The linear and nonlinear melt state viscoelastic properties for a series of layered silicate based intercalated polymer nanocomposites are studied to elucidate the role of highly anisotropic nanometer thick layers in altering the flow properties of such hybrids. The steady shear viscosities for the nanocomposites exhibit enhanced shear-thinning at all shear rates, with the viscosity at high shear rates being almost independent of silicate loading and comparable to that of the unfilled polymer. Further, the elasticity, as measured by the first normal stress difference, when compared at constant shear stress is surprisingly independent of the silicate loading and identical to that of the unfilled polymer. This unique combination of unfilled polymerlike viscosity and elasticity for these filled nanocomposites, is attributed to the ability of the highly-anisotropic layered silicates to be oriented in the flow direction and results in a minimal contribution by the silicate layers to both the viscosity and the elasticity of the hybrids.

The influence of a highly anisotropic layered silicate ͑organically modified montmorillonite͒ in directing the mesoscopic self-assembly of a block copolymer blend is studied as a model for the development and tailoring of templated inorganic-organic hybrid materials. The potential for nanometer thick layers to induce large-scale mesoscopic ordering of cylindrical and spherical microdomains in asymmetric block copolymers is studied using a combination of rheology, electron microscopy, and small angle neutron scattering. Spherical microdomains arranged on a bcc lattice are templated by the anisotropic layered silicate and the kinetics of their growth are dramatically accelerated by the presence of even 0.1 wt.% ͑0.04 vol.%͒ of the filler. However, for cylindrical microdomain ordering, the kinetics are essentially unaffected by the addition of layered silicates and the development of three-dimensional mesoscopic order is possibly even disrupted. These results suggest that for the development of three-dimensional well-ordered nanostructures, the surface defining the pattern has to be significantly larger than the leading dimension of the structure being templated.

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The influence of the addition of small quantities of anisotropic layered silicates on the ordering of block copolymers is studied by a combination of linear viscoelasticity and small angle neutron scattering. Specifically, we examine the influence of varying the lateral dimensions of thermodynamically roughly equivalent layered silicates on the development of cylindrical and spherical microdomain order in a blend of a matched diblock and triblock copolymer. The kinetics for the development of spheres arranged on a bcc lattice from an initial disordered state are dramatically accelerated by the two larger layered silicates with equivalent diameters of ϳ1 and 10 m, while the incorporation of an organically modified laponite, with an equivalent diameter of ϳ30 nm, has no influence on these kinetics. On the other hand, the addition of layered silicates, irrespective of layer dimensions, has no influence on the development of cylindrical ordered microdomains and the epitaxial transformation of shear-aligned cylindrical microdomains to spherical microdomains.

ABSTRACT:Commercially available fourth and fifth generation poly(propyleneimine) (PPI) dendrimers were functionalized with acetyl chloride and deuterated acetyl chloride. Their solution properties in water and D 2 O were measured with dilution viscometry, densitometry, rheology, and small-angle neutron scattering (SANS) and compared to molecular modeling. Both the acetylated and PPI dendrimers exhibited Newtonian rheology in solution at all concentrations, but the functionalized dendrimers were less viscous than the nonacetylated dendrimers at an equal weight fraction (50 wt %). The acetylated dendrimers exhibited a pronounced structure peak in SANS, however, that was not evident for PPI in solution and a greatly enhanced solubility. This structure peak, evident at concentrations as low as 0.2 wt %, was evidence for long-range electrostatic interdendrimer forces, which were screened by added salt. A quantitative agreement was obtained between the dilute-limiting absolute scattering spectra of both the nonacetylated and acetylated dendrimers in solution with model calculations via a homogeneous spherical model and input parameters independently obtained from dilution viscometry or direct calculation. The combined measurements verified significant solvent penetration for both dendrimer types. The form factors measured in this manner were also in good quantitative agreement with the results of molecular dynamics simulations, which pointed to significant backfolding of the terminal groups. SANS and rheology measurements at higher concentrations suggested dendrimer clustering and interpenetration with increasing concentration, leading to less structure and lower viscosity than would be predicted from the dilute-limiting behavior.