Brewster angle microscopy study of poly(ε-caprolactone) crystal growth in Langmuir films at the air/water interface

Surface pressure-induced crystallization of poly(ε-caprolactone) (PCL) from a metastable region of the surface pressure-area per monomer (∏ - A) isotherm in Langmuir monolayers at the air/water (A/W) interface has been captured in real time by Brewster angle microscopy (BAM). Morphological features of PCL crystals grown in Langmuir films during the compression process exhibit four fully developed faces and two distorted faces. During expansion of the crystallized film, polymer chains slowly detach from the crystalline domains and diffuse back into the monolayer as the crystals "melt". Typical diffusion-controlled morphologies are revealed by BAM during the melting process as the secondary dendrites melt away faster, that is, at a higher surface pressure than the principal axes. Electron diffraction on Langmuir-Schaefer films suggests that the lamellar crystals are oriented with the polymer chain axes perpendicular to the substrate surface, while atomic force microscopy reveals a crystal thickness of ∼7.6 nm.