Reversible polymerization for the development of smart polymers in renewable energies

Introduction

The demand for sustainable and highly efficient materials has driven research into smart polymers, capable of adapting to changing conditions. In this context, reversible polymerization, which allows the controlled formation and breaking of bonds, has emerged as a technology with great potential in renewable energy industry applications.

This article explores the fundamentals of this technology, its differences with traditional techniques, and its applications in sectors such as energy storage and solar and wind energy production.

What is reversible polymerization and its fundamentals?

It is a chemical process through which structural changes are achieved at the molecular level and therefore in the properties of polymeric compounds, in response to different stimuli such as temperature, pH or light. This characteristic confers them ideal properties of: regenerating, self-assembling or self-repairing, depending on the specified applications for which they have been synthesized.

Their basis lies in the controlled formation and breaking of covalent and non-covalent bonds, enabling the development of materials with advanced properties for applications in energetic devices that require precise structural changes. In addition, polymers with reversible bonds allow materials to be reprocessed without losing their properties, which fits well with sustainability and recycling objectives, contributing to a circular economy and minimizing waste.

Comparison with traditional polymerization

Unlike traditional polymerization, which produces irreversible bonds, reversible polymerization allows the breaking and reformation of bonds under external stimuli, which gives adaptable characteristics to polymers. This property is used in advanced applications where versatile behavior is required, such as biomedicine, energy and electronics.

In addition, in the traditional process it is difficult to precisely control variables such as molecular weight, in the reversible process much more detailed control is allowed due to techniques such as RAFT (Reversible Chain Transfer Polymerization) and ATRP (Atomic Transfer Radical Polymerization), which provide precise control over the molecular structure, allowing the design of tailor-made polymers for specific needs¹.

Development of reversible polymers for new energy applications

Currently, a series of strategic research is being conducted to improve the performance of these materials in different applications in the energy sector such as energy storage and the production of clean energy, some types of reversible polymers are specified below:

• Photopolymers, also called light-activated resins, photosensitive polymers or photoresistors, are composed of various formulations of monomers, oligomers, polymers, and other additives that may be specific to the individual process, are efficient as organic solar cells².
• Thermo-responsive polymers are smart materials whose physicochemical properties change in a controlled manner in response to temperature alterations. Useful in thermal energy storage, they modify their solubility or shape in response to temperature³.
• Electroactive polymers are polymers that exhibit some activity (usually a change in shape or size) when stimulated by an electric field. Their main applications are as actuators and sensors. Another main application is the generation of artificial muscles used in robotics⁴.
• pH-responsive polymers effective in water purification and controlled drug release systems⁵.

Advanced approaches in the synthesis of reversible polymers are based on specific structural modifications, such as the incorporation of photo- and thermoresponsive functional groups, which allow controlling their stimuli-responsive properties. The formation of copolymers and the integration of nanocomposites enhance the mechanical strength and the electrical and thermal properties of the materials, essential for their use in energy systems.

In addition, molecular self-assembly enables the creation of ordered arrays, optimizing performance in energy storage and generation applications. These approaches innovate the energy field, bringing us closer to a more sustainable future through smart materials that optimize energy production and storage.

Basic principles of reversible polymerization in smart polymers

Process chemistry

Reversible polymerization is based on dynamic reactions, where covalent and non-covalent bonds have the ability to adapt to external stimuli such as temperature, pH, or light. In this context, disulfide bonds and Diels-Alder reactions (addition reactions of conjugated dienes with certain alkenes and alkynes) are representative of dynamic covalent bonds that allow reversible processes.

Polymers with disulfide bonds, for example, can self-repair after thermal damage, while those with hydrogen bonds, which are used in all-polymer solar cells, have been shown to improve energy efficiency. These bonds enable phase separation and molecular alignment, optimizing the performance of solar devices and energy storage systems.

Types of smart polymers based on reversible polymerization

Reversible polymers fall into several categories according to their response to stimuli:

• Thermo-reversible polymers: Change their state or shape in response to temperature, optimizing energy efficiency in applications such as thermal energy storage systems.
• pH-sensitive polymers: They respond to changes in pH, being useful in water purification processes or controlled drug release.
• Photo-reversible polymers: Modify their properties when exposed to light, making them an ideal choice for organic solar cells and smart windows that adjust light transmittance.

These composites offer versatility for renewable energy applications and also allow the design of functional materials that respond precisely to external stimuli.

Reversible polymers as self-repairing materials

The concept of self-repair requires the material to be able to detect a crack and seal it without external intervention. The basic idea consists of dispersing in the starting formulation used to generate the conventional polymer, microcapsules containing a reactive liquid (monomer or mixtures of monomers) capable of forming a solid by polymerization at room temperature in the presence of a suitable catalyst.

The latter is dissolved in the starting formulation and dispersed in the polymer. The microcapsules have a coating of sufficient thickness to survive the formation process of the final material intact and must be uniformly dispersed in the crosslinked polymer.

One of the most important properties of reversible polymers is their ability to self-repair, allowing them to repair damage at the microscopic or macroscopic level when exposed to certain stimuli, such as heat or light. This ability has an impact on the energy industry, where materials must withstand extreme conditions over time. For example, in wind turbines, self-repairing polymers reduce wear due to adverse climatic conditions, extending the useful life of the blades and reducing maintenance costs.

Reversible polymerization applications in the energy industry

Use of reversible polymers in energy storage

Reversible polymers stand out as a solution in the development of energy storage devices, such as batteries and supercapacitors. Their self-healing properties enable these devices to maintain their performance over an extended period, even when faced with internal microfracture.

Furthermore, these materials can offer additional advantages such as weight reduction and greater flexibility, which facilitates their integration into a variety of energy applications, from domestic storage systems to large-scale energy infrastructures.

Solar energy applications

In the field of solar energy, this type of polymers has been used to improve the efficiency of organic solar cells, adjusting their properties according to light and temperature to maximize the conversion of solar energy into usable electricity. In addition, their use is being investigated for the development of smart windows that allow automatic or manual control of light intensity and heat in closed environments.

Polymers that employ non-covalent bonds, such as hydrogen bonds, are used in all-polymer solar cells. These bonds improve molecular alignment and phase separation, leading to an increase in energy conversion efficiency by up to 19%, optimizing device performance under varying conditions. Together, these chemical dynamics enable the design of highly functional and sustainable materials for the next generation of energy technologies.

The following video explains the importance of the use of polymers in organic solar cells, highlighting their ability to adjust their properties in response to light and temperature.

Wind energy applications

In wind applications, wind turbine blades coated with reversible polymers are capable of self-repairing after damage caused by adverse conditions, which reduces maintenance costs and increases their durability. Damping systems based on these polymers also minimize vibrations and improve operating efficiency.

In addition to those mentioned above, reversible polymers are also being used in flexible energy capture devices, such as triboelectric generators, which convert mechanical energy into electricity. Their ability to adapt to deformation and recover from damage makes them an effective option for portable devices and future applications in energy infrastructure.

Circular economy and sustainability

Reversible polymers improve the performance of energy storage devices while reducing the carbon footprint. Being recyclable, these materials help reduce dependence on raw materials and allow products to be reprocessed without loss of properties. As part of a circular economy, reversible polymers can reduce the energy required to manufacture solar devices and batteries by up to 20%, aligning with long-term sustainability goals.

Controlled polymerization techniques in the synthesis of smart polymers

Controlled Radical Polymerization (CRP) allows better control of molecular weight, molecular weight distribution, functionality, and polymer composition. CRP can be used with a wide variety of vinyl monomers to build polymers with diverse properties for different applications. The following are presented below:

• Atomic Transfer Radical Polymerization (ATRP): In particular, ATRP has been adapted for the preparation of polymers coated with nanoparticles, facilitating the surface modification of polymers, necessary to improve the functionality of materials used in renewable energies, such as functional nanoparticles.
• Reversible Addition-Fragmentation Chain Transfer Polymerization (RFAT): This is a more complex methodology than ATRP, through which polymers with precise molecular structures are synthesized, which are essential for applications in energy storage and self-healing coatings, which also contributes to the sustainability of the materials.
• Nitroxide Mediated Polymerization (NMP): The intermediates formed in this type of polymerization during the reaction disable the polymer radicals formed during the propagation stage of the polymerization reaction, which promotes more controlled polymer growth and greater ease in synthesizing polymers with well-defined geometries. The copolymers developed by this technique are considerable for pigment dispersion, memory devices, and composite fabrication.

These techniques enable the development of smart polymers capable of self-repairing and adapting to varying environmental conditions, optimizing energy technologies through advanced and customizable materials.

Conclusions

Reversible polymerization facilitates the development of smart polymers with renewable energy applications. Properties such as self-repair and adaptability to external stimuli contribute to efficiency in energy storage and capture systems, such as batteries and solar cells, by reducing wear and tear and the need for maintenance.

The reversibility of these polymers allows recycling and reprocessing without loss of structural properties, favoring sustainability by minimizing waste and promoting a more circular life cycle. These characteristics position reversible polymers as a viable solution for optimizing resources in various industrial applications, supporting the energy transition.

References

1. Ramírez M, Leticia A; y colaboradores; “Polimerización Radicálica Controlada por Transferencia de Átomos (ATRP), una alternativa para la elaboración de materiales poliméricos controlados”; AFINIDAD LXXII, 571, July – September 2015.
2. Universal Laser System; “https://www.ulsinc.com/es/materials/fotopol%C3%ADmeros” ; Accessed 20-10-2024.
3.  Musarurwa H, Tawanda T, Nikita; “Polímeros sensibles a estímulos y sus aplicaciones en la ciencia de la separación”; Polímeros reactivos y Funcionales, 2022.
4. Wikipedia; Polímeros electroactivos; https://es.wikipedia.org/wiki/Pol%C3%ADmero_electroactivo; consultado el 20-10-2024.
5. https://www.researchgate.net/profile/Carlos_Hernandez81/publication/311680954_UTILIZACION_DE_HIDROGELES_COMO_LIBERADORES_DE_FARMACOS/links/589a444faca2721f0db12529/UTILIZACION-DE-HIDROGELES-COMO-LIBERADORES-DE-FARMACOS.pdf