Dual thermo-responsive diblock copolymer thin films


... and other nerdy things



That is the title of my PhD thesis. Sounds fancy, right? But do you know instantly what it is about? And why it might be relevant and contributing to our society?

This section aims to answers these questions in an accessible and comprehensible way. I will try to convey the basic principles of polymer physics without using too much of scientific gobbledygook, which is found in many publications.

And what's more, I want to write about the things that are not published in my papers. Actually, they are quite important, especially for young researchers. I want to tell about the challenges, the failures, the frustration, and how we (as a team) finally overcome these obstacles and succeeded. 

I want to speak about the daily life of a scientist and not just refer to the final publication, which is condensed on 6 pages. To me, this is important because it enables a different and more personal perspective on science and research. It might help to reduce fears and prejudices and to strengthen the trust in scientific work. 

To achieve this, two things are required: transparency and dialogue. I will describe my work as transparent as possible, while I highly appreciate any exchange, be it questions, remarks, criticism, praise, or a friendly hello. 

That's a lot to get under one roof, but I think this way, you and I will have a lot of fun. 😊 
Day 1 as a PhD student
Lord of the rings meme showing king Theodin and his men facing an orc army at Helm's Deep. It starts to rain, the first orc gets killed and Theodin states his famous words: And so it begins.
I started my PhD in November 2016 at the Chair of Functional Materials at the Technical University of Munich. The focus of this chair is the investigation of responsive and functional polymer films and how the nanoscopic properties, meaning the structure and dynamics of these films on a nano (or molecular) scale, influence their macroscopic behavior (thickness, stiffness, conductivity, absorption behavior, etc.). And only a few hours in, I already got my first hard deadline: Samples had to be ready in two days for experiments at the neutron research reactor FRM II (it's also at the TUM campus, so at least no traveling was involved).

As a trained chemist, I had zero experience in preparing polymer film samples (quite challenging for my polymer systems), I didn't know too much about neutron scattering techniques, and I knew nothing about how challenging a neutron beamtime can be. But still, I had everything I needed for these first tasks: caring and supportive colleagues! And that was already the first lesson I have been taught. No matter how big the challenge is, as a team you can do it.

In the end, it couldn't have started any better. Despite all the failures and frustration during the first days/weeks, these neutron experiments were the basis for my first posters, talks, and publications. Regarding the fact that someone else has written the proposal for the beamtime (who left the chair before doing these experiments), this was pure luck. And that is the second lesson, I have been taught, even though I didn't realize it back then: There is a lot of luck involved in your research. Being at the right location at the right time is plain luck. Or if you want to see it from another perspective: If something doesn't work out as planned, it might not be because of you, it might be because of bad luck (still sucks though).

So, let's talk about some science: As I mentioned earlier, I investigated polymers during my PhD. To be a bit more specific, I investigated diblock copolymers (DBC), which simply means a linear polymer (a long chain of repetitive subunits called monomers) that consists of two different blocks. Figure 1a gives you an impression how these things look like. This polymer system didn't change too much during my PhD, so let me explain what's so special about it (if you are interested in my papers, you can skip these explanations). One of the two polymer blocks (the black one) is a polysulfobetaine. I have written a Wikipedia article about this polymer class, which you can find here.  Polysulfobetaines (PSBs) are zwitterionic polymers, meaning they have some charged groups, while the overall charge is zero. The PSBs I used, have a negatively charged sulfonate group and a positively charged amine group. Figure 1b shows the chemical structure and where the charged groups are located. PSBs are quite useful and by now they are commercially applied as anti-fouling surfaces, ultrafiltration membranes, blood-contacting devices, and drug delivery materials. 
The figure has three subfigures a, b, and c. A shows the exemplified chemical structure of a diblock copolymer. B and C show the chemical structure of a poly(sulfobetaine) and PNIPMAM, respectively. Figure by Lucas Kreuzer
Figure 1: a) schematic structure of PSB-b-PNIPMAM with exemplary block lengths of 15 and 39, respectively. The chemical structures of a b) PSB and c) PNIPMAM.
But what makes them really interesting for us, is their special solution behavior: While almost all PSBs are insoluble in water at low temperatures (low means room temperature or below), many of them (for example the PSB shown in Figure 1b) become soluble at higher temperature. This is due to the formation of inner salts by the charged groups. As a result, attractive and repulsive interactions are present between and within the individual polymer chains. Eventually, this leads to a hybrid behavior combining features of ionic and nonionic hydrophilicity. This is exemplarily sketched in Figure 2.
The figure highlights the presence of attractive and repulsive inter- and intramolecular interactions of poly(sulfobetaine) in aqueous solution. Figure by Lucas Kreuzer.
Figure 2: Attractive and repulsive polymer-polymer and polymer-water interactions present in PSBs.
Thus, PSBs are thermo-responsive, since their solution behavior is dependent on the ambient temperature.  Upon cooling, the polymer chains undergo a phase transition, and transform from an expanded coil to a collapsed globule state. The exact temperature, at which the phase transition occurs is dependent on the polymer concentration and usually is called the clearing point CP'. This will be important later on, since, in polymer films, the polymer concentration is much higher than for example, in an aqueous solution. The highest possible value of CP' is called upper critical solution temperature (UCST). Thus, sometimes these polymers and the phase transitions are called UCST-type polymers and UCST-type phase transitions, respectively. Figure 3a displays the schematic phase diagrams of such UCST-type polymers.
The Figure shows exemplary phase diagrams of polymers with a lower critical solution temperature and an upper critical solution temperature. Figure by Lucas Kreuzer
Figure 3: a) UCST- and (b) LCST-type polymers feature a soluble and insoluble phase. The arrows mark the concentration-dependent transitions between the soluble and insoluble phase. CP' and CP indicate the clearing and cloud point. respectively.
In addition to temperature, PSBs are responsive to the ionic strength (or salt concentration) of the solution, which is also due to the charged groups. Thus, upon increasing electrolyte concentration, the clearing point CP' decreases. Interestingly, for some PSB architectures, this effect turned out to be non-monotonic: Besides a monotonic decrease of the CP'  with added salt amount,  it was also observed that the CP' first increases at very salt concentrations, passes through a maximum, and subsequently decreases at higher salt concentrations. And besides temperature and ionic strength, PSBs also feature a sensitivity (I would not call it a responsive behavior) towards the different water isotopes H2O and D2O. Thus, the CP' of some PSBs varies in H2O and D2O. 

Summarized, PSBs are highly interesting polymers, which are responsive/sensitive to a lot of different environmental parameters. In combination with their great biocompatibility and chemical stability, there is great potential for applying them as responsive or functional surfaces, sensors, switches, artificial muscles or pumps, or as drug delivery agents.
But let's not forget the other polymer block within the DBC, which is a poly(N-isopropylmethacrylamide) or short PNIPMAM (its chemical structure is shown in Figure 1c). In contrast to the PSBs, it is not charged and therefore, is called a nonionic polymer block. Its solution behavior is also dependent on the temperature, thus, PNIPMAM is also a thermo-responsive polymer block. However, compared to the PSBs, it features the opposite solution behavior as it undergoes a coil-to-globule phase transition upon heating. This phase transition is also concentration-dependent and the respective temperature, at which this transition occurs is called cloud point (CP). Accordingly, there exists a lower critical solution temperature (LCST) and thus, these polymers are called LCST-type polymers. Figure 3b displays the respective phase diagram.

At this point, I would like to refer to my thesis (Section 2.2.1), which is about thermo-responsive polymers. There you find the basic principles and further references, which go a bit more into detail. 
That means, our DBC consists of two polymer blocks. Both blocks are thermo-responsive, whereas they feature the opposite temperature response. I called this dual orthogonal thermo-responsive DBC. Basically it means, with increasing temperature, the PSB block becomes soluble and the PNIPMAM block insoluble. Decreasing the temperature reverses the whole process. 

In addition, the PSBs block is responsive to ionic strength and shows a sensitivity to different water isotopes. Therefore, we have a highly (multi-) responsive DBC. And with that, we can do quite a lot! 
One last thing before we go into detail. Have another look at the title of this page: Dual thermo-responsive diblock copolymer thin film. 
The only thing unmentioned so far is the term 'thin films', which describes the sample geometry of my polymer system. Instead of a dilute or semi-dilute aqueous solution, I am using the PSB-b-PNIPMAM diblock copolymer as thin films. Thus, the DBC is confined, meaning it cannot move as freely as for example in a dilute solution. Furthermore, polymer-polymer interactions become dominant in thin films. Also, surface or interfacial effects become more important. As a consequence, it is impossible to extrapolate the behavior of a polymer thin film from its solution behavior. And that is why thin films are so interesting and unpredictable. 

When such (hydrophilic) films are exposed to a humid vapor atmosphere, they absorb a certain amount of the solvent from the vapor and start to swell. By changing the temperature, we can provoke phase transitions (by crossing CP or CP'), which has direct consequences on the thin film behavior, e.g., by crossing the CP of the PNIPMAM block (upon heating), the PNIPMAM block becomes 'insoluble' and some solvent molecules are expelled from the thin film. That means, by adjusting parameters such as temperature or the relative humidity, distinct thin-film states, in terms of solvent content and thickness (or swelling degree) can be obtained. That makes them quite interesting for nano-sensors, or -switches. 
Project #1: Getting started with swelling and exchange behavior of polymer thin films
So let's jump right into my very first project (do you remember the beamtime during my first week?). These experiments were about the swelling behavior of PSB-b-PNIPAM thin films in water vapor. The trick here was, that the water vapor was either H2O or D2O vapor. Since the PSB block is sensitive to different water species, we expected a different thin film behavior in different water vapors. In a second step, we exchanged the water vapor, i.e., the H2O became a D2O vapor and vice versa. In this way, we could also investigate the exchange behavior. Eventually, this work was published in Macromolecules. It was in the middle of the night in Bangkok, when I got the message that my first first-author paper (as a PhD) got published. I was on vacation and couldn't sleep because of the jetlag. When I read the email (yeah, I checked my emails during vacation...I was a greenhorn back then, and probably I still am 🙃), I couldn't sleep even more so, but we managed to celebrate a bit during breakfast a couple of hours later! It's funny, how well I can remember that special moment. Is it the same for you?

Anyhow, this is the paper, which you can find here:

https://pubs.acs.org/doi/full/10.1021/acs.macromol.9b00443

If you cannot access it, send me a message on Twitter or on Researchgate.
The Figure shows a screenshot of the publication 'Swelling and Exchange Behavior of Poly(sulfobetaine)-Based Block Copolymer Thin Films' by Lucas Kreuzer and coworkers, which is published in the Journal Macromolecules.
Original abstract
The humidity-induced swelling and exchange behavior of a block copolymer thin film, which consists of a zwitterionic poly(sulfobetaine) [poly(N,N-dimethyl-N-(3-(methacrylamido)propyl)ammoniopropanesulfonate) (PSPP)] block and a nonionic poly(N-isopropylacrylamide) (PNIPAM) block, are investigated by time-of-flight neutron reflectometry (TOF-NR). 

We monitor in situ the swelling in the H2O atmosphere, followed by an exchange with D2O. In the reverse experiment, swelling in the D2O atmosphere and the subsequent exchange with H2O are studied. 
... and what it actually means
The polymer of interest is the PSB-b-PNIPAM diblock copolymer (just ignore lengthy polymer block names in the original abstract). The thin films are placed in a custom-made sample chamber, where we can control the relative humidity (RH). Upon increasing the RH, the thin films start to swell (humidity-induced swelling). After the swelling process has equilibrated, the vapor atmosphere is exchanged (from H2O to D2O and vice versa). That means a PSB-b-PNIPAM thin film, that has absorbed a lot of H2O molecules is now surrounded by D2O molecules (and vice versa). Consequently, H2O molecules are pushed out of the film, while D2O molecules are absorbed. I called this process 'exchange behavior'. Both dynamic processes, 'swelling' and 'exchange' are followed in situ with neutron reflectometry (NR). It is a non-destructive technique, and in this case, it reveals the water content within the films and the degree of swelling (film thickness). It is quite beneficial since H2O and D2O feature extremely different neutron scattering length densities, which simply means, we can distinguish quite well between an H2O and D2O molecule with neutron scattering/reflectometry techniques. Using NR in time-of-flight mode, means, we are using neutrons with a single wavelength (monochromatic) but with different wavelengths (wavelength band). This way, we can probe different regions of the thin films: Neutrons with higher wavelengths (roughly in the order of 9-14 Angström)  are rather scattered at the thin-film surfaces, while neutrons with lower wavelengths penetrate deeply into the thin films and provide information about the 'bulk' (I put bulk in quotations mark since it still is a thin-film and bulk is actually not an accurate term...).
Both, static and kinetic TOF-NR measurements indicate significant differences in the interactions between the PSPP80-b-PNIPAM130 thin film and H2O or D2O, which we attribute to the different H- and D-bonds between water and the polymer.
'Static' means here, that the thin films are in equilibrium when ToF-NR is applied to them. In the measurement protocol, there are three equilibrium states of the thin films: 1) a dry state right in the beginning (we dried them in a nitrogen atmosphere, 2) after the swelling process has equilibrated and 3) after the exchange process has equilibrated. With 'kinetic' ToF-NR measurements we follow the swelling and exchange processes, where the thin films are not in equilibrium.

Basically, this sentence is the key result of our study. It matters if the thin films are exposed to an H2O or D2O vapor atmosphere. It also matters if the thin films are exposed first to H2O (swelling) and then to a D2O (exchange) or vice versa. We think this is due to differences in H- and D-bonds of the water molecules to the polymer chain. Even though H2O and D2O are very similar, they still differ in mass, electronegativity, and dipole moment. Apparently, these small differences result in surprisingly significant differences in the behavior of the thins films (regarding their water content and film thickness). 
Changes in the chain conformation and hydrogen bonding are probed with Fourier transform infrared spectroscopy during the kinetics of the swelling and exchange processes, which reveals the key roles of the ionic SO3– group in the PSPP block and of the polar amide groups of both blocks during water uptake and exchange.
The dynamic swelling and exchange processes are also followed with Fourier-transform infrared (FTIR) spectroscopy. In my opinion, ToF-NR and FTIR is an extremely beneficial combination of techniques, when it comes to polymers. ToF-NR provides information on a mesoscale (something between micro- and macroscale). That includes basically how much water is within the thin films and how thick are the thin films. FTIR gives information on the molecular level, e.g., chain conformation and hydration states or rates of individual functional groups. In combination, we have several length scales that can be correlated, so we get quite a complementary insight into what is going on during the swelling and exchange processes.

In this case, we get specific information about the role of the sulfonate and amide groups during the swelling and exchange processes. This information can then be correlated with the water content and the thickness of the thin films.

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Short disclaimer: 

This page is still under construction. Whenever I find the time, I will add the next chapter on polymer science and my PhD journey. If you have specific questions, which are not answered somewhere here, please feel free to drop a message. 

Thanks ✌🏽
The image shows a meme of Patrick Star, who tries to build something out of wood, but apparently has no idea what he is doing. Somehow this meme feels like my first years as a phd student.
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