The James Franck Institute and Department of Chemistry, The University of Chicago, 929 East 57th Street, Chicago, IL 60637, USA.
Proc Natl Acad Sci U S A. 2011 Jan 18;108(3):977-82. doi: 10.1073/pnas.1008268107. Epub 2010 Aug 16.
The surface dynamics of amorphous and semicrystalline polymer films have been measured using helium atom scattering. Time-of-flight data were collected to resolve the elastic and inelastic scattering components in the diffuse scattering of neutral helium atoms from the surface of a thin poly(ethylene terephthalate) film. Debye-Waller attenuation was observed for both the amorphous and semicrystalline phases of the polymer by recording the decay of elastically scattered helium atoms with increasing surface temperature. Thermal attenuation measurements in the specular scattering geometry yielded perpendicular mean-square displacements of 2.7•10(-4) Å(2) K(-1) and 3.1•10(-4) Å(2) K(-1) for the amorphous and semicrystalline surfaces, respectively. The semicrystalline surface was consistently ∼15% softer than the amorphous across a variety of perpendicular momentum transfers. The Debye-Waller factors were also measured at off-specular angles to characterize the parallel mean-square displacements, which were found to increase by an order of magnitude over the perpendicular mean-square displacements for both surfaces. In contrast to the perpendicular motion, the semicrystalline state was ∼25% stiffer than the amorphous phase in the surface plane. These results were uniquely accessed through low-energy neutral helium atom scattering due to the highly surface-sensitive and nonperturbative nature of these interactions. The goal of tailoring the chemical and physical properties of complex advanced materials requires an improved understanding of interfacial dynamics, information that is obtainable through atomic beam scattering methods.
使用氦原子散射技术测量了无定形和半结晶聚合物薄膜的表面动力学。收集飞行时间数据以解析中性氦原子从薄聚对苯二甲酸乙二酯薄膜表面漫散射中的弹性和非弹性散射分量。通过记录弹性散射氦原子随表面温度升高的衰减，观察到聚合物的无定形和半晶相的德拜 - 沃勒衰减。在镜面散射几何中进行的热衰减测量得出无定形和半晶表面的垂直均方根位移分别为 2.7×10^(-4) Å^2 K^(-1) 和 3.1×10^(-4) Å^2 K^(-1)。在各种垂直动量传递下，半晶表面始终比无定形表面软约 15%。还在离镜面角度测量德拜 - 沃勒因子以表征平行均方根位移，发现对于两个表面，平行均方根位移比垂直均方根位移增加一个数量级。与垂直运动相反，在表面平面中，半晶态比无定形相硬约 25%。由于这些相互作用具有高度的表面敏感性和非侵入性，因此通过低能中性氦原子散射才能获得这些结果。需要通过原子束散射方法获得有关界面动力学的信息，这是定制复杂先进材料的化学和物理性质的关键。
