New and exciting applications are tantalizing goals for many researchers and, with uses ranging from new sensitive pregnancy testing kits to military camouflage, polymer chemistry has an intriguing future.
The Sussex Polymer Group is based in the School of Chemistry, Physics and Environmental Science at the University of Sussex. It has two full-time faculty members who undertake extensive collaborative work with industry. One area of research has focused on conducting polymer-coated latex particles with ‘core-shell’ morphology. Other interests include water-soluble polymers and metal-coated latexes. A critical factor in many of these projects has been the determination of particle sizing using a Brookhaven disc centrifuge.
The Preparation of Novel Composites
Air-stable conducting polymers, such as polyaniline or polypyrrole, can be combined with inorganic particles to produce organic-inorganic hybrid particles, or nanocomposites. These are produced by carrying out the conducting polymer synthesis in water in the presence of a dispersion of 20 nm inorganic particles, typically silica. The precipitating conducting polymer coats these silica particles and glues them together. The morphology of these nanocomposite particles resembles that of a raspberry, with the outside “berry” pieces made up of silica.
These nanocomposites have the characteristic surface chemistry of silica. This is significant, as silica can be easily adapted by having additional functional groups attached by well-established silane coupling chemistry. Abbot Laboratories in Chicago has already shown that this technology can be used to develop a series of immunodiagnostic assays for use in pregnancy testing kits and for the early diagnosis of the HIV virus and hepatitis B. The nanocomposite particles are important in two ways. Firstly, the silica component enables facile functionalization of the particle surface. Secondly, the intense black color, which is an intrinsic property of the conducting polymer component (in this case polypyrrole), eliminates the need to use an extrinsic dye to achieve the visual agglutination test. In principle, this increasses the sensitivity of the test, allowing lower analyte concentrations to be detected.
When these nanocomposites synthesis are carried out using conventional vinyl monomers, the particle morphology changes and the final composite particles resemble currant buns, with the silica particles arranged like raisins embedded within and on the surface of the polymer bun.
In principle, with this currant bun morphology it should be possible to produce a nanocomposite film that is both tough and transparent. Such films are expected to have excellent mechanical properties and the silica component should provide good scratch resistance. BASF, the German chemical conglomerate, is funding research in this field.
Polymer-Latex “Core Shell” Paticles
The research group is also investigating how to coat submicrometer- and micrometer sized, sterically-stabilized, polystyrene or poly(methyl methacrylate) latex particles with a very thin layer of conducting polymer. The conducting polymer, typically polypyrrole or polyaniline, is deposited inside the solvated steric stabilizer layer; if the new layer is kept thin enough, then reasonably good colloidal stability is maintained. When looking at these “core-shell” conducting polymer-cated latex particles, the group found that if the conducting polymer layer is too thick, then the compostie particles start to weakly aggregate. The disc centrifuge is a superb technique for detecting the onset of aggregation – superimposed on the normal particle size distribution are additional features at higher mass due to the formation of doublets, triplets, and more general flocculation. More traditional sizing methods such as dynamic light scattering do not have sufficient resolution to show these features.
Commercial applications for these types of dispersions are in anti-static paints and anti-corrosion coatings where, unlike sterically stabilized particles with a rigid core produced at Sussex, the particles have soft cores and can form films at room temperature. This film-formation capability is vital if resilient paints and coatings are to be produced. DSM Research of the Netherlands holds patents in this area and has recently funded a basic research program conducted by the Sussex Polymer Group. Particle characterization is generally difficult with soft-core particles, so the Sussex team have concentrated on understanding the behavior of model composite particles with rigid polystyrene cores. The elucidation of synthesis-structure-property relationships should lead to further understanding of these fascinating “core-shell” particles.
An area that is currently being looked into is the coating of polystyrene latex with a thin conductive layer of copper or nickel. Unfortunately, this has not yet been achieved as the final coating is rather patchy. Partial oxidation of the thin metal overlayer occurs in some cases, leading to composite particles, which are superparamagnetic rather than electrically conductive. These metal-coated particles are also highly agglomorated and further research is required to optimize the metal coating in order to maintain a high degree of dispersion. Future applications for this type of research could include immunodiagnostic assays, military camouflage – where the particles can reflect or absorb radar or infra-red beams – and in printed circuit boards- where the particles could as as a form of “micro-solder”. A one-year ROPA grant has been recently recieved from the UK government to continue work in this field.
Water Soluble Polymers
Possible future applications for these types of polymers include helping to disperse other substances such as pigments, ceramics or drugs. Various controlled-structure copolymers have been designed so that one part is water-insolube and the other is water-soluble. For example, the polymer would be designed so that it binds to the drug by the insolube end whilst the other end remained solvated. here the degree of dispersion of the particulate phase is easily determined using the disc centrifuge. The smaller the mean particle size, the greater the degree of dispersion achieved: this enables the performance of various copolymer dispersants to be assessed and compared. Glaxo-Wellcome is currently funding the drug dispersion aspects of this project and earlier work on ceramic disperions was funded by Cooksons.
The Vital Role of Particle Sizing
In the past, new polymer colloids and their properties have been discovered more by chance than by design. An important objective of the Sussex Polymer Group is to establish the synthesis-structure-property relationship of these particles. By determining these, feature composites can be designed in response to the properties required. Characterization may include polymer content and morphology, but particle size is one of the most fundamental parameters in determining properties.
Sedimentation techniques are among the most widely used methods of particle size analysis and disc centrifuge represent the most highly developed form of this technique with a capability to analyze particles ranging in size from approximately 10 nanometers to 100 micrometers. Resolution is also extremely good with the Brookhaven Instruments, able to resolve peaks in multimodal particle size distributions differing in mean size by as little as 13 percent. In a study carried out on the particle size distribution of polyaniline-silica colloidal composites¹, it was shown that, by using disc centrifuge photosedimentometry, the true particle size distribution (PSD) curve could be readily determined. The disc centrifuge method was also shown to produce the most reliable results within the fastest time, and was established as the Sussex groups’ preferred particle sizing technique.
A centrifuge operates using the principle of Stokes’ law. According to this, a small particle falls more slowly through a fluid than a larger one. Given that the particle density is known, timing the descent of a particle through a fluid will, when Stokes’ law is applied, give you the size of the particle. Centrifuges extend this technique to tiny particles. Colloidal particles would normally be too light to sink and would remain suspended in the fluid, their tendency to sediment being counteracted by their thermal or Brownian motion. They need a stronger force than gravity to make them fall. In a centrifuge, this is generated by spinning the particles and the fluid at high speed. Centrifugal force then makes the particles more radially outwards and downwards, so that just like falling rock, timing their descent will reveal their size.
In a high-resolution disc centrifuge, a hollow transparent disc spins at a selected speed of between 500 and 15000 rpm. The disc cavity may contain the diluted dispersed sample to be tested or alternatively it may contain a spin fluid into which the sample is injected as the dis spins. Selecting different types of spin fluid will change the viscosity and relative density and, by doing this, it is possible to arrive at a system that is suitable for virtually any particle size and particle density.
Brookhaven Instruments supplied the disc centrifuge that has been vital to the research carried out by the Sussex Polymer Group. The group has used the Brookhaven BI-DCP Disc Centrifuge Photosedimentometer for over five years and has been satisfied with the wide dynamic range and excellent resolution. The centrifuge is digitally controlled with a a scanning detector, which significantly reduces measurement time. The incorporation of a powerful software model to predict and optimize experimental condition is of great benefit.
The ability of the Brookhaven BI-DCP instrument to produce true weight average particle size distributions directly without calibration, has been a vital part of the research carried out in Sussex.
Brookhaven Reliability and Service
Brookhaven has been at the forefront of particle sizing instrumentation for over twenty years and is proud of its worldwide reputation for excellence. Its policy of ‘no shortcuts’ will continue to produce particle-sizing instruments that keep pace with technological developments and user requirements. Accuracy and speed are assured but not at the expense of value for money and ease of use.