Benita J. Dair

Mechanical properties of the double gyroid phase in oriented thermoplastic elastomers

We investigate the mechanical properties of triblock copolymers with oriented double gyroid (DG) morphology in poly(styrene-b-isoprene-b-styrene) (SIS) triblock copolymers by deforming textured samples along both the [111] direction and transverse to this direction. The modulus anisotropy for the two directions of this cubic material is approximately a factor of 5. Deformation along [111] causes the sample to form a distinct neck and draw, while the deformation in the transverse direction proceeds without neck formation. In addition, the mechanical hysteresis of the [111] stretch is 50% higher than that transverse to the [111] direction. Upon unloading and annealing above the polystyrene Tg, the DG structure recovers fully, both macroscopically and microscopically. The mechanical properties of the DG are compared to those of the classical block copolymer morphologies to gain insight into the deformation mechanism.

Swelling behavior of ordered miktoarm star block copolymer–homopolymer blends

Abstract We report the morphological characterization of asymmetric miktoarm star block copolymers of the (PS-b-PI)nPS type where n=2,3 (denoted 2DB and 3DB miktoarm stars, respectively) and a symmetric super H-shaped block copolymer of the (PS-b-PI)3PS(PI-b-PS)3 type (denoted SH) which were synthesized by anionic polymerization. The initial volume fraction of PS (φPS) for each copolymer was 0.51–0.56, giving a lamellar morphology. Addition of homopolystyrene (hPS) with a molecular weight lower than the respective PS blocks in the neat materials lead to a transition from the lamellar structure to hexagonally packed cylinders. Addition of low molecular weight homopolyisoprene (hPI) on the other hand, only resulted in swollen lamellae even when the overall composition was highly asymmetric (80/20). Changes in the lamellar spacing as well as in the respective PS and PI layer thickness were measured by SAXS. The transition from lamellae to cylinders with increased PS content occurred without the observation of an intervening cubic morphology for the 2DB and 3DB miktoarm stars. However, blends with 30 and 35% hPS ((φPS)total=0.68–0.70) with the super H-shaped block copolymer lead to the observation of lamellar-catenoid structures.

Oriented double gyroid films via roll casting

Films of an isoprene-rich poly (styrene-b-isoprene-b-styrene) (SIS) triblock copolymer thermoplastic elastomer having a highly textured double gyroid (DG) morphology were produced via roll casting and annealing. The symmetries of the extensional and shear flows involved in the roll casting process are not commensurate with those of the cubic Ia 3d symmetry of the tricontinuous, triply periodic cubic DG phase. Upon roll casting the microdomain morphology produced is predominantly that of cylinders, oriented with the axes along the roll cast direction. Upon annealing, the DG phase nucleates and grows with the [111] direction oriented along the roll cast direction. The resultant textured films are suitable for investigating the directional dependence of physical properties. q 2000 Elsevier Science Ltd. All rights reserved.