Poly(styrene-co-1-octene) behavioral differences in aliphatic and aromatic solvents: molecular dynamics simulation and DFT-D quantum calculations

Abstract

Copolymerizing aromatic and aliphatic comonomers helps to create efficient superabsorbents to remove oil from seawater. In this work, the best sequence of styrene and 1-octene at different 1-octene molar percentages (x) was found for poly(styrene-r−1-octene) (CP-x) using neural network potential. According to the calculated characteristics of CP-x using molecular dynamics simulation, increasing the x amount up to 6% in the chain aromatic structure caused the chain expansion by ∼ 20% and the reduction of its diffusion coefficient in n-heptane by ∼ 70%. The determined Helmholtz free energy via thermodynamic integration formula showed a decrease from −2762.7 to −3818.3 kcal mol−1 (∼ 38% reduction) and entropy changes illustrated an increase from 5.12 to 8.21 kcal mol−1.K (∼ 60% increase) indicating a partial conversion of the copolymer nature from aromatic to aliphatic with raising x. This issue led to a ∼ 15% enhancement in the interaction energy between the chain and solvent media meaning a better tendency to n-heptane and the increment of the interfacial density of the solvent molecules around the copolymer chain with higher x. The quantum calculations also proved that toluene with the chain styrene ring, and n-heptane with the chain 1-octene have created a parallel orientation, due to the charge transfer energy arising from an occupied C-H bonding orbital to an adjacent one. This phenomenon moved the electron density to the boundary area between them and changed the chain conformation in the media. Compression of the CP-x characteristics in n-heptane and toluene unmasked the contrariwise behaviors of the copolymer in aliphatic and aromatic solvents, except CP-4, which is the best candidate to use in the oil absorption from water surfaces. The performed theoretical investigation of the CP-x has revealed the hidden molecular insights into the copolymer chain which can help experimentalists reduce their trial and error to manufacture efficient supper oil absorbents.