Temperature-rising elution distribution

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]. 

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 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. 

SEC is routinely used to produce narrow distribution samples by fractionation. However, even with preparative SEC columns the procedure is not particularly effective, and many repeated fractionations are required to produce more than milligram samples. Nevertheless, such samples are invaluable in crystallization-rate [70] and other studies where molecular mass exclusion is very important. Coupling molecular mass with temperature-rising elution fractionation, in which molecular species separate by composition, in copolymers, or by degree of branching and tacticity, in homopolymers, makes this a most important method for separating molecular mass and chemical effects in crystallization studies [17]. 

The SCBDI = wt% of macromolecules having a comonomer content within 50 % of the median total molar comonomer content, calculated from TREF (temperature rising elution fractionation) data. The elastic, substantially linear C2-Cg copolymer has 0.01 < LCB/IOOOC < 3, M /Mn = 1.5-2.5, 2 < SCB (CH3/IOOOC) < 30), and short-chain branch distribution index SCBDI > 50 %. The homogeneously branched copolymer may he produced as described in C. T. Elston (DuPont Canada Ltd.) patent. Films produced from the bimodal MWD new copolymers show good impact and tensile properties... 

J.B.P. Soares, A.E. Hamielec, Temperature rising elution hactimiatimi, in Modem Techniques for Polymer Characterization, ed. by R. A. Pethrick, J. V. Dawkins (Wiley, New Yrak/ChichestCT, 1999) J.B.P. Soares, B. Monrabal, J. Nieto, B.J. Javier, Ciystallization analysis Ixactionation (CRYSTAF) of poly(ethylene-co-l-octene) made with single-site-type catalysts a mathematical model for the dependence of composition distribution on molecular weight. Macromol. Chem. Phys. 199, 1917-1926 (1998)... 

Temperature rising elution fractionation (TREF) (161,162) is a useful fractionation technique which gives information on the compositional distribution. 

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. 

Abstract The synthesis and characterization of polyolefins continues to be one of the most important areas for academic and industrial research. One consequence of the development of new tailor-made polyolefins is the need for new and improved analytical techniques for the analysis of polyolefins with respect to molar mass, molecular topology and chemical composition distribution. This review presents different new and relevant techniques for polyolefin analysis. The analysis of copolymers by combining high-temperature SEC and FTIR spectroscopy yields information on chemical composition and molecular topology as a function of molar mass. Crystallization based fractionation techniques are powerful methods for the analysis of short-chain branching in LLDPE and the analysis of polyolefin blends. These methods include temperature-rising elution fractionation, crystallization analysis fractionation and the recently developed crystaUization-elution fractionation. 

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. 

Copolymers are becoming increasingly important for high performance and new materials with specific mechanical, optical, and electrical properties. The bivariate composition and mass distribution controls many aspects of the materials behavior, such as tensile strength, processability, surface, phase stabihty, and so on. Nonetheless, determining the bivariate distribution can be time consuming and costly, and usually requires the use of complementary techniques, such as thermal field-flow fractionation [8], temperature rising elution fractionation (TREE) [9], Fourier transform infrared (FTIR) spectroscopy [10], and other methods [11] for... 

In order to analyze the SCBD of these two samples, the first step is to use temperature rising elution firactionation (TPJEF) to physically separate the two HDPE samples and get their firactions for the subsequent characterizations. The weight distribution of fractions versus crystalline temperature of the two HDPE samples is shown in Fig. 3.4. Both TREF curves are narrow as typical HDPE samples with very small amount of low and high temperature firactions. 

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