Effects of compositional asymmetry in phase behavior of ABA triblock copolymer melts from Monte Carlo simulation

Abstract

Abstract. We simulate ABA triblock copolymer melts using a lattice Monte Carlo method, known as cooperative motion algorithm, probing various degrees of compositional asymmetry. Selected order-disorder transition lines are determined in terms of the segment incompatibility, quantified by product χN , and the triblock asymmetry parameters, α and β. We correlate the results of the simulation with the selfconsistent field theory and an experimental study of polyisoprene-polystyrene-polyisoprene triblock melt by Hamersky and coworkers. In particular, we confirm the mean-field prediction that for highly asymmetric triblocks the short A-block is localized in the middle of the B-domain due to an entropic advantage. This results in the middle block relaxation and is consistent with the experimental data indicating that as the relatively short A-blocks are grown into AB diblock, from the B-block side, the order-disorder transition temperature is considerably depressed.

Monte‐carlo simulations of the order–disorder transition depression in ABA triblock copolymers with a short terminal block

“…The details of this procedure have been provided earlier. 25 Of particular interest here is the decrease in energy that occurs upon cooling. This reduction is accompanied by an increase in the chain gyration radius and a maximum in the specific heat.…”

“…Values of T ODT are calculated from these MC simulations by monitoring the same criteria identified previously, 25 namely, E Ã n (eq 5), the specific heat calculated as…”

“…Recently, we have performed Monte Carlo (MC) simulations of molecularly asymmetric triblock copolymers,25 and the main findings of that study are summarized below: In a highly asymmetric triblock copolymer melt, the short A block remains mixed in the middle of the B ‐block domain, as predicted by MF theory. Values of T ODT and χ N follow the same trends as the corresponding SCFT results for three selected triblock copolymer cases. Comparison of T ODT values from SCFT with those from MC simulations reveals that the SCFT predictions are significantly shifted toward higher values of χ N (or, conversely, lower values of T ODT ). …”

“…24 It should be noted that the theory developed by Mayes and de la Cruz 24 in its more advanced form is conceptually different from SCFT (which is, in principle, exact within the MF framework), because it is based on a 4th-order expansion of the free energy as introduced by Leibler, 10 as well as on a one-loop approximation, which goes beyond standard MF by including fluctuations as proposed by Fredrickson and Helfand. 13 Recently, we have performed Monte Carlo (MC) simulations of molecularly asymmetric triblock copolymers, 25 and the main findings of that study are summarized below:…”

Incipient microphase separation in short chain perfluoropolyether-block-poly(ethylene oxide) copolymers

“…To account for the polydispersity of our samples, we regard the samples as mixtures of monodisperse, coupled components. The scattering of the mixture, I dis,mix (q), is taken to be the sum of scattering from each coupled component, I dis,k (q), weighted by the volume fraction of the component, j k (eqn (12) 29 The expressions for S k (q) and W k (q) are given in eqn (13)- (16).…”