Title: Multiscale structure and dynamics in matrix-free polymer nanocomposites
Description: The addition of fillers to a polymer matrix to endow soft materials with desirable properties has been a focused area of study over many decades and composite materials based on this idea are being increasingly incorporated into several end use products. Yet, almost always, the focus is on maximizing a particular property set for a unique polymer/filler combination for a specific application, which might not necessarily be translatable into another application. To exploit possible synergies, there is a need to develop materials that have the potential to perform multiple functions at the same time rather than a singular function. In this vein of thought, materials constructed using only polymer grafted nanoparticles (GNPs) have the potential to be one such class of materials as they have been shown to display a whole host of unique property sets – ranging from improved mechanical strength, enhancements in the gas and condensable penetrant transport properties, improvements in thermal conductivity, tunability of impact mitigation to more exotic behavior related to development of phononic bandgaps and quasi-crystalline materials.

This thesis explores some of the structure-dynamics-property relations of some of the unique property sets described above and aims to provide insights into the nanoscale properties that lead to the improvements observed in macroscopic properties. In the first 2 chapters, we study the effect of tethering polymer chains to a spherical surface on the segmental and local vibrational dynamics of grafted polymer chains in an ensemble of GNPs. In the field of gas transport, the hopping motion of gas molecules inside a non-porous polymer matrix is facilitated by the motion of polymer segments, yet the understanding between the coupling of the two is very poor. By utilizing GNPs in which the diffusivity of gases is controlled by varying graft chain molecular weights, we can show that segmental dynamics of the polymer chains operating on a length scale of ~ 1 nm are positively correlated with the observed enhancements in diffusivities observed previously. We also propose that the inefficient packing of polymer chains leads to a decrease in the barriers of motion of the polymer segments, which is ultimately responsible for allowing penetrant molecules to move through the polymer phase much faster than a corresponding homopolymer melt.

By utilizing a similar time and length scale approach, we can also explain the observed increases in thermal conductivities through the vibrational motion of polymer chains. This reaffirms the important role nanoscale polymer dynamics plays in both mass and thermal transport. In the next few chapters, we switch gears and focus on the microscopic structure and dynamics of the nanoparticles and how they impact the mechanical properties in suspensions. By studying the translational and vibrational motion of the GNPs, we find that the vibrational amplitude of a singular GNP decreases with increasing chain length all while the motion of the NP becomes faster, a phenomenon that we can associate with unjamming of the GNPs. This transition from jamming to unjamming is also visible in the local and long wavelength structure of the GNPs as well as the sound velocity through the material. Through these observations we can show that there is an intricate link between the structure and the relevant mechanical properties.

Lastly, by building on the understanding laid out in the first few chapters, we propose that static features measurable through scattering are indicators of the enhanced transport properties of GNP based membranes. This also provides structural insights into the correlation between the structure of the polymer phase and the transport of penetrants. Each of the chapters touch upon a unique aspect of the structure and dynamics of different components of a GNP at different time and length scales, and how they are possibly linked to the several different property sets or dynamic features exhibited by the constructs, while also providing possible microscopic explanations for the same.