(131a) Thermodynamic Insights into the Reentrant Phase Behavior of a Copolymer Model

Proteins and nucleic acids consist of multiple, linearly arranged building blocks. Depending on the length and the sequence in which the building blocks are connected, biomacromolecules can form diverse three dimensional structures which play unique roles in various biological functions. In this work, we performed molecular simulation of a copolymer model developed by Statt et al. [J. Chem. Phys. 152, 075101 (2020)] that consists two types of beads (one attractive and one repulsive) to mimic the hydrophilic and hydrophobic interaction between residue side chains of a polypeptide. While some random copolymer sequences exhibit simple fluid behavior, certain “blocky” sequences exhibit a reentrant phase behavior with density anomaly and negative thermal expansion coefficient upon further cooling. In addition, at low temperature, these blocky sequences form “structured” liquids with three dimensional network morphology similar to those observed for microemulsions and patchy colloid systems. Furthermore, we show that a modified Flory-Huggins theory, called double-lattice theory, can be applied to predict the coexistence density across the whole relevant temperature range, suggesting that these blocky sequences exhibit a closed loop phase behavior with a second critical point. This work provides thermodynamic insights into the reentrant phase behavior of a simple copolymer model, shedding light on the sequence-dependent phase behavior of biomacromolecules.