We present results of numerical Self-Consistent Field (SCF) calculations for the equilibrium mechanical unfolding of a globule formed by a single flexible polymer chain collapsed in a poor solvent. In accordance with earlier scaling theory and stochastic dynamics simulations findings we have identified three regimes of extensional deformation: (i) a linear response regime characterized by a weakly elongated (ellipsoidal) shape of the globule at small deformations; (ii) a tadpole structure with a globular "head" co-existing with a stretched "tail" at intermediate ranges of deformations and (iii) an uniformly stretched chain at strong extensions. The conformational transition from the tadpole to the stretched chain is accompanied by an abrupt unfolding of the depleted globular head and a corresponding jump-wide drop in the intra-chain tension. The unfolding-refolding cycle demonstrates a hysteresis loop in the vicinity of the transition point. These three regimes of deformation, as well as the first-order like transition between the tadpole and the stretched chain conformations, can be experimentally observable provided that the number of monomer units in the chain is large and/or the solvent quality is sufficiently poor. For short chains, on the other hand, at moderately poor solvent strength conditions the unfolding transition is continuous. Upon an increase in the imposed end-to-end distance the extended globule retains a longitudinally uniform shape at any degree of deformation. In all cases the system exhibits a negative extensional modulus in the intermediate range of deformations. We anticipate that predictions of patterns in force-deformation curves for polymer molecules in poor solvent can be observed in single molecule atomic spectroscopy experiments.
We present theoretical arguments and experimental evidence for a longitudinal instability in core-shell cylindrical polymer brushes with a solvophobic inner (core) block and a solvophilic outer (shell) block in selective solvents. The two-gradient self-consistent field Scheutjens-Fleer (SCF-SF) approach and Monte Carlo (MC) simulations are employed to study a conformational transition which occurs upon a decrease in the solvent strength for the inner block from Θ to poor solvent conditions. It is found that a decrease in the solvent strength for the core block leads to an instability in the cylindrically uniform structure and the appearance of longitudinal undulations in the collapsed core of the molecular brush. This result of our modeling is in excellent agreement with experimental observations on core-shell brushes with poly(acrylic acid) (PAA) core and poly(nbutyl acrylate) shell, where the core forms pearl-necklace-like structures due to either a bad solvent for PAA or complexation with multivalent ions.
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