Study On The Nature Of Free Surface/Interfacial Effects And Their Perturbed Distance Of Polymer Dynamics In Supported Thin Films

The miniaturization of functional materials to nanometer scale was enabled by the rapidly developing nanotechnology.Many applications of the materials relied on the physical properties of the molecular system confined in a nanometer scale environment.It was accepted that when the size of material is approximated to the size of polymer coil,or in another words the spatial confinement on chains conformation is induced,the dynamics and associated physical properties of materials deviate substantially from their bulk counterparts.The theory describing the dynamics of bulk polymer may not be appropriate to be used to predict the dynamics behavior of polymer chains in the nanoconfined system.It was found that free surface effect and polymer/substrate interfacial effect play significant roles in determining the molecular dynamics of polymer thin film.The mobility of polymer chain near the surface is enhanced by free surface effect,while the chain mobility near the substrate is suppressed by interfacial effect.Moreover,because of the long-range propagation feature of free surface/interfacial effect,a mobility gradient can be observed near the surface and interface region.With the decreasing of film thickness,the volume fraction of surface and interfacial region in the film increases dramatically,leading to significant changes in the global dynamics of the thin films due to the surface and interface effect.Hence,polymer dynamics at free surface and substrate interface and the associated utmost distance of dynamics perturbation caused by surface/interfacial effect are extremely important to understand how the confinement dictates the properties of materials.However,because of the limitation of the experimental methods,the understanding of the nature of free surface/interfacial effect is not clear and even there are a lot of inconsistent results reported in literatures.Now,it is still lack of direct and effective methods to measure the utmost distance of dynamics perturbed by free surface/interfacial effect.In this paper,the free surface/interfacial effects on polymer film dynamics were investigated systematically.A series of experiments were designed to detect the utmost distance of dynamics perturbed by free surface/interfacial effect;to explore the propagate mechanism for their long-range;and to clarify the effect of surface/interface chain conformation on polymer film dynamics.The conclusions are presented as follow:?1?In the foremost,we explored the length scale of chain mobility enhanced by free surface effect.Since poly?ethylene terephthalate??PET?film exhibits the stepwise crystallization behavior,i.e.,surface crystallization and bulk crystallization process were separated due to the different chain mobility on the surface layer and that in the bulk,the characteristic of mobile surface layer can be obtained by detecting the crystallization in the surface layer.In this study,ellipsometry and X-ray reflection were employed to directly measure the thickness of surface crystallization layer(h_s^cry).It is found that h_s^cry is independent with film thickness and molecular weight,while increasing with temperature.When the temperature is increased from 343 to 373 K,h_s^cry increases from 4 nm to 13 nm,which is fitted well by the cooperative string model.It is believed that the value of h_s^cry is related to the length scale of large-scale segmental rearrangement enhanced by the free surface.The result shows a clear picture on how mobile surface layer is evolved with increasing temperature.?2?The effect of chain conformation on surface dynamics and polymer film dynamics was investigated.The surface chain conformation of PET film was modulated by adding water in corresponding trifluoroacetic acid?TFA?spin-coating solution,which was confirmed by surface-sensitive sum frequency generation?SFG?vibrational spectroscopy.Molecular dynamics?MD?simulation was utilized to explore the mechanism of effect of water on PET chain conformation in the solution.It is shown that the water molecules partly interrupted the hydrogen bond network between TFA molecules and PET chains and make it possible to preserve the average bulk-like spherical conformation at the film surface during the formation of PET film.When the water content increased from 0 to 1.1 vol%,surface chain conformation changed from oblate ellipsoidal conformation to spherical conformation.Such variation of surface conformation leaded to the increase of the surface glass transition temperature(T_g^surface)and surface crystallization temperature(T_c^surface),indicating a suppressed surface chain mobility.It is believed that the variations in the surface chain conformation may reduce the average number of contact sites between free surface and polymer chains and enhance the chain entanglement,which suppress the chain dynamics in surface layer.Continuing to increase the water content,surface conformation did not change any more,and therefore T_g^surface and T_c^surface remained constant.Such result shows the chain conformation plays an important role in determining the surface chain dynamics.?3?Fluorinated tracer-labeled technique was developed to detect the utmost distance of dynamics suppression caused by interfacial effect.Using atom transfer radical polymerization?ATRP?and the click reaction between alkynyl and azide group?CuAAC?,fluorinated tracer-labeled linear and cyclic poly?methyl methacrylate??PMMA?with various molecular weights were synthesized,which are denoted as l-PMMA_m-b-FMA_n and c-PMMA_m-b-FMA_n,respectively?FMA:2-perfluorooctylethyl methacrylate?.The PMMA//PMMA_m-b-FMA_n bilayer films were prepared by spin-coating and floating techniques.The bottom layer is PMMA_m-b-FMA_n layer,and the top layer is PMMA layer.By monitoring the changes of water contact angle of the bilayer film or monitoring the variation of F/C ratio by X-ray photoelectron spectroscopy with annealing time,the time when PMMA_m-b-FMA_n diffuse from bottom layer to the surface of top layer can be easily detected,labeled as t^*.By fixing the thickness of top layer as 50 nm and changing the thickness of bottom layer?h?,the curve of t^*versus h can be obtained.When h is thinner than the utmost distance of dynamics suppressed by substrate,t^*will increases.By this way,the utmost distance of dynamics suppressed by substrate can be directly observed.The utmost distance of dynamics?h_c^*?suppressed by Si/SiO_x substrate in l-PMMA_m-b-FMA_n and c-PMMA_m-b-FMA_n films with different molecular weight at various annealing temperature were studied.It was shown that h_c^*increases with molecular weight?M_n?and the values of h_c^*is equal to about 10.5 R_g?R_g is radius of gyration?in l-PMMA_m-b-FMA_n film when M_n>M_e?M_e:entanglement molecular weight?.When M_n<M_e,h_c^*decreases rapidly.The molecular weight dependent of h_c^*in c-PMMA_m-b-FMA_n film is similar to the situation when M_n<M_e in l-PMMA_m-b-FMA_n film.The reason for such result is that c-PMMA_m-b-FMA_n has specific topological structure and the entanglement of polymer chain is almost forbided.The h_c^*in both l-PMMA_m-b-FMA_n and c-PMMA_m-b-FMA_n films decrease with increasing temperature,but that in l-PMMA_m-b-FMA_n film has a weak temperature dependence.Such results imply that the entanglement is very important to the propagation of interfacial effect from substrate to inner of polymer film.?4?The effect of adsorbed layer thickness(h_ads)on the mobility gradient of polymer chains near substrate was investigated.h_ads/R_g was proposed as a parameter to describe the effect of interfacial effect on polymer dynamics.Based on the stepwise crystallization behavior of poly?ethylene terephthalate??PET?film,the effect of h_ads on polymer chain mobility near substrate was investigated and three characteristic film thicknesses(h_s^*?h^*_sb?h_n^*)can be observed.When the film thickness?h?was smaller than h_s^*(13.6 h_ads+0.62 R_g),the mobility of surface chain started to be suppressed,the surface crystallization temperature(T_g^surface)began to increase.When h<h^*_sb(7.0 h_ads+0.62 R_g),the suppression on chain mobility enhanced,free surface effect was neutralized by interfacial effect and T_g^surface=T_g^bulk.When h<h_n^*(4.2 h_ads),the motion of polymer chain almost absolutely be hindered,no crystallization process can be observed.The three characteristic thicknesses(h_s^*?h^*_sb?h_n^*)are all linearly increased with h_ads/R_g,presented a clear depth profile of mobility gradient of polymer chains near substrate.It is also confirmed that h_ads/R_g can be used as a parameter to describe the effect of interfacial effect on polymer dynamics.Because h_ads/R_g is related to the conformation of adsorbed chain,such results indicate the conformation of adsorbed chain plays a significant role on the mobility gradient of polymer chains near substrate.