Three-Dimensional Cross-Linking

Hydrogen bonding takes place in three dimensions. Molecules are not planar but have various atoms and groups protruding at various angles. The hard segments tend to form cylindrical clusters with flexible chains in between. 

Some cross-linking takes place in the form of side reactions such as biuret and allophanate bonds. Deliberate introduction of covalent cross-linking can be carried out by the introduction of multifunctional agents (mainly triols) into either the prepolymer or chain extension system. 

At this point, the chains have not fully developed and the hard segments will have the joined chain at random lengths in the structure. This has the effect of increasing the viscosity of the mix as well as improving the compression set and swelling in solvents. This is important in very hard compounds. A trifuctional isocyanate such as Tolonate HDT (Rhone Poulenc) will do the same as TMP but with less decrease in dynamic properties. 

When the TMP is used as a curative, the reaction speed is slowed down and the material is softened as a result of the disruption of the chains. The bonds basically will only be at the end of the chains. 

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Performance Characteristics Polyester resins undergo a rapid transformation from a viscous Hquid to a soHd plastic state that comprises a three-dimensional cross-linked polymer stmcture. The level of polyester polymer unsaturation determines essential performance characteristics (Table 7), although polymer components can influence subtle features that affect thermal, electrical, and mechanical performance as defined by ASTM procedures. 

Clearly a wide range of phenols and amines are possible. Where appropriate multi-ringed phenols are used such as 4,4-dihydroxybenzophenone or bisphenol A then three-dimensional cross-linked polymers will be obtained. Amines may be aliphatic such as methylamine or aromatic such as aniline. 

Polyurethane foams are generally made using a polyalcohol rather than a diol as the monomer, so the polymer has a high amount of three-dimensional cross-linking. The result is a rigid but very light foam suitable for use as thermal insulation in building construction and portable ice chests. 

Baekeland had to make important discoveries before he could bridge the gap between the initial concept and final products. In particular, he found that the base-catalysed condensation of phenol and formaldehyde can be carried out in two parts. If the process is carefully controlled, an intermediate product can be isolated, either as a liquid or a solid, depending on the extent of reaction. At this stage, the material consists of essentially linear molecules and is both fusible and soluble in appropriate solvents. When heated under pressure to 150 °C, this intermediate is converted to the hard, infusible solid known as bakelite . This second stage is the one at which the three-dimensional cross-linked network develops. 

The first widely used synthetic polymer was phenol formaldehyde (Bakelite). It is made by heating phenol (C6H5OH—hydroxybenzene) together with formalde-hyde (H2CO).These react to yield a three-dimensionally cross-linked polymer. To reduce the brittleness of Bakelite, it is usually filled with fibers or platelets of an inert solid. It is a good electrical insulator, relatively hard, and thermally stable to a few hundred degrees Centigrade. Its hardness is 50-60 kg/ mm2 (Mott, 1956). 

Smith and coworkers recently proposed a specific and novel mineral-based solution to the problem of dilution and diffusion of prebiotic reactants. They have suggested [132-134] the uptake of organics within the micron-sized three-dimensional cross-linked network of pores found to exist within the top 50 xm, or so, of alumina-depleted, silica-rich weathered feldspar surfaces. These surfaces incorporate cavities typically about 0.5 pm in diameter along with cross inter-connections of about 0.2 pm. The nominal area of the weathered feldspar surface is apparently multiplied by a factor of about 130 arising from this network. The similarity of these pores to the catalytic sites in zeolite-type materials is pointedly mentioned. 

Thermosets cure into nonmelting, insoluble polymers. Frequently, the curing needs heat, pressure, or catalyst to proceed. Often the final cure, which is nothing more than completion of the cross-linking, takes place in the fabrication or molding operations. The chemistry is about the same as you saw in the thermoplastics, but there are more reactive sites per monomer. (They are polyfunctional.) Consequently, more three-dimensional cross-linking takes place. 

The simplest way to achieve three-dimensional cross-linking is to use monomers with three or more reactive sites. Examples are maleic anhydride, butadiene, isoprene, epichlorohydrin, pyromellitic dianhydride, and tri-methylol propane. 

In the early 1930 s, a second type of resin prepared from formaldehyde was introduced to the market—namely, urea-formaldehyde resins. A few years later, melamine-formaldehyde resins also appeared. The same basic process is employed in polymerization of all these resins it consists of the catalyzed reaction of formaldehyde with the second ingredient—phenol, urea, or melamine—to evolve water and produce three-dimensional, cross-linked thermosetting polymers. 

Gel A semisolid system in which a liquid phase is constrained within a three dimensional, cross-linked matrix. The drug substance may be either dissolved or suspended within the liquid phase. 

Three-dimensionally cross-linked poly(l,4-phenylene vinylene)... 

In another approach, a fluorescent conjugated polymer was used as the material for the preparation of a chemosensor to detect 2,4,6-trinitrotoluene (TNT) and its related nitroaromatic compounds. To this end, microparticles, made of three-dimensionally cross-linked poly(l,4-phenylene vinylene) (PPV) via emulsion polymerization, were synthesized [61]. This material was chosen due to its high fluorescence intensity and sensitivity to changes in its microenvironment. The chemosensor was exposed to vapour containing different amounts of TNT and quenching of the polymer luminescence at 560 nm was observed after excitation at 430 nm. The dependence of the fluorescence signal in response to the analyte was described by a modified Stem-Volmer equation that assumes the existence of two different cavity types. The authors proposed the modified Stem-Volmer equation as follows ... 

With urea, formaldehyde forms two stable IV.O-hemiacetals (Figure 9.25) a 1 1 adduct ( methylol urea ) and a 1 2 adduct ( dimethylol urea ). When they are heated, both compounds are converted to macromolecular IV.lV-acetals (Figure 9.26). A three-dimensionally cross linked urea/formaldehyde resin is produced it is an important plastic. 

The heat treatment of the Alcalase-treated hydrolysates does not cause any changes in the viscosity, in contrast to the Neu-trase-treated samples which show an irreversible increase in viscosity, in particular at DH = 1.5%. No gel formation was observed in these experiments, whereas Pour-el and Swenson (26) were able to introduce gel formation capacity in soy protein hydrolysates. This indicates that the removal of the iso-electric soluble phase as carried out in their work (26), is necessary for obtaining gelation ability. Pour-el also proposed that the presence of small peptides would hinder the three-dimensional cross-linking necessary for gel formation (27). 

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