Hypercrosslinked microporous

Sorption isotherms for -valeric acid on hypercrosslinked micropor-ous Styrosorbs 1 crosslinked with p-xylylene dichloride, biporous Styrosorb IBP, and, for comparison, macroporous Amberlite XAD-2 are given in Fig. 11.3. Again, one cannot see any correlation between the specific surface area of dry hypercrosslinked sorbents and their capability to take up the acid. Indeed, Styrosorb 1 with X — 25% is a non-porous material when in the dry state, but it swells in water (0.15 mL/g) and, accordingly, retains a noticeable amount of the sorbate. On the other hand, the apparent specific surface area of dry Styrosorb 1 and Styrosorb IBP, both having a 100% degree of crosslinking, are of the same order of magnitude, about lOOOm /g, but the biporous Styrosorb IBP takes up twice as much water as Styrosorb 1 does (2.90 and 1.31 mL/g, respectively) and exhibits an exceptional sorption capacity at equal equilibrium... 

Recent Developments of Hypercrosslinked Microporous Organic Polymers... 

Hypercrosslinked polymers (HCPs) which are obtained by polymerization of monomers with crosslinking molecules to give a highly microporous material (Figure 2.6) [11]. 

In conclusion, it should be pointed out that none of the physicochemical techniques discussed above permits the direct measurement of the elements of the polymeric materials porous structure we measure the properties of the systems where the polymers interact with certain test substances (nitrogen, mercury, water, polystyrene standards, ions, etc.), and not the dimensions of the pores or other supramolecular elements of the material. Therefore, the evaluation of the surface area and diameters of pores available to the molecules of these substances must be considered as indirect methods of examining the porous structure. Because of this, all calculations are based on assuming certain models of the structure of the material and accepting certain assumptions as to the mechanism of interaction between the material and test molecules. Only transmittance, scanning, and, in particular, atomic force microscopy can be considered as direct methods of measuring dimensions and distances. However, up to now the last technique has not been appHed to microporous hypercrosslinked polymers. 

Nevertheless, some generalized conclusions may be drawn at this stage. Hypercrosslinked polystyrene sorbents represent basically microporous... 

Rigid microporous hypercrosslinked networks were also derived through SchifF base chemistry by condensation of trifimctional melamine with aromatic di- and trialdehydes [271]. If appropriate experimental conditions are chosen (heating at 180°C for 72h in dimethyl sulfoxide under inert atmosphere), the imine double bonds of the initially formed Schiff bases are attacked by primary amines, resulting exclusively in the generation of aminals ... 

Up to now four main groups of hypercrosslinked sorbents have been developed and intensively tested. The first group, Styrosorb 1, incorporates laboratory samples of nanoporous (microporous) single-phase sorbents prepared by intensive post-crosslinking Hnear polystyrene of about 300,000 Da molecular weight, dissolved in ethylene dichloride, with monochlorodi-methyl ether or p-xylylene dichloride. The irregular particles of these sorbents have pores with a diameter of about 20—30 A and display an apparent specific surface area as high as 1000—1500 m g. The pore volume of Styrosorb 1 materials usually amounts to 0.4—O.Scm g. 

Isolation and processing of plant proteins has become an important branch of the food industry. Often, the proteins have to be purified from residual fats and oils, which reduces the shelf time of isolated proteins. HypercrossHnked polystyrene is the material of choice for the removal of all lipids from protein-containing extracts, as well as from isopropyl alcohol where the latter is used for the extraction of lipids from protein masses [50]. Figure 11.5 shows sorption isotherms on Styrosorb 2 for lecithin and oleic acid from pure isopropyl alcohol and its mixtures with water. The equilibrium of lecithin sorption on the microporous Styrosorb 2 from pure isopropyl alcohol is estabhshed within 4h. Addition of water accelerates the sorption process in a water—alcohol mixture of 1 1 (v/v), the equilibrium is estabhshed within 2 h. By reducing the thermodynamic affinity of the medium to both polystyrene and lipids, water also enhances the sorption capacity, so that from the 50% isopropyl alcohol solution as much as 600—700mg/g of lecithin can be taken up by the hypercrosslinked polymer, compared to 400mg/g taken by the XAD-4 resin under the same conditions [51]. Sorption of oleic acid by Styrosorb 2 is smaller than that of lecithin, but, stiU, it reaches 400 mg/g. 

A comparison of Chinese hypercrosslinked sorbents, non-fiinctionalized NG-100 and NG-99 containing tertiary amine groups, with XAD-4 in terms of their ability to retain phenylhydrazone derivatives showed that the capacity of XAD-4 yields by 20-70% to that of both NG-99 and NG-100 [81], although the specific surface area of the hypercrosslinked sorbents is by no means higher. The authors attributed this difference to uniform micropore structure of NG-99 and NG-100 resins, their partial polarity, enhanced 71—71 interactions, and better compatibility of sorbate molecules with the sorbent matrices. 

On the other hand, really large organic molecules may prove to be totally excluded from microporous hypercrosslinked polystyrene sorbents if they were prepared from gel-type copolymer precursors. In these cases, only biporous sorbents can compete with the macroporous AmberHte XAD-4. 

Another type of materials is poly-DVB, which receives microporous texture if the monomer is polymerized in the presence of an appropriate amount of a thermodynamically good solvent, such as toluene. However, in our ISE experiments, these were inferior to hypercrosslinked polystyrene sorbents. 

The development of hypercrosslinked polystyrene, the first neutral microporous polymeric material, and the recognition of the size exclusion mechanism of the differentiation of ions migrating through the porous medium immediately opened new perspectives for SEC in processing... 

In general, the size-sieving property of the inherently microporous hypercrosslinked polystyrene-based HPLC columns should be the most useful in the direct analysis of drugs and drug metabolites in blood and plasma matrices. 

Another basic difference between the two materials is that Oasis HLB is macroporous (80 A pores) while LiChrolut EN and Isolut ENV+ are basically nanoporous materials with the maximum pore size distribution located at 20—30 A. For this reason, the latter sorbents perform much better in extracting smaller molecules with molecular masses less than 500 Da, as is the case with the above halogenated acetic acid derivatives [326, 327] or aryl sulfonates [328]. On the contrary, macroporous Oasis HLB is the material of choice when larger analytes are the target of pre-concentration procedures, as was the case with tetracycline and macroHde antibiotics [321-324]. Another vivid example of this kind is pre-concentration of soy isoflavones [331], where Oasis HLB and macroporous Strata X display by far higher recovery values. On the other hand, the size exclusion effect can be exploited purposefully when only smaller molecules have to be retained with the elimination of the major sample matrix. The best example of this type of applications is the analysis of drugs and drug metaboHtes in whole blood or blood plasma [273-275]. Here the microporous hypercrosslinked polystyrene Purosep-270 is the best possible SPE material since it functions as an RAM. 

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