  Observations of protoplanetary disks (PPDs) at sub-mm wavelengths suggest
that polarisation is caused by scattering of thermal radiation. Most of the
dust models that are used to explain these observations have major drawbacks:
They either use much smaller grains than expected from dust evolution models,
or result in lower polarisations than observed. We investigate the effect of
dust grain porosity on the polarisation due to scattering at sub-mm wavelengths
arising from grains of up to mm sizes, as they are expected to be present in
the midplane of PPDs. Using the effective medium theory, we calculated the
optical properties of porous dust and used them to predict the behaviour of the
scattering polarisation. Subsequently, radiative transfer simulations for PPDs
with porous dust were performed to analyse the additional effect of the optical
depth structure, and thus the effect of multiple scattering events and
inhomogeneous temperature distributions on the polarisation. We find that
porous dust with moderate filling factors of about 10\% increase the
polarisation compared to compact grains. For higher grain porosities, that is,
grains with a filling factor of 1\% or lower, the extinction opacity decreases,
as does the optical depth of a disk with constant mass. Consequently, the
unpolarised direct radiation dominates the scattered flux, and the polarisation
drops rapidly. Even though the simulated polarisation is higher than in the
case of compact grains, it is still below the typical observed values for
face-on disks. However, the polarisation can be increased when crucial model
assumptions derived from disk and dust evolution theories, for instance, dust
settling and mm-sized dust grains, are dropped. In the case of inclined disks,
however, our reference model is able to achieve polarisation degrees of about
1\%, and using higher disk masses, even higher than this.
