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Thesis: M.S., Massachusetts Institute of Technology, Department of Chemical Engineering, 1980 / Includes bibliographical references. / by Ravinder Kumar Oswal. / M.S. / M.S. Massachusetts Institute of Technology, Department of Chemical Engineering

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Materials in contact with the biological milieu (biomaterials) spontaneously and nonspecifically adsorb constituent proteins which may lead to unwanted cell adhesion and responses or hinder device performance. These interactions and their related phenomena lead to complications in ~3% of implant surgeries. Thus, resistance to these nonspecific interactions is critical to the performance of many implanted biomaterials and biosensing surfaces. Further, these interactions have widespread importance to industrial materials in contact with biological environments such as food packaging, and agricultural and nautical surfaces. Thin film coatings of antifouling polymers are one of the leading methods for reducing nonspecific interactions. Both polymer composition (chemical composition and molecular weight) and polymer grafting density are the principal determinants of coating performance. For applications requiring specific bioactivity, such as selective ligand-analyte interactions for sensors, the polymer coating must remain antifouling and be amenable to functionalization with capture ligands. Tethered polymer coatings can be made by surface initiated polymerization (“graft-from”) which results in higher density coatings, but complex fabrication limits commercialization and capacity of functionalization with capture ligands. Simpler “graft-to” procedures, where pre synthesized polymers are immobilized to a surface, are more amenable to translation but suffer from inferior antifouling properties due to lower density coatings. New fabrication methods are therefore required to improve both graft-to and graft-from coatings. Herein, the effects of polymer density on material performance are explored and leveraged to produce novel functional surfaces using two classes of polymers, namely amphiphilic and thermoresponsive poly(oligo(ethylene glycol)) methyl ether methacrylate, and zwitterionic, functionalizable poly(carboxybetaine methacrylamide) (pCB), as well as copolymers thereof. Specifically, polymer grafting techniques which exploit grafting density effects on surfaces were developed, leading to surfaces: 1) that are both high-loading and antifouling due to two different grafting densities within bimodal architectures, and (2) with enhanced anti-fouling properties despite being prepared via a “grafting-to” method using shrinkable or expandable substrates. Interestingly, shrinking substrates with antifouling polymers resulted in a novel LSPR biosensor with high translation potential. Chapter 2 describes the pH controlled, one-pot production of two-layer brushes composed of an antifouling dense layer and a high-loading lower density layer where capture ligand immobilization was improved by 6 times compared to a single high density layer. Towards improving fouling and bioactivity of graft-to surfaces, Chapter 3 describes the first demonstration of Graft-then-Shrink where a stretched polystyrene (PS) substrate coated in a thin gold layer modified with thiol-terminated pCB was thermo-shrunk to one sixth in footprint to increase polymer surface coating content for enhanced antifouling properties and the production of micro/nano gold wrinkles to generate a localized surface plasmon resonance (LSPR) active surface. The low-cost sensors can vi detect biomolecular interactions by tracking changes in absorbance in the visible spectrum using ubiquitous plate readers. In Chapter 4, Graft-then-Shrink was extended to elastomeric materials, where thiol terminated polymers were grafted onto solvent swollen silicone via thiol-maleimide click chemistry, producing strongly antifouling materials. Taken together, these developments represent significant advances in the preparation and application of antifouling polymer coatings towards the improvement of antifouling surface properties of medical devices and resulted in the development of a novel, low-cost LSPR sensor without the need for specialized equipment. / Thesis / Doctor of Philosophy (PhD) / When a material, such as a medical implant or sensor, is placed in contact with tissues and biological fluids, biomolecules stick to the exposed surfaces through nonspecific interactions. It is important to minimize nonspecific interactions because they can lead to bacterial infections, inflammation, implant failure and loss of device performance. Coatings to minimize nonspecific interactions therefore remain an active area of research. In this thesis, we explored new methods to create biomolecule and cell repellent coatings of long, chainlike molecules known as polymers grafted onto surfaces. Specific types of polymers, known as antifouling, were particularly effective at reducing these interactions. Although it is important to block nonspecific interactions, many devices require bioactive surfaces through selective interactions. For example, sensors for analysis of blood products require the selective binding of the target ligand with minimal binding of non-target agents. To this end, functionalizable antifouling polymers are often modified with a capture or binding agent corresponding to the target ligand. Polymer coatings which are both antifouling and functionalizable for specific interactions, are called “romantic” because of their selective love of a single interaction. To synthesize these romantic polymer coatings, two main methods have been reported: 1) “grafting-from” where the polymer is grown from the surface, producing a very dense coating, and 2) “grafting-to” where the polymer is synthesized in solution, and then immobilized onto the material surface, which produces coatings of lower density. For antifouling polymer coatings to be as effective as possible, polymers should be tethered densely on the material surface, but to maximize the loading of capture agents, polymer density must be lower to allow for grafting within the layer. Further, the grafting-from method is typically more synthetically challenging hindering commercialization. To improve the selective bioactivity of graft-to and graft-from coatings as well as antifouling properties of graft-to coatings, we present two methods to improve the specific bioactivity of anti-fouling polymer coatings and the first description of Graft-then-Shrink, a method to enhance the antifouling properties of graft-to coatings for medical implants and label-free in vitro sensors. For graft-from coatings, we produced a hierarchical romantic surface that consists of two polymer layers, the lower of which is dense and antifouling, and the upper of which is low-density and can accommodate high-levels of bioactive agents, resulting in a best of both worlds; the density of the layers is controlled by a novel pH controlled polymerization procedure. A method to improve the less labor intensive “grafting-to” strategy was then devised, called “Graft-then-Shrink” where the antifouling polymers are grafted onto a shrinkable material, and then the material is shrunk, leading to an increase in grafted polymer content over grafting-to alone. This method was successfully applied to a heat shrinkable material and an elastomeric silicone material, a common material for medical devices, for improved antifouling properties. Finally, a method for combining the Graft-then-Shrink technique iv with a novel localized surface plasmon resonance (LSPR) biosensor was found, that provides a simple route to access romantic surfaces on high-sensitivity, easy to fabricate LSPR biosensors. Together, these fabrication methods will simplify and expedite the translation of antifouling and romantic surfaces for medical devices and sensors.

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Structure-property behavior of free radical synthesized polydimethylsiloxane (PDMS) - polystyrene and PDMS-styrene derivative block polymers have been studied. The block polymers were provided by Dr. J. V. Crivello from GE. Two different types of segmented polytetramethyleneoxide (PTMO) based ionene elastomers were also investigated. The PTMO-dihalide ionenes were obtained through the courtesy of Dr. C. M. Leir in 3M, while the PTMO-dipyridinium ionenes were synthesized by Dr. B. Lee in Prof. McGrath’s research group at VPI&SU. In the free radical synthesized PDMS-PS block polymers, the molecular weight (MW) and the molecular weight distribution (MWD) of the PS blocks varied with the PDMS block length (block MW) comprising the macroinitiators, and the styrene conversion level. As the PDMS block length or the conversion level increased, the average PS block MW increased, and the molecular weight distribution of the PS block became broader. Multi-modal molecular weight distribution of the PS blocks was observed on the high conversion polymers with large PDMS blocks. As the MW and the MWD of the PS blocks changed, the morphology, the degree of phase mixing, and bulk properties of these PDMS-PS block polymers were altered as expected. At constant conversion level, the morphology of these block polymers changed from spherical PS domains in the PDMS matrix to a lamellar structure as the PDMS block length increased. As expected, their mechanical properties were also changed as morphology varied. At constant PDMS content, the systems with shorter PDMS blocks displayed elastomeric properties, while the polymers having large PDMS blocks behaved like a plastic due to a continuous lamellar morphology. The degree of phase mixing also decreased with an increase of the PS block length because of the increased incompatibility between the two block components. For a constant PDMS block length, the PS block length increased and the MWD of the PS blocks became broader when the styrene conversion level increased. Consequently, the morphology, the degree of phase mixing as well as the bulk properties of the block polymer also varied with conversion level. Addressing the segmented PTMO based ionene elastomers, these materials displayed excellent elastomeric properties which result from the ion clustering or ionic domain formation in a continuous PTMO matrix. The morphology and bulk properties of these ionene systems were strongly dependent on the strength of ionic association. By varying the ion content, the type of counter ion, or hard segment, the ionic association was changed. Therefore, the morphology and the bulk properties were also altered. Morphological textures of these ionene systems were studied by Transmission Electron Microscopy (TEM) and Small Angle X-ray Scattering (SAXS). Due to the strong ion clustering, an ionene rod-like morphology was observed in the PTMO-dihalide ionene elastomers by both TEM and SAXS at low volume fraction of ionene content (<7 vol%). It is the first time that these two analytical methods have distinctly led to the same end result for any ionomer system! This morphological structure is not predicted by any of existing theories of ion clustering in ionomers nor the classical theories of block/segmented polymers. Finally, the morphology of these ionene systems was altered with ion content. When the ion content was decreased by increasing the PTMO segment length, the long-range ordered structure disappeared as well as the rod-like microphase structure. A very unique phenomenon, a highly reversible modulus "jump" with increasing temperature, has been observed for these ionene materials which has not been reported before. This "jump" is directly related to the ion content, type of counter ion and the hard segment. Based on experimental evidence, the "jump" is tentatively speculated to be caused by a conformational change in the ionene hard segments. However, further investigations are needed to support or disprove this speculation. / Ph. D.

This thesis was a study of the effect of the variation of cross-linkage on the mechanical properties of a polyurethane elastomer prepared from 33’ Bitolyene 44’ diisocyanate and the polyester of adipic acid and ethylene glycol. The investigation consisted of the preparation of polyurethane elastomer, solvent absorption and swelling tests, tensile properties test and hardness test. Three groups of polymer material were prepared. In the first group the R value was varied from 1.0 to 4.5, (the ratio of 33’ Bitolyene 44’ diisocyanate to polyester) and fixed amounts of excess diisocyanate were used. Ethylene glycol was used as the cross-linking agent. In the second and third groups the R value was fixed at 1.65 and 2.5, and the amount of excess diisocyanate after chain extension was varied. The preparation of the elastomer was accomplished by a method based on the literature and preliminary experiments carried out for the purpose of determining optimum conditions for the specific polymer under investigation. All mechanical tests were accomplished by conventional and standard methods. All tests were conducted at 75° F. In addition to these tests the hardness test was carried out at 300° F. The preparation of polyurethane elastomer by bulk polymerization was conducted at atmospheric pressure and 130° C with the reaction requiring a period of 30 minutes for completion. For the molding process, casting, cold molding, hot molding and curing were used. The various degrees of cross-linkage could be obtained by using different ratios of 33’ Bitolylene 44’diisocyanate to polyester and by varying the amount of cross-linked agent, ethylene glycol. The properties associated with cross-linking varied in such a way as to indicate an increase in cross-linking as the R value increased. The solvent absorption and swelling properties decreased with an increase of the R value. The mechanical properties improved with an increased R value. Also the hardness increased with an increase of the R value. / Master of Science