Title

Surface and supramolecular chemistry studies of selected cross-linked macromolecular systems

Date of Award

2002

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Committee Member

Roger M. Leblanc, Committee Chair

Abstract

A principal focus of this thesis is the study of the surface chemistry of enzymatically (DHP) and photochemically (PCP) polymerized lignin model compounds, and a novel modified PEG-NC hydrogel. ESEM and SEM imaging, surface force spectroscopy, and Langmuir film studies of lignin model compounds were performed. It is found that the enzymatic lignin model compound has at least four different organizational orders of supramolecular structures. Modules of about 20 monomers are formed first in nanometer size range, and subsequently polymerize into supermodules containing about 500 monomers stretching to the tens of nanometers in size. Such supermodules subsequently aggregate into globular or spherulitic assemblies, averaging about 400 nm in diameter. Finally, the globules form clusters and large superstructures, stretching several micrometers in size. A formation of complex regular motifs, such as long chains, triangular, hexagonal and spiral-like structures, as well as regular colloidal crystal lattice assemblies of DHP lignin model compound spherulites has been observed. Organization of the enzymatic lignin model compound is strongly influenced by substrate surface properties, with most ordered structures achieved on cellulose film substrates and the least ordered on the graphite. On the other hand, the photochemical lignin model compound expresses less ordered structure incapable of forming any of the complex regular motifs. Evidence of the existence of a strong intermolecular force between two enzymatically polymerized lignin model compound macromolecules is revealed by AFM force spectroscopy, and this force is considered responsible for arranging lignin globules together in higher ordered structures. Based on combined AFM, Langmuir film and ESEM observations, a hypothesis is provided that the DHP lignin model compound globule is not just a simple block-copolymer micelle, but that it consists of at least two individual spherical layers, with space in between filled with solvent or gas.Using the ESEM for topographical and AFM for rheological study, a structural change of a surface of a novel PEG-NC potentially biocompatible hydrogel has been probed as a function of reversible photocross-linking/photocleavage. A reversible significant change in both gel surface topography and in the modulus of elasticity of the hydrogel as a function of the reversible photo-crosslinking (at 365 nm) and photocleavage (at 254 nm) has been observed. The initial photo-crosslinked hydrogel possesses a modulus of elasticity in about E = 600,000 Pa, while the modulus of elasticity of the photocleaved hydrogel is approximately E = 3,000 Pa which is about the same as the modulus of elasticity of a mammalian cells. Such properties indicate possible applicability of this novel material as a potential biomaterial for cartilage tissue replacement and other similar biomedical application were morphing of the material and its solidifying/annealing need to be done in-situ.

Keywords

Chemistry, Physical; Chemistry, Polymer; Biophysics, General

Link to Full Text

http://access.library.miami.edu/login?url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3050738