No description available.

Tethered polymer layers at solid-fluid interfaces are used widely in a variety of surface science applications. Although many of these applications require exposure to dynamic flow conditions, flow field penetration into densely grafted polymer brushes, is still a question open to debate despite the fact that it is a fundamental process crucial to mass transport through these polymer films. Although most theoretical work has indicated flow field penetration into polymer films, with varying predicted penetration depths predicted, the limited experimental attempts to investigate this phenomenon have resulted in inconsistent conclusions due to lack of a proper analytical method. To help resolve this controversy, in this Dissertation, a new spectroscopic method, FRET-TIRFM, based on a combination of Förster resonance energy transfer (FRET) and total internal reflectance fluorescence microscopy (TIRFM), is developed to provide the first direct, quantitative measurements on flow field penetration by measuring linear diffusion coefficients of small molecules through densely grafted, thin poly(N-isopropylacryl-amide) (pNIPAM) films. Decay curves from FRET of the acceptor with a donor covalently attached at the substrate surface were fit to a combined Taylor-Aris-Fickian diffusion model to obtain apparent linear diffusion coefficients of the acceptor molecules for different flow rates. These values can then be used to obtain quantitative estimates of flow field penetration depths. For a pNIPAM surface of 110 nm dry thickness, with a 0.6 chain/nm² grafting density, apparent diffusion coefficients ranging from 1.9-9.1 × 10-12 cm²/s were observed for flow rates ranging from 100 to 3000 μL/min. This increase in apparent diffusion coefficient with applied fluid flow rate is indicative of flow field penetration of the polymer film. The depth of penetration of the flow field is estimated to range from ~6% of the polymer film thickness to ~57% of the film thickness in going from 100 to 3000 μL/min flow rate of a good solvent. Factors other than flow rate that may impact flow field penetration were also investigated using this new FRET-TIRFM method. Solvent quality and polymer brush grafting density are the two most important parameters due to the fact that they influence changes in tethered polymer chain conformation. This work demonstrates that polymer films are most penetrable in a good solvent and least penetrable in a poor solvent under identical flow conditions. These findings are consistent with polymer chain conformational changes going from extended brushes to compact globules. For flow rates ranging from 100 to 3000 μL/min, flow field penetration depth ranges from ~6% of the film thickness to ~57% of film thickness for a good solvent compared to ~4% to ~19% for a poor solvent. Thus, by simply changing solvent quality from good to poor, flow field penetration decreases by about 38%. Grafting density has a less pronounced effect than solvent quality on penetration depth, probably due to the small range of grafting densities chosen for study. However, a roughly 10-20% difference in penetration depth was observed between high density (0.60 chain/nm²) and low density (0.27 chain/nm²) pNIPAM surfaces in the same solvent. Changes in grafting density have a less significant impact in a good solvent compared to a poor solvent. This is most likely caused by the fact that grafting density impacts polymer chain conformation mainly through polymer-polymer repulsion, which becomes less significant in a solvent that better solvates the polymer. For the two extreme cases studied here at flow rates ranging from 100 to 3000 μL/min, the penetration depth is estimated to range from ~4-19% of the original solvent-swollen film thickness for high density pNIPAM films in a poor solvent and from ~7-67% for low density films in a good solvent. One important assumption that underlies all of this work is that the dominant mass transport mechanism for small molecules in dense polymer brushes is diffusion. This assumption was further validated through the use of two different small molecule quenchers, RhB and 2-nitrobenzylalcohol. These molecules are significantly different in size, charge, and structure, and operate by different quenching mechanisms. Despite these differences, the results for flow field penetration are statistically the same for both. These observations validate the assumption of diffusive mass transport in these films.

Cette thèse rapporte le greffage chimique de brosses de polymères neutres de poly(acrylate de tert-butyle) (PtBA) et de brosses chargées d’acide polyacrylique (PAA) sur des substrats de mica afin d’étudier leur conformation en fonction de la densité de greffage, du pH et de la force ionique. Le greffage est réalisé par polymérisation contrôlée par transfert d’atome (ATRP) initiée depuis la surface de mica afin de contrôler la croissance du polymère et sa densité de greffage. L’étude de la conformation des brosses de PtBA et de PAA a été menée avec la technique AFM en mesurant les épaisseurs des films à sec et gonflés sous différentes conditions de solvant, de pH et de force ionique. Une monocouche d’amorceurs est tout d’abord greffée sur du mica porteur de groupes hydroxyles créés par plasma (Ar/H2O). Cette couche a été caractérisée par des mesures d’angle de contact et par la technique TOF-SIMS. L’amorceur greffé a ensuite permis d’initier l’ATRP directement depuis la surface pour former des brosses neutres de PtBA liés de façon covalente au mica. La croissance linéaire de l’épaisseur du film avec la masse molaire du polymère en solution et le taux de conversion montre que la polymérisation est contrôlée. De plus, la ré-initiation des chaînes greffées atteste du caractère vivant de la polymérisation. L’hydrolyse des brosses de PtBA, confirmée par des mesures d’angle de contact, d’épaisseur et par FT-IR, conduit à des brosses de PAA. Les différentes couches greffées sont stables à l’air, en milieu organique et en milieu aqueux et leur gonflement est réversible. Le degreffage de la couche de PAA est observé suite à une longue exposition à pH basique. Cette étude représente le premier exemple de brosses greffées chimiquement sur du mica par polymérisation initiée depuis la surface. La variation des paramètres de la réaction de greffage de l’amorceur, tels que la concentration et la durée de réaction, a permis de contrôler le taux de recouvrement de l’amorceur et la densité de greffage du polymère. Une grande gamme de taux de recouvrement de l’amorceur est accessible et se traduit par un intervalle de densités de greffage allant de faibles à élevées (e.g. 0,04 chaîne/nm2 à 0,5 chaîne/nm2). L’étude de la conformation des chaînes de PtBA dans le DMF montre que cet intervalle de densités recouvre le régime crêpe au régime brosse. Le gonflement de brosses de PAA et la variation de la hauteur de la brosse L ont été étudiés en fonction de la densité de greffage, du pH et du sel ajouté cs (NaCl). Une transition brusque de collapsée à étirée est observée avec l’augmentation du pH, indépendamment de la densité de greffage. A pH neutre, les brosses sont collapsées et se comportent comme des brosses neutres en mauvais solvant. A pH basique, les brosses sont gonflées et chargées et se trouvent dans un régime de Pincus caractéristique des polyélectrolytes forts. En présence de sel, les charges sont partiellement écrantées et les répulsions électrostatiques dominent toujours dans la brosse. Cette étude contribue à une meilleure compréhension du comportement complexe des brosses de polyélectrolytes faibles et apporte un soutien expérimental à la théorie sur le comportement de ces brosses. / Neutral polymer brushes of poly(tert-butyl acrylate) (PtBA) and polyelectrolyte brushes of poly(acrylic acid) (PAA) were chemically grafted on mica in order to study their conformation as a function of grafting density, pH and ionic strength. The brushes were prepared via surface-initiated atom-transfer radical polymerization (SI-ATRP). This approach allows the control of the polymer growth and of the polymer grafting density. The study of brush conformation was investigated by measuring the dry and the swollen film thicknesses by Atomic Force microscopy (AFM) under various environmental conditions. Mica substrates were activated by a plasma method leading to OH-functionalized surfaces to which a radical initiator was covalently bound using standard siloxane protocols. The initiator grafting on the mica substrate was confirmed by water contact angle measurements and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The covalently immobilized initiator successfully underwent SI-ATRP with tert-butyl acrylate, yielding end-grafted PtBA brushes that are covalently linked to mica. A linear relationship of the polymer thickness with respect to the molecular weight of the free polymer and with respect to the monomer conversion is obtained confirming that SI-ATRP is well controlled. The PtBA brushes were then hydrolyzed to PAA as confirmed by water contact angle measurements, polymer thickness measurements and FT-IR. The grafted layers show a good stability in air, organic solvent and aqueous solution and a reversible swelling. Film cleavage is observed for long exposure time in basic solution. This study represents the first example of chemically grafted brushes on mica via SI-ATRP. The polymer grafting density of poly(tert-butyl acrylate) was controlled by tuning the initiator surface coverage. By adjusting parameters such as the immobilization reaction time and concentration of the initiator, a wide range of initiator surface coverages and hence polymer densities on mica were possible ranging from 0.04 chain/nm2 to 0.5 chain/nm2). The swelling behavior of PAA brushes was investigated as a function of the polymer grafting density, pH and ionic strength cs (NaCl). A sharp transition from collapsed to stretched conformation was found with increasing pH. At neutral pH, the brushes are collapsed and behave like neutral brushes in poor solvent. At high pH, the brushes are swollen and charged and belong to the Pincus regime. In the presence of salt, the Coulombic repulsions are screened. The observed behavior of PAA brushes resembles the one of strong polyelectrolyte brushes. This study contributes to a better understanding of the complex behaviour of weak polyelectrolyte brushes and brings experimental support to theoretical studies.

No description available.

Here, we report on a new temperature responsive polymer brush system with a terminal “click” functionality. Bifunctionalized poly(N-isopropylacrylamide) (PNiPAAm) with distinct functional end groups was synthesized by atom transfer radical polymerization (ATRP) and grafted to a modified silicon substrate. The presence of the active terminal alkyne functionality is validated using an azide-modified rhodamine B (N3-RhB) via copper(I) catalyzed alkyne–azide cycloaddition (CuAAC). The optical properties and swelling dynamics of an N3-RhB modified PNiPAAm brush are analyzed in dry state and in situ by VIS-spectroscopic ellipsometry (SE). The best-fit results are obtained using a Gaussian oscillator model and are confirmed by UV/VIS-spectroscopy. We observed evidence of interactions between the aromatic residues of the dye and the PNiPAAm amide groups, which significantly affect the swelling behavior of the modified polymer brush. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

Here, we report on a new temperature responsive polymer brush system with a terminal “click” functionality. Bifunctionalized poly(N-isopropylacrylamide) (PNiPAAm) with distinct functional end groups was synthesized by atom transfer radical polymerization (ATRP) and grafted to a modified silicon substrate. The presence of the active terminal alkyne functionality is validated using an azide-modified rhodamine B (N3-RhB) via copper(I) catalyzed alkyne–azide cycloaddition (CuAAC). The optical properties and swelling dynamics of an N3-RhB modified PNiPAAm brush are analyzed in dry state and in situ by VIS-spectroscopic ellipsometry (SE). The best-fit results are obtained using a Gaussian oscillator model and are confirmed by UV/VIS-spectroscopy. We observed evidence of interactions between the aromatic residues of the dye and the PNiPAAm amide groups, which significantly affect the swelling behavior of the modified polymer brush. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.