) have been identified. Three different methods, namely, Bashir’s, Cools’ and the SEC-LAC plot approach, have been used to determine critical conditions for polypropylene at 160 °C. 2-octanol and 2-ethyl-1-hexanol (2E1H) have been used as adsorption promoting solvents and 1,2-dichlorobenzene (ODCB) and 1,2,4-trichlorobenzene (TCB) as desorption promoting solvents. From the obtained data it was observed that the concentration of TCB required to desorb it-PP is lower compared to that of ODCB in the mobile phase composition at LCCC of it-PP. In the same sense 2-octanol was found to be a slightly weaker eluent compared to 2E1H. The effect of temperature on the critical conditions of it-PP has been probed and the dependency between the fractions of desorption promoting solvent and temperature was found to be linear. A new method to determine conditions for LCCC has been proposed, which requires only a single polymer sample of high molar mass (similar to Bashir’s method), yet at the same makes it possible to identify LCCC at isocratic conditions. This new method does not depend on a series of polymer samples with well defined shape of elugrams or precise values of elution volume. This new method was applied to determine conditions for LCCC of it-PP and st-PP in 2-octanol/ODCB and 2E1H/ODCB, and of Polyethylene (PE) in n-decane/TCB. The use of a solvent gradient in HT-LAC makes it difficult to use a quantitative detector (e.g., Infra-Red (IR) spectroscopy) to monitor the effluent from the chromatographic column as most of the adsorption promoting mobile phases are not IR transparent. Hyphenation of HT-LAC with HT-SEC (HT-LAC x HT-SEC), also known as High Temperature Two Dimensional Liquid Chromatography (HT 2D-LC), has been used to study the bivariate CCD x MMD of polyolefins. HT 2D-LC offers the potential to use an IR detector (HT 2D-LC-IR) after the 2nd dimension of the separation as the latter uses a quasi-isocratic flow of the mobile phase. In this work, a model high impact PP (PP-Hi i.e., it-PP/ EP equal to 50/ 50 wt. %) has been separated into its constituents by applying LCCC for it-PP in the first dimension of HT 2D-LC-IR. The semi-crystalline it-PP eluted in the LCCC mode, while the amorphous EP portion was adsorbed on the stationary phase and eluted when the eluent was changed to pure desorption promoting solvent (ODCB). In this way the need of applying a linear solvent gradient for deformulation of PP-HI has been eliminated, thus enabling a separation by using an isocratic pump. EP copolymers produced with different catalysts may contain unique (i.e., of different molar mass and chemical composition) and identical (i.e., of similar molar mass and chemical composition) segments in varying amounts. Until now no method existed to quantify these unique and identical segments. A new method (Matrix approach – explained in next paragraph) for the quantitative evaluation of identical and unique segments present in EP copolymers has been proposed using HT 2D-LC-IR. As first step, the parameters of HT 2D-LC-IR were optimized with the aim to eliminate the interference of the solvent peak with that of the eluting polymer. An HT-LAC flow rate of 0.05 mL/min and a HT-SEC flow rate of 2.5 mL/min were chosen as optimum to separate EP copolymers according to their CCD x MMD. Calibrations of the elution in HT-SEC and LAC dimension and of the detector response were carried out. Therefore, a new method for calibrating the IR response with respect to the injected mass of polymer was developed. An advantage of this new method is that only a single injection is needed, which reduces the sources of errors and giving a lower value for the limit of quantification (0.003 mg/ 100 µL). Generally, the data obtained from HT 2D-LC-IR analyses of polymers are plotted as contour plot. Transforming these into mathematical matrices allows to perform mathematical operations (e.g., addition and subtraction) with the aim to investigate differences in the molecular heterogeneities of polymers in a quantitative manner. The matrix approach has been applied not only to obtain information about the overall fractions of identical and unique segments in EP copolymers, but also to determine the average molar mass and chemical composition of these segments. Such quantitative data are a prerequisite to establish structure-property relationships for copolymers. The quest for the discovery of alternative stationary phases, which might show improved selectivity compared to HypercarbTM is of utmost interest for analytical scientists working in the field of HT-LAC. It is hypothesized that stationary phases, which exhibit an Atomic Level Flat Surface (ALFS) similar to HypercarbTM, are the most suitable candidates (Boron Nitride (BN) and Molybdenum Disulfide (MoS2)) for the adsorption of polyolefins. In this work, these stationary phases in combination with different mobile phases have been probed for HT-LAC of polyolefins. The order of elution (it-PP < at-PP < st-PP < PE) with these stationary phases has been found to be similar to that of HypercarbTM in the tested mobile phases, yet the general extent of adsorption on the tested stationary phases was found to be weaker compared to HypercarbTM.