The membranes were ultrasonicated for 0.5 h after immersion for 7 and 14 days. Photographs of GOMs pieces after ultrasonication were taken to examine whether they were peeled off from substrate or any cracks were created. As shown in Figure , these GOMs showed excellent attachment to PVDF substrate after soaking in water for 7 days and 14 days. No visible cracks were found on the membrane surface.