Effect of molecular properties of block copolymers and nanoparticles on the morphology of self-assembled bulk nanocomposites

Self-assembly of thiol-terminated polystyrene-tethered Au nanoparticles in microphase-separated diblock copolymers composed of poly(styrene-b-2vinylpyridine) (PS-PVP) as a function of particle concentration and composition of block copolymers was investigated using anomalous small-angle X-ray scattering (ASAXS) and transmission electron microscopy (TEM). Results reveal that the self-assembly of nanoparticles in the PS domain causes swelling and increases the interfacial curvature that, in turn, induces order-order transitions. At intermediate loading, the presence of nanoparticles amplifies the local compositional fluctuations, hence the roughness at the PS and PVP interface, which creates conditions to induce disorder in the polymer morphology. The system thus undergoes an order-disorder transition. At high particle loading, packing constraints prevent all particles from assembling in the PS domain, and the excess nanoparticles undergo macrophase separation. The present systematic study augments experimental data to the scarce literature on the phase behavior of bulk nanocomposites. We present a generalized phase map for the bulk composites as a function of effective volume fraction of PS (F-PS) for a given nanoparticle size. We believe that the results from this study will enable comparison of the phase maps from various studies and will serve to validate the simulation studies of inorganic particle/block copolymer composites.