Association behavior of polyisoprenes having cholesterol and phosphatidylcholine analogous end groups.

Linear polyisoprenes having dimethylamine end groups were prepared by high vacuum anionic polymerization techniques using 3-dimethylaminopropyllithium as the initiator. The amine group was reacted with 2-cholesteryl-2-oxo-1,3,2-dioxaphospholane to provide polymer chains having end zwitterionic groups chemically connected with cholesterol. The association behavior of these end-functionalized polymers was studied in cyclohexane by low angle laser light scattering, dynamic light scattering, and viscometry. The aggregation numbers, Nw were found to decrease by increasing the molecular weight of the precursor polymer, due to excluded volume repulsions. The ability of cholesterol to form liquid crystal mesophases facilitated the association process leading to higher Nw values. The hydrodynamic behavior of the aggregates was similar to that of star polymers. The dependence of the Nw values on the molecular weight of the base polymer, the polydispersity of the associates and the absence of critical micelle concentration, cmc are compatible with the linear head-packing model.

High frequency acoustic excitations in ordered diblock copolymer studied by inelastic x-ray scattering.

The phonon propagation in lamellar nanostructures formed via self-assembling of short styrene-b-isoprene (SI) as well as of its more incompatible styrene-b-(ethylene-alt-propylene) (SEP) counterpart was studied by inelastic x-ray scattering. Irrespective of the physical state of the block copolymers, a single acoustic phonon was observed in SI (ordered and disordered) and SEP (ordered). At GHz frequencies, inelastic light scattering from the same samples revealed very small dispersion in the sound phase velocity but a short phonon lifetime.