Computer simulation studies of a single polyelectrolyte chain in
poor solvent
Macromolecules, 32 (10):
3264--3278 (1999)Abstract
The conformational behavior of a single, intrinsically flexible, weakly
charged polyelectrolyte chain in poor solvent is analyzed by extensive
computer simulations combining Monte Carlo and molecular dynamics
techniques. After determining the Theta point for the charge-free
case, Re focus on the weak screening Limit, corresponding to low
salt concentration in the solution. We study the dependence on both
the solvent strength, characterized by the relative deviation from
the Theta point, tau, and the fraction of charged monomers in the
chain, which is effectively tuned by varying the Coulomb interaction
parameter. The conformations are discussed in terms of global properties
(such as the end-to-end distance, the inertia tensor components,
etc.) and functions revealing more detailed information, such as
the density distribution around the center of mass and the structure
factor. For chains in the Theta regime our data confirm the picture
of a string of electrostatic blobs. For poorer solvents (up to tau
= 0.4) we observe, upon increasing the intrachain Coulomb repulsion,
a splitting of the spherical globule into a dumbbell-type structure,
accompanied by a sharp increase in the chain's gyration radius. For
sufficiently large tau, a further splitting is observed as well.
Such a necklace globule (a sequence of transitions) had been predicted
by Dobrynin, Rubinstein and Obukhov (Macromolecules 1996, 39, 2974),
with a nontrivial scaling of the gyration radius with chain length
and interaction parameters, which is confirmed by our data. By means
of a scaling analysis, we argue that the transitions can be interpreted
as thermodynamic first-order phase transformations, when taking the
appropriate thermodynamic limit, which implies a scaling of the electrostatic
coupling with inverse chain length.