Dewetting Dynamics In The Complex System Of Thin Polymer Film

The polymer thin film is important to many technology areas, such asprotective coating, lithography, selectively permeable membrane, optoelectronicdevices, data storage, micro-reactors and so on. However, the dewetting behavioraffects the function of the polymer thin films. Thus, the study of the dewettingbehavior of the thin polymer film will help us to stabilize thin films or regulate theaggregation structure of the polymer film, and has an important significance onthe fundamental study and the technological applications.Much of the previous work relates to thermally triggered dewetting where asingle layer of polymer film on a smooth solid substrate in the air has a rathergood understanding. Nevertheless, the dewetting dynamics in the complex systemof thin polymer film still has many unresolved issues, for example, the dewettingdynamics of the polymer multi-layers, solvent vapor induced dewetting and thedewetting progress of thin polymer film in solvent. In this work, we use opticalmicroscopy and atomic force microscopy to investigate the dewetting behavior ofthe polymer thin film in the complex system and understand some factors ofaffecting the dewetting behavior of the thin polymer film, such as the molecularweight, polydispersity, substrate, film thickness, solvent and so on.Firstly, we investigate the influence of the polymer with high molecularweight on the solvent vapor induced dewetting dynamics of the polymer thin filmwith the low molecular weight. We compared the dewetting dynamics of themonodipersity PS with low molecular weight (PSL), high molecular weight (PSH)and the mixture of the PSL and PSH (PSM). It was observed that the dewetting behavior depending on the film thickness. In the thin polymer film, the dewettingvelocity of the mixed polymer film between the PSL and PSM. The anomaly ofthe deweting velocity with the addition of PSH happens due to effect of the chainconnectors which were partially in the entangle polymer film and partially in thetop film. In the thick polymer film, the little PSH promotes the dewetting and thevelocity is faster than the PSL. The velocity of holes growth decreases with theincrease of PSM. The number of forming holes reduced with the increase of thefilm thickness.Secondly, we investigate the influence of the polydispersity of the molecularweight on the dewetting dynamics: The model under investigation has beencomposed of two different molecular weights of the PS with the same weightaverage [PS-Mw(mix)]or the number average[PS-Mn(mix)] as the monodiperse PS.It is found that when the thin blend film contains less PS long-chains, theirstrongly confined between the PS short-chains’ film and the substrate: on the onehand, the PS long-chains showed the disentanglement, increased the mobility ofPS chains; on the other hand, the disentanglement of PS long-chains can greatlydecrease the viscous friction on the liquid/substrate interface. When the thin blendfilm contains more PS long-chains, their confined between the PS short-chains’film and the substrate: on one hand, the confinement restricts the motion of PSlong-chains, decreased the velocity of dewetting; on the other hand, in the thinblend film will formd the chain connectors, and their also inhibited thedewetting behavior. When the thin blend film under the free environment, theentangled chains of the film surface play a similar role as the entanglementangents, interwined with the PS long-chains, suppressed the dewetting behavior.Thirdly, we investigate the influence of different mixed solvents on thedewetting dynamics: The two mixed solvents (acetone and n-hexane, acetone andethanol) are chosen in our experiments. It is found that the affinity of non-solvents with the solid substrate strongly influences the dewetting behavior in the mixture.The presence of non-solvents can decrease the mobility of PS chains and slowdown the dewetting velocity. However, when the non-solvent is more affinitywith the solid substrate, it may form a enrich layer at the solid substrate andconsequently change the dewetting pathway and the kinetic behavior.Finally, we investigate the dewetting behavior in the bilayer film of PS andPMMA: circular holes appear on the PS film do not change with the filmthickness and molecular weight. When the molecular weight of PMMA is low, thedewetting rates of PS film increase with increasing film thickness of PMMA.Increasing with the molecular weight of PMMA，the dewetting rates of PS filmdecrease with increasing film thickness of PMMA. When the molecular weight ofPMMA is high, similar with the solid layer, the molecular weight of PS plays animportant role in the dewetting progress: when the molecular weight of PS is alittle higher than the entanglement molecular, the velocity of PS film increaseswith increasing film thickness of PMMA; when the molecular weight of PS ismore higher than the entanglement molecular, the velocity of PS film decreaseswith increasing film thickness of PMMA. It may be due to the deformation of theinterface.