The exfoliated montmorillonite (MMT) nanoplatelets tended to re-stack with each other after casting the MMT/poly(methylacrylate-co-methylmethacrylate) P(MA-co-MMA) latex solutions fabricated by soap-free emulsion polymerization into films as revealed by X-ray diffraction and transmission electron microscopy. As the content of MMT was increased from 0 to 20 wt %, the T-g measured by differential scanning calorimetry was slightly decreased from 19.2 to 17.2 degrees C, whereas that measured by dynamic mechanical analysis was increased from 22 to 32 degrees C, indicating that the local motion of polymer segments has been retarded by MMT nanoplatelets. Besides, the elongated elliptical voids appeared during stretching of 1 wt % MMT/P(MA-co-MMA) film to cracking also illustrated the pinning effect provided by the exfoliated MMT. As the content of MMT was increased more than 10 wt %, the mechanical behavior of MMT/P(MA-co-MMA) nanocomposite films was changed from ductile to brittle nature with significant increase of Young's modulus and tensile strength owing to the restacking of exfoliated MMT nanoplatelets. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1064-1069, 2010