Chain Entanglements and Interlamellar Links in Isotactic Polybutene-1: The Effect of Condis Crystals and Crystallization Temperature
Yanan Qin, Wenbo Song, Ming Chen, V. M. Litvinov, Yongfeng Men
Abstract
Chain entanglements, which reside in the amorphous phase of semicrystalline polymers, influence many macroscopic properties of polymers. There are different opinions about the role of crystallization and subsequent melting in the entanglement density. Isotactic polybutene-1 (iPB-1) was selected for this study. Crystallization of this polymer from an equilibrated melt led to the formation of conformationally disordered (condis) form II crystals with high chain mobility enabling long-range chain diffusion, which facilitates chain disentangling. After the crystallization was accomplished, the entanglement network was frozen during the phase transition to stable form I with the polymer backbone fixed in the crystals. All experiments were performed for samples with form I crystals. A significant decrease in the entanglement density occurred during slow crystallization of iPB-1 with Mw = 115 kg/mol. The disentangling degree was lower at faster crystallization and for iPB-1 with a higher molecular weight Mw = 711 kg/mol. The entanglement density increased during melting and approached its equilibrium value at the end of melting. The estimated entanglement density of the equilibrated iPB-1 melt coincided within approximately 8% with the value reported in a rheological study. In addition to the entanglement density, the relative density of chain segments, which are anchored to crystallites and form interlamellar links, was estimated. The density was found to increase with increasing molecular weight, and it was larger at a faster crystallization rate due to thinner lamellae and a smaller long period on the periodic lamellar structure.