CROSS-LINKED POLYMERS Subject

Oil-field chemistry has undergone major changes since the publication of earlier books on this subject Enhanced oil recovery research has shifted from processes in which surfactants and polymers are the primary promoters of increased oil production to processes in which surfactants are additives to improve the incremental oil recovery provided by steam and miscible gas injection fluids. Improved and more cost-effective cross-linked polymer systems have resulted from a better understanding of chemical cross-links in polysaccharides and of the rheological behavior of cross-linked fluids. The thrust of completion and hydraulic fracturing chemical research has shifted somewhat from systems designed for ever deeper, hotter formations to chemicals, particularly polymers, that exhibit improved cost effectiveness at more moderate reservoir conditions. 

The structure of hydrogels that do not contain ionic moieties can be analyzed by the Flory Rehner theory (Flory and Rehner 1943a). This combination of thermodynamic and elasticity theories states that a cross-linked polymer gel which is immersed in a fluid and allowed to reach equilibrium with its surroundings is subject only to two opposing forces, the thermodynamic force of mixing and the retractive force of the polymer chains. At equilibrium, these two forces are equal. Equation (1) describes the physical situation in terms of the Gibbs free energy. 

As shown originally by Malcolm Dole, polyethylene molecules may be cross-linked when subjected to high-energy radiation. These three-dimensional network polymers may be represented by the structure shown in Figure 1.5. 

There are two major routes to molecular damage when systems are subjected to mechanical forces. The most obvious is simple bond breaking when linear or cross-linked polymer chains are subjected to shear forces, for example, stretching, cutting, bending, breaking or... 

Other relationships between % and an observable physical property such as osmotic pressure [20, 43], freezing point depression of polymer [20, 52] or solvent [20, 53], and gas liquid chromatography [46-54], were established in like fashion. The relationship determined for swelling of cross-linked polymer to thermodynamic equilibrium in excess liquid has particular significance for the subject of this review. It is given here in the form of the Flory-Rehner equation. 

Because these fire-retardant salts are water-soluble and subject to leaching, several new methods have been developed which provide a water-resistant or permanent treatment for cotton fabrics and cellu-losic materials. Perkin developed a process involving successive treatment with sodium stannate and ammonium sulfate, which precipitates stannic oxide in the cellulose libers. Antimony oxide in combination with vinyl chloride or other chlorinated polymers has also been found effective. A more recent approach to this problem involves application of fire-retardant resin-forming or cross-linking compounds. " These include materials and methods based on the copolymerization of tetrakis(hydroxymethyl)phosphonium chloride and methylol — melamine, the reaction of bromoform and triallyl phosphate to form a cross-linked polymer, and the cross-linking reaction of tris(l-aziridinyl)phosphine oxide with cellulose, or its copolymerization with tetrakis(hydroxymethyl)phosphonium chloride and other materials. ... 

Oakenftill (1984) developed an extension of Equation 6.1 for estimating the size of junction zones in noncovalently cross-linked gels subject to the assumptions (Oakenfull, 1987) (1) The shear modulus can be obtained for very weak gels whose polymer concentration is very low and close to the gel threshold, that is, the polymer chains are at or near to maximum Gaussian behavior. (2) The formation of junction zones is an equilibrium process that is subject to the law of mass action. Oakenfull s expression for the modulus is (Oakenfull, 1984) ... 

Possible alternatives to cross-linked polymer supports are soluble and colloidal polymers. They would require large scale ultrafiltration for industrial use. Although ultrafiltration is not yet economical for desalination of seawater, it might be for a separation of a more expensive product. One example is the catalytic partial hydrogenation of soybean oil (361 with soluble polymer-bound transition metal complexes. Solid inorganic supports such as silica gel and alumina are usually not subject to these physical attrition and filtration problems. 

Fire retardancy of fabrics treated by using the same procedure as given above was evaluated by limiting oxygen index (LOI) measurements according to ISO 4589-2 (Tsafack et al., 2004). The results obtained for the acrylates phosphates and phosphonates monomers show that LOI values increase with the relative phosphorus content in the monomer and therefore in the grafted polymer. This can be explained by the fact that phosphate monomers compared to phosphonate monomers are subject to more chain transfers that lead to highly cross-linked polymers and decrease the flammability (Tsafack et al., 2(X)4). 

Initially, hyperbranched and cross-linked polymers were the subject of investigation in the field of polymer science (both polymer chemistry and physics). Gradually, since the... 

The chemistry of typical free-radical polymerizations involves an initiation, propagation, chain transfer, and termination step leading to the formation of a cross-linked polymer system (36). The initiation step (radical formation step) utilizes chemistries that when subjected to thermal or ultraviolet radiation form radicals that react with activated monomers, such as a methacrylate. A wide variety of thermal, ultraviolet, visible, and redox initiators are commercially available. Typical thermal initiators include the class of azo compounds, such as azobisisobutylonitrile (AIBN), and peroxide initiators, such as the per-oxydicarbonates and the hindered peroctoates. Polymerization conditions vary... 

The extension of Rouse s approach from linear chains to other polymer systems is quite straightforward and leads eventually to the concept of generalized Gaussian structures (GGS), which are the subject of this review. In the framework of the GGS approach, a polymer system is modeled as a collection of beads (subject to viscous friction), coimected to each other by means of elastic springs in a system-spedfic way. Initially, the GGS concept was inspired by the study of cross-linked polymer networks however, its applications have turned out to cover large classes of substances, such as dendritic polymers, hybrid polymers, and hierarchically-built structmes. 

Sijbesma et al. [69] recently demonstrated a ROMP reaction initiated by compressing imbedded mechanochemically activated catalyst and monomers in a solid matrix with a KBr press. When samples containing the catalyst 102 (34 kDa) and the monomer 106 or the di-functional monomer 107 (Figure 12.23) within a high molecular weight (170 kDa) polymer matrix (PolyTHF) were subjected to mechanical strain (0.8 GPa), up to 25% conversion of monomer 106 was obtained after five compression cycles, while the production of a cross-linked polymer from 107 was proven by solid-state NMR techniques. 

The polymer is cross-linked. In this case the dotted line in Figure 8.2 is followed, and improved rubber elasticity is observed, with the creep portion suppressed. The dotted line follows the equation E = 3nRT, where n is the number of active chain segments in the network and RT represents the gas constant times the temperature see equation (9.36). An example of a cross-linked polymer above its glass transition temperature obeying this relationship is the ordinary rubber band. Cross-linked elastomers and rubber elasticity relationships are the primary subjects of Chapter 9. 

Data on the biodegradation of a series of homo- and copolymers of e-capro-lactone, cross-linked with varying degrees 2,2-bis(s-caprolactone-4-yl)propane (BCP), were reported later by Pitt et aU as previously mentioned [45]. These elastomeric polymers were found to be bioabsorbed by an enzymatic surface erosion process. Concurrent with the surface erosion, the cross-linked polymers were subject to slower non-enzymatic hydrolysis of the ester groups. 

Figure 26.7 shows the chemical structures of an NLO chromophore (APAN) and an epoxy-based polymer (BPAZO) where NLO moieties are attached to the backbone [81]. Both the dye and the polymer are functionalized with thermally cross-linkable acryioyl groups. As the dye-doped polymer is subjected to heat as part of the simultaneous poling/curing process, the inter- and intramolecular cross-linking reactions occur simultaneously (Fig. 26.8). The 7g of the cross-linked polymer-dye network is lower than that of the undoped polymer network because of the plasticizing effect of the dissolved dye. However, the temporal stability at 100°C of the polymer-dye network is better than that of the undoped polymer network (Fig. 26.9) as a direct result of the increased cross-linking density in the cross-linked guest-host system. Therefore, the addition of the thermally cross-linkable NLO dye not only increases the...

Condensation hinders. Condensation binders are based primarily on resins that interact to form cross-linked polymers when subject to sufficient thermal energy. These binders are also called high-baked materials and are commonly used as tank and pipe linings. Condensation is essentially the release of water during the polymerization process. 

Another approach undertaken by Lin and co-workers towards heterogeneous catalysis involved doping metal-organic frameworks (MOFs) with the metal complex chromophores (Figure 10.3). The authors incorporated Ru and Ir complexes U.2L and H2L , respectively, into porous Zr604(0H)4(bpdc)s (bpdc =para-biphenyldicarbo)ylic acid) frameworks to produce MOF-1 and MOF-2, respectively, which were used in nitro-Mannich reactions of THIQ derivatives (Scheme 10.16). Conversions were generally lower for MOF-1 and MOF-2 than for cross-linked polymers or their monomeric counterparts however, these catalysts could be readily filtered off and re-used for subsequent reactions. For example, N-phenyl THIQ was subjected three times to the photocatalyzed nitro-Mannich reaction in the presence of MOF-2, which provided consistent conversions (57-59%) for each run. 

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