What is the volume shrinkage of methyl methacrylate monomer when polymerized?

The volume shrinkage of methyl methacrylate monomer during polymerization is approximately 21%. This phenomenon occurs because as the monomer transforms into a polymer, the simple molecular structure of the monomer expands into a more complex three-dimensional structure with new cross-links. During this transition, some volume is lost due to the reduction of free volume between the molecules as they bond together, resulting in a net decrease in volume. The figure of 21% is a well-established value in dental materials science, especially in the context of dental resins and acrylics, where understanding shrinkage is critical for achieving accurate fits in restorations and minimizing gaps that could lead to microleakage. This is particularly important in dental applications, as proper curing and shrinkage rates can significantly impact the longevity and performance of dental materials.