External Crosslinking Systems

Scheme 2. Crosslinking chemistry in a three-component system comprising poly(4-hydroxystyrene), external crosslinker (powderlink) and photoacid generator (PAG).

It was shown that the stress-induced orientational order is larger in a filled network than in an unfilled one [78]. Two effects explain this observation first, adsorption of network chains on filler particles leads to an increase of the effective crosslink density, and secondly, the microscopic deformation ratio differs from the macroscopic one, since part of the volume is occupied by solid filler particles. An important question for understanding the elastic properties of filled elastomeric systems, is to know to what extent the adsorption layer is affected by an external stress. Tong-time elastic relaxation and/or non-linearity in the elastic behaviour (Mullins effect, Payne effect) may be related to this question [79]. Just above the melting temperature Tm, it has been shown that local chain mobility in the adsorption layer decreases under stress, which may allow some elastic energy to be dissipated, (i.e., to relax). This may provide a mechanism for the reinforcement of filled PDMS networks [78]. 

One candidate polymer system is the poly(ortho esters), which are pH-sensitive and undergo enhanced hydrolysis rates with decreasing external pH values (6). Furthermore, this system has demonstrated surface erosion characteristics and can be prepared in both linear and crosslinked forms (7., 8). The crosslinked form is particularly interesting because sensitive macromolecules can be incorporated into the polymer under very mild conditions and without denaturation. These mild conditions are important because both insulin and glucose oxidase are proteins. 

The first method used to gel the external continuous phase was the production of a crosslinked poly(acrylamide) gel. Apart from varying the ratio and concentration of monomer and crosslinking agent, little control could be achieved over the reaction - rigid gels were produced resulting in immobile systems. 

System tunability. The interaction potential typically depends on external conditions (e.g., temperature, ionic strength) and internal system parameters (e.g., molar mass, relative core-to-corona size, degree of crosslinking). It is important to be able to access these parameters easily, in order to obtain maximum flexibility. 

Another system similar to the microbaU is called inverse polymer emulsion. In this case, the polymer used is polyacrylamide (PAM). The inverse PAM emulsion is a W/O type of emulsion. The dispersed phase contains 6.4 to 10.5 million Daltons PAM and 1000 mg/L crosslinkers for a sample product. The external continuous phase is white oil. There is a surfactant interfacial film between the disperse phase and continuous phase, as shown in Figure 5.16. The emulsion is stable at the surface. When it is injected into a target formation, it is inverted into an 0/W type of emulsion under certain temperature and salinity, with the help of a phase inversion agent. Thus, the name inverse emulsion is used. 

Mechanical equilibrium between two phases or between a system and an external reservoir can be achieved by allowing the volume of the system to fluctuate. Conventional volume moves [46], although expensive for molecular systems, are easy to implement. Specialized variants have been proposed to speed up the equilibration in systems of flexible polymers, including branched and crosslinked polymers [51,52]. 

Within the different types of epoxies, are found epoxy diacrylates or vinyl ester resins, used to produce specific corrosion and chemical resistant composite systems. Vinyl ester resins are produced by either reacting epoxy resins of glycidyl derivatives with methacrylic acid, or from BPA and glycidyl methacrylates, where an active monomer (usually styrene) as crosslinker, hardener (usually organic peroxides), accelerators (cobalt) are added to the system. In the thermoset epoxy systems, there are also the mould releasers , which can be either internal such as, lecithin, or stearates of zinc and calcium, certain organic phosphates that are mixed in the resin, or, external - such as, fluorocarbons, silicone oil, and certain waxes, that are directly laid on the mould. 

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