Synthesis and crystal structure study of (R,R)-TMCDA ethanol derivatives doubly protonated with FeCl4‒ and Cl‒ as counter-ions Franziska Dorothea Klotz, Clara Alonso Felipe, Fernando Villafañe, Carsten Strohmann Acta Crystallographica Section E Crystallographic Communications, 2025 The synthesis and structural characterization of the crystal forms of (R,R)-TMCDA and its ethanol derivative, both doubly protonated with FeCl4 − and Cl− as counter-ions, are reported, namely, (R,R)-N 1,N 1,N 2,N 2-tetramethylcyclohexane-1,2-bis(aminium) tetrachloridoferrate chloride, (C10H24N2)[FeCl4]Cl (1a), and (R,R)-N 1-(2-hydroxyethyl)-N 1,N 2,N 2-trimethylcyclohexane-1,2-bis(aminium) tetrachloridoferrate chloride (C11H26N2O)[FeCl4]Cl (2a). A notable feature across both synthesized compounds is the presence of N—H...Cl hydrogen bonds of moderate strength in the solid state. In the case of the ethanol derivative of (R,R)-TMCDA, the structure also reveals the formation of intermolecular O—H...Cl hydrogen bonds.
Reaching a near zero radiative heat transfer by the inclusion of modified multiwalled-carbon nanotubes (MWCNTs) in polyurethane-polyisocyanurate aerogels Beatriz Merillas, Tomás Enrique Gómez Álvarez-Arenas, Fernando Villafañe, Miguel Ángel Rodríguez-Pérez Materials Today Chemistry, 2023 This article studies the effect of adding different contents of surface-modified multiwalled carbon nanotubes (CNTs) on the structure and final properties of polyisocyanurate-polyurethane aerogels. The produced samples were characterized in terms of density, porosity, shrinkage, textural properties, mechanical behavior, ultrasonic behavior, and thermal conductivity. Low-weight aerogels were obtained with densities between 89 and 95 kg/m3. The inclusion of homogeneously dispersed carbon nanotubes protected the aerogel structure during supercritical drying decreasing the final shrinkage of these samples and, therefore, increasing their porosity. The porous structure of the aerogels containing CNTs was modified, and slightly larger pores were formed. The structural modifications contribute to decrease the stiffness of the samples while improving their resilience and elasticity. Finally, a significant enhancement on the insulating performance of the aerogels has been found when CNTs were added reaching values as low as 12 mW/mK. These fillers were proved to act as infrared opacifiers by absorbing and scattering the infrared radiation, a relevant contribution in these lightweight materials. In fact, the radiation contribution was strongly reduced when the content of CNTs reached 3 wt%, being this contribution near zero. Thus, this content was considered as the optimum for the final properties-balance in these aerogels.
1,2-Azolylamidino ruthenium(ii) complexes with DMSO ligands: electro- and photocatalysts for CO2 reduction Murphy Jennings, Elena Cuéllar, Ariadna Rojo, Sergio Ferrero, Gabriel García-Herbosa, John Nganga, Alfredo M. Angeles-Boza, Jose M. Martín-Alvarez, Daniel Miguel, Fernando Villafañe Dalton Transactions, 2023 Ru(ii) 1,2-azolylamidino-DMSO complexes are herein described. Their electrochemical behavior in CO2 media is consistent with CO2 electrocatalyzed reduction, whereas photocatalytic CO2 reduction experiments lead to CO and trace amounts of formate.
Polyurethane foam scaffold for silica aerogels: effect of cell size on the mechanical properties and thermal insulation B. Merillas, A. Lamy-Mendes, F. Villafañe, L. Durães, M.Á. Rodríguez-Pérez Materials Today Chemistry, 2022 Silica-based aerogels have been successfully reinforced by means of reticulated polymeric polyurethane (PU) foams with different cell sizes. The resultant silica aerogel-PU foam composites (Sil-PU composites) were fully characterized (density, shrinkage, aerogel percentage, and porous structure), and the mechanical properties and thermal conductivities were analyzed. Moreover, the effect of the application of a surface modification was assessed. A clear influence of the foam pore size on the final properties was found, and the mechanical properties of the aerogels have been notably improved reaching higher elastic modulus (from 130 to 307 kPa), excellent recovery ratios (above 95%), and significant deformations (more than 70%) without breaking. Therefore, the synthesized composites showed a great elasticity (high recovery ratios), tenacity, resilience, and stiffness in comparison with the non-reinforced aerogels. The obtained samples also showed excellent insulating capacities, reaching values between 14.0 and 12.3 mW/(m·K) for the surface-modified composites that were dried under supercritical conditions. Thus, using reticulated PU foams as a skeleton for aerogels is a promising strategy for a broad spectrum of applications in which silica aerogels are suitable candidates.
Improving the Insulating Capacity of Polyurethane Foams through Polyurethane Aerogel Inclusion: From Insulation to Superinsulation Beatriz Merillas, Fernando Villafañe, Miguel Ángel Rodríguez-Pérez Nanomaterials, 2022 A novel synthesis of polyurethane foam/polyurethane aerogel (PUF–PUA) composites is presented. Three different polyurethane reticulated foams which present the same density but different pore sizes (named S for small, M for medium, and L for large) have been used. After the characterization of the reference materials (either, foams, and pure aerogel), the obtained composites have been characterized in order to study the effect of the foam pore size on the final properties, so that density, shrinkage, porous structure, mechanical properties, and thermal conductivity are determined. A clear influence of the pore size on the density and shrinkage was found, and the lowest densities are those obtained from L composites (123 kg/m3). Moreover, the aerogel density and shrinkage have been significantly reduced through the employment of the polyurethane (PU) foam skeleton. Due to the enhanced mechanical properties of polyurethane aerogels, the inclusion of polyurethane aerogel into the foam skeleton helps to increase the elastic modulus of the foams from 0.03 and 0.08 MPa to 0.85 MPa, while keeping great flexibility and recovery ratios. Moreover, the synthesized PUF–PUA composites show an excellent insulating performance, reducing the initial thermal conductivity values from 34.1, 40.3, and 50.6 mW/(m K) at 10 °C for the foams S, M, and L, to 15.8, 16.6, and 16.1 mW/(m K), respectively. Additionally, the effect of the different heat transfer mechanisms to the total thermal conductivity is herein analyzed by using a theoretical model as well as the influence of the measurement temperature.
Super-Insulating Transparent Polyisocyanurate-Polyurethane Aerogels: Analysis of Thermal Conductivity and Mechanical Properties Beatriz Merillas, Fernando Villafañe, Miguel Ángel Rodríguez-Pérez Nanomaterials, 2022 A family of transparent polyisocyanurate-polyurethane (PUR-PIR) aerogels with an interesting combination of physical properties were synthesized. First, their textural properties were analyzed aiming to study catalyst influence on the final porous structures and densities. Their thermal conductivities were measured at different temperatures allowing observation of a clear trend relating the initial formulation with the porous structure and reaching values as low as 12 mW/mK, the lowest found in the literature for aerogels based on this polymer matrix. Contributions to thermal conductivity were calculated, improving the understanding of the porous structure-insulating performance relationship. Moreover, their mechanical properties were studied (elastic modulus, stress at different strains and elastic behavior). The aerogels showed tunable stiffness (elastic modulus from 6.32 to 0.13 MPa) by changing the catalyst concentration and significant elasticity. Thus, super-insulating transparent PUR-PIR aerogels with tailored mechanical properties were obtained opening a wide range of potential applications in the energy, building, automotive and aeronautical sectors, among others. The exceptional insulation of silica aerogels was reached at the same time that their general brittleness was improved while keeping good transparency to visible light (85%, 650 nm). Therefore, these aerogels may constitute an alternative to silica aerogels.
Silica-Based Aerogel Composites Reinforced with Reticulated Polyurethane Foams: Thermal and Mechanical Properties Beatriz Merillas, Alyne Lamy-Mendes, Fernando Villafañe, Luisa Durães, Miguel Ángel Rodríguez-Pérez Gels, 2022 In this work, silica aerogel composites reinforced with reticulated polyurethane (PU) foams have been manufactured having densities in the range from 117 to 266 kg/m3 and porosities between 85.7 and 92.3%. Two different drying processes were employed (ambient pressure drying and supercritical drying) and a surface modification step was applied to some of the silica formulations. These composites, together with the reference PU foam and the monolithic silica aerogels, were fully characterized in terms of their textural properties, mechanical properties and thermal conductivities. The surface modification with hexamethyldisilazane (HMDZ) proved to improve the cohesion between the reticulated foam and the silica aerogels, giving rise to a continuous network of aerogel reinforced by a polyurethane porous structure. The samples dried under supercritical conditions showed the best interaction between matrixes as well as mechanical and insulating properties. These samples present better mechanical properties than the monolithic aerogels having a higher elastic modulus (from 130 to 450 kPa), a really exceptional flexibility and resilience, and the capacity of being deformed without breaking. Moreover, these silica aerogel-polyurethane foam (Sil-PU) composites showed an excellent insulating capacity, reaching thermal conductivities as low as 14 mW/(m·K).
A New Methodology Based on Cell-Wall Hole Analysis for the Structure-Acoustic Absorption Correlation on Polyurethane Foams Beatriz Merillas, Fernando Villafañe, Miguel Ángel Rodríguez-Pérez Polymers, 2022 Polyurethane foams with a hybrid structure between closed cell and open cell were fabricated and fully characterized. Sound absorption measurements were carried out in order to assess their acoustic performance at different frequency ranges. The cellular structure of these systems was studied in detail by defining some novel structural parameters that characterize the cell wall openings such as the average surface of holes (Sh), the number of holes (h), and the area percentage thereof (%HCW). Therefore, these parameters allow to analyze quantitatively the effect of different structural factors on the acoustic absorption performance. It has been found that the parameters under study have a remarkable influence on the normalized acoustic absorption coefficient at different frequency ranges. In particular, it has been demonstrated that increasing the surface of the holes and the percentage of holes in the cell walls allows increasing the acoustic absorption of these types of foams, a promising statement for developing highly efficient acoustic insulators. Additionally, we could determine that a suitable minimum value of hole surface to reach the highest sound dissipation for these samples exists.
Optical Properties of Polyisocyanurate–Polyurethane Aerogels: Study of the Scattering Mechanisms Beatriz Merillas, Judith Martín-de León, Fernando Villafañe, Miguel Ángel Rodríguez-Pérez Nanomaterials, 2022 Highly transparent polyisocyanurate–polyurethane (PUR–PIR) aerogels were synthesized, and their optical properties were studied in detail. After determining the density and structural parameters of the manufactured materials, we analyzed their optical transmittance. It was demonstrated that the catalyst content used to produce the aerogels can be employed to tune the internal structure and optical properties. The results show that the employment of lower catalyst amounts leads to smaller particles forming the aerogel and concomitantly to higher transmittances, which reach values of 85% (650 nm) due to aerogel particles acting as scattering centers. Thus, it was found that the lower this size, the higher the transmittance. The effect of the sample thickness on the transmittance was studied through the Beer–Lambert law. Finally, the scattering mechanisms involved in the light attenuation were systematically evaluated by measuring a wide range of light wavelengths and determining the transition between Rayleigh and Mie scattering when the particles were larger. Therefore, the optical properties of polyurethane aerogels were studied for the first time, opening a wide range of applications in building and energy sectors such as glazing windows.
Characterization and Properties of Water-Blown Rigid Polyurethane Foams Reinforced with Silane-Modified Nanosepiolites Functionalized with Graphite Mercedes Santiago-Calvo, María Carracedo-Pérez, María Luisa Puertas, Antonio Esteban-Cubillo, Julio Santaren, Fernando Villafañe, Miguel-Ángel Rodríguez-Pérez Materials, 2022 In the present study, a promising flame retardant consisting of 80 wt% silane-modified nanosepiolites functionalized with 20 wt% graphite (SFG) is used to obtain a synergistic effect principally focussed on the thermal stability of water-blown rigid polyurethane (RPU) foams. Density, microcellular structure, thermal stability and thermal conductivity are examined for RPU foams reinforced with different contents of SFG (0, as reference material, 2, 4 and 6 wt%). The sample with 6 wt% SFG presents a slightly thermal stability improvement, although its cellular structure is deteriorated in comparison with the reference material. Furthermore, the influence of SFG particles on chemical reactions during the foaming process is studied by FTIR spectroscopy. The information obtained from the chemical reactions and from isocyanate consumption is used to optimize the formulation of the foam with 6 wt% SFG. Additionally, in order to determine the effects of functionalization on SFG, foams containing only silane-modified nanosepiolites, only graphite, or silane-modified nanosepiolites and graphite added separately are studied here as well. In conclusion, the inclusion of SFG in RPU foams allows the best performance to be achieved.
