My research focuses on developing new high-performance hybrid nanocomposite materials that contain molecules in extreme confinement. Confinement of the organic phase is a common phenomenon in composite materials, which often use intimately mixed hard and soft components to achieve desirable properties. Our research focuses on the smallest molecular length scales of this confinement, where new mechanisms of strengthening and toughening exist that are not found in traditional composite materials. By focusing on the behavior of molecules confined at length scales of 1-10 nm, we are able to probe the fundamental limits of strengthening and toughening in these nanostructured low-density materials and find new avenues for innovation.
Figure 1: Strenght-density map showing the enormous potential for strong (mechanically robust) hybrid nanocomposite films with low density compared to other engineering materials.
The strength and toughness of low density molecular hybrids have typically been low compared to other engineering materials (Fig. 1). However, mechanically robust nanocomposite films can be made with dramatic improvements in strength and toughness by exploiting confinement-induced reinforcement mechanisms. This opens the door to a wide variety of applications that require reliable operation in harsh mechanical, chemical, and thermal environments, including use as high-temperature materials for aerospace, microelectronic interlayer dielectrics, biosensors, and membranes in fuel cells.
Figure 2: Pore backfilling approach enables uniform nanoscale mixing of nanoporous materials and organic or inorganic phases. The quantity of second phases can be precisely controlled to any fill level.
We have developed the capability to uniformly fill nanoporous hybrids with polymers ranging in molecular weight from 103 to more than 106 Da (Fig. 2). This enables mixing at extremely small length scales (~1 nm) and unprecedented levels of molecular confinement, in which chains are up to ten times larger than the pores in which they are confined. We used these materials to perform the first-ever measurements of the fracture and mechanical properties of polymers in molecular-scale confinement.
Figure 3: An artistic representation of the molecular bridging process in action. Here, individual polymer chains (red) that are confined in a nanoporous glass (gray) pull out from their pores and bridge the faces of a crack.
Our research has shown that polymers confined at molecular length scales dissipate energy through a confinement-induced molecular bridging mechanism that is distinct from existing entanglement-based theories of polymer deformation and fracture (Fig. 3). We have developed models that capture the nanomechanical processes associated with this molecular bridging mechanism. The combination of these models and the unique nature of the confined molecular bridging process has allowed us to measure the pullout force, pullout length, and even the tensile strength of individual polymer chains. We demonstrate that the toughening is controlled by the molecular size and the degree of confinement, but is ultimately limited by the strength of individual molecules (Fig. 4).
The confinement-induced molecular bridging reinforcement effect is far from fully understood and shows promise for even greater improvements in other nanocomposite films. In addition to improving the strength and toughness of low-density hybrids, future studies will also attempt to integrate novel optical or electrical properties into these materials by leveraging the many possible second phase polymers at our disposal.
Figure 4: Behavior of nanocomposites with confined polymers. a) The cohesive fracture energy of the nanocomposite increases with the molecular weight of the confined polymers. We use mechanical models to describe this toughening effect (solid line). b) The force expereinced by individual polymer chains during the molecular bridging process.
Interfaces between polymers and inorganic oxides are found in a vast array of products and devices. In photovoltaic devices in particular, polymer-oxide interfaces are present at all levels of the device architecture, including in the active layer, at the electrical contacts, and within the protective encapsulation and transparent barrier layers. The integrity of these polymer-oxide interfaces are of critical importance to the lifetime of outdoor electronic devices, since damage to these interfaces can degrade device performance, adversely affect mechanical properties, and create fast diffusion pathways for potentially harmful environmental species such as moisture and oxygen. The need to understand the environmentally-assisted degradation of polymer-oxide interfaces is underscored by the adverse operating conditions experienced by outdoor electronics, including diurnal temperature cycling, mechanical stresses, active chemical species, surface weathering, and exposure to ultraviolet light.
Field studies have shown that these environmental effects severely weaken the adhesion of polymer-oxide interfaces, and cause delamination of thin-film structures that contain these interfaces (Fig. 5). However, the effects of harsh operating conditions on defect evolution at these interfaces are poorly understood.
Figure 5: Degradation of structures containing polymer-oxide interfaces in outdoor environments can proceed by many mechanisms. Often, these mechanisms are coupled, leading to greatly reduced product lifetimes.
My research interests are focused on developing and advancing methodologies for quantifying the effect of the environment on the adhesion and reliability of polymer-oxide interfaces. Often, it is found that the effects of temperature, humidity, and ultraviolet light on the delamination of these interfaces are coupled, and can even depend on the type of mechanical loading experienced by the structure. Presently, a major area of study is the characterization of the adhesive energy of polymer-oxide interfaces after exposure to ultraviolet light and correlating changes in adhesive properties to local chemical changes at the interface. Also under investigation are the in situ effects of environmental conditions on debond propagation rates, which can give insight into the kinetics of defect evolution and provide the basis for kinetic models that can allow for quantitative predictions of device lifetime.