Anomalous behavior of polystyrene blends using thermally induced surface wrinkling

The wrinkling of a 50:50 blend of a high molecular mass (M _n = 990 kg/mol) and low molecular mass (M _n = 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 M _n 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 (T _g) of each component. During thermal wrinkling, the high M _n PS vitrifies first, while the surface containing primarily low M _n 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 M _n 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.