Characterization of branched ultrahigh molar mass polymers by asymmetrical flow field-flow fractionation and size exclusion chromatography

Abstract

The molar mass distribution (MMD) of synthetic polymers is frequently analyzed by size exclusion chromatography (SEC) coupled to multi angle light scattering (MALS) detection. For ultrahigh molar mass (UHM) or branched polymers this method is not sufficient, because shear degradation and abnormal elution effects falsify the calculated molar mass distribution and information on branching. High temperatures above 130 °C have to be applied for dissolution and separation of semi-crystalline materials like polyolefins which requires special hardware setups. Asymmetrical flow field-flow fractionation (AF4) offers the possibility to overcome some of the main problems of SEC due to the absence of an obstructing porous stationary phase. The SEC-separation mainly depends on the pore size distribution of the used column set. The analyte molecules can enter the pores of the stationary phase in dependence on their hydrodynamic volume. The archived separation is a result of the retention time of the analyte species inside SEC-column which depends on the accessibility of the pores, the residence time inside the pores and the diffusion ability of the analyte molecules. The elution order in SEC is typically from low to high hydrodynamic volume. On the contrary AF4 separates according to the diffusion coefficient of the analyte molecules as long as the chosen conditions support the normal FFF-separation mechanism. The separation takes place in an empty channel and is caused by a cross-flow field perpendicular to the solvent flow. The analyte molecules will arrange in different channel heights depending on the diffusion coefficients. The parabolic-shaped flow profile inside the channel leads to different elution velocities. The species with low hydrodynamic volume will elute first while the species with high hydrodynamic volume elute later. The AF4 can be performed at ambient or high temperature (AT-/HT-AF4). We have analyzed one low molar mass polyethylene sample and a number of narrow distributed polystyrene standards as reference materials with known structure by AT/HT-SEC and AT/HT-AF4. Low density polyethylenes as well as polypropylene and polybutadiene, containing high degrees of branching and high molar masses, have been analyzed with both methods. As in SEC the relationship between the radius of gyration (R_g) or the molar mass and the elution volume is curved up towards high elution volumes, a correct calculation of the MMD and the molar mass average or branching ratio is not possible using the data from the SEC measurements. In contrast to SEC, AF4 allows the precise determination of the MMD, the molar mass averages as well as the degree of branching because the molar mass vs. elution volume curve and the conformation plot is not falsified in this technique. In addition, higher molar masses can be detected using HT-AF4 due to the absence of significant shear degradation in the channel. As a result the average molar masses obtained from AF4 are higher compared to SEC. The analysis time in AF4 is comparable to that of SEC but the adjustable cross-flow program allows the user to influence the separation efficiency which is not possible in SEC without a costly change of the whole column combination.

Introduction

Ultrahigh molar mass (UHM) polymers have typically a weight average molar mass 〈M_w〉 of above 500 kg/mol. Especially ultrahigh molar mass polyethylene (UHMPE) is superior to traditional materials with regard to mechanical stability and specific weight and, therefore, used for special applications such as ultra-strong fibers, implants or toothed wheels. For the development of new products made from UHMPE, knowledge of the molar mass distribution and the chain structure is extremely important. These parameters influence the mechanical properties of the final product, the morphology as well as the rheological behavior of the melt [1].

The most common method for the molar mass determination of semi-crystalline polyolefins is high temperature size exclusion chromatography (HT-SEC) [2], [3], [4], [5], [6]. Amorphous materials like polybutadiene can be analyzed with SEC at ambient temperature [7], [8], [9]. In SEC the separation of the polymers takes place in a porous stationary phase. Unfortunately, the polymers, particularly those with very high molar masses, may undergo shear degradation during the chromatographic separation process [2], [10], [11], [12], [13], [14], [15], [16], [17]. Furthermore, branched chains often co-elute with smaller linear ones which then prevents a correct analysis of the chain structure [18], [19], [20], [21], [22], [23], [24], [25], [26]. Field-flow fractionation (FFF), which was discussed by Giddings et al. [27], [28], [29] for the first time in the 1960s, has the potential to overcome the above mentioned drawbacks of SEC for the analysis of very large molecules. The separation in asymmetrical flow field-flow fractionation (AF4) is provided by a cross-flow perpendicular to the solvent flow as it is shown in Fig. 1.

The solvent flow passes the empty channel and forms a parabolic velocity profile which is deformed towards the accumulation membrane like it is visible in Fig. 1. The cross-flow leaves through a semi-permeable membrane. The inlet streams are always higher than the cross-flow and will automatically be adjusted if the cross-flow changes, e.g., if a gradient is applied, to provide a constant outlet stream through the detectors. As a consequence of the cross-flow a field force perpendicular to the carrier-flow is formed which depends on the cross-flow velocity. As a result of this field force the analyte molecules or particles will move towards the accumulation membrane. The material of the membrane is mostly cellulose for ambient temperature and ceramic for high temperature use. The arrangement of the analyte at the membrane causes a concentration gradient over the channel cross-section which leads to an increased diffusion of the analyte molecules. The equilibrium between movement induced by the field-force and the diffusion leads to an arrangement of the analyte molecules or particles in different channel heights. The position of the molecules is influenced by the cross-flow velocity, the carrier flow velocity and the diffusion coefficients of the analyte structures. As the ability to diffuse depends on hydrodynamic size, i.e., small molecules diffuse faster than larger ones of similar structure, the large molecules will be situated closer to the membrane where the flow velocity is lower due to the parabolic-shaped flow velocity profile in the channel (Fig. 1). As a result the polymer molecules will elute according to their size beginning with the smallest and fastest moving molecules if all species have the same chain structure and constant chemical composition [27], [28], [29], [30], [31], [32], [33]. Only a narrow zone above to the accumulation membrane contains analyte molecules, while the remaining layers contain pure solvent.

AF4 has been applied for characterization of large biological molecules like proteins [34] or vesicles [35], as well as synthetic polymers like polystyrene [36], styrene–butadiene-rubber [37] and polyacrylamide [38]. The dissolution of semi-crystalline polyolefins requires temperatures above 130 °C and chlorinated solvents and, therefore, the AF4 has to be carried out at these harsh conditions. The first high temperature FFF separation of PS was done by Giddings et al. [39]. The authors mentioned the possibility to separate PE by HT-FlowFFF but did not report any results. Several years later, Mes et al. [40] described a successful separation of polyolefins with HT-AF4. The first commercial instrument for HT-AF4 was developed in cooperation with Postnova Analytics (Landsberg/Lech, Germany) and Polymer Laboratories (Church Stretton, England).

In this paper the applicability of AF4 for the characterization of polyolefins and polybutadienes with partially high degrees of branching and high molar masses is shown. The complete differential molar mass distributions from AF4-MALS are presented for the samples, which were never published for PE material before. The AF4-measurements are compared with SEC-separations using standard conditions and standard columns like they are used in many laboratories all over the world with the aim to show the huge error which can occur easily in molar mass and branching determination with SEC–MALS. A huge variety of theoretical papers dealing with FFF is present in the literature, but until now only a few papers exist which demonstrate the successful use of AF4 for the analysis of polymers with complex chain structures and the additional information which is accessible by AF4-MALS. The new results from AF4-MALS presented in this work, e.g., the correct information on branching and the visualization of ultrahigh molar mass material in LDPE and branched PB, offer the possibility to figure out new structure–property-relationships for those important samples.