In this study, synchrotron small-angle X-ray scattering (SAXS) and dynamic mechanical thermal analysis (DMTA) were used to evaluate the aging behavior of microfibrils and nanovoids. The effects of such structures on the tensile strength were also investigated. We investigated the correlation length of the fibril interface by fitting the SAXS intensity using the Debye-Bueche method. The orientation and size of the voids were determined with Ruland's streak method. The results show that the correlation length decreased with aging time at 90 degrees C. Voids formed after aging at high temperatures for prolonged periods. In addition, the orientation of the 10 angstrom voids changed with the degree of degradation. DMTA results revealed a new transition temperature for the aged fibers. A model based on the SAXS and DMTA results is proposed to illustrate the hydrothermal aging mechanism. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 1291-1296, 2013