Content area
Abstract
Two random copolymers of ethylene and propylene behaved very differently over temperature in solution. Polymer E/P:60/40, with 60 mole % ethylene and 40 mole % propylene, showed little microstructural change in two solvents used in this study (methylcyclohexane and tetralin) as temperature was changed from 50$\sp\circ$C to $-$10$\sp\circ$C. Polymer E/P:80/20 showed a large degree of microstructural change.
The microstructure of these polymers in solution was studied using viscometry, dynamic light scattering, static light scattering and microscopy. Microstructural parameters measured were, average polymer density, hydrodynamic radius, aggregation number, cluster excluded volume and radius of gyration. The 60/40 polymer showed aggregation numbers of one to ten polymers per cluster with a radius of gyration between 25 and 50 nm in both solvents over the entire temperature range. The 80/20 polymer showed aggregation numbers from 100 to 5000 polymers per cluster as the temperature varied from 50$\sp\circ$C to $-$10$\sp\circ$C respectively in both solvents. While this tendency to aggregate was similar in both solvents the change in cluster radius with temperature was different. In methylcyclohexane the radius increased from 250 nm to 500 nm as the number of polymers per cluster increased. This caused the average polymer density within the cluster to remain constant over temperature. In tetralin the radius of 200 nm remained virtually unchanged over temperature. This resulted in an increased average polymer density at $-$10$\sp\circ$C compared to 50$\sp\circ$C.
In both solvents the high temperature excluded volume of these 80/20 clusters was significantly larger than the volume calculated from the radius of gyration. At lower temperature the excluded volume was closer to the volume calculated from the radius of gyration. This suggested a microstructure that consisted of a dense core surrounded by a less dense corona at high temperature. At low temperature the corona volume diminished and the core volume increased. We postulate that the high ethylene content segments of the chain form "fringed micelles" in both the corona and the core and are responsible for the physical crosslinks of these highly aggregated "microgel" clusters.
