Deformation Behavior of a Roll-Cast Layered-Silicate/Lamellar Triblock Copolymer Nanocomposite

Abstract

We have produced layered-silicate−lamellar triblock copolymer nanocomposites possessing a uniaxial texture. Solution blending of the layered-silicate suspension with a block copolymer solution and subsequent roll-casting or static casting and annealing resulted in a partially immiscible and intercalated structure with only a low degree of exfoliation. Isotropic polygranular nanocomposite samples prepared by static solution casting exhibit increase in modulus, higher yield stresses, earlier onset of strain hardening, increased strain hardening rate, and higher residual strains upon unloading with increasing layered-silicate content. In-situ and ex-situ deformation studies were performed along various directions on roll-cast nanocomposite samples to elucidate the effects of anisotropic particles upon deformation of the globally textured nanocomposite system. Deformation parallel to the roll-cast direction shows an earlier onset of strain hardening with modest increases in the modulus for particle-loaded films compared to that of the roll-cast neat triblock. The evolution of the triblock copolymer microstructure with applied strain shows the breakup of the glassy polystyrene domains as was found for pure triblock copolymers. Upon unloading, the residual strain increases with increased loading content, and the unloaded SAXS patterns show a distribution of lamellar spacings, suggesting significant unloading effects due to the presence of the clay layers. Deformation perpendicular to the texture axis shows a dramatic increase in the tensile modulus compared to the unreinforced material. The evolution of microstructure with strain in this direction is reminiscent of a neat triblock copolymer where at high strains (∼100%) the layers kink to form the chevron morphology, and the lamellar normals continue to rotate away from the deformation direction with increasing strain at constant lamellar spacing. Unlike a pure triblock copolymer, however, the chevron morphology remains locked in place upon unloading due to the presence of the layered-silicate particles.