Mechanism And Application Of Polymer And Nanocomposites Self-Assembly

With the increase of social demands and development of economy, scientists in chemical engineering and material science have made great efforts to design the novel functional materials, especially those with ordered structure. To face this challenge, the bottom-to-up technology of self-assembly comes into being and attracts great interests. The self-assembly is the spontaneous association of subunits under weak interactions and appropriate conditions into stable, structurally well-defined aggregates. Self-assembly promises great potential applications in fabrication of nanomaterial and nanodevices. Theoretic calculation and molecular simulation are effective approaches to provide detail information of complex processes, including self-assembly, in molecular, atomic, and nano scales. In this dissertation, the mechanism of polymer and nanocomposites self-assembly was studied by density functional theory and molecular dynamics simulations, and the application of self-assembly in designing nanodevices was also explored. The research contents and findings are summarized as follows.(1) The self-assembly of dendrimers and rod-coil copolymers in nanoslit was studied by density functional theory. The effects of concentration in bulk, pore width, molecular attraction, and molecular flexibility on self-assembly were considered. When the bulk molecules reaches to a critical concentration, molecules in the nanoslit undergo a first order phase transition where molecules transit from disordered state to ordered state, and dendrimers and rod-coil copolymers self-assemble into lamellar structures in the nanoslit. The critical bulk concentration is very low, which illustrates that the nanoslit can serve as a template of self-assembly for preparing lamellar nanostructures.(2) Brownian dynamics simulations were carried out to explore the self-assembly of amphiphilic copolymers composed of a linear hydrophilic head and a hydrophobic tail with different architectures. The effects of tail architecture on micellization were considered from the critical micelle concentration (CMC), dynamics of aggregation, aggregate size distribution, shape anisotropy and thermal stability. The branching parameter was employed to characterize the tail architecture. The results reveal that copolymers with larger branching parameter forms micelles with larger and size, better monodispersity, longer characteristic relaxation time, higher geometrical symmetry and thermal stability. The CMC is inversely proportional to the branching parameter, and the size of polymeric micelle exhibits an exponential increase with the branching parameter.(3) The method of preparing polymer nanocomposites with ordered structure by self-assembly of star-polymer-attached nanoparticles was proposed. The results reveal that attaching a star polymer to nanoparticle induces the self-assembly to form polymer nanocomposites with different morphologies, such as hollow sphere, porous structure, lamella, perforated lamella, cylinder, core-shell micelle and gyroid-like network, depending on temperature, concentration and nanosphere size. To give a framework of these mesostructures, a temperature versus concentration phase diagram is presented for each case of nanosphere. By comparing with linear chain-attached soluble nanosphere, the specialty of star polymer-attached nanosphere is also discussed.(4) Brownian dynamics simulations were carried out to study the aggregation behavior of PEG-grafted gold nanoparticles (GNPs) in solutions by using the coarse-grained model derived from the all-atom force field. Generally, grafting PEG to the surface of GNPs is to protect them from aggregation in the solution. However, our results reveal that PEG-grafted GNPs may also aggregate when concentration increases. Our simulations indicate that there exists a critical aggregating concentration (CAC), beyond which the PEG-grafted GNPs will aggregate. It is also found that there exists an optimized length of grafted chain, at which the system has the maximal CAC. Furthermore, the aggregate size of self-assembled mesostructures increases with the concentration. Interestingly, it is observed that the aggregation favors to form linear gold nanowires rather than compact gold nanoclusters.(5) Owing to the important roles of chemical gates in biological systems, the biomimetic design of artificial switchable nanodevices has been attracting tremendous interests. A thermo-sensitive channel was designed, in which nanofliudic transport properties can be controlled by manipulating environmental temperature. The switchable channel is formed by a polystyrene-b-poly(acrylic acid)-b-polystyrene (PS-PAA-PS)-like triblock copolymer brush whose conformation and phase behavior are dependent on temperature. With the increase of temperature, the designed channel exhibits the "close→open→lose" behavior, which can serve as a kind of excellent switchable nanodevices for nanofluidic controllable transportation.