Crystallization-Based Fractionation Techniques

The vast majority of commercial polyolefins are semi-crystalline materials. Depending on the chemical composition and tacticity, their melting temperatures range from ambient to temperatures exceeding 160 C. Polyolefins form various crystalline structures that can be investigated with microscopic, spectroscopic and scattering techniques. When a semi-crystalline polyolefin is dissolved in a solvent at high temperature (usually above the melting temperature), followed by a continuous or stepwise decrease of the solution temperature, the polyolefin starts to form crystals that will precipitate out of the solution. The crystallization temperature, the shape and the amount of crystals depend on the molecular structure of the polyolefin, mainly its chemical composition, its tacticity and the degree of branching. Highly crystalline materials will crystallize out of the solution at higher temperatures than materials with lower crystallinity. Flory–Huggins statistical thermodynamic treatment accounts for melting point depression due to the presence of a diluent in a crystallizing system. The diluent can be a solvent or a comonomer. In either case, the crystallization temperature decreases with increasing diluent concentration. Therefore, for copolymers that do not have long chain branches, the separation by crystallizability can be regarded as a separation according to chemical composition. A precondition is that the concentration of the diluent is low and it does not enter into the crystal lattice of the crystallizing polymer. For copolymers where the non-crystallizing comonomer is the diluent, a linear dependence of the melting or crystallization temperature on the amount of comonomer incorporated is observed. Such linear dependencies have been seen in temperature rising elution fractionation (TREF), differential scanning calorimetry (DSC) and crystallization analysis fractionation (CRYSTAF) experiments [1–4]. The potential of crystallization behaviour of semi-crystalline polymers as an analytical tool was recognized by Desreux and Spiegels [5] in the early history of polyolefin fractionation. TREF, CRYSTAF and crystallization elution fractionation (CEF) are the main techniques that are used today in this category. The differences