Komoda Yoshiyuki, Leal L Gary, Squires Todd M
Department of Chemical Science and Engineering, Kobe University , Kobe, Hyogo 657-8501, Japan.
Langmuir. 2014 May 13;30(18):5230-7. doi: 10.1021/la5001733. Epub 2014 Apr 29.
Oscillatory microdisk rheometry was applied to evaluate the evolution of the viscoelastic properties at the surface of a film of an aqueous solution of poly(vinyl alcohol) (PVA) during drying. The drying rate was measured concurrently, based upon measurements of the variation of film thickness. A fully hydrolyzed PVA solution shows a constant drying rate, while a less hydrolyzed PVA solution exhibits a decreased drying rate in the latter part of the drying process, which occurred at the same time as an increase of the elastic modulus. We suggest that this difference in behavior is a consequence of the fact that both the configuration of the PVA molecule and the strength of interaction with water depend on the degree to which the PVA is hydrolyzed. The polymer concentration at the film surface can be estimated from the measured viscosity at the surface for the fully hydrolyzed PVA solution, and this result then can be compared with two theoretical calculations: one in which the polymer concentration is assumed to remain uniform throughout the film, and the other in which the polymer concentration distribution is determined via a one-dimensional diffusion model. This comparison suggests that the polymer is first concentrated locally near the surface but later in the drying process the distribution of polymer becomes increasingly uniform, possibly due to a spontaneously generated convective flow inside the film.
采用振荡微盘流变仪来评估聚乙烯醇（PVA）水溶液薄膜在干燥过程中表面粘弹性性质的演变。基于薄膜厚度变化的测量，同时测定干燥速率。完全水解的PVA溶液显示出恒定的干燥速率，而水解程度较低的PVA溶液在干燥过程的后期干燥速率降低，这与弹性模量的增加同时发生。我们认为这种行为差异是由于PVA分子的构型以及与水的相互作用强度都取决于PVA的水解程度这一事实导致的。对于完全水解的PVA溶液，可以根据在表面测得的粘度估算薄膜表面的聚合物浓度，然后将该结果与两种理论计算进行比较：一种假设聚合物浓度在整个薄膜中保持均匀，另一种通过一维扩散模型确定聚合物浓度分布。这种比较表明，聚合物首先在表面附近局部浓缩，但在干燥过程后期聚合物分布变得越来越均匀，这可能是由于薄膜内部自发产生的对流流动所致。
