ABSTRACT
The value of Young's modulus found here, 0.1 GPa, is markedly less than the value of bulk polystyrene, E = 3 GPa [32], and is consistent with the fact that the surface of polystyrene films is less glassy than the bulk [33-39]. It is also consistent with the value of 0.1 GPa found by Overney et al [39] for the storage modulus of a polystyrene film in a poor solvent. Although not stated explicitly in Ref. [39], it appears that the AFM photodiode calibration factor was taken from the slope of the force curve at the highest applied load where the tip was thought to have fully penetrated the 5-nm-thick polystyrene film and to have been in contact with the underlying silica substrate. The zero of separation was taken to be the beginning of the saturation of the AC response, and was said to represent contact with the underlying silica substrate. The sample indentation was estimated by assuming that the polystyrene behaved as a simple spring, although it was noted that the spring constant changed with applied load. The value of the zero-shear rate storage modulus was deduced using a Maxwell rheological model from the AC measurements made in water, a poor solvent for polystyrene. The present measurements give the material properties of the surface of the film because the amount of deformation is only 10 nm, and deformation decays away from the point of contact. The thickness of the soft surface layer of polystyrene has been estimated to be greater than 4 nm [39]. Because of the small amount of deformation, the thickness of the film (order of /xm), the rigidity of the underlying substrate, or the bulk properties of the film do not have much influence on the measured data. Previous measurements on sub-micrometer polymer colloids revealed the effect of the substrate at high loads when the deformation was greater than about 10% of the radius of the colloid [8].
