Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States.
Langmuir. 2012 May 29;28(21):8100-9. doi: 10.1021/la301300h. Epub 2012 May 14.
Layer-by-layer (LbL) assemblies have attracted much attention for their functional versatility and ease of fabrication. However, characterizing their thermal properties in relation to the film thickness has remained a challenging topic. We have investigated the role of film thickness on the glass transition temperature (T(g)) and coeffecient of thermal expansion for poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) and PEO/poly(methacrylic acid) (PEO/PMAA) hydrogen-bonded LbL assemblies in both bulk and ultrathin films using modulated differential scanning calorimetry (modulated DSC) and temperature-controlled ellipsometry. In PEO/PAA LbL films, a single, well-defined T(g) was observed regardless of film thickness. The T(g) increased by 9 °C relative to the bulk T(g) as film thickness decreased to 30 nm because of interactions between the film and its substrate. In contrast, PEO/PMAA LbL films show a single glass transition only after a thermal cross-linking step, which results in anhydride bonds between PMAA groups. The T(g), within error, was unaffected by film thickness, but PEO/PMAA LbL films of thicknesses below ~2.7 μm exhibited a small amount of PEO crystallization and phase separation for the thermally cross-linked films. The coefficients of thermal expansion of both types of film increased with decreasing film thickness.
层层自组装因其多功能性和易于制造而受到广泛关注。然而，描述其热性质与膜厚的关系仍然是一个具有挑战性的课题。我们使用调制差示扫描量热法（调制 DSC）和温度控制椭圆偏振法研究了膜厚对聚环氧乙烷/聚丙烯酸（PEO/PAA）和聚环氧乙烷/聚甲基丙烯酸（PEO/PMAA）氢键层层自组装体在体相和超薄膜中的玻璃化转变温度（T(g)）和热膨胀系数的影响。在 PEO/PAA 层层自组装体中，无论膜厚如何，都观察到了单个明确的 T(g)。由于薄膜与基底之间的相互作用，当薄膜厚度降至 30nm 时，T(g)相对于体相 T(g)升高了 9°C。相比之下，PEO/PMAA 层层自组装体仅在热交联步骤后才显示出单个玻璃化转变，这导致 PMAA 基团之间形成了酸酐键。在误差范围内，T(g)不受膜厚的影响，但厚度低于约 2.7μm 的热交联 PEO/PMAA 层状自组装体的 PEO 结晶和相分离程度较小。两种类型的薄膜的热膨胀系数都随着膜厚的减小而增加。
