Aggregation And Phase Transition Of Polymer Chains

1.Using the replica-exchange multicanonical Monte Carlo simulation, the intra-association of hydrophobic segments in a heteropolymer was numerically investigated by microcanonical analysis method. We demonstrated that the microcanonical entropy shows the features of one or multiple convexes in the association transition region depending on the number and distribution of hydrophobic segments in the chain. We found that one or multiple negative specific heats imply a first-order-like transition with the coexistence of multiple phases.2.Using the replica-exchange multicanonical Monte Carlo simulation, the aggregate of two homopolymers was numerically investigated by microcanonical analysis method. We demonstrated that the microcanonical entropy shows the features of one convex in the transition region, leading to a negative microcanonical specific heat. The origin of backbending of temperature is re-arrangement of segments in the process of aggregation. The process of aggregation proceeds via a nucleation and growth mechanism. Further studying we observed that the segments which the sequence number in polymer chain is from 9 to 12 have a leading effect on the aggregation in this system.3.The adsorption process of a homopolymer chain on an attractive surface is numerically investigated using replica-exchange multicanonical Monte Carlo simulation. Based on the microcanonical analysis, the microcanonical entropy in the adsorption transition shows convex features. Correspondingly, with the coexistence of two phases, negative specific heat is also observed in the region, implying first order-like transition. The origin of the negative specific heat is due to the non-extensitivity of the energy in the system. Further study reveals that this adsorption process has some similarities to the nucleation and growth mechanism in the crystalline process.4.Monte Carlo simulations are used to study the translocation of a flexible polymer through a pore in a membrane, assuming an attractive interaction between the monomers and the membrane on the trans side of the membrane and no interaction on the cis side. For the cases where T c(T_c:the temperature corresponding to the minimum in the translocation timeÏ„), the value ofÏ„decreases upon increasing the temperature, whereas for T > T_c, the value ofÏ„increases with increasing temperature. The translocation timeÏ„depends on the absorbed energy u₀ in a nontrivial way. The value ofÏ„increases initially upon increasing u₀ before it begins to decrease. The variation of the translocation time with respect to the solvent quality has been studied. There exists a transition, as the solvent quality improves from "poor" to "good": whenε_AB <ε_C (the interaction energy corresponding to the minimum inÏ„),Ï„decreases upon increasing the value ofε_AB;whenε_AB >ε_C,Ï„increases upon increasingε_AB. When we varied the chain length, we found when the absorbed energy u₀ was greater than u_c,Ï„was proportional to N^2.248; for u₀ < u_c,τ∝N^1.602. As the solvent quality improved from "poor" to "good," the translocation probability increased initially before becoming stable.5.The thermodynamics of a long homopolymer chain with the Lennard-Jones (LJ) potential was studied by the replica-exchange multicanonical Monte Carlo method. The results demonstrate that the collapse transition of a long homopolymer is able to be separated in three steps in the low temperature: First, liquid-to-solid like transition; then, molten globule-to-solid transition; finally, solid-to-solid-like transition. In particular, the second process of the collapse transition can not be observed in the collapse process of short polymer chains.