Traditionally the dispersion of particles in polymeric materials has proven difficult and frequently results in phase separation and agglomeration. We show that thermodynamically stable dispersion of nanoparticles into a polymeric liquid is enhanced for systems where the radius of gyration of the linear polymer is greater than the radius of the nanoparticle. Dispersed nanoparticles swell the linear polymer chains, resulting in a polymer radius of gyration that grows with the nanoparticle volume fraction. It is proposed that this entropically unfavorable process is offset by an enthalpy gain due to an increase in molecular contacts at dispersed nanoparticle surfaces as compared with the surfaces of phase-separated nanoparticles. Even when the dispersed state is thermodynamically stable, it may be inaccessible unless the correct processing strategy is adopted, which is particularly important for the case of fullerene dispersion into linear polymers.

Nanoparticles have been shown to influence mechanical properties; however, transport properties such as viscosity have not been adequately studied. This might be due to the common observation that particle addition to liquids produces an increase in viscosity, even in polymeric liquids, as predicted by Einstein nearly a century ago. But confinement and surface effects provided by nanoparticles have been shown to produce conformational changes to polymer molecules, so it is expected that nanoparticles will affect the macroscopic viscosity. To minimize extraneous enthalpic or other effects, we blended organic nanoparticles, synthesized by intramolecular crosslinking of single polystyrene chains, with linear polystyrene macromolecules. Remarkably, the blend viscosity was found to decrease and scale with the change in free volume introduced by the nanoparticles and not with the decrease in entanglement. Indeed, the entanglements did not seem to be affected at all, suggesting unusual polymer dynamics.

A novel approach is presented for the controlled intramolecular collapse of linear polymer chains to give well-defined single-molecule nanoparticles whose structure is directly related to the original linear polymer. By employing a combination of living free radical polymerization and benzocyclobutene (BCB) chemistry, nanoparticles can be routinely prepared in multigram quantities with the size being accurately controlled by either the initial degree of polymerization of the linear chain or the level of incorporation of the BCB coupling groups. The latter also allows the cross-link density of the final nanoparticles to be manipulated. In analogy with dendritic macromolecules, a significant reduction of up to 75% in the hydrodynamic volume is observed on going from the starting random coil linear chains to the corresponding nanoparticles. The facile nature of the living free radical process also permits wide variation in monomer selection and functional group incorporation and allows novel macromolecular architectures to be prepared. Furthermore, the use of block copolymers functionalized with benzocyclobutene groups in only one of the blocks gives, after intramolecular collapse, a hybrid architecture in which a single linear polymer chain is attached to the globular nanoparticle.

The effect of nanoscopic, shape persistent polystyrene (PS) nanoparticles on the rheological properties of linear PS is studied and a dramatic viscosity reduction is observed. This is an ideal blend which simplifies enthalpic interactions between the components and can be used to delineate the effect of particle size on the properties of the blends. Homogeneous blends are assured through small-angle neutron scattering (SANS) experiments which establish the absence of phase segregation (depletion flocculation) in the nanoparticle-polymer blend. We previously found that nanoparticles reduce the viscosity of high molecular mass linear PS when the interparticle gap is smaller than the linear polymer size. In the present study, we find that such confinement of entangled polymers is necessary for the viscosity reduction since lower concentrations provide a viscosity increase. Furthermore, the behavior is found to be dependent on the presence or absence of entanglements and confinement is seemingly not important for unentangled polymers. It is proposed that constraint release caused by the addition of nanoparticles is responsible for some of the observed changes in viscosity although it is suspected this is a very complicated phenomenon and introduction of free volume by the nanoparticles is certain to play a key role.

Surface‐initiated living free‐radical polymerization is employed in a multistep procedure to prepare hollow polymeric nanocapsules. Initially, trichlorosilyl‐substituted alkoxyamine initiating groups are attached to the surface silanol groups of silica nanoparticles. This surface layer of initiating groups is then used to grow functionalized linear chains leading to a core–shell morphology. The choice of functional groups is governed by their ability to undergo facile crosslinking reactions, with both active ester and benzocyclobutene groups being examined. Under either chemical or thermal conditions, the reaction of these functionalities gives a crosslinked polymeric shell that is covalently attached to, and surrounds, the central silica core. Removal of the silica core with HF then gives the hollow polymeric nanocapsules, which are stable under solvent dissolution and thermal treatment because of their crosslinked structure. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1309–1320, 2002