Order−Order and Order−Disorder Transitions in a Polystyrene-<i>block</i>-Polyisoprene-<i>block</i>-Polystyrene Copolymer

Abstract

We investigated the order−order transition (OOT) and the order−disorder transition (ODT) in a polystyrene-block-polyisoprene-block-polystyrene copolymer (Vector 4111, Dexco Polymers Co.), which has a weight-average molecular weight of 1.4 × 105 and a 0.183 weight fraction of polystyrene (PS) blocks, using small-angle X-ray scattering (SAXS) and rheological measurements. Specifically, SAXS experiments reveal that Vector 4111 undergoes an OOT at 179−185 °C with (i) hexagonally-packed cylindrical microdomains of PS at T ≤ 179 °C, (ii) the coexistence of cylindrical and spherical microdomains of PS at 180 °C ≤ T ≤ 185 °C, and (iii) spherical microdomains of PS in a cubic lattice at 185 °C < T ≤ 210 °C, and (iv) the cubic lattice of the spheres is distorted with a further increase of temperature so that the lattice disordering transition occurs at temperatures between 210 (onset) and 214 °C (completion) and the spherical microdomain structure of PS with a liquidlike short-range order persists even up to 220 °C, the highest experimental temperature employed. The SAXS results are confirmed by independent rheological measurements. Transmission electron microscopy (TEM) was employed to investigate the microdomain structures of Vector 4111 at elevated temperatures by rapidly quenching specimens, which had been annealed at a predetermined temperature, into ice water. TEM results reveal that (i) at 170 °C Vector 4111 has hexagonally-packed cylindrical microdomains of PS, (ii) at 185 °C the cylindrical and spherical microdomains of PS coexist, and (iii) as the temperature is increased further to 220 °C, only spherical microdomains of PS persist; i.e., the order−disorder transition temperature (TODT) of Vector 4111 is higher than 220 °C. We found from the present study that the dynamic temperature sweep experiment under isochronal conditions failed to accurately determine the TODT of Vector 4111 having a low volume fraction of PS blocks, while log G' vs log G'' plots were very effective to investigate both OOT and ODT in Vector 4111.