Understanding Dynamics of Polymers Under Confinement: A Molecular Dynamics and Neutron Scattering Study

The current study probes the structure, dynamics, and rheological behavior of associating polymers including ionomers in melts and solutions as well as conjugated polymers confined into nanoparticles, using molecular dynamics (MD) simulations and neutron scattering techniques. The study focuses on two families of associative polymers, ion containing macromolecules and conjugated polymers. Polymers that consist of ionizable groups along their backbone found uses in a broad range of applications. Examples include light weight energy storage and generation systems, and biomedical applications, where the polymers act as ion exchange membranes, and actuators. The ionic groups tend to form clusters that are in the core of many of the applications. Understanding the relationship of cluster properties and the structure and dynamics of ionizable polymers is crucial to optimize current applications and develop new materials with controlled transport, mechanical stability, and desired response to external stimuli. The first part of the study focuses on understanding the structure and dynamics of polystyrene sulfonate melts as the distribution of the ionizable groups varies with random, precise (number of carbons between ionizable groups is exact), and blocky distributions along the backbone, using atomistic MD simulations. We find that the shape and size distribution of clusters as well as the number of unique chains associated with each cluster are affected by the distribution of the ionic groups. The dynamics of the polymer and the mobility of the counterions are affected by both the number and size of the clusters as well as the number of polymer chains associated with each cluster. Following the understanding of the effects of the clusters on polymer melts, the study proceeds to probe the effects of nonlinear elongational flow on associating polymer melts, which are processed into viable materials under elongational flows. This effort contains two components a coarse grain, and an atomistic MD studies. We find that the response of the melts to elongational flows results from the evolution of both the ionic clusters and Van der Waals domains. The coarse grain study shows that clusters break and reform continuously as the chain stretches heterogeneously in the presence of elongational flow. The atomistic study provides details regarding the effect of chain and cluster rearrangements on the response to the flow. Following melts studies, the work probed the segmental motion of slightly sulfonated polystyrene in cyclohexane solutions using the quasi-elastic neutron scattering technique. We find constraint dynamics at larger length scales however the polymer remains mobile on smaller length scales. Adding a small amount of alcohol is enough to release the constraints within the ionic clusters and results in an increase in segmental polymer motion on all length scales. The last part of the study focused on understanding the effects of the number of rigid luminescent polymer molecules, their chemistries, and initial orientation, on the structure and dynamics of soft nanoparticles (referred to as “polydots”). We find that increasing the number of chains confined affects the internal conformation of the polymer chains where side chains substituting the polymer backbone affect the polydots’ shape and stability. Similar to a single macromolecule polydots, these NPs exhibit a glass-like dynamics with relaxation times in a range of microseconds