Perfectly-alternating linear (AB)n multiblock copolymers:: Effect of molecular design on morphology and properties

Perfectly-alternating linear (AB), multiblock copolymers consist of n AB block pairs covalently linked in an alternating sequence. Although these copolymers can microphase-order in the same fashion as their lower-order(n = I) diblock analogs, the 2(n - 1) biconformational midblocks comprising each copolymer molecule have a considerable impact on microstructural characteristics and bulk properties. We have applied transmission electron microscopy, differential scanning calorimetry (DSC), and extensional rheometry to examine and compare the morphologies and properties of two series of compositionally symmetric (lamellar) poly(styrene-b-isoprene)(n) (SI)(n) (1 less than or equal to n less than or equal to 4) multiblock copolymers. In one series, chain length was held constant allowing block mass (M,) to decrease with increasing n. In the second copolymer series, M, remained relatively invariant. Increasing n in these two series generally promoted reductions in both the lamellar period and upper (styrenic) glass-transition temperature, but noticeable increases in tensile modulus and yield strength. These observed trends are more pronounced in the copolymer series with constant chain length due to the coupled relationship between n and M-b. (C) 2001 John Wiley & Sons, Inc.