  We study numerically the tightness of prime flat knots in a model of
self-attracting polymers with excluded volume. We find that these knots are
localised in the high temperature swollen regime, but become delocalised in the
low temperature globular phase. Precisely at the collapse transition, the knots
are weakly localised. Some of our results can be interpreted in terms of the
theory of polymer networks, which allows to conjecture exact exponents for the
knot length probability distributions.
