Modeling of Intermediate Structures and Chain Conformation in Silica-Latex Nanocomposites Observed by SANS During Annealing

The evolution of the polymer structure during nanocomposite formation and annealing of silica-latex nanocomposites is studied using contrast-variation small angle neutron scattering. The experimental system is made of silica nanoparticles (Rsi \approx 8 nm) and a mixture of purpose-synthesized hydrogenated and deuterated nanolatex (Rlatex \approx 12.5 nm). The progressive disappearance of the latex beads by chain interdiffusion and release in the nanocomposites is analyzed quantitatively with a model for the scattered intensity of hairy latex beads and an RPA description of the free chains. In silica-free matrices and nanocomposites of low silica content (7%v), the annealing procedure over weeks at up to Tg + 85 K results in a molecular dispersion of chains, the radius of gyration of which is reported. At higher silica content (20%v), chain interdiffusion seems to be slowed down on time-scales of weeks, reaching a molecular dispersion only at the strongest annealing. Chain radii of gyration are found to be unaffected by the presence of the silica filler

Molecular Arrangements and Conformations of Liquid Unbranched Alkanes In Narrow Slits

Realistic, atomistic models of liquid tridecane in broad slits (> 3 nm) and in narrow slits of thickness 1,2 nm and 1,0 nm have been obtained using the Monte Carlo technique. The setup of the models is such that the molecules in the slits are in equilibrium with the bulk liquid. The surfaces of the plates are modelled as two-dimensional arrays of hexagonally packed units having the same size and interaction parameters of a methylene group. The regions adjacent to the plates in slits with thickness > 3 nm are characterized by a well defined tendency to form partially ordered layer structures, while molecules at a distance from the plates larger than 1,5 nm are unperturbed. The simultaneous presence of two plates increases the tendency to form layer structures when their distance is 1,2 nm, while this tendency is almost totally destroyed when the slit is squeezed down to a thickness of 1,0 nm. This is also associated with a 10% decrease of the density in the latter slit

Theories and simulations of polymer-based nanocomposites: From chain statistics to reinforcement

A survey of the present understanding of particle-filled polymers is presented, as obtained from either theoretical or computational approaches. We concentrate on composites in which the nanoparticles are either spherical or statistically isotropic aggregates, and the matrix is a homopolymer melt or a cross-linked elastomer. Recent progress has been prompted by the preparation and careful characterization of well-defined model systems, as well as by theoretical developments and the application of computer simulation to increasingly realistic models. After an introduction providing the main motivations (Section 1), an overview of the basic phenomenology and recent experimental results is presented (Section 2), with special emphasis on the Payne effect and related aspects. In Section 3, we discuss results of equilibrium molecular dynamics and Monte Carlo simulations of polymer chains in the presence of nanoparticles. After a concise theoretical description, these are compared with those obtained from integral equation and density functional approaches (Section 4). The molecular origins of the inter-particle-depletion interaction are discussed, as well as the phase-separation diagram of the nanoparticle/polymer system. The related issue of polymer chains and networks compressed between planar surfaces is also dealt with. In Section 5 simulations and theories of polymer dynamics at the interface are discussed, with special emphasis on the effects of surface roughness and on the vicinity of the glass transition. In Section 6 the overall viscoelastic response of polymer nanocomposites is considered, both from the point of view of molecular-level simulations and of continuum mechanics approaches. The concluding remarks (Section 7) discuss some of the open challenges in the field

CONFORMATIONAL SOLID-SOLID PHASE-TRANSITIONS IN LONG-CHAIN ALKYLAMMONIUM TETRACHLOROMERCURATES(II)

Long-chain alkylamine tetrachloromercurates(II) ((CnH2n+1NH3)2HgCl4, with n = 8-16) show high-enthalpy solid-solid phase transitions at 293-360 K. These are assocd. with the melting of the hydrocarbon parts of the crystal structure while the inorg. layers remain practically unchanged. The behavior of these salts, as compared with that of analogous compds. with different metals, is discussed in terms of the structural parameters of the inorg. layers