Effects of Cooperative Association on Polymer Conformational Transition in Solutions*

Numerous endeavors have been dedicated to unraveling the physical intricacies underlying polymer conformational transitions, which acts as an important role in polymer engineering. One well-known example of it is the coil-globule transition accompanied with the polymer dehydration, that is lower critical solution temperature (LCST) transition. The abrupt conformation change at the critical temperature as well as the hysteresis observed upon heating and cooling signifies a first-order transition nature. One characteristic of the first-order transition is the coexistence of the collapsed and swollen state at the transition point. However, our coarse-grained model investigation demonstrates that two-state behavior cannot be observed without the explicit inclusion of association correlation among neighbored segments, indicating the insufficiency of models focusing solely on hydrophobic and hydrophilic interactions in studying LCST transitions. Consistently, previous theoretical, computational, and experimental findings all suggest the pivotal role of cooperative association during the transition. To delve deeper, we integrate the cooperative association directly into coarse-grained model to run Monte Carlo simulation and employ several techniques to improve sampling efficiency. Consequently, we not only observe the coexistence of two-states within chain conformation distributions but also confirm it through free energy calculations.