RESUMEN
Pure melts of asymmetric diblock copolymers are studied by means of the off-lattice Gaussian disphere model with Monte-Carlo kinetics. In this model, a diblock copolymer chain is mapped onto two soft repulsive spheres with fluctuating radii of gyration and distance between centers of mass of the spheres. Microscopic input quantities of the model such as the combined probability distribution for the radii of gyration and the distance between the spheres as well as conditional monomer number densities assigned to each block were derived in the previous work of F. Eurich and P. Maass [J. Chem. Phys. 114, 7655 (2001)] within an underlying Gaussian chain model. The polymerization degree of the whole chain as well as those of the individual blocks are freely tunable parameters thus enabling a precise determination of the regions of stability of various phases. The model neglects entanglement effects which are irrelevant for the formation of ordered structures in diblock copolymers and which would otherwise unnecessarily increase the equilibration time of the system. The gyroid phase was reproduced in between the cylindrical and lamellar phases in systems with box sizes being commensurate with the size of the unit cell of the gyroid morphology. The region of stability of the gyroid phase was studied in detail and found to be consistent with the prediction of the mean-field theory. Packing frustration was observed in the form of increased radii of gyration of both blocks of the chains located close to the gyroid nodes.
RESUMEN
A soft particle model for diblock (AB) copolymer melts is proposed. Each molecule is mapped onto two soft spheres built by Gaussian A- and B-monomer distributions. An approximate analytical expression for the joint distribution function for the distance between both spheres and their radii of gyration is derived, which determines the entropic contribution to the intramolecular free energy. Adding a mean-field expression for the intermolecular interactions, we obtain the total free energy of the system. Based on this free energy, Monte Carlo simulations are carried out to study the kinetics of microphase ordering in the bulk and its effect on molecular diffusion. This is followed by an analysis of thin films, with emphasis on pattern transfer from walls with a periodic structure. It is shown that the level of coarse graining in the soft particle model is suitable to describe structural and kinetic properties of copolymers on mesoscopic scales.