Modeling the Fractionation Process in TREF Systems. III. Model Validation With Low Molecular Weight Homopolymers

Abstract

SYNOPSISLow molecular weight semicrystalline homopolymers are used as a model system for temperature rising elution fractionation (TREF) analysis. An already proposed thermodynamic model for TREF analysis is used to characterize TREF fractions from low molecular weight polyethylenes M n = 500 to 3000 and some of their mixtures. In this molecular weight range it is possible, under appropriate crystallization conditions, to form extended-chain crystals, and therefore lamellar thicknesses become comparable to extended-chain lengths. Lamellar thicknesses calculated from TREF spectra permit calculations of the molecular weights of the fractions, up to a limit of about 142 CH,, where partially folded-chain crystallites appear under these operating conditions. Also homopolymers blends are fractionated and the TREF spectra analyzed to test model predictions. It is shown that appearance of chain folding may set a resolution limit to the analysis of commercial copolymers by TREF. 0

Characterization of commercial linear low-density polyethylene by TREF-DSC and TREF-SEC cross-fractionation

“…The results also provide another possible interpretation of the TREF separation mechanism which has not yet been fully understood in the open literature 24–26. Although it is generally agreed that TREF fractionates semicrystalline polymer based on crystallizability, and so most of the TREF calibrations have been based on the average SCB generated from PTREF,1, 2 there has been suggestions that TREF separates macromolecules based on the length of the crystallizable sequence 25, 26. This can be easily understood by considering a crystallized molecule in the TREF column; only when the longest sequence in the molecule dissolves does the whole molecule elute from the column.…”

Characterization of commercial linear low-density polyethylene by TREF-DSC and TREF-SEC cross-fractionation

“…24 -26 Although it is generally agreed that TREF fractionates semicrystalline polymer based on crystallizability, and so most of the TREF calibrations have been based on the average SCB generated from PTREF, 1,2 there has been suggestions that TREF separates macromolecules based on the length of the crystallizable sequence. 25,26 This can be easily understood by considering a crystallized molecule in the TREF column; only when the longest sequence in the molecule dissolves does the whole molecule elute from the column. As shown in Figure 7, the shifting of the methylene sequences toward high elution temperatures or long sequences implies that the longest methylene sequence length may dictate the separation by TREF.…”

Effects of low molecular weight compounds with hydroxyl groups on properties of poly(L‐lactic acid)

“…The first approach describes TREF profiles based on the Stockmayer bivariate distribution assuming that all polymer molecules with a specific comonomer content are fractionated at the same elution temperature 9–11. The second approach assumes that the solid–liquid equilibrium between crystallized polymers and polymers in a dilute solution at each elution temperature governs the TREF fractionation process 21–25. Both modeling approaches intrinsically assume that fractionation takes place at thermodynamic equilibrium and cannot describe the important crystallization and dissolution kinetics effects (cooling, heating, and solvent flow rates) that have been observed experimentally in TREF 18…”