Engineering Stable Glass Films Using Molecular Design and Surface-Mediated Equilibration

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: Nanoscopic thin films of small molecule amorphous organic materials are widely used in applications that range from protective coatings to organic photovoltaics and resist materials in nanoimprint lithography. These films are frequently manufactured through use of physical vapor deposition (PVD) onto a substrate held below the materials' glass transition temperature, Tg. Tg signifies the temperature where a system is unable to equilibrate on laboratory or computational time scales. Since glassy systems are out of equilibrium, the precise method of their fabrication, including substrate temperature, its properties, and rate of deposition can profoundly affect the materials properties and function in these applications. This project employs a combination of molecular synthesis, high-throughput characterization, and molecular simulation to design and characterize a library of synthetic glass-forming materials as a function of deposition variables. Addressing fundamental questions of the formation of highly stable glasses during PVD will have a transformative effect on the community's ability to engineer the properties of amorphous organic thin films and open the door to new applications of stable glasses for various industries. In addition to the project's impact on our fundamental understanding of stable glass formation and industrial applications, this project will impact the education of junior scientists from the undergraduate level through the PhD level. Undergraduate education is integrated into all aspects of the project. The starting material for the synthesis of glass formers is prepared as part of an undergraduate organic chemistry laboratory course. Advanced undergraduates and graduate students participate in the synthesis of the glass-formers as well as the characterization of PVD films using various experimental and computational techniques. TECHNICAL DESCRIPTION: When held at a constant temperature a glass very slowly evolves towards a more stable, higher density state. This process, called physical aging, can take millions of years to reach equilibrium and only result in modest improvement in properties. Recent breakthrough studies have shown that PVD onto a substrate held just below Tg leads to a glass with properties that appear to be that of a glass that has aged hundreds or even thousands of years. It is hypothesized that this is a result of the enhanced mobility at the free surface of the film during deposition. Through PVD, each deposited molecule experiences this enhanced mobility upon condensation, allowing it to find a low energy state. As such, this process is referred to as surface mediated equilibration (SME). The remarkable kinetic stability of SME-generated glasses opens the door for their use in a number of new applications, but several fundamental challenges hinder their adoption. Most notably, a systematic understanding of the role of the chemical structure and intermolecular interactions, the interactions of the organic molecule with the substrate, and the effect of film thickness remain poorly understood. The synthesis capabilities previously developed by the PIs allows one to dial in particular structural motifs and intermolecular interactions. High-throughput characterization methods will enable rapid determination of a materials' kinetic stability as well as the relationship between stability and enhanced surface dynamics. Finally, molecular-level insights will be provided through coarse-grained simulations of the molecules synthesized and characterized experimentally. Specifically, the primary goals of this project are to i) determine the influence of chemical structure on surface mobility and SME glass stability; ii) determine the effect of film thickness on stability; and iii) determine the role of substrate interactions on altering materials' packing and ability to form a stable glass. Addressing these questions will have a transformative effect on the community's ability to engineer the properties of amorphous organic thin films and open the door to new applications of stable glasses.