Fluctuations, phase transitions, and latent heat in short diblock copolymers: Comparison of experiment, simulation, and theory

We present a detailed quantitative comparison of experimental results and theoretical predictions for the structure and thermodynamics of low molecular weight symmetric (f_L ≈ 1/2) poly(1,4-isoprene-b-dl-lactide) (IL) diblock copolymers near the order-disorder transition (ODT). Small-angle neutron and X-ray scattering (SANS and SAXS) measurements obtained in the disordered phase with IL degree of polymerization N = 39 were fit to the renormalized one-loop (ROL) theory in order to estimate the effective interaction parameter χ_e(T). Calorimetric measurements of the latent heat of the ODT for the same copolymer compare well with that obtained from corresponding coarse-grained simulations, when the comparison is based on this estimate of χ_e(T). The corresponding estimate of (χ_eN)_ODT at the experimental ODT of this polymer is much closer to the value obtained from simulations than to any theoretical prediction but differs from the simulation result by somewhat more than the bounds implied by experimental uncertainties. A larger discrepancy between simulation and experimental results for (χ_eN)_ODT is obtained for longer chains, with N ≥ 50, when also calculated using χ_e(T, N = 39). We discuss possible reasons for this discrepancy, including the possibility of a significant end-group effect for these polymers. These results confirm the overwhelming importance of fluctuation effects in short diblock copolymers, and the usefulness of coarse-grained simulations as a starting point for quantitative modeling, but also indicate the need for attention to nonuniversal features of specific polymers that can also become more important with decreasing chain length.