Mechanical properties and interrelationships of poly(methyl methacrylate) following hydration over saturated salts

Three types of specimens were machined from a model unfilled linear poly(methyl methacrylate) (PMMA), which was nominally 1.5 mm thick. After pre-condition annealing and pre-drying, the specimens were equilibrated at one of eight relative humidities (RH) at 22 or 37°C. Thereafter, the parallelepipeds were deflected in 3-point bending, the dumbbells were pulled in tension or deformed using a Knoop (HK) microhardness indenter, and the disks were deformed using a Vickers (HV) microhardness indenter. As the RH increased from 0 to 100%, the samples exponentially sorbed 2% w/w of water. Elastic moduli in bending and tension (EB and ET), ultimate tensile strength (UTS), and hardnesses (HV and HK) were inversely and linearly dependent on water uptake (p<0.001). Strain at UTS (εUTS) was independent of weight change; whereas, strain at fracture (εF) was directly and linearly dependent on water uptake (p<0.02). Under these equilibrium conditions of sorption, no evidence was found that sustained the concept that a break in mechanical properties occurred at about 1% sorption as a result of plasticization leading to clustering. After logarithmic transformations of selected mechanical properties, linear correlations were found between HV versus EB (p<0.02) and strength (UTS or YS) versus HV (p<0.001). The results paralleled the relationship found for pure face-centered-cubic (FCC) metals in the former case and was coincident with the relationship for FCC metals in the latter. These interrelationships suggest that the effects of plastic anisotropy are absent in hydrated PMMA and that water continues to facilitate long-range elastic interactions.