Molecular-dynamics simulations of polyampholytes: Instabilities due to excess charges and external fields

In this paper we study the conformational properties of polyampholytes (PAs; polymers with positively and negatively charged monomers) by molecular-dynamics (MD) simulations and by scaling arguments. As is well known, in the absence of external electrical fields PAs with a small total charge ${Q}_{\mathrm{tot}}$ collapse into spherical globules, whereas PAs whose ${Q}_{\mathrm{tot}}$ exceeds a critical value ${Q}_{c}$ are expanded; our MD simulations confirm this fact. For ${Q}_{\mathrm{tot}}=0$ we study the influence of external electric fields on single PAs. We find that the PA globule is unstable above a critical field strength ${E}_{c1};$ for $E>{E}_{c1}$ the PA becomes highly stretched. Lowering the external field again, one observes hysteresis; the PA collapses back into a globule only for $E<{E}_{c2}$ where ${E}_{c2}<{E}_{c1}.$ In the weak coupling limit (when the intrachain electrostatic interactions can be neglected) the PA attains in an external field a trumpetlike shape, whose end-to-end distance obeys Pincus scaling.