Laser Surface Thermal Lensing (STL) as a Tool for Probing the Dynamic-Mechanical Properties of Polymer Thin Films: Applications in Detecting Solid-State Phase Transitions, etc.

9901306snyder从脉冲调制激光束吸收的能量压在透明材料上导致暴露在光束下的表面的循环热膨胀和收缩。这种表面的畸变形成了一种瞬态光学透镜结构，可以通过监测以透镜曲率一定角度反射的第二束连续激光束的变化来探测。EMU实验激光物理实验室在探测和定量反射探针光束方面取得了重大进展，这导致了一种高灵敏度的技术，称为激光表面热透镜（STL），用于研究表面材料的热机械响应。应用在光学材料表面的微米厚薄膜或有机聚合物被发现比玻璃背景产生更强的STL信号。由于STL信号的大小和相位应该是聚合物的吸收率和动态/机械性能的函数，因此可以合理地推断，影响这些性能的外部条件的任何变化都应该导致STL信号的可检测变化。本课题将研究聚合物的激光STL特性与其结构、力学性能和相关外部条件之间的关系。方法学将包括对仪器参数的初步研究，如光学工作台配置、泵浦光束频率、功率和波长，以优化合适测试聚合物的STL响应。然后，STL信号的行为将作为不同聚合物温度的函数进行研究，以确定使用该技术研究玻璃相变和其他固态相变的可行性。还将研究从气相吸附到聚合物表层的小有机分子对聚合物膜STL响应的影响，以确定是否可以通过STL信号的变化来量化塑化效果。激光STL技术在聚合物结构分析和相关领域有许多潜在的应用，因为它可以研究聚合物作为加工薄膜的动态/机械性能。测量聚合物表面微小区域的玻璃化转变温度的能力将有助于研究基于聚合物共混物的商业涂层系统中由于相分离而产生的结构缺陷。该传感器用于空气中的物质，如溶剂蒸气和有机污染物。具有可调谐红外激光器的STL也可以使该技术用作表面化学成分分析光谱的敏感探测器。

9901306SnyderAbsorption of energy from a pulse-modulated laser beam impringing on a transparent material leads to cyclic thermal expansion and contraction of the surface exposed to the beam. This distortion of the surface forms a transient optical lens structure which can be probed by monitoring changes in a second, continuous laser beam reflected at an angle off the lens curvature. Major improvements in detection and quantitation of the reflected probe-beam developed at the EMU Experimental Laser Physics Laboratory have resulted in a highly sensitive technique, termed laser Surface Thermal Lensing (STL), for study of the thermomechanical response of surface materials. Micron-thick films or organic polymers applied over the surface of optical materials have been found to give much stronger STL signals than the glass background. Since the magnitude and phase of the STL signals should be a function of both the absorbtivity and the dynamic/mechanical properties of the polymer, it is reasonable to theorize that any changes in external conditions which affect these properties should result in detectable changes in the STL signal. This research project will study the relationship between laser STL characteristics of a polymer, its structure, mechanical properties and relevant external conditions. Methodology will involve initial study of instrument parameters such as optical bench configuration, pump-beam frequency, power and wavelength in order to optimize the STL response of suitable test polymers. The STL signal behavior will then be studied as a function of temperature for different polymers to determine the feasibility of using this technique to investigate glass-transition and other solid-state phase transitions. The effect of small organic molecules adsorbed from the gas phase into the surface layer of the polymer on the STL response of the polymer film will also be investigated to determine it plasticization effects can be quantified by changes in the STL signal.The laser STL technique has numerous potential applications in polymer structural analysis and related fields since it allows studies of dynamic/mechanical properties for polymers as processed thin films. The ability to measure glass-transition temperatures over minute areas of the polymer surface would be useful in studying structural defects due to phase-separation in commercial coating systems based on polymer blends. The sensors for airborne materials such a solvent vapors and organic pollutants. STL with tuneable infrared lasers could also allow the technique to be used as a sensitive detector for analytical spectroscopy of surface chemical composition.