IN-SITU SPECTROSCOPIC ELLIPSOMETRY FOR STUDIES OF THIN FILMS AND MEMBRANES
Promotion date: March 21.
Promotor: Prof.dr.-ing. Matthias Wessling
Assistant Promotor: Dr.ir. Nieck Benes
The properties of a thin polymer film can be significantly affected by the presence of a penetrant. It is also known that behavior of ultra-thin polymer films (<100 nm) may deviate from bulk behavior.
This sole impact of film thickness reduction is often referred to as a nano-confinement effect. Superposition of the penetrant and the nano-confinement can have potential implications for many technological applications, such as protective and functional coatings, sensors, microelectronics, surface modification and membrane separations.
Particularly in dense, non-porous membranes - used for gas separation, water desalination or organic solvent purification - the drive to use ultra-thin polymer films is related to maximization of process efficiency. The thinner the films are, the faster the transport of a given medium is.
However, such membranes are known to suffer from undesired slow changes in their properties over time, such as plasticization or physical aging.
In-situ ellipsometry is a powerful optical technique for the characterization of ultra-thin films in contact with penetrants, due to its high precision and non-invasive character. This thesis explores the applicability of the technique to study fundamentals of various physical phenomena occurring in thin and ultra-thin polymer films in the presence of interacting penetrants. The investigated macromolecular systems include model glassy and rubbery structures, polystyrene and poly(dimethyl siloxane) respectively, as well as zwitterionic thin films and complex composite membranes. Penetrants range from water, aqueous salt solutions and liquid organic solvents, to high pressure fluids.
What was the main characteristic of your work?
The multidisciplinary aspects were very interesting. Spectroscopic ellipsometry is a well-established technique in, for instance, semiconductor research. However, it is very suitable to study the physics of light reflection, transmission and absorption in thin inorganic films as well. We felt that the potential of the technique has not been fully explored yet in the in-situ investigations of swollen organic films and membranes. In particular because modern ellipsometry instruments - in addition to their high precision and non-invasive character - allow for vary fast, continuous measurements. These features make analysis of fast diffusion phenomena, which often occur in very thin films, possible.
Polymer physics, membrane technology and optical techniques represent the three worlds I had to manage and merge in this project. Optical modeling of the applied physical systems in various ambient conditions took a lot of work, starting almost from scratch. After measurements took place, it often took months to make sense out of the vast amount of data and unexpected results I was confronted with. This also was an unexplored area of research at the time.
I am proud of bringing ellipsometry more within reach of polymer science, expanding its horizon in general, applying this technique to membranes science more specifically. I think this technique has a lot of potential in future applications, as one is able to study the membranes during operation. This line of research will continue, I am sure. The technique is very versatile and, therefore, applicable to various kinds of film materials.
The combination of fundamental topics and experimental techniques suits me very well. In a post-doc project I will continue to work on in-situ ellipsometry even further, trying to find application domains within hybrid, partly inorganic, materials as well as looking for new material properties. I am sure it will work, as there are some promising preliminary results obtained already.
Were you able to publish some nice articles?
In total twelve papers appeared, and a few are still to come. Eight of them are directly related to the PhD thesis, and the others originated from cooperation with other researchers. Articles appeared for example in: Journal of Membrane Science, Polymer, Macromolecular Chemistry and Physics, ACS Applied Materials & Interfaces and Analytical Chemistry. The journal titles represent the multidisciplinary character of my work, which matches modern science, bringing together different worlds and fields of expertise.
Apart from these publications, I was happy to win some presentation prizes: one during the Euromembrane Conference in London, and one on the Spectroscopic Ellipsometry Workshop in Leipzig. I also had a chance to present my findings to many colleagues during the Mesa+ Day at the Twente stadium.
What are your future plans?
To stay in academics or to move to industrial research, is still an open question for me. I found the teaching part of the PhD project very inspiring. It was really easy to motivate master and bachelor students, introducing them to ellipsometry techniques. Some of the students contributed a great deal to the contents of my project as well, and I had some very positive feedback. I like to transfer my knowledge and passion of research, and see how the students now are thinking independently on the matters involved here. On the other hand, as teaching positions are scarce, working in industry is still an option to consider carefully.
Did you feel part of the Mesa+ community during your PhD period?
Most of my work was centered within the Membrane Technology Group and Inorganic Membranes Group. Special thanks I owe to Mesa+ expert Dr. ir. Herbert Wormeester. He contributed a lot to my understanding of the optics and physics involved in ellipsometry. The atmosphere at Mesa+ is very open. Colleagues are willing to share their knowledge and expertise. Also the interactions with numerous guests visiting the Institute, I perceived as very motivating. I met a lot of expert people eager to talk about their work.