Micromechanical Model for the Orthotropic Elastic Constants of Polyetheretherketone Composites Considering the Orientation Distribution of the Hydroxyapatite Whisker Reinforcements

Abstract

Hydroxyapatite (HA) whisker reinforced polyetheretherketone (PEEK) composites have been investigated as bioactive materials for load-bearing orthopedic implants with tailored mechanical properties governed by the volume fraction, morphology, and preferred orientation of the HA whisker reinforcements. Therefore, the objective of this study was to establish key structtire-property relationships and predictive capabilities for the design of HA whisker reinforced PEEK composites and, more generally, discontinuous short fiber-reinforced composite materials. HA whisker reinforced PEEK composites exhibited anisotropic elastic constants due to a preferred orientation of the HA whiskers induced during compression molding. Experimental measurements for both the preferred orientation of HA whiskers and composite elastic constants were greatest in the flow direction during molding (3-axis, C^j), followed by the transverse (2-axis, C22) and pressing (Iaxis, Cji) directions. Moreover, experimental measurements for the elastic anisotropy and degree of preferred orientation in the same specimen plane were correlated. A micromechanicai model accounted for the preferred orientation of HA whiskers using two-dimensional implementations of the measured orientation distribution function (ODF) and was able to more accurately predict the orthotropic elastic constants compared to common, idealized assumptions of randomly oriented or perfectly aligned reinforcements. Model predictions using the 3-2 plane ODF, and the average of the 3-1 and 3-2 plane ODFs, were in close agreement with the corresponding measured elastic constants, exhibiting less than 5% average absolute error. Model predictions for C// using the 3-1 plane ODF were ¡ess accurate, with greater than 10% error. This study demonstrated the ability to accurately predict differences in orthotropic elastic constants due to changes in the reinforcement orientation distribution, which will aid in the design of HA whisker reinforced PEEK composites and, more generally, discontintious short fiberreinforced composites. [DOI: 1O.1115/1,4(X)5421]

Effects of the mold temperature on the mechanical properties and crystallinity of hydroxyapatite whisker‐reinforced polyetheretherketone scaffolds

“…These results suggest that further improvement of scaffold mechanical properties could be possible by achieving a more uniform dispersion of HA reinforcements within the PEEK matrix through compounding PEEK and HA reinforcements before compression molding or inducing flow during molding. HA whiskers were previously observed to be uniformly distributed within the PEEK matrix, with no evidence of segregation, in dense composites which were compression molded in channel die to induce flow 14, 16, 17. Alternatively, chemical coupling agents can improve reinforcement dispersion and composite mechanical properties,31 but would certainly be subject to enhanced regulatory scrutiny due to leaching from exposed HA/PEEK interfaces.…”

“…Therefore, a number of studies over the last decade began to investigate bioactive PAEK composites, and these are reviewed in detail elsewhere 11. Dense HA‐reinforced PEEK composites have been engineered to exhibit mechanical properties similar to those of human cortical bone 15–17. The bioactivity of HA‐reinforced PEEK was confirmed in vitro after immersion in simulated body fluid 18.…”