Polymerization of styrene in DODAB vesicles : a small-angle neutron scattering study

The polymerization of styrene, located in the bilayer of dioctadecyldimethylammonium bromide (DODAB) vesicles, gives rise to phase separation between the growing polymer and the bilayer. The result is a small (20-30 nm) bead of polymer located in the bilayer of each vesicle giving them a "parachute-like" appearance. Small-angle neutron scattering (SANS) has been used to monitor the evolution of the polymer morphology in situ, on the time scale of polymerization, and thus to characterize all intermediate morphologies. Analysis of the scattering profiles for the case of both photo and thermal initiation reveals that a phase separation sets in immediately after the onset of polymerization involving a fast nucleation interval followed by a slower growth phase. Above the phase transition temperature, the data support an even distribution of monomer within the vesicle prior to initiation. Growth of a single, oblate ellipsoid polymer particle then ensues with the asymmetry of the particle decreasing with increasing extent of polymerization. Neither the reaction temperature nor the mode of initiation (photo or thermal) was found to have a significant effect on this underlying mechanism of particle growth. Conversely, below the phase transition temperature SANS supports an uneven monomer distribution within the bilayer resulting in multiple nucleation and growth of more than one particle per vesicle