  Millimeter and sub-millimeter observations of continuum linear dust
polarization provide insight into dust grain growth in protoplanetary disks,
which are the progenitors of planetary systems. We present the results of the
first survey of dust polarization in protoplanetary disks at 870 $\mu$m and 3
mm. We find that protoplanetary disks in the same molecular cloud at similar
evolutionary stages can exhibit different correlations between observing
wavelength and polarization morphology and fraction. We explore possible
origins for these differences in polarization, including differences in dust
populations and protostar properties. For RY Tau and MWC 480, which are
consistent with scattering at both wavelengths, we present models of the
scattering polarization from several dust grain size distributions. These
models aim to reproduce two features of the observational results for these
disks: (1) both disks have an observable degree of polarization at both
wavelengths and (2) the polarization fraction is higher at 3 mm than at 870
$\mu$m in the centers of the disks. For both disks, these features can be
reproduced by a power-law distribution of spherical dust grains with a maximum
radius of 200 $\mu$m and high optical depth. In MWC 480, we can also reproduce
features (1) and (2) with a model containing large grains ($a_{max}$ = 490
$\mu$m ) near the disk midplane and small grains ($a_{max}$ = 140 $\mu$m) above
and below the midplane.
