The microphase separation transition in symmetric diblock copolymer melts: A Monte Carlo study

We use Monte Carlo methods to study a lattice model of a dense melt of symmetric diblock copolymers. The calculations are performed in three dimensions using a slithering snake algorithm. In this work we present results for temperatures at and above the microphase separation transition (MST). The location of the MST is found through a study of the relaxation time associated with lamellar compositional fluctuations. A range of copolymer chain lengths are studied, and from our results, approximate scaling relationships are established. A collective length scale characterizing the compositional fluctuations is found which results from a nontrivial compositional mesostructure. At infinite temperature (athermal conditions), the copolymers exhibit the expected Gaussian conformational behavior. We find, however, that at finite temperatures, the copolymers exhibit non-Gaussian behavior indicative of a stretched or elongated state. The magnitude of this stretching, and hence of the deviation from Gaussian behavior, is found to increase as the temperature decreases towards the MST.