Study On The Relationship Between The Viscosity Of Ultrathin Polystyrene Films Supported On Various Substrates And Molecular Weights

Because of its excellent performance and wide application,thin polymer films have been attracted wide attention in the field of nanomaterials.When the polymer film is confined in nano-scale,the physical properties of the polymer will deviate from the bulk.The lifetime,thermostability and structural stability of nano-scale polymer films are closely related to the viscoelasticity of polymer thin films.It is of great theoretical and practical significance to study the viscoelasticity of confined polymer films.Meantime,the introduction of free surface effect,interfacial effect,chain confinement effect and non-equilibrium effect due to ultrathin polymer films,usually results in sufficiently more complex rheological behaviors.At present,the results for the thickness dependence of the viscoelasticity of ultra-thin polymer films can not be well unified,and there is no unified theory to explain it.Therefore,it is very urgent to develop a sensitive technique to measure the viscoelasticity of nano-scale polymer films.A novel method has been developed to measure the viscosity of thin polystyrene films by monitoring the growth of the wetting ridge on polymer film surface when a drop of ionic liquid placed on the film surface in our previous work.It was found that the height of wetting ridge increases linearly with time when the droplet depositing time was long enough.The slope(k)and the vicosity of polystyrene films obeys the following formulation: ? = 0.37?sin?/k.In this thesis,the viscoelasticity of polystyrene ultrathin films supported on Si/SiOx substrate and H-Si substrate was investigated respectively.The results are shown as follows:(1)The viscosity of PS film supported on Si/SiOx substrate decrease with the reducing film thickness when film thickness is less than a critical value.However,when H-Si is used as the substrate,the viscosity of PS film increase with the decreasing film thickness.More importantly,the critical thickness for viscosity to start to deviate from the bulk are different.It was found that the critical thickness is7.5 Rg for PS film on Si/SiOx when the film thickness is normalized.But for H-Si substrate,the critical thickness is 10.8 Rg.In addition,we find that the viscosity of PSfilm on Si/SiOx substrate and molecular weight meet following formulation: ? ~Mw3.4?For H-Si substrate,the viscosity and molecular weight obeys the following formulation: ? ~ Mw3.0?(2)Whether it is Si/SiOx substrate or H-Si substrate,the temperature dependence of PS ultrathin film viscosity satisfies Arrhenius equation,and the viscous flow activation energy(Ea)of different PS molecular weight can be obtained by fitting.When Si/SiOx is the substrate,Ea is related to the molecular weight,and the smaller the film thickness,the greater the molecular weight dependence of Ea.For H-Si substrate,Ea is independent of the molecular weight,and Ea increases with decreasing film thickness.(3)For Si/SiOx substrates,when the film thickness decreases to ~140 nm,the viscous flow temperature decreases as the film thickness decreases.For the H-Si substrate,when the film thickness decreases to ~205 nm,the viscous flow temperature increases with the decreasing of the film thickness.Besides,it was found that the molecular weight dependence of the viscous flow temperature of the PS film supported by the H-Si substrate was weaker than that of the Si/SiOx substrate.This is consistent with the molecular weight dependence of viscosity.(4)It was found that the depth of interfacial effect from the substrate is affected by the strength of interfacial interaction.The greater the interfacial interaction,the farther the depth of interfacial effect from the substrate.When Si/SiOx is the substrate,the interfacial interaction between the polymer and the substrate is weaker,the depth of interfacial effect from the substrate is smaller,and the free surface effect plays a decisive role in the molecular motion of the polymer film,resulting in which the viscosity,viscous flow activation energy and viscous flow temperature of PS film decrease with the decreasing of the film thickness,and the critical thickness of the viscosity reduction is also smaller.The viscosity of the PS film is mainly dominated by entanglement,so viscous flow activation energy increases with the increasing of PS molecular weight.When the H-Si is the substrate,the interfacial interaction between the polymer and the substrate is stronger,the impact of the depth of the substrate effect may exceed the thickness of the film.The free surface effect isweakened or even counteracted,the viscosity of the film is mainly controlled by the interface effect,resulting in which the viscous flow activation energy has nothing to do with the molecular weight.