Local conformation controlled crystallization of isotactic poly(butene-1)

The slow spontaneous solid-solid transition from metastable form II to stable form I in typical isotactic polybutene-1 (iPB-1) homopolymer after melt crystallization may be unavoidable and it is challenging to obtain form I directly from bulk iPB-1 melt. In this work, by DSC and WAXS, form I was observed to directly crystallize from bulk iPB-1 while cooling to T far below the crystallization temperature T_c of form II. It is further proposed that the local formation of 3/1 helix conformation is the determining process of direct formation of form I and there exists a temperature window for 3/1 helix conformation to form at low T due to the competition between the entropy effect and the synergistic effect of enthalpy and the energy barrier. The key of polymorph selection of iPB-1 lies in the T-dependent formation of the crystal-favored helix conformations. The results could shed lights on developing charming industrial approaches to obtain the stable form I directly in bulk iPB-1 as well as on understandings to the physics behind polymer crystallization and polymorph selection processes.