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Author(s):
M. Tanaka, A.Yu Grosberg and T. Tanaka
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Title:
Condensation and Swelling Behavior of Randomly Charged Multichain Polymers by Molecular Dynamics Simulations
Date of publication:
Dec. 1999
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Key words:
condensation, reptation, Coulombic crystal, hysteresis, multichain, reentrant volumetric change, salt and temperature effects, molecular dynamics.
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Abstract:
The behavior of polyampholytes over a wide range of temperatures is studied with the use of molecular dynamics simulations. It has been found that biased fluctuations of charge density due to aggregation of oppositely charged monomers causes the Coulombic interactions more attractive than repulsive for a globally near-neutral polyampholyte. Such a polyampholyte collapses to a globule, in which the polymer chains inter-penetrate with each other and reptate through the whole globule. It further condenses to an NaCl-like cubic crystal for widely extensible chains, but crystallization is suppressed for finite extensible chains. Hysteresis in the volume-temperature diagram is prominent for multichain polyampholytes, which is absent for individual chains. Multichain effects always dominate over the single-chain effects in swelling processes of polyampholytes. A non-neutral polyampholyte still forms a globule if the amount of charge offset is less than {1/2}N^{1/2} (N: the number of charged monomers) at low temperatures. Otherwise, it consists of scattered chains and is swollen. At high temperatures, it shrinks to the thermal state where the Coulomb force plays little role. A polyampholyte with slight charge offset has a non-monotonic (re-entrant) phase boundary with temperature. These results in the temperature domain are in good agreement with polyampholyte experiments with variable salt concentrations.
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