Arlette R. C. Baljon

Simulation of polymer melt intercalation in layered nanocomposites

Polymer layered silicates form an important class of nanocomposite materials. These structures may be formed by annealing layered silicate particles, whose surfaces have been chemically modified to render them organophilic, with a polymer melt. During intercalation, polymer molecules leave the bulk melt and enter the galleries between the silicate layers. An essential feature of this process is the flow of macromolecules from a bulk fluid to a confined environment. To model this phenomenon, we have performed molecular-dynamics simulations of the flow of polymer molecules from a bulk melt into a rectangular slit. The simulations are consistent with a diffusive description of the transport, and show qualitative agreement with time-dependent x-ray diffraction measurements of intercalation kinetics in layered nanocomposites.

Spontaneous swelling of layered nanostructures by a polymer melt

Polymer layered silicate nanocomposites may be formed by annealing layered silicate particles with a polymer melt. Polymer molecules leave the bulk melt and intercalate between the silicate layers, producing a structure in which polymers are confined on the nanometer scale by the silicate layers. We report here molecular dynamics simulations of this formation process, which is modeled by the flow of polymer from a bulk melt into a slit whose walls are maintained at constant pressure and whose surfaces are decorated by grafted short hydrocarbon chains. The results are compared with x-ray diffraction studies of the intercalation of high molecular weight polymers into organically modified silicates, and with a previous simulation of the flow of polymer molecules from a bulk melt into a slit of fixed dimension.

Molecular dynamics study of the intercalation of diblock copolymers into layered silicates

Polymer-layered silicate nanocomposites may be formed by annealing layered silicate particles with a polymer melt. Polymer molecules flow from a bulk melt into the galleries between silicate sheets, swelling the silicate structure. The use of an amphiphilic intercalant raises possibilities of forming novel structures and enhancing the intercalation kinetics relative to the case of homopolymer intercalants. We perform molecular dynamics simulations of the flow of a symmetric diblock copolymer from a bulk melt into a slit whose surfaces are modified by grafted surfactant chains, and whose walls are maintained at a constant pressure to permit the slit to open as polymer intercalates. Intercalation kinetics are examined for a variety of polymer–surface and interblock interactions and for thermodynamic states in which the bulk polymer occupies either a lamellar or disordered phase. Comparison to previous simulations of homopolymer intercalation demonstrates that diblock copolymers may be used to intercalate a ...

MOLECULAR VIEW OF POLYMER FLOW INTO A STRONGLY ATTRACTIVE SLIT

We present molecular dynamics simulations of the flow of macromolecules from a bulk melt into a slit of nanometer dimension with strongly attracting walls. Such flow is central to the formation of polymer-layered silicate nanocomposites by direct melt intercalation. In this process, polymer molecules flow from a melt into the galleries between the sheets that compose a mica-type layered silicate. We present a systematic study of the effects of polymer molecular weight and polymer-surface interactions on the flow dynamics.

Adhesion and Friction of Thin Films

Polymers are routinely placed between solid walls to provide lubrication or adhesion. Their function in these roles depends critically on the degree of dissipation within the polymer film and at the film/wall interface as the film shears or ruptures. Good lubrication is achieved by minimizing frictional dissipation while dissipation increases the strength of adhesive bonds. Gent and Schultz suggested a direct link between frictional losses and the adhesive performance of polymers. This correspondence has been supported by recent experiments and by some of the molecular-dynamics simulations to be described. However we find that the correspondence breaks down when the molecular motion producing dissipation occurs at different locations during shear and rupture. In the following sections, we discuss the types of rate-dependent dissipation observed in thin films and the different factors that control whether dissipation occurs within the polymer or at the wall/film interface. The results suggest an origin for interesting memory effects observed in surface-force-apparatus (SFA) experiments on thin films and expose the atomic-scale processes that produce dissipation during internal rupture of a thin film.

A molecular view of bond rupture

Abstract Molecular mechanisms of rupture will be discussed in the light of recent computational studies. Bonds formed by materials as diverse as simple crystalline solids, glassy polymers, and biological ligands and receptors, reveal similar behavior. When these bonds are pulled apart at constant velocity, they rupture through a series of sudden yield events during which the material reorganizes. Yield events are separated by periods of elastic deformation where the stress builds until the system becomes unstable. The nature of the structural change at yield events varies from system to system. Small cavities form in the polymer film, an additional atomic layer is formed in the crystal, and hydrogen binding sites rearrange in the biological system. The work required to rupture these bonds is determined by the full sequence of yield events.

Interfacial and topological effects on the glass transition in free-standing polystyrene films

United-atom molecular-dynamics computer simulations of atactic polystyrene (PS) were performed for the bulk and free-standing films of 2 nm-20 nm thickness, for both linear and cyclic polymers comprised of 80 monomers. Simulated volumetric glass-transition temperatures (Tg) show a strong dependence on the film thickness below 10 nm. The glass-transition temperature of linear PS is 13% lower than that of the bulk for 2.5 nm-thick films, as compared to less than 1% lower for 20 nm films. Our studies reveal that the fraction of the chain-end groups is larger in the interfacial layer with its outermost region approximately 1 nm below the surface than it is in the bulk. The enhanced population of the end groups is expected to result in a more mobile interfacial layer and the consequent dependence of Tg on the film thickness. In addition, the simulations show an enrichment of backbone aliphatic carbons and concomitant deficit of phenyl aromatic carbons in the interfacial film layer. This deficit would weaken the strong phenyl-phenyl aromatic (π-π) interactions and, hence, lead to a lower film-averaged Tg in thin films, as compared to the bulk sample. To investigate the relative importance of the two possible mechanisms (increased chain ends at the surface or weakened π-π interactions in the interfacial region), the data for linear PS are compared with those for cyclic PS. For the cyclic PS, the reduction of the glass-transition temperature is also significant in thin films, albeit not as much as for linear PS. Moreover, the deficit of phenyl carbons in the film interface is comparable to that observed for linear PS. Therefore, chain-end effects alone cannot explain the observed pronounced Tg dependence on the thickness of thin PS films; the weakened phenyl-phenyl interactions in the interfacial region seems to be an important cause as well.