In order to understand more fully how polymer matrix attributes influence polymer nanocomposite properties, nanocomposites containing hydroxyapatite nanoparticles and a poly(3-hydroxybutyrate) matrix were prepared and compared to results for a chemically-similar nanocomposite system with a lesser degree of matrix crystallinity. Experimental results indicated that the higher degree of matrix crystallinity hinders nanoparticle dispersion at loadings above 0.5 wt.% and together these structural factors, high matrix crystallinity and nanoparticle aggregation, produced different mechanical reinforcement behavior below and above the glass transition temperature than has been seen previously in amorphous matrices or matrices with moderate crystallinity levels. Overall, these results suggested that the amorphous character of the polymer does not govern the properties at all crystallinity levels in polymer nanocomposite matrices. (C) 2011 Elsevier Ltd. All rights reserved.