A new technique to characterize mono-molecular micelles in random ethylene–propylene copolymers

Abstract

A new technique to investigate the nano-structure of ethylene–propylene (EP) random copolymers has been developed. It consists in the measurement of the turbidity which develops at a lower critical solution temperature (LCST) in pentane solutions. The information on the solution comes from different types of turbidity obtained during a step-by-step temperature increase. The transient turbidity (hi) is associated with random coils (I) and structured coils (II) while the stable turbidity comes from aggregates (III). The proportion of (I), (II) and (III) depends on the solution history and on the solvent. The Mw distribution can be obtained from the set hi (Ti) of (I). Turbidity (II) has an unexpected gap in the hi (Ti) trace. The gap (10–20 K) is explained by the presence of two entities in solution. Their temperatures of phase separation permit their identification as monomolecular micelles, whose outer core is either E-rich or P-rich. This nano-structure is thought to exist in the solid and also in solution as a metastable state. The technique can differentiate between mobile chains in solutions (I, II) and attached chains in a network (III) through the sedimentation behaviour of the concentrated phase. Three samples with a similar (EP) content (0.75) made with different catalysts have been analysed by LCST and slow calorimetry.