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A new thermodynamically stable, aperiodic“bricks-and-mortar”(B&M) cellular mesophase structure is reported in PS1-b-(PI-b-PS2)3miktoarm copolymer and PS homopolymer blends [PS1, longpolystyrene; PI, poly(isoprene); PS2, short polystyrene],where PS comprises discrete hard “bricks” and PI thecontinuous soft “mortar”. The mesophase is unique inits extreme domain volume fractions, its lack of positional order,and quasi-long-range orientational order. On the basis of this unusualmesophase structure, a series of PS-based thermoplastic elastomersare realized, combining rigidity from an exceptionally high contentof discrete glassy PS domains (up to 82 wt %) and high extensibilitywith recoverable elasticity from a low content of continuous rubberyPI (down to 18 wt %). The new elastomers show sharp yielding behaviorwhile maintaining good elasticity at large strains. Tensile-SAXS experimentsreveal that voiding plays an important role for the mechanical behaviorand voids can open/close reversibly with/without loading. Plasticdeformation only results in a slight loss of recoverable elasticity. [ABSTRACT FROM AUTHOR]