Step polymerization Crosslinking Prepolymer

Simultaneous luterpenetrating Network, SIN. The monomers and/or prepolymers plus crosslinkers and activators of both components are mixed, followed by simultaneous polymerization via non-interfering reactions, see Figure 6.1B. Typical syntheses involve chain and step polymerization kinetics. While both polymerizations proceed simultaneously, the rates of the reactions are rarely identical. 

Precipitate can also form fiom a step reaction. Precipitation polymerizations of prepolymers to form crosslinked, thermoset polymers is a very common commercial reaction of the epoxy resins and phenol-methanal, network polymers covered in this book. These reactions work best when the reaction is slow, mildly exothermic, and undergoes a viscous to solid transformation late in the synthesis. 

Formaldehyde-based resins were the first network polymers prepared by step polymerization to be successfully commercialized. They are prepared in two stages. The first involves the formation of a prepolymer of low molar mass which may either be liquid or solid. In the second stage the prepolymer is forced to flow under pressure to fill a heated mould in which further reaction takes place to yield a highly crosslinked, rigid polymer in the shape of the mould. Since formaldehyde is difunctional, the coreactants must have a functionality, /, eater than two and those most commonly employed are phenol (/= 3), urea (/= 4) and melamine (/= 6)... 

Step growth polymerization can also yield highly crosslinked polymer systems via a prepolymer process. In this process, we create a prepolymer through a step growth reaction mechanism on two of the sites of a trifunctional monomer. The third site, which is chemically different, can then react with another monomer that is added to the liquid prepolymer to create the crosslinked species. We often use heat to initiate the second reaction. We can use this method to directly create finished items by injecting a mixture of the liquid prepolymer and additional monomer into a mold where they polymerize to create the desired, final shape. Cultured marble countertops and some automotive body panels are created in this manner. 

From a general point of view, chemical crosslinking processes can be classified into four major types of reactions step-growth polymerization, free radical polymerization, vulcanization, and end-linking of prepolymers. 

Direct Casting The direct method is to cast the polymer material onto the mold and then peel off the 3D structure after the material is polymerized. Figure 10 illuminates this simple process. The reverse 3D structure is first prepared by microfabrication techniques as a mold. Then, a prepolymer with crosslink is cast onto the mold. After the polymer is well cured by heat or UV, the 3D polymer structure is produced by peeling off the polymer structure from the mold. To construct a microfiuidic device, bonding the 3D polymer structure with a substrate is the next step to seal the fluidic channels and reservoirs. 

It is therefore very difficult to use the lathe cut method to satisfy this need. An appropriate response is to adopt the direct method using complete precision polymerization of an aqueous solution with a single step process in a mold. It is also necessary to develop an appropriate monomer, prepolymer, or photopolymerizable materials. At this point, optimization of design concept as shown in Fig. 3 becomes necessary. The raw material must be water-soluble monomers or polymers that can be aqueous-solution polymerized. During polymerization, 3D crosslinking must take place. Furthermore, the lens must be molded while it contains water and therefore dimensional accuracy will be guaranteed. 

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