Short chain branching distribution of LLDPE by tref and NMR

Abstract

In this study, a preparative temperature rising elution fractionation apparatus (TREF) was constructed. Three commercial grade LLDPE samples, i.e. ethylene/l-butene copolymers (C₄-LLDPE), ethylene/1-hexene copolymers (C₆-LLDPE), andethylene/l-octene copolymers (C₈-LLDPE) were fractionated by this TREF. Solvent type and cooling rate in the crystallization step were the two major parameters investigated in this study. With the aim to reduce the fractionating time, choices of solvents included xylene, xylene/ethylene glycol and xylene/glycerol. The cooling rate was varied from 1-10°C/h. The crystallized polymer fractions were subsequently eluted out of the column. Each of the eleven fractions was analyzed for short chain branching content by 13C-NMR performed at l30°C. The highest cooling rate for fraetionating C₄-LLDPE was found to be 5°C/h when xylene was used during crystallization. The cooling rate could be increased to 7°C/h when using 0.3% ethylene glycol/xylene while obtaining the same fractionating efficiency. For C6-LLDPE and C8-LLDPE, the highest cooling rate was found to be 2°C/h with xylene, but 2°C/h and 1°C/h with 0.3% ethylene glycol/xylene, respectively. When 0.1% glycerol in xylene was employed, the cooling rate could not be higher than 1°C/h for all three LLDPE samples. DSC was also used to determine the thermal characteristics of every fraction obtained from TREF using xylene in crystallization step.