Copolymers 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. 

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. 

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... 

Mirabella, F.M., Correlation of the elution behavior in temperature rising elution fractionation and melting in the solid-state and in the presence of a diluent of polyethylene copolymers, J. Poly. Sci. Pol. Phy. 2001, 39 2819-2832. 

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. 

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)...

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... 

Crystallization fraaionation (CRYSTAF) " and temperature-rising elution fractionation (TREF) are used for chemical composition or crystallinity analysis. For copolymers, CRYSTAF and TREF provide information about the CCD. The drawbacks of these methods are that (1) they are very time consuming and (2) they work only for crystal-lizable polyolefins. 

---