Probing the Role of Side-Chain Interconnecting Groups in the Structural Hydrophobicity of Comblike Fluorinated Polystyrene by Solid-State NMR Spectroscopy.

Abstract

In order to probe the role of side-chain interconnecting groups (-O-, -S-, and -SO2- linkages between the polystyrene (PST) main chain and fluorooctyl side chain) in the hydrophobicity of the comblike fluorinated polystyrenes, the molecular motion and structure of polymers are explored using the spin-lattice relaxation times (T1 and T1ρ) by solid-state (1)H and (19)F nuclear magnetic resonance spectroscopy. The chain-end motions of the polystyrene main chain and the fluorooctyl side chain are homogeneous, regardless of the interconnecting groups, which means that the chain-end motions of the main chain and the side chain maintain consistency, and these are irrelevant to each other. However, the local dynamic of the main chain shows the structural heterogeneity composed of the mobile and rigid regions, attributed to the rigidity of the side chain. The mobile dynamic portions of the main chain for PST-O and PST-S increase, and their rigid dynamic portions decrease as the temperature increases, whereas the ratio of structural heterogeneity for PST-SO2 is maintained despite increasing temperature. The activation energies (Ea) corresponding to the local motion of fluorooctyl side chains for PST-O and PST-S are drastically increased on the fast motion side compared to the slow motion side, suggesting the motional transformation of side chains for PST-O and PST-S from the small local motion into the large-scale movements related to a cooperative segmental motion when heated. Also, the local motion of the fluorooctyl side chain for PST-SO2 has similar Ea values on both sides, indicating that the relaxation time of PST-SO2 does not change with temperature. Therefore, PST-SO2 is structurally more stable than PST-O or PST-S, which can be attributed to the densely packed fluorooctyl side chain structure caused by the large dipole moment of the sulfone interconnecting group.