Cross fractionation

Cross-fractionation is a combination of two or more fractionation techniques, each of which separates polymer chains according to a different microstruc-tural characteristic. By combining different techniques, cross-fractionation can probe information on chain microstructiue in greater detail than any single characterization technique and is an especially important tool for understanding polymers with complex chain microstructiues. In this review, we considered only cross-fractionation techniques that involve chain crystalliz-ability as one of the fractionation mechanisms. More details on a wide range of cross-fractionation techniques are available in the literatiue [35]. 

Besides being used to understand complex microstructiual distributions, cross-fractionation can also be used for sample preparation. Before singlesite-type catalysts were commonly available, it was extremely difficult to study the independent effect of molecular weight or comonomer composition on polyolefin properties, because polyolefins produced with Ziegler- 

Natta catalysts have very broad and interdependent molecular weight and comonomer composition distributions (for these resins, the molecular weight decreases with increasing comonomer content). 

SSF/solution crystallization fractionation [36] are old techniques used to prepare fractions with narrow molecular weights and comonomer composition distributions. Unfortunately, these techniques are very time consuming and generally require significant amounts of solvent. More recently, P-Tref/SEC apparatuses have been used to perform this type of cross-fractionation in a more efficient, but still rather involved, way [37-40]. 

In P-Tref/SEC cross-fractionation, copolymer chains are first fractionated according to comonomer composition into a series of fractions using P-Tref. Each fraction is then analyzed using SEC to obtain its MWD. P-Tref/SEC is a very powerful cross-fractionation technique because it provides information on the bivariate comonomer composition and MWD. Although the process is still time-consuming, the information obtained with P-Tref/SEC crossfractionation provides an almost complete map of chain microstructures. This cross-fractionation technique has been used for various ethylene/1-olefin copolymers (1-butene, 1-hexene, 1-octene, and l-pentene-4-methyl). 

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The SCB distribution (SCBD) has been extensively studied by fractionation based on compositional difference as well as molecular size. The analysis by cross fractionation, which involves stepwise separation of the molecules on the basis of composition and molecular size, has provided information of inter- and intramolecular SCBD in much detail. The temperature-rising elution fractionation (TREE) method, which separates polymer molecules according to their composition, has been used for HP LDPE it has been found that SCB composition is more or less uniform [24,25]. It can be observed from the appearance of only one melt endotherm peak in the analysis by differential scanning calorimetry (DSC) (Fig. 1) [26]. Wild et al. [27] reported that HP LDPE prepared by tubular reactor exhibits broader SCBD than that prepared by an autoclave reactor. The SCBD can also be varied by changing the polymerization conditions. From the cross fractionation of commercial HP LDPE samples, it has been found that low-MW species generally have more SCBs [13,24]. 

Figure 5 Cross fractionation of a 1-octene LLDPE by crystallization/dissolution treatment after direct extraction. Source Ref. 32.

In homopolymer analysis this meant a closer study of the accuracy and reproducibility of data from GPC to see how resolution correction techniques could be either circumvented or practically applied. In copolymer analysis the limitation of conventional molecular size fractionation emerged as the fundamental difficulty. An orthogonal coupling of GPCs operated so as to achieve the desired cross fractionation before detection is presented as a novel approach with considerable potential. 

Figure 15. Cross fractionation by orthogonal chromatography arrangement of...

D. Recommendations. Cross-Fractionation using Orthogonal Chromatography has high potential in the analysis of complex polymers and even polymer latices (with Hydrodynamic Chromatography). Multi-detector analysis, particularly utilizing spectrofluorometry, should be very useful in developing the technique. 

Glockner G (1991) Gradient HPLC of copolymers and chromatographic cross-fraction. Springer, Berlin Heidelberg New York... 

G. GlOckner, Gradient HPLC of Copolymers and Chromatographic Cross-fractionation, Springer-Verlag, Heidelberg (1991). 

Table 5.1 lists several heart-cut and comprehensive techniques. Heart-cut 2DLC is very common and has great application for the increased resolution of one or several components from the first dimension (Augenstein and Stickler, 1990 Majors, 1980 Pasch et al., 1992 and Dixon et al., 2006). Heart-cut 2DLC for the analysis of polymers is often referred to as cross-fractionation (Balke and Patel, 1980). Protein digest analysis with MS/MS identification has been called multidimensional protein identification technology or MUDPIT. This is described in detail in Chapter 11. 

Glockner, G. (1991). Gradient HPLC and Chromatographic Cross-Fractionation. Springer, Berlin, Heidelberg, New York. 

Figure 1. Compositional distribution of a predominantly syndiotactic copolymer bearing COCHiSOsN(CHs)t units (DPn = 650, DSm = O 366) from precipitation fractionation data. System 1 (6) CHCls—Et2C) system II (O) DMF—Hs0 + O, 5% NH4Cl cross fractionation ( ) intermediate fractions obtained from system II are further fractionated according to System I.

Cross-Fractionation. Complex polymers contain more than one broad property distribution, if molecular weight and composition are the only two property distributions present then an example of cross-fractionation would be the separation of the polymer first according to molecular weight and the separation of each single molecular weight fraction obtained according to composition. This cross-fractionation provides a two-dimensional answer to a two-dimensional distribution problem. It has typically been accomplished for polymers using solvent/non-solvent precipitation. 

Multi-dimensional Chromatography. Multi-dimensional chromatography is the term used to describe a variety of methods where fractions from one chromatographic system are each transferred to another for further separation. Combinations of SEC with thin-layer chrcmatography have been shown to enable separation of copolymers by composition in a "cross-fractionation". OC utilizes a combination of two SECs in a cross-fractionation approach. 

Once it was realized that multiple columns in the first SEC really did not offer cmy advantage in terms of greater injection amounts because of increased dilution in the columns, smaller injections and less columns reduced emalysis times 50% with no loss in sensitivity. For the analyses shown in Figure 8, only three columns were used in the first SEC and three in the second. With this system the first analysis by both SBC instruments required a total of 30 minutes and subsequent analyses of the same sample eibout 15 minutes each. However, despite these significant reductions in analysis times in comparison to the initial work, complete analysis of even one complex polymer required many cross fractionations and generated much data. 

The diode array UV/vis spectrophotometer was used to both Identify the polymer exiting and to obtain a quantitative analysis of the copolymer composition distribution. Figure 9 (6) shows the result of summing many individual fraction analyses to see the total copolymer composition distribution. The result had the correct average composition but not the skewed shape expected from theory. Part of the difficulty was the relatively small number of cross fractionations done. 

Figure 9 Copolymer composition distribution for Whole polymer as sum of distributions obtained from individual cross fractionations. (Reproduced from Ref. 6. Copyright 1983,...

The term cross-fractionation (CF) refers to analyses of distributions in differing directions by means of separation processes. Cross-fractionation is a significant tool for the evaluation of the complex distribution which copolymers normally have with respect to molar mass (MMD) and chemical composition (CCD). The idea of CF implies separation by one parameter and subsequent analysis of the fractions obtained for the distribution of the other parameter through another separating process. 

Fig. 30. Chromatographic cross-fractionation of300 mg poly(styrene-6-methyl methacrylate) with 47 wt % styrene. SEC curves of the fractions obtained by column adsorption chromatography. The large SEC curve is from the non-fractionated sample. (From Ref.1321 with permission)...

The reason for this discrepancy can be deduced from Fig. 30 all the constituents differing in chemical composition had approximately equal SEC distributions. Dual detection SEC can provide only information on the average composition of the eluted portions. The broad but symmetric CCD of the sample yield roughly constant values for the average at any point of the SEC curve for the whole sample. The investigation is a fine demonstration for the advantage of cross-fractionation over dual-detection... 

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