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(me thyl methacrylate) (PMMA), which was nominally 1.5 mm thick. After pre-cond ition annealing and pre-drying, the specimens were equilibrated at one of e ight relative humidities (RH) at 22 or 37 degreesC. Thereafter, the paralle lepipeds were deflected in 3-point bending, the dumbbells were pulled in te nsion or deformed using a Knoop (HK) microhardness indenter, and the disks were deformed using a Vickers (HV) microhardness indenter. As the RH increa sed from 0 to 100%, the samples exponentially sorbed 2% w/w of water. Elast ic moduli in bending and tension (Eg and ET). ultimate tensile strength (UT S), and hardnesses (HV and HK) were inversely and linearly dependent on wat er uptake (p < 0.001). Strain at UTS (<epsilon>(UTS)) was independent of we ight change; whereas, strain at fracture (epsilon (F)) was directly and lin early dependent on water uptake (p < 0.02). Under these equilibrium conditi ons of sorption, no evidence was found that sustained the concept that a br eak in mechanical properties occurred at about 1% sorption as a result of p lasticization leading to clustering. After logarithmic transformations of s elected mechanical properties, linear correlations were found between HV ve rsus E-B (p < 0.02) and strength (UTS or YS) versus HV (p < 0.001). The res ults paralleled the relationship found for pure face-centered-cubic (FCC) m etals in the former case and was coincident with the relationship for FCC m etals in the latter. These interrelationships suggest that the effects of p lastic anisotropy are absent in hydrated PMMA and that water continues to f acilitate long-range elastic interactions. (C) 2000 Elsevier Science Ltd. A ll rights reserved.