Sharp J S, Teichroeb J H, Forrest J A
School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, UK.
Eur Phys J E Soft Matter. 2004 Dec;15(4):473-87. doi: 10.1140/epje/i2004-10078-0. Epub 2004 Dec 15.
We present a detailed study of free polymer surfaces and their effects on the measured glass transition temperature (T(g)) of thin polystyrene (PS) films. Direct measurements of the near-surface properties of PS films are made by monitoring the embedding of 10 and 20 nm diameter gold spheres into the surface of spin-cast PS films. At a temperature T = 378 K( > T(g)), the embedding of the spheres is driven by geometrical considerations arising from the wetting of the gold spheres by the PS. At temperatures below T(g) (363 K < T < 370 K), both sets of spheres embed 3-4 nm into the PS films and stop. These studies suggest that a liquid-like surface layer exists in glassy PS films and also provide an estimate for the lower bound of the thickness of this layer of 3-4 nm. This qualitative idea is supported by a series of calculations based upon a previously developed theoretical model for the indentation of nanoscale spheres into linear viscoelastic materials. Comparing data with simulations shows that this surface layer has properties similar to those of a bulk sample of PS having a temperature of 374 K. Ellipsometric measurements of the T(g) are also performed on thin spin-cast PS films with thicknesses in the range 8 nm < h < 290 nm. Measurements are performed on thin PS films that have been capped by thermally evaporating 5 nm thick metal (Au and Al) capping layers on top of the polymer. The measured T(g) values (as well as polymer metal interface structure) in such samples depend on the metal used as the capping layer, and cast doubt on the general validity of using evaporative deposition to cover the free surface. We also prepared films that were capped by a new non-evaporative procedure. These films were shown to have a T(g) that is the same as that of bulk PS (370+/-1 K) for all film thicknesses measured (> 7 nm). The subsequent removal of the metal layer from these films was shown to restore a thickness-dependent T(g) in these samples that was essentially the same as that observed for uncapped PS films. An estimate of the thickness of the liquid-like surface layer was also extracted from the ellipsometry measurements and was found to be 5+/-1 nm. The combined ellipsometry and embedding studies provide strong evidence for the existence of a liquid-like surface layer in thin glassy PS films. They show that the presence of the free surface is an important parameter in determining the existence of T(g) reductions in thin PS films.
我们对自由聚合物表面及其对聚苯乙烯（PS）薄膜测得的玻璃化转变温度（T(g)）的影响进行了详细研究。通过监测直径为10和20纳米的金纳米球嵌入旋涂PS薄膜表面的情况，对PS薄膜的近表面性质进行了直接测量。在温度T = 378 K（> T(g)）时，纳米球的嵌入是由PS对金纳米球的润湿性所产生的几何因素驱动的。在低于T(g)的温度下（363 K < T < 370 K），两组纳米球都嵌入PS薄膜3 - 4纳米并停止。这些研究表明，玻璃态PS薄膜中存在类似液体的表面层，并且还给出了该层厚度下限为3 - 4纳米的估计值。基于先前开发的纳米尺度球体压入线性粘弹性材料的理论模型进行的一系列计算支持了这一定性观点。将数据与模拟结果进行比较表明，该表面层的性质类似于温度为374 K的PS本体样品的性质。还对厚度范围为8纳米 < h < 290纳米的旋涂PS薄膜进行了T(g)的椭偏测量。测量是在聚合物顶部热蒸发5纳米厚金属（Au和Al）覆盖层的PS薄膜上进行的。此类样品中测得的T(g)值（以及聚合物 - 金属界面结构）取决于用作覆盖层的金属，这对使用蒸发沉积覆盖自由表面的普遍有效性提出了质疑。我们还制备了通过新的非蒸发方法覆盖的薄膜。对于所有测量的薄膜厚度（> 7纳米），这些薄膜显示出与本体PS相同的T(g)（370±1 K）。从这些薄膜中随后去除金属层后，这些样品中恢复了与未覆盖PS薄膜中观察到的基本相同的厚度依赖性T(g)。还从椭偏测量中提取了类似液体表面层的厚度估计值，发现为5±1纳米。椭偏测量和嵌入研究相结合为玻璃态PS薄膜中存在类似液体的表面层提供了有力证据。它们表明自由表面的存在是决定PS薄膜中T(g)降低是否存在的一个重要参数。
