Density-Functional Theory For Thermodynamic Properties Of Polymers With Complex Chain Architechture

Chemical engineering materials are these materials used in the production of chemical industry and process industry. With the worsening of the environment issue and the energy crisis, it is important to develop novel and non-toxic chemical engineering materials. It is a challenging topic to predict how molecular microstructure could be adjusted to modify physical properties of chemical materials.Molecular architecture is one of the most important factors to determine the physical properties of polymeric material, which is one of the most important chemical engineering materials. Density-functional theory (DFT) is capable of explicit description on important microscopic details of polymeric systems, such as the molecular excluded-volume effects, associating interactions, van der Waals attraction, Coulomb forces, and inter- and intra-molecular correlations. By using DFT, this dissertation investigates the thermodynamic properties of polymeric fluids by exploring the effects from various chain architectures, including linear, star, branched, dendritic, hyperbranched, rodlike and helical structures. The main finding and novelty are summarized as follows.1 The DFT for inhomogeneous polyatomic fluids with complex architecture is proposed by introducing the tree-type structure to represent a polymer chain. An efficient hierarchal algorithm is proposed for the first time to calculate the direct bonding connectivity integral for the complex architecture. In comparison with molecular simulations, the DFT combines the advantages of computational efficiency and versatile application for various molecular architecutures, including linear, star, branched, dendritic and hyperbranched structures.(i) By using the theory proposed, the self-assembly of diblock copolymers in a slit is investigated. It is found that the linear copolymer self-assembles into a tri-layer film structure, while the copolymers with branched and dendritic tails self-assemble into a five-layer film structure. The copolymer with star tail self-assembles into a tri-layer film structure, and then the tri-layer structure evolves into a five-layer structure with the further increase of bulk packing fraction. Moreover, the bulk packing fraction, corresponding to the phase transition from a disordered state to an ordered state, increases with the complexity of the chain architecture(ii) The adsorption of colloids on the surfaces grafted by polymers of different architectures is investigated by considering the architectural effect from linear, star, branched and dendritic structures. It is shown that the complex architectural polymer brush is much more effective in preventing adsorption than linear polymer brush.(iii) The predictions of the DFT on the structural and surface properties of polymer brushes and polymer nanocomposites are reviewed for their potential application in antifouling control. In comparison to alternative theoretical methods, the DFT exhibits versatile features that are ideal for investigating various polymer-mediated interactions, self-organization of nanoparticles, and surface-induced phase transitions. It is shown that DFT is the ideal approach to investigate the interfacial properties of polymers in aqueous environment. The theoretical descriptions of surface forces may provide helpful guide in the design and development of polymeric materials for preventing non-specific adsorption of biological substances.2 By constructing the distribution function of architectral equivalent classifications, a new DFT for inhomogeneous hyperbranched polymers is developed by using the second order thermodynamic perturbation theory and Percus-Yevick closure relation to represent the Helmholtz free energy contribution from triple connection of polymer chain. The theory is able to describe the polydisperse effect of degree of branching quantitatively. The topological contributions of the polymer chains to the Helmholtz free energy take into account the effect of triple connections that are absent in previous DFT theory. One key advantage of the new theory is that the computational cost shows only a linear relationship with the molecular weight, rather than a exponential relationship in direct molecular simulation approach. The practical utility of the new DFT is illustrated by investigating colloidal stability in the presence of monodisperse and polydisperse hyperbranched polymers. Oscillatory barriers of surface interaction are present at small surface separation in monodisperse cases, but these diminish as the degree of polydispersity increases to polydisperse cases. 3 The two dimensional DFT for helical polymers is proposed by introducing the orientational potential of a molecule. A multi-scaled finite elements approach is incorporated into DFT for the first time to reduce the computational time-consuming efficiently. The main computations are performed by the coarse mesh, and the errors are corrected and smoothed by the fine ones.(i) The molecular orientation of rodlike fluids in a slit is investigated by using the theory proposed. It is found that, with the increase of the surface energy, the rod molecules adsorbed on the wall present the perpendicular orientation gradually, and assemble into a homeotropic monolayer finally. The phase behavior of rodlike molecules confined in a slit is strongly dependent on the width of the silt.(ii) By using the theory proposed, the self-assembly of helical polymers on a hydrophobic surface is investigated. It is found that a homeotropic monolayer would be formed on the surface directly as the ratio between helical radii (R) and height (H) is less than 0.5. At the range of 0.5<R/H<1, a planar homogeneous monolayer would be first formed at low attractive strength, and then transmitted to a homeotropic monolayer at high attractive strength. For the case of R/H≥1, the helical polymers only assemble into the planar homogeneous monolayer with no transition to the homeotropic monolayer.(iii) The thermodynamic stability of the polypeptides folding within modeled ribosomal exit tunnel is explored. Results indicate that too long polypeptides (N>100) cannot fold into a helix state within the modeled nanopore, and the helix polypeptides favor folding into a negative coiled coil rather than a positive one, because the negative coiled coil has a lower grand potential than the positive one, and the polypeptide folding into the negative coiled coil therefore needs less driving force than the positive one. To fold into the positive coiled coil, the helix polypeptides must hold a small minor radius or a short chain length, which provides helpful insights into the design of nanodevices for manipulating the positive coiled coil.