Study On Molecule Dynamics Of Glass Polymer Film Surface At Multiscale

Due to the unbalanced force the surface molecules experienced,the properties of polymer free surface,including the chain conformation,free volume and chain density,are significantly deviated from the bulk,resulting in a very complex surface molecular dynamic.In addition,as the significant increment of surface-to-bulk volume fraction of nanomaterials,molecular mobility on polymer surface significantly affect the physical properties of ultrathin films and nanostructured polymers,such as rheology,viscoelasticity,glass transition,and crystallization.However,the characterization of the surface molecular motion is pretty difficult because the free surface layer is extremely thin,normally below 10 nm.Thereof,there desperately needs special characterization methods which are able to investigate the molecular motion of polymer at surfaces.In this dissertation,the molecular dynamics of the polymer film surface was investigated by probing the dynamics of surface deformation formation under the vertical component of the surface tension of droplets atop the polymer surface.We divide the researches in this dissertation into three parts:（1）relationship between the molecular dynamics with various length scales,（2）effect of chemical structures of polymer on the thickness of surface layer,（3）the molecular weight dependence of the surface chain relaxation.The main conclusions are summarized as following:（1）A new method that can study the chain relaxation on the polymer surface was developed.In the experiment,a droplet which is incompatible with the polymer was placed on the surface of the polymer film.The vertical component of the droplet surface tension(₇)4)9);₇)is the surface tension of the liquid;is the contact angle)induced the formation of nanometer scale deformation on polymer surface,called wetting ridges.It was found that the rheological behavior of the polymer surface including Rouse relaxation and reptation relaxation could be tracked by investigation of the change of the wetting ridge height with time.It was found that the molecular chain movement on the PS surface could be observed even at a temperature below the PS bulk T_g（T<T_g^b）,suggesting the existence of enhanced surface mobility.The Rouse relaxation and reptation relaxation times on the PS surface can be described with the Arrhenius equation.More importantly,the reptation relaxation exhibits a stronger temperature dependence and a larger relaxation activation energy,than the one the Rouse time did.The difference on temperature dependences of surface Rouse and reptation dynamics suggests that the decoupling of surface dynamics with various length scale,leading to breakdown of the empirical principle of time-temperature superposition.Further,the Ngai’s coupling model was used to explain the decoupled surface dynamics.The results show that the increased activation energy of the reptation relaxation is due to the greater coupling and interaction of the surrounding molecules.By considering the unique“surface mobile layer/glassy bulk”structure of glassy polymer,it is probably that there is a weak interaction between the polymer chain on the mobile surface layer and in the glassy layer below.The close contact between the surface chains and glassy chain underneath brings about an increased friction for the surface layer to move,resulting in reduced surface mobility.In addition,this effect is more remarkable for the large-scale chain movement,because of a larger number of contacts.This results in an activation energy,which is higher for surface chain relaxation than that for entanglement segments relaxation.This study provides a new possibility to study the multiscale dynamics of polymers and their rheological behavior,as well as a new mechanism that dominate surface chain motion.（2）Probing the thickness of free surface of glassy polymer films by using the method of droplet-induced deformation on the polymer surface.It was found that the Laplace pressure generated by an ionic liquid droplet induces a nanometer-depth grooves of PS surface inside the wetting ridge.On the temperature below T_g（T<T_g）,the measured depth of nano-groove was gradually increased with the increasing of the droplet action time at the beginning and kept in a platform finally.the finally value of the groove depth was proper to represent the thickness of the mobile surface layer.The results showed that the thickness of mobile surface layer increase is in direct proportion to the annealing temperature and no relevant with the molecular weight of polymer.The results of PS with different side groups were also measured.For the result on poly-α-methylstyrene（PαMS）,the thickness of the mobile surface layer is zero,which suggest there is no free surface effect on this thin film.However,for the results on poly-t-butyl styrene（Pt BS）which t-butyl（t B）locates in the para position of the phenyl ring,the surface molecular motion activity was enhanced as the increased thickness of free surface layer.On the temperature of T_g of polymer,the thickness of free surface layer on PS and Pt BS films are 7 nm and 11 nm,respectively.It was believed that the synergistic size and chain rigidity of the polymer may be responsible for the different thickness of the surface layer.The spatial hinderance of the added methyl group on theα-site carbon limits the free rotation of the main chain C-C bond.As a result,the rigidity of polymer chains was increased and the chain packing density on the surface was similar to the bulk.These changes cause the disappeared mobile surface layer and lead to a similar surface dynamic to the bulk.In addition,the large side groups of Pt BS increases the size of the cooperative rearrangement of the polymer and then increases the thickness of the mobile surface layer.These results provide a theoretical basis for the regulation of polymer surface motion and rheological behavior.（3）Studying the dependence of molecular weight of the glass PS and the entire chain mobility by using the method of wetting ridges.It was found that there is a scale relationship of 3(τ_rep,sur～M_w^2.93.1)as the PS molecular weight below 52.5 kg/mol,which shows an similar trend with that of the bulk;When 52.5 kg/mol>M_w>300 kg/mol,τ_rep～M_w^1.21.4;When M_w>785 kg/mol,τ_rep,surhas no molecular weight dependence(τ_rep～M_w⁰).In conclusion,the reptation relaxation time on the surface of the glassy PS gradually was weakened as the molecular weight of the polymer increases and disappeared finally.These results significantly deviate from the classical polymer dynamics theory.We believe that the relaxation of the polymer chain in the mobile surface layer is affected by the underlying glassy polymer,and physical interactions such as entanglement between the surface molecular chain and the bulk molecular chain will make the surface molecular motion more complicated.The reason of such complex molecular weight dependence of the relaxation time on the surface in the further and the mechanism of surface molecular chain relaxation will be further investigated in the future.