Crystal growth rates of diblock copolymers in thin films: influence of film thickness

Using optical and atomic force microscopy we determined the growth rate of polymer crystals in thin films of low molecular weight poly(styrene-ethyleneoxid) block copolymers. We focused in particular on films either thinner or slightly thicker than the thickness of a crystalline lamella (L). At a given temperature, three distinctly different growth rates were observed for i) crystals grown from adsorbed monolayers thinner than L, ii) "primary" compact lamellar crystals in films thicker than L and iii) "secondary" crystals formed from molecules remaining on top of ii). The growth rate of primary crystals did not indicate a dependence on film thickness. Crystals from monolayers grew by a factor of 30 more slowly than primary crystals. Secondary crystals grew faster than crystals from monolayers but slower than primary crystals and the degree of deviation from the growth rate of primary crystals was found to depend on the value of L, which varied with temperature. The mechanisms responsible for the differences in growth rate are discussed in terms of the amount of available polymers and the controlling influence of diffusion towards the crystal front.