  ALMA observations revealed recently polarised radiation of several
protoplanetary disks in the (sub-)millimetre wavelength range. Besides
self-scattering of large particles, thermal emission by elongated grains is a
potential source for the detected polarisation signal. We calculate the
wavelength dependent absorption and intrinsic polarisation of spheroidally
shaped, micrometre and sub-millimetre sized dust grains using the discrete
dipole approximation. In particular, we analyse the impact of dust grain
porosity which appears to be present in disks when small grains coagulate to
form larger aggregates. For the first time our results show that (a) the
intrinsic polarisation decreases for increasing grain porosity and (b) the
polarisation orientation flips by 90 degree for certain ratios of wavelength to
grain size. We present a new method to constrain grain porosity and the grain
size in protoplanetary disks using multi-wavelength polarisation observations
in the far-infrared to millimetre wavelengths. Finally, we find that moderate
grain porosities ($\mathcal{P}\lesssim0.7$) potentially explain the observed
polarisation fraction in the system HD 142527 while highly porous grains
($\mathcal{P}>0.7$) fail unless the grain's axis ratio is extraordinarily
large.
