And Frequency Vibrational Spectroscopy (sfg) Study Polymer Surface Relaxation Behavior

Polymer surface should be defined as the plane between the bulk and vacuum (or another phase), in other words, the last atom layer before the adjacent phase begins. There exists distinct differences between the mobility of molecules on polymer surface and in bulk. At the beginning of 1990s, the glass transition on surface of amorphous polymer has gradually got great attention. The debate on"Is the Molecular Surface of Polymer Glass Really Glassy"lasts until today. What kinds of discrepancy on the glass transition between polymer surface and bulk has become a fundamental scientific issue of condensed matter physics.As one of the second order nonlinear optical techniques, sum frequency gen- eration vibrational spectroscopy (SFG) has interfacial selectivity and sub-monolayer sensitivity. In this thesis, spin-coating method was selected to produce a unstable Polystyrene( PS) and Polymethyl methacrylate( PMMA) structure on surface. SFG together with other techniques was employed to track the structure relaxation of polymer chain on surface. Some main conclusion can be summarized as below:The transformation of the surface structure of PS film from a nonequilibrium state to equilibrium state was investigated utilizing SFG. When PS film was prepared by cast-coating method, CH2 group dominated on surface, resulting in a surface structure staying in equilibrium state. Whereas, when spin-coating was used , relative polar phenyl group almost covered the entire surface, having surface structure unstable. SFG results indicated that the structure relaxation on surface from spin-coating film (staying in nonequilibrium state) will occurred, if only the polymer is above the glass temperature (Tg) . The results revealed that the glass transition of PS on surface was in the range of 55-60℃,which was 50-55℃lower than that of bulk. Enhanced mobility at free surface was observed. Meanwhile, a segments relaxation of the surface at temperatures well below the bulk glass temperature was observed. However, a complete relaxation of the surface structures occurred only when temperatures was equal to or greater than the bulk glass transition temperature. The depth distribution of the glass transition has been obtained by a fluorinated end-group labeled method. Fluorinated moiety was attached to the tail of poly(methyl methacrylate) PMMA chain . Upon annealing , the fluorinated end group would migrate toward the outmost surface, simultaneously accompanying segments movement of PMMA chain . The temperature at which the surface composition started to change was defined as the glass transition temperature (Tg) at the surface region. SFG with sub-monolayer sensitivity,contact angle measurements(CA) which can reflect structure information of the outmost 0.5-1nm surface and angle-resolution X ray photo-electron spectroscopy(XPS) (5-10nm) were employed to study the glass transition from outmost surface to 10nm depth. Results shown that glass transition on the outmost surface was about 70℃,which was much lower than the Tg of PMMA in bulk. The mobility of PMMA on surface gradually reduced with a decrease in distance from the outermost surface. In the case of the analytical depth of about 9 nm, the surface Tg increased to 81℃.Micellar thin films composed of several densely packed monolayers of micelles was formed by casting kinetically stable micelles from a solution of 2-perfluorooctyl -ethyl methacrylate end capped PMMA(PMMA-ec-PFMA) in toluene onto glass substrates. On surface, PMMA chain in micelles was connected together by the"anchor chain"-PFMA. The mobility of PMMA chain on surface confined in spherical micelle was studied. Nuclear magnetic resonance experiments have found that a longer PFMA chain gave rise to heavier entanglement and stronger interaction between PFMA segments in core of micelles, resulting in tightly"anchor point". The temperature of the polymer chain on surface began to rearrange for PMMA confined in micelles was higher than that of free polymer chain, which implied reduced mobility for the confined PMMA chain. Meanwhile, the mobility of PMMA chain on surface declined, as the interaction between PFMA segments became stronger. Atom force microscopy(AFM) tracked the topography alteration of surface micelles at different temperature. Results demonstrated that in case of temperature below Tg of PMMA bulk, partial segments movement occurred, causing cavities on surface; in case of temperature near Tg of PMMA bulk, PMMA segments was fully activated, producing a flat surface.