  In this work we extend previous theoretical works to gain a better
understanding of the origin of recently observed polarisation degree spectra of
molecular clouds, which show a so-called V-shape, i.e. a pronounced minimum
around 350 $\mu$m. For this purpose, we present results of semi-analytical dust
polarisation models. We benchmark our model against dust polarisation radiative
transfer calculations performed with POLARIS. We show that V-shaped
polarisation spectra can only be obtained if two dust phases, one dense and
cold and one warm and dilute phase, are present along the line of sight. In
contrast to previous results, no correlation between the alignment efficiency
of silicate grains and the dust temperature is required; carbon grains are
assumed to be not aligned with the magnetic field. We find that the V-shape is
the stronger pronounced the larger the density and temperature contrast between
both phases is. Moreover, the destruction of carbon grains by UV radiation in
the warm and dilute phase leads to a significantly more pronounced V-shape in
the polarisation spectrum. Reducing the alignment efficiency in the cold and
dense phase also results in a more pronounced V-shape, its effect, however, is
smaller than that of the UV-induced carbon grain destruction. Furthermore, we
present a first, self-consistent polarisation spectrum obtained from a 3D,
magneto-hydrodynamical molecular cloud simulation. The spectrum matches well
with our semi-analytical prediction demonstrating the potential of such complex
3D simulations to study polarisation spectra. Comparing our model results with
actual observations indicates that carbon grain destruction in illuminated
regions might be required to match these observations. Reducing the alignment
efficiency of silicate grains in the cold and dense phase would further improve
the match between both data, however, it appears to not be a necessity.
