TREF and CRYSTAF

Factors contributing to the CCD include catalyst type, polymerization conditions, and reactor nonhomogeneity. Ziegler-Natta multiple-site catalysts produce polymers with broad distributions of chemical composition in copolymers and of tacticity in polypropylene, and such distributions are important in plastics applications. For example, low-crystaUinity fractions are extractable and render a material unsuitable for food packaging, while high-crystallinity fractions result in haze and low impact strength in plastic films. A thorough review of TREF and the closely related technique, CRYSTAF, by Soares and Hamielec was recently published [92]. 

The basic principle of TREF is that material with a high-level of crystalUzability crystallizes from solution at a higher temperature than material with a lower level. Thus, crystalline material 

Because a reliable model of the process is not available, no universal calibration method exists, and a calibration curve is required to determine the distribution of tacticity or chemical composition. Because regio-regularity, stereoregularity, and tacticity all affect crystallizability, it is essential that the microstructure of the polymer used for calibration be the same as that of the material to be analyzed. A typical calibration curve for an olefin polymer is a plot of methyl groups per thousand carbon atoms versus temperature. The result for a given solvent is usually a straight line with a negative slope. 

As a first step toward the modeling of the TREE and CRYSTAF processes, Costeux etal [93] compared analytical solutions for LES distributions with the results of Monte Carlo simulations for copolymers made using single-site catalysts. 

Crystallization analysis fractionation (CRYSTAF) provides the same information as TREF but is much faster, as it uses only the dissolution process to accomplish the separation. The basic principle is that material with a low-level of crystallinity dissolves in a solvent at a lower temperature than material with a higher level. It also avoids the use of a column and thus the peak broadening that occurs there and requires no support. However, CRYSTAF involves very small quantities of material and is therefore not useful as a preparative technique. The sample is placed in a small sample vial equipped with a stirrer and a sampling line with a filter that prevents crystals from leaving. The vial is placed in an oven whose temperature is gradually increased. Samples are collected at small temperature intervals by nitrogen pressurization, and the polymer concentration is detected by an IR sensor. A cumulative curve of polymer concentration versus temperature of crystallization is obtained. Taking the derivative, a TREF-type curve can be obtained, and for conversion to CCD the calibration procedure is the same as in TREF. 

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Both techniques provide similar results the comparison of TREF and CRYSTAF has aheady been discussed [84] and the most significant difference is the temperature shift due to the undercooling, as analytical conditions are far from equilibrium CRYSTAF data are obtained during the crystallization whereas TREF data are obtained in the melting-dissolution cycle. Both techniques, however, can be calibrated and the results expressed in branches/lOOOC will be similar for PE copolymers. 

Fig. 25 TREF and CRYSTAF analysis of PE-PP combinations. TREE separates iPP + PE better and CRYSTAF separates EP + PE better...

Monrabal B (2004) Microstructure Characterization of Polyolefins. TREF and CRYSTAF. In Proceedings of the 17th International symposium on polymer analysis and characterization (ISPAC), Heidelbergh, 6-9 June 2004... 

The CCD is the second most important microstructural distribution in polyolefins. Differently from the MWD, the CCD carmot be determined directly only the distribution of crystallization temperatures (CTD) in solution can be measured and one can try to relate this distribution to the CCD using a calibration curve. Two techniques are commonly used to determine the CTD or CCD of polyolefins TREF and Crystaf. Both operate based on the same principle chains with more defects (more comonomer molecules or stereo-and/or regioirregularities) have lower crystallization temperatures than chains with fewer defects. Figure 2.11 compares the TREF and Crystaf profiles of an ethylene/1-butene copolymer made with a heterogeneous Ziegler-Natta catalyst. Notice that they have very similar shapes the Crystaf curve is shifted toward lower temperatures because it is measured as the polymer chains crystallize, while the TREF curve is determined as the polymer chains dissolve (melt) and are eluted from the TREF column, as explained in the next few paragraphs. 

Figure 2.11 Comparison between TREF and Crystaf profiles for an ethylene/1-butene copolymer made with a heterogeneous Ziegler atta catalyst. The rectangular region shown in the Crystaf curve is proportional to the fraction of polymer remaining soluble at room temperature. This fraction was not reported for the TREF profile shown here.

Similarly to GPC, the amount of information obtained with TREF and Crystaf can be increased by adding more detectors to the system. For instance, LS and VlSC detectors have been used to determine molecular weight averages as a function of crystallization/elution temperature or comonomer content in the copolymer. The analytical results shown in Figure 2.4, for instance, were measured with a TREF/IR-LS system. Another TREF/IR-LS profile is depicted in Figure 2.12 for a rather complex trimodal polyolefin resin. 

High-temperature GPC, TREF and Crystaf are used almost exclusively to analyze polyolefins. Other more general polymer analytical techniques are also commonly used for polyolefin analysis. Because they are less specific to polyolefins, they will be described only very briefly in the remaining part of this section. 

However, in the normal growth pattern, the technology becomes progressively more complex. New catalysts, new multi-catalyst/multi-reactor processes, blending of various PO and additives result in complex structures, especially in terms of CCD, severely complicating their characterization. Certain combinations of POs may result in ambivalent results if only TREF or CRYSTAF is used. To obtain unequivocal results for complex PP or PE copolymer or blend, both TREF and CRYSTAF should be conducted. 

Fortunately, as the complexity of macromolecules increases, the precision and sophistication of instruments designed for their characterization improve. The analytical and preparative fractionating columns of GPC used in the 1950s are transformed into automatic multicolumn, multi-detector, high-temperature SEC. Similar advance is observed for TREF and CRYSTAF, now fused into CFC. At the same time, new methods are being developed for faster and more precise characterization by means of the multi-detector liquid chromatography, HTLC or HPLC. 

Various mathematical models have been proposed for TREF and CRYSTAF, but none predicts crystallization and co-crystallization effects during the fractionation with sufficient precision. Thus, for the accurate determination of semicrystaUine copolymer microstmcture, one should use the two optimized complementary... 

Fig. 20. Comparison of TREF and Crystaf blend peak resolution for ethylene/1-hexene copolymers as a function of cooling rate-----Tref ... Crystaf. From Ref 14.

Fig. 5 Comparison of TREF and CRYSTAF curves for the copolymer having 1 -butene content of 5.1 wt% dashed line CRYSTAF, dolled line TREF. (Reprinted frran [44] with permissirai of Springer Science -I- Business Media)...

The classical techniques for chemical composition analysis of polyolefins are based on crystallization behaviour of different components of these materials. These techniques are only apphcable for the crystalline part of the sample and the amorphous part is obtained as a bulk fraction. Nevertheless, these techniques are stiU the analytical workhorse in most polyolefin research laboratories. The reason behind this is that most of the commercially important polyolefin materials are semi-crystalline. There has been a number of recent advances in these techniques that have enabled a reduction in analysis time, better resolution and mathematical modelling etc. The most fascinating innovation in this regard is the development of CEF. CEF combines the separation powers of both TREF and CRYSTAF, resulting in better separation of fractions along with considerable reduction in analysis time. CEF has the promise and potential to be the major technique in crystallization analysis in future. 

Information about the distribution of short-chain branches cannot be obtained from intrinsic viscosity measurements but can be investigated using carbon-thirteen NMR and crystallization techniques such as TREF and CRYSTAF, as explained in Section 2.6.6. 

The use of NMR makes it possible to probe details of molecular features not accessible using other techniques. TREF and CRYSTAF are techniques based on crystaUizability in solution and are essential in the determination of the level and distribution of the short-chain branches resulting from the introduction of a comonomer. Much of the material in this chapter is presented in more detail by Sperling [101] and in the book edited by Pethrick and Dawkins [102],... 

CRYSTAF profiles of the examined homo-polyethylene samples, Pasch showed that CRYSTAF is a powerful technique for the analysis of short-chain branching and blends of high-and low-density polyethylene. On the other hand, Gabriel and Lilge came to the conclusion that short-chain branching can be analyzed best by combining TREF and CRYSTAF with a special kind of differential scanning calorimetry (DSC). 

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