Molecular Weight Dependency of Crystallization Line, Recrystallization Line, and Melting Line of Polybutene-1

Isothermal crystallization and subsequent melting processes in semicrystalline polymers can be described by several linear dependencies termed as crystallization line, recrystallization line, and melting line in a phase diagram of inversed crystalline lamellar thickness as a function of temperature. The crystallization line denotes the lamellar thickness dependency on crystallization temperature. Upon subsequent heating, the system either undergoes a process involving a continuous change in lamellar thickness following a recrystallization line or a direct melting at a constant thickness defining a melting line. The limiting temperatures of infinite lamellar thickness for crystallization line and recrystallization line coincide with each other and are higher than that of melting line. The above-mentioned experimental results are interpreted as a consequence of involvement of a mesomorphic phase during crystallization. In this work, the effect of molecular weight on the crystallization line, recrystallization line, and melting line was investigated for three polybutene-1 (PB-1) samples in metastable form II by means of temperature-dependent small-angle X-ray scattering experiments. Three crystallization lines with different slopes but same limiting temperature were observed for the three PB-1 samples. However, a common recrystallization line and melting line were observed for all three samples. The results can be understood as follows. The surface free energy of the native crystals formed from the mesomorphic blocks increases with molecular weight in PB-1, leading to an increase of the slop of the crystallization line for samples with higher molecular weight. However, during the stabilization process after the formation of native crystals forming stable crystals the surface free energy decreases to such an extent that the differences among samples of different molecular weight vanish so that a common recrystallization line and melting line were observed.