Expansion during polymerization

FIGURE 54 Micrograph of polymer particle, showing structural evidence of its growth and expansion during polymerization. 

Bending beam theory calculation of elastic modulus, 361-362 calculation of glass temperature, 362 calculation of thermal expansion coefficient, 362 layer stress determination, 361 Benzophenone-3,3, 4,4 -tetracarboxydi-anhydride-oxydianiline-m-phenylenediamine (BTDA-ODA-MPDA) polyimide, properties, 115-116 Bilayer beam analysis schematic representation of apparatus, 346,348/ thermal stress, 346 Binary mixtures of polyamic acids curing, 116-124 exchange reactions, 115 Bis(benzocyclobutenes) heat evolved during polymerization vs. 

FIGURE 53 Pore volumes of recovered catalyst ash obtained by burning away the polymers produced at various yields. The expansion of the catalyst during polymerization is apparent in the pore volume of the ash, before it finally collapses. 

Table 8.11. Volume changes during polymerization and/or isomerization of TBH calculated from bulk densities of monomer and polymer. (+ expansion, — shrinkage)47)...

According to these results expansion in volume during polymerization occurs only when both fused rings open upon polymerization. 

The thermal expansion of dental composites, as with other dental materials, exceeds that of tooth structure. Moreover, there is a contraction during polymerization of 1.2 to 1.6%. These effects are thought... 

A proper balance among the catalyst structure, polymerization conditions, and the nature, number, and distribution of the active centers will allow for a progressive expansion throughout the granule during polymerization (Fig.2). 

Shrinkage during polymerization reduces die forces and should always be arranged to be greater than the thermal expansion caused by the temperature rise. 

It was reasoned from this study that if monomers were available in which at least two rings were opened for every new bond that was formed in the backbone, materials with either no change in volume during polymerization or slight expansion would be possible. It should be emphasized that this concept would eliminate from consideration the polymerization of a monomer, such as a dl-epoxlde or a dlanhydrlde, because, although two rings are opened during the polymerization, two new bonds are also formed at the same time. 

When the polymerization was carried out In a dllatometer In which the bath was held at a constant temperature (142 0), the meniscus. Instead of falling as Is the usual case during polymerization, actually rose quite substantially. A calculation of the extent of change in volume Indicates an expansion In excess of 2%. This compares very favorably with the very slight Increase (0.14%) in volume reported earlier for the polymerization of a spiro ortho ester. This example, then, represents the first reported case In which a substantial amount of expansion in volume occurs during polymerization. 

It is obvious from the data that the conversion of a crystalline monomer to an amorphous polymer represents the ideal case for the large expansion in volume since in most cases the crystalline monomer would be expected to be considerably more dense than the corresponding liquid monomer. This is just the opposite of the case in which a liquid monomer is converted to a crystalline polymer. For example, when liquid ethylene monomer is converted to crystalline polyethylene at 5 C, a 66% shrinkage occurs, appears that the conversion of a liquid monomer to a crystalline polymer represents the ideal case to get the largest shrinkage during polymerization (3). 

The POW has quite a different morphology from the SGC or MGC. For the morphology of the POW, the major structure consists of particles having a radius of 3 pm, and several fibrils befween fhese particles. The origin of these cobweb structures, composed of particles and fibrils, is ascribed fo the internal expansion stress during polymerization [29,30], Much polymer chains synthesized inside the earlier powder globule causes extensive force, which stretches the outer polymer membrane in the polymerization process. 

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