Temperature rising elution

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 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]. 

Anantawaraskul, S., Soares,. B. P. and Wood-Adams, P. M. Fractionation of Semicrystalline Polymers by Crystallization Analysis Fractionation and Temperature Rising Elution Fractionation. Vol. 182, pp. 1-54. 

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. 

Whereas in the example just described the sample amount was about 50 mg, a similar procedure developed by another group 129) started with 4 g polyethylene copolymer. The sample was applied as a dilute solution in xylene and precipitated by very slow cooling (1.5 K/h) onto the Chromosorb P packing of a 500 x 127 mm column. The first separation was temperature-rising elution fractionation at a flow-rate of 20 ml/min and a Unear temperature increase by 8 K/h. The MMD of the fractions was measured by SEC at 145 °C in o-dichlorobenzene at 0.7 ml/min flow rate. The column set included a pair of bimodal columns 100 A and 1000 A plus a 4000 A column. The apparatus was equipped with an IR detector. The experimental data is computed to show the distribution of short-chain branching and of molar mass simultaneously. 

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 (TREE) to expand fundamental understanding of polymer structure-property relationships [8]. 

Polymers by Crystallization Analysis Fractionation and Temperature Rising Elution... 

Watkins et al. (1992) demonstrate how semicrystalline LLDPE can be fractionated both by molecular weight and backbone structure. The first step of this double fractionation is to fractionate with respect to molecular weight. In this step the fractionation is carried out as an isothermally increasing pressure profile. The second step is to fractionate with respect to backbone structure using an isobarically increasing temperature profile. They termed the second step of the double fractionation CITREF, which is an acronym for critical isobaric temperature rising elution fractionation, a supercritical fluid variation of TREF, a liquid phase fractionation called temperature rising elution... 

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... 

The temperature rising elution fractionation (TREE) (3-5) and temperature gradient extraction fractionation (TGEF) (6) techniques have been proven to be the powerful tools to analyze the fractions, M, MWD, and stereo regularities of both amorphous and crystalline components of PE or PP. The separated fractions can be examined further by incorporation of light scattering, gel permeation... 

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. 

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