Abstract
Adsorption of polymers from the solution phase has been extensively studied to cope with many demands not only for separation technologies, but also for the development of coatings, adhesives, and biocompatible materials. Most studies hitherto focus on adsorption on flat surfaces and mesoporous adsorbents with open frameworks, plausibly because of the preconceived notion that it is unlikely for polymers to enter a pore with a diameter that is smaller than the gyration diameter of the polymer in solution; therefore, sub-nanoporous materials are rarely considered as a polymer adsorption medium. Here we report that polyethylene glycols (PEGs) are adsorbed into sub-nanometer one-dimensional (1D) pores of metal–organic frameworks (MOFs) from various solvents. Isothermal adsorption experiments reveal a unique solvent dependence, which is explained by the balance between polymer solvation propensity for each solvent and enthalpic contributions that compensate for potential entropic losses from uncoiling upon pore admission. In addition, adsorption kinetics identify a peculiar molecular weight (MW) dependence. While short PEGs are adsorbed faster than long ones in single-component adsorption experiments, the opposite trend was observed in double-component competitive experiments. A two-step insertion process consisting of (1) an enthalpy-driven recognition step followed by (2) diffusion regulated infiltration in the restricted 1D channels explains the intriguing selectivity of polymer uptake. Furthermore, liquid chromatography using the MOFs as the stationary phase resulted in significant PEG retention that depends on the MW and temperature. This study provides further insights into the mechanism and thermodynamics behind the present polymer adsorption system, rendering it as a promising method for polymer analysis and separation.
Highlights
Adsorption of polymers from solution on solid surfaces is a ubiquitous phenomenon that has been studied extensively due to practical needs in a variety of technological aspects, for separation and puri cation methods, and for the development of coatings and adhesive materials.[1]
Isothermal adsorption experiments reveal a unique solvent dependence, which is explained by the balance between polymer solvation propensity for each solvent and enthalpic contributions that compensate for potential entropic losses from uncoiling upon pore admission
To investigate the general adsorption capability of the metal–organic frameworks (MOFs), adsorption isotherms of polyethylene glycols (PEGs) were measured for both 1 and 2 using nine typical solvents: ethanol (EtOH), 1-propanol, 1-butanol (n-BuOH), toluene, acetonitrile (MeCN), THF, ethyl acetate (EtOAc), DMF, and chloroform (CHCl3) at 40 C (Fig. 2). 1 and 2 in powder form were synthesized with slight modi cation according to literature procedures (Fig. S1–S3, see Electronic supplementary information (ESI)†)[49,50] and evacuated prior to use
Summary
Adsorption of polymers from solution on solid surfaces is a ubiquitous phenomenon that has been studied extensively due to practical needs in a variety of technological aspects, for separation and puri cation methods, and for the development of coatings and adhesive materials.[1]. Similar pore size effects have been reported for polymer adsorption in other porous materials.[10,12,17] For insertion of PEG into 2, an effect of the MW on the insertion rate was observed (Fig. 5b), with the shorter PEG 2k exhibiting faster insertion compared to the longer PEG 20k.
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