This blog post was adapted from a research project with properly supervised experiments conducted at City University of New York.
Plastic is ever-present all around us, and much of it is visible in our everyday lives (like plastic water bottles, bags, toys, etc). However, there are also a variety of plastics that remain less visible to us as they are able to hide in the complex engineering of device technology. One of these types of non-biodegradable plastics are a group called fluoropolymers, which possess unique electrical properties that make them suitable for electronic devices. There are very few types of commercially available fluoropolymers, and the design or discovery of new polymers that might have the same desirable properties is complicated. In this blog post, we discuss the application of fluoropolymers in current technology, and the possible bioinspired approaches that could come in their replacement.
Fluoropolymers - What are they? What is “Ferroelectric”?
Fluoropolymers are an important class of polymers with a wide variety of applications. The term fluoropolymer is as it suggests - a polymer with carbon-fluorine bonds in it. How much fluorine and how it is bonded along the carbon chain determines which type of polymer and its properties. Fluoropolymers include teflon (PTFE) - the non-stick coating, which we won’t discuss further here. Certain types of fluoropolymers can be designed to be ferroelectric, which is a special type of electrical behavior that occurs when a polymer is subjected to an applied electric field. In general, if a polymer responds to an electric field by a change in a physical property, such as a mechanical change in shape or volume, the polymer is said to be electroactive. Common types of electroactive behavior include piezoelectricity - a very useful property that occurs when a mechanical stress creates an electric field in a substance or vice versa. A ferroelectric polymer is piezoelectric and also possesses the characteristic of spontaneous electrical polarization, meaning the polymer remains polarized even after the electric field (voltage) is switched off. The ferroelectric response occurs when molecular dipoles within the polymer chain can align with the electric field to create polarization. Ferroelectricity is a property desired for electronic devices, including computing memory, radio-frequency, sensors in screens, and energy storage. It is predicted that ferroelectric polymers will become more important in the future.
What fluoropolymers do we use in current technology?
The most common ferroelectric polymers are the family based upon PVDF, polyvinylidene fluoride. Only certain types of crystalline phases of PVDF are ferroelectric, namely the beta-phase. PVDF has very useful electrical properties overall that makes it a material found in electronics, radio engineering, medicine, pharmaceuticals.The molecular structure of PVDF can be altered to create different versions which can improve its ferroelectric behavior, forcing it to more readily adopt the beta-phase. Two common types are PVDF and PVDF-TrFE (shown below).
PVDF (top) and PVDF-TrFE (bottom) are two types of electroactive polymers used in electronics, and predicted to be of growing importance for the future.
Bioinspired Solutions
We can ask the question, is it possible to design bioinspired ferroelectric or electroactive polymers? In order to answer the question we first need to understand the underlying molecular behavior within the polymer that gives rise to the property. Then we can ask if it’s possible to use or design polymers from bioderived resources as opposed to using petrochemicals, which is the source of fluoropolymers. The key feature of polymers like PVDF and PVDF-TrFE is that they contain C-F bonds which are very polar. This is because the fluorine is very electronegative and creates a dipole along the C-F bond, with F taking the lion’s share of the electron density. When you combine thousands, or hundreds of thousands, of these dipoles together in the polymer (in aggregate) you can get an overall very large polarization in the material. How these dipoles bend, twist and lock together along the polymer chain is the basis for the ferroelectricity.
Do we have to have C-F bonds to make a ferroelectric material? Plenty of other materials with fluorine are known to be ferroelectric, and therefore the answer is no. What we do need is polarization: dipoles across the bonds. There are bioderived polymers that have dipoles in their molecular structure, so if they can bend and twist together along the polymer chain in the right way, then we could have a chance at making a ferroelectric bioinspired polymer! For example, the bioinspired polymer PHA is the group of polyhydroxyalkanoates. In this case they are a family of polymers with different R-groups. Modifying the R group might be a means to affect how the dipoles align in the polymer chain. We performed a quick experiment in the lab to see if PHA is ferroelectric. In order to do this we needed a ferroelectric tester (an unusual and specialized piece of equipment!) and some polymer films. We compared films of PVDF-TrFe with films of commercially purchased PHA which is used for 3D printing. The results are shown below.
Results & Hysteresis Loops
Hysteresis loops of PVDF-TrFE (left) and PHA (right). The PVDF-TrFE clearly shows a large open loop at high voltage, called a hysteresis loop. The PHA behaves as a linear dielectric. The data was recorded by me on a ferroelectric tester at the City University of New York, with help from some scientists(!).
The way the experiment works is you apply an electric field (a voltage) across the polymer film. The field is a voltage sweep, going from +V to -V and back again. The PVDF-TrFE clearly shows a large open loop at high voltage, called a hysteresis loop. This is the indication of ferroelectric behavior because as you sweep the voltage (the x-axis), the polymer polarization (the y-axis) responds in non-linear fashion - you get this opening loop. The PHA does not have an open loop. It responds in a linear fashion, and is indeed called a linear dielectric. So a quick check shows that PHA is not (yet) ferroelectric, but it is promising to note that the PHA polymer behaves like an insulator and maintains consistent behavior up to hundreds of volts. Therefore, with the right chemical tweaking and fabrication, there is hope we can use a bioinspired approach and make it ferroelectric!
If you are interested to learn more, check out these references!
(1) Dallaev, R.; Pisarenko, T.; Sobola, D.; Orudzhev, F.; Ramazanov, S.; Trčka, T. Brief Review of
PVDF Properties and Applica.ons Potential. Polymers. MDPI November 1, 2022.
hbps://doi.org/10.3390/polym14224793.
(2) Titirici, M. Bioderived and Bioinspired Sustainable Materials. Philosophical Transactions of
the Royal Society A: Mathematical, Physical and Engineering Sciences. Royal Society
Publishing September 20, 2021. hbps://doi.org/10.1098/rsta.2020.0329.
(3) Momeni, S.; Craplewe, K.; Safder, M.; Luz, S.; Sauvageau, D.; Elias, A. Accelerating the
Biodegradation of Poly(Lactic Acid) through the Inclusion of Plant Fibers: A Review of
Recent Advances. ACS Sustainable Chemistry & Engineering 2023, 11 (42), 15146–
15170. hbps://doi.org/10.1021/acssuschemeng.3c04240.
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