Stiffness Gradients in Supported Thin Polymer Films and in Polymer Model Nanocomposites: Evaluation via a Novel Fluorescence Method with Comparison to AFM Results

The issues of how significantly and over what length scales stiffness or modulus is modified by a substrate or nanoparticle interface is important in the performance of polymer nanocomposites and thin films. We provide the first comparison of stiffness gradient length scales in polymers characterized by two methods: fluorescence and AFM. Bilayer film studies show that stiffness is enhanced near a substrate and reduced near a free surface. Fluorescence/trilayer experiments and AFM agree that stiffness gradients in thin polystyrene (PS) model nancomposites, with PS sandwiched between two substrates, depend on whether the PS layer is bulk or nanoconfined. With bulk layers, room-temperature stiffness-confinement effects extend about ~80 nm from a substrate interface. With 266 nm thick PS layers, perturbations to stiffness extend ~200 nm from each interface in a nonlinear compound effect. A small stiffness enhancement is observed at the midpoint of a 266 nm model nanocomposite; in a 60 nm thick model nanocomposite, the midpoint modulus is 50% higher than bulk. Thermal history and the presence of plasticizer are also important factors in tuning the magnitudes and length scales of stiffness-confinement effects.