Effects of Thickness on Time‐Dependent Diffusivity and Water Absorption in Polymethyl Methacrylate ( PMMA ): Experimental and Analytical Studies

ABSTRACT Polymethyl methacrylate (PMMA) is widely used in medical, marine, and engineering structures due to its excellent physical and mechanical properties. However, its susceptibility to water absorption threatens long‐term performance and dimensional stability, necessitating a thorough understanding of its water absorption behavior. This study investigates the effects of specimen thickness on the time‐dependent diffusivity and water absorption behavior of PMMA. Experiments were conducted on samples with five different thicknesses (3.2, 6.4, 9.6, 12.8 and 16.0 mm) to analyze absorption kinetics and determine diffusion coefficients. Results confirmed Fickian diffusion behavior and revealed that thinner specimens reached saturation faster and absorbed a higher final moisture content (1.13% vs. 0.74% for the thickest specimen), demonstrating an inverse relationship between thickness and total water uptake. A critical finding was the non‐steady nature of the diffusion coefficient, which was found to be dependent on both thickness and exposure time, exhibiting an initial high value that decayed as saturation approached. A novel three‐dimensional model derived from Fick's second law successfully captured this time‐dependent diffusion as well as accurately predicted water uptake. Accurate experimental validation confirms the model's utility as a predictive tool for assessing PMMA's long‐term sorption kinetics in water‐exposed applications.