during  membrane fabrication,  the width (not thickness) very narrow which results in shrink to some extent , definetly effect the pore structure inside the membrane , because this width (not thickness) orshrink  determined by the viscoity, cohesive forces, concentration of solute, type of homogenity of the liquid or mixture, further the tability and nature of pore also related to the movement of the width along with opposing forces.

Comparing the membrane pore size in contracted and uncontracted state, I would say as the pore wall shrinks, the pore size increases. We already have seen such an effect in hydrogels (DOI: 10.1002/anie.201004774 and the supporting information)

https://www.researchgate.net/publication/49666643_High-Capacity_Hierarchically_Imprinted_Polymer_Beads_for_Protein_Recognition_and_Capture

Article High-Capacity Hierarchically Imprinted Polymer Beads for Pro...

I dont think narrow cast membrane will affect pore structure. Pore structure depend on phase inversion process. Casting narrow film will result in crumpling or curling which will result  in small surface area which can be useful for testing.

thank you all very much .

I have seen some curling after 1 hour the solution was left cast... does this lead to a separation across the layer (across the thickness while monitoring via SEM)?

The polymer contraction may be be the reason for the membrane shrinkage behaviour

check this article: http://www.sciencedirect.com/science/article/pii/S1383586616315179

thank you so much