Topological Mechanism of Polymer Nucleation and Growth – The Role of Chain Sliding Diffusion and Entanglement
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Direct evidence of nucleation during the induction period of nucleation from the melt is obtained for the first time by means of small angle X-ray scattering (SAXS). This confirmed that the induction period of crystallization from the melt corresponds to the process of nucleation, not to that of spinodal decomposition. This success is due to a significant increase in the scattering intensity (Ix) from the nuclei (104 times as large as is normal), which was achieved by adding a nucleating agent (NA) to a “model polymer” of polyethylene (PE). Ix increased soon after quenching to the crystallization temperature (Tc) and saturated after the induction time (τi). Lamellae start stacking later than the Mn.
Power laws of the molecular weight (Mn) dependence of the primary nucleation rate (I) and the growth rate (V) of PE, i.e., I or V ∝ Mn−H where H is a constant, were found for both morphologies of folded chain crystals (FCCs) and extended chain crystals (ECCs). As the power law was also confirmed on isotactic polypropylene (iPP), universality of the power law is suggested. It is to be noted that the power H increases significantly with increase of the degree of order of the crystal structure. The power law confirms that the topological nature of polymer chains, such as chain sliding diffusion and the chain entanglement within the interface between the nucleus and the melt or those within a nucleus, adopts a most important role in the nucleation and growth of polymers. This is theoretically explained by improving the “chain sliding diffusion theory” proposed by Hikosaka.
Entanglement dependence of the nucleation rate I is qualitatively obtained for the first time by changing the number density of entanglement (νe) within the melt. An experimental formula of I as a function of νe was obtained on PE, I(νe) ∝ exp(−γνe) where γis a constant.
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The authors are grateful to Prof. Akihiko Toda, Dr. Isao Wataoka, Dr. Swapan K. Ghosh of Hiroshima University, Dr. K. Yamada of SunAllomer Co. Ltd., Dr. Katsuaki Inoue of the Japan Synchrotron Radiation Institute (JASRI) and Dr. Zdenek Kozisek of the Institute of Physics, Academy of Sciences of the Czech Republic for their help with the experiments and discussions. SAXS experiments were carried out at the BL40B2 of SPring8 (SP8) at JASRI (Proposal No. 2001B0187-NDL-np—2004A0224-NL-2b-np) in Harima and at the BL-10C small angle installation of the Photon Factory (PF) at KEK in Tsukuba. The authors also thank Asahi Denka Kogyo K.K. for supplying the nucleating agent. This work was partly supported by the Grant-in-Aid for Scientific Research on Priority Areas B2 (No.12127205) and Scientific Research A2 (No. 12305062). The authors are grateful to the financial support from the International Joint Research grant, NEDO, 1996–1998.
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