CRYSTAF Crystallization analysis fractionation

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],... 

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. 

Methods based on differences in crystallizability of macromolecules - temperature rising elution fractionation, TREF and crystallization analysis fractionation, CRYSTAF. 

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. 

Fig.l Cumulative and differential crystallization analysis fractionation (Crystaf) profiles of a blend of two polyolefins... 

The chemical composition distribution of polyolefins is measured (indirectly) by either temperature rising elution fractionation (Tref) or crystallization analysis fractionation (Crystaf). These two techniques provide similar information on the chemical composition distribution of polyolefins and can be used interchangeably in the vast majority of cases. Both methods are based on the fact that the crys-tallizability of HOPE and LLDPE depends strongly on the fraction of a-olefin comonomer incorporated into the polymer chains, that is, chains with an increased a-olefin fraction have a decreased ciystallizability. A similar statement can be made for polypropylene and other polyolefin resins that are made with prochiral monomers resins with high stereoregularity and regioregularity have higher crystalliz-abilities than atactic resins. 

S. Anantawaraskul, J. B. P. Soares, P. M. Wood-Adams, Fractionation of semi-crystalline polymers by crystallization analysis fractionation (Crystaf) and temperature rising elution fractionation (Tref). Adv. 

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. 

Crystallization Analysis Fractionation (CRYSTAF) It is based on the segregation of crystals of different morphology or comonomer content by crystallization, In CRYSTAF, the separation and analysis are performed in a single step (the crystallization cycle), where concentration of the polymer solution is intermittently sampled and analyzed as the temperature goes down. The temperature-concentration data which are obtained correspond, in the cases of branched poly(olefins) or their copolymers with a-olefins, to the cumulative curve of the side chain branching distribution (SCBD). The last point at the lowest temperature of the experiment is the soluble or noncrystallizable fiaction. 

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. 

B. Monrabal, Analysis of Metallocene Type Polyolefins by CRYSTAF (Crystallization Analysis Fractionation), International GPC Symposium, San Diego, 1996. 

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. 

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. 

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. 

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). 

Similarly to Crystaf, Tref is an analytical technique that fractionates semicrystalline polymers on the basis of chain crystallizabilities. However, Tref involves two consecutive steps, crystallization and elution, while Crystaf can perform a similar analysis in a single crystallization step. 

Ideally, it would be preferable to operate Crystaf in conditions that fractionate the polymer chains according to their crystallizabilities at thermodynamic equilibrium in order to eliminate any crystallization kinetics effects. Practically, this idealized condition is imtenable because very long analysis times would be required. Recent investigations [29] have shown that the fractionation process in Crystaf is, in fact, very far from thermodynamic equilibrium. 

Cocrystallization during Crystaf analysis can be investigated by comparing experimental Crystaf profiles of blends with their predicted Crystaf profiles, assuming the absence of cocrystallization. The Crystaf profiles of the blends in the absence of cocrystallization can be estimated as the summation of the Crystaf profiles of each parent sample, measured alone, multiplied by its weight fraction in the blend. Deviations from the predicted profile are a measure of the extent of co crystallization taking place during the analysis. 

Two methods for preparing the calibration curve have been reported. Both methods were done by performing Crystaf analysis in a series of narrow-CCD copolymer samples with known comonomer contents with crystallizabilities covering a broad range of crystallization temperatures. The only difference between these two methods is the type of samples used in the calibration. The first method uses a series of polymer samples synthesized with single-site-type catalysts [58,68], while the second method uses a series of fractions from broad-CCD Ziegler-Natta copolymers obtained with P-Tref [1,49]. After the whole series of samples has been analyzed, the relationship between Crystaf peak temperature and CC is used as the cahbration curve. 

As pointed out by Monrabal and coworkers, the TREF is an operational complex method requiring more than one day to perform. The CRYSTAF method is also a separation method that fractionates samples of differing crystallizability by slowly cooling a polymer solution in a single crystallization cycle by monitoring the decrease in solution concentration as the temperature is lowered. Because of this one-step approach, analysis time is reduced to around 6 hours for five simultaneous samples [23]. 

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