Influence of the interphase on block copolymer thermodynamics: Extension of the leary model

The Leary–Williams model for the microphase thermodynamics of triblock ABA copolymers has been modified to accommodate deviations from homogeneous random-coil configurations in the B-chain dimensions as well as in those of the A chains, and has also been extended to cover the case of diblock AB copolymers. Only planar morphology is considered, but qualitative conclusions reported herein are expected to hold for other morphologies as well. The focus is on interphase thickness ΔT, with predictions made also for separation temperature T_s and planar repeat distance D. Results are presented as systematic functions of copolymer composition (0 ≤ ?_A ≤ 1), total molar volume (25,000 ≤ ? ≤ 4 × 10⁶ cm³/g mol), block architecture (AB vs. ABA), temperature (298, 373 K), and for five different interphase composition profiles. In most cases, A represents a polystyrene block and B a butadiene block in these calculations. Predictions for ΔT increase with temperature and depend on architecture, profile, and ?; comparisons with data are close, in the range 15–30Å. It is shown that T_s depends strongly on profile choice and ?_A, reaching a maximum in the ?_A midrange but always with ?_A > 0.5. The major parameter influencing D (at constant ?) is architecture, with D(SB) ≈ 2D(SBS), and D(?) varies from D ∝ ?^0.75 at low ? to D ∝ ?^0.5 at high ?.