Abstract: When polymer crystallized from its dilute solution, the formation of small crystallites is difficult because of the balance between the favorable energy decrease as the more stable crystal formed and the unfavorable interface energy increasing with the forming of crystallites, thus the crystallites smaller than a minimum critical size are unstable and will be disappear or grow to larger size crystals. In this paper, High Resolution Transmission Electron Microscopy(HRTEM) technology was used in the study of polyethylene(PE) and polypropylene(PP) nano-sized crystals obtained from their dilute solution of xylene. The results showed that the molecular packing in these crystals was far from perfect one. There were many defects such as distortion, branches, and dislocation in the crystals. After isothermal crystallization at the temperature of 413 K, the crystallinity was improved greatly. All these proved that the high resolution transmission electron microscopy technique was very powerful in the study on polymer crystal at the level of molecules, and the nano-sized crystals were another stable crystal state of PE and PP.

Key words: Polyethylene, Polypropylene, Nano-sized Crystal, HRTEM