Polymers cross-fractionation

Balke ST, Patel RD (1983) Orthogonal chromatography. Polymer cross-fractionation by coupled gel permeation chromatographs. In Craver CD (ed) Polymer characterization. Adv Chem Ser 203, American Chemical Society, Washington, DC... 

The advantage of this sequence lies in the fact that SEC of complex copolymers responds to both molecular weight and chemical composition, while the influence of molecular weight on HPLC fractionation can be minimized by the selection of adequate analysis conditions. The importance of polymer cross-fractionation techniques is increasing because of the rapidly growing use of polymers with complex microstructures and would require a separate chapter for complete discussion. The interested reader is referred to the article by Pasch for a review on recent advances in the field of cross fractionation of polymers (96). 

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 4. Entrance region polymer weight fraction (relative to value at the tube centerline) profiles in the tube cross section for a zeroth order reaction and uniform viscosity at GrSc = 10 and = 0.05 and 0.1.

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. 

In the absence of adsorption, inclusion, or exclusion, a polymer is fractionated on a GPC column according to the hydrodynamic volume.40138 The hydrodynamic volume is a function of monomer identity, as well as polymer molecular weight, branching, and cross-linking. The polymer chains in any given chromatographic fraction have roughly the same hydrodynamic volume. 

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. 

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. 

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,...

Glockner, G., van den Berg, J. H. M., Meijerink, N. L., Scholte, T. G. Characterization of copolymers chromatographic cross-fractionation analysis of styrene-acrylonitrile copolymers , in Kleintjens, L., Lemstra, P. (ed) Integration of Fundamental Polymer Science and Technology , Elsevier Applied Science Publ., Barking, UK (1986), p. 85... 

In mechanistic models these interactions can be directly simulated. Thus the issue of kinetic coupling (molecular interactions) may well be somewhat artificial and only introduced by analysis at the less detailed molecular or global levels. Likewise, the intrusions of diffusion may also be somewhat artificial and a result of modelling at the molecular or global level. That is, mechanistic simulations can now account for the movement as well as reaction of molecules and active centers (54). This becomes especially convenient when the device of a percolation lattice is used. Molecules can then be assembled, moved and reacted on the lattice which, in addition to allowing for simulation of the mechanism of diffusion in reaction, can also provide information about global product fractions, such as polymer gel fraction and cross-link density. The literature of polymer science is rich in these types of applications. 

By the use of different modes of liquid chromatography it is possible to separate polymers selectively with respect to hydrodynamic volume (molar mass), chemical composition or functionality. Using these techniques and combining them with each other or with a selective detector, two-dimensional information on different aspects of molecular heterogeneity can be obtained. If, for example, two different chromatographic techniques are combined in a cross-fractionation mode, information on CCD and MMD can be obtained. Literally, the term chromatographic cross-fractionation refers to any combination of chromato-... 

Recent studies [111,214] indicate that Th-FFF can even be used to determine the relative chemical composition of two components in random copolymer and linear block copolymers whose monomers do not segregate due to solvent effects. However, this application is limited by the unpredictable nature of thermal diffusion. Nevertheless, combining information from Th-FFF with those derived on fractions by independent detectors selective to composition (such as an IR spectrometer) can yield further insight into the dependence of DT on the chemical composition. Even more powerful is the combination of Th-FFF with SEC as, here, the chemical composition (from Th-FFF) can be studied as a function of the molar mass (from SEC). This was demonstrated by van Asten et al. by cross fractionating copolymers and polymer blends with SEC and Th-FFF [358]. 

Cross-linked pre-polymers having fractionalized molecular weights between 35,000 and 70,000 daltons with polydispersity indexes of less than 3.4 have been prepared by the free radical addition of 2-hydroxethyl methacrylate, HEMA, with 2% methacrylic acid or glycerol methacrylate. Once crosslinked, these materials are particularly useful as contact lenses because of their limited shrinkage and expansion. 

Teramachi S and Fukao T (1974) Cross fractionation of styrene-acrylonitrile copolymer. Polym J 6 532-6. 

This book covers some of the significant advances in hyphenated chromatographic separation methods for polymer characterization. Chromatographic separation techniques in this volume include size-exclusion chromatography, liquid chromatography, and field flow fractionation methods that are used in conjunction with information-rich detectors such as molecular size-sensitive or compositional-sensitive detectors or coupled in cross-fractionation modes. 

To cross-fractionate a given TREE fraction in terms of MWD, online TREE-SEC-ET-IR instrumentation has been developed to obtain three-dimensional plots of polymer concentration. My, and composition (short-chain branching) (18). 

CCC. A schematic diagram of the CCC system is shown in Figure 4 [this technique also has been named orthogonal chromatography 10-J3)]. Cross-fractionation on the polymer solution injected into the SEC column was performed on the solution passing through the switching valve in the time interval 830-850 s. This fraction was separated by isocratic elutions with column two. 

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