Date of Award

12-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemistry

Committee Member

Prof. Dvora Perahia, Committee Chair

Committee Member

Dr. Gary S. Grest

Committee Member

Dr. Brian N. Dominy

Committee Member

Dr. Jason D. McNeill

Committee Member

Dr. Robert A. Latour Jr.

Abstract

The conformations and dynamics of luminescent polymers, polymeric nanoparticles or polydots and polymers grafted onto inorganic nanoparticles using molecular dynamics (MD) simulation have been studied. Rigidity of the backbones of polymers effects their properties. Model polymer, dialkyl poly para phenylene ethynylene (PPE), comprises rigid backbones substituted by flexible side chains. The degree of conjugation of these polymers depends on the conformation of their backbones, which in turn affects their electronic and luminescence characteristics. The results of single chain of PPEs followed by polydots and polymers grafted onto inorganic nanoparticles. PPEs as a function of molecular weight and the nature of side chains in toluene and in implicit poor and good solvents were studied. Toluene is a good solvent for the polymer backbone and a poor solvent for the side chains whereas implicit poor solvent is poor for the overall polymer. Independent of solvent quality and molecular weight, PPE chains remained extended and the end-to-end distance scales with molecular weight. There was no correlation between aromatic rings of the backbones of the polymer chains. The polymeric nanoparticles or polydots, formed by collapsing the rigid polymers into nano dimensions, retain their stability over extended period. The internal structure, stability and dynamics of polydots as a function of solvents, side chains and temperatures were studied. Static properties such as the radius of gyration, and the structure factor were calculated which showed that these polydots had a spherical shape consistent with the visual findings. These polydots remained in their confined state in a poor solvent whereas in a good solvent they unraveled. Unraveling was faster in bare polydots compared to substituted polydots. With increasing temperature, polydots expanded and became aspherical but remained confined with no internal correlations between the aromatic rings These studies have shown that these polydots retain their overall shape and dynamics over an extended temperature range The conformation of PPEs confined to five nm diameter silica nanoparticle as a function of solvent quality, coverage and molecular weights was studied. In water, which is a poor solvent for PPEs, the polymer chains aggregated to form clusters, which became more defined with increasing molecular weight and coverage whereas the chains remained extended and did not aggregate in toluene and decane, which are the good solvents. The distribution of the chains around the nanoparticle's core was homogeneous in good solvents whereas in poor solvents the distributions were heterogeneous, independent of molecular weight and coverage. In comparison with grafted flexible hydrocarbon chains, the PPE backbones remained stretched out away from the surface of the NP. Conjugated polymers in the confined geometry in have different conformation in polydots whereas the chains confined to NP's surface have same conformation as the PPEs chains in solutions.

Share

COinS