Effect of Chain Microstructures and Operation Conditions

Comonomer content significantly affects Tref profiles. This is to be expected, as the comonomer units are known to reduce chain regularity, thus lowering chain crystallizability [26,27]. Linear relationships between average comonomer contents (CC) and elution peak temperatures are generally observed for ethylene/1-olefin copolymers [5,6]. It is important that these relationships reflect only the influence of comonomer content because they are used as calibration curves to convert Tref profiles into CCDs. 

Operation conditions also influence Tref results. Our recent work [29] showed that crystallization kinetics plays an important role in the fractionation process. The CR, the heating rate, and the solvent flow rate can affect Tref profiles to a large extent. We demonstrated that there is a general relationship between Tref profiles and the relative ratio among the CR, the heating rate, and the solvent flow rate  

A too fast CR can significantly reduce the fractionation efficiency, as polymer chains will not have sufficient time to separate according to their microstructures. 

Since the solvent flow rate during the elution step determines the residence time of the polymer solution in the column, slow flow rates broaden Tref profiles and increase the Tref peak temperature, most likely due to an increase in residence time and axial dispersion in the column. Although higher solvent flow rates can help reduce axial dispersion, they also reduce the signal-to-noise ratio. 

Fast heating rates can broaden Tref profiles, as a given volume of solvent will elute polymer over a wider range of crystallinities. 

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