Mechanical properties and deformation behavior of the double gyroid phase in unoriented thermoplastic elastomers

The mechanical properties of the double gyroid (DG) cubic phase in glassy-r ubbery block copolymer systems are examined. The stress-strain properties o f an isoprene-rich polystyrene/polyisoprene/polystyrene (SIS) triblock and a polystyrene/polyisoprene (SI) starblock DG, both comprised of two separat e interpenetrating glassy networks embedded in rubbery matrices, are compar ed to those of the sphere, cylinder, and lamellar morphologies. This 9-dime nsionally interpenetrating periodic nanocomposite is found to have superior properties over those of its classical counterparts, attributable to the m orphology rather than to the volume fraction of the glassy component, the a rchitecture of the molecule, or the molecular weight. The DG is the only po lygranular/isotropic thermoplastic elastomer morphology which exhibits neck ing and drawing and which requires considerably higher stresses for deforma tion up to 200% strain than any of the three classical microdomain morpholo gies. The deformation behavior of the DG is further investigated as a funct ion of applied strain using in situ synchrotron small-angle X-ray scatterin g. Yielding and necking are observed at similar to 20% strain, accompanied by sudden changes in the SAXS patterns: the characteristic Bragg rings of t he DG disappear and are replaced by a lobe pattern containing streaks and d iffuse scattering. Analysis of the {211} reflection in the SAXS data indica tes that PS networks play a large role in governing the deformation behavio r. The necking behavior of the DG suggests a different deformation mechanis m. The DG samples recover both microscopically and macroscopically upon unl oading and annealing, indicating that the complex interconnected nanocompos ite structure was not permanently damaged, even after having been stretched to 600% strain.