Crosslinked polymer discs were immersed into deionized water at room temperature to reach equilibrium state, and then quickly frozen in liquid nitrogen and lyophilized in a freeze-drier at -40 °C. The freeze-dried samples were fixed on stubs and sputter coated with 4 nm platinum before observation. The morphology of the fractured sample surface was observed by SEM ('Crossbeam 340' von Zeiss). The SEM analysis was done with an acceleration voltage (EHT) of 2kV and by detecting secondary electrons (SE).