(5ci) Control of Heterogeneous Polymer Properties through the Use of Nanoparticle and Antiplasticizer Additives

As polymeric materials such as photoresists and nanocomposites are designed with features approaching the order of tens of nanometers, it becomes crucial for one to control the transport and mechanical properties of polymeric solids on such length scales. It is well known that amorphous materials exhibit dynamic heterogeneity, and several recent studies have shown that amorphous solids also exhibit mechanical heterogeneity on the nanoscale. In my work, I have employed molecular simulations to study how nanoparticles and molecular additives can be used to control the heterogeneous properties of polymers on the nanoscale.

When one adds a low molecular weight additive to a polymeric system, one expects to observe a class of behaviors known as plasticization, whereby the glass transition temperature (Tg), density, and elastic constants are all decreased. However, there is a second class of materials, known as antiplasticizers, which will decrease (Tg) while simultaneously increasing both the density and the stiffness of the polymer. Using molecular dynamics simulations, I have explored the fundamental origins of antiplasticization and its consequences on the nanoscale properties of glass-forming polymers. We have found that the addition of antiplasticizers improves the packing efficiency in bulk polymeric materials, and this leads to a more dynamically homogeneous material. When a dynamically heterogeneous, pure polymer is confined to a free-standing thin film, the dynamics of the polymer near the free surfaces lead to substantial changes in the properties of the entire thin film. However, when the homogeneous, antiplasticized polymer is confined to a free-standing thin film, the effects of the free surface are greatly reduced, leading to improved properties in nanoscale thin polymer films.

More recently, my research has focused on understanding how aging and deformation changes the properties of polymer nanocomposite materials, which is essential to understand as nanocomposite materials are finding increased use in critical applications where they will be subject to deformation. Recent experiments and simulations have shown that the dynamics in glassy solids can be enhanced by several orders of magnitude. Using molecular dynamics simulations on a model nanocomposite material, I have studied how the presence of nanoparticles changes the dynamic enhancement. I have shown that the changes due to the presence of the particles arise from the nanoparticles' ability to stiffen the polymer. Additionally, by examining the entanglement network in a polymer glass and in the nanocomposite, we have found that nanoparticles have the ability to increase the entanglement density in the polymer, leading to increases in the plateau modulus, a key design parameter in many applications.