Ellipsometry measurements of glass transition breadth in bulk films of random, block, and gradient copolymers

Spectroscopic ellipsometry as a route to thermodynamic characterization

Strategies for synthesizing molecularly designed materials are expanding, but methods for their thermodynamic characterization are not. This shortfall presents a challenge to the goal of connecting local molecular structure with material properties and response. Fundamental thermodynamic quantities, including the thermal expansion coefficient, α, can serve as powerful inputs to models, yielding insight and predictive power for phenomena ranging from miscibility to dynamic relaxation. However, the usual routes for thermodynamic characterization often require a significant sample size (e.g. one gram), or challenging experimental set-ups (e.g. mercury as a confining fluid), or both. Here, we apply spectroscopic ellipsometry, which is an optical technique for thin film analysis, to obtain thermodynamic data. We clarify issues in the scientific literature concerning the connection between ellipsometric and volumetric thermal expansion coefficients for substances in both the glass and melt states. We analyze temperature-dependent data derived using both ellipsometry and macro-scale dilatometric techniques for ten different polymers. We find superb correlation between the α values obtained via the two techniques, after considering the effects of mechanical confinement by the substrate for a glassy thin film. We show how the ellipsometric α can serve as input to the locally correlated lattice theory to yield predictions for the percent free volume in each polymer as a function of temperature. We find that the ellipsometric α at the glass transition temperature, T_g, is not only material dependent, but it is linearly correlated with T_g itself. Spectroscopic ellipsometry, which requires only very small quantities of sample and is straightforward to perform, will significantly expand the range of systems for which thermodynamic properties can be characterized. It will thus advance our ability to use theory and modeling to predict the miscibility and dynamic relaxation of new materials. As such, ellipsometry will be able to underpin materials synthesis and property design.

Temperature-Dependent Spectroscopic Ellipsometry of Thin Polymer Films

Thin polymer films have found many important applications in organic electronics, such as active layers, protective layers, or antistatic layers. Among the various experimental methods suitable for studying the thermo-optical properties of thin polymer films, temperature-dependent spectroscopic ellipsometry plays a special role as a nondestructive and very sensitive optical technique. In this Review Article, issues related to the physical origin of the dependence of ellipsometric angles on temperature are surveyed. In addition, the Review Article discusses the use of temperature-dependent spectroscopic ellipsometry for studying phase transitions in thin polymer films. The benefits of studying thermal transitions using different cooling/heating speeds are also discussed. Furthermore, it is shown how the analysis and modeling of raw ellipsometric data can be used to determine the thermal properties of thin polymer films.

Glassy state molecular mobility and its relationship to the physico-mechanical properties of plasticized hydroxypropyl methylcellulose (HPMC) films

Changes in tensile properties and the glass transition temperature (T_g) of plasticized polymer films are typically attributed to molecular mobility, often with no empirical data to support such an assertion. Herein solvent cast HPMC films containing varying amounts of PEG, as the plasticizer, were used to assess the dependence of tensile properties and the T_g on glassy state molecular mobility. Molecular mobility (molecular relaxation time and temperature) parameters were determined by Thermally Stimulated Current Spectroscopy (TSC). The tensile properties and T_g of the HPMC films were determined by texture analysis and DSC, respectively. Molecular mobilities detected by TSC were cooperative and occurred at temperatures (T_g′) well below (113 to 127 °C) the bulk T_g. The relaxation times (τ) were 71 ± 1, 46 ± 1, 42 ± 1, 36 ± 1 and 29 ± 1 s for HPMC films containing 0, 6, 8, 11 and 17% (w/w) PEG, respectively. The T_g and glassy state molecular mobility were found to be intimately linked and demonstrated a linear dependence. While tensile strength was found to be linearly related to molecular relaxation time, tensile elongation and elastic modulus exhibited a non-linear dependence on molecular mobility. The data presented in this work demonstrates the complex nature of the relationship between plasticizer content, molecular mobility, T_g and tensile properties for plasticized polymeric films. It highlights the fact that the dependence of the bulk physico-mechanical properties on glassy state molecular mobility, differ greatly. Therefore, empirical characterization of molecular mobility is important to fully understand and predict the thermo-mechanical behavior of plasticized polymer films. This work demonstrates the unique capability of TSC to provide key information relating to molecular mobility and its influence on the bulk properties of materials. Data generated using TSC could prove useful for stability and performance ranking, in addition to the ability to predict materials behavior using data generated at or below typical storage conditions in the pharmaceutical, food, and polymer industries.

Glass transition of polymers in bulk, confined geometries, and near interfaces

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass-a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

Progress in reactor engineering of controlled radical polymerization: a comprehensive review

Controlled radical polymerization (CRP) represents an important advancement in polymer chemistry. It allows synthesis of polymers with well-controlled chain microstructures.