MICROGELS - MODEL POLYMERS FOR THE CROSS-LINKED STATE

Polystyrene microgels, small network particles with an overall size in the region of 100 nm, are synthesized via radical copolymerization of styrene and m-diisopropenylbenzene in microemulsion. The resulting structures are characterized by viscosimetry, static and dynamic light scattering (LS), and dynamic mechanical experiments. These microgels represent strictly spherical particles with a moderate polydis- persity where the size of the microemulsion droplets can only be adjusted in the restricted range 60 nm < d < 200 nm. This is possibly due to the polymeric surfactant used. The intrinsic viscosity data are related to molecular swelling where the internal density of the particles in dependence of the cross-linking density obeys the rules of rubber elasticity. From LS experiments it is concluded that the swelling occurs heterogeneously even on the size scale given by the microgel dimensions. The application of microgels as a model for macroscopic networks is extended to the examination of the influence of dilution during the cross- linking reaction. The efficiency of cross-linking decreases with increasing amount of solvent. Dynamic mechanical shear experiments of microgels result in a typical rubbery plateau at high frequencies and small strains known also from macroscopic networks. The absolute plateau value indicates a large amount of defects reducing the cycle rank of the network. Dilution during cross-linking causes a qualitative change of the complete spectrum, which exhibits a continuous loss of the moduli toward lower frequencies, a behavior known for systems close to gel transition. © 1990, American Chemical Society. All rights reserved.