Confinement effects on the crystallization and SSA thermal fractionation of the PE block within PE-b-PS diblock copolymers

Abstract

Well defined polyethylene-b-polystyrene (PE-b-PS) diblock copolymers have been synthesized by anionic polymerization in a wide composition range, keeping the length and the microstructure of the PE block constant. These copolymers provide a model system to study the confined crystallization of PE. A fractionated crystallization behavior (i.e., multiple exotherms are observed upon cooling from the melt) was observed for the PE block within block copolymers with 26% and 11% PE (i.e., Click to view the MathML source and Click to view the MathML source diblock copolymers, where the subscripts indicate the composition in wt% and the superscript the molecular weight in kg/mol). For confined PE, annealing is always observed when the material is self-nucleated. In the case of Click to view the MathML source, most of the PE spheres crystallize at very large supercoolings, a behavior previously reported in the literature and associated with homogeneous nucleation. However, in our case, the peak crystallization temperature (i.e., Tc = 46.6 °C) is much lower (16 °C) than that reported for similar size PE nano-droplets but in diblock copolymer whose second block is chemically different to PS. We therefore conclude that the nature of the interphase between the two neighboring blocks may be responsible for such low temperature nucleation, since this Tc is still quite high with respect to the vitrification temperature of PE (in comparison with other polymers whose homogeneous nucleation temperature has been found close to Tg), a fact that could indicate that the homogeneous nucleation temperature has not been reached because surface (or interfacial) effects can dominate. Thermal fractionation takes advantage of the distribution of methyl sequence length in hydrogenated polybutadiene in order to produce by successive thermal annealing a distribution of lamellar thickness within each PE block. Such a distribution of thermal fractions is affected by confinement and therefore these experiments demonstrate the influence of morphological restrictions on the crystallization of the PE block within the PE-b-PS diblock copolymers. As the PE content in the copolymer decreases, topological confinement effects limit the size of the lamellar crystals that can be formed within the reduced dimensions of the microdomains (MD). By the use of the Gibbs-Thomson equation and the thermal fractionation results, a distribution of crystalline lamellar thickness within each MD was obtained and the orientation of the chains within the MD was deduced.