Segmental Dynamics And Its Depth Profile At Polystyrene Film Surface

The research on surface dynamics is especially important, due to the promising applicationfor polymer films in a wide industrial field. Molecules on polymer surface, which is in anasymmetrical force field, have a more complicated relaxation behavior. A great deal ofevidences, such as the lower glass transition temperature (Tg), faster relaxation rates and lowerrelaxation activation energies have verified that the molecules at a surface exhibit enhancedmobility compared with molecules in the bulk. In addition, some reports point out that thesurface Tgdepend on depth away from the outmost surface. But there are also manycontroversial reports. Due to limitation of characterization technique these debates are stillunresolved.In this thesis, fluorinated groups labeling method was used to study the segmentaldynamics and its depth profile at polystyrene (PS) film surface. PS chain terminated by variousnumbers of2-perfuorooctylethyl methacrylate (FMA) units (PS428-b-PFMAn(n=5、9、14、20、27)) were synthesized by atom transfer radical polymerization (ATRP). Sum frequencygeneration vibrational spectroscopy (SFG), contact angle goniometry (CA) and angle-resolutionX-ray photoelectron spectroscopy (XPS) were employed to investigate the molecular dynamicsand its profile at PS surface through detecting the relaxation process of PS428-b-PFMA5spin-coating films from a thermodynamic non-equilibrium state to equilibrium state. In addition,micelles with various degree of confinement were prepared by labeling PFMA with differentpolymerization degree to PS chain ends. And the molecular motion on surface was studied bycontact angle goniometry and atom force microscopy (AFM). Experimental results are asfollows：(1) The depth profile of the PS surface dynamics had been systematically studied.Fluorinated units were labeled to the tail of PS chains. Thermodynamic non-equilibrium statefilms were prepared through spin-coating film formation method. Upon annealing, thefluorinated groups would migrate to the outmost surface, simultaneously accompanyingsegments movement of PS chain. The Tonsetwas defined as the critical temperature wheremolecurlar motion occurred and accessed by measuring molecular structure information duringthe relaxation process of PS films. Experimental results of contact angle goniometry showed that the molecular motion on surface of PS428-b-PFMA5spin-coated films were independent onthe film thickness and substrates when the film thickness was within the scope of30to160nm.(2) CA with0.5nm sensitivity, SFG which reflects structure information of1nm and XPSwhich can probe molecular structure changes from4.5to9nm were employed to study themolecular motion. Results showed that the Tonsetat0.5nm depth was68℃, which was muchlower than the Tgof PS bulk. The Tonsetgradually reduced with increase of the distance from thesurface. In the case of the analytical depth of9nm, the Tonsetincreased to76℃. The resultsindicated that there was a “liquid-like” layer with thickness of3.1nm on the surface, andmolecules within this layer have the highest mobility and homogeneous dynamics. The“liquid-like” surface dynamic can be transported32nm into the film. The thickness of“liquid-like” layer is likely related to the size of polymer chains.(3) The molecular motion of PS tethered by a micellar core was studied by labelingdifferent number of FMA units to PS chain end. PS428-b-PFMAnchains formed sphericalmicelles in mixed solvent (cyclohexanone/ethanol) constituting of PS which acted as coronaand PFMA as core. The results of nuclear magnetic resonance experiment (1H-NMR) anddynamica light scattering (DLS) pointed out that more FMA units gave rise to a more compactPFMA core, which resulted in reduction of PS chains mobility. The onset temperature ofsurface rearrangement (TRonset) increased from68℃to78℃along with the number of FMAunits increasing from5to27. The TRonsetof the micellized PS are higher than that of free PSchains, and the mobility of PS chains tethered by a micellar core reduced, which suggested thatthe tethering of PS chains to a micelle core reduced their chain dynamics on surface. Atomforce microscopy (AFM) was employed to track the morphology evolution of surface micelleson PS428-b-PFMA27surface under annealing. The temperature for the PS corona opening isconsistent with the Tonsetmeasured by CA. Thus, the Tonsetof fluorinated PS surface was relatedto the Tgof PS surface. The decrease of PS segmental mobility on micellar surface indicatedthat the segmental dynamics on surface was affected by aggregative state.