| Although polymers are ubiquitous, their microscopic properties are poorly understood. The vast heterogeneity and microscopic thermodynamic instability of amorphous polymers give rise to undesirable long-term effects. Polymer aging, mechanical failure, small molecule diffusion, and other deficiencies in the physical properties of polymers are, by definition, not homogeneous phenomena but, rather, effects that occur on localized sites. Conventional ensemble averaged testing and characterization techniques measure bulk properties rather than localized phenomena, and therefore, often cannot address polymer performance deficiencies. Undesirable mobility of small molecules, resulting from polymer dynamics well below the glass transition temperature, often causes the decay of useful polymer properties. These uncontrolled mobilities lead to effects such as relaxation mechanisms in nonlinear optical polymers and spatial migration of conductive dopants in polymer electronics, both of which are detrimental to the long-term reliability. Thus, not only do polymer host dynamics require elucidation, but also it is imperative that small molecule mobilities within these systems be understood. Therefore, a fundamental understanding of amorphous solids remains an elusive goal that can be answered by an individual survey of the spatially heterogeneous materials. Implementation of wide-field total internal reflection (TIR) microscopy, allowed simultaneous tracking of many individual x, y, and z-oriented fluorescent DilC18 molecules embedded within glassy state poly (methylmethacrylate) films. Because molecules emit with a characteristic sine-squared emission pattern with respect to the transition dipole direction, observation of fluorescent patterns indicative of both single molecule behavior and true 3-D molecular orientation. Three-dimensional rotational motion of molecules allow for the analysis of dynamics and heterogeneity within polymer films. The vast differences in rotational motion and the exquisite sensitivity of single molecule orientational changes provide a glimpse of the heterogeneous environmental distributions within glassy state amorphous solids. Rotational relaxation time distributions, constructed from individual probe molecule rotational mobility measurements, were manipulated using polymer matrix temperature and glass transition temperature to elucidate the temperature dependencies 40K below the glass transition temperature. Complementing bulk studies, the measured distributions of nanoscale barriers to rotational motion afforded by our single molecule orientational methods directly probe the spatial heterogeneity and nanoscopic α-relaxation dynamics deep within the glassy state. |
