Three-dimensional study of cylindrical morphology in a styrene-butadiene-styrene block copolymer

When a block copolymer undergoes microphase separation, well ordered microstructures result which give the copolymer unique thermal and mechanical properties. In relating these properties quantitatively to the microstructure, and in devising models for the phase separation, theoreticians have always invoked highly idealized structural elements. However, variations in the actual microstructure may also play a significant role in these properties, so we describe here a method for highly detailed examination of these elements. By using transmission electron microscopy (TEM), we have obtained projections of a styrene-butadiene-styrene (SBS) copolymer (30 wt ~o styrene) at tilt angles ranging from 61.5 ° to + 48.5 ° in 5 ° increments. Applying a filtered back-projection reconstruction method to this set of tilted projections yielded a three-dimensional view of the polymer, revealing two different types of structures. The first type resembled cylinders in hexagonal packing with some deviations from geometric ideality. In these cases, the microstructures were found to extend through the thickness of the TEM sample (approximately 36 nm) and were oriented normal to the surface. In the second type of structure, globules of polystyrene were observed in a clearly nonequilibrium condition. Qualitative information concerning the evolutionary process of cylindrical domain formation was obtained from the shapes of these globules.