Forcefield development and validation for Styrene-Isoprene Copolymers

Abstract

Copolymers are interesting kind of macromolecules and have found many applications. Styrene-Isoprene copolymers are used as additives in the fuels to improve their Viscosity Index (VI). A fuel with a good VI is the one which has low viscosity-temperature dependence.

I this masters thesis, we have developed a all-atomistic forcefield for these copolymers. The dihedrals and partial charges were calculated using quantum chemical calculations. MD simulations of the Styrene-Isoprene and Styrene-Isoprene (Hydrogenated) copolymers with different compositions. The MD simulation data have been analyzed to provide information about density dependence on the copolymer chain length in order to find minimum chain length that needs to be used in order to get a polymeric behaviour. The simulated annealing technique was used for calculating the glass transition temperatures of different compositions of copolymers. It was found that both the copolymers have linear density dependence on the copolymer compositions. Glass transition temperature follows a similar trend for both the copolymers.

N-decylbenzene and n-hexadecane were used as a base oil (diesel surrogates) to study the viscosity modification property of these copolymers. Our calculations using Green-Kubo method shows that the copolymers indeed enhance the viscosity of the base oil. Comparing the block and tapered copolymers, we observed that block copolymer increases the viscosity comparatively higher than the corresponding tapered copolymer. Tapered copolymer works better as Viscosity Index Modifier (VIM), as the viscosity-temperature dependence is lower compared to the block copolymers. The radius of gyration was observed to be smaller for tapered than block copolymer which results in a lower interaction of the tapered copolymer with the base oil, hence lower viscosity enhancement compared to a block copolymer.