Crystaf fractionation

Crystallization analysis fractionation (Crystaf) fractionates polymer chains according to differences in crystallizability. Crystaf can be used to fractionate polymers due to differences in chemical composition, comonomer sequence length, and tacticity. It may also respond to long-chain branching, provided that the polymer is branched enough to affect its crystallinity. The fractionation principle operative in Crystaf was discussed in the section on batch fractionation for the case of slowly cooling (or warming) solutions of semicrystalline polymers. 

All microstructural features impacting chain crystallizability can potentially influence the Crystaf fractionation process. The main microstructural properties of interest are (1) number average molecular weight, (2) CC, and (3) comonomer type. Each of these factors will be discussed below. 

Two main approaches have been proposed to model Crystaf fractionation (1) models based on Stockmayer s bivariate distribution [58,80], and (2) models based on the distribution of chain crystallizabilities using Monte Carlo simulation [57,81]. 

Unfortunately, most of these assumptions have recently been proved to be inaccurate [14,29]. Even though modeling Crystaf profiles with Stockmayer s distribution can provide an adequate fit of the data, these models are, at best, only semiempirical. It is clear that a truly phenomenological model of the Crystaf fractionation process is essential to obtain the details of the correct distribution. 

Rytter et al. reported polymerizations with the dual precatalyst system 14/15 in presence of MAO [30]. Under ethylene-hexene copolymerization conditions, 14/MAO produced a polymer with 0.7 mol% hexene, while the 15/MAO gave a copolymer with ca. 5 mol% hexene. In the mixed catalyst system, the activity and comonomer incorporation were approximate averages of what would be expected for the two catalysts. Using crystallization analysis fractionation (CRYSTAF) and differential scanning calorimetry (DSC) analysis, it was concluded in a later paper by Rytter that the material was a blend containing no block copolymer [31],... 

In addition to the above, the intermolecular co-monomer distribution, that is the extent of the co-monomer s incorporation in different molecular mass fractions of the polymer, is an important aspect. When feasible, this is evaluated by fractionating the polymer and measuring the co-monomer content in each fraction, by solubility (TREF),511 crystallizability (CRYSTAF),512,513 or by GPC-IR.514 Whereas ZN catalysts tend to produce LLDPE co-polymers of broad co-monomer distribution (with more co-monomer incorporated in the lower molecular mass fractions), single-center catalysts always have a nearly perfect intermolecular co-monomer distribution, and this aspect is taken as implicit in most co-polymerization studies with the latter systems. 

Crystallization analysis fractionation (CRYSTAF) was developed by Monrabal [99] in 1991 as a process to speed up the analysis of the CCD, which at that time lasted around 1 week per sample with the TREF technique. CRYSTAF shares with TREF the same principle of separation according to crystallizability. In CRYSTAF, the samples are not crystallized in a column but in a stirred vessel with no support, and only a temperature cycle (crystallization) is required [64], thus speeding up the analysis process and simplifying the hardware requirements. 

In CRYSTAF, the analytical process is followed by monitoring the polymer solution concentration during crystallization by temperature reduction. Aliquots of the solution are filtered (through an internal filter inside the vessel) and analyzed by a concentration detector at different temperatures, as shown in Fig. 21. The whole process is similar to a classical stepwise fractionation by precipitation with the exception that, in this new approach, no attention is paid to the polymer being precipitated but to the one that remains in solution. 

Both techniques share the same principles of fractionation on the basis of crystallizability. TRFF is carried out in a packed column and demands two full temperature cycles, crystallization and elution (dissolution), to obtain the analysis of the composition distribution. In CRYSTAF, the analysis is performed in a single step, the crystallization cycle, which results in faster analysis time and simple hardware requirements. 

Crystallization elution fractionation (CEF) is a new separation technique developed by Moiuabal [102] for the analysis of the CCD that combines the separation power of CRYSTAF and TREF. The CEF technique is based on a new and patented separation principle, referred to as dynamic crystallization (DC) [87], that separates fractions inside a column according to crystallizabUity while a small flow of solvent passes through the column. The separation by DC occurs during the crystallization step. CEF combines the separation power of DC in the crystallization step with the separation during dissolution of the TREF technique. 

Important data on physical architecture of crystallable polymers can be obtained from temperature rising elution fractionation, TREF and crystallization fractionation, CRYSTAF. 

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.

The company Polymer Char (Valencia, Spain) was created for developing fully automated PO characterization instruments. The first device, commerciahzed and patented in 1994, was the CRYSTAF, crystallization analysis fractionation, for the fast measurement of the chemical composition distribution (CCD) in PE, PP, copolymers, and blends. Next came the SEC (with a quadruple detector system) and then SEC/a-TREF and p-TREF instruments. The first commercial, fully automated cross-fractionating SEC/TREF apparatus for microstructure characterization of POs was described by Ortin et al. (2007). The instrument yields a bivariate distribution CCD by TREE fractionation and then SEC fraction analysis in a single run. A schematic diagram of this new cross-fractionation instrument is shown in Fig. 18.7. 

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... 

Fractionation by crystallizability takes place by either decreasing or increasing the temperature, similarly to what is done in the methods of fractional precipitation or coacervate extraction for molecular weight fractionation. However, differently from those, solid-liquid phase separation occurs during fractionation by crystallizability. We will call Crystaf-mode the fractionation that takes place upon cooling the polymer solution, and TREF-mode the fractionation that takes place by dissolving previously precipitated polymer crystallites. This nomenclature is consistent with the continuous analysis techniques of Ciystaf and TREF, which will be described later in this article, and is more adequate for fractionation by crystallizability. 

Fractionation Equipment. Similar to molecular weight fractionation, most of the conventional equipment for crystallization fraction is rather simple and requires significant operator time. Fully automated crystallization fractionations are possible using mc2-PREP, in both Crystaf-mode and TREF-mode (the c2 in mc2-PREP stands for these two fractionation modes). 

The two fractionation vessels are required to fractionate one sample in Crystaf-mode. The polymer solution is initially placed in one of the fractionation vessels at high temperature (vessel 1). The temperature is then decreased, causing part of the polymer to crystallize out of solution. The polymer solution... 

Several of the issues related to the fractionation efficiency of these two techniques have been investigated with their continuous coimterparts, Crystaf and TREF, and will be discussed in the next section. 

Usually, Crystaf is operated at a cooling rate of 0.1°C/min. It will be shown later that cooling rates can significantly influence the results of Crystaf analysis. The concentration of the polymer solution is not considered to affect the fractionation results, provided it is kept in the range of 0.2-1.0 mg/mL (14). The t5q>e of solvent only affects the crystallization temperature polymers crystallize at lower temperatures in the presence of better solvents, but the effect on fractionation efficiency is negligible (15). 

One of the main difficulties for the quantification of Crystaf is the nonimiver-sality of its calibration curves. Even for a series of ethylene/a-olefin copolymers, the calibration curves will vary as a function of comonomer type, as illustrated in Figure 15. The general rule of thumb for these copolymers (from propene to 1-octene) is, the longer the a-olefin, the lower the crystallization temperature for a given a-olefin molar fraction. This has been explained by several authors on the basis of the difference in the degree of inclusion of the a-olefin in the crystalline lattice shorter a-olefins are more likely to cocrystallize with ethylene and therefore depress the crystallization temperature to a lesser extent. 

Temperature rising elution fractionation (TREF) is based on the same principles of Crystaf, but involves two consecutive steps precipitation (or crystallization) and elntion. In the precipitation step, polymer is crystallized from a dilute solution by slowly decreasing the temperature inside a column packed with an inert support. Alternatively, the precipitation step can be done in a stirred vessel in the presence of the support and the polymer-coated support is subsequently transferred to the TREF colnmn for the elution step. In the elution step, the polymer deposited onto the snpport is eluted from the column with a continuous flow of solvent at increasing temperatures. An on-line detector measures the concentration of the polymer solution exiting the column as a function of elution temperature. Figure 18 shows a schematic of a TREF apparatus and a typical TREF cnrve for linear low density polyethylene. 

Polymer fractionation takes place dnring the precipitation step according to crystallizability. The precipitation step in TREF is essentially analogons to the crystallization that takes place in Crystaf. As for Crystaf, a slow cooling rate is the most important requirement to minimize cocrystallization of polymer populations with different crystallizabilities and enhance peak resolntion. Since there is no monitoring of the polymer solution concentration dnring the precipitation step, the elution step is required to measure the concentration of polymer recovered as a function of elution temperature. 

Given that the fractionation mechanism in Crystaf and TREF are very similar, they both provide analogous information about polymer microstructure, with the TREF profiles shifted to higher temperatures due to the supercooling effect in Crystaf, as exemplified in Figure 19. Crystaf analysis times tend to be shorter because of the extra time required by the additional elution step in TREF. 

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