  By large-scale Monte Carlo simulations of semiflexible polymers in $d=2$
dimensions the applicability of the Kratky-Porod model is tested. This model is
widely used as "standard model" for describing conformations and force versus
extension curves of stiff polymers. It is shown that semiflexible polymers in
$d=2$ show a crossover from hard rods to self-avoiding walks, the intermediate
Gaussian regime (implied by the Kratky-Porod model) is completely absent. Hence
the latter can also describe force versus extension curves only if the contour
length is only a few times larger than the persistence length. Consequences for
experiments on biopolymers at interfaces are briefly discussed.

Soft Condensed Matter