Vertical pattern decay and lateral in-phase capillary breakup of nanoimprinted bilayer polymer films

We report the morphological evolutions of nanoimprinted polystyrene (PS)/polymethyl methacrylate (PMMA) bilayers during thermal annealing. At an annealing temperature above the glass transition temperatures of both polymers, the PS/PMMA bilayer patterns undergo a sequential event of morphological evolutions. At the early stage, the bilayer patterns decay vertically, which leads to a continuous PS layer over the PMMA patterns. Further annealing leads to an intriguing "in-phase" capillary breakup of the PS stripes. As a result, strings of PS droplets develop at the direction orthogonal to the original pattern line directions. At the final stage, these PS droplets anisotropically coalesce into thicker stripes, which eventually break up into larger discrete droplets due to the Rayleigh instability. Our study demonstrates that the mobile and corrugated PS/PMMA interfaces dictate both the kinetics of the vertical pattern decay and the correlated lateral instability. Both polymers directly participate in the pattern evolutions, different from the well-documented dewetting of polymers confined on elastic topographic substrates.