Order-disorder transition temperature depression of a diblock copolymer induced by the addition of a random copolymer

Abstract

The change in the order-disorder transition temperature (T-ODT) of a diblock copolymer, induced by the addition of a random copolymer, was investigated with synchrotron small-angle X-ray scattering and differential scanning calorimetry techniques. The studied block copolymer was poly(styrene-b-butadiene) containing 52 wt% styrene, and the added random copolymer was a small amount of poly(styrene-r-butadiene) containing 50 wt% styrene. The observed transition from the isotropic melt to the lamellar ordered structure appeared as a pronounced discontinuity in the peak's height and shape, and the estimated T-ODT was found to decrease as the fraction of a random copolymer increased within the solubility limit (about 15 wt%). It was noted that the interdomain distance (D) decreased slightly upon the addition of a random copolymer, suggesting that the added random copolymer may have dissolved into the interfacial region of the microdomain. The plot of the reciprocal scattering maximum S-1(q*) as a function of the reciprocal absolute temperature T-1 can be fitted satisfactorily to the theoretical prediction based on the work of Fredrickson and Helfand. Also, the shift of T-ODT in the blend is calculated with the theoretical prediction developed by Whitmore and Noolandi. The spinodal temperature (T-S) and the T-ODT decreased along a similar slope with the addition of a random copolymer.