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" Revisiting the Colloidal Fundamentals of Ionic Polymers for Engineering of Dispersions and Nanocoatings with Tailored Properties "
Islam, Sabina
Khan, Saad
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Latin Dissertation
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English
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| Record Number
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1056943
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TL56060
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| Main Entry
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Islam, Sabina
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| Title & Author
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Revisiting the Colloidal Fundamentals of Ionic Polymers for Engineering of Dispersions and Nanocoatings with Tailored Properties\ Islam, SabinaKhan, Saad
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| College
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North Carolina State University
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| Date
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2019
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| Degree
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Ph.D.
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2019
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| Note
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223 p.
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| Abstract
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Self-assembly is a predominant hallmark of nature. Without the right self-assembly and folding of biopolymers such as proteins, enzymes, nucleic acids, and cellulose, our existence as we know now, might not be possible. Similar to proteins, when placed in water, synthetic polymers with ionic groups self-assemble into defined nanostructures programmed by their composition and the characteristics of the medium such as dielectric constant, pH, electrolyte concentration, etc. One such polymer system is the family of sulfopolyesters. Water-dispersible sulfopolyesters are a major class of film-forming and solution-modifying polymers, which are routinely used in applications such as inks, adhesives, coatings, and personal care products. These ionic polyesters are developed for replacing the solvent-borne coating polymers in response to the environmental legislations imposed upon coating industries for reducing the emission of volatile organic compounds (VOCs). Since these polyesters are designed to be used as waterborne dispersions, understanding their colloidal interactions in dispersions is critical for their successful application. By using a range of commercially available water-dispersible sulfopolyesters as a model system, we investigated the relationship between their molecular composition, colloidal interactions, and phase equilibria. We established how these polyesters undergo different molecular configurations and nanoaggregated states, depending on the nature of the liquid medium. For example, the polyesters are in a solvated molecular form in certain organic solvents, whereas they self-assemble into compact nanoaggregates in water. We found that the interactions of these nanoaggregates follow the classical DLVO theory of critical colloidal coagulation where the stability of these nanoparticles is extremely sensitive to multivalent electrolytes (i.e., Ccrit ∝ z-6). By using static, dynamic, and electrophoretic light scattering, we correlate their nanoscale intermolecular and interparticle interactions with corresponding macroscale phase behavior in both organic medium and water, based on the theoretical framework of second virial coefficients. Furthermore, we present a model for nanoaggregate formation in water based on the critical surface charge density of these polyester nanoparticles. These ionic sulfopolyesters were originally developed for various packaging and coating applications. Despite their widespread commercial use, the fundamental understanding of their film formation properties is still limited. While incorporation of sufficient ionic groups into the polyester backbone ensures the formation of stable dispersion, an excess of ionic groups are deleterious for film performance such as water sensitivity. Therefore, it was important to understand how the ionic composition affects the interaction of the films with water. We deposited polyester nanofilms of brilliant structural colors, which allowed us to investigate the structural integrity of the films upon exposure to water. To test the properties of the dried films, we deposited sessile water drops on their surface, which lead to observation of evaporation-induced “coffee-ring erosion”. The polyester nanoparticles spontaneously get partially redispersed in the sessile water droplets and driven by convective evaporation flows, become redistributed forming multiple colorful ring patterns whose colors correspond to the different thicknesses of the films. We found that the generated erosion patterns are dictated by the ionic composition and not governed by molecular weight. Furthermore, we show that the integrity of these thin polyester films can be significantly improved by thermal densification without any further chemical curing. Another prominent example of polymer self-assembly is the formation of polyelectrolyte complexes (PECs) from the electrostatic interactions of two polyelectrolytes with oppositely charged groups. When mixed in solution, they spontaneously neutralize each other to form insoluble complexes. This process is routinely used for making waterborne coatings by sequential layer-by-layer (LBL) assembly. The compact structure of LBL films often offer superior barrier property which can be used for drug and food packaging application. However, one of the inherent challenges of commercial application of LBL barriers remains in that the barriers are typically not stretchable due to having tight intermolecular complexation of the polyelectrolytes. In the last section of this thesis, we investigated the effect of the solution pH on the ionization state of the weak polyelectrolyte Chitosan (CS) and Polyacrylic acid (PAA) on the final coating properties of CS/PAA LBL films. We found that the barrier and stretchability of the deposited films were crucially dependent on the deposition pH conditions. The films were oxygen impermeable and stretchable when the PAA solution pH was higher than that of CS solution. In contrast, the coatings were brittle and leaky when pH of the PAA solution was less than that of CS solution. This way, such fundamental understanding of colloidal parameters and interactions could be used to efficiently control and improve the film-formation ability of ionic polymers and may enable the design of novel high-performance waterborne coating systems.
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Chemical engineering
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Nanotechnology
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Khan, Saad
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North Carolina State University
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