The Molecular Mechanism Of Adsorption And Environmental Responsive Behavior Of Polymer Brushes:Density Functional Method

Adsorption and environmental responsive behavior of grafted polymer brushes have close relationship with its practical application, thus cause the extensive atention of the experimental and theoretical researches. In this thesis, a density functional theory (DFT) approach based on a weighted density approximation has been employed to study the structure and property of the polymer brushes. The calculation shows that:the thickness of homo-polymer brushes is nonlinear positive relation with the grafting density and the temperature. By studying the effect of grafting density, chain length and the interaction potential parameters on selective adsorption of protein and nutrition molecules, it is found that the exclusion of protein enhances with the grafting density and chain length increasing but would be saturated, whereas adsorption of small nutritious molecules decreases. We also found the grafted chains with strong interaction can achieve impactful selective adsorption. In study of selective adsorption of binary mixed system in polymer grafted pore. the effect of the density of grafting, grafting chain length and curvature on adsorption capacity and adsorption selectivity, results shows that the adsorption selectivity are increased with the increasing of chain length and the grafting density but with the decreasing of the pore size.To explain binary solvent response of the polymer brush and lock/unlock mechanism, we study the perpendicular microphase separation of symmetric binary polymer brushes with compatibility or weak incompatibility in explicit solvents with different selectivity. Characterized by the relation between grand potential and layer structures (including non-layered and layered structures), a dry binary brush can be categorized as W-type or U-type according to whether the relation contains a structure of spontaneous symmetry breaking. W-type brush can memorize the selectivity of the induced solvent in one of its two layered structures after removal of solvent, which can be seen as a kind of lock state with the non-selective solvent used as its key to unlock. U-type brush is lockless but can adapt to the environment without non-selective solvent’s triggering. Also the boundary described in chain length-incompatibility space is investigated by our DFT approach, which also verifies that the spontaneous symmetry breaking of W-type brush originates from the molecular contributions against symmetry, such as enthalpic contribution of incompatibility and entropic contribution of chain connectivity. Asymmetric binary polymer brushes with weak incompatibility are also studied. Results show that as the asymmetry in density and chain length increasing, the responsive and switch property would change, when difference of two concentrations is limited, it is still like the symmetric brushes, which is responsive and memorable, once the less concentration is lower than the critical components, although still showes responsive, but after solvent evaporated, the chain of lower components will become more stretch in order to achieve greater entropy. Binary brushes of different concentration have different distributions of grand potential, lower asymmetry appears an asymmetric W-type while high asymmetry appears an asymmetrical U-type. Finally the difference in chain length of binary brushes is studied, it shows long chain has a tendency to extend outward, as one of the chain length shorten, memory effect would shutdown, when continue to shorter chain length, solvent-responseive effect would vanish.