CRYSTAF method

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]. 

Carbon-13 nuclear magnetic resonance NMR) is important in imder-standing more detailed structural information in the backbone of the polyethylene sample. For example, l.T.DPE is produced commercially with either 1 -butene, 1 -hexene or 1 -octene as the comonomer. Copolymers with a low content of 1-olefin contain only isolated branches, however copolymers containing higher levels of comonomer (e.g., 2-20 mol%) contain a wide variety of complex sequence distributions making C NMR a particularly important characterization tool. Information into the sequence distribution of the comonomer in ethylene/1-olefin pentads provides data to determine the reactivity constants K, k, k and k, where e represents ethylene and h represents 1-hexene. 

Some possible Pentad sequences involving ethylene (e) and 1-hexene (h) that may be determined from C-NMR data are shown below  

Such sequence structures are required to reduce the crystallinity of the polyethylene sample as the comonomer n-butyl branch from incorporation of 1 -hexene into the polymer backbone interrupts the chain-folding mechanism responsible for the crystallinity in polyethylene. Long units of ethylene along the polymer backbone are able to crystallize the polymer sample and are undesirable in polyethylene products that require a large degree of elasticity for particular applications. 

Details of the examination of ethylene/1-butene [24, 25] ethylene/1-hexene [26] or ethylene/1-octene copolymers [27] by NMR to determine various sequence distributions have been reported. 

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

Dry samples are placed in the crystallization vessels, dissolved, crystallized, and sampled. The polymer concentration is measured by an FT-IR at increasing T. The instrument may be converted into a CRYSTAF-TREF system capable of running both types of measurements in the same hardware. Each method provides complementary information on the CCD in some complex resins. 

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. 

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. 

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. 

A number of calibration curves have been reported for Crystaf (Table 2, Fig. 40). Unfortunately, similar to calibration curves for Tref, calibration curves for Crystaf depend on polymer type, solvent type, CR, and method of sample preparation. Published calibration curves should only be used if care is taken in trying to replicate as closely as possible the conditions under which they were obtained. 

They obtained good agreement with the experimental Crystaf profiles for their limited sample set of ethylene/1-octene copolymers. Later, Cos-teux et al. [23] derived an analytical expression for this distribution of the longest monomer sequences. This analytical solution can be used to dramatically shorten the computational time required for Crystaf modehng using the method of Beigzadeh et al. [81]. 

Crystaf has also started to gain recognition as an efficient technique to analyze polyolefin blends quantitatively, as it is considered to be superior to the conventional DSC method. It is quite certain that its use for blend analysis will become more common in the near future. 

The amount of the polymer crystallizing at each temperature can be obtained by differentiation of the integral CRYSTAF profile at each temperature. The plot of amount of polymer crystallized as a function of temperature is the most common and the clearest reporting method of CRYSTAF results. Both types of plots for a blend of HOPE and PP are shown in Fig. 8. Similar to TREF, there are several factors which can affect CRYSTAF profiles and the reliability of results including the molar mass of the polymer, the comonomer type and content, the cooling rate (CR), co-crystallization effects. Soares et al. wrote two comprehensive reviews about crystallization based techniques in 2005 [13, 51]. In this review we focus on the work that has been published more recently. 

Gemoets and Hagen [52] derived a mathematical expression / for CRYSTAF curves to describe the longest ethylene sequence (LES) distribution of random copolymer chains. The comparison of the LES-distribution of a random copolymer having a polydispersity of 2, as calculated by /poiymer. with Monte-Carlo simulations and a method previously described in the literature showed that the derived... 

As described in the previous sections, there are a number of fractionation techniques that are used very successfully in polyolefin analysis, including HT-SEC, CRYSTAF and TREE. For copolymers, CRYSTAF and TREF provide information about the chemical composition distribution. The drawbacks of these methods are that (1) they are very time-consuming and (2) they work only for crystallizable polyolefins. The latest development in this field, CEF, is able to obtain similar results to TREF in less than 1 h and is, therefore, a significant improvement. Still, CEF is based on crystallization and can only address the crystallizable part of a polyolefin sample. 

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. 

Once the alkaloid is purified, the absolute structure and stereochemistry can be determined by a combination of spectroscopic methods including H and C NMR spectroscopy, and high resolution mass spectrometry. All three methods should be in agreement on one structure for the compound in question. Additional structural data and confirmation can be obtained from the analysis of the trimethylsilyl ether derivative by GC-MS as indicated above [13]. The absolute stereochemistry can be determined by X-ray crystallography, which can be used whenever well-defined crystaf structure is obtained, either from the alkaloid itself or from its hydrochloride salt. 

Crystallization fraaionation (CRYSTAF) " and temperature-rising elution fractionation (TREF) are used for chemical composition or crystallinity analysis. For copolymers, CRYSTAF and TREF provide information about the CCD. The drawbacks of these methods are that (1) they are very time consuming and (2) they work only for crystal-lizable polyolefins. 

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