Effect Of Chain Conformation And Aggregation State On Polymeric Thin Film Dynamics

The processing and performance of polymer nanomaterials are closely correlated with the rheology and molecular motion of thin polymer films.Significantly,the dynamics of thin polymer films deviate from their bulk counterpart.Consensus is emerging that surface and interface,where the packing density,chain conformation and molecular mobility were significantly modified by the interfacial constraints and unbalanced forces,are the underpinnings of confinement effects on molecular dynamics.In this paper,the vitrification and interfacial dynamics of thin films were studied systematically from the perspective of chain conformation at surface and interface.Various experimental systems and methods were designed to explore the effect of polymer surface and interface on molecular dynamics of polymer thin films including the chain conformation,aggregation structure and packing density.Then,we clarified the molecular mechanism underpinned the vital role of surface/interface on thin film dynamics.The main conclusions are as follows:(1)We investigated the vitrification of a series of poly(ethylene terephthalate)(PET)films covered by various amount of dendritic surface crystals.Meanwhile,the free-volume hole diffusion(FVHD)model was adopted to illuminate how glass transition temperature(T_g)of PET films depended on the fraction of amorphous area at the film surfaces,i.e.,the exposed free surface.As evidenced in our previous works,amorphous PET films exhibit stratification in crystallization kinetics.A nanometer-thick mobile surface layer had a faster crystallization rate than the underlying bulk materials.Upon heating the films,the thin surface layers crystallized at lower temperature,and subsequently so did the bulk after surface crystallization was completed.Accordingly,a series of surface-crystallized PET films with various coverages could be prepared by heating the film to different temperatures.It was observed that the T_g decreased with increasing exposed free surface of PET films and recovered to the bulk value as the films fully covered by surface crystals,which indicated that the vitrification of thin films was controlled by the surface aggregation structure.By correlating the surface crystallization/amorphous region with the free-volume holes annihilation efficiency,we modeled quantitatively the correlation between T_g and surface coverage of crystals using the FVHD model.The estimated apparent activation energy for hole diffusion in the thin films decreased linearly with the inverse film thickness,suggesting that free surface enhanced the hole diffusion in thin films,which in turn led to the depression of T_g.Thus,a new mechanism for the delayed vitrification of thin polymer films dominated by free hole diffusion was proposed.(2)The nontrivial role of intramolecular dynamic coupling in the near-surface region were revealed by investigating the activation effect of surface chain loops with various size on surface dynamics.Statistical random copolymers of methyl methacrylate and pentafluorostyrene(PMMA-sta-PPFS)were synthesized and blended to PMMA matrix to prepare thin films.Upon annealing above T_g,the fluorinated segments(PFS)with lower surface tension in the copolymer were driven towards the surface,and MMA segments were depleted from surface to the film interior simultaneously,resulting in the formation of surface loops.The size of the surface loops was easily tuned by adjusting the mole fraction of PFS in copolymers.These surface loops could effectively activate the surface dynamics.The smaller loops,the stronger activating effect.This conformational dependence of surface dynamics could be justified by invoking the interplay between the dominant intramolecular coupling along the surface loops and the steep surface mobility gradient.Within the same loop,the segments at different depth experienced huge dynamic heterogeneity due to the steep mobility gradient and exhibited mutual influence to each other through the dominant intra-chain coupling along the loop contour.The excess mobility at the outmost surface could propagate into the bulk interior along the loop and activate chain dynamics in the near-surface region.The slow-relaxing segments far away from the film surface,in turn,retarded the relaxation of segments near the surface in the same loop.Due to the intramolecular coupling,larger surface loops penetrating deeper into the steep mobility gradient would induce stronger constraints to the faster segments near the surface,and cause lower degree of enhancement of the surface dynamics.The above experiments demonstrated the intramolecular dynamic coupling mechanism which is absent in small molecule system,further enabling us to reach a better understanding of the microscopic mechanism of the surface dynamics.(3)The T_g of polystyrene(PS)thin film and the propagation of suppressed dynamics induced by the highly stacked adsorbed layer at the graphene/PS interface via the strongπ-πinteraction between graphene and aromatic rings were explored.The T_g of the PS thin film and propagation distance of suppressed interfacial dynamics both showed non-monotonic behaviors during thermal treatment—increased firstly and then decreased.This phenomenon was attributed to the gradually increase of the density of adsorbed layer as the thermal treatment time goes.In the early stage of thermal treatment,interfacial molecular mobility was sluggish with gradually increasing the amount and packing density of adsorbed chains,resulting in the increase of the propagation distance and the T_g of PS thin film.However,with further thermal treatment,there would be an adsorbed layer with extremely high packing density,which repelled the unadsorbed chains away from the adsorbed chains.Thus,an entropic repulsive interface was generated between adsorbed chains and unadsorbed chains above,weakening their interpenetration and motional coupling,and resulting in suppression of the T_g and the propagation distance of suppressed dynamics originating at the interface.These results indicated that the chain conformation with highly stacked adsorbed layer suppressed the interaction between the adsorbed chains and the unadsorbed chains,and prevented the propagation of interfacial dynamics.