  Understanding the mechanics of detrimental convective instabilities in drying
polymer solutions is crucial in many applications such as the production of
film coatings. It is well known that solvent evaporation in polymer solutions
can lead to Rayleigh-B\'enard or Marangoni-type instabilities. Here we reveal
another mechanism, namely that evaporation can cause the interface to display
Rayleigh-Taylor instabilities due to the build-up of a dense layer at the
air-liquid interface. We study experimentally the onset time ($t_p$) of the
instability as a function of the macroscopic properties of aqueous polymer
solutions, which we tune by varying the polymer concentration ($c_0$),
molecular weight and polymer type. In dilute solutions, $t_p$ shows two
limiting behaviors depending on the polymer diffusivity. For high diffusivity
polymers (low molecular weight), the pluming time scales as $c_0^{-2/3}$. This
result agrees with previous studies on gravitational instabilities in miscible
systems where diffusion stabilizes the system. On the other hand, in low
diffusivity polymers the pluming time scales as $c_0^{-1}$. The stabilizing
effect of an effective interfacial tension, similar to those in immiscible
systems, explains this strong concentration dependence. Above a critical
concentration, $\hat{c}$, viscosity delays the growth of the instability,
allowing time for diffusion to act as the dominant stabilizing mechanism. This
results in $t_p$ scaling as $(\nu/c_0)^{2/3}$.
