Scanning electron microscopy (SEM) was used to visualize the surface morphology of the crosslinked polymers. The surface morphology of pressed discs of PEtOx powder blends with 20 wt. % of P[PM-co-MA] was assessed after heat treatment. Furthermore, samples heated at 120 -200 °C were studied. Before analysis heated samples were washed with water for several days to extract the sol fraction followed by lyophilization (Figure ). Samples incubated at 120 °C exhibit the largest pores with a wide pore size distribution, the determined average pore areas range decreases with increasing incubation time from 205 to 51 µm 2 (Figure ). In addition, at higher incubation temperatures, smaller pore sizes were obtained, but no significant differences in pore size for samples crosslinked at 160 °C or 200 °C were observed. Interestingly, with increasing temperature and incubation time, an increasing number of agglomerates consisting of small, spherical particles were observed (Figure ). The origin of this phenomenon is unclear at this moment but may be connected to an increased degradation of the polymer, as evidenced by the polymer discoloration. It has become clear in recent years that freezing hydrogels at an insufficient cooling rate or increasing the temperature after freezing for e.g. freeze drying may have a strong impact on the morphology due to the formation of cubic or hexagonal ice . By rapid freezing of small hydrogel samples in liquid nitrogen slush (LNS), liquid propane (LP) or ethane (LE) one can avoid water crystallization (vitrification). To keep this in mind is of great importance for cryogenic (cryo-) SEM analysis, as such experimental artifacts should be avoided . Accordingly, we analyzed vitrified swollen hydrogel samples crosslinked at different temperatures (Figure ). Clearly, the hydrogel morphology is indeed affected by the sample preparation. After vitrification, the observed pore size is much smaller than it was obtained from the SEM analysis of the lyophilized samples (Figure ). Also, the morphology is quite heterogeneous with the presence of rather large pores (a few hundred nanometers to a few micrometers) which are filled with another, finer network in the range of several tens of nanometers resulting in a double-pore structure. Interestingly, the hydrogel obtained at 160 °C shows the largest pores size compared to the hydrogels obtained at 120 °C and 200 °C, which clearly contradicts results obtained for lyophilized samples, showing once again the importance of sample preparation. Equilibrium water content and Sol-Gel fraction