Surface nanomechanical properties of polymer nanocomposite layers

The nanomechanical behavior of molecularly thick (8-10 nm) compliant polymeric layers with the nanodomain microstructure from poly [styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS or Kraton) was probed with micromechanical surface analysis based on scanning probe microscopy. The microprobing with high lateral resolution revealed the bimodal character of the nanomechanical surface behavior with different elastic responses shown by the rubber matrix and the glassy nanodomains. High-resolution probing showed virtually constant elastic response for the compliant layer compressed to 60% of its initial thickness followed by a sharp increase of the resistance when the tip reached within 3 nm from a stiff solid substrate. Application of the double-layer model allowed the estimation of the actual elastic moduli of different nanophases within the grafted polymer monolayer: 7 +/- 3 MPa for the rubber phase and 20 +/- 7 MPa for the glassy domains. The relatively high elastic modulus of the rubber matrix is caused by a combination of chemical cross-linking/branching and spatial confinement within a < 2R(g) layer. On the other hand, the observed low modulus of the glassy nanodomains can be attributed to both the low molar weight of polystyrene segments and the presence of rubber layers in the probed volume.