Periodic Reporting for period 1 - CoopHeal (Enhancing Self-healing Properties in Polymer Materials through Cooperative Supramolecular Interactions)
Reporting period: 2019-09-01 to 2021-08-31
As opposed to many natural materials (skin, tree bark, etc.), which can autonomously heal after damage, synthetic polymers unavoidably degrade and their original properties decay over time upon long-term exposure to environmental and working conditions. Modern society demands for materials that are reliable, for safety reasons, durable, to offer prolonged service lifetimes, and cost- and energy-efficient, to preserve natural resources and produce minimum waste and environmental impact. In this context, self-healing polymers are “smart” materials with the ability to repair themselves autonomously or to heal on-demand upon exposure to an external stimulus such as heat, light or pressure. One of the most promising directions considers the use of supramolecular polymers, in which polymer chains are endowed with chemical functions that can associate selectively by means of reversible and dynamic noncovalent interactions. The polymer chains in these materials form a network that is crosslinked through weak, transient, physical bonds, able to connect and reconnect multiple times, which leads to properties that are actually associated with bulk covalent polymers of much higher molecular weight. After damage, these noncovalent bonds are dissociated and separated apart. The generated new fractured interface contains now multiple unbound bonds, which remain “sticky” at the surface for a period of time. Thus, recombination of the two fragments leads to reformation of the bonds closing the gap and healing the damaged site. However, the broad commercialization and universal application of self-healing polymers is still hampered by a main problem that resides in the balance between self-healing ability and mechanical properties at working conditions.
CoopHeal is focused on giving solutions and alternatives to this problem by introducing cooperative ‘all-or-nothing’ interactions in supramolecular polymers. Hence, our strategy is the synthesis of polymers that combine supramolecular motifs able to strongly bind through multiple and cooperative non-covalent interactions, with polymer chains that are flexible enough to facilitate recombination after damage. This type of materials is able to emulate traditional resins, where their main feature is a high degree of crosslinking (necessary for applications where hardness and rigidity play an important role) by means of hydrogen bonding located at the chain ends, easy to unreticulate by an external stimulus –heat or light- for reprocessing.
CoopHeal is focused on giving solutions and alternatives to this problem by introducing cooperative ‘all-or-nothing’ interactions in supramolecular polymers. Hence, our strategy is the synthesis of polymers that combine supramolecular motifs able to strongly bind through multiple and cooperative non-covalent interactions, with polymer chains that are flexible enough to facilitate recombination after damage. This type of materials is able to emulate traditional resins, where their main feature is a high degree of crosslinking (necessary for applications where hardness and rigidity play an important role) by means of hydrogen bonding located at the chain ends, easy to unreticulate by an external stimulus –heat or light- for reprocessing.
CoopHeal research line initially focused on linking a cooperative, multivalent supramolecular motif, like our G-C dinucleoside able to form H-bonded cyclic tetramers, to both ends of soft polymers with low Tg, such as poly(dimethylsiloxane) (PDMS). A protocol was optimized in which a dinucleobase molecule was endowed with a reactive pentafluorophenol ester to be coupled to telechelic diamino-substituted silicones of diverse molecular weights. The final amide-linked polymers were subjected to different studies by 1H NMR and CD spectroscopies to confirm the formation of cyclic tetramer cross-linked networks, which influences enormously materials’ properties: the amino-terminated PDMS pristine material is a liquid, but forms relatively resistant transparent films upon functionalization with the dinucleoside. Preliminary thermomechanical experiments with some of these materials were also very promising, but we soon learned that we needed to change two important elements in our original design: 1) the mechanical properties of the final material are too “soft”, and we need to use polymer chains of higher Tg, and 2) the dinucleoside motif is too “expensive” and it takes too many steps and synthetic efforts to arrive to 1 gram of material, which is only enough to produce a few polymer samples for complete rheological and thermomechanical analysis.
Therefore, we planned to develop a new improved version of these (potentially) self-healing thermoplastics. On one hand, we substituted the polymer chains by polyurethanes based on hydroxyl-terminated PDMS and a diisocyanate, frequently used polymer in the coating industry. The introduction of urethane groups in the main polymer chain brought as expected additional intermolecular H-bonding interactions that increased Tg and hence reinforced thermomechanical properties. On the other, we changed our terminal dinucleoside motif able to form cyclic tetramers with guanosine units able to associate in G-quadruplexes in the presence of alkaline salts. The main advantages are that guanosine is an extremely cheap compound that is bought in kg amounts, whose functionalization is simple and straightforward, and that mechanical properties could be modulated as a function of the salt employed.
The polymers produced have been characterized by common H-NMR, FT-IR, MS, GPC, DSC and TGA measurements, whereas the cross-linking process was studied by NMR and optical spectroscopy as a function of temperature. The rheological and mechanical properties, as well as the qualitative evaluation of the self-heling properties, have been carried out in collaboration with the group of Dr. Daniel López at the Institute of Polymer Science and Technology (Spanish Research Council) in Madrid. The results of all these techniques showed the formation of a reversible network based on G-quadruplex assemblies. The network is practically stable until ca. 150 °C where the network dissociation starts, in agreement with the disassembly of a supramolecular polymer network into lower molecular weight species. The network structure is completely recovered when the material is cooled down to room temperature, as well as their mechanical properties. This behaviour has already been observed in other supramolecular polymers (for example, polymers carrying UPy motifs). However, the mechanical properties are clearly improved with lower functionalization degree, as well as more stable at higher temperatures due to the steady quadruplex assembly.
Therefore, we planned to develop a new improved version of these (potentially) self-healing thermoplastics. On one hand, we substituted the polymer chains by polyurethanes based on hydroxyl-terminated PDMS and a diisocyanate, frequently used polymer in the coating industry. The introduction of urethane groups in the main polymer chain brought as expected additional intermolecular H-bonding interactions that increased Tg and hence reinforced thermomechanical properties. On the other, we changed our terminal dinucleoside motif able to form cyclic tetramers with guanosine units able to associate in G-quadruplexes in the presence of alkaline salts. The main advantages are that guanosine is an extremely cheap compound that is bought in kg amounts, whose functionalization is simple and straightforward, and that mechanical properties could be modulated as a function of the salt employed.
The polymers produced have been characterized by common H-NMR, FT-IR, MS, GPC, DSC and TGA measurements, whereas the cross-linking process was studied by NMR and optical spectroscopy as a function of temperature. The rheological and mechanical properties, as well as the qualitative evaluation of the self-heling properties, have been carried out in collaboration with the group of Dr. Daniel López at the Institute of Polymer Science and Technology (Spanish Research Council) in Madrid. The results of all these techniques showed the formation of a reversible network based on G-quadruplex assemblies. The network is practically stable until ca. 150 °C where the network dissociation starts, in agreement with the disassembly of a supramolecular polymer network into lower molecular weight species. The network structure is completely recovered when the material is cooled down to room temperature, as well as their mechanical properties. This behaviour has already been observed in other supramolecular polymers (for example, polymers carrying UPy motifs). However, the mechanical properties are clearly improved with lower functionalization degree, as well as more stable at higher temperatures due to the steady quadruplex assembly.
The promising results obtained with the telechelic guanine PDMS-based polyurethanes encourage us to carry out further analysis of the self-healing properties of the materials. For this reason, their ability to self-heal through different heat treatments and conditions is currently being analysed by a quantitative study of their mechanical properties (toughness and tensile strength) by stress-strain testing in collaboration with the group of Dr. Daniel López at the Institute of Polymer Science and Technology in Madrid. Additionally, the healing process is being monitored by optical microscopy at different temperature treatments in order to understand deeply the mechanisms of the healing process.
Our intention is to scale up the entire process, and change the formulation of the polyurethane composition in order to obtain different materials that will encompass a wide range of high performance materials with self-healable capability. For that goal, and due to the excellent results obtained along the MSCA Grant CoopHeal, we have been able to attract National funding with a project named: “Analysis of the Market Possibilities of Novel Self-Healing Plastic Coatings” (PDC2021-121487-I00). Through this “proof-of-concept”-type project, we now plan to focus on evaluating the viability of our approach for producing commercial self-healing thermoplastic coatings.
Our intention is to scale up the entire process, and change the formulation of the polyurethane composition in order to obtain different materials that will encompass a wide range of high performance materials with self-healable capability. For that goal, and due to the excellent results obtained along the MSCA Grant CoopHeal, we have been able to attract National funding with a project named: “Analysis of the Market Possibilities of Novel Self-Healing Plastic Coatings” (PDC2021-121487-I00). Through this “proof-of-concept”-type project, we now plan to focus on evaluating the viability of our approach for producing commercial self-healing thermoplastic coatings.