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UNDERSTANDING THE MACROSCOPIC PROPERTIES OF POLYMER

Author
Goyal, Sushmit Sunil Kumar
Abstract
Polymer nanocomposites have been a topic of interest in recent years for their potential
in applications such as water desalination, CO2 capture, photovoltaics, battery
membranes and immersion lithography. Unlike colloids which tend to agglomerate
irreversibly, polymer grafted colloids are stabilized by polymer-polymer steric
interactions. Polymer grafted nanoparticles(PGNs) are a class of such materials which
consist of an inorganic nanoparticle core, functionalized with a corona of organic
oligomers. These differ from common nanocomposites in that the tethered corona can
be used as the sole suspending medium for the cores. The hybrid nature of the
suspension allows the fabrication of materials with tunable properties by varying
parameters such as nanoparticle chemistry, shape and size, as well as the polymer
molecular weight, grafting density and chemistry. The range of properties exhibited by
these composites vary from solids, stiff waxes, and gels for high core content to single
component solvent free fluids for low core content.
While PGNs have been extensively studied experimentally by several groups at Cornell,
this research focuses on the use of molecular simulations to help elucidate the effect of
molecular design on the properties of PGNs. We studied the effect of grafting density,
corona thickness and core volume fraction on equilibrium and non-equilibrium
properties like diffusivity, rotational diffusivity, equilibrium structure, rheology and
molecular origin of stress. We find that increasing the chain length and grafting density
decreases the viscosity and structural order, which makes the system to have a more
liquid-like behavior. While these trends have also been observed in experiments and
predicted by analytical theories, our results complement simulations data from other
groups to provide a molecular basis for these phenomena and to create phase diagrams
to encapsulate the behavior of a large number of systems.
We also compare the properties of solvent-free PGNs with those suspended in a solvent,
and examine the effect of dilution in these systems. We find that solvent-free systems
have higher viscosity and a larger shear thinning coefficient. On studying the phase
behavior of PGNs in chemically identical polymeric solvents, we find that changing the
ratio of polymer length to nanoparticle size can result in a transition from well-mixed
systems to phase-separated systems, a phenomenon that could be attributed to the
interplay between entropic forces acting on the grafted and free polymers. Our
simulations reveal trends in structural packing for low curvature PGNs that are
consistent with those observed in experiments and predicted by theory (e.g., as
pertaining to the first peak of structure factor), while predicting that for high curvature
PGNs macrophase separation can occur (a trend yet to be tested experimentally).
Date Issued
2017-01-30Subject
dispersion; Polymers; Chemical engineering; Nanoparticles
Committee Chair
Escobedo, Fernando
Committee Member
Cohen, Itai; Archer, Lynden A.
Degree Discipline
Chemical Engineering
Degree Name
Ph. D., Chemical Engineering
Degree Level
Doctor of Philosophy
Type
dissertation or thesis