Force-dependent change of structural behavior upon pulling a grafted flexible polymer

Manipulating polymer chains by means of experimental instruments such as optical tweezers has helped us understand better the effects of molecular forces in macromolecular structures. In this study, we simulate the influence of a pulling force on structural phases of a coarse-grained model for flexible polymers using generalized-ensemble Monte Carlo methodologies. The recently introduced microcanonical inflection-point analysis method is employed to identify and classify structural phases of the system. Specific order parameters help characterize the transition behavior in response to the present pulling force. We show that, upon increasing the pulling strength, the coil-globule transition turns from second to first order and then merges with the liquid-solid transition. An intermediate phase, separating the coil and globule phases, is identified for certain force strengths. Clear evidence of the Gough-Joule effect is found as well. Transitions do not occur at all if the pulling force exceeds a certain threshold value.