We report the identification of a new equilibrium microdomain morphology in an intermediate to weakly segregated diblock copolymer melt. A polystyrene-polyisoprene (SI) diblock copolymer consisting of 37 wt % styrene and of total Afw = 27 400 was observed to transform from the lamellar morphology (in equilibrium at low annealing temperatures) to a new morphology at annealing temperatures approximately 50 °C below the order-disorder transition (ODT). The transformation was observed to be thermoreversible. Investigation of the new morphology via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed the new structure to have remarkable three-dimensional long-range order, to belong to the cubic space group Ia3d, and to possess a bicontinuous cubic microstructure. From computer simulations of model structures and comparison with microscopy results, we propose models for the new morphology based on the triply periodic G minimal surface (gyroid) discovered by Schoen;1 similar morphologies have been observed in a variety of microphase-separated surfactant-water systems. Blends of this diblock with various short-chain homopolymers were used to investigate the compositional extent of the region of IaZd stability on the phase diagram; the results indicate that the Ia3d phase is stable over a wide range of minority component volume fractions.ogy, the ordered bicontinuous double diamond (OBDD), in a strongly segregated melt;3 the new structure has since been observed in a variety of block copolymer systems.4-6Recently, Olmsted and Milner7 have developed methods for calculating the free energy of bicontinuous morphologies in the strong segregation limit. A variety of new structures not predicted by the early theories have been observed in the weak segregation regime, including the lamellar-catenoid,8'9 hexagonally modulated lamellae, and hexagonally packed lamellae.10 As part of a study on block copolymer thermal behavior, Gobran11 observed an unusual microphase-separated morphology in a polystyrene-polyisoprene (SI) diblock copolymer. After casting from toluene, the sample formed a lamellar phase; upon heating to temperatures above 120

In contrast to other types of segmented multiblock thermoplastic elastomers, simple ABA block copolymers represent a class of well-defined nanostructured materials. Due to the inherent block lengths built in during the polymerization, the microdomain structure of block copolymers exhibits a size scale of typically 10-100 nm. The ability to control the individual chemistry of each block as well as the size and the shape of the domains in a block copolymer affords enormous advantages to tailor physical properties. By globally orienting the microdomains, a well-defined initial morphological state aids greatly in the interpretation and modeling of mechanical deformation and allows for exploitation of the inherent anisotropy of the cylindrical and lamellar structures. Several types of orientation techniques are reviewed. Experiments investigating structure-mechanical properties in styrene-diene triblock copolymers with spherical, cylindrical, and lamellar morphologies are discussed, with emphasis on the clarifying role of global morphological orientation in data interpretation. Composite theory which treats each microphase as a continuum describes small strain behavior of cylinders and lamellae quite well. Molecular variables such as the number of effective bridge vs loop conformations in the rubber midblock become more important at large strains. With controlled chemistry and morphology as well as with improved dynamic probes, further understanding between the interplay of molecular and morphological structure in influencing the deformation process is expected.

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Recently, a new equilibrium microstructure, a second bicontinuous cubic morphology similar in many respects to the ordered bicontinuous double diamond (OBDD) structure, has been identified in a weakly segregated polystyrene-polyisoprene (SI) diblock copolymer melt.1'2 X-ray diffraction indicated that the new cubic phase was more consistent with a microstructure based on the Schoen G or "gyroid" minimal surface3 than with an analogous model of the OBDD morphology based on the Schwarz D minimal surface.4 This new cubic phase was therefore entitled the "gyroid*", with the "*" symbol serving to distinguish the block copolymer morphology from the G minimal surface.Model microstructures of the OBDD and gyroid* morphologies were developed to determine the characteristics of the new cubic phase.1'2,7 In these models, the majority component material (e.g., polyisoprene) is confined to lie within a constant distance from the underlying minimal surfaces (Schwarz D for OBDD, Schoen G for gyroid*). The resulting majority component phase has constant thickness (CT), and so the * To whom correspondence should be addressed.

The mechanical properties of the double gyroid (DG) cubic phase in glassy−rubbery block copolymer systems are examined. The stress−strain properties of an isoprene-rich polystyrene/polyisoprene/polystyrene (SIS) triblock and a polystyrene/polyisoprene (SI) starblock DG, both comprised of two separate interpenetrating glassy networks embedded in rubbery matrices, are compared to those of the sphere, cylinder, and lamellar morphologies. This 3-dimensionally interpenetrating periodic nanocomposite is found to have superior properties over those of its classical counterparts, attributable to the morphology rather than to the volume fraction of the glassy component, the architecture of the molecule, or the molecular weight. The DG is the only polygranular/isotropic thermoplastic elastomer morphology which exhibits necking and drawing and which requires considerably higher stresses for deformation up to 200% strain than any of the three classical microdomain morphologies. The deformation behavior of the DG is further investigated as a function of applied strain using in situ synchrotron small-angle X-ray scattering. Yielding and necking are observed at ∼20% strain, accompanied by sudden changes in the SAXS patterns:  the characteristic Bragg rings of the DG disappear and are replaced by a lobe pattern containing streaks and diffuse scattering. Analysis of the {211} reflection in the SAXS data indicates that PS networks play a large role in governing the deformation behavior. The necking behavior of the DG suggests a different deformation mechanism. The DG samples recover both microscopically and macroscopically upon unloading and annealing, indicating that the complex interconnected nanocomposite structure was not permanently damaged, even after having been stretched to 600% strain.