Complex Phase Behavior of Polyisoprene-Polystyrene Diblock Copolymers Near the Order-Disorder Transition

Abstract

The phase behavior of polyisoprene-polystyrene (PI-PS) diblock copolymers near the orderdisorder transition was investigated using dynamic mechanical measurements, transmission electron microscopy, and small-angle X-ray and neutron scattering at polyisoprene volume fractions 0.33 < /pi < 0.42. Two new ordered morphologies have been documented between hexagonally packed cylinders (HEX) and lamellae (LAM). At low temperatures a hexagonally perforated layered (HPL) microstructure occurs, that transforms into a bicontinuous cubic phase characterized by laid space group symmetry near the orderdisorder transition (ODT). The ordered-bicontinuous double diamind (OBDD) phase was not found at any temperature or composition studied. We demonstrate that the bicontinuous laid phase exists at equilibrium over a narrow range of compositions, 0.35 5 /pi 5 0.40 bounded by the HEX, LAM, HPL, and disordered states, and is restricted to within about 60 °C of the ODT, suggesting the importance of finite size effects. This cubic phase also exhibits pronounced metastability, remaining intact for extended periods of time after cooling below the Iaid-HEX or 7a3d-HPL phase boundary. Cooling from above the ODT to below the PS glass transition temperature leads to a vitrified form of the laid state.

Phase Behavior of a Block Copolymer in Solvents of Varying Selectivity

“…Upon heating to 15°C, an OOT is observed as the higher order C 2 reflections disappear, and a pronounced shoulder appears on the q* peak. The first two peaks are spaced at ratios of 6: 8, in agreement with the first two allowed reflections of G. 39,40 Additional allowed reflections are indicated at higher q values, but their assignment is difficult due to the number of possible reflections and the broad peaks. Heating to 25°C leads to another thermotropic OOT into L, as indicated by the three reflections at peak ratios of 1:2:3, and heating to 55°C causes the higher order peaks to disappear and the principal peak broadens considerably.…”

“…This is presumably attributable to the subtle balance of chain stretching, packing frustration, and conformational asymmetry that ultimately determines the precise location of the L/G/C phase boundaries. [48][49][50][51] Experiment 39,40 and theory [48][49][50] show that the G phase is only stable in weakly segregated block copolymers; it is destabilized at stronger segregations due to a heightened free energy penalty of packing frustration in both the minor and major domains. Hajduk et al attributed the near absence of G in aqueous solutions of a PEO-poly(ethylethylene) diblock to the increase in segregation imparted by the strong selectivity of the water for PEO.…”

Enthalpy-driven selective loading of CdSe 0.75 S 0.25 nanoalloys in triblock copolymer polystyrene- b -polyisoprene- b -polystyrene

“…The formation of self-assembled structures is driven by the microphase separation of different blocks of PS-SIS competing with the chemical linking between them. The findings observed here are consistent with those of the previous studies that have examined the effect of the volume ratio f PS of the block copolymers on a variety of copolymer morphologies [25,[51][52][53][54][55][56].…”

Morphology and elasticity of polystyrene-block-polyisoprene diblock copolymers in the melt

“…Furthermore the critical stress is a function of the lattice spacing and the temperature in relation to T ODT (Sebastian et al (2002a).) The phase behavior of various PS-b-PI diblock copolymers was 3 investigated by Förster et al (1994) using transmission electron microscopy (TEM), scattering techniques and rheological experiments. Their study led to a detailed phase diagram of these diblock copolymers with a volume fraction of polyisoprene between 0.33 and 0.42.…”