Theoretical And Computer Simulation Study On The Wetting Phenomenon Of Polymer Solutions

Polymer solution is one type of polymeric systems commonly used in a variety of fields.Due to the important role in many applications such as stabilization of colloidal suspensions and surface coating,numerous efforts have been devoted to examine the physical and chemical properties of polymer solution close to surface(s),and one of the most important phenomena is the wetting properties of the polymer solution when close to a single surface.Controlling the wetting behavior of polymer solutions is one of the core technical problems that required to be solved in many technological applications,such as microfluidic control,nano-printing,and ink-jet printing However,the understanding on the wetting properties of polymer solutions is far from mature.In this thesis,to disclose the physical mechanisms underlying the wetting phenomenon of polymer solution,we adopt the methods of materials calculation and simulation to systematically examine the wetting phenomenon of polymer solutionThe wetting phenomenon of polymer solution close to surface(s)is not only related to the inherent physical properties of the solid surface,but also depends on the characteristics of polymer and solvent in polymer solution.In the first part of this thesis,we use the lattice self-consistent field theory to systematically study the wetting phenomenon of homopolymer solution close to a solid surface.We mainly focus on the influence of the following four important parameters:polymer chain length N,the Flory-Huggins parameter ? that characterizes the interaction between the polymer and the solvent,the volume fraction of the polymer segment in the bulk solution ?b,and the interaction energy between the surface and polymer segments ?s.We calculate the polymer volume fraction profile,excess adsorption amount,and thickness of surface layer.With these quantities,we quantitatively determine the wetting point,pre-wetting line and adsorption-depletion transition point at various N and ?s.Finally,we summarize the general adsorption properties and rules of wetting and prewetting by constructing the wetting phase diagram of homopolymer solutions.The main results can be summarized as follows.First,at fixed chain length N,the ? value corresponding to the wetting transition point of a homopolymer solution is positively correlated with ?s.It means that a larger adsorption strength of the surface corresponds to a higher ? value of the wetting phase transition point.With the decrease of ?s,the type of wetting transition gradually changes from the first-order to the second-order.With further decreasing ?s,the system undergoes an adsorption-depletion transition,and polymer segments start to be depleted from the surface rather than being adsorbed.Second,when the polymer solution deviates from the dilute phase branch of the coexistence line,with increasing the polymer concentration,a prewetting phenomenon will occur in the first-order wetting transition regime.The prewetting line starts from the first-order wetting transition point and extends towards some point in the homogeneous region of the bulk phase diagram.The range of the prewetting line increases monotonically with ?s.Third,for homopolymer solutions with different chain lengths,with the increase of N,both the ?s values corresponding to the transition point from the first-order wetting transition to the second-order wetting transition and that corresponding to the adsorption-depletion transition decrease gradually Moreover,the region where the second-order wetting transition occurs also decreases,but the range of prewetting line gradually increasesIn addition to the above self-consistent field theory calculations,we also use Monte Carlo simulation to construct the bulk phase diagram of homopolymer solution with N=100,and compare it with that by self-consistent field theory in the fourth chapter.We find that the two phase diagrams are qualitatively consistent with each other.First,both phase diagrams show that phase separation occurs only when ? is sufficiently large.Second,both phase diagrams exhibit a similar asymmetry These similarities indicate that the self-consistent field theory is qualitatively correct.However,there are still some quantitative differences,and the most significant one is that the phase boundary predicted by the self-consistent field theory is lower than that by the Monte Carlo simulation.We attribute this difference to the composition fluctuations ignored by the self-consistent field theory Nevertheless,the self-consistent field theory can be used to qualitatively describe the interfacial phenomena of polymer solutions.