Particle orientation, influence

Many of the fillers used in industry are anisotropic in character. Depending on the shape of fillers, they are subdivided into isotropic particles, flakes, and fibers. Anisotropic particles may take on states of orientation because of flow and packing processes. Whether developed during flow or processing, particle orientation influences phenomena ranging from rheological properties to compound processability in industrial processing equipment, electrical characteristics, and mechanical performance. 

The physical parameters for the metallic effect, which influence the ratio between reflection and scattering and thereby the flop effect, the lightness and brightness, the brilliance and sparkle, the gloss and the distinctiveness of image (DOI) etc. etc. can simply be described in terms of particle size, particle shape (morphology), particle size distribution and particle orientation in the film... 

Asymmetric particles, such as ellipsoids or discs, do not generally fall vertically, but tend to drift to the side. Thin, flat, triangular laminae fall edgeways unless equilateral. Few particles possess high symmetry and small local features exert an orienting influence. 

Fig. 1 Influence of particle orientation on statistical diameters. The change in Feret s diameter is shown by the distances, df. Martin s diameter (dm) corresponds to the dashed lines.

Polymer blends and alloys have more complex behavior in the presence of fillers than the binary mixtures of polymer and filler. The same factors, such as filler distribution, filler-matrix interaction, filler-matrix adhesion, particle orientation, nucleation, chemical reactivity, etc. have influence on properties, but this influence is complicated by the fact that there are two or more polymers present which compete for the same filler particles. These complex interactions result in many interesting phenomena discussed below. 

Quemada (1978a, 1978b) examined the rheology and modelling of concentrated dispersions and described simple viscosity models that incorporate the effects of shear rate and concentration of filler and separate effects of Brownian motion (or aggregation at low shear) and particle orientation and deformation (at high shear). The ratio of structure-build-up and -breakdown rates is an important parameter that is influenced by the ratio of the shear rate to the particle diffusion. A simple form of viscosity relation is given here ... 

Scattering of electrons, fast atoms, or ions with laser-excited atoms A can result in elastic, inelastic, or superelastic collisions. In the latter case, the excitation energy of A is partly converted into kinetic energy of the scattered particles. Orientation of the excited atoms by optical pumping with polarized lasers allows investigations of the influence of the atomic orientation on the differential cross sections for A + B collisions, which differs for collisions with electrons or ions from the case of neutral... 

The above presentation illustrates the similar bases of the electro-optical methods yl]. The components of the scattering matrix can differ in their sensitivity to the various sources of particle anisotropy, but they yield similar information on the induced anisotropy fluctuations, particularly on their relaxation. This scheme is true in so far as noninteracting particles are concerned. Though the correlations in particle orientation [42] have been accounted for in recent electro-optical theories, the influence of the applied field on the space distribution of the scattering elements is generally neglected. 

The flexural modulus is of most interest in many applications. This is much more complex than tensile, compressive or shear moduli. It not only has elements of both tensile and compression, but is very much influenced by particle orientation and by the surface layers. These are often polymer rich, an effect that becomes greater as the filler size increases. 

Several researchers [28, 29] attempted to investigate how the particle shape influences the sizing analysis results. The investigations suggest that the main effect is on the tails of the distribution (D,o and Dgo) rather than the median diameter (D50) therefore the spans (Dge-Dio) / D o are dissimilar. In the case of laser diffraction techniques, as particles with different aspect ratios tumble though the beam at random orientations, the laser at times sees the maximum projected area and at times the minimum projected area and every orientation in between. Therefore, the distribution curve by this technique could be broader than microscopy or other techniques. 

The presence of fillers in viscous polymer melts not only increases their viscosity but also influences their shear rate dependency, especially with non-spherical particles (fibrous or flake-like) which become oriented in the flow field. As Fig. 6 shows, particle orientation increases the non-Newtonian behaviour which commences at a lower rate of shear than for unfilled melt. 

Fibers and anisotropic particles reinforce polymers, and the effect increases with the anisotropy of the particle. In fact, fillers and reinforcements are very often differentiated by their degree of anisotropy (aspect ratio). Plate-like fillers, like talc and mica, reinforce polymers more than spherical fillers and the influence of glass fibers is even stronger [17]. Anisotropic particles orientate during proeessing, and the reinforcing effect depends very much also on orientation distribution. 

To proceed we need to know how the functional A[f(fi-, 22 Ri 2)] varies when the equilibrium state of the liquid crystal is elastically distorted. A macroscopic strain will not influence Mi 2 or g, p. since these are dependent only on molecular parameters of the model the free energy changes because the single particle distribution functions change. We assume that for the small distortions described by the Frank elastic constants, the single particle orientational distribution function, defined with respect to a local director axis, is also independent of strain i. e. elastic torques do not change the molecular order parameters. The product of distribution functions /(i2j, R ) f, Q2i R2) will change with strain because the director orientations at R1 and R2 will differ, and the evaluation of the strain dependence of the... 

Various models for composite permeability as they relate to nanocomposites have been reviewed and different models have been proposed [41—44]. The simplest way to model any composite property is to use a rule of mixtures approach. Polymer nanocomposite properties, however, do not generally follow this rule. Instead, fillers with high aspect ratio particles will influence the permeability of gases through the matrix more than filler particles with lower aspect ratios. Alignment/orientation of the filler particles (with respect to the axis of gas permeation) also plays a significant role in bulk permeability. Five models are briefly described in Sections 8.5.1-8.5.5. Predictions from these models are later compared to experimental mass loss rates. 

---