| Abstract
| - The structure and dynamics of structure formation have been studied in model triarm starblock copolymers composed of two crystallizable blocks (poly(ethylene oxide) (PEO) and poly(ε-caprolactone)(PCL)) and one amorphous block (polystyrene (PS)). Crystallization starts from the homogeneous phase.For the structure investigation, X-ray scattering, optical microscopy, and atomic force microscopy havebeen employed, whereas for the kinetics, we have used differential scanning calorimetry, opticalmicroscopy, and rheology. In the stars, there is a competition for crystallization between the twocrystallizable blocks which have similar mobilities and melting temperatures but crystallize in differentunit cells (monoclinic vs orthorhombic). When the crystallizable block length ratio is 3 or higher, onlythe longer block will crystallize. For comparable lengths both blocks can crystallizehowever, not withinthe same moleculebut the crystallinity, long period, and crystalline lamellar thickness are reduced withrespect to the pure PEO and PCL. The latent heats, obtained in the isothermal crystallization calorimetricexperiments, are analyzed in terms of the Avrami theory. Although similar Avrami exponents were foundfor all stars (n = 2, reflecting a disklike growth from heterogeneous nuclei), the crystallization timeswere different depending on the nature of the crystallizable blocks. Optical microscopy revealed theformation of different superstructures (spherulites/axialites) depending on the type of crystallizable block(PEO/PCL). The growth rates of these superstructures were obtained and analyzed in terms of a kineticnucleation theory, and the fold surface free energies were extracted. Notwithstanding the larger specificsurface of bulk PCL as compared to that of PEO, the fold surface free energies in the stars were similarto that in PEO, indicating a pure PEO crystal and mixing of the amorphous blocks with the PCL crystal.This is supported from the results of the atomic force microscopy measurements on thin films, whichhave indicated the formation of perforated PCL crystals.
|
