Graduation Year

2019

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemical Engineering

Major Professor

Norma Alcantar, Ph.D.

Co-Major Professor

Ryan Toomey, Ph.D.

Committee Member

Nathan Gallant, Ph.D.

Committee Member

Mark Jaroszeski, Ph.D.

Committee Member

Maj-Linda Selenica, Ph.D.

Keywords

Thin Coatings, Responsive Hydrogels, Natural Materials, ATR-FTIR, Ellipsometry

Abstract

Pectin polysaccharides provide promising potential as all-natural, non-toxic “green” coatings. Pectin polysaccharides have been drawing growing attention as elements of stimuli-responsive systems and as source materials for functionalized robust coatings. They can be extracted from several natural sources and are intrinsically biocompatible. The majority of the applications of pectin gels require a better understanding of the properties of thin layers of gels at surfaces and interfaces. Despite the prevalence of bulk gels of pectin, coatings of pectin have not been reported. In bulk hydrogels, swelling of polymer networks is controlled by a diffusion limited transport process in which the rate of response is determined by the volume of solvent to be absorbed. Therefore, reducing the size of the gel accelerates the response rate due to the high surface-to-volume ratio.

The overall thrust of this dissertation is to perform a comprehensive study to delineate the factors which govern the responsive behavior of thin coatings of naturally occurring pectin polysaccharides. For this, coatings of pectin were fabricated and their responsive behaviors were evaluated by assessing their swelling behaviors to elucidate the main factors governing their responsive behaviors.

Gelling properties of pectin networks are strongly governed by their chemical structure, particularly their methoxyl content. Pectins with low content of methoxyl groups can be readily cross-linked in the presence of divalent ions. Pectins with a high content of methoxyl groups, however, have been rarely studied due to the complications associated with their cross-linking. The available methods to cross-link high-methoxyl pectin require high sugar concentrations as well as an acidic environment which limit their functionality and applicability. Here, a simple technique is presented to fabricate and cross-link high-methoxyl pectin in a controlled manner without altering the chemical properties of pectin. This approach is based on cross-linking through electrostatic interactions rather than relying on the hydrophobic forces. Employing the proposed strategy, high-methoxyl pectin coatings with degrees of esterification of approximately 70% were cross-linked and their swelling responses were investigated with ellipsometry and ATR-FTIR. Moreover, the possibility of employing such coatings for cell culture applications were also investigated.

Moreover, responsive behaviors of pectin coatings in relation to the degree of esterification were investigated. This was performed to provide an understanding of the thermo- and pH-responsive behaviors of thin pectin coatings in relation to their degree of esterification with the eventual aim of achieving tunable surface properties and functionalities through well-controlled chemistry.

The structural and molecular properties of pectin vary widely based on their extraction source, encouraging the exploration of diverse sources of pectins to achieve versatile functionalities. In this sense, the mucilage of Opuntia ficus-indica cactus was addressed as an underexplored source of pectin. The pectin extracted from the mucilage of Opuntia ficus-indica is capable of holding high volumes of water, rendering hydrophilic gels suitable for multiple applications. Accordingly, thin coatings of cactus pectin were fabricated and their chemical properties were assessed by ATR-FTIR. The responsive behaviors of these coatings were assessed using ellipsometry.

The results of this research will provide an improved understanding of the responsive properties of naturally occurring pectin polysaccharides in relation to chemical composition and system confinement. Pectin coatings established in this research combine the promising properties of pectin with the thermo- and pH-responsive properties and may, therefore, be used in biomedical applications.

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