Abstract
AbstractCrystallization‐based fractionation techniques are powerful methods for the analysis of short‐chain branching (SCB) in linear low‐density polyethylene. Recently, thermal gradient interaction chromatography (TGIC) has been developed for SCB determination of semi‐crystalline and amorphous polyolefins. In TGIC, the fractionation mechanism relies on the interaction of polyolefin chains with a graphite surface upon temperature change for a given solvent strength. Using molecular dynamics simulations, both the enthalpic and entropic contributions responsible for the separation of short‐chain branched polyethylene are estimated. A new thermodynamic framework for understanding the fractionation mechanism of TGIC is proposed. It is confirmed that at high SCB content, the decrease of the adsorption enthalpy is mostly due to the increased steric hindrance of short branches. However, for low chain branching density, strong increase of the persistence length observed during adsorption decreases the conformational entropy of the polymer and thus counter‐balances the favorable adsorption enthalpy. This entropic contribution may also explain the narrowing of the peak observed experimentally with low short‐chain branching density at high elution temperature.
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