The pathway and kinetics for the cylinder-to-sphere order−order transition in a mixture of a matched diblock and triblock copolymer of styrene and ethylene−butene-1 is reported. The microstructure transformation was monitored by viscoelastic measurements, and the structural assignments of the intermediate states were performed by electron microscopy. The kinetics of transformation from macroscopically unaligned wormlike cylindrical microdomains to spherical microdomains arranged on a bcc lattice were extremely slow. The wormlike cylinder-to-sphere transition slowed with decreasing quench depth from the order−disorder transition. Additionally, these order−order kinetics were in quantitative agreement with those for the development of spherical microdomains from an initial disordered state. Further, during the initial induction time following a temperature jump from a wormlike cylindrical order to a spherical state, the sample exhibited liquidlike viscoelastic characteristics and structurally showed the absence of long-range order. In contrast, shear-aligned cylinders rapidly transformed to spheres, adopting a viscoelastic pathway distinct from that of the unaligned samples. The cylinder-to-sphere transition is thermotropically reversible, with the viscoelasticity-based kinetics of the sphere-to-cylinder transition being slower than the cylinder-to-sphere transition.

The pathway and kinetics of the order-order transition from shear-aligned cylindrical microdomains to spheres arranged on a bcc lattice in a highly asymmetrical block copolymer are monitored using small-angle neutron scattering (SANS). Shear-aligned hexagonally closed-packed cylindrical microdomains were heated to the spherical microdomain region under quiescent conditions, and the microstructure evolution associated with the order-order transition was followed by SANS. The data indicate a rapid decrease in the cylindrical microdomain order and a gradual rise in the signatures associated with the spherical microdomains, thereby suggesting the existence of an intermediate state where the sample is poorly ordered. The duration of this intermediate state increases with increasing distance from the order-order transition and is consistent with previous rheological measurements on unaligned wormlike cylinders. The results of the scattering measurements are compared to the predictions of recent theories describing the cylinder-to-sphere transition.

The phase behavior and linear viscoelasticity of a mixture of a matched diblock and triblock copolymer of styrene and ethylene−butene-1 are reported. The mixture undergoes reversible thermotropic transitions from a cylindrically ordered state at low temperatures to a spherically ordered state at intermediate temperatures to a disordered state at high temperatures. The morphological assignments were based on TEM and SANS measurements, which also revealed a decrease in the domain size in the spherically ordered state as compared to the cylindrically ordered state. The kinetics of the order−order transition are extremely slow, with the sample exhibiting rheological characteristics similar to a disordered material immediately after heating from the cylindrical to the spherical state. The transition exhibits a long induction period wherein the sample continues to behave like a disordered material followed by a rapid increase in the storage modulus and a final plateau value. The kinetics and rheological characteristics of a material quenched from the disordered state to the spherical state are remarkably similar to those obtained after heating from the cylindrical to the spherical state, suggesting that the transition from cylindrically ordered to spherically ordered microdomains proceeds via a mesoscopically disordered state.

Physical, mechanical and thermal properties of a new class of isobutylene based model block copolymers are studied and contrasted with those of traditional thermoplastic elastomers based on polydienes and saturated polydienes (polyolefins) such as Kraton (or Vector) and Kraton G block copolymers. Melt state rheological and dynamic mechanical measurements confirm that thermodynamic interactions between polystyrene (S) and polyisobutylene (iB) or poly‐p‐tert‐butylstyrene (tbS) and iB are comparable to, if not larger than, those between S and polybutadiene (B) or S and ethylene/butene‐1 copolymer (EB). Physical properties for the S‐iB‐S and tbS‐iB‐tbS block copolymers, particularly the injection‐moldability and cut‐growth characteristics, are found to be considerably different from those found for the S‐B‐S and S‐EB‐S systems. We attribute this behavior to the longer entanglement chain length of iB compared with those for B and EB.