| Because of its excellent performance and wide application,thin polymer films have become the hot research 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 chain dynamics determine the physical properties of polymer nanomaterials.Study on diffusion behavior of polymer chain confined in a thin film would be significant in the design of high performance polymer nanomaterials.Meanwhile,it has been accepted that the substrate has a significant effect on the chain dynamics of supported polymer thin films.However,the mechanism of how the substrate affects the motion of chain resided several nanometers far away the substrate is not clear so far.It is of great significance to study the effect of substrate on the diffusion of polymer chains,and elucidate the associated mechanism.In this thesis,a bilayer system which consist of an upper unlabeled poly(methyl methacrylate)layer and a bottom fluorinated tracer-labeled poly(methyl methacrylate)layer was prepared and the chemical and physical properties of its surface was characterized by contact angle measurement(CA)and X-ray photoelectron spectroscopy(XPS).The diffusion of labeled PMMA(PMMA-ec-FMA)and the depth about interfacial effect from the substrate was investigated.The conclusions are gotten as follow:(1)The critical time(t*)required for PMMA-ec-FMA diffusing to the surface of upper PMMA layer could be obtained accurately by observing the change of the wettability and F/C ratio of the bilayer surface.According to the Fickian equation,the diffusion coefficient(D)could be calculated by t*.The diffusion coefficient was found to be dependent on the thickness of upper layer(h).With the change of h,there were three stages of D: i)when the film thickness was higher than a critical value(hup),D kept as constant and would not change with film thickness;ii)when the film thickness decreased to below hup,D decreased with the decrease of h;iii)when the film thickness continued to decrease to below another critical thickness(hlow),D reached to another constant value.The results showed that the mobility of polymer chain decreases with the increase of the degree of nano-confinement.The influence of molecular weight of PMMA-ec-FMA on the critical thickness was systematically investigated.We have constructed two kinds of bilayer systems.When the molecular weight of the upper and bottom layers was the same,it was called molecular weight asymmetric system.When the molecular weight of the upper and bottom layers was different,it was called molecular weight molecular weight symmetric system.Comparing the two kinds of bilayer systems,it is found that the critical thickness of hlow is the same,which is about 30 nm.However,hup/Rg(mean square radius of gyration)of two systems are found to be different.In symmetric system,hup/Rg kept constant.This indicated that the influence of film confinement on the chain diffusion is closely related to the size of the polymer chain.However,hup/Rg of molecular weight symmetric system decreased with the increase of molecular weight of labeled PMMA,which showed that there was another factor besides chain size in this system.(2)The diffusion coefficient of PMMA-ec-FMA was found to be dependent on its molecular weight.When the thickness of the upper layer reached to hup,the relationship between the diffusion coefficient De and the molecular weight could be described by De~M-0.58.As well,The diffusion coefficient of PMMA-ec-FMA was found to be depend on temperature.D increased with the increase of temperature,and the activation energy(Ea)was deduced to be 49±3 kJ·mol-1.We proposed that during the diffusion process,the fluorinated end group pulled the PMMA chain and thus changed the conformation of chain,resulting in the larger diffusion coefficient,weaker molecular weight dependence and lower active energy.(3)Based on the method of bilayer,the depth about the interfacial effect from the substrate was studied by changing the thickness of the upper layer.The critical time(t*)for the change of physical and chemical properties of the surface of bilayer film was used to reflect the time required for the migration of the bottom PMMA across the upper PMMA.The critical thickness(h*)of t* which began to change with the decrease of the film thickness was the depth of the interfacial effect from the substrate.The effect of the surface properties of substrate was studied.It is found that the depth of the interfacial effect from the substrate of native oxide-covered silica wafer(Si/SiOx)and the HF etched silica wafer(Si-H)in the PMMA(Mn=67kg·mol-1)thin film is 69 nm and 37 nm,respectively.While the depth of interfacial effect in the PS(Mn=43kg·mol-1)thin film is 27 nm and 65 nm,respectively.This indicated that the depth of the interfacial effect from the substrate was closely related to the surface properties of the substrate.The hydrogen-bond interaction between Si/SiOx substrate and PMMA is much stronger than that of Si-H substrate.Meanwhile,the interaction between Si-H substrate and PS was confirmed to be stronger than that of Si/SiOx.The interaction between the polymer and the substrate would be a main factor determined the depth of the interfacial effect from the substrate.(5)The relationship between the influencing depth of interfacial effect and molecular weight was studied.It was found that the influence depth of Si/SiOx to PMMA increased with the increase of molecular weight,which could be normalized to 9.5Rg.It is shown that the influence of substrate on the dynamics of thin films was dependent on the size of polymer chain and the interfacial effect from the substrate is propagated by the entanglement of polymer chains.Meanwhile,the relationship between the depth of substrate effect and temperature was studied.The result showed that the depth of interfacial effect from the substrate decreases with the increase of temperature,which indicated that increase of temperature could weaken the substrate effect by activating the mobility of polymer chains. |
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