  We study the thermodynamics of an exactly solvable model of a
self-interacting partially directed self-avoiding walk (DSAW) in two
dimensions, when a force is applied on one end of the chain. The critical force
for the unfolding is determined exactly, as a function of the temperature,
below the $\Theta$-transition. The transition is second order and characterized
by new critical exponents which are determined by a careful numerical analysis.
The usual polymer critical index $\nu$ on the critical line, and another one,
which we call $\zeta$, take a non-trivial value which is numerically close to
2/3.
