Polymer-particle interactions, macroscopic

With nanometric particles the problem of polymer adsorption is different from the usual adsorption on macroscopic surfaces (Spalla O, Cabane B. Coll. Pol. ScL, in press). For one thing, nanometric dispersions have an extremely large surface area, hence they can adsorb a substantial amount of polymer before being saturated. In fact the amount of bound polymer is often comparable to the amount of particles in the system thus the system may be better described as a two-component system, similar in nature to mixed solutions of two polymers which interact with each other. This paper presents an analysis of such mixed particles+polymer dispersions from the point of view of two-component systems. 

Adsorbed on the surface of dispersed particles, polymer chains lessen the attraction energy by steric reasons (the minimum distance to which particles can approach increases) and because they change the efficient Hamaker s constant value. The attraction energy in expressions for Ur dependence on A is the function of not only interaction constants of dispersed phase A, dispersion medium A2 and the phase with the medium A 2, but of Gamaker s constant for adsorption layer 3 too. The effect of polymer adsorption layers on molecular attraction of particles has been described theoret-ically. Below is an expression for Ur, based on the Lifshits macroscopic theory... 

For polymer solutions, a decrease in the solvent thermodynamic quality tends to decrease the polymer-solvent interactions and to increase the relative effect of the polymer-polymer interactions. This results in intermolecular association and subsequent macrophase separation. The term colloidally stable particles refers to particles that do not aggregate at a significant rate in a thermodynamically unfavourable medium. It is usually employed to describe colloidal systems that do not phase separate on the macroscopic level during the time of an experiment. Typical polymeric colloidally stable particles range in size from 1 nm to 1 xm and adopt various shapes, such as fibres, thin films, spheres, porous solids, gels etc. 

Nevertheless, in the presence of polymer-particle interaction, the polymer chains can affect the interaction between the particles even if the particles are of macroscopic size " For example, if the polymer chains have an attractive interaction with the particle surface, the chains tend to be adsorbed on the surface and may bridge neighboring particles, which results in interparticle attraction. We may include such polymeric effects in the interparticle interaction, as explained below. 

Multiparticle collision dynamics describes the interactions in a many-body system in terms of effective collisions that occur at discrete time intervals. Although the dynamics is a simplified representation of real dynamics, it conserves mass, momentum, and energy and preserves phase space volumes. Consequently, it retains many of the basic characteristics of classical Newtonian dynamics. The statistical mechanical basis of multiparticle collision dynamics is well established. Starting with the specification of the dynamics and the collision model, one may verify its dynamical properties, derive macroscopic laws, and, perhaps most importantly, obtain expressions for the transport coefficients. These features distinguish MPC dynamics from a number of other mesoscopic schemes. In order to describe solute motion in solution, MPC dynamics may be combined with molecular dynamics to construct hybrid schemes that can be used to explore a variety of phenomena. The fact that hydrodynamic interactions are properly accounted for in hybrid MPC-MD dynamics makes it a useful tool for the investigation of polymer and colloid dynamics. Since it is a particle-based scheme it incorporates fluctuations so that the reactive and nonreactive dynamics in small systems where such effects are important can be studied. 

One would like to see more experiments carried out with mixed dispersions in the presence of polymers (leading to selective flocculation ), and on the interaction of particles with macroscopic surfaces. Both of these areas have long-term implications in biological studies. (Selective cell ahesion adhesion of microorganisms to surfaces.)... 

On the basis of what has been discussed, we are in the position to provide a unified understanding and approach to the composite elastic modulus, yield stress, and stress-strain curve of polymers dispersed with particles in uniaxial compression. The interaction between filler particles is treated by a mean field analysis, and the system as a whole is macroscopically homogeneous. Effective Young s modulus (JE0) of the composite is given by [44]... 

For more practical purposes, therefore, one should take recourse to metal particles as produced by other means, in particular on supports or in matrices. The advantage is the availability of macroscopic amounts of sample the disadvantage is that interaction with the supporting medium must be assessed. A great variety of synthetic methods exists, of which we can mention only a few. Metal clusters can be produced by aerosol techniques, by vapor deposition, by condensation in rare-gas matrices, by chemical reactions in various supports, e.g. zeolites, SiOi, AI2O3, or polymer matrices. Many different metal-nonmetal composites, such as the ceramic metals (cermets) have been obtained with metal particles with sizes varying from nanometers upward. In alternative approaches, metal particles are stabilized by chemical coordination with ligand molecules, as in metal colloids and metal cluster compounds. 

One often investigates the nature of surfaces and interfaces by measuring the forces of interaction between two macroscopic surfaces For charged systems, these surfaces may be coated with a charged surfactant or polymer and they are separated by a polar solvent in which the counterions are dissolved. It is thus of interest to calculate the force between two surfaces. These forces are defined via the changes in free energy of the system as the volume of the system is varied, but the number of particles (e.g., fixed charges and counterions) is kept constant. 

The introduction of nanoceramics in thermoplastics or thermosets seems to be a promising route to improving mechanical moduli, particularly of elastic matrices. The macroscopic properties are governed by the nature of polymer-nanoparticle interactions. In this chapter, specific attention will be paid to nanocomposites of polyamide (PA) with HAp. Considering their attractive viscoelastic properties, semiaromatic polyamides (e.g., PA-llTlO) have been chosen as the matrix [Choe et al., 1999]. Recently, several publications have been devoted to one-dimensional nanostructures with high-aspect-ratio particles. Promising data have been obtained for the polymer/nanowire-nanotube nanocomposites. 

Self-assembly is the process in which a system s components—be it molecules, polymers, colloids, or macroscopic particles—organize into ordered and/or functional structures or patterns as a consequence of specific, local interactions among the components themselves, without external direction. 

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