Weight distribution curve, TREF

Figure 3.4 Weight distribution curve of fractions after TREF fractionation of the HDPE samples (A) PE-1 and (B) PE-2, a X) 1 / b W /(A7).

One aspect of the data obtained for TREF of EVA copolymers is their increasingly distorted and truncated distribution curves for the higher VA copolymers. This behavior is reminiscent of SEC of materials in which the molecular weights exceed the exclusion limits of the column system. Here, it seems likely that it is due to the fact that in the normal operation of the TREF analysis, room temperature represents both the lowest crystallization temperature as well... 

In principle, SEC can be calibrated with standards of narrow molecular weight distribution of the polymer that one wishes to analyze. In this way, a calibration curve relating molecular weight to elution volume is obtained, in a fashion similar to what is done for the calibration of TREF or crystallization analysis fractionation. This approach, however, suffers from the limitation that the calibration curve is only applicable to one specific polymer. Additionally, narrow standards are not available for many different types of polymers. In principle, the batch fractionation techniques discussed above could be used to obtain such narrow standards, but these processes are time-consuming and, as explained, do not permit the isolation of samples with very narrow molecular weight distributions. 

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. 

Because of the complexity of the fractionation mechanism, not many mathematical models have been proposed to describe separation with Tref. Soares and Hamielec [47] used Stockmayer s distribution (Eq. 7) to simiflate the CCD of Hnear binary copolymers synthesized with miflti-site-type catalysts. Under the assumption that the fractionation process of Tref was controlled only by comonomer composition, the CCD was directly converted into the Tref profile using a calibration curve. For the case of ethylene/1-olefin copolymers made with multiple-site catalysts, the CCD of the whole polymer is described as the weighted summation of the CCDs of the copolymers produced by each active site ... 

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