Rheological Evidence of Composition Fluctuations in an Unentangled Diblock Copolymer Melt near the Order-Disorder Transition

Rheological and small-angle X-ray scattering (SAXS) measurements were conducted on a symmetric, low molar mass (M-n = 17.6 kg/mol), poly(tert-butylstyrene-block-methyl methacrylate) (PtBS-PMMA) diblock copolymer near the order-disorder transition temperature (T-ODT = 193 +/- 1 degrees C). Evidence of composition fluctuations is apparent in the low frequency elastic (G') and loss (G '') moduli and in the temperature dependence of the peak scattering intensity, I(q*), up to 50 degrees C above the T-ODT. These findings demonstrate that chain entanglements are not responsible for the well-documented fluctuation mode in the terminal viscoelastic regime of block copolymer melts.