Static and Dynamic Mechanical Behavior of Hydroxyapatite-Polyacrylic Acid Composites Under Simulated Body Fluid

In this work, we have investigated mechanical response of hydroxyapatite/polyacrylic composites under dry, wet and simulated body fluid conditions. Hydroxyapatite (HAP) is mineralized under two conditions; one, in presence of polyacrylic acid (in situ HAP), second, in absence of polyacrylic acid (ex situ HAP). Further, in situ and ex situ HAP are mixed with polyacrylic acid to make HAP/PAAc composites. Interfacial interactions between PAAc and HAP have been studied using photoacoustic Fourier transform infrared spectroscopy (PAS-FTIR). The mechanical response of the composites under wet condition is studied by soaking composite samples in simulated body fluid (SBF). Under wet conditions, SBF and water weaken the HAP-HAP interface significantly. PAS-FTIR data suggests that PAAc attaches to HAP through the dissociated carboxylate groups. The water and SBF soaked samples showed creep-like behavior and exhibit large residual strain after unloading. Loading under different strain rates has significant effect on mechanical properties of these composites. Both in situ and ex situ 70:30 composites exhibit highest elastic modulus at strain rate of 0.01 sec-1. XRD study indicates formation of Ca₂P₂O₇ phase in ex situ composite after soaking in SBF and water for 3 hours, whereas in situ composites showed presence of only hydroxyapatite phase after soaking in SBF and water for same duration of time.