Probing carbonyl–water hydrogen-bond interactions in thin polyoxazoline brushes

Temperature-responsive oxazoline-based polymer brushes have gained increased attention as biocompatible surfaces. In aqueous environment, they can be tuned between hydrophilic and hydrophobic behavior triggered by a temperature stimulus. This transition is connected with changes in molecule–solvent interactions and results in a switching of the brushes between swollen and collapsed states. This work studies the temperature-dependent interactions between poly(2-oxazoline) brushes and water. In detail, thermoresponsive poly(2-cyclopropyl-2-oxazoline), nonresponsive hydrophilic poly(2-methyl-2-oxazoline), as well as a copolymer of the two were investigated with in situinfrared ellipsometry. Focus was put on interactions of the brushes’ carbonyl groups with water molecules. Different polymer–water interactions could be observed and assigned to hydrogen bonding between C=O groups and water molecules. The switching behavior of the brushes in the range of 20–45°C was identified by frequency shifts and intensity changes of the amide I band

Probing carbonyl–water hydrogen-bond interactions in thin polyoxazoline brushes

Temperature-responsive oxazoline-based polymer brushes have gained increased attention as biocompatible surfaces. In aqueous environment, they can be tuned between hydrophilic and hydrophobic behavior triggered by a temperature stimulus. This transition is connected with changes in molecule–solvent interactions and results in a switching of the brushes between swollen and collapsed states. This work studies the temperature-dependent interactions between poly(2-oxazoline) brushes and water. In detail, thermoresponsive poly(2-cyclopropyl-2-oxazoline), nonresponsive hydrophilic poly(2-methyl-2-oxazoline), as well as a copolymer of the two were investigated with in situinfrared ellipsometry. Focus was put on interactions of the brushes’ carbonyl groups with water molecules. Different polymer–water interactions could be observed and assigned to hydrogen bonding between C=O groups and water molecules. The switching behavior of the brushes in the range of 20–45°C was identified by frequency shifts and intensity changes of the amide I band

Probing carbonyl-water hydrogen-bond interactions in thin polyoxazoline brushes

Temperature-responsive oxazoline-based polymer brushes have gained increased attention as biocompatible surfaces. In aqueous environment, they can be tuned between hydrophilic and hydrophobic behavior triggered by a temperature stimulus. This transition is connected with changes in molecule–solvent interactions and results in a switching of the brushes between swollen and collapsed states. This work studies the temperature-dependent interactions between poly(2-oxazoline) brushes and water. In detail, thermoresponsive poly(2-cyclopropyl-2-oxazoline), nonresponsive hydrophilic poly(2-methyl-2-oxazoline), as well as a copolymer of the two were investigated with in situ infrared ellipsometry. Focus was put on interactions of the brushes' carbonyl groups with water molecules. Different polymer–water interactions could be observed and assigned to hydrogen bonding between C=O groups and water molecules. The switching behavior of the brushes in the range of 20–45 °C was identified by frequency shifts and intensity changes of the amide I band

Infrarotellipsometrie an gemischten funktionalen Polymerbürsten zur Kontrolle von Oberflächeneigenschaften

The present work deals with the optical characterization of temperature-responsive polymer brushes. It is part of a DFG-funded project to study responsive surfaces that aims to use these for substrates in tissue engineering. The used polymers are poly(2-cyclopropyl-2-oxazoline) [PcPrOx] and the well-studied poly(N-isopropylacrylamide) [PNIPAAm], that both exhibit a lower critical solution temperature (LCST). Focus of this work was put on the examination of the temperature-responsive behavior of these brush systems in aqueous environment. With the use of in situ infrared ellipsometry (IR-SE) it was possible to probe the solid–liquid interface and to correlate the measured vibrational bands with interactions between polymer and water, that take place during the transition within the brush. The method is very sensitive to changes at the interface as well as to the adsorption of organic molecules even at submonolayer thickness. This is a special advantage for experiments of protein adsorption or to study anti-fouling properties. POx are a class of polymers that have become of increased interest for biomedical applications in the past years. Their temperature-responsive behavior was very interesting for the project in comparison to PNIPAAm, due to their similar structure to acrylamides but without the presence of an N–H group. PcPrOx was studied both as pure polymer brush as well as in form of a copolymer with the hydrophilic methyl-2-oxazoline [MeOx]. Results show that the POx brushes display a transition from a swollen to a collapsed state in water, that expands over a wide temperature range. This stands in contrast to the behavior of POx in solution and to that of PNIPAAm both in solution and as a thin film or brush. Identification of the different components in the C=O band as well as simulations showed, that during the collapse of the brushes the amount of polymer–water interactions decreases. However, complementary in situ VIS ellipsometry measurements indicate that the water molecules diffuse more slowly out of the brush than for example in PNIPAAm brushes. This is probably caused by the inability of the brush to form hydrogen bonds between the polymer chains in form of N–H∙∙∙O=C, as well as by the higher hydrophilicity of the copolymer. PNIPAAm was used in the form of a block-copolymer with the anchoring polymer poly(glycidyl methacrylate) [PGMA] and the resulting brushes were compared with traditionally prepared PNIPAAm brushes. It turned out that due to the presence of a cross-linked PGMA network the mobility of the PNIPAAm chains is strongly limited. As a result, the PNIPAAm-b-PGMA brushes hardly swell in water. A more detailed analysis of the in situ IR-SE spectra showed that there are still changes of interactions taking place within the brush. These are less pronounced compared to the traditional PNIPAAm brushes and stretched over a wider temperature range, leading to the conclusion that during the transition the interactions with water decrease but the water molecules hardly diffuse out of the brush. Experiments of protein adsorption of fibrinogen (FIB) showed a protein-resistant behavior of the PNIPAAm-b-PGMA brushes both above and below the LCST. We concluded that the surface of the brushes is dominated by PNIPAAm chains, with the result that no attractive interactions between PGMA and FIB are possible.Die vorliegende Arbeit beschäftigt sich mit der optischen Charakterisierung von temperaturschaltbaren Polymerbürsten. Sie ist Teil eines DFG-geförderten Projektes zur Untersuchung von responsiven Oberflächen mit dem Ziel, diese als Substrate für Zellwachstum einsetzen zu können. Die verwendeten Polymere sind Poly(2-cyclopropyl-2-oxazolin) [PcPrOx] und das ausgiebig untersuchte Poly(N-isopropylacrylamid) [PNIPAAm], welche eine untere kritische Lösungstemperatur (LCST) aufweisen. Der Fokus der Arbeit lag auf der Untersuchung der temperaturabhängigen Eigenschaften der Bürstensysteme in wässriger Umgebung. Mittels in situ Infrarotellipsometrie (IR-SE) war es möglich, die fest–flüssig Grenzfläche zu untersuchen und die gemessenen Schwingungsbanden den Wechselwirkungen zwischen Polymer und Wasser zuzuordnen, die während des Schaltvorgangs der Bürsten auftreten. Die Methode ist sehr empfindlich für Änderungen an der Grenzfläche sowie für die Adsorption organischer Moleküle, sogar bei submolekularen Schichten. Dies ist ein besonderer Vorteil in Experimenten zur Proteinadsorption oder zu Antifouling-Eigenschaften. POx sind eine Klasse von Polymeren, die in den letzten Jahren vermehrt in den Fokus für biomedizinische Anwendungen gerückt sind. Durch ihre ähnliche Struktur zu Acrylamiden, jedoch der fehlenden N–H Gruppe, war ihr Verhalten im Vergleich zu dem von PNIPAAm für das Projekt sehr interessant. PcPrOx wurde sowohl als reine Polymerbürste als auch in Form von einem Copolymer mit dem hydrophilen Methyl-2-oxazolin [MeOx] untersucht. Die Ergebnisse zeigen, dass die POx Bürsten einen Übergang vom geschwollenen zum kollabierten Zustand aufweisen, der sich über einen weiten Temperaturbereich erstreckt. Dies steht im Gegensatz zu dem Verhalten von POx in Lösung und dem von PNIPAAm. Identifikation der verschiedenen Komponenten der C=O Bande sowie Simulationen konnten zeigen, dass sich beim Kollaps der Bürsten die Polymer–Wasser Wechselwirkungen verringern. Komplementäre in situ VIS Ellipsometrie Messungen deuten jedoch darauf hin, dass die Wassermoleküle langsamer aus der Bürste diffundieren als z.B. in PNIPAAm. Ursache ist wahrscheinlich die fehlende Möglichkeit, Wasserstoffbrücken zwischen den Polymerketten (N–H∙∙∙O=C) zu Bilden, sowie im Copolymer die erhöhte Hydrophilizität. PNIPAAm wurde als Block-Copolymer mit dem Anker-Polymer Poly(glycidyl methacrylat) [PGMA] kombiniert und die daraus erhaltenen Bürsten mit klassisch angebundenen PNIPAAm Bürsten verglichen. Es stellte sich heraus, dass aufgrund des vorliegenden PGMA Netzwerkes die Mobilität der PNIPAAm Ketten stark eingeschränkt ist. Als Folge schwellen die PNIPAAm-b-PGMA Bürsten kaum in Wasser an. Eine genauere Analyse der in situ IR-SE Spektren zeigte, dass dennoch in der Bürste deutliche Änderungen der Wechselwirkungen stattfinden. Diese sind gegenüber der klassischen Bürste verringert und auf einen weiten Temperaturbereich ausgedehnt und zeugen von einer Reduktion der Wechselwirkungen mit Wasser aber kaum Diffusion der Wassermoleküle aus der Schicht heraus. Experimente zur Proteinadsorption mit Fibrinogen (FIB) zeigten eine Proteinresistenz sowohl unter als auch über der LCST. Daraus wurde geschlossen, dass die Oberfläche der Bürsten von PNIPAAm Ketten dominiert ist, sodass keine attraktiven Wechselwirkungen zwischen PGMA und FIB möglich sind.DFG, Hi 493/7-1, World Materials Network/Switchable polymer interfaces for bottom-up stimulation of mammalian cell