Abstract
Molecular recoiling stresses in polystyrene ultrathin films (4 -100 nm) were measured through wetting instability at above Tg (100 ℃) to explore the physical state and condensation process. To obtain the molecular recoiling stress, the measurement of Young’s modulus was needed. By measuring the force curve using an indenting AFM tip on the film surface, the Young’s modulus was found to decrease linearly with decreasing film thickness if thickness was lower than about 1μm. The Young’s modulus of thin films with different molecular weight show different dependent on film thickness. Moreover, the face and back of thin films show different Young’s modulus. With the measurement of Young’s modulus, the molecular recoiling stress could be calculated. Molecular recoiling stress emerges when the film thickness is less than the unperturbed molecular diameters. Thin films with better solvent trapping entangle chain would constrain larger molecular recoiling stress. As aging temperature lowered but still above Tg, film stability increased but recoiling stress underwent significant changes by stress relaxation. A small fraction of MEH-PPV added in films manifested photoluminescence (PL) following the same trend as recoiling stress confirming stress-enhanced PL characteristic of conjugated polymers. In dewetting films, the sheared molecules emit light very efficiently, giving rise to the large PL enhancement.