Tuning the water intrinsic permeability of PEGDA hydrogel membranes by adding free PEG chains of varying molar masses

TL;DR

This study investigates how varying PEG molar mass and content in PEGDA hydrogel membranes affects water permeability, revealing a maximum permeability at a specific PEG molar mass and structural insights into the membrane's nanoscale features.

Contribution

It demonstrates the ability to precisely tune hydrogel membrane permeability by adjusting PEG molar mass and content, and uncovers the structural mechanisms behind these effects.

Findings

Water permeability varies over several orders of magnitude with PEG content and molar mass.

Maximum permeability occurs at PEG molar mass of 35,000 g/mol near the critical overlap concentration.

Structural analysis shows PEG chains induce nanoscale defects and surface arrangements affecting permeability.

Abstract

We explore the effect of poly (ethylene glycol) (PEG) molar mass on the intrinsic permeability and structural characteristics of poly (ethylene glycol) diacrylate PEGDA/PEG composite hydrogel membranes. We observe that by varying the PEG content and molar mass, we can finely adjust the water intrinsic permeability over several orders of magnitude. Notably, we show the existence of a maximum water intrinsic permeability, already identified in a previous study to be located at the critical overlap concentration C^* of PEG chains, for the highest PEG molar mass studied. Furthermore, we note that the maximum intrinsic permeability follows a non-monotonic evolution with respect to the PEG molar mass and reaches its peak at 35 000 g.mol-1. Besides our results show that a significant fraction of PEG chains is irreversibly trapped within the PEGDA matrix even for the shortest molar masses down to 600 g.mol-1. This observation suggests the possibility of covalent grafting of PEG chains to the PEGDA matrix. CryoSEM and AFM measurements demonstrate the presence of large micron-sized cavities separated by PEGDA-rich walls whose nanometric structure strongly depends on the PEG content. By combining our permeability and structural measurements, we suggest that the PEG chains trapped inside the PEGDA rich walls induce nanoscale defects in the cross linking density, resulting in an increased permeability below C^*. Conversely, above C^*, we speculate that partially-trapped PEG chains may form a brush-like arrangement on the surface of the PEGDA-rich walls, leading to a reduction in permeability. These two opposing effects are anticipated to exhibit molar-mass-dependent trends, contributing to the non-monotonic variation of the maximum intrinsic permeability at C^*. Overall, our results demonstrate the potential to fine-tune the properties of hydrogel membranes, offering new opportunities in separation applications.