  We study pore nucleation in a model membrane system, a freestanding polymer
film. Nucleated pores smaller than a critical size close, while pores larger
than the critical size grow. Holes of varying size were purposefully prepared
in liquid polymer films, and their evolution in time was monitored using
optical and atomic force microscopy to extract a critical radius. The critical
radius scales linearly with film thickness for a homopolymer film. The results
agree with a simple model which takes into account the energy cost due to
surface area at the edge of the pore. The energy cost at the edge of the pore
is experimentally varied by using a lamellar-forming diblock copolymer
membrane. The underlying molecular architecture causes increased frustration at
the pore edge resulting in an enhanced cost of pore formation.
