Structural and stability characteristics of agglomerated clusters

We investigated the cohesion of agglomerates formed by sticking two fractal clusters, each cluster having been previously generated by particle aggregation on a 3D lattice. The degree of cohesion of an agglomerate of a given configuration was defined by the number of connections simultaneously established on the two stuck clusters. All the possible nonoverlapping configurations were investigated and the corresponding porosity and brittleness as well as the pore volume and connection frequencies were determined. The numerical study showed the greater internal cohesion of agglomerates issued from sticking of reaction-limited aggregation (RLA) clusters compared to that of diffusion-limited-aggregation (DLA) clusters. DLA and RLA agglomerates presented continuously decreasing pore volume frequency curves, the latter agglomerates being characterised by a greater frequency of large pores. Comparison with typical controlled fragmentation experiments showed the number of connections to be the prevailing factor in the cohesion of aggregates formed in aqueous suspensions under various conditions. Received: 25 January 2001 Accepted: 16 May 2001     

Structural properties of atactic polystyrene adsorbed onto solid surfaces

In the present work, we are studying the local conformation of chains in a thin film of polystyrene adsorbed on a solid substrate by using atomistically detailed simulations. The simulations are carried out by using the readily available and massively parallel molecular dynamics code known as LAMMPS. In particular, a special emphasis is given to the density and orientation of side chains (which consist of phenyl groups and methylene units) at solid/polymer and polymer/vacuum interfaces. Three types of substrates were used in our study: α-quartz, graphite, and amorphous silica. Our investigation was restricted to atactic polystyrene. Our results show that the density and structural properties of side chains depend on the type of surface. An excess of phenyl rings is observed near the α-quartz substrate while the film adsorbed on graphite is depleted in C(6)H(5). Moreover, the orientation of the rings and methylene units on the substrate/film interface show a strong dependence on the type of the substrate, while the rings at the film/vacuum interface show a marked tendency to point outward, away from the film. The results we obtained are in a large part in good agreement with previous experimental and simulation results.     

Cluster-cluster aggregation controlled by the number of intercluster connections: kinetics of aggregation and cluster mass frequency

The aggregation of colloids in the presence of hydrodynamic forces was investigated, employing a numerical model that took into account the masses of the individual clusters and the number of intercluster connections established when two clusters stuck together. The number of possible connections was determined by analyzing all the possible nonoverlapping configurations of stuck clusters. This operation was done for a couple of clusters of various masses, taking into account the assembly of clusters of even and uneven masses. The formulation of the constraints established a certain hierarchy in the sticking on a basis compatible with the irregular fracture model of Horwatt and co-workers. As a result, the permanent sticking of large clusters required the formation of a large number of connections, whereas that of small clusters might be realized even with a small number of connections. Thus, the aggregation started with the features of the standard reaction-limited process and this cluster growth became progressively inhibited as a result of the prevailing effects of the connection constraints. The cluster-mass frequency showed the emergence at least of a second population whose bell-shaped mass distribution was superimposed on the monotonically decreasing distribution resulting from the reaction-limited aggregation process. The results of the numerical study were confronted with those previously obtained in the aggregation of hydrated polystyrene latex particles dispersed in 1 M sodium chloride solution. The two striking features--the aggregate growth kinetics and the mass distribution function--were common to the computer-generated clusters and the latex aggregates.     

Origin of Bends in Unperturbed Vinyl Polymers: An Illustration with Polystyrene

Previous experimental works have shown that dendronized vinyl polymers exhibit bends when adsorbed onto a surface. Two different mechanisms are believed to be responsible for the formation of these bends. These mechanisms are the temperature dependent random fluctuations of torsional bond states on one hand, and the intramolecular interactions due to the randomness in the stereochemical sequence of side chains on the other hand. Investigation of the amplitude and scope of the above mechanisms has been made by studying the conformational space of PS chains via RIS based Monte Carlo sampling. It was found that at low temperature bend formation is due to tacticity, whereas it was thermally driven at high temperature. The existence of a transition temperature between these two bend formation modes was demonstrated. It was also shown that for atactic chains, the maximum of bend formation occurs at P_m ≈ 0.7.