Polymer-coated silica particles

Sprycha, R. et al.. Characterization of polymer-coated silica particles by microelectrophoresis, Colloid Polym. Sci., ILi, 693, 1995. 

Scheme 7.4 Schematic diagram of filler-matrix interaction in the polymer-coated silica particles (adapted from ref. 20).

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

It was apparent that the dense adsorption layer of HPC which was formed on the silica particles at the LCST plays a part in the preparation of new composite polymer latices, i.e. polystyrene latices with silica particles in the core. Figures 10 and 11 show the electron micrographs of the final silica-polystyrene composite which resulted from seeded emulsion polymerization using as seed bare silica particles, and HPC-coated silica particles,respectively. As may be seen from Fig.10, when the bare particles of silica were used in the seeded emulsion polymerization, there was no tendency for encapsulation of silica particles, and indeed new polymer particles were formed in the aqueous phase. On the other hand, encapsulation of the seed particles proceeded preferentially when the HPC-coated silica particles were used as the seed and fairly monodisperse composite latices including silica particles were generated. This indicated that the dense adsorption layer of HPC formed at the LCST plays a role as a binder between the silica surface and the styrene molecules. 

Silica-base stationary phases have also been employed for enantiomeric separations in CEC [6,72-81]. In the initial work on chiral CEC, commercially available HPLC materials were utilized, including cyclodextrins [6,74,81] and protein-type selectors [73,75,80] such as human serum albumin [75] and ai-acid glycoprotein [73]. Fig. 4.9, for example, depicts the structure of a cyclodextrin-base stationary phase used in CEC and the separation of mephobarbital enantiomers by capillary LC and CEC in a capillary column packed with such a phase. The column operated in the CEC mode affords higher separation efficiency than in the capillary LC mode. Other enantiomeric selectors are also use in CEC, including the silica-linked or silica-coated macrocyclic antibiotics vancomycin [82,83] and teicoplanin [84], cyclodextrin-base polymer coated silicas [72,78], and weak anion-exchage type chiral phases [85]. Relatively high separation efficiency and excellent resolution for a variety of compounds have also been achieved using columns packed with naproxen-derived and Whelk-0 chiral stationary phases linked to 3 pm silica particles [79]. Fig. 4.10 shows the... 

Recently, interest in polymer-coated silica phases has been renewed, with investigators (Chen and Lee [2]) exploring the use of more efficient deactivation techniques and more polar polymers to coat silica particles for neat CO2 chromatography. Polyethyl-eneimine-coated silica and amino-terminated polyethylene oxide-coated silica appear promising for pSFC of moderately polar basic compounds. Similarly, hydroxy-terminated polyethylene-oxide-coated silica has been used successfully for pSFC of alcohols and acids. Optimization and commercial production of these stationary phases could significantly extend the polarity range of compounds that can be chromatographed with neat supercritical CO2. 

The adhesion of the polymer to the electrode surface is another consideration. For example, the polymer can be generated at an electrode such as tantalum, where it will not deposit. In our laboratories38 we have used the fact that deposition of PPy onto tantalum is difficult in the design of an electrochemical slurry cell to coat silica particles. The polymer generated at the tantalum anode does not deposit there instead, it deposits on the more receptive (silica particle) surfaces in the electrochemical cell. This represents a unique polymerization process whereby the polymer is generated electrochemically in an environment that allows nonconductive substrates to be coated, resulting in unique composite structures. 

Polymeric crown ether resins are mechanically unstable and are, therefore, operated with low flow rates between 0.05 and 0.1 mL/min resulting in long analysis times. Typical for all polymeric crown ether phases is the relatively low chromatographic efficiency, which does not meet today s requirements. However, this problem can be overcome by immobilizing crown ether polymers on the surface of a solid support. Modified and non-modified silica are predominantly used as support materials. Igawa et al., for instance, coated silica particles with the above-mentioned polyamide crown ether resin and obtained significantly better separations then with the rmcoated resin. 

Particle sedimentation is often a problem in ER fluids containing the solid particle. As mentioned above, additive and surfactant are frequently used for enhancing both the stability of the ER suspension and the ER effect linally. One way to resolve this problem is to make a polymer coated microballoon particle, matching the density between the particle and the carrier liquid and thus reducing the particle sedimentation. An example is the poly(vinyl alcohol) (PVA) coated silica microballoon dispersed in the mixture of heptane and toluene (63]. The shear stress against the electric field is shown in Figure 11 for such a system. In both particle concentrations, 10 wt% and 30 wt%, the coated samples show a much better sedimentation property and much stronger ER effect, also. However, it may be hard to solely attribute the enhanced ER performance to the improved sedimentation property, as the PVA may act as an additive to enhance the ER effect. 

One disadvantage of all silica-based stationary phases is their instability against hydrolysis. At neutral pH and room temperature the saturation concentration of silicate in water amounts to lOOppm. Solubility increases with surface area, decreasing particle diameter drastically with pH above 7.5. This leads also to a reduction of the carbon content. Hydrolysis can be recognized during the use of columns by a loss in efficiency and/or loss of retention. Bulky silanes [32], polymer coating [33], or polymeric encapsulation [34] have been used in the preparation of bonded phases to reduce hydrolytic instability, but most of the RPs in use are prepared in the classical way, by surface silanization. Figure 2.3 schematically shows these different types of stationary phases. 

Fig. 23 Confocal laser scanning microscopic image of rhodamine-labeled SiP coated with PMMA brush The diameter of silica particle core is 230 nm, and the Mn of the graft polymer is 256000...

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