Analytical TREF

As the data in Fig. 7 suggests there is certainly room for improvement in terms of the resolution achieved by preparative TREF. It is anticipated that future developments in the preparative TREF area will concentrate on strategies for achieving greater fractionation efficiency. 

Anal3Ttical TREF is a relatively recent development and there are only a few reports describing in detail the systems used [8], [9], [10], [12]. As noted earlier the instrumentation used is derived directly from the already well-developed SEC. The pumps and detector systems are the same for both SEC and analytical TREF. However, because of the nature of the separation in TREF there is a different sample preparation step, and some device is needed to provide the necessary temperature rise control. Also if one wants to express the separation temperature as a comonomer content (or short chain branching) a calibration is needed which requires different standards from those used for SEC calibration. 

An analytical TREF system was first described by Wild and Ryle [8], Their system (Fig. 8) used components taken from a Waters model 200 size exclusion chromatograph. The solvent reservoir, degasser and pump from the Waters unit was used as was the refractive index detector system. In place of the SEC oven a temperature programmed oil bath provided the temperature gradients. A 0.2 g polymer sample was loaded in a hot trichlorobenzene solution into a small column packed with 40-60 mesh Chromosorb P. Crystallization was achieved by slow-cooling the polymer solution (0.2 g in 5 ml) in the packed column at a rate of 1.5 K/hour down to room temperature. The temperature rising elution was carried out at a flow rate of 6 ml/min and a rate of temperature rise of 8 K/hour. The refractive index response and the separation temperature were recorded continuously on a two-pen recorder. A calibration curve of methyl content vs. elution 

In subsequent publications further refinements of this system were described [9], [14], These included a reduction in both column and sample size to improve resolution. The column dimensions were reduced to 7 x 0.9 cm and the sample size of polymers was 20 mg loaded in 3 ml of solvent. A temperature programmed gas chromatography oven was used to replace the less convenient oil bath to provide a more reproducible temperature gradient. The detector was also changed to an infra-red (IR) detector which provides adequate sensitivity with much improved base-line stability due to the relative insensitivity of the IR detector to temperature fluctuations. The resultant TREF curves clearly demonstrated the much improved resolution achievable by these refinements. 

More recently Kelusky et al. [11] described a system which included similar experimental improvements to those described by Wild et al. Their system also incorporated a liquid chromatography oven as an alternate way of imposing a temperature program on the column system. The use of an air oven is effective due to the small size of the column system which eliminates the normal problem of heat transfer in an oven system. The advantage over an oil bath system is the convenience and ease of changing sample column as well as the decreased 

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Fig. 20 Analytical TREF profiles of a random copolymer, an OBC, and a blend of HDPE with ultra-low density polyethylene (ULDPE) of similar overall densities [46]...

Fig. 25 Comparison of solution solubility by analytical TREF of the blend and diblock OBC...

The solvents used in analytical TREF are limited to chlorinated solvents, mainly ort/io-dichlorobenzene and 1,2,4 tri-chlorobenzene (perchloroethylene and a-chloronaphtalene have also been used), which can dissolve the polyolefins at high temperature and are transparent enough in the IR region of measurement. These solvents are the same as used in GPC/SEC analysis of polyolefins and are also appropriate for detection by refractive index, although this detector has not... 

Solution concentrations of 0.5% are usually prepared in vials or dissolution vessels and injection of 1-5 mg of polymer are loaded onto the column in analytical TREF. The more sensitive detectors should be used to allow for the lowest concentration possible in order to reduce co-crystallization and entrapment effects. Polyolefin homopolymers, which elute in a narrow temperature range, may often result in column plugging, especially if they have large molar mass in those cases, a lower concentration of sample should be used for injections. 

The current trend is toward the use of analytical TREF systems and once again the impetus came from developments in the molecular weight fractionation area. In this case it was SEC which led the way to the establishment of TREF methods which operated on a small scale, relatively rapidly, automated and using on-line detectors initially developed for SEC. The various TREF systems are described in the following section. 

Because of the significant experimental differences involved, a discussion of techniques is best separated into two parts, one concerning preparative TREF and the other relating to the operation of analytical TREF. For the purpose of this review we define preparative as a finctionation in which fractions are recovered for further evaluation and, thus, relatively large samples (1-10 g) are separated. 

Fig. 7. Analytical TREF analysis of preparative fractions obtained using the stepwise continuous temperature program shown in Fig. 6 [13]...

Fig. 8. Analytical TREF system uswl by Wild an Ryle [8]...

Fig. 9. Calibration curve for calculating short-chain branching distributions from analytical TREF data (9). Reprinted with permission...

Fig. 10. Schematic of an LC oven-based Analytical-TREF used for SCB distributions [11]. The capital T denotes thermocouple monitors...

Perhaps the most ambitious attempts to build a TREF instrument that will operate automatically and in addition provide molecular weight information along with comonomer content are those described by Nakano and Goto [15] and Hazlitt and Moldovan [12]. The approach of Nakano et al. promises the most complete structural information by couphng a SEC to an analytical TREF. A diagram of their system is shown in Fig. 11. 

Fig, 14. Data output from a dual detector analytical TREF for determining molecular weight as a function of separation temperature [12]... 

For reference, a summary is given in Table 2 of the various analytical TREF systems used, along with the polymers they were used to analyze. 

Analytical-TREF temperature (°C) at which 50% of the material eluted. 

Fig. 23. Analytical TREF profiles of six EVA copolymer listed in Table 4...

From the point of view of establishing realistic comonomer distributions for various LLDPE resins it is clear that data obtained through analytical TREF is... 

It is interesting to note that analytical TREF does not seem to have been applied to any extent to polypropylene analysis, possibly because of the need to subject fractions to further analysis, C-NMR for example. The study of Kakugo etal. [34] does suggest that analytical TREF with a calibration based on pentad might... 

In a more recent study, Kelusky, Elston and Murray [11] demonstrated the use of analytical TREF for the analysis of a wide range of polyethylene-based blends. As with the work described above, their analytical TREF system used very small sample and column size, a programmed oven and an IR detector to achieve the necessary reproducibility and resolution. In addition to confirming the effectiveness of TREF for the characterization of blends of LLDPE and LDPE, they showed examples of LDPE/medium density PE blends (Fig. 38) and of ethylene vinyl acetate copolymer/LLDPE blends (Fig. 39). 

Kelusky et al. also indicated that by use of a semi preparative TREF system one could achieve an effective characterization of PE/EPDM terpolymer and PE/polyisobutylene blends. The approach for this type of analysis of blends, in which one of the components was non-crystalline, was to first collect the PIB or EPDM by elution at 30 °C followed by elution of the PE component as an analytical TREF or it could be eluted off, precipitated and recovered for subsequent analysis by SEC or NMR. They point out that the TREF separation of this type of mixture is superior to the usual solvent extraction methods which invariably remove some highly branched or low molecular weight PE in addition to the PIB or EPDM. One would anticipate that such a separation scheme would be effective for other crystalline/amorphous polymer blends such as impact polypropylenes which contain ethylene-propylene rubber. 

Nakatio and Goto used a highly automated system speciaUy designed for cross fractionation in which a SEC is interfaced with a semi-preparative TREF. This has been described in the analytical TREF section above. They demonstrated the principle of complete cross fractionation for analysing a polyethylene blend. The results are shown in Fig. 40 and earlier in Fig. 12 and 13. 

Fig. 42. Analytical TREF of PE resins used in cross fractionation shown in Fig. 41...

Fig. 1 Schematic diagram of analytical TREF. (Reprinted from [11] with permission of Springer Science + Business Media)...

The initial use of analytical TREF was to characterize polyethylene fractions [120]. Mirabella and Ford used TREF to fractionate LDPE and LLDPE for further characterization using SEC, X-ray C-NMR, DSC and viscosity measurements [121]. These investigators determined that the melting behavior of LLDPE correlated well with a multimodal SCB distribution and the distribution became narrower with increasing molecular weight. Mingozzi and collaborators [122] have applied both analytical and preparative TREF to the analysis of tacticity distribution in polypropylene. The results showed that both methods could be used successfully analytical TREF gave faster qualitative results on the polymer microstructure, while preparative TREF, with subsequent analysis of the fractions, could yield detailed quantitative tacticity information. Hazlitt has described an automatic TREF instrument to measure SCB distributions for ethylene/ a-olefin copolymers [123]. 

Development of high performance analytical TREF for polyolefin analysis has recently been reported by Wild and Blatz [124]. Their goal was to develop TREF to the point where it is economical, fast, and easy to carry out, making it approach analytical SEC in convenience. These authors ... 

Soares and Elamielec recently reviewed TREF of polyolefins [125] and compared analytical and analytical TREF (Table 6). TREF is a powerful tool that can be used to fractionate complex polymer systems according to composition, tacticity, and crystallinity. Once the materials have been fractionated, conventional methods may be used to characterize the molecular weight and molecular weight distribution of the fractions. Recently Dawkins and Montenegro have coupled TREF with SEC to characterize styrene/... 

OBCs show very different solution crystallization behavior than statically RCPs. Figure 28 shows the analytical TREF profile comparing an OBC to a commercially available RCP (AFFINITY VP8770 from The Dow Chemical Company) and a polymer blend with components that are representative of the HS and SS within the OBC. Table 12 summarizes the analytical characteristics of these polymers and as shown, these polymers have similar crystallinity and within a similar molecular weight range. Figure 28 shows that for this particular OBC, which was made using precatalysts 3 and 4 with DEZ as CSA, 90 wt.% of... 

These differences in block architecture of the multiblock and diblock CBCs are apparent in a comparison of the solubility characteristics. Figure 36 shows analytical TREF traces for a physical blend, multiblock OBC, and diblock OBC with similar composition. Both OBCs have lower purge fractions and elute at lower temperatures than the high-density fraction of the blend. However, the multiblock OBC elutes at a lower temperature and over a broader temperature range than the diblock OBC. 

Figure 36 Comparison of solution solubility by analytical TREF of a blend, diblock, and multiblock OBC with similar overall density ( 0.90 g cm ). Adapted with permission from Wenzel, T. T. Arriola, D. J. Carnahan, E. M. etai. In Meta/ Catalysts In Olefin Polymerization. Topics in Organometallic Chemistry, Guan, Z., Ed. Springer-Verlag Berlin, Germany, 2009 Vol. 26. ...

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