Interfacial properties and phase transitions in ternary symmetric homopolymer–copolymer blends: A dissipative particle dynamics study

Abstract

We use dissipative particle dynamics simulations to study the interfacial properties and their relevance to the phase transitions in ternary symmetric blends. For the blend in the lamellar (LAM) state, by analyzing the distribution of inter-bilayer distances as well as the position fluctuations and correlations of bilayers, we found that the “Discrete Harmonic” theory can describe the fluctuations of the LAM phase in ternary blends in a satisfactory way when the LAM phase is not too close to the phase transition to the bicontinuous microemulsion (BμE). In particular, as the LAM stack is swollen increasingly with the addition of homopolymers, the bilayers in the LAM phase experience a crossover from the single coherent fluctuation mode to the coexisting of coherent and incoherent fluctuation modes wherein the incoherent free membrane regime starts at larger length scales and becomes more pronounced. Meanwhile, the in-plane correlation length increases, the bending modulus and compressibility modulus decrease, indicating that as the more homopolymers are added, fluctuations between adjacent bilayers become less coherent, bilayers become more flexible, and the highly swollen LAM system becomes more susceptible to thermal fluctuations. Therefore, the phase transition from LAM to BμE is expected to occur when the bending modulus turns out to be within KBT or when the persistent length is no larger than the lamellar spacing. Also the two criteria yield the same predictions for the transition point. While for the macrophase-separated (2P) phase, we found that Helfrich model gives a good description of the fluctuation spectrum of the copolymer monolayer at long wavelengths if the 2P systems are not too close to the phase transition to BμE. With the addition of copolymers the interfacial tension reduces but the bending modulus increases. Moreover, the phase transition of 2P to BμE coincides with the saturation of the interface between homopolymer phases, i.e. attaining a vanishing interfacial tension. In a word, our results justify the notion that there exists a direct correlation between peculiar changes in interfacial properties and the phase transitions from 2P or LAM to the BμE state.