Polyacid in ionomer is unable to diffuse through dentin due to its

• A. high molecular weight
• B. low molecular weight
• C. solubility
• D. polymerization rate

Answer: (A)

Diffusion through dentin mainly depends on the size of the diffusing molecule relative to the porous structure of dentin. The polyacid in ionomer is a large polymer chain with high molecular weight, so its movement is hindered by steric barriers as it tries to pass through dentinal tubules and the mineralized matrix. As molecular weight increases, the diffusion coefficient decreases, making diffusion through dentin effectively limited. Solubility would affect how much dissolves, not how easily large molecules move through the solid dentin, and polymerization rate relates to curing within the cement rather than diffusion across dentin. Hence, the inability to diffuse is due to the high molecular weight.