Miyazaki Tsukasa, Miyata Noboru, Yoshida Tessei, Arima Hiroshi, Tsumura Yoshihiro, Torikai Naoya, Aoki Hiroyuki, Yamamoto Katsuhiro, Kanaya Toshiji, Kawaguchi Daisuke, Tanaka Keiji
Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society, 162-1 Shirakata, Tokai, Naka, Ibaraki 319-1106, Japan.
Kurashiki Research Center, Kuraray Co., Ltd., 2045-1, Sakazu, Kurashiki, Okayama 710-0801, Japan.
Langmuir. 2020 Apr 7;36(13):3415-3424. doi: 10.1021/acs.langmuir.9b03964. Epub 2020 Mar 25.
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.
我们通过在潮湿条件下的中子反射测量法，详细研究了聚（乙烯醇）（PVA）在硅衬底上吸附层的结构。该吸附层是在将相对较厚的30 - 50 nm薄膜浸入热水后留在衬底上的。对于皂化度超过98 mol%的PVA，吸附层在厚度方向上呈现出三层结构。底层被认为是所谓的内吸附层，它没有被水蒸气完全溶胀。这可能是因为内吸附层中的聚合物链被强烈地限制在衬底上，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硅衬底表面的硅醇基团之间的氢键与衬底有许多接触点，因此表现出极其缓慢的动力学。所以，可以推断底层是完全无定形的。此外，我们认为中间层有些无定形，因为部分分子链被固定在中间层和底层之间的界面以下。顶层中的分子链由于与强烈受限的底层距离较大而变得更具流动性且更有序；因此，与中间的无定形层相比，它们的溶胀程度要低得多。同时，对于皂化度低得多的PVA，吸附层结构由两层组成。底层形成内吸附层，它会因聚合物链与衬底的接触点较少而适度地被水蒸气溶胀。因此，中间层中的分子链由于这种较弱的限制而有些可结晶。
