What drives polymer adsorption into MOF nanopores? Insights into the role of solvents from titration calorimetry

Spontaneous infiltration of polymer chains into nanometer-sized pores of metal–organic frameworks (MOFs) has emerged as an intriguing phenomenon with potential applications in polymer recognition, separation, and nanocomposite fabrication. Despite the substantial conformational entropy loss expected upon confinement within sub-nanometer channels, polymers are known to penetrate MOF nanopores and adsorb from solution. However, the thermodynamic origin of this counterintuitive process remains unresolved, largely due to the difficulty of disentangling the interplay of interactions among the polymer, framework, and solvent. Here, we study the thermodynamics governing polymer adsorption into MOF nanopores using isothermal titration calorimetry (ITC) in a series of organic solvents. Estimation of apparent ΔG, ΔH, and ΔS from ITC analysis using simple Langmuir-type fitting models indicates that, although adsorption is exothermic, the overall free energy change is accompanied by a favorable entropic contribution. Notably, the apparent entropy gain is largely independent of solvent identity, whereas the enthalpic term varies significantly with solvent. These findings suggest that solvent release from the nanopores and polymer desolvation plausibly contribute to the adsorption driving force, with their relative importance governed by solvent choice rather than polymer–framework interactions alone. This result overturns the conventional enthalpy-driven interpretation. Analysis of the individual titration profiles further provides kinetic insight, supporting a two-step adsorption mechanism consisting of initial surface association followed by intrapore diffusion. This study establishes a solvent-mediated thermodynamic framework for understanding polymer adsorption in nanoporous materials and offers guiding principles for the rational control of polymer infiltration in confined environments.