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PNIPAM-Coated Brushy Beads and How They Collapse

Duinen, David van (2021)
PNIPAM-Coated Brushy Beads and How They Collapse.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00020041
Ph.D. Thesis, Primary publication, Publisher's Version

Abstract

Stimuli-responsive polymers are a group of powerful switchable materials with a broad range of applications. Induced by a stimulus, the properties of such polymers can change dramatically. One popular polymer is poly(N- isopropylacryamide) (PNIPAM). PNIPAM is interesting for a number of applications, including in technologies as sensors, actuators, microfluidics, and mineral retrieval, but also in biology, and in medicine. One powerful place to use them is in colloids. By themselves, colloids are a center point of interest, owing to the fact that their properties depend on their surfaces rather than on their bulk. Adding to this fact a stimuli-responsiveness will open up new possibilities for applications. What is missing to make full use of PNIPAM is a thorough understanding of its properties and how they respond to stimuli. Understanding PNIPAM requires understanding its response to stimuli. To this end, I investigated two of PNIPAM’s main stimuli: temperature and solvent. With temperature, PNIPAM mostly displays a simple collapse while above a certain temperature, the lower critical solution temperature (LCST). In some cases, however, PNIPAM has been reported to show a rather complex, not fully understood, two-stage collapse. With solvent, PNIPAM exhibits the co-non-solvency effect. PNIPAM swells well in either water or in alcohols, yet it collapses in intermediate mixtures. The exact cause of the co-non- solvency effect is still debated, however. The study of the effects of these two stimuli on PNIPAM-based colloids is not straightforward: many techniques are technologically quite involved, average over a multitude of colloids, or are invasive and change the response of the sampled colloids to stimuli. Hence, a better understanding of these two stimuli would be greatly benefited by the ability to study them in a simple, non-invasive manner. In this thesis, I present a new method to observe responses of PNIPAM-based colloids to stimuli. The method is optical, and is based on interference and microscopy. Specifically, I studied colloidal glass beads that were coated with an end-grafted PNIPAM brush (51 ± 3 nm thick) near a glass surface. The Brownian motion of such beads is dominated by the brush layer’s viscoelastic properties, which change as a response to stimuli. As a result, monitoring the Brownian motion through interference allows observing viscoelastic changes of the PNIPAM brush in a simple and non-invasive manner. Consequently, this method allowed me to study how various stimuli affected the PNIPAM brush coating. Taking temperature as a stimulus, I observed a two-stage collapse of the PNIPAM brush-coated beads. Upon increasing the temperature, I first ob- served a change at 36°C. This change was attributed to the LCST volume collapse of PNIPAM, which induced an increase of polymer brush density and subsequent increase of viscosity of the brush layer. Then, increasing temperature above 46°C induced a second transition. I attributed this second transition to the complete collapse of the brush layer. Upon this complete collapse, the brush layer became stiffer throughout, which made the Brownian motion more elastic. These results indicate that PNIPAM undergoes a type II-phase transition. The better understanding will play a role towards proper application of PNIPAM brush coatings. Furthermore, I investigated the co-non-solvency effect using the same method. For this effect, there exist a few hypotheses regarding the underlying cause. One leading hypothesis is based on the preferential binding of alcohol to PNIPAM, rather than water to PNIPAM. Through monitoring the viscoelastic changes in my experiments, I provide support for the theory of preferential binding. These viscoelastic responses to stimuli provide us with a better insight into responsive thin coatings. Being non-invasive, simple, flexible, repeatable, yet measuring single coated colloids in-situ, the optical method that I developed and described proved to be a useful tool. This method can be integrated into the standard set of techniques to investigate changes in stimuli-responsive colloids.

--- For the non-scientists --- Stimuli-responsive polymers are a group of powerful switchable materials with a broad range of applications. One popular responsive polymer is PNIPAM, or poly(N-isopropylacryamide) in full. It is interesting for a range of applications as a technology in sensors, actuators, microfluidics or for mineral retrieval but also as an in-body medicine carrier. What makes PNIPAM so interesting is that it responds to stimuli: changes of its environment. For example, changing the temperature can dramatically change the properties of PNIPAM. However, exactly why and how PNIPAM responds is not completely known. Therefore, before we can fully use its potential, we first need to understand PNIPAM better. There are several forms and shapes of polymers and of PNIPAM. I looked at PNIPAM brushes. Such brushes are basically like the brush of a broom, but very short; only about a hundred nanometer thick – a thousand times thinner than printer paper. Such brushes can be coatings. What is nice in the case of a PNIPAM brush coating, is that the coated surface becomes stimuli-responsive. I investigated such PNIPAM brush-coatings on small glass beads. To look at the response of these PNIPAM brush-coated beads, I exposed them to different temperatures. Through a special microscope set-up that I developed, I was able to study the response of the PNIPAM brush coating. The response of PNIPAM to temperature showed an interesting result. Most of the time, PNIPAM responds at a single temperature. In my experiments, I found that there are two distinct transitions, at two distinct temperatures. I could explain these results by looking at the thin layer of the PNIPAM brush and how it changed its thickness and stiffness. In a further study, I also looked at the influence of another stimulus: solvent, and these experiments support for one of the possible theories. All in all, we now understand thin responsive coatings a little bit better, which was made possible because of the new experimental optical method that I developed.

Item Type: Ph.D. Thesis
Erschienen: 2021
Creators: Duinen, David van
Type of entry: Primary publication
Title: PNIPAM-Coated Brushy Beads and How They Collapse
Language: English
Referees: Butt, Prof. Dr. Hans-Jürgen ; Klitzing, Prof. Dr. Regine von
Date: 2021
Place of Publication: Darmstadt
Collation: xx, 128 Seiten
Refereed: 16 December 2020
DOI: 10.26083/tuprints-00020041
URL / URN: https://tuprints.ulb.tu-darmstadt.de/20041
Abstract:

Stimuli-responsive polymers are a group of powerful switchable materials with a broad range of applications. Induced by a stimulus, the properties of such polymers can change dramatically. One popular polymer is poly(N- isopropylacryamide) (PNIPAM). PNIPAM is interesting for a number of applications, including in technologies as sensors, actuators, microfluidics, and mineral retrieval, but also in biology, and in medicine. One powerful place to use them is in colloids. By themselves, colloids are a center point of interest, owing to the fact that their properties depend on their surfaces rather than on their bulk. Adding to this fact a stimuli-responsiveness will open up new possibilities for applications. What is missing to make full use of PNIPAM is a thorough understanding of its properties and how they respond to stimuli. Understanding PNIPAM requires understanding its response to stimuli. To this end, I investigated two of PNIPAM’s main stimuli: temperature and solvent. With temperature, PNIPAM mostly displays a simple collapse while above a certain temperature, the lower critical solution temperature (LCST). In some cases, however, PNIPAM has been reported to show a rather complex, not fully understood, two-stage collapse. With solvent, PNIPAM exhibits the co-non-solvency effect. PNIPAM swells well in either water or in alcohols, yet it collapses in intermediate mixtures. The exact cause of the co-non- solvency effect is still debated, however. The study of the effects of these two stimuli on PNIPAM-based colloids is not straightforward: many techniques are technologically quite involved, average over a multitude of colloids, or are invasive and change the response of the sampled colloids to stimuli. Hence, a better understanding of these two stimuli would be greatly benefited by the ability to study them in a simple, non-invasive manner. In this thesis, I present a new method to observe responses of PNIPAM-based colloids to stimuli. The method is optical, and is based on interference and microscopy. Specifically, I studied colloidal glass beads that were coated with an end-grafted PNIPAM brush (51 ± 3 nm thick) near a glass surface. The Brownian motion of such beads is dominated by the brush layer’s viscoelastic properties, which change as a response to stimuli. As a result, monitoring the Brownian motion through interference allows observing viscoelastic changes of the PNIPAM brush in a simple and non-invasive manner. Consequently, this method allowed me to study how various stimuli affected the PNIPAM brush coating. Taking temperature as a stimulus, I observed a two-stage collapse of the PNIPAM brush-coated beads. Upon increasing the temperature, I first ob- served a change at 36°C. This change was attributed to the LCST volume collapse of PNIPAM, which induced an increase of polymer brush density and subsequent increase of viscosity of the brush layer. Then, increasing temperature above 46°C induced a second transition. I attributed this second transition to the complete collapse of the brush layer. Upon this complete collapse, the brush layer became stiffer throughout, which made the Brownian motion more elastic. These results indicate that PNIPAM undergoes a type II-phase transition. The better understanding will play a role towards proper application of PNIPAM brush coatings. Furthermore, I investigated the co-non-solvency effect using the same method. For this effect, there exist a few hypotheses regarding the underlying cause. One leading hypothesis is based on the preferential binding of alcohol to PNIPAM, rather than water to PNIPAM. Through monitoring the viscoelastic changes in my experiments, I provide support for the theory of preferential binding. These viscoelastic responses to stimuli provide us with a better insight into responsive thin coatings. Being non-invasive, simple, flexible, repeatable, yet measuring single coated colloids in-situ, the optical method that I developed and described proved to be a useful tool. This method can be integrated into the standard set of techniques to investigate changes in stimuli-responsive colloids.

--- For the non-scientists --- Stimuli-responsive polymers are a group of powerful switchable materials with a broad range of applications. One popular responsive polymer is PNIPAM, or poly(N-isopropylacryamide) in full. It is interesting for a range of applications as a technology in sensors, actuators, microfluidics or for mineral retrieval but also as an in-body medicine carrier. What makes PNIPAM so interesting is that it responds to stimuli: changes of its environment. For example, changing the temperature can dramatically change the properties of PNIPAM. However, exactly why and how PNIPAM responds is not completely known. Therefore, before we can fully use its potential, we first need to understand PNIPAM better. There are several forms and shapes of polymers and of PNIPAM. I looked at PNIPAM brushes. Such brushes are basically like the brush of a broom, but very short; only about a hundred nanometer thick – a thousand times thinner than printer paper. Such brushes can be coatings. What is nice in the case of a PNIPAM brush coating, is that the coated surface becomes stimuli-responsive. I investigated such PNIPAM brush-coatings on small glass beads. To look at the response of these PNIPAM brush-coated beads, I exposed them to different temperatures. Through a special microscope set-up that I developed, I was able to study the response of the PNIPAM brush coating. The response of PNIPAM to temperature showed an interesting result. Most of the time, PNIPAM responds at a single temperature. In my experiments, I found that there are two distinct transitions, at two distinct temperatures. I could explain these results by looking at the thin layer of the PNIPAM brush and how it changed its thickness and stiffness. In a further study, I also looked at the influence of another stimulus: solvent, and these experiments support for one of the possible theories. All in all, we now understand thin responsive coatings a little bit better, which was made possible because of the new experimental optical method that I developed.

Alternative Abstract:
Alternative abstract Language

’Stimuli-responsive’ Polymere sind eine Gruppe leistungsstarker schaltbarer Materialien mit einem breiten Anwendungsspektrum. Induziert durch einen Stimulus können sich die Eigenschaften solcher Polymere dramatisch verändern. Ein populäres Polymer ist Poly(N-isopropylacryamid) (PNIPAM). PNIPAM ist für eine Reihe von Anwendungen interessant, u.a. in Technologien wie Sensoren, Aktoren, Mikrofluidik und Mineraliengewinnung, aber auch in der Biologie oder in der Medizin. Ein wirkungsvoller Ort für ihre Verwendung sind Kolloide. Die Kolloide selbst stehen im Mittelpunkt des Interesses, da ihre Eigenschaften von ihrer Oberfläche statt von ihrem Volumen abhängen. Wenn man zu dieser Tatsache noch eine ’Stimuli-responsiveness’ hinzufügt, eröffnen sich neue Anwendungsmöglichkeiten. Was fehlt, um PNIPAM in vollem Umfang nutzen zu können, ist ein gründliches Verständnis von den Eigenschaften und der Art und Weise, wie PNIPAM auf Stimuli reagiert. Um PNIPAM zu verstehen, muss man die Reaktion auf Stimuli verstehen. Zu diesem Zweck untersuchte ich zwei der wichtigsten Stimuli von PNIPAM: Temperatur und Lösungsmittel. Bei der Temperatur zeigt PNIPAM meist einen einfachen Kollaps, während oberhalb einer bestimmten Temperatur, der unteren kritischen Lösungstemperatur (Englisch: ’lower critical solution temperature’, LCST), ein Kollaps stattfindet. In einigen Fällen wurde jedoch berichtet, dass PNIPAM einen ziemlich komplexen, nicht vollständig verstandenen, zweistufigen Kollaps zeigt. Mit Lösungsmittel zeigt PNIPAM den ’co-non-solvency’-effekt. PNIPAM quellt in Wasser oder in Alkoholen gut auf, kollabiert jedoch in Zwischenmischungen. Die genaue Ursache des ’co-non-solvency’-effekts wird jedoch immer noch diskutiert. Die Untersuchung der Auswirkungen dieser beiden Stimuli auf PNIPAM-basierte Kolloide ist nicht einfach: Viele Techniken sind technologisch aufwendig, über eine Vielzahl von Kolloiden gemittelt, oder sie sind invasiv und verändern die Reaktion der untersuchten Kolloide auf Stimuli. Daher wäre ein besseres Verständnis dieser beiden Stimuli durch die Möglichkeit, sie auf einfache, nicht-invasive Weise zu untersuchen, von großem Vorteil. In dieser Arbeit stelle ich eine neue Methode zur Beobachtung der Reaktionen von PNIPAM-basierten Kolloiden auf Stimuli vor. Die Methode ist optisch und basiert auf Interferenz und Mikroskopie. Insbesondere untersuchte ich kolloidale Glasmurmeln, die in der Nähe einer Glasoberfläche mit einer ’end-grafted’ PNIPAM-Bürste (51 ± 3 nm dick) beschichtet wurden. Die Brownsche Bewegung solcher Murmeln wird durch die viskoelastischen Eigenschaften der Bürstenschicht dominiert, die sich als Reaktion auf Stimuli verändern. Infolgedessen ermöglicht die Überwachung der Brownschen Bewegung durch Interferenz die Beobachtung viskoelastischer Veränderungen der PNIPAM- Bürste auf einfache und nicht-invasive Weise. Folglich konnte ich mit dieser Methode untersuchen, wie sich verschiedene Stimuli auf die Beschichtung der PNIPAM-Bürste auswirkten. Bei der Verwendung der Temperatur als Stimulus beobachtete ich einen zweistufigen Kollaps der PNIPAM-Murmeln mit Bürstenbeschichtung. Beim Erhöhen der Temperatur beobachtete ich zunächst eine Veränderung bei 36°C. Diese Änderung wurde dem LCST-Volumenkollaps von PNIPAM zugeschrieben, der einen Anstieg der Polymer-Bürstendichte und eine an- schließende Erhöhung der Viskosität der Bürstenschicht bewirkte. Dann induzierte ein Temperaturanstieg über 46°C einen zweiten Übergang. Ich führte diesen zweiten Übergang auf den vollständigen Zusammenbruch der Bürstenschicht zurück. Nach diesem vollständigen Zusammenbruch wurde die Bürstenschicht durchgehend steifer, wodurch die Brownsche Bewegung elastischer wurde. Diese Ergebnisse weisen darauf hin, dass PNIPAM einen Typ-II-Phasenübergang durchläuft. Das bessere Verständnis wird eine Rolle bei der korrekten Anwendung von PNIPAM-Bürstenbeschichtungen spielen. Außerdem untersuchte ich mit der gleichen Methode den ’co-non-solvency’- Effekt. Für diesen Effekt gibt es einige Hypothesen bezüglich der zugrunde liegenden Ursache. Eine führende Hypothese basiert auf der präferenziellen Bindung von Alkohol an PNIPAM anstelle von Wasser an PNIPAM. Indem ich die viskoelastischen Veränderungen in meinen Experimenten beobachte, unterstütze ich die Theorie der präferenziellen Bindung. Diese viskoelastischen Reaktionen auf Stimuli geben uns einen besseren Einblick in ’Stimuli-responsive’ dünne Polymerschichten. Da die von mir entwickelte und beschriebene optische Methode nicht invasiv, einfach, flexibel und wiederholbar ist und dennoch einzelne beschichtete Kolloide in-situ misst, erwies sie sich als nützliches Werkzeug. Diese Methode kann in den Standardsatz von Techniken integriert werden, um Veränderungen bei auf ’Stimuli-responsive’ Kolloiden zu untersuchen. Der englische Text wurde mit Hilfe von DeepL (Link) ins Deutsche übersetzt. Anschließend habe ich die Übersetzung überprüft.

German
Status: Publisher's Version
URN: urn:nbn:de:tuda-tuprints-200413
Classification DDC: 500 Science and mathematics > 530 Physics
Divisions: 05 Department of Physics
05 Department of Physics > Institute for Condensed Matter Physics
05 Department of Physics > Institute for Condensed Matter Physics > Soft Matter at Interfaces (SMI)
Date Deposited: 15 Dec 2021 13:40
Last Modified: 16 Dec 2021 12:22
PPN:
Referees: Butt, Prof. Dr. Hans-Jürgen ; Klitzing, Prof. Dr. Regine von
Refereed / Verteidigung / mdl. Prüfung: 16 December 2020
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