We investigated in detail the structures in the poly(vinyl alcohol) (PVA) adsorption layers on a Si substrate, which remained on the substrate after immersing the relatively thick 30–50 nm films in hot water, by neutron reflectometry under humid conditions. For the PVA with a degree of saponification exceeding 98 mol %, the adsorption layer exhibits a three-layered structure in the thickness direction. The bottom layer is considered to be the so-called inner adsorption layer that is not fully swollen with water vapor. This may be because the polymer chains in the inner adsorption layer are strongly constrained onto the substrate, which inhibits water vapor penetration. The polymer chains in this layer have many contact points to the substrate via the hydrogen bonding between the hydroxyl groups in the polymer chain and the silanol groups on the surface of the Si substrate and consequently exhibit extremely slow dynamics. Therefore, it is inferred that the bottom layer is fully amorphous. Furthermore, we consider the middle layer to be somewhat amorphous because parts of the molecular chains are pinned below the interface between the middle and bottom layers. The molecular chains in the top layer become more mobile and ordered, owing to the large distance from the strongly constrained bottom layer; therefore, they exhibit a much lower degree of swelling compared to the middle amorphous layer. Meanwhile, for the PVA with a much lower degree of saponification, the adsorption layer structure consists of the two-layers. The bottom layer forms the inner adsorption layer that moderately swells with water vapor because the polymer chains have few contact points to the substrate. The molecular chains in the middle layer, therefore, are somewhat crystallizable because of this weak constraint.

中文翻译：

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溶剂蒸汽诱导溶胀的Si基底上聚乙烯醇吸附层的详细结构研究

我们详细研究了Si衬底上聚乙烯醇（PVA）吸附层的结构，在相对湿润的条件下，通过中子反射法将相对厚的30–50 nm薄膜浸入热水中后，该吸附层仍保留在Si衬底上。对于皂化度超过98mol％的PVA，吸附层在厚度方向上呈现三层结构。底层被认为是没有被水蒸气完全溶胀的所谓的内部吸附层。这可能是因为内部吸附层中的聚合物链强烈地约束在基材上，从而抑制了水蒸气的渗透。该层中的聚合物链通过聚合物链中的羟基与Si衬底表面上的硅烷醇基团之间的氢键与衬底形成许多接触点，因此动力学表现极慢。因此，可以推断出底层是完全非晶的。此外，我们认为中间层有些无定形，因为分子链的某些部分固定在中间层和底层之间的界面下方。由于与强约束底层的距离较大，顶层中的分子链变得更易流动和有序。因此，与中间非晶层相比，它们的溶胀度要低得多。同时，对于皂化度较低的PVA，吸附层结构由两层组成。底层形成内部吸附层，该内部吸附层随着水蒸气而适度溶胀，因为聚合物链与基材的接触点很少。因此，由于这种弱约束，中间层中的分子链有些可结晶。