| Polymer brushes, forming a bridge between polymers and colloids, have invoked considerable interest due to the ability to pursue well-dispersed polymer nanocomposites, a new paradigm over the conventional macro-composite. This work investigates understanding the structure and dynamics of silica polymer brush nanocomposites. In this context, well-defined homopolymer and block copolymer based hybrid materials were studied. Use was made of a wide range of structural and dynamics tools of x-ray and neutron scattering, transmission electron microscopy, melt-state dynamic viscoelasticity and in-situ shear-x-ray scattering.; In the case of homopolymer grafted chains, these particles were seen to arrange themselves onto a crystalline lattice, attributable to purely entropic reasons, and exhibit a solid-like rheological response. On the other hand, replacing the homopolymer chains with strongly-segregated block copolymer chains, led to worm-like micellar structures. In the homopolymer brush system, experimental evidence indicates that the polymer chains on these brushes behave in accordance with the physical description of star polymers, as given by the classical Daoud-Cotton model. In addition, diluting these brushes with free matched polymer chains, led to a wetting of the brushes, and to a melting of the long-range underlying structure, in synergy with a weakening of the viscoelastic properties. The nanoparticles were seen to dominate the viscoelastic behavior (∼&phis;4), with a weak contribution (∼ Mn1) of the polymer chains, thereby delineating the effects of each. The nanoparticles were also found to interact via a strong interparticle (Lennard-Jones-like) (∼ r-12) potential.; Under large strains and steady shearing on the hybrids and their dilutions, the crystalline lattice was found to melt via defect motion along slip-planes. However, due to entropic effects, upon cessation of shear, the crystalline lattice is seen to quickly recover. |
