Monomers Polymerization Shrinkage

Dilatometry utilizes the volume change that occurs on polymerization. It is an accurate method for some chain polymerizations because there is often a high-volume shrinkage when monomer is converted to polymer. For example, the density of poly(methyl methacrylate) is 20.6% lower than that of its monomer. Polymerization is carried out in a calibrated reaction vessel and the volume recorded as a function of reaction time. Dilatometry is not useful for the usual step polymerization where there is a small molecule by-product that results in no significant volume change on polymerization. 

Another type of thermoset polymers is based on epoxy monomers. These thermosets are cured by use of a two-component system or by photo initiators. Disadvantages of epoxies are high water uptake in service and polymerization shrinkage (22). 

A typical example of frontal polymerization is the polymerization of methyl methacrylate (or an oligomer), placed inside a long aluminum tube 249 these tubes continuously dip into a bath with a liquid heated up to temperature of 70 - 80°C. The part at the tubes above the bath are cooled so that the reactive material does not polymerize. Polymerization shrinkage is compensated by continuous injection of a monomer or oligomer into the reaction zone. The appropriate combination of injection rate, velocity of tube movement through the reaction zone, and tube diameter are chosen according to experimental studies of the process. 

Diacrylate monomers have been prepared that are photocurable in visible light and that have small polymerization shrinkage and high X-ray contrast properties. When polymerized with 0.01 to 0.04 pm glass powder, these dental composites were easily machined into artificial teeth. 

Continuous Dilatometric Measurement of y-Emulsion Polymerization. Polymerization of vinyl monomers causes a volume contraction up to 27% (vinyl acetate) of the volume of the monomer. This shrinkage is proportional to conversion and can be linearly interpolated, however consult Treloar (37). The dila-... 

The use of a diluent monomer gives problems, however. Polymerization shrinkage is increased leading to problems of bonding at the adhesive interface with tooth dentin (Puckett et al., 2007). 

While these approaches did provide for reductions in polymerization shrinkage and improvements in adhesion, the oxaspiro monomers were more sluggish in free... 

Recently, Garska et al. took an interest in the preparation of methacrylated calix[4]arene PAs (Scheme 8.5). Moszner et al. had previously demonstrated that the incorporation of modified calix[4]arenes into dental materials results in a significant decrease of the polymerization shrinkage. As an extension of this work, the phosphonic acid PA-7 and the diphosphonic acid PA-8 were prepared in three steps, starting from j-tert-butylcalix[4]arene. To evaluate their adhesive properties, those two monomers were... 

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). 

The resin matrix of commercial dental composites has bis-GMA (bisphenol-A-glycidyldimethacrylate) as its predominant base monomer. Due to its high viscosity, bis-GMA is mixed with other dimethacrylates, such as TEGDMA, UDMA or other monomers of lower molecular weight [26, 50] to reduce viscosity. The monomers are heavily reinforced with filler particles, which add dimensional stability, improve wear and strength of the material, also reducing polymerization shrinkage [51]. 

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