Reversibly cross-linked gels

Although increasing the pH can delay the onset of peak viscosity, it is nevertheless the case that a gel that is cross-linked to the maximum extent at the target reservoir temperature will be over cross-linked at surface temperatures. A fluid that is aoss-linked to the correct level at reservoir temperatures could easily experience a fivefold increase in the concentration of the mono-borate ion at surface temperatures and this can result in the so-called gel syneresis, the formation of a densely cross-linked gel phase in contact with excess water. Although this process is reversible due to the labile nature of the borate ester bonds, an over-cross-linked gel can stiU cause high-friction pressures during initial pumping operations. 

Borate cross-linked fracture fluids are also believed to cause less damage to the reservoir and less likely to impair permeability than rival cross-linkers [26,50,68], This is partly due to the fact that borate cross-links can be broken down after fracturing simply by reducing pH. That is not to say that chemical (oxidative) or enzymatic means for effecting cleanup of the reservoir are not required to break down the polymer chains and flush away the fluid residues, but this process is more effective with borates because of the reversible nature of the cross-link bond. Some metal ion cross-linked gels have poor cleanup properties and soluble precipitates can be formed when they react with certain chemical breakers. ... 

One interesting feature of PE solutions is the ability of PEs to form cross-linked gel networks [83, 96]. These networks can be formed either by chemical or by ionic thermo-reversible bonds formed by e.g. multivalent ions as cross-linkers [83, 97]. A Hory theory based theoretical study of the PE gelation points using di- and... 

Fig. 10. Composite hoUow-fiber membranes (a) polysulfone boUow fiber coated witb fiiran resin. A and B denote fiiran resin surface and porous support, respectively (b) cross section of composite boUow fiber (PEI/TDI coated on polysulfone matrix). C, D, and E denote tightly cross-linked surface, "gutter" gel layer, and porous support, respectively. Both fibers were developed for reverse osmosis appHcation (15).

Molded polyamide surfaces can be hardened by grafting with Ai,Ai-diallylacrylamide [3085-68-5] monomer under exposure to electron beam (159). AijAZ-DiaHyltartardiamide [58477-85-3] is a cross-linking agent for acrylamide reversible gels in electrophoresis. Such gels can be dissolved by a dilute periodic acid solution in order to recover protein fractions. 

Recovery and Purification. The dalbaheptides are present in both the fermentation broth and the mycelial mass, from which they can be extracted with acetone or methanol, or by raising the pH of the harvested material, eg, to a pH of 10.5—11 for A47934 (16) (44) and A41030 (41) and actaplanin (Table 2) (28). A detailed review on the isolation of dalbaheptides has been written (14). Recovery from aqueous solution is made by ion pair (avoparcin) or butanol (teicoplanin) extraction. The described isolation schemes use ion-exchange matrices such as Dowex and Amberlite IR, acidic alumina, cross-linked polymeric adsorbents such as Diaion HP and Amberlite XAD, cation-exchange dextran gel (Sephadex), and polyamides in various sequences. Reverse-phase hplc, ion-exchange, or affinity resins may be used for further purification (14,89). 

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