Lamellar crystal thickness lc of isotactic polybutene-1 (it-PB1) have been investigated for crystallization in the melt over a wide range of crystallization temperature T from 40°C to 90°C by small angle X-ray scattering experiments and density measurements. The crystal thickness lc demonstrates two linear dependences on inverse supercooling and a transition from one dependence to the other has been observed around T = 65°C. Each of the two dependences obeys the nucleation theory in the high and low supercooling ranges, respectively. Chain folding free energy q determined from the low supercooling range is larger than that determined from the high supercooling range. Possible mechanisms for the transition are discussed taking account of entropy of chain folding directions.
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