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
