Effects of sample preparation method and thermal history on phase transition in highly asymmetric block copolymer: Comparison with symmetric block copolymers

Symmetric and highly asymmetric polystyrene-block-polyisoprene-block-polystyrene (SIS triblock) copolymers were synthesized via sequential anionic polymerization using difunctional initiator, and symmetric and highly asymmetric polystyrene-block-polyisoprene (SI diblock) copolymers were also synthesized using monofunctional initiator. The molecular weight of the triblock copolymer was approximately twice that of the diblock copolymer. The phase transition temperatures of the SIS triblock and SI diblock copolymers were determined using oscillatory shear rheometry and small-angle X-ray scattering (SAXS), respectively. It has been found that the sample preparation methods (solvent casting vs compression molding) and annealing conditions employed have a profound influence on the "apparent" phase transition temperatures of the highly asymmetric block copolymers that undergo order-disorder transition (ODT) involving lattice disordering/ordering transition (LDOT) followed by the demicellization/ micellization transformation (DMT) process, but little effect on the ODT temperature (T-ODT), where transition occurs between the ordered lamellae or cylinders and micelle-free disordered state with only thermal composition fluctuations, for the symmetric or nearly symmetric block copolymers. It has been observed, via transmission electron microscopy and SAXS, that highly asymmetric SIS triblock and SI diblock copolymers form thermodynamically stable disordered micelles between T-ODT and DMT temperature (T-DMT), and the disordered micelles transform into the micelle-free homogeneous state only with thermally induced composition fluctuations at temperatures above T-DMT. It has been found that T-DMT is much higher than T-ODT, that the T-ODT of symmetric SIS triblock copolymer is 24 +/- 2 degreesC higher than that of the corresponding SI diblock copolymer, and that the T-DMT of highly asymmetric SIS triblock copolymer is 23 +/- 2 degreesC higher than that of the corresponding SI diblock copolymer.