Dual thermo-responsive diblock copolymer thin films
... and other nerdy things
That is the title of my PhD thesis. Sounds fancy, right? But what is it really about? And why is it relevant to our society?
This section aims to answer these questions in an accessible and understandable way. I will try to convey the basic principles of polymer physics without using too much 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, overcame 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 into 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 trust in scientific work.
To achieve this, two things are required: transparency and dialogue. I will describe my work as transparently 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
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 e.g., their thickness, stiffness, conductivity, or absorption behavior. 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. This neutron source is located at the TUM campus, so at least no traveling was involved).
As a trained chemist, I had zero experience in preparing polymer film samples (which turned out to be quite a challenge for my polymer systems). I didn't know too much about neutron scattering techniques, and I knew nothing about how challenging neutron beamtimes can be. But still, I had everything I needed for these first tasks: Supportive and experienced 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 of what 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. 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.
As a trained chemist, I had zero experience in preparing polymer film samples (which turned out to be quite a challenge for my polymer systems). I didn't know too much about neutron scattering techniques, and I knew nothing about how challenging neutron beamtimes can be. But still, I had everything I needed for these first tasks: Supportive and experienced 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 of what 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. 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.
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.
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.
In addition to temperature, PSBs are responsive to the ionic strength (or salt concentration) of the solution. This is also due to the charged groups. Thus, upon increasing electrolyte concentration, the clearing point CP' decreases. Interestingly, for some types of PSB, this effect turned out to be non-monotonic. It was observed that the CP' first increases at low salt concentrations (a few mmol), passes through a maximum, and subsequently decreases at higher salt concentrations. Besides temperature and ionic strength, PSBs also feature a sensitivity towards different water isotopes, H2O and D2O. Thus, the CP' of some PSBs varies in H2O and D2O.
Summarized, PSBs are super interesting polymers, as they 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.
Summarized, PSBs are super interesting polymers, as they 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, it 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 the 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.
If you want to know more about thermo-responsive polymers, I would like to refer to Section 2.2.1 of my PhD thesis. There you find the basic principles and further references, which go a bit more into detail.
If you want to know more about thermo-responsive polymers, I would like to refer to Section 2.2.1 of my PhD thesis. There you find the basic principles and further references, which go a bit more into detail.
Summarized, my 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 (this is basically the title of my thesis, I hope you understand it a bit better now 😊). 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 a lot of interesting stuff!
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 a lot of interesting stuff!
One last thing before we go into detail. Have another look at the title of this page: Dual thermo-responsive diblock copolymer thin films.
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 thin films. Thus, the DBC is confined, meaning it cannot move as freely as for example in a dilute solution. As a consequence, polymer-polymer interactions become more dominant in thin films compared to solution. Also, surface or interfacial effects become more important. Therefore, it is impossible to extrapolate the behavior of a polymer thin film from its solution behavior. Thus, the thin film behavior is unpredictable. And very exciting.
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 the 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 PNIPMAM parts of the thin film. That means, by adjusting parameters such as temperature or 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 nano-switches.
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 thin films. Thus, the DBC is confined, meaning it cannot move as freely as for example in a dilute solution. As a consequence, polymer-polymer interactions become more dominant in thin films compared to solution. Also, surface or interfacial effects become more important. Therefore, it is impossible to extrapolate the behavior of a polymer thin film from its solution behavior. Thus, the thin film behavior is unpredictable. And very exciting.
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 the 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 PNIPMAM parts of the thin film. That means, by adjusting parameters such as temperature or 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 nano-switches.
Project #1: Getting started with swelling and exchange kinetics 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 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 because of the jetlag, I couldn't sleep. 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.
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.
Let's go through the abstract and see how this study was about!
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-isopropyl acrylamide) (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 the 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 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 from D2O to H2O). 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 very different neutron scattering length densities. This means we can distinguish quite well between an H2O and D2O molecule with neutron scattering techniques. Using NR in time-of-flight (TOF) mode, means, we are using neutrons 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 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 at 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 the 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 serrious differences in the behavior of the thin films. So we are able to control the film thickness and the water content by using different water isotopes. A cool application of this polymer could be a humidity sensor that is able to distinguish between an H2O and D2O (or even further solvents) vapor.
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 the 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 serrious differences in the behavior of the thin films. So we are able to control the film thickness and the water content by using different water isotopes. A cool application of this polymer could be a humidity sensor that is able to distinguish between an H2O and D2O (or even further solvents) vapor.
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 are complementary 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 the thin films are. 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.
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.
By the way: if you wondered, how we control the surrounding of the thin films, e.g., the humidity, the type of the vapor atmosphere, meaning H2O or D2O, or the temperature, which will become relevant in later projects, you can have a look at these publications:
- Tobias Widmann, Lucas Kreuzer et al., Review of Scientific Instruments 91, 113903 (2020)
- Tobias Widmann, Lucas Kreuzer et al., Applied Science, 11(9), 4036 (2021)
Tobias and I have developed a sample environment for neutron scattering experiments on thin polymer films, which is called FlexiProb. The heart of FlexiProb is a 3D printed (from the metal alloy AlMgSi) measurement chamber called UniCORN (Universal Chamber for the Observation of the Reflectivity of Neutrons) as shown in Figure 4.
- Tobias Widmann, Lucas Kreuzer et al., Review of Scientific Instruments 91, 113903 (2020)
- Tobias Widmann, Lucas Kreuzer et al., Applied Science, 11(9), 4036 (2021)
Tobias and I have developed a sample environment for neutron scattering experiments on thin polymer films, which is called FlexiProb. The heart of FlexiProb is a 3D printed (from the metal alloy AlMgSi) measurement chamber called UniCORN (Universal Chamber for the Observation of the Reflectivity of Neutrons) as shown in Figure 4.
Project #2: Co-nonsolvency effect in thin films
This was my favorite project!
We submitted a proposal about the potential co-nonsolvency effect in polymer thin films at the Institute Laue-Langevin (ILL) in Grenoble, France, and got 5 days of beamtime. Co-nonsolvency is a phenomenon where two solvents that can typically readily dissolve a polymer, when mixed, at certain compositions of these two solvents, are no longer able to dissolve the polymer. I wrote a Wikipedia article about this effect, which you can find here.
So we prepared for the beamtime, packed all our equipment into a rental car, drove from Munich to Grenoble, installed our setup, and started measuring.
We submitted a proposal about the potential co-nonsolvency effect in polymer thin films at the Institute Laue-Langevin (ILL) in Grenoble, France, and got 5 days of beamtime. Co-nonsolvency is a phenomenon where two solvents that can typically readily dissolve a polymer, when mixed, at certain compositions of these two solvents, are no longer able to dissolve the polymer. I wrote a Wikipedia article about this effect, which you can find here.
So we prepared for the beamtime, packed all our equipment into a rental car, drove from Munich to Grenoble, installed our setup, and started measuring.
The beamtime went so incredibly well, I still can hardly believe it. Everything was so well prepared (thanks to Christina!), everyone knew exactly what to do, and our setup worked perfectly . We not only measured all planned samples (five) we also measured ALL backup samples we had brought to ILL (another five)! In the end, we had data from three different sample systems, which was extremely helpful because suddenly, the results started to make sense to us. It was like puzzle pieces coming together (a very wholesome science moment). The lesson I learned during that beamtime: Always come overly prepared (if possible)! Everything else will work out (or won't...).
Eventually, we managed to publish all three sample systems! Let's go through the abstract of my paper to see, what we have done there.
Eventually, we managed to publish all three sample systems! Let's go through the abstract of my paper to see, what we have done there.
Original abstract
The swelling of thin diblock copolymer (DBC) films is investigated in situ at 22 °C in pure water vapor as well as in mixed water/methanol vapor.
... and what it actually means
I must admit that this is not precisely worded! Yes, we follow the swelling behavior of DBC thin films (again a PSB-b-PNIPMAM thin film) in pure water vapors (either H2O or D2O). After the swelling process, when the thin films are in equilibrium, we add methanol, either partly deuterated CD3OH or regular CH3OH, to the H2O or D2O vapor, respectively. This is not a swelling but rather an exchange process, which we follow as well in situ at a constant temperature of 22 °C.
The DBC consists of a zwitterionic poly(sulfobetaine) block, poly[3–((2-(methacryloyloxy)ethyl)dimethylammonio) propane-1-sulfonate] (PSPE), and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block.
This is the IUPAC name of the polymer of interest. In the following, it will be abbreviated with PSB-b-PNIPMAM.
The swelling in water vapor (either H2O or D2O) and the thin-film response to methanol vapor exchange (i.e., a part of the H2O vapor is exchanged by CD3OH vapor and a part of the D2O vapor is exchanged by CH3OH vapor) are followed with simultaneous time-of-flight neutron reflectometry (ToF-NR) and spectral reflectance (SR) measurements. In situ Fourier transform infrared (FTIR) spectroscopy complements these data
Here, the measurement protocol is stated in a bit more detail. In total, we performed two experiments: one PSB-b-PNIPMAM thin film is swollen in H2O before some of the H2O vapor molecules are exchanged by CD3OH molecules, while a second PSB-b-PNIPMAM thin film is swollen in D2O before some of the D2O vapor molecules are exchanged by CH3OH molecules. Both dynamic processes, the swelling and exchange, are followed in situ with time-of-flight neutron reflectometry (ToF-NR), spectral reflectance (SR), and Fourier-transform infrared (FTIR) spectroscopy.
Exposure to H2O vapor leads to a slightly higher degree of swelling and amount of absorbed H2O compared to D2O.
This is the first result. Swelling in H2O vapor leads to higher swelling degrees of the thin films than in D2O vapor. This is due to the isotope sensitivity of the PSB block and fits perfectly with the results of project #1.
Upon methanol exchange, the PSPE-b-PNIPMAM thin film undergoes two contractions, which are assigned to the specific responses of the individual polymer blocks of the DBC. Due to its isotope sensitivity, FTIR confirms these two separate contraction processes of the blocks on a molecular level and reveals the role of each polymer block during swelling in water vapor and upon the methanol exchange.
This is the interesting part! Once some of the water vapor is exchanged by methanol the thin films undergo two individual contractions. These contractions could be addressed to the individual polymer blocks of the DBC: PSB and PNIPMAM. The change in the vapor composition leads to a first contraction (or collapse transition) of the PSB block, which is followed by a second collapse of the PNIPMAM block. The contractions occur not simultaneously but after each other. While ToF-NR and SR follow the decrease of the film thickness during the contractions, FTIR reveals the hydration state of the PSB and PNIPMAM blocks. It shows, that upon methanol exchange first the PSB dehydrates and collapses (the film contracts for the first time) and subsequently the PNIPMAM block dehydrates (the film contracts for the second time). Different measurement techniques, different length scales, and thus different perspectives of the same processes. I am a big fan of this ToF-NR and FTIR combination.
Thus, four distinct film regimes with different thicknesses dependencies on the vapor composition can be established, thereby enabling a quarternary nanoswitch.
We are able to achieve four distinct thin film regimes, with respect to the film thickness: 1) dry film, 2) water-swollen film, 3) after the first contraction, and 4) after the second contraction. Furthermore, these states can be tuned by the deuteration degree of water and methanol. All in all, a very complex but also versatile thin film system, which might be suited for nano-switches, where you need more than two states like in a classic on-off scenario.
To better understand what's going on in these films (we didn't know too much back then), we performed similar experiments on the homopolymer thin films (PSB and PNIPMAM). These were simpler experiments as we used only regular water and methanol and skipped the deuterated solvent species.
Both, the PSB and the PNIPMAM thin film swelled in water vapor and contracted in mixed water/methanol vapor, before re-swelling in pure methanol vapor. Classic co-nonsolvency behavior, but in thin films and not in solution.
When I wanted to wrap up the results, I had a funny moment with my supervisor. He looked at me and asked: "Don't you see it?" I replied: "Yeah, pretty awesome results, right...?". "No, no, there are two papers, not just one. Make two individual stories out of it. One about the homopolymers, and one about the diblock copolymers. This way you have two (shorter) stories, which are easier to grasp by the readers. And you get one paper for free".
He was right and I am quite happy with the decision to split this co-nonsolvency study into two parts. Here is the second paper about the homopolymers. I won't go into detail here but have a look at the thin film contraction processes (I marked them with red circles.) Both contractions, the one of the PSB and of the PNIPMAM thin film, have a quite characteristic shape (straight 1-step contraction vs. 2-step contraction), which you also find in a combined way in the contraction process of the PSB-b-PNIPMAM thin films (even though the 1-step contraction process is not as pronounced as for the PSB homopolymer thin film). This way, we knew which polymer block collapses first, and which one second.
Both, the PSB and the PNIPMAM thin film swelled in water vapor and contracted in mixed water/methanol vapor, before re-swelling in pure methanol vapor. Classic co-nonsolvency behavior, but in thin films and not in solution.
When I wanted to wrap up the results, I had a funny moment with my supervisor. He looked at me and asked: "Don't you see it?" I replied: "Yeah, pretty awesome results, right...?". "No, no, there are two papers, not just one. Make two individual stories out of it. One about the homopolymers, and one about the diblock copolymers. This way you have two (shorter) stories, which are easier to grasp by the readers. And you get one paper for free".
He was right and I am quite happy with the decision to split this co-nonsolvency study into two parts. Here is the second paper about the homopolymers. I won't go into detail here but have a look at the thin film contraction processes (I marked them with red circles.) Both contractions, the one of the PSB and of the PNIPMAM thin film, have a quite characteristic shape (straight 1-step contraction vs. 2-step contraction), which you also find in a combined way in the contraction process of the PSB-b-PNIPMAM thin films (even though the 1-step contraction process is not as pronounced as for the PSB homopolymer thin film). This way, we knew which polymer block collapses first, and which one second.
Project #3: Phase transitions in polymer thin films upon changing temperature
Finally! After talking so much about thermo-responsive polymers, we finally change the temperature and look what the thin films are doing. Do you remember how in solution these dual orthogonally thermo-responsive DBCs (meaning two thermo-responsive polymer blocks with an LCST- and UCST-type transition, respectively) self-assemble into micelles and inverse their structure, once the temperature is increased? This is often called 'schizophrenic' behavior and in this project #3 we wanted to check if such a structural inversion also occurs in thin films.
It was a fun project, since we analyzed the behavior of the thin films on different length scales to obtain the most complete picture possible. Therefore, it is a very detailed study and the final paper turned out to be incredibly long (13k words...). Can you imagine the pain of my poor co-authors going through such a long manuscript? Can you imagine my pain, of going through hundreds of co-author comments over and over again? It was at that moment that I decided to work on my writing skills and focus on the main story and not some random side quests...
Anyhow, let's jump into the abstract, and if you are brave enough to read all 13k words click here.
It was a fun project, since we analyzed the behavior of the thin films on different length scales to obtain the most complete picture possible. Therefore, it is a very detailed study and the final paper turned out to be incredibly long (13k words...). Can you imagine the pain of my poor co-authors going through such a long manuscript? Can you imagine my pain, of going through hundreds of co-author comments over and over again? It was at that moment that I decided to work on my writing skills and focus on the main story and not some random side quests...
Anyhow, let's jump into the abstract, and if you are brave enough to read all 13k words click here.
Original abstract
... and what it actually means
The swelling and phase transition behavior of a doubly thermoresponsive diblock copolymer thin film upon increasing temperature in steps above the characteristic cloud points (CPs) of the blocks is studied.
We measured first the swelling behavior of PSB-b-PNIPMAM in D2O vapor at a constant temperature, analogously to the projects before. Once the thin films are swollen, we increase the temperature in two steps (or rather quick temperature jumps). Upon the first temperature jump, we cross the phase transition temperature of the PSB block, while during the second temperature jump, we exceed the phase transition temperature of the PNIPMAM block. The wording of the abstract is not really consistent with the text on this webpage: doubly thermo-responsive means dual orthogonally thermo-responsive. Also, the PSB-b-PNIPMAM DBC does NOT have two cloud points. The PSB block exhibits a clearing point (in solution it undergoes a globule-to-coil transition, meaning a cloudy solution becomes clear upon heating). The PNIPMAM block features the opposite behavior and shows a coil-to-globule transition upon heating, and therefore a clear, transparent solution becomes cloudy with increasing temperature. Summarized, PSB has a clearing point, and PNIPMAM has a cloud point.
An upper critical solution temperature (UCST)-type zwitterionic poly-
(sulfobetaine), poly(N,N-dimethyl-N-(3-methacrylamidopropyl)-ammonio-
propane sulfonate) (PSPP, CPUCST = 31.5 °C), is combined with a lower
critical solution temperature (LCST)-type nonionic poly(N-isopropyl-/
methacrylamide) (PNIPMAM, CPLCST = 49.5 °C) block.
(sulfobetaine), poly(N,N-dimethyl-N-(3-methacrylamidopropyl)-ammonio-
propane sulfonate) (PSPP, CPUCST = 31.5 °C), is combined with a lower
critical solution temperature (LCST)-type nonionic poly(N-isopropyl-/
methacrylamide) (PNIPMAM, CPLCST = 49.5 °C) block.
This sentence states the exact name of the polymer of interest as well as the respective clearing and cloud point of the PSB and PNIPMAM block in solution. These temperatures are most likely to change in thin film geometry since there, polymer-polymer interactions are altered and typically more dominant than in solution.
Using time-of-flight neutron reflectivity (ToF-NR), we observe the swelling in D2O vapor at a constant temperature of 20 °C, followed by two subsequent temperature jumps, from 20 to 40 °C (above CPUCST) and from 40 to 60 °C (above CPLCST).
ToF-NR is used to follow first the swelling in D2O vapor and the subsequent two temperature jumps (from 20 to 40 °C and from 40 to 60 °C), thereby exceeding first the clearing point of the PSB block (here denoted with CPUCST), and then the cloud point of the PNIPMAM block (denoted with CPLCST).
The observed response of the diblock copolymer films deviates from the aqueous solution behavior, which is mainly attributed to the increased polymer concentration.
And indeed, we found significant changes in the behavior of the PSB-b-PNIPMAM thin film compared to the PSB-b-PNIPMAM solution. In particular, the clearing and cloud points are strongly shifted.
Temperature-induced changes in the thin-film nanostructure are investigated with ToF grazing-incidence small-angle neutron scattering (GISANS).
A really cool thing about this study was that during the experiment (swelling + two temperature jumps) we were able to switch between the ToF-NR and GISANS techniques. ToF-NR follows the film thickness and provides information about the water content inside the films. In contrast, GISANS gives information about the lateral thin film morphology, meaning are there characteristics and periodically appearing distinguishable regions (for example a microphase separated DBC with PSB-rich and PNIPMAM-rich regions).
Spoiler: the thin film is microphase separated with a low degree of order. However, a structural, 'schizophrenic' inversion upon increasing temperature as seen in solution, was not observed.
Spoiler: the thin film is microphase separated with a low degree of order. However, a structural, 'schizophrenic' inversion upon increasing temperature as seen in solution, was not observed.
Alterations in the chain conformation and hydrogen bonding are probed by Fourier transform infrared (FTIR) spectroscopy. The ionic SO3–groups (in the PSPP block) and the nonionic hydrophilic amide groups (in both blocks) are found to affect the mechanisms of D2O uptake and release significantly.
To complement the neutron study, we followed the swelling and the temperature jumps with FTIR spectroscopy, thereby obtaining information about the molecular hydration behavior of specific functional groups of the polymer (e.g., the amide or the sulfonate group). This information can then be correlated with the absorbed water content, the thickness of the thin films, and the lateral morphology.
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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.
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