Impact of Morphological Orientation in Determining Mechanical Properties in Triblock Copolymer Systems

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.