Bulk-imprinting technique

These imprinted micro spheres can then be packed more efficiently into chromatography columns or into solid-phase extraction (SPE) cartridges than the particles prepared by bulk polymerization techniques. Larger spherical imprinted polymer particles can be prepared by modification of preformed latex particles either by reswelling with a secondary polymerization mixture or by coating a spherical core particle with an imprinted polymer shell. 

The main benefit of this technique lies in the fact that it is, in chemical terms, essentially identical to bulk polymerisation. Any recipe which has been optimised by grinding and sieving of bulk materials could be transferred directly to the pores of preformed beads. The main drawback is that quite careful experimental technique is required when filling the pores and carrying out the polymerisation to avoid undue aggregation of beads. The final volume of imprinted polymer is also obviously limited by the space occupied by the original bead structure. This could range from about 5% to 40% or more. Suitable beads with low polydispersity can also be quite expensive, which makes the technique unattractive for some applications. 

In summary, no perfect and universally applicable solution to making beaded imprinted polymers has yet been invented, but a number of promising techniques have been introduced during the last few years which will doubtless be further developed and refined alongside a range of novel approaches. Only time will tell whether any of these techniques will displace crushed bulk polymer as the production method of choice, either in the laboratory or on a production scale. 

In situ-imprinted polymers [39] To further simplify construction of such sensors we investigated the use of in situ or rod imprinted polymers using the technique pioneered by Matsui et al. (see Chapter 13). Solutions of template, either DEAEMA or AA as functional monomers, EDMA and initiator in the porogen (4 1 octanohdodecanol, w/w) were introduced into 100 x 4.6 mm (i.d.) HPLC columns and polymerised in situ at either 50 or 70°C. These rods were then flushed with MeCN until a stable absorbance and back pressure were obtained. The most selective polymers were those polymerised at 50°C using DEAEMA as the functional monomer, with a functional monomer/template ratio of either 2 1 or 4 1. The latter was chosen for use in the sensor so as to directly compare the results obtained using bulk and in situ imprinted polymers. 

Polymer stabilized liquid crystals are formed when a small amount of monomer is dissolved in the liquid crystal solvent and photopolymerized in the liquid crystal phase. The resultant polymer network exhibits order, bearing an imprint of the LC template. After photopolymerization, these networks in turn can be used to align the liquid crystals. This aligning effect is a pseudo-bulk effect which is sometimes more effective than conventional surface alignment. Several characterization techniques... 

As stated above, columns packed with irregular materials are less than ideal in terms of chromatographic performance. Thus, in recent years much effort has been dedicated to develop alternative methods to prepare imprinted stationary phases that are superior in terms of efficiency, mass transfer characteristics, and sample load capacity. Micrometer-sized spherical-imprinted polymers with narrow size distribution have been prepared through several techniques reported in Table 2. It should be considered that all these procedures show serious limitations. These are high sensitivity to small changes in polymerization conditions, a polymerization medium that is not compatible with weak noncovalent interactions between functional monomers and their template, high costs or procedure complexity (which can hinder a wide application of these techniques as valid substitutes to bulk polymerization method). 

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