Lattice disordering/ordering and demicellization/micellization transitionsin highly asymmetric polystyrene-block-polyisoprene copolymers

Two kinds of transitions involving a change in molecular assembly, lattice disordering/ordering transition (LDOT) and demicellization/micellization tr ansition (DMT), in highly asymmetric polystyrene-bloch-polyisoprene (SI dib lock) copolymers were investigated using oscillatory shear rheometry, trans mission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). For the study, three highly asymmetric SI diblock copolymers having the vo lume fractions of polystyrene (PS) block of 0.11, 0.13, and 0.16, respectiv ely, were synthesized using anionic polymerization. In highly asymmetric bl ock copolymer, LDOT is defined as a transition where the long-range order o f microdomains is lost during heating, giving rise to a disordered arrangem ent of spheres with short-range liquidlike order (termed disordered spheres or micelles), and DMT is defined as a transition where all microdomains di sappear during heating and are transformed into the micelle-free homogeneou s state in which the component polymers are mixed on a molecular level and only thermally induced composition fluctuations may exist DMT may be regard ed as a pseudo-phase transition, involving effectively a finite number of m olecules and hence occurring over a finite range (possibly a narrow range) of temperatures. It was found that the three highly asymmetric SI diblock c opolymers underwent, during heating, first lattice disordering and then dem icellization transitions. The values of LDOT temperature (T-LDOT) and DMT t emperature (T-DMT) for the highly asymmetric SI diblock copolymers were det ermined by careful analysis of SAXS results, and the value T-DMT determined by oscillatory shear rheometry was found in good agreement with SAXS resul ts. The LDOT determined from SAXS was confirmed by TEM, showing the existen ce of disordered spheres at thermal equilibrium at T-LDOT less than or equa l to T less than or equal to T-DMT.