Study On The Relationship Between The Thickness Of Active Layer At Polymer Thin Film Surface And Its Molecular Weight

Polymer surface dynamic is one of the most important discipline in polymer physics. Compare to the bulk, polymer chains at surface have different state of aggregation structure and higher mobility. The most striking feature of the surface dynamic is the depth dependence: the chain mobility changes as the corresponding distance from surface increasing. This phenomenon attracts a lot of attention in recent years. However, how the molecular weight affects the depth dependence of surface glass transition behavior remains controversial.In this thesis, we applied an fluorinated labeled method in combination with three characterization instruments with different analysis depth to investigate the surface glass transition behavior and their molecular weight dependence of PMMA and PS. Moreover, we have associated the molecular weight dependence of surface Tg to the glass transition behavior of polymer films. The main conclusions are summarized as below:(1) The surface molecular motion ability is depended on the depth from surface. Plotting the Tg of surface obtained by CA, SFG and XPS against the sensing depth of the corresponding technique, the lateral distribution of Tg within a 9.0 nm depth was obtained. The depth profile of surface Tg effectively divides a film into three layers: a “surface layer” with Arrhenius dynamics on the uppermost film surface, a“gradient region”layer underneath the top layer transporting “liquid-like” dynamics into the internal film, and the remaining bulk layer. This observed depth profile of the surface dynamics may suggest that the polymer chains near the surface adopt an oblate conformation and have lowered entanglements.(2) The thickness of surface layer and the surface Tg are both affected by molecular weight, as molecular weight increasing, the thickness of surface layer grows and the surface Tg rises. Thickness of surface layer is approximately 0.45 times of the chain radius of gyration(Rg) which is due to the elliptic spherical conformation of surface chains. The relationship between surface Tg and chain Rg is as follows: Tg PMMA=2.1 Rg+57.6, Tg PS=1.57 Rg+59.4. As can be seen in the equation, the higher molecular weight, the weaker mobility of surface, which could be explained by ―sliding motion‖ behavior of the surface polymer chains. This study directly confirms the rationality of ―sliding motion‖ model proposed by de gennes vice-versa.(3)We deposited a PMMA absorption layer on silicon dioxide substrate and then casting another PMMA layer on it. Since the chemical structure of the PMMA absorption layer and the upper PMMA layer are the same, the interface effect of them could be ignored. Ellipsometer experiments show Tg of these PMMA films exist a molecular weight dependence. With the thickness decreasing, the influence of molecular weight to film Tg becomes stronger. The Tg of PMMA surface obtained by CA, SFG and XPS is perfectly matched to the film Tg obtained by ellipsometer. This is due to the surface molecular weight dependent affect: as the film thickness decreasing, the volume fraction of surface layer grows, the molecular weight dependence of surface Tg leads to a strong influence to the whole film.