Polystyrene Nanoparticles with Anionically Polymerized Polybutadiene Brushes

Model hairy nanoparticles of highly cross-linked polystyrene (PS) cores and linear polybutadiene (PBd) brushes were synthesized. The PS cores were obtained using conventional microemulsion polymerization. The linear PBd brushes were synthesized using surface initiated living anionic polymerization. The PS core nanoparticles of 44 ± 12 nm in diameter (volume weight average) were first lithinated using a combination of butyllithium and N,N,N‘,N‘-tetramethylethylenediamine. Butadiene was subsequently polymerized at the surface of the particles. Two series of samples were synthesized with different degrees of polymerization (N) and grafting densities (σ). In the first series, while σ was constant at ∼0.14 chains/nm2 (∼500 chains per particle), N changed from 37, 74, 139, to 198. Accordingly, the M w of brushes changed from 2.0, 4.0, 7.5, to 10.7 kg/mol. In the second series, the weight ratio between PS and PBd was fixed at PS/PBd ≅ 5/2 (σ N was constant in this case) and σ varied from 0.15 to 0.08 chains/nm2 and lower. On the basis of the atomic force microscope (AFM) results, it was found that these hairy particles appeared to obtain better dispersion in PBd matrix as N of grafted chains was increased in the first series. More pronounced changes were observed, such as severe aggregation of particles, when σ decreased from relatively high to low with fixed total weight of brushes in the second series. The results were explained using theoretical calculations for grafted planes in contact with a chemically identical melt. In the system of grafted particles with short brushes (N < P, P is the degree of polymerization of the melt) studied in this paper, it was realized that the parameter σ can represent the surface characteristics of the particles and be correlated to the dispersion of particles. A higher σ value corresponds to better dispersion. The properties of cured rubber samples containing hairy particles were also examined using a rubber process analyzer (RPA) and a dynamic mechanical analyzer (DMA). RPA results are consistent with the morphological changes observed with AFM in terms of the “Payne effect”. DMA results show that the glass transition temperatures, associated with the core and brushes, shift when the morphology changes. The AFM, RPA, and DMA results prove the strong correlation among the nanostructure of brushes, the microscopic dispersion of particles, and macroscopic properties of nanocomposites.