The wrinkling of a 50:50 blend of a high molecular mass (Mn = 990
kg/mol) and low molecular mass (Mn = 1.3 kg/mol) polystyrene (PS)
film is studied as a function of annealing temperature and film thickness.
Both thermal and mechanical wrinkling are utilized to elucidate the apparent
modulus of these PS blend films. The PS blend shows a modulus comparable to
the high molecular weight PS, ≈ 3.2 GPa for mechanical wrinkling at ambient
and thermal wrinkling for T ≤ 50 °C. A sharp decrease in the apparent
modulus of the film occurs when thermal wrinkling occurs at 60 °C or higher.
The calculated modulus in this case is 0.5 GPa, which is significantly below
the modulus determined the neat PS for either Mn when thermally
wrinkled at T > 60 °C. This behavior is attributed to a combination of
surface segregation of the low molecular weight PS as well as the large
difference in bulk glass transition temperature (Tg) of each
component. During thermal wrinkling, the high Mn PS vitrifies
first, while the surface containing primarily low Mn PS is
rubbery; this leads to only the underlayer of PS wrinkling initially and
selection of a shorter wavelength due to the effective thickness. The
increased thermally induced stresses during cooling when the low
Mn PS is vitrified do not change the selected wavelength and
instead only leads to an increase in the wrinkle amplitude. These results
illustrate a potential method to modulate the wrinkle wavelength without
changing the overlayer, which could be useful for patterning
applications.