ORDER, DISORDER, AND COMPOSITION FLUCTUATION EFFECTS IN LOW MOLAR-MASS HYDROCARBON-POLY(DIMETHYLSILOXANE) DIBLOCK COPOLYMERS

Four block copolymer melts-two poly(isoprene-bloch-dimethylsiloxane) ( PI-PDMS) and two oly(ethylene-alt-propylene-block-dimethylsiloxane) (P EP-PDMS)-have been investigated through rheological measurements, SANS , and SAXS. In both systems, lamellar to disorder and <Ia(3)over bar d > bicontinuous to disorder transitions have been observed. Upon heatin g, the sequence of phases lamellae, modified layers, bicontinuous <Ia( 3)over bar d>, disorder is observed in the f = 0.65 PEP-PDMS sample. C omposition fluctuations dominate the disordered state properties over an unprecedented wide temperature window in the f = 0.49 PEP-PDMS samp le. The fluctuations are evident both in SANS measurements and through the temperature dependence of the viscosity. The phase behavior of th ese relatively low molar mass block copolymer melts, which range from 6 to 10 kg/mol, is qualitatively anticipated by the trends reported ea rlier for progressively lower molar mass diblock copolymers.