Microdomain structures and phase transitions in binary blends consisting of a highly asymmetric block copolymer and a homopolymer

Microdomain structures and phase transitions in binary blends consisting of a highly asymmetric block copolymer and a homopolymer were investigated us ing transmission electron microscopy (TEM), small-angle X-ray scattering (S AXS), and oscillatory shear rheometry. For the study, a polystyrene-block-p olyisoprene-block-polystyrene copolymer (Vector 4111) was mixed with a low molecular weight polystyrene (PS), where Vector 4111 has a 0.16 volume frac tion of PS blocks. TEM images show that during heating from room temperatur e each of the binary blends prepared (86/14, 73/27, 62/38, and 49/51 Vector 4111/PS) exhibits "distorted microdomains" that have lost long-range order and yet have retained a distinct interface before reaching the micelle-fre e (or microdomain-free) homogeneous phase. SAXS measurements show that, dur ing heating, each of the binary blends first undergoes lattice disordering transition and then demicellization transition. Following the terminologies introduced in our recent paper (Macromolecules 2000, 33, 3767), lattice di sordering/ordering transition (LDOT) is a transition where ordered microdom ains, during heating, transform into distorted microdimains (micelles witho ut long-range order) and the micelles transform, during cooling, into order ed microdomains, and demicellization/micellization transition (DMT) is a tr ansition where the micelles disappear, during heating, transforming into th e micelle-free homogeneous state and micelles are formed, during cooling, f rom the micelle-free homogeneous phase. Analysis of SAXS results enabled us to determine the LDOT temperature (T-LDOT) and DMT temperature (T-DMT). It was found that the T-DMT determined by SAXS agreed well with those determi ned by oscillatory shear rheometry and that the addition of PS to Vector 41 11 lowered the T-DMT of the block copolymer, the extent of which increased with increasing amounts of PS added. It is concluded. that a simple theolog ical study is not sufficient in order to unambiguously determine different transitions in highly asymmetric block copolymer and its blends with a homo polymer and that it is imperative to combine rheological measurements with structural investigation.