Multiple transitions between various ordered and disordered states of a helical polymer under stretching.

ABSTRACT

Using coarse-grained molecular dynamic simulations, we systematically investigate the conformational transitions of a helical polymer chain under tension. While a typical helix-coil transition is derived by our simulation with the absence of the stretching and varying temperature, the chain behaviors become more interesting and complicated when the force is applied. Specifically, when the temperature is low enough relative to the chain rigidity, the polymer is solid-like and displays a series of stepwise conformational transitions on the force-extension curve. We introduce a chain disorder parameter to capture the essence of these transitions. Detailed investigation indicates that the first few transitions correspond to the breaking of the helices, while the last one denotes a transition from a fully disordered state to an all-trans ordered conformation. By increasing the temperature, the thermal fluctuation makes the chain enter a liquid-like state, in which the initial weak stretching induces extra helix formation, followed by the force-induced helix breaking and the transition to the all-trans state. In contrast to the solid-like state, the liquid-like chain always adopts a mixed conformation with both helical and disordered regions. Further increasing the temperature makes the chain fully flexible and thus no helices can form at such a gas-like stage. We further study the relaxation behaviors of the polymer by decreasing the force and find hysteresis for the solid-like cases. Finally, we compare our simulation results with experiments in a semi-quantitative fashion and get quite good agreement.