  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" coexisting 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-wise drop in the intrachain 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, under 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 predicted patterns in force-deformation curves
for polymer molecules in poor solvent can be observed in single molecule atomic
spectroscopy experiments.
