Chemical and Biomolecular Engineering - Theses

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    Supramolecular polymers as building blocks for the formation of particles
    Tardy, Blaise Leopold ( 2014)
    Over the last two decades, there has been a growing interest in the development of supramolecular polymers, linear macromolecules whose monomeric components are held together by non-covalent interactions. Such supramolecular assemblies are commonly found in nature and are crucial for the function of living tissues and cells. The recent development of synthetic supramolecular polymers has shown promise for enhancing the properties of polymeric materials. Indeed, studies have shown that such materials have significant benefits when compared to conventional, covalently bound, polymers. These benefits are due to the ability of supramolecular assemblies to respond to stimulus, and to dynamically rearrange their structure in a manner unachievable using conventional, covalently bound polymers. Resemblances between the dynamics of synthetic supramolecular polymers and naturally occurring supramolecular polymers are suggestive of their potential for biomedical applications. In this trend, the most promising supramolecular polymer, cyclodextrin (CD) based polyrotaxanes (PRXs), is now emerging as a potential tool to synthetically form dynamic interfaces for applications in the biomedical field. The recent popularity of these polymers in this field is not only due to their inherent, non-covalent properties but also to the low cost, high engineerability and low toxicity of the components they are made of. In this work, CD-based PRXs have been used as building blocks to form particles that were designed for developments in drug delivery. Specifically, the properties specific to PRXs have been exploited to design particles with degradation or stimuli-based response. The unique characteristics of PRXs were found to translate into similarly unique characteristics of the assembled particles. Different approaches have been studied and their advantages and limitations are highlighted. Initial investigations were aimed at designing particles fitting the requirements in properties and specific characteristics highlighted by recent in vivo and in vitro studies. In this direction, we demonstrated controlled degradation of self-assembled PRX-based structures through stimuli triggered disassembly. Such control was also shown for PRX particles dynamically formed using a templated approach, for which disassembly through judicious selection of specific building blocks is highlighted. The use of the templated approach was shown to be more straightforward and versatile in its applications, laying out a framework to form and engineer particles using PRXs as a building block. Lastly, by using CD’s molecular mobility in the PRX as an additional handle for tuning; a “one block” polymer, able to reversibly segregate into multi-blocks leading to the formation of nanoparticles, was developed. This approach is particularly interesting as many responsive polymeric materials have their response due to a stretched-to-coiled transition of individual chain while we show here a transition between a mono-block like architecture to a multi-block like architecture. The preliminary results highlight the potential of PRXs as building blocks for applications in drug delivery systems.
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    Soft polymeric nanoparticles as additives for CO2 separation membranes
    Halim, Andri ( 2014)
    The use of polymeric membranes for gas separation has experienced a major expansion in the past few decades with current applications which include the separation of CO2 from flue gas. Various approaches have been explored to fabricate membranes with superior separation performance that can exceed the current upper performance limit. These include the incorporation of hard inorganic nanoparticles into polymers to form mixed-matrix membranes (MMMs). The performance of MMMs can be further enhanced if they can be fabricated into asymmetric morphology. The fabrication of asymmetric membranes, in the form of a thin film composite (TFC) membrane, is more commercially viable due to the increased flux and reduced consumption of expensive nanoparticles. TFC membranes are typically composed of a porous support coated with a highly permeable gutter layer, which is in turn coated with a thin active layer. However, the development of effective asymmetric MMMs has been limited. This is due to the difficulty in fabricating nanoparticles in a size that does not exceed the thickness of the active layer and in avoiding defects in the resulting composite structure. The fabrication of next generation mixed-matrix gas separation membranes is also hampered by the need to ensure a defect-free polymer/inorganic particle interface. A similar approach can be applied to the addition of soft polymeric nanoparticles into a selective polymer matrix. In this case, the problem of defects occurring between the particle and the matrix can be avoided through the engineering of particles that are compatible with the polymer matrix. Hence, this thesis aims to synthesize novel soft polymeric nanoparticles with well-defined architectures and utilize these as additives to be incorporated into the thin active layer of TFC membranes. The requirements for these nanoparticles include (a) a soft and CO2 permeable core and (b) a corona which is compatible with the polymer matrix. The best candidate nanoparticles are then blended with a selective polymer matrix to form the active layer of TFC membranes, which are tested for their CO2 separation from N2. The size of the soft polymeric nanoparticles are significantly smaller than the thickness of the active layer and overcome the problem of blending larger inorganic nanoparticles to form asymmetric MMMs. The first soft polymeric nanoparticles studied were based on triblock copolymers containing polyimide (PI) and poly(dimethylsiloxane) (PDMS). Well-defined difunctional PI was initially prepared via step-growth polymerization. Subsequently, PI was functionalized and chain extended with different molar ratios of PDMS-monomethacrylate (PDMS-MA) via atom transfer radical polymerization (ATRP) to form a series of triblock copolymers. Self-assembly of triblock copolymers in a selective solvent for PI, followed by cross-linking via hydrogen abstraction, resulted in the formation of well-defined nanoparticles with a soft PDMS core. The second soft polymeric nanoparticles developed in this study was based on diblock copolymers containing poly(ethylene glycol) (PEG) and PDMS. Commercially available PEG was utilized as a substitute for the PI block due to the difficulty in synthesizing well-defined polymers via step-growth polymerization. Three different molecular weights of monomethyl ether PEG were initially functionalized to form macroinitiators suitable for ATRP. These macroinitiators were then chain extended with PDMS-MA and photoactive anthracene moeities in different molar ratios to afford a series of photoresponsive diblock copolymers. Self-assembly of diblock copolymers in a selective solvent for PEG, followed by photocross-linking via [4+4] photodimerization of anthracene moeities, resulted in the formation of another well-defined soft polymeric nanoparticles with various structures that range from spherical micelles to large compound micelles. The preparation of soft polymeric nanoparticles through the self-assembly of block copolymers is generally carried out in low concentration to avoid aggregation of nanoparticles. This hinders the preparation of nanoparticles on a larger scale. The third soft polymeric nanoparticles explored in this thesis were based on PEG and PEG-b-PDMS grafted star polymers that were synthesized via the ‘core-first’ approach. This method allows the preparation of nanoparticles in high yields as the crude reaction mixture only requires separation from unreacted monomers. Various grafted star polymers with different PEG and PDMS molar ratios were synthesized in high yields and high conversions utilizing a four-arm ATRP initiator. These grafted star polymers were then utilized as additives for existing gas separation membranes. TFC membranes were prepared from commercially available selective poly(amide-b-ether) (Pebax® 2533) that was blended with a series of PEG and PEG-b-PDMS grafted star polymers. These blends formed a thin film on microporous polyacrylonitrile substrates which have been pre-coated with a PDMS gutter layer. Their ability to selectively separate CO2 from N2 was studied at 35°C and an upstream pressure of 3.4 bar. The addition of soft polymeric nanoparticles into the thin active layer of TFC membranes resulted in greatly improved flux as these particles are able to form localized, high flux, soft domains within a selective polymer matrix. These results create an interesting route to further develop and utilize soft polymeric nanoparticles as additives in membranes for gas separation processes.