Study On Mechanism Of Interfacial Effect Affecting Mobility Of Thin Polymer Film

Recently,thin polymer film has become a new research hotspot of nanomaterial because of its unique properties and widespread applications.It was widely reported that if polymer film is confined in a nanometer scale environment,its physical properties,such as glass transition temperature,crystallization behavior and viscoelasticity,deviate substantially from bulk counterparts.The increasingly significant roles of free surface effect and interfacial effect with decreasing film thickness are usually used to describe such phenomena.It was found that the dynamics of polymer chain near the surface are enhanced by free surface effect,while chain dynamics near substrate are suppressed by interfacial effect.For a substrate supported polymer film,the free surface effect is usually determined and hard to be tuned so that the physical properties of polymer thin film are dominated by interfacial effect.Hence,the dynamics of polymer chains near the polymer/substrate interface,the mobility gradient near substrate caused by interfacial effect and the mechanism of long-range propagation are extremely significant to understand how interfacial effect works in nanoconfined polymer films.However,because there is no effective method to measure the mobility gradient near substrate perturbed by interfacial effect,the picture of the nature of interfacial effect and its long-range propagation mechanism are still not clear.In our previous work,we successfully obtained the mobility gradient of polymer chains near a substrate by stepwise crystallization behavior of poly?ethylene terephthalate??PET?film and found the mobility gradient can be described by the thickness of adsorbed layer.It was observed that there are three characteristic film thicknesses(h_s^*,h_sb^*,h_n^*)that are linearly related to h_ads/R_g.When the film thickness?h?was thinner than h_s^*(13.6h_ads+0.617R_g),the chain mobility starts to be suppressed.When h<hsb(6.97h_ads+0.623R_g),the high polymer mobility caused by the top free surface effect is neutralized by the low polymer mobility caused by the substrate effect.When h<h_n^*(4.2h_ads),the motion of polymer chain was highly suppressed.Based on these results,we will further study the universality of the relationship between the mobility gradient caused by interfacial effect and the thickness of the adsorbed layer.At the same time,the effect of free surface effect on the mobility gradient near a substrate is investigated to give a new sight to understand interfacial effect.The main conclusions are presented as follows:?1?The universality of the relationship between the mobility gradient near substrate and the thickness of the adsorbed layer was investigated.The effect of adsorbed layer thickness(h_ads)on mobility gradient of polymer chain near substrate in poly?L-lactic acid??PLLA?film was studied.It was found that the effect of h_ads on the molecular motion of PLLA chains also has three critical thicknesses(h_s^*?h_sb^*?h_n^*),which still satisfy the above linear relationships:h_s^*=13.6h_ads+ 0.61 7R_g,h_sb^*=6.97h_ads+0.623R_g,h_n^*=4.2h_ads.Because the value of h_ads/R_g is related to the conformation of adsorbed polymer,and the long-range effect is propagated to the interior of the film through intermolecular entanglement,the law has certain universality.Comparing the results of PLLA and PET systems,the mobility gradient of the bulk layer that was affect by adsorbed layer thickness was consistent,while the crystallization temperature of the surface layer in PLLA film was independent of adsorbed layer thickness.Maybe the sample preparation conditions and test conditions created a good environment for mesophase generation,promoting the formation of surface crystals.The physical meaning of the intercept?0.6R_g?in the h_sb^*and h_n^*linear relationship was initially clarified.It was found that the surface crystallization temperature of 7 nm PET film on the Si-H substrate was about 16 K higher than that of 16 nm,which shows the mobility of 7 nm?<0.6R_g=8.2 nm?film was still suppressed by substrate at the situation that polymer/substrate interfacial interaction was weak enough to ignore.There are two possible reasons to explain this experimental result.On one hand,the lamellar thickness?l?of the PET crystal is approximately 8.2 nm under experimental heating conditions.When h<l,polymer chains have to motion a long distance to drain into crystalline region.That is,the crystallization behavior is controlled by diffusion,and a higher temperature is needed to form crystal.On the other hand,chain conformation is changed by the substrate which acts as an impenetrable wall,affecting polymer film dynamics in the 0.6R_g range,even if the substrate does not interact with the polymer chain?2?The effect of free surface effect on the mobility gradient near substrate caused by interfacial effect was studied.The mobility gradient in polymer films influenced by the interfacial effect was investigated by the cold crystallization behavior of thin poly?ethylene terephthalate??PET?film covered by a thin poly?2,6-dimethyl-1,4-phenyleneoxide??PPO?layer to eliminate the free surface effect.Two characteristic film thicknesses?h_s^*,h_n^*?were observed,which were related to the depth profile of local dynamics reflecting long-range perturbations by the interfacial effect.Below the h_s^*(1 8.3h_ads+1.6R_g),the bulk-like crystallization temperature of PET films begins to shift upward,the chain mobility starts to be suppressed.Below the h_n^*(4.2h_ads),crystallization of PET films is completely suppressed.These two characteristic thicknesses are proportional to the adsorbed layer thickness.We found that h_s^*is much greater than the thickness of the PET film with a free surface,while h_n^*is independent of the free surface.This result indicated that the distance for the suppression of polymer dynamics can be increased by removing the contribution of the free surface.The value of h_n^*independent of the free surface may be attributed to that the chain conformation within this distance adopts the chain conformation at the interface.There may be less interfacial free volume remaining between the polymer and the substrate,which constrains the dynamics in ultrathin films,resulting in the disappearance of "free surface".This result suggested that the chain conformations affected by the substrate may be another cause of the long-range perturbations through which the inhibitory effect of molecular motion is propagated to the interior of the film by the interfacial free volume arising from the adsorbed chain conformation.