Study On Structures And Flow Regulation Properties Of Functional Nanoporous Membranes

For the development of advanced nanotechnology, it is important to develop nanoporous membranes with capacity of accurate separation. Design and manufacturing of functional nanoporous membranes are crucial for the development of separation technology. For the nanoporous membrane, the transmission rate and selectivity mainly depend on the structures and size of pores as well as fluid viscosity. Nanoporous membranes are widely used in MEMS, implantable medical devices, fuel cells, desalination and so on. On one hand, it is a hot issue to create nanoporous membranes which can control the fluid in response to external signals. On the other hand, due to environmental friendly characteristics of fuel cells, it is a key subject of research to study the structures of the proton exchange membranes in the field of nanoporous membranes.The dissipative particle dynamics method was used to investigate the structures and flow regulation properties of nanopores grafted with environment-sensitive polymer brushes. In addition, the effects of different parameters on the membrane structures for two kind of hydrated PFSA proton exchange membranes were studied. The following results were obtained:1. Study on conformation of polymer brushes grafted in a nanopore.In order to understand the properties of nanoporous membranes with environment-responsive polymer brushes, the conformation of polymer brushes needs to be investigated primarily. However, there are few theoretical and computational study of polymer brushes grafted onto curve surface and hardly theoretical models accepted widely. In this paper, the DPD method has been used to study the influence on the conformation of polymer brushes grafted in a nanopore due to varying length of chains and grafted density. The results show that the distribution of polymer monomers becomes diffuse and extends to the center of the cylindrical pore when the length of polymer chains becomes longer. The polymer chains can extend to the other hemicylinder and interact in the middle region of the pore. The height of polymer coating is linear to the length of polymer chains under the condition of lower grafting density, which is consistent with Alexander-de Gennes models of polymer brushes. This result showed that the polymer brush was compressed when the grafting density was high. When the grafting density increases, polymer chains move away from the wall. The stretching degree of polymer chains in the direction perpendicular to local grafted surface is less than the prediction of Alexander-de Gennes models, which reflects the restricting effect of nanopores.The mesoscale model for polymer brushes grafted into nanopores was firstly established in this paper. Due to the soft-repulsive nature of the conservative interactions between beads, the nanopore wall consisted of two components:an imaginary cylinder boundary and two layers of wall beads. A no-slip boundary condition was achieved.2. Study on flow regulation properties of nanoporous membranes grafted with solvent-sensitive polymer brushes.Solvent flow was controlled by a nanopore grafted with polymer brushes. Results for monomer and solvent beads density for different solvent quality were obtained, as well as the height and inclination angle of the polymer brushes in Poiseuille flow. The velocity profile and solvent permeability in Poiseuille flow for varying solvent quality were also examined. The polymer brushes expand and inhibit flow under good solvent conditions. However, under poor solvent conditions the chains collapse to open the channel. Additionally, the dependence of the permeability on the grafting density was given. The permeability reduces as the grafting density increases for the same solvent quality. However, although a large permeability in the open pore can be gotten when the grafting density is low, the pore leaks even in the closed state and this fails to control the flow.The DPD simulation was firstly performed in this paper for solvent flow regulated by nanopores grafted with polymer brushes under different solvent conditions. The solvent-sensitive permeabilities of nanopore range from 0.05 to 0.405 when the grafting densityσ=0.4036, while the range of the permeabilities calculated with MD simulation by Adiga et. al. was 0.045-0.47. The results indicated that the simulation of mesoscale models had high efficiency as well as considerable accuracy.3. Study on structures of proton exchange membranes for fuel cellsThe DPD method was used to investigated hydration structures of Nafion and SSC PFSA membranes. Each kind of PFSA polymers consists of three kinds of interconnected DPD beads based on composition. The hydration structures of two kinds of PFSA membranes were investigated under one high EW condition and one low EW condition. The structures of pores were reflected by distributions of water clusters in membranes. The results shows that the water clusters become interconnected and form the channels from isolated state when the hydration coefficient become larger under low EW condition. In addition, the boundary between water-rich region and polymer-rich region is more clear in Nafion than SSC PFSA membranes, which means a higher degree of phase separation. With the increasing in volume content of water, phase separation of PFSA membranes increased. Water clusters are irregular in shape rather than spherical shape in theoretical prediction. With high volume content of water, the water-rich regions consist of water clusters and interconnected channels between them. The degree of phase separation becomes larger with high EW, which is more evident for Nafion. The EW has less impact on membrane structures in low water content conditions while when the water content is high, the EW has greater impact on membrane structures.