  We study the model of a partially directed flexible or semi-flexible
homopolymer on a square lattice, subject to an externally applied force, in a
direction either parallel to, or perpendicular to the preferred direction. The
polymer is self-interacting and can therefore undergo a collapse transition. We
show that this model can be solved and we obtain the force-temperature phase
diagrams which, for the case of flexible polymers, agree with that of Brak et
al obtained using a different method. At sufficiently low temperatures, the
polymer conformation changes from compact to coil state as the force is
increased beyond a critical value. This transition is second or first order for
the completely flexible or semi-flexible polymer, respectively.
