Molecular Dynamics Simulation of Ethylene/Hexene Copolymer Adsorption onto Graphene: New Insight into Thermal Gradient Interaction Chromatography

• Published in: Macromolecular chemistry and physics 2019-04, Vol.220 (8), p.n/a
• Main Authors: Brunel, Fabrice, Boyron, Olivier, Clement, Arnaud, Boisson, Christophe
• Format: Article
• Language: English
• Subjects: Adsorption; Chain branching; Chemical Sciences; Chromatography; Crystallization; Density; Elution; Enthalpy; Fractionation; Graphene; Low density polyethylenes; Molecular dynamics; Polyethylene; Polymers; Polyolefins; Steric hindrance; Temperature gradients
• ISSN: 1022-1352; 1521-3935
• DOI: 10.1002/macp.201800496

Crystallization‐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. Molecular dynamics simulations of polyethylene adsorption onto graphene provide new insights into the fractionation mechanism of thermal gradient interaction chromatography (TGIC). Steric hindrance from short branches decreases the adsorption enthalpy. However, for low short‐chain branching, the increase persistence length during adsorption decreases the conformational entropy of the polymer. This entropic contribution explains the narrowing of the peak characteristic of TGIC profiles.