Polymer particle properties

This article addresses the synthesis, properties, and appHcations of redox dopable electronically conducting polymers and presents an overview of the field, drawing on specific examples to illustrate general concepts. There have been a number of excellent review articles (1—13). Metal particle-filled polymers, where electrical conductivity is the result of percolation of conducting filler particles in an insulating matrix (14) and ionically conducting polymers, where charge-transport is the result of the motion of ions and is thus a problem of mass transport (15), are not discussed. 

Although both of these models provide a reasonable description of the precipitation polymerization process, they do not illustrate the relationship between the reactor variables and the polymer particle properties. 

Water-soluble polymers in general, and especially polyelectrolytes, are often difficult due to their specific and long range electrostatic interactions, which complicate all analytical techniques that rely on single particle properties that are usually realized by high dilution. In most cases the ionic strength of the solution must be increased by the addition of salt in order to screen electrostatic forces. Ideally, SEC separation is predominantly governed by entropic interactions,... 

Kandori, K. Tamura, S. Ishikawa.T. (1994) Inner structure and properties of diamondshaped and spherical a-Fe203 particles. Colloid Polym. Sci. 272 812-819 Kandori, K. Uchida, S. Kataoka, S. Ishikawa, T. (1992) Effects of silicate and phosphate on the formation of ferric oxide hydroxide particles. J. Mater Sci. 27 719-728 Kandori, K. Yasukawa, A. Ishikawa,T. (1996) Influence of amines on formation and texture of uniform hematite particles. J. Colloid Interface Sci. 180 446-452 Kaneko, K. Inouye, K. (1974) Electrical properties of ferric oxyhydroxides. Bull. Chem. 

These stabilizers are added to the formulation in order to stabilize the emulsion formed during particle preparation. These stabilizers, however, can also influence the properties of the particles formed. The type and concentration of the stabilizer selected may affect the particle size. Being present at the boundary layer between the water phase and the organic phase during particle formation, the stabilizer can also be incorporated on the particle surface, modifying particle properties such as particle zeta potential and mucoadhesion (203). Other polymers have also been evaluated as stabilizers in earlier studies such as cellulosic derivatives methylcellu-lose (MC), hydroxyethylcellulose ( ), hydroxypropylcellulose (HPC), and hydroxypropylmethylcellulose (HPMC), as well as gelatin type A and B, carbomer and poloxamer (203). 

Statistical mechanics was originally formulated to describe the properties of systems of identical particles such as atoms or small molecules. However, many materials of industrial and commercial importance do not fit neatly into this framework. For example, the particles in a colloidal suspension are never strictly identical to one another, but have a range of radii (and possibly surface charges, shapes, etc.). This dependence of the particle properties on one or more continuous parameters is known as polydispersity. One can regard a polydisperse fluid as a mixture of an infinite number of distinct particle species. If we label each species according to the value of its polydisperse attribute, a, the state of a polydisperse system entails specification of a density distribution p(a), rather than a finite number of density variables. It is usual to identify two distinct types of polydispersity variable and fixed. Variable polydispersity pertains to systems such as ionic micelles or oil-water emulsions, where the degree of polydispersity (as measured by the form of p(a)) can change under the influence of external factors. A more common situation is fixed polydispersity, appropriate for the description of systems such as colloidal dispersions, liquid crystals, and polymers. Here the form of p(cr) is determined by the synthesis of the fluid. 

Colloidal systems are generally of a polydispersed nature - i.e. the molecules or particles in a particular sample vary in size. By virtue of their stepwise build-up, colloidal particle and polymer molecular sizes tend to have skew distributions, as illustrated in Figure 1.2, for which the Poisson distribution often offers a good approximation. Very often, detailed determination of relative molecular mass or particle size distribution is impracticable and less perfect experimental methods, which yield average values, must be accepted. The significance of the word average depends on the relative contributions of the various molecules or particles to the property of the system which is being measured. 

Y. Suetsugu, State of Dispersion - Mechanical Properties Correlation in Small Particle Filled Polymer composites, Int. Polym. Process., 5, 184—190 (1990). 

Fillers are added to polymers either to improve the polymer properties or to reduce the price of the compound. The properties of filled polymers are heavily influenced by the interaction between particles and polymers as well as by particle size, particle size distribution, and the homogeneity of the particle distribution. The smaller the particles and the more homogenous their distribution in the polymer matrix, the better in general the properties of the compound. Therefore, dispersion plays a key role. 

Nanocomposites are already making an impact on the choice and use of polymeric materials. As the dimensions of the particles diminish into the range of a few nanometers, surface area effects dominate, changing fundamentally the interactions between particle and polymer. Often nanocomposites containing less than 5% additive have substantially improved properties with no adverse effects. 

In conventional composites filled with carbon black or silica, the increase in stiffness is mainly associated with a change in the structure and dynamics of the polymer at the filler surface. On account of the enormous surface-to-volume ratio of the particles, the polymer in the interfacial region represents a significant fraction of the materials and its behavior significantly affects or even governs the properties of the composite. 

In summary, concentrated emulsions have been used to prepare latexes or composites, to encapsulate solid particles, for polymer blending and to generate molecular reservoirs. The mechanical properties could be controlled by combining suitable monomers in the latexes. The concentrated emulsion pathway was also employed in our laboratory in other directions than those presented in this review, and we would like to mention them in this final section without details, but with suitable references. 

Continuons emulsion polymerization is one of the few chemical processes in which major design considerations require the use of dynamic or unsteady-state models of the process. This need arises because of important problems associated with sustained oscillations or limit cycles in conversion, particle number and size, and molecular weight. These oscillations can occur in almost all commercial continuous emulsion polymerization processes such as styrene (Brooks et cl., 1978), styrene-butadiene and vinyl acetate (Greene et cl., 1976 Kiparissides et cl., 1980a), methyl methacrylate, and chloropene. In addition to the undesirable variations in the polymer and particle properties that will occur, these oscillations can lead to emulsifier concentrations too low to cover adequately the polymer particles, with the result that excessive agglomeration and fouling can occur. Furthermore, excursions to high conversions in polymer like vinyl acetate... 

In general, polymers have low stiffness and strength in comparison with other materials, e.g., metals and ceramics, and consequently these materials present serious difficulties in structural applications. To improve their mechanical properties, polymers are reinforced by the addition of rigid particles or fibers to form composite materials (1). Thus, polymer matrix composite materials are made up of a low modulus phase, the polymer matrix, and a high modulus phase, the reinforcement, which is usually carbon or glass. The modulus of the composite is higher than that of the polymer matrix, and the increment is proportional to the volume fraction of the reinforcement. In general, the properties of the composite depend not... 

Ocana, M., Preparation and properties of uniform praseodymium-doped ceria colloidal particles. Colloid Polym. Sci., 280, 274, 2002. 

Although the first reports of this approach involved studies with metal alloys [3] and minerals [4], within a few years the technique has been extended to a wide variety of research areas. As these findings have been summarized in several reviews [5-8] and also in a monograph [9], attention will be focused here on more recent developments, notably on the mechanical immobilization of particles on electrodes. Today, a huge amount of information is available for electrochemical systems comprising particles enclosed in polymer films or other matrices (see Refs [10-16]). Originally, the main aim of such particle enclosure was to achieve specific electrode properties (e.g., functionalized carbon/polymer materials as electrocatalysts [17, 18] solid-state, dye-sensitized solar cells [19]), rather than to study the electrochemistry of the particles. This situation arose mainly because the preparation of these composites was too cumbersome for assessing the particles properties. The techniques also suffered from interference caused by the other phases that constituted the electrode. 

Utracki and Fisa (1982) and Metzner (1985) review the rheology of (asymmetric) fibre-and flake-filled plastics, noting the importance of the filler-polymer interface, filler-filler interactions, filler concentration and filler-particle properties in determining rheological phenomena such as yield-stress, normal-stress and viscosity profiles (thixotropy and rheo-pexy, dilatancy and shear thinning). 

N ew opportunities and future directions in the area of microchannel emulsification are most likely in the areas of scale-up [140,141], encapsulation/polymeriza-tion [123, 125, 158, 164—169], rapid quenching of droplets [135], and the use of emulsions as templates for uniform macroporous particle structure formation [172]. MicroChannel emulsification is also likely to open up new opportunities with systems that are highly shear-sensitive [120, 135, 173]. The ability to scale up the process will spur new markets that require high production rates and the production of monodisperse capsules and polymer particles. Such developments will be useful in drug delivery applications and will contribute to the further quantification of micro-particle properties. Additionally, the use of monodisperse emulsions as particle templates is likely to enhance the utility of highly functional nanoparticles in need of a deployment mechanism [172]. 

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