TREF temperature rising elution fractionation

The compositional distribution of ethylene copolymers represents relative contributions of macromolecules with different comonomer contents to a given resin. Compositional distributions of PE resins, however, are measured either by temperature-rising elution fractionation (tref) or, semiquantitatively, by differential scanning calorimetry (dsc). Table 2 shows some correlations between the commercially used PE characterization parameters and the stmctural properties of ethylene polymers used in polymer chemistry. 

The authors conducted a similar investigation of precatalysts 7 and 11 using TiBA and trityl tetrakis(pentafluorophenyl)borate as the cocatalyst. They concluded that this material contained no fraction that could be characterized as blocky. It was therefore proposed that reversible chain transfer occurred only with MAO or TMA and not with TiBA. This stands in contrast to the work of Chien et al. [20] and Przybyla and Fink [22] (vida supra), who claim reversible chain transfer with TiBA in similar catalyst systems. Lieber and Brintzinger also investigated a mixture of isospecific 11 and syndiospecific 12 in attempts to prepare iPP/sPP block copolymers. Extraction of such similar polymers was acknowledged to be difficult and even preparative temperature rising elution fractionation (TREF) [26, 27] was only partially successful. 

Temperature rising elution fractionation (TREF), 26 537 method, 20 204... 

Temperature-rising elution fractionation (tref) is a technique for obtaining fractions based on short-chain branch content versus molecular weight (96). On account of the more than four days of sample preparation required, stepwise isothermal segregation (97) and solvated thermal analysis fractionation (98) techniques using variations of differential scanning calorimetry (dsc) techniques have been developed. 

Takahashi, T., 320, 330, 331 Tea decaffeination plant, 7 Temperature-entropy diagram, 138 Temperature rising elution fractionation (TREF), 197, 202-203 Ternary mixtures, phase diagrams, 71-84 Testosterone, 340 Tetracyclic steroids, 293 Thermodynamic modeling, 99-134 Thies, M. C., 88-90... 

On the basis of the IR spectral analysis of the grafted products, Chaoqin Li et al. (44) have concluded that in the PP/PE melt, MAH gets grafted to chains of both PE and PP. This conclusion is based on the fact that the values of characteristic band frequencies of carbonyl absorption of MAH grafted to PE/PP blends (1864.1 and 1785.6 cm ) are between frequency values for MAH grafted to PE (1865.2 and 1784.9 cm ) and to PP( 1862.5 and 1785.8 cm ). However, if Chaoqin Li etal. (44) had separated the (PEZPP)-g-MAH blend into separate PE and PP fractions by some known methods used in polymer blend fractionation (e.g., temperature rising elution fractionation, TREF, or some others), then determination of grafting location by IR spectral analysis would have been much more certain and precise. 

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

The technique of temperature rising elution fractionation (TREF) [76,77] has been developed to measure the compositional distribution of semicrystalline polymers. Polymer is dissolved off a substrate as temperature is raised through the melting region, so that discrimination is based on differences in crystallizability of the fractions. A similar method uses supercritical fluids [78]. TREF can also provide information about the sequence distribution, since longer sequences of a monomer unit are more crystaUizable. 

E.C. Kelusky, C.T. Elston, R.E. Murray, Characterizing polyethylene-based blends with temperature rising elution fractionation (TREF) techniques. Polym. Eng. Sci. 27(20), 1562-1571 (1987)... 

A more accurate characterization of PP can be obtained by fractionations based on solvent/non-solvent mixtures or by a progressive dissolution of carefully crystallized samples (temperature rising elution fractionation, TREF). 

Two methods have been developed that are in common usage and aid the fractionation of polymer materials. These are temperature rising elution fractionation (TREF) (6-8) and Field Flow Fractionation (FFF) (9). 

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. 

Crystaf was developed as an alternative to temperature rising elution fractionation (Tref). Although both techniques are based on similar fractionation mechanisms and provide comparable results, Tref operation tends to be more time-consuming because it involves two fractionation steps, crystallization and elution, while Crystaf requires only the crystallization step. Similarly to Crystaf, the most important fractionation step in Tref occurs during the crystallization step, but data collection in Tref is done only during the elu-... 

Fig.4 Elution step of temperature rising elution fractionation (Tref) analysis and typical Tref profiles of different polymers [5]. LLDPE linear low-density polyethylene, PP polypropylene...

The SEC technique has been in existence for 10 years. It is a relative newcomer to the analytical arena. The amount of information (molecular weight, conformational, and branching) produced, given the ease with which it can be generated, makes SEC a very attractive technique. Recently, the triple detector system has been used in conjunction with temperature rising elution fractionation (TREF) to expand fundamental understanding of polymer structure-property relationships [8]. 

In 1982, Wild and coworkers [17] reported the short-chain branching distribution of LDPE produced in each type of reactor design by utilizing the temperature rising elution fractionation (TREF)analytical technique. The autoclave LDPE sample had a 0.924 g/cc density and a 3.0 Melt Index. The tubular LDPE sample had a 0.921 g/cc density and a 2.2 Melt Index. Ihe branching distribution data from each sample is summarized in Figure 5.7. 

Temperature rising elution fractionation(TREF) is a solvent fractionation technique in which detailed polymer structure information may be obtained by separating molecular species according to their branching content, usually given as number of branches/1000 carbon atoms in the polymer backbone. 

Fig. 6 Temperature rising elution fractionation (TREF) data for olefin block copolymers showing the influence of a-olefin content in the hard block on polymer solubility sample code indicates catalyst composition. (Reprinted from [47] with permission of American Chemical Society)...

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. 

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