Significant reductions in linear thermal expansion coefficients in the flow and transverse directions of injection-molded specimens of thermoplastic polyolefin, or TPO, nanocomposites were achieved by controlling the maleated polypropylene (PP-g-MA)/organoclay ratio. Linear thermal expansion behavior was examined using a thermomechanical analyzer (TMA). The trends in thermal expansion for the nanocomposites are discussed in terms of the morphology of both dispersed clay and elastomer phases by means of transmission electron microscopic (TEM) and atomic force microscopic (AFM) observations and subsequent particle analyses. A higher PP-g-MA/organoclay ratio causes an increase in the aspect ratio of clay particles along the flow direction (FD) and transverse direction (TD) for the injection-molded specimens; however, the aspect ratio along the FD was higher than that along the TD. On the other hand, the aspect ratio of elastomer particles along the FD was much higher than that along the TD. Furthermore, highly elongated elastomer particles along the FD were observed. The combined effect of the mechanical constraint by organoclay and the highly elongated elastomer particles caused at high PP-gMA contents was responsible for the significant reduction of thermal expansion for these materials. (c) 2008 Elsevier Ltd. All rights reserved.