Thermoresponsive polymers for environmentally friendly electrospinning

How we can make the future of electrospinning greener

The concern about the environment is real, yet having all industrial processes more environmentally friendly is still fiction. Also the production of nanofibers is a burden for our planet and for our own health at this moment. But there might be a solution for electrospinning. Thermoresponsive polymers allow for nanofiber production from aqueous solutions. This is a large improvement compared to the toxic solvents used today.

Many studies have already proven the value of nanofibers, very fine filaments that can be processed as a coating or into a membrane with unique properties such as high porosity and a large specific surface area. Nanofibers even made the jump to industry already as they are, for instance, used as a breathable but water-repellent layer in rain coats. If the nanofibers are on the market, so is the process to make them. The most common production technique to produce nanofibers is called solvent electrospinning and a few companies around the world produce the equipment needed for this process. In fact, the equipment you need is quite straightforward, especially on a lab scale. The combination of a pump, a needle, a voltage source and a grounded collector in the form of an aluminum plate, will do the trick. The pump will make sure that droplets of the spinning solution are formed at a constant rate at the tip of the needle. This droplet is subjected to the electrical field caused by the voltage source and consequently attracted towards the grounded collector. If the spinning solution contains a viscous polymer and all the parameters are right, the droplet will be drawn into a continuous filament which is solidified and stretched enormously before it collides with the collector. If all the parameters are right, indeed, because it doesn’t need much explanation that a lot of parameters play an important role in this process. The flow rate of the spinning solution, the strength of the electrical field, the distance between the needle-tip and collector, are just a few examples. Eventually the result is a random deposition of the filaments which forms a membrane with a thickness of your liking.

But what about that spinning solution? Perhaps, this is the most crucial parameter of all. First, the solution should contain a macromolecular structure, for no continuous fiber can be formed without it. If enough macromolecular structures are present, they can form entanglements which keep the filament together during stretching under influence of the electrical field. Some very well-known macromolecular structures are polymers. If the polymer chains are long enough and their concentration in the spinning solution is sufficiently high, the spinning solution will reach a viscosity suitable for producing uniform nanofibers rather than incoherent droplets or beads. This viscosity will also be the crucial factor determining the final fiber morphology, including the nanofiber diameter. Second, the polymer should be dissolved in a solvent system. Nanofibers can be produced from melt electrospinning, but this is not a universally applicable technique simply because not all polymers melt. Solvent electrospinning is more generally applicable, you just have to find a solvent system in which the polymer is soluble. And this is where the problem lies, indeed. A lot of commonly used polymers are hydrophobic and only soluble in toxic or deleterious solvents such as chloroform, DCM, DMF or THF. This is not good for the environment and holds safety and health issues.

Wouldn’t it be possible to electrospin from more ecofriendly solvents, such as water or ethanol? The answer is yes. And one of the answers to the question of how, is thermoresponsive polymers. Thermoresponsive polymers have different properties depending on temperature. A typical example of thermoresponsive behavior is the occurrence of a Lower Critical Solution Temperature or LCST, which can be assessed by in-depth turbidimetry experiments. If water is taken as the Solution, the LCST is defined as the highest temperature for which all concentrations of the polymer are still soluble in water. Based on the figure below, this means that the polymer is soluble in water below this threshold temperature, but insoluble above. There you have it. Below the LCST, the thermoresponsive polymer can thus be electrospun from water. Problem solved.

Or not? In reality it’s not that straightforward, indeed. First of all, electrospinning must be carried out under controlled temperature conditions, because, if the temperature of the solution crosses the LCST, phase separation occurs and electrospinning becomes impossible. A second parameter that must be controlled, is the humidity of the environment. Remember that in electrospinning fibers are formed as a consequence of the evaporation of the solvent in the polymer solution. If the air surrounding the electrospinning process is already full of water, the water from the polymer solution will not be evaporating sufficiently, indeed. It doesn’t need further explanation that these environmental parameters can have a crucial impact on the electrospinning process and the resulting fiber morphology. This requires some intensive process optimization which is aided by thorough rheology experiments. By investigating the visco-elastic behavior of the polymers at different concentrations, shear rates and temperatures and linking these results to processing, we get a better understanding of how polymer properties and fiber formation through electrospinning are intertwined.

Water is not the only solvent that is environmentally friendly. Ethanol or combinations of water and ethanol are good options as well. We have already gained breakthrough insights into processing-related polymer properties and succeeded in electrospinning different kinds of polymers, e.g., poly(N-isopropyl acrylamide) and several poly(2-oxazoline)s, in a non-toxic and ecofriendly way, opening up possibilities for greener electrospinning. For specific applications, such as biomedicine, this is a great benefit as no toxic or harmful solvent residues will be found in these materials.

But, about those applications, the polymers are still water-soluble, aren’t they? True, the LCST-behavior remains present in the nanofibrous end-material. This is an interesting feature for the design of smart or responsive systems. Yet, for some applications the water solubility of the polymers, and consequently the nanofibers, is a liability, as they will undoubtedly come into contact with water when used. Of course, when the application temperature is well above the LCST, there is no problem. But what if it’s below? Then we apply crosslinking. We link the polymer chains together with the help of strong bonds so they form one big network that cannot fall apart in solution. We have already developed several physical as well as chemical crosslinking strategies to produce nanofibers that are stable in aqueous media at all temperatures, without compromising the eco-friendliness of the process. Interestingly, the thermoresponsive behavior here leads to swelling or de-swelling of the nanofibrous structures without dissolution of the material. This opens doors towards future applications in biomedicine, sensors or climate control.

Research towards green electrospinning is strongly interdisciplinary and based on a collaboration between the Department of Materials, Textiles and Chemical Engineering and the Department of Organic and Macromolecular Chemistry at Ghent University. This way, expertise in materials science, chemical engineering and supramolecular chemistry is combined to produce, investigate, process and characterize several polymer systems. Production and electrospinning of the polymers is performed and optimized in-house. A thorough understanding of the produced materials is achieved by several analysis techniques. While the hydrophilic or hydrophobic properties of the material – it’s affinity for water – can be assessed by contact angle measurements, Dynamic Vapor Sorption and wicking experiments, optical and electron microscopy reveal what the microstructure of the material looks like before and after contact with water. Modulated Dynamic Scanning Calorimetry gives us insight in the thermal properties of the material enabling analysis of the crosslinking procedures.

Further information

Publications:

• Ella Schoolaert (UGent) , Paulien Ryckx, Jozefien Geltmeyer (UGent) , Samarendra Maji (UGent) , Paul Van Steenberge (UGent) , Dagmar D'hooge (UGent) , Richard Hoogenboom (UGent) and Karen De Clerck (UGent) (2017) ACS APPLIED MATERIALS & INTERFACES. 9(28). p.24100-24110
• Ella Schoolaert (UGent) , Ronald Merckx (UGent) , Jana Becelaere (UGent) , Serge Rijssegem, Richard Hoogenboom (UGent) and Karen De Clerck (UGent) (2022) ADVANCED FUNCTIONAL MATERIALS. 32(1).
• Ella Schoolaert (UGent) , Ronald Merckx (UGent) , Jana Becelaere (UGent) , Melissa Everaerts, Joachim Van Guyse, Ondrej Sedlacek (UGent) , BRUNO DE GEEST (UGent) , Guy Van den Mooter, Dagmar D'hooge (UGent) , Karen De Clerck (UGent) , et al.(2020) MACROMOLECULES. 53(17). p.7590-7600
• Ella Schoolaert (UGent) , Luisa Cossu, Jana Becelaere (UGent) , Joachim Van Guyse, Ali Tigrine (UGent) , Maarten Vergaelen (UGent) , Richard Hoogenboom (UGent) and Karen De Clerck (UGent) (2020) MATERIALS & DESIGN. 192.
• Yin Li, Maarten Vergaelen (UGent) , Ella Schoolaert (UGent) , Richard Hoogenboom (UGent) and Karen De Clerck (UGent) (2019) EUROPEAN POLYMER JOURNAL. 112. p.24-30

Parts of this research received an award:

Ella Schoolaert - 1st prize for poster at 6th International Conference on Electrospinning (China, 2019)

Acknowledgements

This research has been and still is part of several Ph.D. projects:

• Ir. Olmo Frateur (funded by FWO strategic basic research grant 1S22722N)
• Ir. Jana Becelaere (funded by FWO strategic basic research grant 1S89120N)
• Dr. Ir. Ella Schoolaert (funded by FWO strategic basic research grant 1S05517N)

Contact

Prof. Dr. Ir. Karen De Clerck (Karen.DeClerck@UGent.be)