Topological mechanism of polymer nucleation and growth - The role of chain sliding diffusion and entanglement

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 (I_x) 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). I_x increased soon after quenching to the crystallization temperature (T_c) and saturated after the induction time (τ_i). Lamellae start stacking later than the τ_i. Power laws of the molecular weight (M_n) dependence of the primary nucleation rate (I) and the growth rate (V) of PE, i.e., I or V ∝ M_n^-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 (ν_e) within the melt. An experimental formula of I as a function of ν_e was obtained on PE, I(ν_e) ∝ exp(-γν_e) where γ is a constant.