Alberto Tena

Porous chitosan films were functionalized via Schiff base formation with four azo-salicylaldehyde derivatives bearing different para substituents (NO2, Br, H, and NMe2). ATR-FTIR spectroscopy supported the incorporation of the azo derivatives while preserving the main features of the chitosan framework, and UV−vis spectroscopy revealed a marked substituent-dependent optical response. All functionalized films showed enhanced CO2 adsorption compared with pristine chitosan at both 273 and 298 K. At 1 atm, Chi-AzoNO2 exhibited the highest uptake at 273 K (1.498 mmol g−1), whereas Chi-AzoBr performed best at 298 K (0.976 mmol g−1). The temperature-dependent ranking indicates that CO2 adsorption cannot be described solely by the electronic nature of the azo substituent, highlighting the combined influence of surface chemistry and accessible adsorption sites.

The selection of suitable solvents for polymer processing remains largely dependent on empirical optimization and thermodynamic approaches based on Hansen solubility parameters (HSPs), which frequently require extensive solvent‐screening campaigns. In this work, a simplified experimental methodology for rapid comparative polymer–solvent evaluation is proposed through dilute‐solution viscometry and intrinsic‐viscosity analysis. Complete Mark–Houwink–Sakurada (MHS) analysis performed for a model polyimide system revealed that the conformational exponent a remains effectively quasi‐invariant within the investigated aprotic solvent family, whereas the hydrodynamic parameter K exhibits substantially stronger solvent dependence. Based on this behavior, a simplified methodology using single‐point Solomon–Ciuta intrinsic‐viscosity estimations was developed and experimentally validated, showing excellent agreement with complete MHS analysis while substantially reducing experimental complexity, solvent consumption, and material requirements. Comparison of the experimentally determined K values with conventional solvent descriptors revealed that dielectric constant, HSPs, R _a , and RED values do not necessarily reproduce the experimentally observed polymer–solvent interaction trends. The methodology was subsequently extended to PVDF and PES systems using literature‐derived Mark–Houwink exponents, demonstrating the transferability of the approach to established polymeric systems under fixed experimental conditions.

Nanocellular polyetherimide (PEI) exhibits enhanced toughness and impact resistance compared to the initial solid precursor, an effect usually attributed to the presence of nanometric cells within a confined glassy polymer matrix. In this work, we examine whether the CO ₂ saturation step used in gas dissolution foaming can affect the mechanical response by modifying the thermodynamic state of the glassy polymer. Thermal annealing without CO ₂ increases the yield stress and reduces ductility, whereas solid PEI saturated with CO ₂ and subsequently fully desorbed shows lower yield stress, an increase in tensile ductility, and an improved impact response. Density, gas transport, WAXS, and PALS measurements were used to further analyze this state, showing that the effect of CO ₂ leads to a subtle modification of the glassy matrix, changing the packing and gas accessible regions of the matrix. It is proposed that this matrix state, together with the nanocellular architecture, contributes to the brittle‐to‐ductile transition observed in nanocellular PEI. Our work highlights that optimization of the mechanical properties of nanocellular polymers should not only focus on the key characteristics of the final cellular morphology, but also on the conditions that define the state of the polymer matrix surrounding the pores before and during foaming.

Mixed-matrix membranes (MMMs) incorporating porous organic fillers into high-performance polyimides were developed to investigate the influence of free volume and molecular architecture on gas transport. Four structurally rigid, intrinsically porous fillers (TFAP-Trp, Is-Trp, TFAP-TPB, and Is-TPB) were incorporated into a range of polymer matrices (P84^®, Matrimid^®, Pi-DAPOH, Pi-DAROH, Pi-HABAc, Pi-DAM, and PIM-1), enabling the development of a matrix-independent methodology for estimating intrinsic filler permeabilities for five gases (He, O₂, N₂, CH₄, and CO₂). This comprehensive multi-matrix, multi-gas study reveals a strong correlation between filler fractional free volume (FFV), BET surface area, and gas permeability, with isatin-based fillers exhibiting particularly high CO₂ permeability. Filler incorporation generally resulted in substantial permeability enhancements (100–350%) while maintaining selectivity, often with only minor losses or even favorable improvements in CO₂/CH₄ and He/CH₄ separation performance. Several MMMs, particularly those based on Pi-DAPOH and Pi-DAROH polyimides, approached or exceeded the Robeson upper bound. Analysis of permeability as a function of gas kinetic diameter further elucidated clear structure–property relationships, confirming that filler-induced disruption of polymer chain packing and the creation of additional transport pathways are the primary factors governing separation performance. Overall, these findings demonstrate that rationally designed porous organic fillers provide a robust and broadly applicable strategy for mitigating the permeability–selectivity trade-off in polymer membranes and enhancing gas separation efficiency.

Hydroxyl functionalization was investigated as a molecular strategy to enhance water vapor transport in 6FDA-based polyimides. Three structurally related polymers containing zero, one, and two hydroxyl groups in the diamine moiety were systematically evaluated as dense self-standing films and thin-film composite membranes (TFCMs). Structure–property relationships were established through fractional free volume (FFV), contact angle, permeability, diffusivity, solubility, and membrane performance.Hydroxyl incorporation progressively increased polymer hydrophilicity, while FFV increased only slightly after the first hydroxyl incorporation, leading to higher water vapor permeability in the order 2 OH > 1 OH > 0 OH. Water permeability increased more strongly than that of H2 or CO2, indicating that polymer–water affinity plays a dominant role in transport. Solution‒diffusion analysis showed that the permeability enhancement was mainly driven by increased water solubility, whereas diffusivity peaked at intermediate hydroxyl content and decreased at the highest hydroxyl loading. This behavior is consistent with stronger water‒polymer interactions, which increase the residence time of sorbed water molecules and reduce the effective diffusional jump frequency between free-volume elements. These results demonstrate a decoupling between FFV and diffusivity for condensable penetrants in hydrophilic polymers.Experiments at varying feed pressure and temperature revealed increasing sensitivity to operating conditions with increasing hydroxyl content. At elevated vapor activities, non-monotonic permeability behavior was observed, suggesting cooperative sorption effects such as clustering or partial occupation of free volume elements by sorbed water molecules. Unlike H2 and CO2, which followed conventional Arrhenius behavior under conditions far from saturation, water vapor deviated from a single Arrhenius trend above 40 °C, confirming the contribution of additional sorption–diffusion mechanisms at elevated vapor activity.Submicrometric TFCMs were successfully fabricated, preserving O2/N2 selectivity close to dense-film values and confirming largely defect-free selective layers. Water permeance increased strongly with hydroxyl content, reaching 21,000 GPU for the polymer containing two hydroxyl groups. Effective thickness analysis indicated near ideal diffusion for H2, whereas H2O and CO2 exhibited lower apparent transport resistance due to preferential sorption-mediated transport. The polymer containing one hydroxyl group showed the best intrinsic balance between water affinity and molecular mobility, while the two-hydroxyl polymer achieved the highest practical water permeance under finite water-vapor activity.These results demonstrate that controlled hydroxyl functionalization of 6FDA-based polyimides is an effective route to tailor water-vapor transport from dense films to practical TFCM architectures for moisture-removal and humid-gas separation applications.

Guidelines for the evaluation of green solvents were revisited and integrated into a decision diagram. GVL, Cyrene™, DMC, NOP, DEC, DMI, GBL, NBP, Rhodiasolv® PolarClean, and DMSO showed the potential to substitute hazardous polar aprotic solvents.

Mixed matrix membranes (MMMs) were formed by using seven polymeric matrices with a wide range of permeabilities. All of the polymeric matrices have been polyimides, namely: P84, Pi-DAPOH, Pi-DAROH, Matrimid, Pi-HABAc, PI-DAM, and PIM-1 in the order of increasing O₂ permeability. A fixed (10%) concentration of a microporous organic polymer (TFAP-Trp), formed by the combination of trifluoroacetophenone and triptycene, was added as a porous filler. The material properties as well as their separation performances for multiple pure gases, specifically the permeabilities of He, N₂, O₂, CH₄, and CO₂, were measured. The correlation between the relative increase in permeability in MMMs and that of the matrix polymeric membrane has been quantitatively analyzed. This study proves that the increased permeability of MMMs is largely linked to the contribution of the high permeability of the filler. The addition of the TFAP-Trp porous filler proves to be especially beneficial for matrices with low to moderate permeabilities, significantly enhancing the matrix permeability overall. The fitted relationship is approximately linear in accordance with the existing models to predict permeability in dual-phase systems for low proportions of the dispersed phase. An extrapolation allows the evaluation of the permeability of the pure microporous organic polymer, which agrees with the previous values described by the group for different filler contents and in other polymeric matrices. In all cases, the selectivity remains approximately constant while the permeability increases. The addition of TFAP-Trp to all the polymeric matrices led to a moderate improvement of the MMM separation performances, mainly centered on their permeabilities.

The relationships between gas permeability and free volume fraction, intersegmental distance, and glass transition temperature, are investigated. They are analyzed for He, CO₂, O₂, CH₄, and N₂ gases and for five similar polyimides with a wide range of permeabilities, from very low to extremely high ones. It has been established here that there is an exponential relationship between permeability and the free volume fraction, and between permeability and the most probable intersegmental distance as measured by WAXS; in both cases, with an exponential coefficient that depends on the kinetic gas diameter as a quadratic polynomial and with a preexponential positive constant. Moreover, it has been proven that the intersegmental distance increases linearly with the free volume fraction. Finally, it has been established that the free volume fraction increases with the glass transition temperature for the polymers tested, and that they depend on each other in an approximate linear way.