Fractionation Process in TREF Systems: Validation of of Thermodynamic Model and Calculation Procedure by Raman LAM Studies

J. PABLO TOMBA, JOSE M. CARELLA, JOSE M. PASTOR

JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS

Año: 2005 vol. 43 p. 3083 - 3092

0887-6266

In this paper, possible sources for the unexpected distributions of crystallinesequence lengths calculated from temperature rising elution fractionation (TREF) calibrationexperiments, as reported in a previous work, are investigated. With this aim,chain folding and cocrystalization phenomena were explored in the conditions of crystallizationas used for TREF or crystallization analysis fractionation (CRYSTAF). Slow crystallizationswere performed from xylene solutions of model low molecular weight ethylenehomopolymers with narrow molecular weight distributions. The same experiments wereperformed with homopolymers having narrow molecular weight distributions and withblends having wide molecular weight distributions. The resulting distributions of thelengths of crystalline methylene sequences were directly studied by Raman in the socalledlongitudinal acoustic mode (LAM) and by DSC. For ethylene homopolymers withmolecular weights below 2000 g/mol, the results from Raman LAM indicate that slowcrystallization in TREF or CRYSTAF systems occurs in the extended-chain mode. Forhigher molecular weights, evidence of chain folding was found. In the case of blends, independentcrystallization was observed for each molecular weight when the molecularweight ranges used for the blends are relatively narrow. Cocrystallization was observedwhen this range was increased. Overall, these results strongly support the inverse techniquecalculation procedure developed by our group for the calculation of distributions oflengths of crystallizable sequences from TREF spectra. In this context, the results con-firm that the unexpected crystallizable sequence lengths found in our previous workreally exist and can be associated to chain folding or cocrystallization phenomena.