  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.
