Effect of particle size on the phase behavior of block copolymer/nanoparticle composites

Effect of particle size on the self-assembly of thiol-terminated polystyrene-stabilized Pd nanoparticles in microphase-separated block copolymers composed of poly(styrene-b-2 vinyl pyridine) (PS-PVP) was investigated using transmission electron microscopy and differential scanning calorimetry. We observe that the state of particle dispersion depends strongly on the relative size between the particle diameter (d) and PS domain spacing (D). As d/D similar to 0.3, the self-assembly of nanoparticles in the PS domains causes swelling and increases the interfacial curvature that induces order-order transitions. At intermediate d/D, the introduction of nanoparticles in the PS domains causes polymer stretching, resulting in the reduction of the conformational entropy. This behavior leads to the particle aggregation, which induces the macrophase separation. Additionally, these particle aggregates increase in size with increasing particle loading, and eventually they create conditions to induce disorder in the polymer morphology. As d is compatible to D, packing constraints prevent particles from assembling in the PS domains, and particles aggregate at low particle loading. This study provides valuable experimental results on the recent emerging field of nanostructured polymer nanocomposites. Our results will enable to gain better insight into the physical properties of these functional materials and will serve to validate the simulation studies of block copolymer/nanoparticle mixtures. (C) 2010 Published by Elsevier B.V.