ABSTRACT We extend previous theoretical works to gain a better understanding of the origin of observed polarization 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 the results of two-phase dust models investigated with polaris. We also provide a guideline to calculate individual dust temperatures for different grain types in polaris. We show that V-shaped polarization spectra can only be obtained if two dust phases, one dense and cold as well as one warm and dilute phase, are present along the line of sight. We find that the V-shape becomes more pronounced as the density and temperature contrast between the two phases increases. In contrast to previous results, no correlation between the alignment efficiency of silicate grains and the dust temperature is required; carbonaceous grains are, in general, assumed to be unaligned with the magnetic field. By matching our model results with actual observations of V-shaped polarization spectra, we show that in ultraviolet-illuminated regions (here, the warm and dilute phase), carbon grain destruction might take place. This leads to a more pronounced V-shape with a minimum around 300 $\mu$m. In addition, we show that the dust spectral index and temperature of silicate grains affect the steepness of the polarization spectrum at long wavelengths. Finally, we present a first polarization spectrum obtained from a 3D magnetohydrodynamical molecular cloud simulation. It shows a flattening or even weakly pronounced minimum around 350 $\mu$m, demonstrating the potential of such complex 3D simulations to study polarization spectra.