Effectively cross-linked reversible gels

Water-based permanent gels are not structurally different from other permanent gel systems. They acquire special character mainly when they are ionized and the ionic strength of the medium is sufficiently low. Suitable copolymers in aqueous solution can form a vast variety of reversible gel systems because of the moderately strong, primary interactions possible within the aqueous environment. Structural organization (mainly cooperative effects) are then used, to a limited extent, to create sufficiently long-lived bonds that form gels of adequately long relaxation and cross-link turnover times. 

Abstract This paper reviews possible phase diagrams of associating polymer solutions in which phase separation and molecular association interfere. Paying special attention on the structure and reorganisation of the network junctions, we study competition between phase separation and gelation. The molecular structure of associating micelles, or multiple cross-link junctions, in the networks is analyzed from the sol/gel transition lines. The effect of added surfactants on the formation of reversible gels in hydrophobically modified polymer solutions is also studied under the assumption of the existenee of a minimum multiplicity required for stable cross links. To describe... 

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. ... 

A response of microparticle gel to an applied electric field was investigated for a particle of cross-linked sodium poly(acrylic acid) (PAA) prepared by inverse emulsion polymerization [68]. The PAA microparticles (about 200 mm) contracted when an electric field was applied, and a 90% volume change was reached within 50 s. Thus, it was confirmed that microgel is very effective for constructing a system with a minimized response time. Moreover, the reversible contraction-expansion behavior was observed with and without applying an electric field. The swelling process after switching off the electric field was almost complete within a few minutes. 

---