Solid + Liquid Equilibria of Isotactic Poly(1-butene) + n-Alkanes by Differential Scanning Calorimetry and Dynamic Methods

Isotactic crystalline low-molecular-weight poly(1-butene), iPBu-1, was synthesized by using a metallocene catalyst. The molecular weight was determined by using gel permeation chromatography. The chemical structure of iPBu-1 was verified by using high-temperature 13C NMR spectroscopy and differential scanning calorimetry (DSC) methods. The solid + liquid phase equilibria (SLE) of iPBu-1 with three long-chain n-alkanes (n-tridecane, n-octadecane, and n-tricosane) were studied by DSC and dynamic methods. It was found that in the first and second heating runs of the mixture the multiple endotherms were detected on heating. This was attributed to the solid + liquid phase transition at melting temperature and to the solid + solid phase transitions of the well-known polymorphic forms, form Iʹ - form II. After the next few heating runs of the mixture, one new endotherm for the same sample was detected. It was assumed that the new endotherm, being observed at higher temperature, was evidence of the transformation of iPBu-1 into the new high-temperature crystalline form of polymer, form I (twinned hexagonal). The same was observed over the systematic thermal conditioning after a few days. By the performance of DSC heating and dynamic method experiments over a large concentration range, the T-x phase diagrams of polymersolvent systems could be constructed. From these diagrams, it was found that the solubility of iPBu-1 is the highest in ntridecane and the lowest in n-tricosane in the mole fraction range measured. A similar conclusion was made from a dynamic method. The excess Gibbs energy models were used to describe the nonideal behavior of the liquid phase and to estimate the solubility of iPBu-1 in the whole mole fraction range.