The present invention provides a nano-particle comprising a shell and a polar core, their preparation, and their applications in ER fluids, polymeric product, rubber composition, tire product, hard disk drive gasket, matrix composition, and engine mount etc. The shell comprises a polymer formed from formula (I) monomers and the core comprises a polymer formed from formula (II) monomers, in which R.sub.1 and R.sub.2 are hydrogen; R.sub.3, R.sub.4, R.sub.5, R.sub.6, and up to three of R.sub.7, R.sub.8, R.sub.9, and R.sub.10 are each independently selected from the group consisting of hydrogen, methyl, ethyl, propyl, and isopropyl; and at least one of R.sub.7, R.sub.8, R.sub.9, and R.sub.10 is a polar group that is more polar than any one of R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6 groups. ##STR00001##