Recently, surface tension has been shown to be important in emerging high technologies, such as in pumping and control of flow in microfluidic devices, in microchemical analysis of complex fluids, and in rapid DNA screening, etc. Advances in these new technologies will depend strongly on the availability of flexible methods for controlling surface tension. Photo-control using a photoresponsive surfactant is a potentially attractive route to accomplishing many of the tasks required in these processes. Photoresponsive surfactants typically incorporate an azobenzene group as the functional unit which experiences reversible trans-to-cis photoisomerization under different irradiation conditions. The photoisomerization usually causes a change in surface tension. Obviously, a large change in surface tension under different illumination conditions will be highly desirable in practical applications. However, the largest change in surface tension as reported in the literature is only 3 mN/m which is too small to generate any significant effect. In this presentation, we report a new class of photoresponsive surfactants which exhibit excellent performance in surface tension control. Under different illumination conditions, the change in surface tension can be as large as 11.0 mN/m. Experimental results are presented for two new photoresponsive surfactants. A discussion of experimental results follows. / Singapore-MIT Alliance (SMA)

We report on the surface properties of a photoresponsive surfactant that incorporates the light-sensitive azobenzene group into its tail. Cis-trans photo-isomerization of this group alters the ability of the surfactant to pack into adsorbed monolayers at an air-water interface or into aggregates in solution, causing a significant variation in bulk and surface properties upon changes in the illumination conditions. NMR studies indicate that a solution left in the dark for an extended period of time contains trans isomer almost exclusively, while samples exposed to light of fixed wavelength eventually reach a photostationary equilibrium with significant amounts of both isomers present. Dynamic surface tension studies performed on this system under different illumination conditions (dark, UV light, visible light) show profoundly different approaches to equilibrium. At concentrations well above the CMC, the same equilibrium tension is reached in all three cases, presumably corresponding to a surface saturated with the trans (more surface active) isomer. The dark sample shows a simple, single-step relaxation in surface tension after creation of a fresh interface, while the UV and visible samples exhibit a more rapid initial decrease in tension, followed by a plateau of nearly constant tension, and end with a final relaxation to equilibrium. It is hypothesized that this behavior of the UV and visible samples is caused by competitive adsorption between the cis and trans isomers present in these mixtures. Presumably the cis surfactant reaches the interface more quickly, leading to a cis-dominated interface having a tension value corresponding to the intermediate plateau, but is ultimately displaced by the trans isomer. Diffusional time scale arguments which consider the extremes of possible micellar dissolution rates are used to analyze the relaxation data of the dark sample, and the results indicate that micellar dissolution in these samples is slow. / Singapore-MIT Alliance (SMA)

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19008号 / 工博第4050号 / 新制||工||1623(附属図書館) / 31959 / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 赤木 和夫, 教授 中條 善樹, 教授 吉﨑 武尚 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20574号 / 工博第4354号 / 新制||工||1677(附属図書館) / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 今堀 博, 教授 秋吉 一成, 教授 白川 昌宏 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

No description available.

<p> Photoresponsive polymers can act as controllable drug delivery systems that may revolutionize ophthalmic drug delivery for disease treatment in the posterior segment of the eye. Localized, controlled drug delivery devices have significant therapeutic advantages for treating diseases of back of the eye by increasing patient compliance and maintaining therapeutic levels of drug in the tissue. Sustained-release delivery systems that respond to light/laser stimuli are under development to control the rate of delivery resulting in a tuneable treatment profile ideal for retinal diseases. The use of light as a crosslinking mechanism has the potential to create unique materials with controllable swelling, degradation and diffusion properties. </p> <p> This thesis investigates the synthesis and development of universal, graftable PEG-anthracene molecules and their applications in photosensitive alginate and hyaluronate (HA) "photogels". Anthracene undergoes reversible dimerization with wavelengths above 300 nm and de-dimerization/dissociation below 300 nm; due to its well-understood chemistry and symmetry, it was used as a starting point and proof-ofconcept for the synthesis of reversible dimerizing crosslinkers that may be generically grafted to different polymers to cause crosslinking/decrosslinking. After synthesis, watersoluble PEG-anthracene macromolecules were grafting via carbodiimide chemistry to the carboxyl groups along the polymer backbone of alginate and HA at various densities to create viscous liquids or gels with good handling properties. </p> <p> Light irradiation can be used to control the swelling and effective crosslinking density of the photogels which in tum can control drug delivery from photocrosslinked hydro gels as illustrated through the decrease or increase in the delivery of a variety of low molecular weight (<1000 Da) and high molecular weight (>10,000 Da) model drug compounds from both alginate and HA photogels with various light treatments. Novel loading mechanisms were developed through the loading of compounds into uncrosslinked gels followed by crosslinking 365 nm exposures to "lock" in the model drug compounds. Diffusion coefficients effectively compared the different systems showing increase exposures of 365 nm resulted in greater decrease in release of compounds demonstrating the ability to fine-tune release rates. Different formulations and control gels demonstrate a variety of different release profiles. The photogels were valuable long-term controlled release systems (>80 days) that also demonstrate high cytocompatibility when grown with ophthalmic cell lines. </p> <p> Novel photoresponsive biomaterials for smart delivery of therapeutics which use light-controlled crosslinking and decrosslinking mechanisms have been developed. The PEG-anthracene graftable photocrosslinkers show the ability to introduce photocontrolled crosslinking into hydrogel systems. While anthracene as the photodimerizer and alginate and HA as the bulk materials are used as a proof-of-concept in this work, this grafting system can be further manipulated to include new photosensitive dimerizers and other applicable polymers. The ability to use light stimuli to control release rates in a continual fashion, rather than having delivery that is strictly on or off, is a valuable finding that may lead to the development of drug delivery systems that can be catered towards individuals and the progression of their disease. </p> / Thesis / Doctor of Philosophy (PhD)

<p>Multi-stimuli-responsive materials with dual sensitivities to both temperature and light were designed and investigated for their responsive properties in aqueous media.</p> <p>Amphiphilic polymers were synthesized by copolymerizing monomers of thermoresponsive N-isopropylacrylamide (NIPAM) with vinyl cinnamate (VC), using different chain transfer agents to both control the molecular weight and impart functionality of an amine-terminal or carboxylic acid- terminal end groups. Linear polymers based on pNIPAM-VC were characterized and their thermo- and photo-responsive properties confirmed by ¹H NMR, GPC, and UV-visible spectroscopy.</p> <p>To obtain desired solubility and phase transition properties for the copolymer, latent variable methods were applied to past polymer data to identify the correlated reaction variables. Using model inversion, the ability to predict polymer properties was possible. The outcomes helped to determine ideal reaction reagents and conditions for future designs, facilitating the synthesis of both amine-capped and carboxylic acid-capped poly(NIPAM-co-VC) polymers with high solubility and phase transition onset below physiological temperature (<37°C)</p> <p>The designed poly(NIPAM-co-VC) polymers were subsequently grafted to a polysaccharide, hyaluronic acid (HA) or carboxymethyl cellulose (CMC), via carbodiimide chemistry. The graft material’s mechanical strength was compromised by both the linear polymer size and the architecture (end-group-grafting) which lead to unsuitable materials.</p> <p>Microgels with multi-responsive properties were synthesized by copolymerizing NIPAM with either acrylic acid (AA) or methacrylic acid (MAA) by conventional precipitation-emulsion methods. These microgels were aminated and subsequently grafted with a cinnamate pendant group. As an alternative, microgels were fabricated by microfluidics using linear polymers precursors. Both types of microgels exhibited significant deswelling upon changes in temperature, light, and pH, suggesting their potential utility as smart, photo-responsive drug delivery vehicles.</p> / Master of Applied Science (MASc)

Künstliche molekulare Schalter, wie beispielsweise Azobenzole, Diarylethene, Fulgide, Stilbene oder Spiropyrane wurden in den letzten Jahren intensiv erforscht, da sie zur Datenspeicherung, in selbstheilenden Materialien, molekularer Elektronik, Energiespeichern und mikromechanischen Anwendungen eingesetzt werden können. Eine der größten Herausforderungen im Forschungsfeld der molekularen Schalter ist die Frage, wie die Photoreaktion isolierter Moleküle in eine kontrollierte Photoreaktion wohldefinierter supramolekularer Systeme, wie z.B. organischer Dünnfilme oder 3D Nanostrukturen, übersetzt werden kann. Die Integration molekularer Schalter, beispielsweise von Azobenzolen, in supramolekulare Anordnungen kann zu emergenten Phänomenen wie kooperativem Schaltverhalten führen. Kooperatives Schalten bedeutet, dass die energetische Landschaft und daher auch die Isomerisationskinetik eines einzelnen molekularen Schalters von den isomerischen Zuständen benachbarter Schalter beeinflusst werden. Kooperatives Schaltverhalten, oder überhaupt Schaltbarkeit in geordneten Ensembles molekularer Schalter auf Oberflächen bewusst zu erzeugen hat sich allerdings aufgrund von sterischer Behinderung oder Delokalisierung angeregter Zustände als schwierig herausgestellt. Deshalb wäre ein besseres Verständnis der Voraussetzungen für Schaltbarkeit und kooperatives Verhalten molekularer Schalter in supramolekularen Systemen ein großer Schritt in Hinblick auf die Entwicklung von Bauelementen, die auf der gemeinsamen Bewegung molekularer Schalter basieren. Die in dieser Arbeit erzielten Resultate gewähren neue Einblicke in das Verhältnis zwischen der Photoisomerisierung einzelner Azobenzole und der Photoreaktion supramolekularer Systeme, was dabei helfen wird, neuartige und optimierte stimulireaktive Materialien zu entwickeln. / Synthetic molecular switches, such as azobenzenes, diarylethenes, fulgides, stilbenes or spiropyranes, have been intensively investigated in recent times because of their possible use in data storage, self-healing materials, molecular electronics, energy and information storage and optomechanics. One of the biggest challenges in the research field of molecular switches is the translation of the photoresponse of isolated molecules into a controlled photoresponse of well-defined supramolecular systems, such as organic thin films or functional nanostructures. The main focus of this thesis lies on the photoisomerization of multi-azobenzene compounds in different structural environments. Incorporation of molecular switches, for example azobenzene, into supramolecular assemblies can lead to emergent phenomena like cooperative switching behavior. Cooperative switching means that the energetic landscape and thus also the isomerization kinetics of a single molecular switch is influenced by the isomeric state of adjacent switches. However, it has proven difficult to establish cooperative switching behavior or even switching functionality at all in ordered ensembles of molecular switches on surfaces due to steric hindrance or delocalisation of excited states. Therefore, understanding the prerequisites for switching functionality and cooperative behavior of molecular switches in supramolecular assemblies is a crucial step towards the development of devices that make use of concerted motion of molecular switches. This thesis yields unprecedented insight into the relation between the photoisomerization of isolated azobenzenes and the photoresponse of supramolecular systems, which will ultimately help to build novel and optimized stimuli-responsive materials.

Stimuli-responsive materials are changing the landscape of actuated materials, optoelectronics, molecular machines, solar cells, temporary memory storage, and biomedical materials. Specifically, photo-responsive polymers have gained acceleration in research and application since the last two decades in the form of a surface coating and micro-patterns. Light as a stimulus can be coherent, mono or polychromatic, tunable for power (intensity) and energy (wavelength), and has precise spatiotemporal control. Conventional surface coating techniques such as spin coating are unable to impart properties to the coatings in terms of sturdiness, homogeneity, uniformity over the complex surface, post deposition modification, and process efficiency. Also, in the field of photoreponsive polymers, there is no simple technique for surface-patterning of photo-responsive polymers, which is an important missing link between current research and future potential applications. This dissertation designs new strategies for light-mediated ring opening metathesis polymerization (ROMP) to synthesize a diverse class of stable photo-responsive polymers and coatings. Firstly, we propose a new synthetic route to functionalize surface-initiated ring opening metathesis polymerization (SI ROMP) coatings. The backbone of ROMP polymers has internal carbon-carbon double bonds which are potential sites to introduce additional functionalities like stimuli-responsive functional groups. We leverage these unsaturated bonds to incorporate functionalized side chains using thiol-ene click chemistry. Thiol-ene chemistry is a versatile approach to attach diverse functional groups at the site of a carbon-carbon double bond. This approach was tested by grafting 3 types of thiols with different functional tail groups and can be readily used for any polyolefin coatings. Secondly, oxidative degradation of SI ROMP coatings in the organic solvent is a common problem resulting in a decrease in the film thickness due to polymer chain cleavage. We incorporated a custom designed crosslinker to the polynorbornene (pNB) coatings to prepare in situ crosslinked pNB coatings. This approach provides a crosslinked coating of pNB with significantly increased stability against organic solvents by decreasing the film loss from 73 % to 28 %. Lastly, a novel approach of making photo-responsive polymer by light mediated ROMP is demonstrated. Light mediated control over rate of polymerization is the key feature required for patterning surface with photoresponsive polymers. We achieved this goal by designing and synthesizing a monomer that effectively controls the activity of the catalyst by temporarily deactivating it on irradiation with UV 365 nm light and reactivating it back by irradiation with blue 455 nm light to resume the ROMP.

No description available.