Shrinkage upon polymerization

All monomers, but a few exceptions exhibit shrinkage upon polymerization since Van der Waals bonds are converted into much shorter covalent bonds. The accompanying reduction of the entropy also favors a closer packing. When a surface with a specified curvature has to be made by replication from a mold, due allowance has to be made for shrinkage of the replication coating. This means that the shape of... 

Approximate volume shrinkage upon polymerization (%) Solubility in water (%)... 

The polymerization of methacrylic monomers is highly exothermic and involves an increase in the density of the forming polymeric material. Both effects must be accounted for in industrial processes. The degree of shrinkage upon conversion from monomer to polsrmer decreases as the side-chain length increases, but on a molar basis, the shrinkage level is relatively constant (40). 

Finally, B5A could be polymerized at room temperature to high conversion and showed little density change upon polymerization. The low polymerization shrinkage with added advantage of room temperature polymerization has important implications if the monomer is used in composites or potting compounds. The interfacial stresses will probably be very low due to low polymerization shrinkage and low polymerization temperature (i.e., no thermal shrinkage). 

Mixtures of monomers and oligomers must possess two attributes in order to be practical for TPP. First, upon polymerization they should undergo as litfle shrinkage... 

Slowing down the polymerization rate is another mechanism that has been utilized to compensate for stresses generated upon polymerization in resin-based composites. These mechanisms increase the material flow capacity, lower stress build-up and thus promote improved interfacial integrity [84]. A recently introduced flow-able resin-based composite material, intended to be used as a Uner in occlusal and posterior proximal restorations, differs from conventional resin by incorporating a Sfress Decreasing Resin (SDR) technology. This material provides an approximate 20 % reduction in volumetric shrinkage and almost an 80 % reduction in polymerization sfress compared to a traditional resin system due to the addition of a urethane... 

Common thermosets are cured by a free radical addition mechanism. These types of composites are cured by heat initiators, such as peroxides, or by photo initiators, such as a-diketones. A characteristic of cured acrylates is large shrinkage in the course of polymerization, which is undesirable for many uses. Another undesirable characteristic of acrylates is the formation of an oxygen-inhibited layer on the surface upon curing. 

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