Early-stage formation of helical single crystals and their confined growth in thin film

We have previously reported on flat (lathlike) and helical single crystals grown from a smectic A* liquid crystalline (LC) phase of a main-chain nonracemic chiral polymer, PET(R*-9), synthesized from (R)-(-)-4'-{omega-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyl-1-nonyloxy}-4-biphenylcarboxylic acid.(1,2) In this paper, we focus on the formation and growth process of these two types of crystals on the surface of a carbon-coated glass substrate. This substrate provides confinement that affects the crystallization process and ultimately prohibits the development of helical crystals when the film thickness is less than a few tens of nanometers. The population of helical crystals increases as the film thickness increases. Furthermore, at the same crystallization temperature, the pitch lengths of these helical crystals vary in films with different thickness. Coexistence of flat and helical forms in the same lamellar crystal can also be found, suggesting the possibilities of sudden changes of chain packing between them. Early stages of the formation of the helical crystals have been studied using a solvent washing technique for partially crystallized droplets. Uncrystallized LC PET(R*-9) is dissolved while helical lamellae are exposed, and at an early stage of growth, these crystals are observed to be saddle-shaped. Primary nucleation initiates planar growth which develops into saddle-shaped crystals as growth proceeds along both long and short axes of these flat crystals. Crystal growth rates are such that molecular diffusion can occur over distances on the order of micrometers in the surrounding mobile LC phase during crystallization.