Surface nanomechanical properties of polymer nanocomposite layers

The nanomechanical behavior of molecularly thick (8-10 nm) compliant polyme ric layers with the nanodomain microstructure from poly [styrene-b-(ethylen e-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 nanomechanica l surface behavior with different elastic responses shown by the rubber mat rix and the glassy nanodomains. High-resolution probing showed virtually co nstant elastic response for the compliant layer compressed to 60% of its in itial thickness followed by a sharp increase of the resistance when the tip reached within 3 nm from a stiff solid substrate. Application of the doubl e-layer model allowed the estimation of the actual elastic moduli of differ ent nanophases within the grafted polymer monolayer: 7 +/- 3 MPa for the ru bber phase and 20 +/- 7 MPa for the glassy domains. The relatively high ela stic modulus of the rubber matrix is caused by a combination of chemical cr oss-linking/branching and spatial confinement within a < 2R(g) layer. On th e other hand, the observed low modulus of the glassy nanodomains can be att ributed to both the low molar weight of polystyrene segments and the presen ce of rubber layers in the probed volume.