Abstract
In this thesis, we discussed dewetting mechanism. How the polymer chain entanglements and molecular recoiling affect dewetting was carefully discussed. First of all, Molecular recoiling force stemmed from non-equilibrium chain conformation was found to play a very important role in the dewetting stability of polymer thin films. Correct measurements and inclusion of this molecular force into thermodynamic consideration are crucial for analyzing dewetting phenomena and nanoscale polymer chain physics. This force was measured using a simple method based on contour relaxation at the incipient dewetting holes. The recoiling stress was found to increase dramatically with molecular weight and decreasing film thickness. The corresponding forces were calculated to be in the range from 9.0 to 28.2 mN/m, too large to be neglected when compared to the dispersive forces ( ~ 10 mN/m) commonly operative in thin polymer films. Furthermore, Stability of high molecular weight polystyrene (PS) thin films (200k≦Mw≦2M, 20 nm≦h≦80 nm) on a soft surface of low energy at 170℃ was investigated. A unique two-stage dewetting process was discovered and the magnitude of total forces driving the dewetting was determined from the strong substrate interaction. The film dewet by first nucleating small holes which grew rapidly soon after their emergence but subsequently ran into a complete rest. With these holes remaining stationary, however, a new type of dewetting took place in the regions of intact surface when additional annealing of more than 4.5 hours was given. The newly initiated holes grew steadily in size with the characteristic dendrite-like fingers developed and lengthened from the retracting edges of the holes. The fingers finally broke down to form droplets to cover the whole surface. The magnitude of the dewetting force was estimated from the substrate deformation to be around 0.036 N/m. Substrate interactions appeared to have a significant effect of increasing surface energy, hampering the rate of dewetting. The two-stage dewetting occurred only for films of high molecular weights (≧ 900k) and with thicknesses close to the coil dimensions. Moreover, investigations of the evolution of opening of dewetting holes in ultra-thin, almost glassy polystyrene films deposited onto silicon wafers were disclosed. These films were aged at temperatures closely to its glassy transition temperature (T<Tg) for varies of periods of time. At this stage, the stress store inside thin films of long-entangled polymers was relaxed. In other words, molecular recoiling forces were released due to polymer chains diffusion. By observing crests height, molecular recoiling forces may easily be calculated. These forces decreased from 12.1mN/m to 3.6mN/m for 200k PS film in 10nm thick after it were aged at 80℃ after 2 weeks. Meanwhile the recoiling forces decreased form 13.2mN/m to 4.5mN/m for 900k PS film in 20nm thick after it were aged at 80℃ after 2 weeks. The dispersive force calculated from total force and molecular recoiling force, however, exhibited time and molecular weight independent. This dispersive force was change only when the film thickness was change. We believe that after some appropriate clarifications this research of dewetting of huge molecules may contribute importantly and shed some lights to our understanding of properties of thin polymer films.