  The dust sub-millimetre polarisation of star-forming clouds carries
information on dust and the role of magnetic fields in cloud evolution. With
observations of a dense filamentary cloud G035.39-00.33, we aim to characterise
the dust emission properties and the variations of the polarisation fraction.
JCMT SCUBA-2/POL-2 data at 850um are combined with Planck 850um (353GHz) data
to map polarisation fractions. With previous SCUBA-2 observations (450um and
850um) and Herschel data, the column densities are determined via modified
blackbody fits and via radiative transfer modelling. Models are constructed to
examine how the polarisation angles and fractions depend on potential magnetic
field geometries and grain alignment. POL-2 data show clear changes in the
magnetic field orientation. The filament has a peak column density of N(H2)~7
10^22 cm-2, a minimum dust temperature of T~12 K, and a mass of some 4300Msun
for the area N(H2)> 5 10^21 cm-2. The estimated average value of the dust
opacity spectral index is beta ~ 1.9. The ratio of sub-millimetre and J band
optical depths is tau(250 um)/tau(J) ~ 2.5 10^-3, more than four times the
typical values for diffuse medium. The polarisation fraction decreases as a
function of column density to p ~ 1% in the central filament. Because of noise,
the observed decrease of p(N) is significant only at N(H2)>2 10^22 cm-2. The
observations suggest that the grain alignment is not constant. Although the
data can be explained with a complete loss of alignment at densities above ~
10^4 cm-3 or using the predictions of radiative torques alignment, the
uncertainty of the field geometry and the spatial filtering of the SCUBA-2 data
prevent strong conclusions. G035.39-00.33 shows strong signs of dust evolution
and the low polarisation fraction is suggestive of a loss of polarised emission
from its densest parts.
